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Explorations and development of bis(N-heterocyclic carbene) pincer complexes for the electrocatalytic… Therrien, Jeffrey A. 2016

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Explorations and Development of  Bis(N-Heterocyclic Carbene) Pincer Complexes for the Electrocatalytic Reduction of Carbon Dioxide  by  Jeffrey A. Therrien B.Sc. (Hons), The University of British Columbia – Okanagan, 2009   A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY  in  The Faculty of Graduate and Postdoctoral Studies (Chemistry)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  December 2016 © Jeffrey A. Therrien, 2016ii  Abstract A variegated series of bis(N-heterocyclic carbene) (NHC) pincer complexes have been tested for the ability to act as homogeneous CO2 reduction electrocatalysts. Structure-activity relation-ships were investigated in order to enable the rational design of improved electrocatalysts and provide a platform for further development of mono- and bimetallic electrocatalysts within the pincer motif.  Pyridine- and lutidine-linked bis-NHC palladium pincer complexes were screened for CO2 re-duction capability with trifluoroethanol, acetic acid, and trifluoroacetic acid as proton sources. The lutidine-linked pincer complexes were found to electrocatalytically reduce CO2 to CO at potentials as low as -1.65 V vs. Fc0/+ in the presence of trifluoroacetic acid. The one-electron reduction of these complexes is shown to be chemically reversible, resulting in a monometallic species in so-lution. Computational models indicate charge transfer from a redox-active ligand upon interaction of the reduced species with CO2, thus potentially addressing a source of deactivation in earlier pincer electrocatalysts. The presence of Lewis acids in solution was also investigated, assisting reactivity with CO2. The NHC moieties of these lutidine-linked Pd pincer complexes were modified by phenanthro- and pyreno-annulation to investigate the effect of an extended NHC π-system on their electro-chemical reactivity with CO2. The polyannulated NHC groups are shown to be additional sites for redox-activity in the pincer ligand, enabling increased electron donation and activation of CO2. Following this, modification of the pyridyl para position is reported (R = OMe, H, Br, and COOR), allowing the first reduction potential to be tuned over a 1 V range in relation to the substituent’s Hammett σp constant, and labilizing the trans ligand in the case of electron-donating substituents, iii  thus improving activity for one-electron reduced species. Finally, analogous Ni and Pt bis-NHC pincer complexes are synthesized, characterized, and compared to Pd, with the Pd bis(benzimid-azol-2-ylidene) pincer complexes exhibiting the best performance with faradaic yields for CO pro-duction approaching 50% in the presence of trifluoroacetic acid. The remaining current resulted in the production of H2, thus producing a CO-rich synthesis gas mixture as the overall product.          iv  Preface In all chapters, Prof. Michael O. Wolf acted in a supervisory role. I am the principal author of the work reported in this thesis and have performed all of the experiments, modelling, calculations, and analysis unless otherwise indicated. Solid state structures from X-ray crystallography were collected and solved by Dr. Brian O. Patrick at the University of British Columbia. Dr. Thamy Sriskandakumar and Wei Xue assisted with EPR spectroscopy experiments and signal simulation. Large portions of Chapters 2 and 3 have been previously published,1,2 with a modified version of Chapter 4 accepted for publication.     The following publications have arisen from this work: Therrien, J. A.; Wolf, M. O.; Patrick, B. O. Electrocatalytic Reduction of CO2 with Palladium Bis-N-Heterocyclic Carbene Pincer Complexes. Inorg. Chem. 2014, 53, 12962–12972. Therrien, J. A.; Wolf, M. O.; Patrick, B. O. Polyannulated Bis(N-Heterocyclic Carbene) Palladium Pincer Complexes for Electrocatalytic CO2 Reduction. Inorg. Chem. 2015, 54, 11721–11732. Therrien, J. A.; Wolf, M. O. The Influence of para Substituents in Bis(N-Heterocyclic Carbene) Palladium Pincer Complexes for Electrocatalytic CO2 Reduction. Inorg Chem. In press.   v  Table of Contents Abstract .......................................................................................................................................... ii Preface ........................................................................................................................................... iv Table of Contents .......................................................................................................................... v List of Tables ................................................................................................................................ ix List of Figures ............................................................................................................................... xi List of Schemes ......................................................................................................................... xxiii List of Symbols and Abbreviations ........................................................................................ xxiv Acknowledgements ................................................................................................................ xxviii Dedication .................................................................................................................................. xxx Chapter 1: Introduction ......................................................................................................... 1 1.1 Background ...................................................................................................................... 1 1.2 Carbon Dioxide, the Molecule ......................................................................................... 5 1.3 Overview of Homogeneous Carbon Dioxide Reduction Electrocatalysts ....................... 8 1.3.1 Nitrogen-Containing Tetracoordinate Ligands on Fe, Co, and Ni.......................... 10 1.3.2 Group 7 Bipyridine Carbonyl Complexes, and Related Complexes ...................... 14 1.3.3 Square Planar Triphosphine Complexes ................................................................. 21 1.4 Summary and Discussion ............................................................................................... 27 1.5 Heterogeneous Electrocatalysts...................................................................................... 28 1.6 Towards Pincer Complexes with a Redox-Active Ligand ............................................. 29 1.7 A Brief Primer to N-Heterocyclic Carbene Bonding ..................................................... 30 1.8 Goals and Scope of Thesis .............................................................................................. 31 Chapter 2: Initial Investigations of Bis-NHC Pincer Complexes for the Electrocatalytic  Reduction of Carbon Dioxide ........................................................................... 33 2.1 Introduction .................................................................................................................... 33 vi  2.2 Results & Discussion ..................................................................................................... 34 2.2.1 Electrochemical Characterization ........................................................................... 35 2.2.2 Electrochemical Behaviour in the Presence of CO2 ............................................... 40 2.2.3 DFT Modelling of Reduced Species ....................................................................... 40 2.2.4 DFT Modelling of Interaction with CO2 ................................................................. 42 2.2.5 Electrochemical Response to Brønsted Acids ........................................................ 44 2.2.6 Preparative-Scale Controlled Potential Electrolysis (CPE) .................................... 47 2.2.7 Effects of Additives: Organic and Alkali Ions ........................................................ 50 2.2.8 Solvento Species ..................................................................................................... 53 2.2.9 Testing for Reactivity by a Heterogeneous Species ............................................... 57 2.3 Conclusions .................................................................................................................... 58 2.4 Experimental .................................................................................................................. 59 2.4.1 General .................................................................................................................... 59 2.4.2 Electrochemistry ..................................................................................................... 59 2.4.3 Electron Paramagnetic Resonance Spectroscopy ................................................... 60 2.4.4 Computational Methods .......................................................................................... 61 2.4.5 Synthesis ................................................................................................................. 61 Chapter 3: Polyannulated Bis-NHC Palladium Pincer Complexes ................................. 64 3.1 Introduction .................................................................................................................... 64 3.2 Results & Discussion ..................................................................................................... 67 3.2.1 Synthesis & Structure ............................................................................................. 67 3.2.2 Electrochemical Characterization under N2 and CO2 ............................................. 71 3.2.3 Electrochemical Effects of Stabilizing Cations ....................................................... 76 3.2.4 DFT Modelling & Investigations ............................................................................ 77 3.2.5 Controlled Potential Electrolysis ............................................................................ 85 vii  3.3 Conclusions .................................................................................................................... 92 3.4 Experimental .................................................................................................................. 93 3.4.1 General .................................................................................................................... 93 3.4.2 Electrochemistry ..................................................................................................... 94 3.4.3 Computational Methods .......................................................................................... 95 3.4.4 Synthesis ................................................................................................................. 95 Chapter 4: The Influence of Pyridyl para Substituents in Bis-NHC Palladium Pincer  Complexes ........................................................................................................ 101 4.1 Introduction .................................................................................................................. 101 4.2 Results & Discussion ................................................................................................... 103 4.2.1 Synthesis and Characterization ............................................................................. 103 4.2.2 Reactivity of R = Br Species with Acid and Base ................................................ 108 4.2.3 Electrochemical Behaviour under N2 and CO2 ..................................................... 109 4.2.4 Density Functional Theory Modeling ................................................................... 116 4.2.5 Controlled Potential Electrolysis (CPE) Experiments .......................................... 125 4.3 Conclusions .................................................................................................................. 129 4.4 Experimental ................................................................................................................ 130 4.4.1 General .................................................................................................................. 130 4.4.2 Electrochemistry ................................................................................................... 130 4.4.3 Computational Methods ........................................................................................ 131 4.4.4 Synthesis ............................................................................................................... 132 Chapter 5: Traversing Group 10 – A Comparison of Nickel, Palladium, and Platinum  Analogues ......................................................................................................... 143 5.1 Introduction .................................................................................................................. 143 5.2 Results & Discussion ................................................................................................... 145 5.2.1 Synthesis of [Pd(bC^N^bC)Cl]OTf and [Pd(bC^N^bC)CH3CN](OTf)2 ............. 145 viii  5.2.2 Synthesis of [Ni(bC^N^bC)Cl]OTf ...................................................................... 145 5.2.3 Synthesis of [Ni(bC^N^bC)CH3CN](OTf)2 ......................................................... 147 5.2.4 Synthesis of [Pt(bC^N^bC)Br]OTf and [Pt(bC^N^bC)CH3CN](OTf)2 ............... 149 5.2.5 Characterization of Complexes ............................................................................. 151 5.2.6 Fluxionality of [Ni(bC^N^bC)Cl]OTf and [Ni(bC^N^bC)CH3CN](OTf)2 .......... 154 5.2.7 Electrochemical Characterization ......................................................................... 156 5.2.8 DFT Modelling ..................................................................................................... 162 5.2.9 Controlled Potential Electrolysis .......................................................................... 164 5.3 Discussion .................................................................................................................... 168 5.4 Conclusions .................................................................................................................. 170 5.5 Experimental ................................................................................................................ 171 5.5.1 General .................................................................................................................. 171 5.5.2 Electrochemistry ................................................................................................... 171 5.5.3 Computational Methods ........................................................................................ 172 5.5.4 Synthesis ............................................................................................................... 173 Chapter 6: Conclusions and Future Work ....................................................................... 179 6.1 Conclusions .................................................................................................................. 179 6.2 Future Work ................................................................................................................. 182 References .................................................................................................................................. 185 Appendix A:  NMR Data .......................................................................................................... 193 Appendix B:  FTIR Data .......................................................................................................... 211 Appendix C:  Electrochemical Data ........................................................................................ 219 Appendix D:  Crystallography Data ....................................................................................... 228 Appendix E:  DFT Input Code and Calculated Geometries ................................................. 232  ix  List of Tables Table 1-1. Equilibrium Potentials of CO2 and H+ Reduction Half-Reactions in Aqueous Solution at pH 7 vs. SHE and Fc0/+ in CH3CN (converted from V vs. SHE)12–14 ......................................... 4 Table 1-2. Solubility of CO2 in Various Polar Solvents24–26 .......................................................... 7 Table 2-1. Electrochemical Peak Potentials vs. Fc0/+ ................................................................... 39 Table 2-2. Preparative-Scale Controlled Potential Electrolysis Resultsa ..................................... 47 Table 2-3. Preparative-scale CPE Results with Cation Additivesa .............................................. 52 Table 2-4. Peak Potentials of the First Cathodic Wave for Solvento vs. Halido Complexes ...... 54 Table 3-1. Peak Potentials for [Pd(phC^N^phC)Cl]+ and [Pd(pyC^N^pyC)Cl]+ ........................ 72 Table 3-2. Peak Potentials vs. Fc0/+ of First Cathodic Wave for Solvento and Chlorido Complexesa ................................................................................................................................... 76 Table 3-3. Calculated Degree of CO2 Activation when Bound to Reduced Complexes ............. 83 Table 3-4. Correlation between Calculated Degree of CO2 Activation at Bonding Potential Energy Minimum and Required Proton Source18,19,64,111 .............................................................. 83 Table 3-5. Controlled Potential Electrolysis Results for [Pd(phC^N^phC)Cl]+ a ........................ 86 Table 3-6. Controlled Potential Electrolysis Results for [Pd(pyC^N^pyC)Cl]+ a ........................ 88 Table 3-7. Controlled Potential Electrolysis Results for [Pd(phC^N^phC)(CH3CN)]2+ a ........... 89 Table 3-8. CPE Results for [Pd(phC^N^phC)(CH3CN)]2+ with 10 mM HPF6 a .......................... 90 Table 3-9. Controlled Potential Electrolysis Results for [Pd(pyC^N^pyC)(CH3CN)]2+ a ........... 91 Table 4-1. Peak Potentials of Complexesa versus Fc0/+ ............................................................. 111 Table 4-2. Calculated CO2 Bond Angles at Pd-CO2 Distance of 2.0 Å ..................................... 123 Table 4-3. Results of CPE Experimentsa ................................................................................... 126 Table 4-4. Results of CPE Experiments with Mg2+ Addeda ...................................................... 127 x  Table 5-1. Peak Potentials of Halido Complexesa versus Fc0/+ ................................................. 159 Table 5-2. Peak Potentials of Solvento Complexesa versus Fc0/+ .............................................. 160 Table 5-3. Calculated NPA Charges and NBO dz2 Orbital Energies ......................................... 163 Table 5-4. Results from CPE Experimentsa ............................................................................... 167  Table C-1. Steady-state currents of a 5 mL CO2-saturated solution of 10 mM TFA in 0.10 M [n-Bu4N]PF6/DMF with and without 2 mM [Pd(C^N^C)Cl]BF4 present, and resulting proportions of current with [Pd(C^N^C)Cl]BF4 present resulting from direct reaction of H+ and/or CO2 at the glassy carbon electrode. .............................................................................................................. 224 Table C-2. Preparative controlled potential electrolysis results from alkali and organic cations in CO2-saturated 0.10 M [n-Bu4N]PF6/DMF solutions.a ................................................................ 225  Table D-1. Crystal and Refinement Data for [Pd(C^N^C)(CH3CN)](BF4)2 .............................. 229 Table D-2. Crystal and Refinement Data for [Pd(phC^N^phC)Cl]OTf ..................................... 231    xi  List of Figures Figure 1-1. Greenhouse gas emissions by type and economic sector (left) and country (right). Adapted from ref.3 .......................................................................................................................... 1 Figure 1-2. Global carbon dioxide emissions over time. Adapted from ref.4 ................................ 2 Figure 1-3. Energy densities of various materials.6,7 ..................................................................... 3 Figure 1-4. Dipole moments and partial charges in CO2 (top left), DFT-calculated isosurfaces of degenerate π* LUMOs (ωB97xD/D95(d)) (left), and qualitative MO diagram of CO2 adapted from ref (right).24 ............................................................................................................................ 6 Figure 1-5. Representative examples of CO2 reduction electrocatalysts bearing tetracoordinate ligands. .......................................................................................................................................... 10 Figure 1-6. CO2 bound to trans-I and trans-III conformations of Ni(cyclam). Adapted from ref.38....................................................................................................................................................... 12 Figure 1-7. NHC-containing tetradentate nickel complexes.16,17 ................................................. 14 Figure 1-8. Examples of group 7 bipyridine carbonyl CO2 reduction electrocatalysts. .............. 17 Figure 1-9. Examples of complexes closely related to group 7 bipyridine carbonyl complexes, where bipyridine has been replaced by another potentially redox-active neutral bidentate ligand........................................................................................................................................................ 19 Figure 1-10. [Pd(triphos)(CH3CN)]2+ electrocatalysts and related complexes.1,69 ...................... 22 Figure 1-11. Variations of [Pd(triphosphine)(CH3CN)]2+ to facilitate cooperative activation of CO2. ............................................................................................................................................... 25 Figure 1-12. Cooperative activation of CO2 by [NiFe] CO dehydrogenase. Adapted from ref.1 27 Figure 1-13. General design of pyridyl-linked bis-NHC pincer complexes. ............................... 30 xii  Figure 1-14. Electronic structure (left) and types of bonding between an NHC and a late transition metal (right).75 ............................................................................................................... 31 Figure 2-1. Cyclic voltammograms of [Pd(C^N^C)Cl]BF4 (left) and [Pd(bC^N^bC)Cl]BF4 (right) under N2 and CO2. ............................................................................................................. 36 Figure 2-2. Overlaid square-wave voltammograms of [Pd(C^N^C)Cl]BF4 (black) and [Pd(bC^N^bC)Cl]BF4 (green) taken at a frequency of 25 Hz with 5 mV potential steps and 25 mV amplitude................................................................................................................................ 36 Figure 2-3. Square-wave voltammograms of [Pd(C^N^C)Cl]BF4 after reduction at the peak potential of the first cathodic wave (-1.8 V) for varying amounts of time (left), and the one-electron reduced product of [Pd(C^N^C)Cl]BF4 after oxidation at the potential of the produced anodic wave (0.5 V) for varying amounts of time (right). ............................................................ 37 Figure 2-4. Cyclic voltammograms of [Pd(C-N-C)Br]Br (left) and [Pd(bC-N-bC)Br]Br (right) under N2 and CO2.......................................................................................................................... 38 Figure 2-5. Overlaid square-wave voltammograms for [Pd(C-N-C)Br]Br (left) and [Pd(bC-N-bC)Br]Br (right). The wave at -0.47 V is due to the internal reference decamethylferrocene. .................................................................................................................... 39 Figure 2-6. Structures and HOMO and LUMO orbital diagrams of model complexes DFT(C^N^C) and DFT(C-N-C). .................................................................................................. 41 Figure 2-7. DFT-calculated energetics for the approach of CO2 to a reduced complex, with CO2 bond angles given for each energy minimum, or in the absence of a minimum, at 2.0 Å. The CPCM solvent model with acetonitrile was employed in each case. The M-CO2 distance is measured from the metal center to the carbon of CO2. ................................................................. 42 xiii  Figure 2-8. Plots of CO2 bond angle and ligand pyridyl C-C bond lengths for the active species of Re(bpy)(CO)3Cl (left) and the one-electron reduced model species of [Pd(C^N^C)Cl]+ (right)........................................................................................................................................................ 44 Figure 2-9. Cyclic voltammograms of complexes [Pd(bC^N^bC)Cl]BF4 (left) and [Pd(bC-N-bC)Br]Br (right) under N2 and CO2 with and without 10 mM TFA. ............................................ 46 Figure 2-10. Cyclic voltammograms of 0.10 M [n-Bu4N]PF6/DMF solutions at 200 mV/s with no complex present, with and without 10 mM TFA under N2 and CO2. ...................................... 46 Figure 2-11. Plot of current over time during the electrolysis of [Pd(C^N^C)(CH3CN)](BF4)2 with 10 mM TFA under CO2 at -1.8 V vs. Fc0/+ after 10 C charge was already passed. .............. 48 Figure 2-12. Mass spectra of CPE headspace products formed from a solution of [Pd(C^N^C)(CH3CN)](BF4)2 sparged with 13CO2 only (left), and with a mixture of 12CO2 and 13CO2 (right). ................................................................................................................................. 50 Figure 2-13. ORTEP representation of solid-state structure of [Pd(C^N^C)(CH3CN)](BF4)2. ... 54 Figure 2-14. Overlaid square-wave voltammograms of [Pd(C^N^C)(CH3CN)](BF4)2 with 10 mM TFA under N2 and CO2, and [Pd(C^N^C)Cl]BF4 with 10 mM TFA under CO2. ................. 55 Figure 2-15. Overlaid square-wave voltammograms of [Pd(C-N-C)(CH3CN)](BF4)2 with 10 mM TFA under N2 and CO2 and [Pd(C-N-C)Br]Br with 10 mM TFA under CO2 (left), and CVs of  [Pd(C-N-C)(CH3CN)](BF4)2 under N2 and CO2 (right). ............................................................... 55 Figure 2-16. Experimental and simulated EPR spectra for the one-electron reduced product of [Pd(C^N^C)Br]PF6 (left) and a stable, computed structure with overlaid SOMO geometry for one-electron-reduced DFT(C^N^C) (right). ................................................................................. 57 Figure 3-1. Overlaid and normalized UV-Vis absorption spectra for [Pd(phC^N^phC)Cl]OTf and [Pd(pyC^N^pyC)Cl]OTf in acetonitrile. ................................................................................ 69 xiv  Figure 3-2. Solid state structure of [Pd(phC^N^phC)Cl]OTf. Triflate anion and acetonitrile within the unit cell are omitted. .................................................................................................... 70 Figure 3-3. Overlaid SWVs of [Pd(C^N^C)Cl]+, [Pd(bC^N^bC)Cl]+, [Pd(phC^N^phC)Cl]+, and [Pd(pyC^N^pyC)Cl]+ at a frequency of 25 Hz with 5 mV potential steps and 25 mV amplitude........................................................................................................................................................ 72 Figure 3-4. Cyclic voltammograms of [Pd(phC^N^phC)Cl]+ (left) and [Pd(phC^N^phC)Cl]+ (right) at 100 mV/s under N2 and CO2. ......................................................................................... 73 Figure 3-5. Cyclic voltammograms of [Pd(phC^N^phC)(CH3CN)]2+ (left) and [Pd(pyC^N^pyC)(CH3CN)]2+ (right) at 100 mV/s under N2 and CO2. ........................................ 75 Figure 3-6. Overlaid cyclic voltammograms of [Pd(phC^N^phC)Cl]+ at 100 mV/s under N2 and CO2 with and without 25 mM Mg(ClO4)2 present. ....................................................................... 76 Figure 3-7. Calculated geometries of low-lying unoccupied molecular orbitals for DFT(pyC^N^pyC). LUMO+2 is similar in energy and geometry to LUMO+1, but also contains some pyridyl character. ................................................................................................................. 79 Figure 3-8. HOMO and SOMO geometries of lowest energy geometry optimized structures of reduced species of DFT(phC^N^phC) (top) and DFT(pyC^N^pyC) (bottom) as pristine, non-halide-dissociated species. ............................................................................................................ 81 Figure 3-9. Overlaid CVs of [Pd(pyC^N^pyC)Cl]OTf at 100 mV/s under N2, CO2, and CO2 + 200 mM 2,2,2-trifluoroethanol. ..................................................................................................... 82 Figure 3-10. DFT-calculated energetics for the activation of CO2 as it approaches a reduced metal complex. Bond angles of CO2 are given at the energy minimum, unless otherwise stated. 84 xv  Figure 3-11. Plot of current over time during electrolysis of [Pd(phC^N^phC)Cl]OTf at -2.00 V under CO2 with TFA present (left) and overlaid cyclic voltammograms taken at various points during the experiment (right, variable concentrations of TFA between 2 – 8 mM). .................... 86 Figure 3-12. ATR-FTIR spectrum of insoluble precipitate formed during controlled potential electrolysis with [Pd(pyC^N^pyC)(CH3CN)](OTf)2 and [Pd(pyC^N^pyC)Cl]OTf. ................... 91 Figure 4-1. Correlation of Hammett σp constant to chemical shift of pyridyl meta-H atom from 1H NMR spectra of complexes taken in (CD3)2CO. ................................................................... 108 Figure 4-2. Calculated HOMO and LUMO geometries of bis(dimethylamino)phenyl [Ni(NCN)Cl] pincer complex. .................................................................................................... 110 Figure 4-3. Overlaid square-wave voltammograms (25 Hz) of 2 mM solutions of complexes in 0.10 M [n-Bu4N]PF6/DMF under N2 using a glassy carbon working electrode. ........................ 112 Figure 4-4. Linear correlation between first reduction potential of Pd(C^N^Cp-R)Cl+ complexes and Hammett σp substituent constant. ......................................................................................... 112 Figure 4-5. Overlaid and normalized cyclic voltammograms of [Pd(C^N^Cp-COOR)Cl]OTf at varied scan rates (left) with [Pd(phC^N^phC)Cl]+ at 800 mV/s as a representative comparison (right). ......................................................................................................................................... 113 Figure 4-6. Cyclic voltammograms of (A) [Pd(C^N^Cp-COOR)Cl]OTf, (B) [Pd(C^N^Cp-Br)Cl]OTf, (C) [Pd(C^N^Cp-H)Cl]BF4, and (D) [Pd(C^N^Cp-OMe)Cl]OTf taken at 100 mV/s under N2, CO2, and CO2 with 10 mM trifluoroacetic acid present. ............................ 114 Figure 4-7. Cyclic voltammograms of [Pd(C^N^Cp-Br)Cl]OTf in a CO2 sparged solution with TFA present after a controlled potential electrolysis experiment, and then with 25 mM Mg(ClO4)2 added. ....................................................................................................................... 116 xvi  Figure 4-8. LUMO geometries and energies (Ha) for DFT-modelled complexes. The HOMO geometries were identical to that of R = H (shown) for all complexes except R = NMe2 (shown)...................................................................................................................................................... 117 Figure 4-9. Linear fits of calculated LUMO and NPA values to experimental first reduction potentials, including varied NHC groups imidazol-2-ylidene, benzimidazol-2-ylidine (benzC), and phenanthroimidazol-2-ylidene (phenC) with R = H on the para position of the pyridyl (crossed entries, ×). ..................................................................................................................... 118 Figure 4-10. Calculated Pd-Cl bond length (left) and LUMO energy and Pd NPA charge (right) as a function of Hammett σp constant of para substituent. ......................................................... 119 Figure 4-11. Calculated LUMO energy and Pd NPA charge vs. first reduction potential......... 119 Figure 4-12. Calculated Pd-Cl bond length in one-electron reduced model species as a function of Hammett σp constant. ............................................................................................................. 120 Figure 4-13. Calculated energies of unreduced (left) and reduced (right) complexes as a function of Pd-Cl distance incremented from 2.3 to 3.0 Å relative to the energy of the optimized structure where the Pd-Cl distance is 2.3 Å, with geometry relaxed at each step. Inset graph retains axes of parent and shows energies of unreduced species from a Pd-Cl distance of 2.85 to 2.95 Å........ 121 Figure 4-14. Calculated energy profiles for approach of CO2 to reduced complex in vacuum, starting from axial position at 3.25 Å. ........................................................................................ 123 Figure 4-15. Calculated interaction energy of CO2 approaching reduced species in CPCM solvent field of acetonitrile.......................................................................................................... 124 Figure 4-16. Calculated energies of dz2 orbitals by Natural Bond Orbital (NBO) methods. ..... 125 xvii  Figure 5-1. 1H NMR spectrum of isolated impurities from column chromatography of as-synthesized [Ni(bC^N^bC)Cl]OTf wherein NiCl2(DME) was slowly added to the silver carbene proligand species. ........................................................................................................................ 146 Figure 5-2. 1H NMR spectrum from 5.0 to 7.5 ppm of as-synthesized [Pt(bC^N^bC)Br]OTf before purification by column chromatography. ......................................................................... 150 Figure 5-3. Stack plot of 1H NMR spectra from 3.5 to 9.0 ppm for [Ni], [Pd], and [Pt] in CD3CN. ....................................................................................................................................... 152 Figure 5-4. Solid state structures of [Ni(bC^N^bC)Cl]OTf (left) and [Pd(bC^N^bC)Cl]BF4 (right), with counterions and solvent molecules omitted. ........................................................... 153 Figure 5-5. Interconversion between conformers in [M(bC^N^bC)X]+ species. ...................... 154 Figure 5-6. Overlaid 1H NMR spectra of [Ni(bC^N^bC)Cl]OTf (left) and [Ni(bC^N^bC)CH3CN](OTf)2 (right) at varied temperatures in CD3CN. .................................. 156 Figure 5-7. Overlaid cyclic voltammograms (left) and square-wave voltammograms (right) of [Ni(bC^N^bC)Cl]OTf (A), [Pd(bC^N^bC)Cl]OTf (B), and [Pt(bC^N^bC)Br]OTf (C) under N2, CO2, and CO2 with 10 mM trifluoroacetic acid present.............................................................. 157 Figure 5-8. Overlaid cyclic voltammograms of [Ni(C^N^C)Cl]OTf taken at a scan rate of 100 mV/s under N2 and CO2. ............................................................................................................. 158 Figure 5-9. Overlaid cyclic voltammograms (left) and square-wave voltammograms (right) of [Ni(bC^N^bC)CH3CN](OTf)2 (A), [Pd(bC^N^bC)CH3CN](OTf)2 (B), and [Pt(bC^N^bC)CH3CN](OTf)2 (C) under N2, CO2, and CO2 with 10 mM TFA present. ............ 161 Figure 5-10. Calculated HOMO, LUMO, and LUMO+1 geometries for model complexes of [Ni(bC^N^bC)Cl]+, [Pd(bC^N^bC)Cl]+, and [Pt(bC^N^bC)Cl]+. ............................................. 162 xviii  Figure 5-11. Calculated energy profiles for the interaction of CO2 approaching one-electron reduced complexes from axial position at an initial distance of 3.25 Å from the metal center. . 164 Figure 5-12. Modeled [Ni] complex viewed side-on (left) and front-on (right), showing the twisted orientation of the pincer ligand. ..................................................................................... 169 Figure 6-1. General design of lutidyl-linked bis-NHC pincer complexes, showing modular components (pyridyl, NHC groups, metal center, monodentate ligand, and NHC N-substituents)...................................................................................................................................................... 181 Figure 6-2. Prospective bimetallic complexes through modification of the NHC N-substituents to include additional donors (left), and addition of a pyridyl-linked bridge (center and right). ..... 182 Figure 6-3. Preliminary DFT-modelling (ωB97xD/D95(d)/SDD, CPCM with CH3CN solvent) of interaction between CO2 and isoelectronic one-electron reduced species [Rh(C^N^C)Cl]- and [Pd(C^N^C)Cl]0. Dashed lines represent the CO2 bond angle at each distance increment. ....... 183 Figure 6-4. Calculated solution-state molecular and MO geometries for a naphthoquinone-annulated NHC species. .............................................................................................................. 184  Figure A-1. 1H NMR spectrum of [Pd(C^N^C)Cl]BF4 in (CD3)2SO. Residual solventa and waterb indicated. .......................................................................................................................... 193 Figure A-2. 1H NMR spectrum of [Pd(bC^N^bC)Cl]BF4 in (CD3)2SO. Residual solventa and waterb indicated. .......................................................................................................................... 193 Figure A-3. 1H NMR spectrum of [Pd(C-N-C)Br]Br in (CD3)2SO. Residual solventa and waterb indicated. ..................................................................................................................................... 194 Figure A-4. 1H NMR spectrum of [Pd(bC-N-bC)Br]Br in CD2Cl2. Residual solventa and waterb indicated. ..................................................................................................................................... 194 xix  Figure A-5. 1H NMR spectrum of [Pd(C^N^C)(CH3CN)](BF4)2 in (CD3)2SO. Residual solventa and waterb indicated. ................................................................................................................... 195 Figure A-6. 1H NMR spectrum of [Pd(C-N-C)(CH3CN)]BF4 in CD3OD. Residual solventa and waterb indicated. .......................................................................................................................... 195 Figure A-7. 1H NMR spectrum of 1-butyl-1H-phenanthro[9,10-d]imidazole in (CD3)2CO. Residual solventa and water signalsb indicated. .......................................................................... 196 Figure A-8. 1H NMR spectrum of 9-butyl-9H-pyreno[4,5-d]imidazole in (CD3)2SO. Residual solventa, waterb, and acetonitrilec signals are indicated. ............................................................. 196 Figure A-9. 1H NMR spectrum of phC^N^phC ∙ 2HBr in (CD3)2SO. Residual solventa and waterb signals are indicated. ........................................................................................................ 197 Figure A-10. 1H NMR spectrum of pyC^N^pyC ∙ 2HBr in (CD3)2SO. Residual solventa and waterb signals are indicated. ........................................................................................................ 197 Figure A-11. 1H NMR spectrum of [Pd(phC^N^phC)Cl]OTf in (CD3)2SO. Residual solventa and waterb signals indicated. Singlet at 1.15 ppmc is due to an impurity in as-received NMR solvent...................................................................................................................................................... 198 Figure A-12. 1H NMR spectrum of [Pd(pyC^N^pyC)Cl]OTf in (CD3)2SO. Residual solventa and waterb signals indicated. Singlet at 1.15 ppmc is due to an impurity in as-received NMR solvent...................................................................................................................................................... 198 Figure A-13. 1H NMR spectrum of [Pd(phC^N^phC)(CH3CN)](OTf)2 in CD3CN. Inset: 1H resonance for CH3CN at 1.96 ppm. Residual solventa and waterb signals indicated. ................. 199 Figure A-14. 1H NMR spectrum of [Pd(pyC^N^pyC)(CH3CN)](OTf)2 in CD3CN. Inset: 1H resonance for CH3CN at 1.96 ppm. Residual solventa and waterb signals indicated. ................. 199 xx  Figure A-15. 1H NMR spectrum of C^N^Cp-OMe ∙ 2HBr in (CD3)2SO. Residual solventa and waterb indicated. .......................................................................................................................... 200 Figure A-16. 1H NMR spectrum of [Pd(C^N^Cp-OMe)Cl]OTf in CD3CN. Residual solventa and waterb indicated. .......................................................................................................................... 200 Figure A-17. 1H NMR spectrum of [Pd(C^N^Cp-OMe)(CH3CN)](OTf)2 in CD3CN. Residual solventa and waterb indicated. ..................................................................................................... 201 Figure A-18. 1H NMR spectrum of C^N^Cp-Br ∙ 2HBr in CD2Cl2. Residual solventa and waterb indicated. ..................................................................................................................................... 201 Figure A-19. 1H NMR spectrum of [Pd(C^N^Cp-Br)Cl]OTf in CD3CN. Residual solventa and waterb indicated. .......................................................................................................................... 202 Figure A-20. 1H NMR spectrum of [Pd(C^N^Cp-Br)(CH3CN)](OTf)2 in CD3CN. Residual solventa and waterb indicated. ..................................................................................................... 202 Figure A-21. 1H NMR spectrum of 4-hydroxybutyl 2,6-bis(bromomethyl)isonicotinate in (CD3)2CO. Residual solventa and waterb indicated..................................................................... 203 Figure A-22. 1H NMR spectrum of C^N^Cp-COOR ∙ 2HBr in (CD3)2SO. Residual solventa, waterb, and diethyl etherc indicated.97 ..................................................................................................... 203 Figure A-23. 1H NMR spectrum of [Pd(C^N^Cp-COOR)Cl]OTf in (CD3)2CO. Residual solventa and silicone greaseb indicated. Water signal suppressed by W5 pulse sequence.166 .................. 204 Figure A-24. 1H NMR spectrum of bC^N^bC ∙ 2HBr in (CD3)2SO. Residual solventa and waterb indicated. ..................................................................................................................................... 204 Figure A-25. 1H NMR spectrum of [Ni(bC^N^bC)Cl]OTf in CD2Cl2. Residual solventa, waterb, and BHTc indicated. .................................................................................................................... 205 xxi  Figure A-26. 1H NMR spectrum of [Ni(bC^N^bC)(CH3CN)](OTf)2 in CD3CN. Residual solventa, waterb, and BHTc indicated. ......................................................................................... 205 Figure A-27. 1H NMR spectrum of [Ni(C^N^C)Cl]OTf in CD2Cl2. Residual solventa and waterb indicated. ..................................................................................................................................... 206 Figure A-28. 1H NMR spectrum of [Pd(bC^N^bC)Cl]OTf in CD2Cl2. Residual solventa and waterb indicated. .......................................................................................................................... 206 Figure A-29. 1H NMR spectrum of [Pd(bC^N^bC)(CH3CN)](OTf)2 in CD3CN. Residual solventa and waterb indicated. ..................................................................................................... 207 Figure A-30. 1H NMR spectrum of [Pt(bC^N^bC)Br]OTf in CD3CN. Residual solventa and waterb indicated. .......................................................................................................................... 207 Figure A-31. 195Pt NMR spectrum of [Pt(bC^N^bC)Br]OTf in CD3CN. .................................. 208 Figure A-32. 1H NMR spectrum of [Pt(bC^N^bC)(CH3CN)](OTf)2 in CD3CN. Residual solventa and waterb indicated. ................................................................................................................... 208 Figure A-33. Stacked 1H NMR spectra of [Pd(phC^N^phC)Cl]OTf, [Pd(pyC^N^pyC)Cl]OTf, and [Pd(bC^N^bC)Cl]BF4 in (CD3)2SO as magnetic gradient strength is varied. ..................... 209 Figure A-34. Plot of monoexponentially fit DOSY data, yielding diffusion rates. .................... 209 Figure A-35. VT 1H NMR spectrum of [Pd(bC^N^bC)Cl]BF4 in (CD3)2SO. ........................... 210  Figure B-1. ATR-FTIR spectra of free CH3CN, [Pd(C^N^C)Cl]BF4, and [Pd(C^N^C)(CH3CN)](BF4)2, revealing a shifted nitrile stretch at 2342 cm-1. .......................... 211 Figure B-2. ATR-FTIR spectrum of [Pd(phC^N^phC)Cl]OTf. ................................................ 211 Figure B-3. ATR-FTIR spectrum of [Pd(phC^N^phC)(CH3CN)](OTf)2. ................................. 212 Figure B-4. ATR-FTIR spectrum of [Pd(pyC^N^pyC)Cl]OTf. ................................................ 212 Figure B-5. ATR-FTIR spectrum of [Pd(pyC^N^pyC)(CH3CN)](OTf)2. ................................. 213 xxii  Figure B-6. ATR-FTIR spectrum of [Pd(C^N^Cp-OMe)Cl]OTf.................................................. 213 Figure B-7. ATR-FTIR spectrum of [Pd(C^N^Cp-OMe)(CH3CN)](OTf)2. ................................. 214 Figure B-8. ATR-FTIR spectrum of [Pd(C^N^Cp-Br)Cl]OTf. ................................................... 214 Figure B-9. ATR-FTIR spectrum of [Pd(C^N^Cp-Br)(CH3CN)](OTf)2. .................................... 215 Figure B-10. ATR-FTIR spectrum of [Ni(bC^N^bC)Cl]OTf. .................................................. 215 Figure B-11. ATR-FTIR spectrum of [Ni(bC^N^bC)(CH3CN)](OTf)2. ................................... 216 Figure B-12. ATR-FTIR spectrum of [Pd(bC^N^bC)Cl]OTf. .................................................. 216 Figure B-13. ATR-FTIR spectrum of [Pd(bC^N^bC)(CH3CN)](OTf)2. ................................... 217 Figure B-14. ATR-FTIR spectrum of [Pt(bC^N^bC)Br]OTf. ................................................... 217 Figure B-15. ATR-FTIR spectrum of [Pt(bC^N^bC)(CH3CN)](OTf)2. .................................... 218  Figure C-1. Cyclic voltammograms of proligands phC^N^phC • 2HBr (left) and pyC^N^pyC • 2HBr (right) at 200 mV/s with added decamethylferrocene as an internal standard. ................. 227   xxiii  List of Schemes Scheme 1-1. Three Binding Modes of CO2a ................................................................................... 7 Scheme 1-2. Homogeneous Electrocatalytic Reduction of CO2a ................................................... 9 Scheme 1-3. Electrocatalytic Cycle for M(bpy)(CO)3X (M = Re, Mn; X = Cl, Br) and Related Complexes..................................................................................................................................... 16 Scheme 1-4. Reported Catalytic Cycle for [Pd(triphos)(sol)]2+ (sol = CH3CN or DMF)1,67 ....... 24 Scheme 2-1. Proposed Equilibria for the Protonation of Pd-bound CO2. .................................... 45 Scheme 3-1. Proposed Equilibria for Protonation of Activated CO2. .......................................... 65 Scheme 3-2. Synthesis of Phenanthro- and Pyreno-annulated Chlorido and Acetonitrilo Species....................................................................................................................................................... 68 Scheme 4-1. Synthetic Route to [Pd(C^N^Cp-Br)Cl]OTf and [Pd(C^N^Cp-OMe)Cl]OTf ............ 104 Scheme 4-2. Synthetic Route to [Pd(C^N^Cp-COOR)Cl]OTf ....................................................... 106 Scheme 5-1. General Synthetic Route to M(bC^N^bC) Complexes ......................................... 151 Scheme 5-2. Putative Electrocatalytic Cycle for Reduction of CO2 with Pd(C^N^C) Complexes..................................................................................................................................................... 170    xxiv  List of Symbols and Abbreviations ∆ difference; heat Å angstrom acac acetylacetonato Anal. Analysis APCI-MS atmospheric pressure chemical ionization mass spectrometry approx. approximately Ar aryl ATR attenuated total reflectance BHT 3,5-di-t-butyl-4-hydroxytoluene BMIM 1-butyl-3-methylimidazolium bpy 2,2’-bipyridine Bu butyl tBu tert-butyl Calcd Calculated CIF crystallographic information file cod 1,5-cyclooctadiene CPCM conductor-like polarizable continuum model CPE controlled potential electrolysis CV cyclic voltammetry Cy cyclohexyl d doublet DFT Density Functional Theory DIPP 2,6-diisopropylphenyl xxv  DME dimethoxyethane DMF N,N-dimethylformamide DMSO dimethyl sulfoxide DOSY diffusion-ordered spectroscopy dt doublet of triplets E° standard redox potential Epc cathodic peak potential esd estimated standard deviation Et ethyl EPR electron paramagnetic resonance equiv. equivalents ESI-MS electrospray ionization mass spectrometry Fc ferrocene FE Faradaic efficiency FID flame ionization detector FTIR Fourier transform infrared GAA glacial acetic acid GC gas chromatography h hour(s) HOMO highest occupied molecular orbital ip peak current IR infrared LC-MS liquid chromatography mass spectrometry LMCT ligand-to-metal charge transfer LUMO lowest unoccupied molecular orbital xxvi  m multiplet M metal; parent ion; molarity m/z mass to charge ratio Me methyl Mes mesityl MO molecular orbital MS molecular sieves; mass spectrometry NBO natural bond orbital NHC N-heterocyclic carbene NMR nuclear magnetic resonance NPA natural population analysis OTf trifluoromethanesulfonate Ph phenyl ppm parts per million quin quintet ref reference rev reversible RuBisCO ribulose-1,5-bisphosphate carboxylase/oxygenase s singlet salen 2,2’-N,N’-bis(salicylidene)ethylenediamine SCE saturated calomel electrode SOMO singly occupied molecular orbital SWV square-wave voltammetry sxt sextet t triplet xxvii  TBA tetrabutylammonium TCD thermal conductivity detector TFA 2,2,2-trifluoroacetic acid TFE 2,2,2-trifluoroethanol THF tetrahydrofuran TLC thin layer chromatography TOF turnover frequency TON turnover number tpy 2,2’:6’,2’’-terpyridine triflate trifluoromethanesulfonate UV ultraviolet Vis visible VT variable temperature wt weight δ chemical shift (ppm) ν scan rate triphos triphosphine; bis(diphenylphosphinoethyl)phenylphosphine    xxviii  Acknowledgements The work presented in this thesis would not have been possible without the support of many people. I would first like to thank Prof. Mike Wolf for the opportunity and freedom to pursue this project, and his consistent and dependable support inside and outside of chemistry. I’ve learned so much over the last five years. I’m grateful to all of the amazing people from the Wolf Group, past and present, with whom I’ve had the privilege of working with and alongside of. Specifically, I’d like to thank Dr. Marek Majewski for getting me started in the lab and Dr. Glen Bremner for training me in the basics of electrochemistry. Prof. Jeff Nagel, Prof. Francesco Lelj, and Dr. Guillaume Lefèvre gave me a lot of help in learning and understanding DFT calculations. I’d also like to thank Dr. Lyndsey Earl, Dr. Ashlee Howarth, Dr. Renee Man, Dr. Pete Christensen, Dr. Yang Cao, Janet Ochola, Chris Brown, Elise Caron, and others in the group for their input and scientific discussions over the years, and for making the lab a genuinely good place to be.  Additionally, I’d like to acknowledge the essential contributions made by the many people who operate support facilities within the department. I want to particularly thank Dr. Brian Patrick for solving the crystal structures in this thesis and always being willing to look at a malformed crystal under the microscope, Brian Ditchburn for custom-building the electrochemical cells used for this project, Dr. Maria Ezhova and Dr. Paul Xia for training and assistance with NMR experiments, Dr. Emily Seo for GC-MS training, and the many people in electronic and mechanical engineering services who have provided cheerful help and fixed so many problems.  Thanks to Prof. David Wilkinson and the Wilkinson group in CHBE (and especially Dr. Arman Bonakdarpour) for use of the gas chromatograph, rotating disk electrodes, general help, and fun xxix  banter. I am also thankful for the timely and helpful feedback for this thesis provided by Prof. Dan Bizzotto and Prof. David Perrin. This research was made possible by the financial support of UBC and the Natural Sciences and Engineering Research Council of Canada, for which I am very grateful. WestGrid and Compute Canada are thanked for providing the software and considerable computational resources required for DFT calculations.  Finally, my deepest gratitude and love is expressed for my wife Suzanne who has been an in-credible source of joy, adventure, and unconditional support throughout this degree. You and Ab-igail help me put everything into perspective.    xxx  Dedication       To Suzanne, Abigail, Dad, Mom, and the rest of my family S.D.G.       1 Chapter 1: Introduction 1.1 Background Atmospheric carbon dioxide is the source of carbon for photosynthetic organisms and thus the prin-cipal source of carbon for all other organisms. It is an abundant carbon resource ubiquitously and equally dispersed across the planet. Through photosynthesis, energy in the form of sunlight is collected and trans-ferred to biomolecular reaction centers where water oxidation occurs and reducing equivalents are pro-duced, ultimately resulting in carbon dioxide reduction and the storage of energy in chemical bonds.1 The primary enzyme for the fixation of carbon dioxide, ribulose-1,5-bisphosphate carboxylase (RuBisCO), is almost certainly the most abundant enzyme in the world.2 Over geological time scales, the stored en-ergy of millions of years of photosynthetic production has been deposited in the concentrated, energy dense forms of coal, petroleum, and natural gas (fossil fuels).        Figure 1-1. Greenhouse gas emissions by type and economic sector (left) and country/union (right). Adapted from ref.3    2 The sustained, widespread, and large scale use of these fossil fuels in recent history as energy sources for electricity, heat production, industry, and transportation, combined with deforestation (Figure 1-1), is continuing to add carbon dioxide to the atmosphere at a faster rate than plants and geological processes can fix it, leading to a strengthened and abiding greenhouse effect as CO2 absorbs infrared radiation over its long atmospheric lifetime. The annual global amount of carbon emitted into the atmosphere from human sources is approximately 10,000 million tonnes, up from approximately 6,000 million tonnes in 1990 (Figure 1-2), with continued growth in emissions coming especially from developing countries.3,4 The increased global temperatures which result could lead to widespread and deleterious effects.3  Figure 1-2. Global carbon dioxide emissions over time. Adapted from ref.4  Ultimately, this problem is one of energy sources where new technologies are required to enable more efficient and economical energy harvesting from the abundant energy source available to and shared by all: the sun. Intimately related to this is the problem of energy storage, where energy dense materials are ubiquitously required in modern societies, from agricultural and construction, to transportation and the 1825 1850 1875 1900 1925 1950 1975 2000 20250200040006000800010000   Global Carbon EmissionsGlobal Carbon Emissions(million tonnes/year)Year0.00.51.01.52.02.5  Global Carbon Emissions per CapitaGlobal Carbon Emissions per Capita (tonnes/year)  3 production of goods. Current technologies allow electrical energy to be stored in rechargeable batteries and capacitors, but at relatively low energy densities and sizeable economic and environmental costs. Hydrocarbon fuels, however, have a significantly higher energy density than current or projected battery technologies (by a factor of approximately 40-100) and have many important applications that will not soon be replaced, such as air travel (Figure 1-3).5 Additionally, much of the world’s energy infrastructure is already based on hydrocarbon fuels. Efficiently generating “solar fuels,” that is, CO2-derived fuels using renewable energy sources, can in principle enable a carbon neutral fuel cycle.5 This can only occur through CO2 reduction, one half of the overall redox reaction for photosynthesis, and advances in the catalysis of CO2 reduction may therefore have a significant impact on our energy systems.  Figure 1-3. Energy densities of various materials.6,7  Interest in CO2 reduction catalysis has increased drastically over the last two decades as researchers are responding to the steady increase in atmospheric CO2 from anthropogenic sources and the need for 0 25 50 75 100 125 1500102030405060708090AluminumDieselEthanolGasolineGlucoseHydrazineHydrogen Gas (700 bar)IronLiquid HydrogenLiquid Natural GasLithium BorohydrideMagnesiumMethanolNatural Gas (250 bar)SiliconSodiumZincZinc-Air BatteryLithium Ion BatteryEnergy density by volume (MJ/L)Energy density by mass (MJ/kg)  4 renewable and sustainable energy sources.1,8 The reduction of CO2 to liquid fuels precursors (CO + H2, synthesis gas)9,10 or directly to hydrocarbon fuels (such as methanol or methane) is thermodynamically feasible with these reactions becoming increasingly favourable concomitant with an increasing number of proton-coupled electrons transferred. The thermodynamic potentials for relevant reduction products are given in Table 1-1 (pH 7 in aqueous solution, 25 °C, 1 atmosphere gas pressure, and 1 M concentra-tions for other species). Performing these reactions near their thermodynamic potentials is challenging, however, requiring the use of catalysts to minimize the energy barriers of CO2 activation and allow or facilitate subsequent proton transfers. Due to the inherent requirement of H+ ions in the CO2 reduction half-reactions and the relative thermodynamic and kinetic ease of H+ reduction to produce H2, successful electrocatalysts must be selective for CO2 reduction even in the presence of H+. In the case of two electron reduction of CO2 to produce CO, a side product of H2 results in synthesis gas (CO + H2) being produced which may be used in Fischer-Tropsch reactions to produce hydrocarbons of varying chain lengths (see overall reaction equation below).11 A composition of 1 CO : 2 H2 is desirable for industrial applications.12 (2n + 1)H2  +  nCO  →  Cn H(2n+2)  +  nH2O Table 1-1. Equilibrium Potentials of CO2 and H+ Reduction Half-Reactions in Aqueous Solution at pH 7 vs. SHE and Fc0/+ in CH3CN (converted from V vs. SHE)13–15 Half-Reaction Potential vs. SHE (V) Potential vs. Fc0/+ (V) CO2(g)  +   e–               →  CO2•– –1.99 –2.61 CO2(g)  +   2H+  +  2e–  →  HCO2H –0.61 –1.23 CO2(g)  +   2H+  +  2e–  →  CO(g)  +  H2O –0.53 –1.14 CO2(g)  +     H+  +  2e–  →  HCO2– –0.43 –1.05 CO2(g)  +   4H+  +  4e–  →  CH2O  +  H2O –0.49 –1.11 CO2(g)  +   6H+  +  6e–  →  CH3OH  +  H2O –0.38 –1.00 CO2(g)  +   8H+  +  8e–  →  CH4(g)  +  2H2O –0.24 –0.86       2H+   +  2e–  →  H2(g) –0.42 –1.04   5 Recent research into homogeneous CO2 reduction catalysts has largely focused on variations of early, established families of catalysts such as porphyrin, salen, and other tetracoordinate ligands on Fe and Ni,16–18 group 7 bipyridine carbonyl complexes,19–22 and square planar phosphine complexes to produce the two-electron reduction product CO. These three general classes of electrocatalysts will be briefly surveyed with a focus on recent work (post-2009). A robust understanding of the underlying mechanisms and structure-activity relationships for CO2 reduction (especially for higher reduction products) is grow-ing, in part due to the application of computational modelling,23,24 but is still nascent. 1.2 Carbon Dioxide, the Molecule Carbon dioxide is a linear, symmetrical, and overall non-polar molecule with two equal and opposing bond dipoles from the central carbon atom to the terminal oxygen atoms, leaving a partial positive charge and partial negative charge on the carbon and oxygen atoms, respectively (Figure 1-4). The atoms are equivalently bound at a distance of 1.16 Å by σ- and π-bonds, leaving two degenerate π* LUMOs with a larger 2pC than 2pO character, susceptible to electron donation at the electrophilic carbon (Figure 1-4, left).25 Population of the π* LUMO results in the molecule bending from linearity, with the LUMO en-ergy decreasing concomitant to a decreasing bond angle.25   6      Figure 1-4. Dipole moments and partial charges in CO2 (top left), DFT-calculated isosurfaces of degen-erate π* LUMOs (ωB97xD/D95(d)) (left), and qualitative MO diagram of CO2 adapted from ref (right).25  Given its structure, carbon dioxide may be described as an amphoteric molecule with weakly Lewis basic oxygen atoms and a Lewis acidic carbon. As an O-nucleophile, CO2 can bind in an η1 end-on coordination mode to oxophilic metals, though an analysis of the cases where this occurs shows that a linear geometry is retained and electron back-donation to CO2 usually does not occur to any significant degree.25 Another coordination mode, the most common, is side-on η2 CO-bonding where CO2 acts as both an electron donor and acceptor in a π-interaction with the d-orbitals of a metal center. Finally, having an electrophilic C atom, CO2 may form an η1 C-bond with nucleophilic species (such as carbanions, amines, hydrides, and electron-rich metal centers).    7 Scheme 1-1. Three Binding Modes of CO2a  a “end-on” O-coordination (left), “side-on” coordination (center), and C-coordination (right)  Carbon dioxide, a gas at temperatures above -78 °C at standard pressure, is soluble in a wide variety of solvents (Table 1-2). Its solubility is lowest and most complex in water, where at pH 6 and below the solvated CO2 will undergo a slow hydration equilibrium to form carbonic acid (H2CO3), which can then be in equilibrium with hydrogen carbonate (HCO3-). Both carbonic acid and hydrogen carbonate are present in lower concentrations than the solvated CO2. At higher pH values, the solvated CO2 can directly react with hydroxide ions to form carbonate species, leaving lower concentrations of molecular CO2.26 Its solvation is more straightforward in organic solvents with solubilities of 0.28 M and 0.20 M in ace-tonitrile and N,N-dimethylformamide, respectively, and the absence of carbonate equilibria. Table 1-2. Solubility of CO2 in Various Polar Solvents25–27 Solvent Solubility (M) Water 0.038 Methanol 0.18 Ethanol 0.12 Acetonitrile 0.28 N,N-Dimethylformamide 0.20 Dimethyl sulfoxide 0.14   8 1.3 Overview of Homogeneous Carbon Dioxide Reduction Electrocatalysts Homogeneous electrocatalysts are molecular, redox-active species which mediate electron transfer from an electrode to a substrate for a specific chemical reaction (Scheme 1-2), as direct reduction at electrode surfaces is often inefficient and a mediating electron transfer agent can be tuned to facilitate specific chemical interactions with the substrate.28,29 As defined for this thesis, homogeneous electrocat-alysts are distinct from redox catalysts, where redox catalysts act only as simple outer sphere electron transfer agents and do not form intermediates where bonding occurs between the catalyst and the sub-strate. Ideally, an electrocatalyst should be redox-active at potentials near that of the thermodynamic potential of the reaction to be performed and have a high rate of electron transfer with both the electrode (ke) and substrate, coupled with fast chemical kinetics for the transformation (Scheme 1-2).28 The over-potential is one metric for an electrocatalyst, defined as the difference between the applied potential at the electrode required for catalytic turnover and the equilibrium potential of the reaction, and should be minimized to achieve energy-efficient conversions. Electrocatalytic activity can often be identified by performing voltammetric experiments, where in an ideal case, a reversible redox couple for the redox-mediating electrocatalyst species will become irre-versible, increase in current, and shift anodically when the substrate is introduced.28,29 In general, a sig-nificant increase in current at the redox potential(s) of the electrocatalyst upon introduction of the sub-strate is indicative of electron transfer to and chemical reactivity with the substrate.    9 Scheme 1-2. Homogeneous Electrocatalytic Reduction of CO2a  aAdapted from refs.8,23,30  The first apparent report of CO2 reduction by a homogeneous electrocatalyst was in 1974 where Tam-aru et al. reported reductive electrochemical activity between Co and Ni phthalocyanine complexes and CO2, and little or no activity for other metal phthalocyanine species.31 Unfortunately, few details were given, including reduction products, but the communication did speculate about the importance of an occupied dz2 orbital in the metal complexes and a possible relationship between activity and ligand π-electron density. Other complexes with tetracoordinate ligands were investigated subsequently, and make up one of three overarching classes of homogeneous CO2 reduction electrocatalysts to date, alongside bipyridine carbonyl complexes, and square planar phosphine complexes. A brief overview of seminal and recent work within these general classes of transition metal complexes will be surveyed in turn. One type of electrocatalyst to be noted which does not fit in the above three classes is the organic pyridinium, imidazolium, and related “biomimetic 2e– + 2H+” transfer agents by Bocarsly and others, where there have been claims to nearly quantitative yields of the six-electron reduction product methanol from CO2 at very low overpotentials in aqueous solution at hydrogenated Pd electrodes, an illuminated   10 p-GaP electrode, or even a Pt electrode.32–34 It is believed that hydride generation may occur at the elec-trode surface, allowing multiple, sequential two-electron reductions of CO2. These reports are electrode surface-dependent and some careful attempts to reproduce the experiments on Pt electrodes have shown no CO2 reduction products at all.35,36 As studies in this area have conflicting and disputed results, these reported electrocatalyst species will not be further discussed. 1.3.1 Nitrogen-Containing Tetracoordinate Ligands on Fe, Co, and Ni The work of Tamaru et al. with Co and Ni phthalocyanines (Figure 1-5A) was followed by Eisenberg and co-workers where tetraazomacrocyclic complexes of Co and Ni (Figure 1-5B) were found to elec-trocatalytically reduce CO2 at a potential of -1.6 V vs. SCE (approx. -2.0 V vs. Fc0/+) in a 2:1 H2O:CH3CN KNO3 or LiClO4 electrolyte solution to produce CO and H2. Faradaic efficiencies for CO2 reduction were approximately 50-67% at best, with most of the remaining current reducing H+, and the turnover fre-quencies were low.37 In wet DMF or CH3CN, the performance was worse, with the overall amount of gaseous product lower, and faradaic efficiencies for CO2 reduction to CO dropping by a factor of 5 in some cases.30   Figure 1-5. Representative examples of CO2 reduction electrocatalysts bearing tetracoordinate ligands.    11 Related Ni(II) cyclam complexes (Figure 1-5C) were investigated in depth by Sauvage et al. over the second half of the 1980s, showing highly selective reduction of CO2 to CO at potentials of -1.2 V vs. SCE (approx. -1.6 V vs. Fc0/+) and with nearly quantitative faradaic efficiencies in aqueous conditions at pH 4-5 with KClO4 or KNO3 electrolytes.38 No interactions with carbonic acid or hydrogen carbonate were reported. Contrastingly, the use of wet DMF or CH3CN as the solvent led to both CO and formate as reduction products in varying ratios, depending on the potential of electrolysis.39 Overall, the electro-catalyst was found to be highly stable and operated at a moderate turnover frequency at more negative potentials. It was also found that the macrocyclic effect was important, as similar complexes with open-chain ligands performed poorly.30,38 These initial studies proved that the one-electron reduced species, Ni(cyclam)+, is the catalytically active species with CO2.  Following studies found that the use of a mercury electrode was important, with adsorbed species of Ni(cyclam)+ responsible for the electrocatalytic reactivity at the potentials investigated (Figure 1-5C).40 More recent work has thoroughly characterized the electrochemical reactivity of the complex with CO2 at an inert glassy carbon electrode,41 showing electrocatalytic carbon dioxide reduction ability starting at -1.2 V vs. NHE (approx. -1.8 V vs. Fc0/+) in 1:4 water/CH3CN to selectively produce CO, but at only 10% of the current density relative to the mercury electrode. The proposed mechanism has been thor-oughly modelled by DFT calculations,39,41 revealing a favoured η1-CO2 binding mode leading to CO production and suggesting that the conformation of the cyclam ligand is important for reactivity, where both oxygen atoms in the trans-I(R,S,R,S) conformer may interact with the amine H’s, leading to a greater number of stabilizing H-bonding interactions in a symmetric arrangement (Figure 1-6). The trans-I conformer is calculated to more favourably reduce CO2. The trans-I and trans-III conformers represent approximately 15% and 85% of the Ni(cyclam) species in solution,41 whereas isomerization is known to occur upon adsorption to mercury, presumably to the more active trans-I form. Additionally,   12 analogous N-methyl-substituted species were synthesized but had severely diminished performance, con-sistent with the H’s being important for reactivity.41 Catalyst deactivation has been shown to be related to the formation of carbonyl species and loss of the cyclam ligand.42 Finally, bimetallic nickel bicyclam complexes were synthesized, showing only similar electrocatalytic activity for CO2 reduction compared to the monometallic species where no cooperative effects were found.43 Precise conformational charac-terization of the complex at the mercury electrode surface remains an ongoing topic of investigation.   Figure 1-6. CO2 bound to trans-I and trans-III conformations of Ni(cyclam). Adapted from ref.39  Another tetradentate ligand, tetraphenylporphyrin, has found extensive utility in homogeneous CO2 reduction electrocatalysis in the form of electrogenerated Fe(0) complexes, first introduced in this context by Savéant et al. in 1991 and studied until the present (Figure 1-5D).44–46 Initial studies found a faradaic yield for CO up to 75% at -1.8 V vs. SCE (approx. -2.2 V vs. Fc0/+) in dry DMF with a tetraalkylammo-nium perchlorate electrolyte at a mercury electrode, though with poor stability as the porphyrin ligand quickly decomposed after only a few cycles due to hydrogenation or carboxylation at the negative po-tentials employed. It was found, however, that the addition of hard, Lewis acidic Mg2+ ions (in the form of the perchlorate salt) to the solution dramatically improved the reaction rate and catalyst stability. Cy-clic voltammograms of the electrocatalyst solution under CO2 at varied concentrations of Mg2+ were carefully analyzed, showing that the catalytic wave observed with higher concentrations of Mg2+ (16   13 mM) split into two closely-spaced waves at low concentrations of Mg2+ (2-4 mM), attributed to the Mg2+ ion interacting with the O-atoms of the activated CO2 moiety and assisting in C-O bond breaking, thus offering an example of bimetallic chemical catalysis between the nucleophilic metal center and an elec-trophilic Mg2+ ion.44 Subsequent studies also investigated CO2 reduction in the presence of weak Brønsted acids such as CF3CH2OH (TFE), yielding additional improvements in faradaic efficiency for CO2 reduction and cata-lyst stability and a slight decrease in the required applied potential, to -1.7 V vs. SCE (approx. -2.1 V vs. Fc0/+). In optimized cells turnover frequencies as high as 350 h-1 were achieved, resulting in catalyst inhibition as the high local concentrations of produced CO adsorbed on the mercury electrode surface. More recently, this system was further optimized where intramolecular Brønsted acid sources were added to the phenyl groups (Figure 1-5D, R = OH), leading to a lower operating potential of -1.2 V vs. NHE (approx. -1.8 V vs. Fc0/+), increased rates to a TOF greater than 100 s-1, and faradaic yields for CO pro-duction up to 90-94% (when operating at a potential slightly more anodic than E0cat, where current den-sities are lower) in DMF with 2 M H2O containing “0.4 M EtNCO2CH3” and 0.1 M [n-Bu]4PF6.46 When the hydroxyl groups were replaced with methoxy groups (Figure 1-5D, R = OMe), activity diminished by orders of magnitude, and the overpotential increased by nearly 400 mV. Water soluble varieties of the catalyst have also been synthesized and tested with promising initial results, though more work is re-quired.47 Lastly, a new ligand for CO2 reduction electrocatalysis has recently been employed, the tetradentate bis(N-imidazolylpyridine)alkane ligand where the carbene-pyridine moieties are bridged by methylene, ethylene, or propylene spacers (Figure 1-7, left). The varying alkyl spacer length results in altered de-grees of ligand distortion away from a square planar geometry and towards a pseudo-tetrahedral geome-try, thereby systematically shifting the second reduction of the complex anodically due to preorganization of the reduced species and added flexibility to adopt the tetrahedral or pseudo-tetrahedral geometry of a   14 d10 or d9 metal center, respectively. Carbon dioxide can be selectively reduced to CO at faradaic efficien-cies up to 90% with these complexes in wet acetonitrile solutions with 0.1 M [n-Bu]4PF6 electrolyte at potentials of -1.9 to -2.1 V vs. Fc0/+, though at low rates, representing the first report of a CO2 reduction electrocatalyst utilizing N-heterocyclic carbene donors.17 The second iteration of these complexes intro-duced benzannulated pyridine and imidazolilydene donors which resulted in improved electrocatalytic responses and tuning of the electrocatalysis onset potential (Figure 1-7, right).18   Figure 1-7. NHC-containing tetradentate nickel complexes.17,18  1.3.2 Group 7 Bipyridine Carbonyl Complexes, and Related Complexes A major class of homogeneous CO2 reduction electrocatalysts is that of bipyridine carbonyl com-plexes, most often with group 7 metal centers (Figure 1-8), and related species. The archetypical complex within this class is Re(bpy)(CO)3Cl, first reported by Lehn et al. in 1984 as they discovered its electro-chemical activity with CO2 while investigating its utility as a photosensitizer. They found that it was an efficient and selective electrocatalyst (and photocatalyst) for the reduction of CO2 to CO beginning at -1.25 V vs. NHE (approx. -1.9 V vs. Fc0/+; but more typically operating at -2.2 V vs. Fc0/+) using a glassy carbon electrode, yielding faradaic efficiencies of 91-98% in DMF or DMF-H2O solutions containing a tetraalkylammonium electrolyte.19 While selectivity and stability were excellent, and turnover frequency   15 good (~1000 h-1),20 the operating potential was still fairly negative. Subsequent studies by other groups investigated other bipyridine and bipyridine/carbonyl complexes of Ru, Os, Rh, and Ir, with electrocata-lytic activity for CO2 reduction observed in some cases, but not with comparable performance and effi-ciency to Re(bpy)(CO)3Cl.8 It took until the late 2000s for interest to be revived in complexes of this type, where Kubiak et al. made electronic variations to Re(bpy)(CO)3Cl, with 4,4’-substituents with varying electronic properties added to the bipyridine ligand: R = COOH, H, Me, tBu, and OMe (Figure 1-8A).9 It was found that in acetonitrile with 0.10 M [n-Bu4N]PF6, the R = COOH complex showed little to no reactivity with CO2, the R = H and R = Me complexes were comparable, the R = tBu complex had increased activity for reducing CO2, and the R = OMe complex was inactive. Compared to the original Lehn catalyst, the tBu-substituted derivative reduced CO2 to CO with a 13-fold increase in rate and nearly quantitative faradaic yields while retaining excellent stability, though at a moderately more cathodic potential (by ap-prox. -0.15 V). This illustrates the large influence electronic modifications on the ligand can have for reactivity, where tBu substituents result in enhanced performance but another electron donating group, OMe, essentially eliminates activity, presumably due to the different nature of its electron donation (OMe is inductively electron withdrawing, but resonance π-donating; tBu is only inductively electron donating). The catalytic cycle for this type of catalyst has been well-defined through iterative mechanistic, infra-red spectroelectrochemical, and computational studies (Scheme 1-3).8,9,48,49 The first reduction of the complex is ligand-centered and creates a radical anion with a weakened Re-halide bond. From there, a ligand-to-metal charge transfer (the rate of which is modulated by the electronic properties of different substituents on the ligand) and halide loss occurs through a dissociative mechanism, followed by the second reduction to form a five-coordinate anion having a high energy HOMO comprised of ligand and M-dz2 character. This is the active nucleophilic species for activation of CO2. The activated and O-basic Re-bound CO2 moiety then undergoes protonation to form a neutral hydroxycarbonyl intermediate which   16 then, for Re species, undergoes reduction and subsequent protonation (“reduction-first” pathway), and C-O bond cleavage to release water. Finally, the CO ligand dissociates as the neutral tetrakis(carbonyl) complex undergoes reduction to reform the active, anionic species. For this class of electrocatalysts, ligand redox-activity is critical to reactivity. Scheme 1-3. Electrocatalytic Cycle for M(bpy)(CO)3X (M = Re, Mn; X = Cl, Br) and Related Com-plexes.  The next development within the class was the exploration of the activity of analogous first-row Mn complexes. In 2011 Deronzier et al. reported that Mn(bpy-R)(CO)3Br (where R = H and Me) complexes reduce CO2 to CO with similar selectivity and faradaic efficiency as compared to Re though at less neg-ative potentials (approx. -2.0 V vs. Fc0/+), though requiring the presence of a Brønsted acid (water), hav-ing decreased stability and current densities, and resulting in dimer formation which complicates the   17 reactivity (Figure 1-8B).50 This work was quickly followed up by Kubiak et al. where experiments were expanded to include a tBu-substituted complex and investigate the effect of different Brønsted acids (TFE, MeOH, and water), quantifying a best TOF of ~250 s-1 in the presence of 1.4 M TFE. Computational and spectroelectrochemical mechanistic studies have suggested a similar mechanism to the Re complexes, though preferring a “protonation-first” pathway before final reduction of the protonated hydroxycarbonyl intermediate (Scheme 1-3).49  Figure 1-8. Examples of group 7 bipyridine carbonyl CO2 reduction electrocatalysts.  Further improvement was realized in 2014 through suppression of dimer formation by the incorpora-tion of bulky 6,6’-mesityl groups on the bipyridine ligand (Figure 1-8C). This resulted in a substantial change in the voltammetric characteristics, from two one-electron reduction events separated by 200-400 mV (depending on the ligand) to one wave corresponding to the transfer of two electrons (acting as two concomitant one-electron reductions, and not a direct two-electron reduction). No catalytic current was seen at this reduction potential, where the anionic [Mn(bpy-Mes)(CO)3]- species is generated and binds CO2 with H+, but only at potentials approximately 400 mV more negative, at -2.0 V vs. Fc0/+ (roughly equivalent to that of previously discussed Mn complexes).21 Improved activities were found compared to the previous best Mn electrocatalyst, Mn(bpy-tBu)(CO)3Br, with a TOF of 700-5000 s-1 and a nearly quantitative faradaic yield for CO production in the presence of weak Brønsted acids. Finally, in 2016 it   18 was discovered that the addition of the hard, Lewis acid Mg2+ led to improved catalytic current for these complexes in the potential range between the first reduction event and where a large catalytic current begins (approx. -400 mV from first reduction, the “slow catalysis” regime).51 Recent work in 2016 has now begun to investigate the formation of bimetallic and heterobimetallic (Re and Mn) supramolecular assemblies by appending H-bonding alkyl amide or tyrosine substituents to the bipyridine ligand, with the idea being that a cooperative effect and subsequent catalytic enhancement may be possible given the amphoteric nature of CO2.52,53 While these studies gave evidence of heterobi-metallic dimers forming in solution at reducing potentials and some additional CV responses in the co-catalyst solution not observed in separate solutions of each complex, the performance for CO2 reduction was not improved.52,53 The phenol residues of tyrosine were able to act as intramolecular proton sources, however, which resulted in larger catalytic responses (TOF ~ 50 s-1) compared to analogous complexes without these residues. Another noteworthy area of research appearing in 2015 has been to investigate the substitution of bipyridine in M(bpy)(CO)3X complexes (M = Mn, Re; X = Cl, Br) with other neutral, redox-active bi-dentate ligands, specifically diimine, pyridine-imine and pyridine-NHC ligands  (Figure 1-9). For exam-ple, a series of M(DAB)(CO)3Br (where M = Mn, Re; DAB = 1,4-diazabutadiene) complexes have been synthesized and electrochemically characterized (Figure 1-9A), showing ligand redox activity and two one-electron reductions at more anodic potentials than bpy complexes (approx. -1.2 V and -1.5 to -1.7 V vs. Fc0/+ for the two reductions, respectively, depending on the ligand substituents). These complexes were found, however, to be poor mediators for the electrocatalytic reduction of CO2. Some CO was produced through reductive disproportionation of CO2 to CO and CO32-, but with faradaic yields less than 10%. This was attributed to less electron donation to the metal center from DAB as compared to bpy while in the reduced state, leading to a less nucleophilic metal center.54 Rhenium complexes with a pyr-idine-monoimine ligand have also recently been reported (Figure 1-9B), though electrochemical activity   19 with CO2 was not discussed.55 We briefly studied some similar Re pyridine-imine tricarbonyl chlorido complexes in unpublished work and found significantly diminished activity with CO2 relative to the bi-pyridine complexes.  Figure 1-9. Examples of complexes closely related to group 7 bipyridine carbonyl complexes, where bipyridine has been replaced by another potentially redox-active neutral bidentate ligand.  The use of pyridine-NHC ligands with Re may be a more promising alternative, however. A number of studies have recently been published investigating the activity of a variety of different NHC groups (Figure 1-9C).56–59 Delcamp et al. reported a series of 8 different pyridine-NHC Re complexes with varied imidazol-2-ylidene substituents (Me, Ph, p-CF3-Ph, p-OC6H13-Ph) and either Cl or Br as the axial halide, and upon characterization by cyclic voltammetry under CO2, large current enhancements were observed which correlated with the electron donating ability of the NHC substituent.58 Bulk electrolysis studies were not performed, however, though photocatalysis studies were, showing greater photocatalytic activity and turnover numbers for CO production with the pyridine-NHC complex with electron with-drawing substituent p-CF3-Ph than with the benchmark bipyridine catalyst (contrary to the current en-hancements observed by cyclic voltammetry). The bromide complexes were found to outperform the chloride complexes, likely due to easier bromide dissociation from the reduced species. The other pyri-dine-NHC complexes also produced CO, where performance was inversely proportional to how electron-  20 donating the NHC substituent was which was attributed to the higher energy required for a ligand-based reduction. This was followed by an electrocatalysis study by the same group, where four other pyridine-NHC ligands with inductively electron-withdrawing substituents (p-CF3-Ph, p-NO2-Ph, p-CN-Ph, and m-CF3-Ph) were surveyed.59 Compared to Re(bpy)(CO)3Br, electrocatalytic activity was seen at the first reduction wave in addition to the second (though the first reduction wave is at near the same potential to the second for the bpy complex). Faradaic efficiencies of 60-100% were obtained for CO production, with a TOF of approximately 100 s-1 with a Brønsted acid present (H2O, TFE, phenol).59 Manganese pyridine-NHC tricarbonyl bromido complexes have been studied by Agarwal et al., with Mn(pyr-NHC)(CO)3Br complexes reported in a brief communication in 2014 where N-methyl imidazol-2-ylidene and benzimidazol-2-ylidene NHCs were employed (Figure 1-9C), representing the second reported example of NHC donors in a CO2 reduction electrocatalyst.22 The two electron reduction of CO2 to CO was achieved at potentials similar to Mn(bpy)(CO)3Br (approx. -2.0 V vs. Fc0/+), though at a reduced faradaic efficiency of 35%. This was followed by another communication in 2016 investigating the difference between Mn(I) pyridine-benzimidazol-2-ylidene and pyrimidine-benzimidazol-2-ylidene tricarbonyl bromido complexes, with the hypothesis that increased π-acidity in the pyrimidine vs. pyri-dine ligand will lower the ligand-centered LUMO energy and enable ligand reduction at less negative potentials. Indeed, the first reduction potential for the pyrimidine complex was anodically shifted by 140 mV (to -1.77 V vs. Fc0/+ as compared to -1.93 V vs. Fc0/+ for the pyridine-NHC analogue) while simul-taneously showing a three-fold enhancement in the catalytic current. This was attributed to the “reduc-tion-first” pathway for Mn now being favoured over the slower “protonation-first” pathway, where the rate-limited step for the [Mn]-hydroxycarbonyl intermediate (required reduction or protonation for cata-lytic turnover) was calculated to have a reduction potential shifted by +260 mV, and thus occurring at a faster rate (Scheme 1-3). Faradaic efficiencies up to 72% for CO production were found in preparative   21 scale electrolysis. These same ligands were tested in analogous Re complexes which had reduction po-tentials cathodically shifted by approx. 200-400 mV for pyrimidine- vs. pyridine-NHCs. Large current enhancements under CO2 were observed, but preparative-scale electrolyses showed 60% faradaic effi-ciencies for CO production at -2.20 V vs. Fc0/+ in both cases, with most of the remaining current produc-ing H2, as compared to 99% FE for Re(bpy)(CO)3Cl. Here, the pyrimidine-substituted Re complex gave the same yields of CO as the pyridine-substituted complex, though with less H2 production.  Finally, recent work has been done to condense Re(5,6-diamino-1,10-phenanthroline)(CO)3Cl to or-tho-quinone edge defects on graphitic electrode surfaces, resulting in a heterogeneous catalyst which retains the high activity of the molecular species.60 In general, there are numerous strategies reported which can convert a tunable and well-defined molecular, homogeneous electrocatalyst to a heterogeneous modified-glassy carbon electrocatalyst.61–64 In sum, these results show the significant effect even small electronic and steric modifications of the ligands can have on electrochemical reactivity with CO2. 1.3.3 Square Planar Triphosphine Complexes The development of another class of homogeneous CO2 reduction electrocatalysts, Pd triphosphine pincer catalysts, has been pursued almost exclusively by DuBois and coworkers1,65–68 following the ear-lier discovery of a Rh bis(diphosphine) electrocatalyst by Wagenknecht et al. in 1984 which could reduce CO2 to formate with a faradaic yield of 40% at approx. -1.9 V vs. Fc0/+ (though with poor stability).69 Research began in the mid-1980s with a systematic study of Fe, Co, and Ni complexes bearing various polyphosphine and solvento ligands. By adjusting the number of phosphine donors in the polyphosphine ligand, the number and relative position of coordination sites occupied by weakly bound solvento ligands could be controlled as potential binding sites for hydrides and CO2. Additionally, by adjusting the elec-tron-donating or withdrawing properties of the P-substituents, the reduction potentials of the complexes   22 could be tuned to a relevant range for CO2 reduction. Late transition metals were chosen as they are not oxophilic given the prospect of stable, catalysis-stopping M-O bond formation.70  Figure 1-10. [Pd(triphos)(CH3CN)]2+ electrocatalysts and related complexes.1,70  From this library of complexes, the Ni species showed the most promising electrochemical character-istics with two closely-spaced one-electron reductions at approx. -0.9 and -1.3 V vs. Fc0/+.66 This led to the synthesis and characterization of analogous Pd and Pt complexes.66 Although the Ni and Pt complexes did not display catalytic activity for CO2 production (only H2 was produced in electrolysis experiments), various [Pd(triphos)(CH3CN)](BF4)2 complexes were found to be efficient and selective electrocatalysts for the reduction of CO2 to CO in acidic (0.1 M HBF4 or DMF-H+) CH3CN and DMF solutions containing 0.3 M tetraethylammonium tetrafluoroborate electrolyte (Figure 1-10A). Faradaic efficiencies from 9 to 99% were achieved at low operating potentials of -1.3 to -1.5 V vs. Fc0/+, depending on the P-substituents in the triphosphine ligand.1,65,70,71 Catalyst turnover numbers (TON) were relatively small at only 10-200, however, due to the formation of an inert bimetallic species through Pd(I)-Pd(I) bonding and ligand bridging (which could be reversed by oxidation of the bimetallic species). Mechanistic studies were performed, showing a catalytic activity dependence on dissociation of the monodentate ligand, where PR3 and DMSO complexes were extremely sluggish compared to CH3CN   23 complexes.65 Furthermore, ligand steric effects were investigated where complexes with mesityl or tri-methoxybenzene substituents on the central phosphorus blocked one prospective axial coordination site, leading to a decrease in rate by a factor of 2. These results strongly suggested an axial approach for CO2 to the metal center in the active species and a five-coordinate transition state, ruling out the involvement of a six-coordinate intermediate.71 Additional electrochemical and kinetic data were collected, leading to the overall mechanism shown in Scheme 1-4. Here, the dicationic complex undergoes one-electron reduction to become the active, Pd(I) species for CO2 activation. Upon coordination of CO2, protonation by the strong acid occurs, producing a dicationic hydroxycarbonyl intermediate which undergoes reduction at the operating potentials. This is followed by loss of the solvento ligand, a second protonation, and C-O bond cleavage to release water. The CO ligand in the fourth coordination site then exchanges with a solvent molecule, completing the cycle.   24 Scheme 1-4. Reported Catalytic Cycle for [Pd(triphos)(sol)]2+ (sol = CH3CN or DMF)1,68    The reaction rates and faradaic efficiencies of CO production were strongly dependent on the phos-phorus substituents of the triphosphine ligand in a manner difficult to predict, leading to efficiencies of 99% where R = Ph and R’ = Cy, 54% where R = R’ = Ph, and 10% where R = Me and R’ = Cy or Et, for example. In all cases the remaining current resulted in H2 production. In general, the preference for CO or H2 production is in part dependent upon the basicity (and reduction potential, relatedly) of the com-plex. Reduction potentials which are more negative favoured protonation at Pd to form a hydride, leading to H2 production, while reduction potentials which are less negative favoured protonation at the O-atom of the activated CO2, leading to CO production.   25 Pyridine-containing ligands were also investigated, with [Pd(terpyridine)(CH3CN)]2+ yielding no de-tectable CO, a tridentate ligand with a central phosphorus and terminal pyridines (2,2'-((phe-nylphosphanediyl)bis(ethane-2,1-diyl))dipyridine) yielding only trace CO, and a tridentate ligand with a central pyridyl and terminal phenylphosphines also yielding only trace CO (Figure 1-10B).65 Indeed, numerous ligand variations were implemented, with neutral pincer ligands synthesized where the central phosphorus group is replaced with NH, O, S, and AsPh (Figure 1-10C), as well as an anionic pincer ligand with a central phenyl group (PCP; Figure 1-10B).72 All complexes were electrochemically char-acterized and tested for reactivity with CO2, with the remarkable result that only the triphosphine ligands resulted in the reduction of CO2 to CO- and even then only with high efficiency for very specific combi-nations of P-substituents.65,72 For the complex with the anionic PCP ligand, CO2 was found to be a co-factor for H+ reduction, however, suggesting that H2 and CO formation can emerge from the same inter-mediate.   Figure 1-11. Variations of [Pd(triphosphine)(CH3CN)]2+ to facilitate cooperative activation of CO2.  Due to the amphoteric nature of CO2, the use of two metal centers to cooperatively activate CO2 was explored within this class of complexes as well. A hexadentate phosphine ligand was synthesized- two   26 triphosphine ligands tethered together through their central phosphorus by a methylene group- and coor-dinated to Pd, thus resulting in a bimetallic complex (Figure 1-11A). Evidence from acid concentration-dependence kinetics experiments gave strong evidence for both Pd atoms being involved to cooperatively activate CO2, where the CO2 is presumably η1-C bound to one Pd and η1-O bound to the other. The most compelling evidence, however, was that the catalytic rate of CO2 reduction increased by three orders of magnitude while operating at the same potential as the analogous mononuclear catalyst (approx. -1.3 V vs. Fc0/+). This activity is near the same order of magnitude as the [NiFe] CO dehydrogenase enzyme, which has a TOF of 31,000 s-1 and is the fastest known catalyst for the two-electron reduction of CO2 to CO (Figure 1-12).1,8,73 In the [NiFe] CO dehydrogenase enzyme, the high activity is due to the cooper-ative activation of CO2 where Ni acts as a Lewis base and Fe acts as a Lewis acid, minimal geometry changes upon CO2 binding (pre-organized transition state), an electron-reservoir effect from the Fe3S4 cluster, and H-bonding from the protein structure.8 In the Pd(triphos) system, an attempt to minimize geometry changes and pre-organize the transition state was made by substituting the methylene bridge for a more rigid group, m-phenylene, resulting in both Pd centers acting as independent electrocatalysts without a cooperative effect (Figure 1-11B).74 Although extremely high activity was observed for the bimetallic species with the methylene bridge, the decisive downside was a very low turnover number (<8) due to a high rate of decomposition to the inactive Pd(I)-Pd(I) species as the active, reduced species has metal radicals positioned closely together.    27  Figure 1-12. Cooperative activation of CO2 by [NiFe] CO dehydrogenase. Adapted from ref.1  Finally, the incorporation of cooperative Lewis base/acid interactions with CO2 by the inclusion of a pendant scorpionate-type phosphonium group to the central phosphorus of the triphosphine ligand was attempted, with the aim that this Lewis acid could stabilize the bound CO2 at the metal center to allow protonation and catalytic turnover (Figure 1-11C). No significant changes in reactivity were detected compared to analogous complexes without the pendant phosphonium group.68  1.4 Summary and Discussion From this survey of homogeneous electrocatalysts for CO2 reduction, a number of general points can be made. First, since the mid-1980s, the same general classes of homogeneous CO2 reduction electrocat-alysts have been iterated upon, with the majority of recent work focusing on group 7 bipyridine carbonyl-type complexes. Besides the group responsible for the discovery, no other group, until ours in the work presented in this thesis, has contributed to the development of new pincer-type complexes for CO2 elec-troreduction despite their capability for selective reduction of CO2 to CO at low operating potentials. Second, ligand redox-activity is a critical feature in the success of Re(bpy)(CO)3X and related com-plexes, and may be broadly beneficial for CO2 reduction electrocatalysis by enabling the storage and separation of reducing equivalents on the metal complex, thereby avoiding the formation of a basic site apt to protonation and alleviating metal radical character which can lead to undesirable dimerization reactions.   28 Third, H-bonding interactions, or an intramolecular proton source or transfer site, may drastically im-prove reactivity as seen with Ni(cyclam)2+ catalysts and the Fe porphyrin system of the Savéant group. Fourth, computational modelling, especially Density Functional Theory calculations, has recently en-abled new advances in homogeneous electrocatalyst design by allowing electronic properties for com-plexes to be tuned in silico based on accurately modeled reaction mechanisms. For example, energy barriers to different reaction pathways can be quantified and then tuned as a function of catalyst design. Fifth, bimetallic or cooperative activation of CO2 is a very promising path forward given the nature of the carbon dioxide molecule as containing both Lewis acidic and Lewis basic atoms while being non-polar overall, the catalytic enhancements observed by the addition of Mg2+ for a number of different catalysts (and its requirement for activity with RuBisCO2), the structure of [NiFe] CO dehydrogenase and its mechanism for CO2 reduction, and the three orders of magnitude rate increase seen for bimetallic methylene-bridged Pd(triphosphine) complexes. At the same time, early investigations of bimetallic Ni(cyclam) complexes resulted in no improvements, and recent work on bimetallic M(bpy)(CO)3X com-plexes (M = Re, Mn) shows only a 10% improvement at best. For the Re and Mn complexes, this is likely due to the fact that they are octahedral complexes and may therefore be inherently ill-suited to simulta-neously act on a single substrate as compared to two square planar metal centers. For the bimetallic Ni(cyclam) complex, this is likely a result of poor ligand design where a longer, more flexible linker between the cyclam rings is required to allow the metal centers to get closer together, although the prob-lem of deactivation through Ni(I)-Ni(I) bond formation would still remain as the redox-activity is local-ized on the metal center in such a complex. 1.5 Heterogeneous Electrocatalysts In varying contexts, homogeneous or heterogeneous catalysts may provide better overall performance for a reaction when considering reaction rate, stability, cost, and ease of product separation. For context,   29 a few recent advancements in heterogeneous electrocatalysts will be mentioned. In general, these are organized into the categories of metals, metal oxides, metal chalcogenides, and carbon-based materi-als.75,76 The two electron reduced products CO and formate are most typical, with H2 as a common by-product, though methane production has recently been reported at a faradaic efficiency of 80% using a Cu electrode operating at -1.25 V vs. SHE. The production of CO at very high current densities (100 mA/cm2) when using a titania-supported 40 wt% Ag nanoparticle electrode with 1.0 M KOH electrolyte solution has also recently been reported,77 though current densities for other heterogeneous electrocata-lysts are typically in the range of 1-10 mA/cm2.75,76 In general, understanding of the underlying reaction mechanism for heterogeneous CO2 is limited, though computational methods are enabling further inves-tigation and the prediction of new materials to test. 1.6 Towards Pincer Complexes with a Redox-Active Ligand Therefore, the investigation and development of a modular platform for late transition-metal pincer complexes with the possibility of ligand redox activity and capacity for development to a bimetallic species was pursued. Pincer ligands containing N-heterocyclic carbene moieties were specifically tar-geted as they are well-known to behave analogously to phosphines and strong electron-donors are re-quired to facilitate sufficient metal nucleophilicity for CO2 activation.78,79 It was reasoned that pincer complexes containing NHCs in extended π-systems could conceivably provide storage of redox equiva-lents away from the metal center akin to the function of the bipyridine ligand in Re(bpy)(CO)3X electro-catalysts, which, combined with non-labile M-NHC bonds, could improve turnover numbers relative to complexes containing phosphine ligands.  Informed by a report containing computationally modelled interaction energies for CO2 with thirty-eight different Rh(I), d8 pincer complexes, a modular, pyridyl-linked bis-NHC pincer was targeted (Figure 1-13).80 Additionally, NHC and pyridyl containing ligands have now recently been used in other   30 designs of CO2 reduction electrocatalysts, demonstrating their stability under relevant reducing condi-tions and redox cycling.17,22 Lastly, group 10 pyridyl-bridged bis-NHC pincer complexes have been pre-viously synthesized and tested for carbon-carbon bond-formation catalysis, but without consideration as potential electrocatalysts for CO2 reduction.  Figure 1-13. General design of pyridyl-linked bis-NHC pincer complexes.  1.7 A Brief Primer to N-Heterocyclic Carbene Bonding As N-heterocyclic carbenes will be continually referred to, some brief background is required. NHCs are neutral, electron-rich σ-donors and non-negligible π-acceptors when bound to late transition metals (Figure 1-14).79,81 They are stronger electron-donors than even alkylphosphines and form robust, non-labile bonds with metals. The carbenic carbon is able to be stabilized as a σ-donor with an empty p-orbital by its neighbouring σ-withdrawing and π-donating N atoms. For unsaturated NHCs, such as imidazolyli-dene and benzimidazolylidenes, there is π-conjugation and some aromaticity in the imidazolium ring, allowing an NHC to connect a metal center to an extended π-system.   31  Figure 1-14. Electronic structure (left) and types of bonding between an NHC and a late transition metal (right).79  Over the last 20 years, N-heterocyclic carbenes have become very well-established ligands in cataly-sis, being featured prominently in the second-generation Grubbs catalyst for olefin metathesis, for exam-ple. In this case, the replacement of a tri(cyclohexyl)phosphine ligand with the saturated NHC 1,3-bis(mesityl)dihydroimidazol-2-ylidene resulted in improved stability to moisture and air in addition to generally higher activity.82,83  1.8 Goals and Scope of Thesis The general strategy employed in this thesis is to use a combined experimental and computational approach to synthesize and screen different bis-NHC pincer complexes for electrochemical properties well-suited to carbon dioxide reduction, such as ligand redox activity, and then iterate upon any promis-ing species. These iterations take the form of modifying different components of the potential catalysts, typically the ligand(s) or metal center, and then looking for structure-activity relationships in order to better understand the system and optimize the framework for reactivity with CO2.  Chapter 2 introduces and screens a new series of potential CO2 electrocatalysts for electrochemical reactivity with CO2. These complexes are electrochemically characterized and compared in terms of the effect of direct bonding (and extended π-conjugation) between the central pyridyl and two types of NHC   32 moieties, and how the presence of varying Brønsted or Lewis acid additives affect reactivity. In this chapter, the ability to electrocatalytically reduce CO2 using Pd bis-NHC pincer complexes is established. From there, Chapter 3 looks primarily at modification of the NHC moieties by phenanthro- and pyreno-annulation, resulting in new electron reservoir functionalities to increase the degree of CO2 acti-vation. Chapter 4 addresses modification of the central pyridyl group of the pincer ligand through syn-thesis of a series of para-pyridyl-substituted complexes, where the influence of these para-substituents on the electrochemical properties and activities of the complexes is studied in relation to their Hammett σp constants. In Chapter 5, analogous Ni and Pt complexes are synthesized and characterized and compared to Pd, thereby traversing the reactivity of all Group 10 transition metals to see if the earth-abundant metal Ni shows activity for CO2 reduction or if Pt shows improved activity relative to Pd. Finally, Chapter 6 summarizes the thesis and presents some new and promising directions for improved electrocatalysis within the bis-NHC pincer ligand framework.     33 Chapter 2: Initial Investigations of Bis-NHC Pincer Com-plexes for the Electrocatalytic Reduction of Carbon Dioxide 2.1 Introduction Interest in CO2 reduction electrocatalysis has increased drastically over the last two decades as re-searchers are responding to the steady increase in atmospheric CO2 from anthropogenic sources and the need for renewable and sustainable energy sources.1,8 The reduction of CO2 to hydrocarbon fuel precur-sors (CO + H2, synthesis gas)9,10 or directly to hydrocarbon fuels (such as methanol or methane) is ther-modynamically feasible with these reactions becoming increasingly favourable concomitant with an in-creasing number of proton-coupled electrons transferred.  Recent research into homogeneous CO2 reduction electrocatalysts has largely focused on variations of established families of catalysts such as group 7 bipyridine tricarbonyl complexes19–22 and porphyrin, salen, and other planar tetracoordinate ligand systems on Fe and Ni16–18 to produce the two-electron reduction product CO. Another class of homogeneous CO2 reduction electrocatalysts, Pd triphosphine pincer complexes, has been developed almost exclusively by DuBois and coworkers throughout the 1990s.1,65–68 Various [Pd(triphos)(CH3CN)](BF4)2 complexes were found to be efficient and selective electrocatalysts for the reduction of CO2 to CO in acidic DMF and CH3CN, exhibiting faradaic efficiencies >95% and operating at low potentials.1,65,70 Catalyst turnover numbers (TON) were relatively small at only 10-200, however, due to the formation of an inert bimetallic species through Pd(I)-Pd(I) bond formation and ligand bridg-ing. Both the mechanism and structure-activity relationships were elucidated for CO formation with these catalysts and it was shown that CO2 coordinates to the axial position of the reduced complexes.70 It was also discovered that a bimetallic version of the catalyst exhibited a reaction rate three orders of magnitude   34 higher than the monometallic complex due to cooperative effects, operating at rates comparable to the [NiFe] CO dehydrogenase enzyme, though with a very low TON (8).67 In an effort to further explore the pincer motif for CO2 reduction electrocatalysts, this chapter reports an investigation of other strongly electron-donating pincer complexes, specifically those incorporating N-heterocyclic carbene (NHC) moieties as they are well-known to behave analogously to phosphines.78,79 It was reasoned that pincer complexes of this type may provide storage of redox equivalents away from the metal center, similar to the function of the bipyridine ligand in Re(bpy)(CO)3X electrocatalysts,8,9,84 which, combined with non-labile M-NHC bonds, could improve turnover numbers relative to complexes containing phosphine ligands. Here, a series of pyridyl-linked and lutidine-linked bis-NHC palladium complexes are reported (see below). Electrochemical characterization and screening for electrocatalytic CO2 reduction ability, as well as DFT calculations and comparisons to other known electrocatalysts are discussed.           2.2 Results & Discussion The complexes under study varied by bridge type (lutidine or pyridine) and NHC-donor type (imid-azolilydene and benzimidazolilydene), as shown above. Two literature approaches were employed for   35 their synthesis: protonolysis of the proligand with palladium acetate at high temperatures in a microwave reactor,85,86 and formation of silver carbene species from the proligand and subsequent transmetallation to dichloro(1,5-cyclooctadiene)palladium.87,88 The transmetallation approach afforded [Pd(C^N^C)Cl]BF4 and [Pd(bC^N^bC)Cl]BF4 in higher purity (Figure A-1, Figure A-2). Methods of purifying the protonolysis product mixtures by column chromatography on SiO2 with KNO3(aq) in the acetonitrile eluent were also developed and utilized, affording comparable purity to the silver carbene transmetallation product as determined by NMR analysis. Complexes [Pd(C-N-C)Br]Br and [Pd(bC-N-bC)Br]Br were obtained without need for further purification via protonolysis at 160 °C in a microwave reactor and subsequent precipitations of the product from addition of a dichloromethane solution to di-ethyl ether (Figure A-3, Figure A-4). The four pincer complexes are thermally-, air-, and moisture-stable.85,87,88 2.2.1 Electrochemical Characterization The complexes were characterized by cyclic voltammetry under both N2 and CO2 atmospheres (2 mM concentrations, 100 mV/s scan rate, and in a 0.10 M [n-Bu4N]PF6/DMF solution, unless otherwise spec-ified; summarized in Table 2-1). Under N2, the lutidine-linked complexes [Pd(C^N^C)Cl]BF4 and [Pd(bC^N^bC)Cl]BF4 exhibited similar behaviour with two irreversible cathodic waves observed at peak potentials of -1.79 V and -2.34 V for [Pd(C^N^C)Cl]BF4 and -1.72 V and -2.20 V for [Pd(bC^N^bC)Cl]BF4 (Figure 2-1, Figure 2-2). Using controlled potential electrolysis at a potential approximately 150 mV more negative than the first cathodic wave, it was determined that the first reduc-tion is a one-electron transfer (0.89 F/mol for [Pd(C^N^C)Cl]BF4 and 0.91 F/mol for [Pd(bC^N^bC)Cl]BF4).     36   Figure 2-1. Cyclic voltammograms of [Pd(C^N^C)Cl]BF4 (left) and [Pd(bC^N^bC)Cl]BF4 (right) under N2 and CO2.  Figure 2-2. Overlaid square-wave voltammograms of [Pd(C^N^C)Cl]BF4 (black) and [Pd(bC^N^bC)Cl]BF4 (green) taken at a frequency of 25 Hz with 5 mV potential steps and 25 mV am-plitude.  As electrolysis of the solution proceeded, a colour change in the solution from pale to golden yellow was observed, and a new anodic wave emerged at 0.43 V for [Pd(C^N^C)Cl]BF4 and 0.50 V for [Pd(bC^N^bC)Cl]BF4.  These new waves have similar peak currents to the original cathodic waves and -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-60-40-20020 Blank, N2 [Pd(C^N^C)Cl]BF4, N2 [Pd(C^N^C)Cl]BF4, CO2Current (µA)Potential (V vs. Fc0/+)-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-60-40-200 Blank, N2 [Pd(bC^N^bC)Cl]BF4, N2 [Pd(bC^N^bC)Cl]BF4, CO2Current (µA)Potential (V vs. Fc0/+)-2.5 -2.0 -1.5 -1.0 -0.5 0.0-20-15-10-50  [Pd(C^N^C)Cl]BF4  [Pd(bC^N^bC)Cl]BF4Current Difference (µA)Potential (V vs. Fc0/+)  37 are diffusion controlled, indicating the production of a relatively stable reduced species. The wave at 0.43 V for [Pd(C^N^C)Cl]BF4 became two overlapping waves at 0.40 V and 0.54 V by 15 minutes after electrolysis was complete, whereas the reduced species of [Pd(bC^N^bC)Cl]BF4 maintained one wave at 0.50 V. Exhaustive oxidation at these potentials led to re-emergence of the originally reduced cathodic wave, indicating a chemically reversible process (Figure 2-3). In both cases, the second cathodic wave was not affected by the one-electron transfer reaction. A one-electron transfer event is desirable as con-current one-electron transfers can be beneficial to selectively reduce CO2 in the presence of H+ ions.65,89   Figure 2-3. Square-wave voltammograms of [Pd(C^N^C)Cl]BF4 after reduction at the peak potential of the first cathodic wave (-1.8 V) for varying amounts of time (left), and the one-electron reduced product of [Pd(C^N^C)Cl]BF4 after oxidation at the potential of the produced anodic wave (0.5 V) for varying amounts of time (right).  The pyridyl-linked complexes [Pd(C-N-C)Br]Br and [Pd(bC-N-bC)Br]Br show different electrochem-ical behavior than the lutidine-linked complexes (Figure 2-4). [Pd(C-N-C)Br]Br exhibits three irreversi-ble cathodic waves of decreasing intensity at -1.67, -1.99, and -2.27 V, and [Pd(bC-N-bC)Br]Br has two cathodic waves at -1.45 and -1.83 V and a third diminished wave at -2.20 V (Figure 2-5). Both display an anodic wave at 0.26 V due to oxidation of the free bromide counterion present in solution. The peak -0.25 0.00 0.25 0.50 0.750510152010 s5 s0 sCurrent Difference (µA)Potential (V vs. Fc0/+)2 s-2.25 -2.00 -1.75 -1.50 -1.25-20-15-10-504 s2 s1 sCurrent Difference (µA)Potential (V vs. Fc0/+)0 s  38 current of the first cathodic wave in both complexes is approximately 85-90% that of the peak current for the oxidation of the bromide counterion, indicating that the cathodic waves are one-electron reduc-tions. A slightly greater relative peak current for bromide oxidation wave is expected as the smaller anion will have a higher diffusion rate than the larger cationic complex according to the Stokes-Einstein equa-tion. Exhaustive controlled potential electrolysis of [Pd(C-N-C)Br]Br at -1.8 V was unsuccessful as the passage of current diminished within minutes due to the formation of a black, insulating solid on the electrode. The decomposition of [Pd(C-N-C)Br]Br into an insulating solid may also cause the decreasing peak current as the complex undergoes successive reductions (C1, C2, C3), as seen in the square-wave voltammogram of the complex (Figure 2-5, left). Likewise, the diminished current for the cathodic wave at -2.20 V in [Pd(bC-N-bC)Br]Br may also be due to decomposition, where the increased ligand π-con-jugation is better able to stabilize the singly-reduced complex as compared to [Pd(C-N-C)Br]Br, but still unable to stabilize the doubly-reduced complex. Nevertheless, each of the cathodic waves for these spe-cies can be assigned as one-electron reductions by comparison to the bromide oxidation wave.   Figure 2-4. Cyclic voltammograms of [Pd(C-N-C)Br]Br (left) and [Pd(bC-N-bC)Br]Br (right) under N2 and CO2.  -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-60-40-2002040 [Pd(C-N-C)Br]Br, N2 [Pd(C-N-C)Br]Br, CO2Current (µA)Potential (V vs. Fc0/+)-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-60-40-2002040 [Pd(bC-N-bC)Br]Br, N2 [Pd(bC-N-bC)Br]Br, CO2Current (µA)Potential (V vs. Fc0/+)  39            Figure 2-5. Overlaid square-wave voltammograms for [Pd(C-N-C)Br]Br (left) and [Pd(bC-N-bC)Br]Br (right). The wave at -0.47 V is due to the internal reference decamethylferrocene.  For each of the four complexes, the cathodic waves were tested for return oxidation waves by potential sweeps up to 1000 mV/s, with none detected. Plots of peak current against the square root of the scan rate are linear in each case, indicating that the electroactive species diffuse freely in solution according to the Randles-Sevcik equation (Appendix C).  Table 2-1. Electrochemical Peak Potentials vs. Fc0/+ Complex Epc1 (V) Epc2 (V) Epc3 (V) Generated Epa1 (V) [Pd(C^N^C)Cl]BF4 -1.79* -2.34* - 0.43 → 0.40/0.54 [Pd(bC^N^bC)Cl]BF4 -1.72* -2.20* - 0.50 [Pd(C-N-C)Br]Br -1.67 -1.99 -2.27* - [Pd(bC-N-bC)Br]Br -1.45 -1.83 - - *Numbers in bold and italics indicate a significant current enhancement upon addition of CO2.  -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5-30-20-100102030C3C1Current Difference (µA)Potential (V vs. Fc0/+)C2-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5-20-1001020C2 C1Current Difference (µA)Potential (V vs. Fc0/+)  40 2.2.2 Electrochemical Behaviour in the Presence of CO2 Upon saturating the electrolyte solutions of the complexes with CO2 by sparging for 10 minutes, a significant current enhancement is observed at both cathodic waves for [Pd(C^N^C)Cl]BF4 and [Pd(bC^N^bC)Cl]BF4, indicative of some reactivity or interaction with CO2 (Figure 2-1). The larger current enhancement at the second cathodic wave is attributed to a greater degree of reactivity between CO2 and the two-electron reduced species. A marginal current enhancement at the second cathodic wave of [Pd(C-N-C)Br]Br and little to no current enhancement at the two cathodic waves of [Pd(bC-N-bC)Br]Br are observed (Figure 2-4). The largest contribution to the broad, ill-defined increase in current observed beyond -2.1 V for [Pd(C-N-C)Br]Br and [Pd(bC-N-bC)Br]Br is attributed to background cur-rent from direct reaction of CO2 at the electrode surface, where carbon dioxide radical anions are pro-duced which dimerize to form oxalate.45 Cyclic voltammograms of [Pd(C^N^C)Cl]BF4 and [Pd(bC^N^bC)Cl]BF4 also exhibit a broad anodic feature from -0.65 V to 0.50 V in the presence of CO2 after reduction at the first or second cathodic wave, indicating the formation of a new species. This broad feature also obeys the Randles-Sevcik equation up to at least 1000 mV/s, indicating a freely diffusing species formed upon reduction of the complex. The results of controlled potential electrolysis experi-ments in the presence of CO2 and a Brønsted acid source are discussed below. 2.2.3 DFT Modelling of Reduced Species To better understand the state of the reduced complexes in solution, DFT modelling was employed. The model complexes DFT(C^N^C) and DFT(C-N-C) were geometry optimized to stationary points in a CPCM solvent model of acetonitrile. The unreduced monocationic species showed LUMO geometries primarily centered on the pyridyl ring but also containing some Pd and imidazolilydene character ( Figure 2-6). Electrons were added to the model complexes in silico, and the calculated changes were observed. For DFT(C^N^C), the addition of one electron resulted in reduction of the pyridyl ring, as   41 evidenced by NBO charge analysis and the resulting contractions of the pyridyl C2-C3 and C5-C6 bonds and corresponding expansions of the other bonds, and a slightly contracted Pd-N bond. For DFT(C-N-C), the same was true to a different degree, with other parts of the conjugated ligand also being affected.     DFT(C^N^C)    DFT(C^N^C) HOMO DFT(C^N^C) LUMO       DFT(C-N-C)    DFT(C-N-C) HOMO DFT(C-N-C) LUMO  Figure 2-6. Structures and HOMO and LUMO orbital diagrams of model complexes DFT(C^N^C) and DFT(C-N-C).  Addition of a second electron resulted in more significant structural changes. For DFT(C^N^C), the pyridyl ring transferred its charge to the Pd and Cl atoms and was oriented 25° out of the plane of the molecule. The Pd-N bond distance increased by about 50 pm as the pyridyl fell out of a bonding orienta-tion, and the chloride dissociated. The M-NHC bonds remained almost unchanged. For DFT(C-N-C), the pyridyl moiety remained reduced but pointed 28° out of plane, moving towards a tetrahedral bonding geometry within the restraints of direct bonding to the tightly Pd-bound NHC moieties. The chloride remained bound in a pseudo-tetrahedral geometry with a Pd-Cl bond distance of 2.51 Å.    42 2.2.4 DFT Modelling of Interaction with CO2 The approach of CO2 to the reduced model complexes is a key step for reactivity, and thus the ener-getics of this process were investigated and compared to the behavior of the known electrocatalysts Re(bpy)(CO)3Cl and [Pd(triphosphine)(CH3CN)]2+ (where the phosphine substituents are methyl groups). Computed structures of the active reduced species which have been found to interact with CO2 were optimized to stationary points and then the energy of the system was recorded as a function of varying M-CO2 distance. The CO2 bond angle was recorded to quantify the degree of CO2 activation (Figure 2-7).  Figure 2-7. DFT-calculated energetics for the approach of CO2 to a reduced complex, with CO2 bond angles given for each energy minimum, or in the absence of a minimum, at 2.0 Å. The CPCM solvent model with acetonitrile was employed in each case. The M-CO2 distance is measured from the metal center to the carbon of CO2.    43 Several interesting results were observed from this method. Firstly, the reduced models of known electrocatalysts and DFT(C^N^C) and DFT(C-N-C) all exhibited some degree of energetic stabilization upon approach of CO2 whereas the unreduced species repelled CO2. Re(bpy)(CO)3– exhibited energetic stabilization starting at an M-CO2 distance of ~3.0 Å and a fully-activated CO2 bond angle of 120° at 2.0 Å, consistent with the presence of strong Brønsted acids being unnecessary for this catalyst; substantial current enhancements (factor of ~4) are seen in the presence of CO2 alone. Reduced species of [Pd(tri-phos)(CH3CN)]2+ did not experience a large stabilization, consistent with the fact that strong acids such as HBF4 are necessary for significant CO2 current enhancement to be observed. The one-electron reduced species of DFT(C^N^C) showed a greater stabilizing interaction with CO2 than [Pd(triphos)(CH3CN)]+ within 2.75 Å, consistent with the moderate CO2 current enhancement (factor of ~2) observed by cyclic voltammetry, whereas the one-electron reduced species of DFT(C-N-C) was comparable to [Pd(tri-phos)(CH3CN)]2+ by this method. Each of the two-electron reduced species was stabilized by weak Pd-C bond formation with CO2 (20 kcal/mol). DFT(C^N^C) and DFT(C-N-C) activated CO2 to 137° and 139° at their respective energy minima when reduced by one electron, and both to 128° when reduced by two electrons. This degree of activation suggests that, similar to the [Pd(triphos)(CH3CN)]2+ catalysts, a relatively strong acid source is required to protonate the moderately activated CO2 adduct. Solvento species of DFT(C^N^C) and DFT(C-N-C) were also investigated, where the chlorido ligand was replaced by an acetonitrile in order to see if this change affects the thermodynamics of the reaction given its im-portance in the triphosphine pincer catalysts of DuBois and co-workers, but little thermodynamic effect was evident. Finally, in the case of Re(bpy)(CO)3– and the one-electron reduced species of DFT(C^N^C) and DFT(C-N-C) where the additional charge is pyridyl-centered, a charge transfer occurred from the ligand, helping to activate CO2 (Figure 2-8). This effect is very pronounced with Re(bpy)(CO)3– as indicated by the significant decrease in CO2 bond angle concomitant with a decrease in bipyridyl C2-C2’ bond length   44 (some additional charge is transferred from the CO ligands as well, as determined by NBO charge anal-ysis). A similar observation is also made with the one-electron reduced species of DFT(C^N^C) and DFT(C-N-C), though this is not observed with the two-electron reduced species. In this case, the excess charge of DFT(C^N^C) is Pd-centered while the excess charge of DFT(C-N-C) is delocalized throughout the complex. DFT(C^N^C) returns to a square planar geometry as CO2 coordinates, whereas DFT(C-N-C) remains pseudotetrahedral.         Figure 2-8. Plots of CO2 bond angle and ligand pyridyl C-C bond lengths for the active species of Re(bpy)(CO)3Cl (left) and the one-electron reduced model species of [Pd(C^N^C)Cl]+ (right).  These results support ligand charge-transfer capabilities in pincer complexes presently studied, akin to group 7 bipyridyl tricarbonyl electrocatalysts, and suggest that the presence of a relatively strong Brønsted acid may be required for complete CO2 activation, as with the group 10 triphosphine electro-catalysts. Additionally, these results correlate with the degrees of electrochemical current enhancement observed in the presence of CO2 for all species investigated. 2.2.5 Electrochemical Response to Brønsted Acids The reduction of CO2 to fuel-relevant products such as CO, CH3OH, and CH4 requires a proton source (Table 1-1). The addition of Brønsted acids of varying strengths, 2,2,2-trifluoroethanol (TFE), glacial 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.61.381.401.421.441.461.481.50bpy C2-C2' bond length (Å)M-CO2 distance (Å)120140160180CO2 bond angle (°)1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.61.371.381.391.40Pyridyl C2/5-C3/6 bond length (Å)M-CO2 distance (Å)140160180CO2 bond angle (°)  45 acetic acid (GAA), and trifluoroacetic acid (TFA) (with pKa values of 23.6, 12.3, and 3.45 in DMSO, respectively;90 TFA has a pKa of approximately 6 in DMF91,92), were investigated to determine if the use of weaker acids would enable the reduction of CO2 while safeguarding selectivity for reduction of CO2 over H+ by limiting the concentration of free H+ (Scheme 2-1). Ideally, the proton source should be strong enough to protonate an activated CO2 intermediate while resulting in minimum background cur-rent at potentials relevant to the complexes.  Scheme 2-1. Proposed Equilibria for the Protonation of Pd-bound CO2.   For [Pd(C^N^C)Cl]BF4 and [Pd(bC^N^bC)Cl]BF4, with or without CO2 in solution, addition of up to 500 mM TFE resulted in only a small increase in current at the most negative cathodic wave and did not affect the first cathodic wave. There was a negligible response to the addition of TFE for [Pd(C-N-C)Br]Br and [Pd(bC-N-bC)Br]Br, even in the presence of CO2. This result is in contrast to electrocata-lysts such as Mn(bpy-tBu)(CO)3Br where even weak acids such as TFE lead to large current enhance-ments with CO2 present.20 The addition of acetic acid gave similar results with lower concentrations, where the addition of 10-100 mM GAA resulted in a small increase in current at the most negative ca-thodic wave for both [Pd(C^N^C)Cl]BF4 and [Pd(bC^N^bC)Cl]BF4, and a negligible increase at the first cathodic wave. Addition of 5 equivalents of TFA to a solution of [Pd(bC^N^bC)Cl]BF4 resulted in a large current response under both N2 and CO2, indicating that the complex is able to reduce H+ ions to form H2 (Figure 2-9). The current response under N2 and CO2 is different in each case, indicating involvement by CO2, and it is notable that there is a large current at potentials near that of the first cathodic wave for   46 [Pd(C^N^C)Cl]BF4 and [Pd(bC^N^bC)Cl]BF4 only in the presence of CO2. To determine whether the increase in current is due to CO2 reduction or perhaps CO2 acting as a cofactor for H+ reduction (or some other interaction affecting H+ reduction), preparative-scale controlled potential electrolysis (CPE) exper-iments were performed. It should be noted that with TFA there is direct background H+ reduction at the electrode surface beginning at about -1.75 V vs. Fc0/+ (Figure 2-10).       Figure 2-9. Cyclic voltammograms of complexes [Pd(bC^N^bC)Cl]BF4 (left) and [Pd(bC-N-bC)Br]Br (right) under N2 and CO2 with and without 10 mM TFA.  Figure 2-10. Cyclic voltammograms of 0.10 M [n-Bu4N]PF6/DMF solutions at 200 mV/s with no complex present, with and without 10 mM TFA under N2 and CO2.  -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-140-120-100-80-60-40-2002040 N2 CO2 N2 / 10 mM TFA CO2 / 10 mM TFACurrent (µA)Potential (V vs. Fc0/+)[Pd(bC^N^bC)Cl]BF4-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-140-120-100-80-60-40-2002040[Pd(bC-N-bC)Br]Br N2 CO2 N2 / 10 mM TFA CO2 / 10 mM TFACurrent (µA)Potential (V vs. Fc0/+)-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-100-80-60-40-2002040 Blank / N2 Blank / CO2 Blank / N2 / 10 mM TFA Blank / CO2 / 10 mM TFACurrent (µA)Potential (V vs. Fc0/+)  47 2.2.6 Preparative-Scale Controlled Potential Electrolysis (CPE) CO2-saturated 0.10 M [n-Bu4N]PF6/DMF solutions containing the complexes in 2 mM concentrations were prepared in an air-tight cell with frit-separated compartments for the reference electrode and counter electrode. Electrolysis was carried out in the presence of 5 equivalents of TFA and at the potentials of the first and second cathodic waves (Table 2-2). A typical current versus time plot is shown in Figure 2-11, representative of electrolysis experiments with [Pd(C^N^C)Cl]BF4 and [Pd(bC^N^bC)Cl]BF4.  Table 2-2. Preparative-Scale Controlled Potential Electrolysis Resultsa Complex Potential (V) Charge Passed (C) FE H2 FE CO Ratio CO in Produced Gas [Pd(C^N^C)Cl]BF4 -1.80 13 93% 9% 9% [Pd(C^N^C)Cl]BF4 -2.35 12 79% 23% 23% [Pd(bC^N^bC)Cl]BF4 -1.70 37 89% 8% 8% [Pd(bC^N^bC)Cl]BF4 -2.00 4.0 83% 10% 11% [Pd(bC^N^bC)Cl]BF4 -2.25 14 64% 28% 30% [Pd(C-N-C)Br]Br -1.70 15 100% 0% 0% [Pd(C-N-C)Br]Br -2.05 13 100% 0% 0% [Pd(bC-N-bC)Br]Br -1.55 8.2 86% 0% 0% [Pd(bC-N-bC)Br]Br -2.05 6.4 81% 3% 4% aThe 80 mL electrochemical cell employed a reticulated vitreous carbon working electrode in a com-partment containing 10 mL solution which was separated by fritted glass from the platinum mesh counter electrode compartment, which contained 5 mL solution. Hydrogen and carbon monoxide production are given in terms of faradaic efficiency.    48  Figure 2-11. Plot of current over time during the electrolysis of [Pd(C^N^C)(CH3CN)](BF4)2 with 10 mM TFA under CO2 at -1.8 V vs. Fc0/+ after 10 C charge was already passed.   [Pd(C-N-C)Br]Br and [Pd(bC-N-bC)Br]Br did not exhibit any significant CO production at either first or second reduction potentials. Both complexes are stable at their first reduction potential in the presence of TFA, though once the acid concentration diminishes due to hydrogen production, a colour change from bright yellow to brown is observed and a black solid eventually precipitates. Reduction at more negative potentials leads to a colour change from bright to dark yellow even in the presence of TFA, and a loss of the characteristic cathodic waves for the complexes. The lack of CO production for these com-plexes is consistent with the observed lack of current enhancement upon addition of CO2 (see Figure 2-4), as well as with the DFT scan calculations, where the original coordination sites in DFT(C-N-C) remain locked in a pseudotetrahedral geometry even as CO2 is forced into a bonding orientation. In contrast, [Pd(C^N^C)Cl]BF4 and [Pd(bC^N^bC)Cl]BF4 exhibit significant CO production upon re-duction at both the first and second cathodic potentials in the presence of CO2 and 5 equivalents TFA. After electrolysis at the first reduction potential, the headspace gas for both complexes contained approx-imately a 1:10 ratio of CO:H2, corresponding to faradaic efficiencies (FEs) of 9% and 8% for CO pro-duction from [Pd(C^N^C)Cl]BF4 and [Pd(bC^N^bC)Cl]BF4, respectively, with the remaining faradaic 0 500 1000 1500 2000 2500 3000-6.0-4.0-2.00.0Current (mA)Time (s)Additional TFA added  49 current producing H2. Electrolysis at the second reduction potential resulted in 23% FE for CO produc-tion from [Pd(C^N^C)Cl]BF4 and 28% FE from [Pd(bC^N^bC)Cl]BF4. Electrolysis of blank solutions at the most cathodic potentials (-2.25 to -2.35 V) yields only trace amounts of CO. It should be noted that with a relatively strong acid such as TFA, direct reduction of H+ at the electrode at these potentials is non-negligible, accounting for approximately 25% of the current passed at -1.80 V and 48% of the current passed at -2.35 V in the case of [Pd(C^N^C)Cl]BF4 (see Appendix C, Table C-1). Thus, for [Pd(C^N^C)Cl]BF4, the inherent selectivity of the complex for CO2 reduction against H+ reduction (TFA) is approximately 12% at -1.80 V and 43% at -2.35 V. If the same ratios of background current are used for [Pd(bC^N^bC)Cl]BF4, selectivities of 12% at -1.65 V and 59% at -2.20 V are ob-tained. The complexes exhibit stability to reduction in the presence of CO2 and H+, retaining well-defined, diagnostic electrochemical features after the passage of 10 C of charge at either the first or second ca-thodic peak potentials, though the formation of some fine, dark precipitate is evident after reduction at the second reduction potential. Samples taken from the headspace during electrolysis gave similar ratios of H2 to CO compared to the final headspace analysis, indicating that there is no significant induction period affecting the selectivity of H2 and CO. Electrolysis was also attempted with 20 mM acetic acid and 1 M TFE for [Pd(C^N^C)Cl]BF4 at -2.35 V, where a small current enhancement had been observed, in order to see if a weaker Brønsted acid source may lead to higher selectivity for CO production. The passage of 11 C of charge yielded hydrogen and only trace CO and CH4. The complex remained intact as shown by a characteristic CV response, though some insoluble decomposition products formed. After electrolysis, the first cathodic wave for the complex was diminished and could be regenerated by oxidation at +0.55 V, indicating the presence of the one-electron reduced species in solution and suggesting slow reactivity in the presence of a weak acid. Colourimetric spot tests with chromotropic acid and methylquinaldinium were performed   50 on the solutions after electrolysis to test for the presence of formate, formic acid, or formaldehyde, how-ever none of these compounds were detected.93,94 To ensure that the CO detected was the product of CO2 reduction and not from other sources of carbon and oxygen present (such as DMF), an isotope labelling electrolysis experiment was performed on a 13CO2-sparged solution with the headspace gas analyzed by GC-MS. The only product detected was 13CO, proving CO2 as the source. Another electrolysis experiment was performed on the solution after being briefly sparged with 12CO2, and in this case the headspace gas showed the presence of a mixture of 12CO and 13CO (Figure 2-12).  Figure 2-12. Mass spectra of CPE headspace products formed from a solution of [Pd(C^N^C)(CH3CN)](BF4)2 sparged with 13CO2 only (left), and with a mixture of 12CO2 and 13CO2 (right). 2.2.7 Effects of Additives: Organic and Alkali Ions Though the lutidine-linked bis-NHC complexes [Pd(C^N^C)Cl]BF4 and [Pd(bC^N^bC)Cl]BF4 reduce CO2 to CO in significant measure, selectivity for CO2 reduction over H+ reduction is mediocre, and weaker acids, though they maintain a lower concentration of free H+, are not able to sufficiently protonate the putative CO2-adduct intermediate to lead to CO production (Scheme 2-1). In order to try and favour CO2 activation and reduction, the use of stabilizing cations was investigated as it has been shown that   51 the presence of alkali ions can have a synergistic effect on the activation of CO2 by stabilizing the nega-tive charge on the O-atoms of a partially activated CO2 molecule,45,95–97 and it is well known that the presence of Mg2+ ions is critical for CO2 reduction in the enzyme RuBisCO.98 Additionally, imidazolium based ionic liquids have been implicated in carbon dioxide reduction chemistry.96,99–101 Thus, the effect of the addition of KPF6, Mg(ClO4)2, and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF6) to the electrochemical behavior of [Pd(C^N^C)Cl]BF4 was investigated. Improved selectivity for reduction of CO2 over H+ was observed in the presence of K+, Mg2+, and [BMIM]+ (Table 2-3). When combined in solution with the complex at -2.35 V, the FE for CO produc-tion increased from 23% with no additive ions to 47% with 50 mM K+ and 30% with 65 mM Mg2+. In a 30% [BMIM]PF6/DMF solution (v/v), the selectivity for CO production decreased at this negative po-tential, perhaps due to excess stabilization of the reduced complex by the high concentration of imidaz-olium ions. (In [BMIM]PF6, little to no decomposition was observed, even after the passage of 27 C of charge.) At the first reduction potential of [Pd(C^N^C)Cl]BF4, the greatest improvements for selectivity were observed, where the FE for CO production improved from 9% with no additive ions to 24% with [BMIM]PF6, 26% with Mg2+, and 34% with K+. If a similar amount of background H2 production occurs without the presence of these additives (similar currents are passed in the blank solutions), and this is subtracted from the total amount of H2 produced, the inherent selectivity for CO2 reduction by [Pd(C^N^C)Cl]BF4 can be estimated to increase from 12% at the first reduction potential (-1.80 V) to approximately 35% with [BMIM]+ or Mg2+ and 49% with K+, and from 43% at the second reduction potential to approximately 58% with Mg2+ and 93% with K+. These results are consistent with stabiliza-tion of a CO2-adduct intermediate by interaction of the cation with the negatively charged O-atoms of CO2, thereby helping to favour reduction of CO2 over H+.  It must be noted that the presence of 50-70 mM K+ at the most negative potentials used in electrolysis (-2.35 V) resulted in a non-negligible amount of CO production with GAA (23% faradaic yield in DMF,   52 7% in CH3CN), though only trace CO was produced at -1.80 V and -2.05 V (Appendix C, Table C-2). The presence of Mg2+ ions or [BMIM]PF6 in solution alone did not result in any significant CO production with TFA or GAA even at -2.35 V. Finally, the presence of Mg2+ with [Pd(bC-N-bC)Br]Br was investi-gated, but no changes in reactivity were observed. The pyridyl-linked species remain almost entirely selective for H+ reduction, with the low overall faradaic efficiency for H2 + CO production attributed primarily to degradation of the complex. Table 2-3. Preparative-scale CPE Results with Cation Additivesa Solution Acid Potential (V) Charge Passed (C) FE H2  FE CO  [Pd] + 50 mM K+ 100 mM GAA -2.35 7.1 27% 52% [Pd] + 50 mM K+ 10 mM TFA -2.35 10 51% 47% [Pd] + 50 mM K+ 10 mM TFA -1.90 5.8 63% 34% [Pd] + 65 mM Mg2+ 10 mM TFA -2.35 7.2 70% 30% [Pd] + 65 mM Mg2+ 10 mM TFA -1.95 7.0 78% 26% [Pd] + 30% [BMIM]PF6 10 mM TFA -2.35 10 88% 7% [Pd] + 30% [BMIM]PF6 b 10 mM TFA -1.95 17 72% 24% [Pd(bC-N-bC)Br]Br + 35 mM Mg2+ 10 mM TFA -2.15 6.6 47% 2% [Pd(C^N^C)(CH3CN)](BF4)2 b 10 mM TFA -1.80 26 83% 17% a [Pd] = [Pd(C^N^C)Cl]BF4. 2 mM concentrations of the respective complex in 0.10 M [n-Bu4N]PF6/DMF solutions. The 80 mL electrochemical cell employed a reticulated vitreous carbon working electrode in a compartment containing 10 mL solution which was separated by fritted glass from the platinum mesh counter electrode compartment, which contained 5 mL solution. Hydrogen and carbon monoxide production are given in terms of faradaic efficiency. All potentials reported vs. Fc0/+. bAddi-tional 3-4 μL increments of TFA were added for every additional 5 C of charge passed. Ratio of CO:H2 in headspace was tested at approximately 7 C increments, and remained similar throughout electrolysis.    53 2.2.8 Solvento Species In another effort to increase selectivity for CO production, the chlorido ligand was replaced with a more labile acetonitrile ligand, forming a dicationic complex. It is known that a labile ligand in the fourth coordination site served an important role in the activity of [Pd(triphos)(CH3CN)]2+ pincer catalysts of DuBois, and reduction of a dicationic complex should occur at less negative potentials. Solvento complexes were readily prepared by halide abstraction with a silver salt in acetonitrile, re-sulting in dicationic complexes with a labile acetonitrile ligand. 1H NMR spectra of these species in DMSO-d6 contained a singlet at 2.07 ppm characteristic of free acetonitrile,102 indicating displacement by the solvent (DMSO or methanol; see Appendix A, Figure A-5 and Figure A-6), and ATR-FTIR spectra revealed nitrile stretches centered at 2342 cm-1 (Appendix B, Figure B-1). X-ray quality crystals of [Pd(C^N^C)(CH3CN)](BF4)2 were grown, unambiguously revealing the presence of a coordinated CH3CN molecule (Figure 2-13). The Pd-NCCH3 bond lengths were 1.978(2) and 1.999(1) Å for the disordered solvento ligand, typical for CH3CN trans to an imine, pyridine, amine, or carbonyl donor.103–105  CV experiments were performed on these solvento complexes and in each case the first cathodic wave shifted anodically by a few hundred mV relative to the parent halido complex (Table 2-4). These shifted cathodic waves could also be generated by the addition of silver hexafluorophosphate to solutions of the analogous halido complexes in the electrochemical cell. Due to the presence of adventitious halide ions in the electrolyte solution and the lability of the solvento ligand, cathodic waves for both the halido and solvento complexes were simultaneously present in some proportion except immediately after in situ halide abstraction with silver hexafluorophosphate. Conversely, the addition of sodium chloride imme-diately removed the anodically shifted waves.   54  Figure 2-13. ORTEP representation of solid-state structure of [Pd(C^N^C)(CH3CN)](BF4)2.  Table 2-4. Peak Potentials of the First Cathodic Wave for Solvento vs. Halido Complexes  Complex Epc1(Y = Cl) (V) Epc1(Y = CH3CN) (V) ΔEpc1 (V) [Pd(C^N^C)Y]n+ -1.79 -1.45 +0.34 [Pd(bC^N^bC)Y]n+ -1.72 -1.57 +0.15 [Pd(C-N-C)Y]n+ -1.67 -1.37 +0.30 [Pd(bC-N-bC)Y]n+ -1.45 -1.24 +0.21  Reduction of [Pd(C^N^C)(CH3CN)](BF4)2 was examined in the presence of CO2 and 10 mM TFA, showing an anodic shift from -1.82 V to -1.70 V for the peak potential of the first cathodic wave with TFA present (Figure 2-14). Electrolysis at the potential of the first wave resulted in CO production with 17% faradaic efficiency, an improvement from 9% with [Pd(C^N^C)Cl]BF4 (Table 2-3). [Pd(C-N-C)(CH3CN)](BF4)2 was also examined (Figure 2-15), with electrolysis at -2.0 V yielding <1% CO.   55  Figure 2-14. Overlaid square-wave voltammograms of [Pd(C^N^C)(CH3CN)](BF4)2 with 10 mM TFA under N2 and CO2, and [Pd(C^N^C)Cl]BF4 with 10 mM TFA under CO2.         Figure 2-15. Overlaid square-wave voltammograms of [Pd(C-N-C)(CH3CN)](BF4)2 with 10 mM TFA under N2 and CO2 and [Pd(C-N-C)Br]Br with 10 mM TFA under CO2 (left), and CVs of  [Pd(C-N-C)(CH3CN)](BF4)2 under N2 and CO2 (right).  Finally, it was noticed that in contrast to [Pd(C^N^C)Cl]BF4, reduction of [Pd(C^N^C)(CH3CN)](BF4)2 at its first cathodic wave did not produce a corresponding anodic wave -2.5 -2.0 -1.5 -1.0 -0.5 0.0-60-45-30-150[Pd(C^N^C)(CH3CN)](BF4)2: N2 N2  + 10 mM TFA CO2 + 10 mM TFA CO2 + 10 mM TFACurrent Difference (µA)Potential (V vs. Fc0/+)[Pd(C^N^C)Cl]BF4:-2.5 -2.0 -1.5 -1.0 -0.5-75-60-45-30-15Current Difference (µA)Potential (V vs. Fc0/+) [Pd(C-N-C)(CH3CN)](BF4)2, N2 [Pd(C-N-C)(CH3CN)](BF4)2, N2 + 10 mM TFA [Pd(C-N-C)(CH3CN)](BF4)2, CO2 + 10 mM TFA [Pd(C-N-C)Br]Br, CO2 + 10 mM TFA-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-80-60-40-20020Current (µA)Potential (V vs. Fc0/+) [Pd(C-N-C)(CH3CN)](BF4)2, N2 [Pd(C-N-C)(CH3CN)](BF4)2, CO2  56 within the scanned potential range. Another complex, [Pd(C^N^C)Br]PF6, containing a bromido ligand in place of the chloro, was also examined and showed a corresponding anodic wave with a peak potential of 0.33 V, shifted relative to an initial peak potential of 0.43 V in [Pd(C^N^C)Cl]BF4. This may indicate the necessity of a halide being present for formation of the reduced species, or else the reduced [Pd(C^N^C)(CH3CN)](BF4)2, being cationic, may be oxidized at a more positive potential outside the scanned range. It should also be noted that the bromido complex had its first reduction potential anodi-cally shifted by 40 mV in comparison to the chlorido complex, with the second reduction potential un-changed. EPR spectroscopy was performed on a solution of [Pd(C^N^C)Br]PF6 after exhaustive reduction at its first reduction potential, indicating the presence of a species with Pd(I) character and an axially elon-gated square planar structure (g(x) = 2.0075, g(y) = 2.0075, g(z) = 2.1110). These results are comparable to those observed for both monometallic and bimetallic bridged Pd(I)-Pd(I) species in the literature.106,107 Rotating disk electrode experiments were performed to differentiate between monometallic and bimetal-lic alternatives, where a bimetallic species would diffuse more slowly. The diffusion rates of the non-reduced and one-electron-reduced species were determined to be 5.1±0.7 × 10-6 cm2/s and 9.2±0.9 × 10-6 cm2/s, respectively, according to the Levich equation. Additionally, DFT optimizations of a putative halide-bridged dimer converged to individual mononuclear species. Thus, a mononuclear spe-cies is supported as the product in contrast to [Pd(triphos)(CH3CN)]2+ which loses activity due to the formation of a Pd(I) bridged species. The SOMO of the one-electron-reduced species of DFT(C^N^C) has both Pd and pyridyl character (Figure 2-16) and it is proposed that this ligand involvement contrib-utes to the stability of a reduced mononuclear species, potentially resolving a major source of deactiva-tion which occurs with triphosphine pincer complexes.     57  Figure 2-16. Experimental and simulated EPR spectra for the one-electron reduced product of [Pd(C^N^C)Br]PF6 (left) and a stable, computed structure with overlaid SOMO geometry for one-elec-tron-reduced DFT(C^N^C) (right).  2.2.9 Testing for Reactivity by a Heterogeneous Species To test whether the observed activity is authentically due to the solution species and not the result of a decomposition product being deposited on the electrode surface, the high surface area electrodes were carefully removed from solution after controlled potential electrolysis experiments had been performed with solutions initially containing [Pd(C^N^C)Cl]BF4, 65 mM Mg2+, and 10 mM TFA, and also [Pd(C^N^C)(CH3CN)](BF4)2 and 10 mM TFA under CO2, and then placed into new CO2-saturated so-lutions containing 10 mM TFA only. No potential was applied to the electrode until the solutions were electrolyzed at -2.0 V. Only trace amounts of CO were detected (0.002-0.004% in the headspace gas) as well as 1.3% H2 after the passage of 6 C, consistent with the active species being the dissolved complex and not a decomposition product deposited on the electrode surface.   58 Additionally, plots of peak current against the square root of the scan rate for these bis-NHC pincer complexes are linear in each case, including for the electrochemically generated anodic waves, suggest-ing that the electroactive species are freely diffusing in solution according to the Randles-Sevcik equation (Appendix C). 2.3 Conclusions Four bis-NHC palladium pincer complexes were electrochemically characterized with the lutidine-linked complexes [Pd(C^N^C)Cl]BF4 and [Pd(bC^N^bC)Cl]BF4 demonstrated to electrocatalytically re-duce CO2 to CO in the presence of strong acids such as TFA. The one-electron reductions of [Pd(C^N^C)Cl]BF4 and [Pd(bC^N^bC)Cl]BF4 were found to be chemically reversible and, due to charge delocalization onto the pyridyl moiety, avoid decomposition into inert bimetallic species thereby circum-venting the primary deactivation pathway limiting the TON of palladium triphosphine electrocatalysts. Computational studies of the initial interaction of CO2 with reduced species of Re(bpy)(CO)3Cl, [Pd(tri-phosphine)(CH3CN)]2+, and DFT(C^N^C) correctly predicted the proton source requirements of the elec-trocatalytic systems by correlation to the degree of CO2 activation and gave insight into charge transfer dynamics of the electrocatalysts with CO2. The reduced species of DFT(C^N^C) exhibited charge transfer from the redox-active ligand to CO2, a characteristic important to the reactivity of Re(bpy)(CO)3Cl and other related CO2 reduction electrocatalysts. Selectivity for reduction of CO2 over H+ with [Pd(C^N^C)Cl]BF4 was improved by the addition of the CO2-adduct-stabilizing cations K+, Mg2+, and [BMIM]+ as well as by anodically shifting the first reduction potential of the complex by synthesis of the dicationic species [Pd(C^N^C)(CH3CN)](BF4)2. This chapter has demonstrated the capacity of lutidine-linked bis-NHC pincer complexes for homogeneous CO2 reduction electrocatalysis, expanding the pal-ladium pincer motif to strongly electron-donating redox-active ligands.    59 2.4 Experimental 2.4.1 General Unless otherwise specified, all reactions were performed under nitrogen using standard Schlenk tech-niques and solvents and reagents were used as received from commercial sources. N-butylbenzimidazole, [BMIM]PF6, pyridyl-linked bis-NHC pincer proligands, and palladium pincer halido complexes were synthesized according to literature procedures, with modifications noted below.85–88,108,109 Potassium hex-afluorophosphate (Aldrich), magnesium perchlorate (Alfa), anhydrous 99.8+% acetonitrile (Alfa), anhy-drous 99.8+% N,N-dimethylformamide (Aldrich), 99.998% carbon dioxide (Praxair), and 99 atom% car-bon-13 dioxide (Aldrich) were used as received for electrochemical experiments. THF was distilled from a sodium benzophenone ketyl still. Microwave syntheses were performed using a Biotage Initiator mi-crowave system. 1H NMR spectra were acquired using Bruker AV300 or Bruker AV400-Inverse spectrometers with chemical shifts referenced to residual solvent signals. Mass spectra were acquired using a Waters LC-MS ESI-MS. IR spectra were collected using a PerkinElmer Frontier FT-IR Spectrometer with ATR attachment. Gaseous products were analyzed using an SRI Model 8610C gas chromatograph equipped with molecular-sieve columns and dual TCD and FID detectors. Mass spectra for headspace gases were acquired using an Agilent 6890N gas chromatograph coupled with a 5975B MS detector. Crystallo-graphic data was acquired using a Bruker X8 APEX II diffractometer with graphite monochromated Mo-Kα radiation. 2.4.2 Electrochemistry Electrochemical experiments were performed using a Pine AFCBP1 bipotentiostat or Metrohm Au-tolab PGSTAT12. Cyclic voltammetry experiments were performed in an air-tight three-electrode cell   60 with a 7 mm2 glassy carbon working electrode (Bioanalytical Systems, Inc.), Pt mesh counter electrode, and Ag wire pseudo-reference electrode in a 0.010 M AgNO3 acetonitrile solution separated from the bulk solution by a Vycor frit. Experiments were performed under N2 or CO2 using 2 mM concentrations of the complexes in 10 mL anhydrous electrolyte solution unless otherwise stated. The electrolyte solu-tion was 0.10 M triply recrystallized [n-Bu4N]PF6 in anhydrous dimethylformamide and sparged with nitrogen prior to use. For experiments with CO2, the solution was sparged with CO2 for 15 minutes. Cyclic voltammograms were recorded at a scan rate of 100 mV s-1 and square-wave voltammograms at a frequency of 25 Hz with 5 mV potential step and 25 mV amplitude unless otherwise stated. Controlled potential experiments used reticulated vitreous carbon (Bioanalytical Systems, Inc.) or glassy carbon rod (Alfa Aesar, 5 mm diameter) working electrodes, with the Pt mesh counter electrode in a separate com-partment with a fritted glass interface. Glassy carbon electrodes were cleaned by successive polishing with 1 μM, 0.3 μM, and 0.05 μM alumina paste, following by rinsing with water, sonication (5 min) in distilled water, and sonication (5 min) in methanol. Reticulated vitreous carbon electrodes were cleaned by electrochemical oxidation at +0.93 V vs. Fc0/+ followed by thorough rinsing with acetone and then methanol, resulting in a reflective carbon surface. A 5.0 mm diameter glassy carbon electrode (Pine) was used for rotating disk electrode experiments. Decamethylferrocene was used as an internal standard with its reversible redox couple observed at -404 ± 5 mV vs. Ag/AgNO3.110 This was referenced to the redox couple of ferrocene at +72 mV vs. Ag/AgNO3. Peak potentials were determined by square-wave voltam-metry at 25 Hz. 2.4.3 Electron Paramagnetic Resonance Spectroscopy The EPR spectrum of [Pd(C^N^C)Br]PF6 after electrochemical one-electron reduction was obtained in solution at 77 K using a Bruker Elexsys E500 series continuous wave EPR. The spectrometer was   61 operated at a frequency of 9.40 GHz (X-band) with 50 kHz field modulation and 8G modulation ampli-tude. 2.4.4 Computational Methods DFT calculations were performed using Gaussian 09 (Revision D.01) using the long range and dis-persion-corrected ωB97xD hybrid functional.111 The D95(d) basis set was used for all atoms except pal-ladium, which employed the Stuttgart-Dresden-Bonn quasi-relativistic effective-core potential and cor-responding correlation-consistent triple zeta basis set.112,113 Frequency calculations were performed on all geometry optimized structures to ensure that energy minima were achieved.  2.4.5 Synthesis 2,6-Bis(3-butylbenzimidazolium)pyridine dibromide, bC-N-bC • 2HBr This compound was prepared via a procedure that was modified from the literature.88 Benzimidazole (4.23 mmol, 500 mg), 2,6-dibromopyridine (1.92 mmol, 456 mg), and K2CO3 (4.23 mmol, 585 mg) were heated together without solvent at 185 °C for 2 h and then at 160 °C for 30 h. Upon cooling, a white solid formed which was ground into a powder and washed with water and Et2O. The powder was taken up in DMF (7 mL) and an excess of 1-bromobutane (3 mL) was added. The mixture was heated to 100 °C and the resulting solution left to stir overnight, resulting in the formation of a white precipitate. Upon cooling, the powder was washed with diethyl ether (25 mL × 2), collected by centrifugation, and dried under vacuum yielding an off-white powder (595 mg, 53.4% yield). 1H NMR ((CD3)2SO), 400 MHz):  ppm 0.99 (t, J=7.39 Hz, 6 H), 1.48 (sxt, J=7.46 Hz, 4 H), 2.04 (quin, J=7.46 Hz, 4 H), 4.69 (t, J=7.23 Hz, 4 H), 7.74 (t, J=7.61 Hz, 2 H), 7.82 (t, J=7.54 Hz, 2 H), 8.30 (d, J=8.22 Hz, 2 H), 8.37 (m, J=8.07 Hz, 2 H), 8.46 (m, J=8.38 Hz, 2 H), 8.77 (t, J=8.07 Hz, 1 H), 10.81 (s, 2 H)   62 [Pd(C^N^C-Bu)Br]PF6 A dark red-orange solution of C^N^C • 2HBr (0.44 mmol, 228 mg) and Pd(OAc)2 (0.44 mmol, 100 mg) in 3.5 mL DMSO was left to stir under vacuum for 1 h at room temperature, then blanketed with N2. The solution was then reacted in a microwave at 160 °C for 30 minutes (40 W). The resulting dark yellow mixture was filtered to remove a black solid, with the solvent removed under vacuum at 60 °C. The bright orange residue was taken up in a minimum of dichloromethane and added dropwise to diethyl ether (12 mL), resulting in the formation of a light orange solid. This process was repeated and the product was then purified by column chromatography over SiO2 using an 85:10:5 (v/v) acetonitrile/water/saturated KNO3(aq) eluent. The eluted product was taken up in methanol, filtered, concentrated to dryness, and reacted with 3 equivalents of NH4PF6 in acetonitrile. The solvent was removed and the residue taken up in dichloromethane and washed with water, with the dichloromethane extracts concentrated and added dropwise to diethyl ether, resulting in the formation of a light orange solid (106.5 mg, 35% yield).  1H NMR ((CD3)2SO, 300 MHz):  ppm 0.88 (t, J=7.31 Hz, 7 H), 1.16 - 1.33 (m, 5 H), 1.70 - 1.87 (m, 4 H), 4.17 - 4.33 (m, 2 H), 4.42 - 4.57 (m, 2 H), 5.54 - 5.72 (m, 4 H), 7.44 (d, J=1.60 Hz, 2 H), 7.59 (d, J=1.48 Hz, 2 H), 7.85 (d, J=7.77 Hz, 2 H), 8.21 (t, J=7.71 Hz, 1 H) ESI-MS [M-Br]+ = 538.2 m/z General synthesis method for solvento complexes Acetonitrile bound complexes were prepared by addition of 1.1 equivalents AgBF4 to stirring solu-tions of [Pd(CNC)X](Y) (where CNC is a lutidine or pyridine linked bis-NHC pincer ligand, X = Cl, Br and Y = BF4, Br) complexes, respectively, in dry acetonitrile in the absence of light. After stirring for 1 h at room temperature, the mixtures were centrifuged to remove a gray powder and then filtered through Celite, with the resulting solution concentrated to saturation and added dropwise to 10 mL Et2O resulting in the formation of a pale yellow precipitate. The hygroscopic powders were isolated and dried under vacuum at 40 °C overnight.   63 [Pd(C^N^C)(CH3CN)](BF4)2 Pale yellow hygroscopic powder (119.7 mg, 93% yield). X-ray quality crystals grown by slow diffu-sion of Et2O into a saturated CH2Cl2/CH3CN solution at -30 °C. 1H NMR ((CD3)2SO), 400 MHz):  ppm 0.89 (t, J=7.35 Hz, 6 H), 1.17 - 1.36 (m, 4 H), 1.75 - 1.87 (m, 4 H), 2.07 (s, 3 H), 4.09 - 4.26 (m, 4 H), 5.69 (d, J=15.19 Hz, 2 H), 5.86 (d, J=15.03 Hz, 2 H), 7.48 - 7.56 (m, 2 H), 7.66 (d, J=1.49 Hz, 2 H), 7.84 (d, J=7.76 Hz, 2 H), 8.22 (t, J=7.68 Hz, 1 H) ESI-MS [M-2BF4-CH3CN+Cl]+ = 494.3 m/z [Pd(bC^N^bC)(CH3CN)](BF4)2 Orange-brown hygroscopic powder (83.2 mg, 77% yield). 1H NMR ((CD3)2SO, 400 MHz):  ppm 0.90 (t, J=7.31 Hz, 6 H), 1.27 (q, J=7.20 Hz, 2 H), 1.37 - 1.53 (m, 2 H), 1.81 - 2.01 (m, 4 H), 2.07 (s, 3 H), 4.44 - 4.72 (m, 4 H), 6.03 (d, J=15.23 Hz, 2 H), 6.30 (d, J=15.23 Hz, 2 H), 7.44 - 7.66 (m, 8 H), 7.93 (d, J=8.22 Hz, 2 H), 8.04 (d, J=7.92 Hz, 2 H), 8.13 (d, J=8.22 Hz, 2 H), 8.24 (t, J=7.77 Hz, 1 H) ESI-MS [M-2BF4-CH3CN+Cl]+ = 592.1 m/z [Pd(C-N-C)(CH3CN)](BF4)2 Yellow hygroscopic powder (80.2 mg, 82% yield). 1H NMR ((CD3)2SO, 400 MHz):  ppm 0.94 (t, J=7.16 Hz, 6 H), 1.34 (m, J= 7.50, 4 H), 1.76 - 1.90 (m, 4 H), 2.08 (s, 3H), 4.22 (t, J=6.85 Hz, 4 H), 7.86 (s, 2 H), 8.03 (d, J=8.53 Hz, 2 H), 8.50 (s, 2 H), 8.62 (t, J=8.38 Hz, 1 H) ESI-MS [M-2BF4-CH3CN+Cl]+ = 466.2 m/z [Pd(bC-N-bC)(CH3CN)](BF4)2 Beige hygroscopic powder (68.0 mg, 70% yield). 1H NMR ((CD3)2SO, 400 MHz):  ppm 0.96 (t, J=7.27 Hz, 6 H), 1.39 - 1.51 (m, 4 H), 1.90 - 2.01 (m, 4 H), 2.08 (s, 3 H), 4.64 - 4.74 (m, 4 H), 7.70 - 7.81 (m, 4 H), 8.18 (d, J=7.60 Hz, 2 H), 8.49 (d, J=8.26 Hz, 2 H), 8.56 (d, J=7.76 Hz, 2 H), 8.69 (t, J=8.10 Hz, 1 H) ESI-MS [M-2BF4-CH3CN+Cl]+ = 566.1 m/z     64 Chapter 3: Polyannulated Bis-NHC Palladium Pincer Complexes 3.1 Introduction There is currently great interest in developing more efficient ways to use CO2 as a chemical feedstock for production of renewable fuels and value-added chemicals.114–116 Hydrocarbon fuels are advantageous due to their ability to be stable, long-term energy storage materials with considerably higher energy den-sities than batteries.5 The two electron reduction of CO2 to produce CO can be coupled with Fischer-Tropsch chemistry to produce long-chain liquid hydrocarbon fuels, creating, in principle, a closed CO2 cycle for combustion processes.5  The use of homogeneous molecular electrocatalysts to enable the reduction of CO2 to CO is an attrac-tive approach as the properties of the electrocatalyst can be tuned through ligand modification, and well-defined complexes can be studied in solution, allowing greater understanding of the mechanism of reac-tion with CO2 to be attained. Research into CO2 reduction electrocatalysts has progressed since early discoveries in the 1980’s, though a robust understanding of the design factors required for highly selec-tive CO2 reduction over H+ reduction, an inherently competitive reaction due to the requirement of pro-tons in the reduction of CO2, is still developing.19,69,116 Previous investigations probing the structure-activity relationships of phosphine-containing palladium pincer electrocatalysts found that selectivity for CO2 reduction was extremely sensitive to variations in ligand substituents, donor-type, bite angle, and changes in the redox potential of the complex.  These factors can act to modify the basicity of a catalytic complex and the energy and stability of hydride formation leading to subsequent hydrogen produc-tion.65,70 In general, less negative reduction potentials have been found to be beneficial for improved selectivity for CO2 reduction.    65 High selectivity for CO2 reduction can also be achieved at more reducing potentials if the active elec-trocatalyst species can activate CO2 to a sufficiently high degree, resulting in negative charge formation on the oxygen atoms of the bound CO2 and higher basicity than the metal center. DFT analysis of the bonding energy and degree of CO2 activation for the electrocatalysts Re(bpy)(CO)3Cl and [Pd(triphos-phine)(CH3CN)]2+ show energetically favourable binding and activation of CO2 with O-C-O bond angles of approximately 123° and 130°, respectively, at their energy minima, which allow the use of water and HBF4, respectively, as weakly and strongly acidic proton sources.19,24,66,117 The greater degree of CO2 activation achieved with Re(bpy)(CO)3Cl allows the use of weak acid sources at highly reducing poten-tials while avoiding H2 production, as the half-cell potential for H+ reduction becomes more negative with increasing pKa.29 Conversely, a lower degree of CO2 activation requires the use of stronger, less discriminating Brønsted acids for protonation of the bound CO2 and subsequent catalytic turnover (Scheme 3-1). Additionally, if a strong acid is required, the catalyst should operate at low potentials to limit background H+ reduction at the working electrode, which can occur at the electrode surface even in the absence of electrocatalyst at rates which increase exponentially with applied overpotential when mass transport is not limiting.29  Scheme 3-1. Proposed Equilibria for Protonation of Activated CO2.  Redox-active ligands have a promising role in the design of improved CO2 reduction electrocatalysts as they can store at least some of the charge required for CO2 activation away from the metal center, reducing the basicity of the reduced complex. This is showcased prominently in M(bpy)(CO)3X type complexes (where M = Mn or Re),9,20 and is an important factor in previous work from our group where   66 lutidyl-linked bis-N-heterocyclic carbene palladium pincer complexes were found to reduce CO2 to CO at moderate potentials with trifluoroacetic acid (TFA; pKa 3.5 in DMSO90) as the proton source.117 One mitigating factor for the performance of these bis-NHC electrocatalysts is the direct reduction of H+ at the electrode surface, accounting for up to half of the current passed at typical operating potentials. With this general bis-NHC ligand framework, less negative reduction potentials and improved selec-tivity for CO2 over H+ reduction were observed for benzimidazol-2-ylidene relative to imidazol-2-ylidene NHC donors.117 Modification of the annulating moiety has been shown to considerably affect the σ-do-nating and π-accepting properties of the NHC donor, both of which are significant factors when bonded to a late transition metal,81,118 providing a means of tuning the electronic properties of the complex. In this vein, we have synthesized lutidyl-linked bis-NHC palladium pincer complexes with further extended π-systems attached to the NHC moiety, namely phenanthro- and pyreno-annulated NHCs, to investigate their relative performance as CO2 reduction electrocatalysts.118,119 Here we demonstrate that these com-plexes are capable of reducing CO2 to CO and have additional redox-activity which enables increased electron donation and activation of CO2.    67 3.2 Results & Discussion 3.2.1 Synthesis & Structure The polyannulated complexes were prepared according to Scheme 3-2. The proligands were synthe-sized by condensation of phenanthrene-9,10-dione or pyrene-4,5-dione with formaldehyde and ammo-nium acetate in acetic acid to yield the polyannulated imidazole, followed by alkylation with 1-iodobu-tane, and then addition of two equivalents to 2,6-bis(bromomethyl)pyridine, yielding the dibromide salt (Figure A-9, Figure A-10). Coordination to palladium was achieved via transmetallation from a silver carbene intermediate, where the proligand was reacted with an equivalent of silver(I) oxide in the pres-ence of 3 Å molecular sieves in DMSO, thus forming a silver carbene species with low solubility, fol-lowed by the addition of an equivalent of both PdCl2(cod) and silver triflate, yielding the phenanthro- and pyreno-annulated chlorido species in 46-66% isolated yields (Figure A-11, Figure A-12). Purifica-tion can be achieved either by repeated washes with small aliquots of methanol or by column chroma-tography on silica gel using a DMF/CH3CN/KNO3(aq) solvent mixture as the eluent. Once synthesized, these complexes were functionalized by halide abstraction using an excess of silver triflate in the presence of a coordinating neutral ligand, in this case acetonitrile, yielding the analogous solvento complexes in 66-80% yields (Figure A-13, Figure A-14). It should be noted that both proligands are highly fluorescent under UV irradiation, whereas the palladium complexes are non-emissive, providing a convenient means to detect impurities. The complexes were characterized by MS, 1H NMR (see referred figures above), and ATR-FTIR spectroscopy, with the IR spectra indicating the presence of the bound acetonitrilo ligand for [Pd(phC^N^phC)(CH3CN)](OTf)2 and [Pd(pyC^N^pyC)(CH3CN)](OTf)2 (Figure B-2 to Figure B-5). Microanalysis of [Pd(pyC^N^pyC)Cl]OTf yielded inconsistent results. Over four analyses, %N and %H were consistent and within the acceptable range, but %C was consistently 2.7-3.0% below calculated   68 amounts. Microanalysis of the derivative product [Pd(pyC^N^pyC)(CH3CN)](OTf)2 matched the pre-dicted %N, %H, and %S, but was also 2.6-2.7% below calculated amounts for %C which cannot be accounted for by the presence of unreacted starting materials or silver carbene intermediates. Incomplete combustion of electropositive complexes with M-C bonds has been observed previously and attributed to carbide formation.120–122  Scheme 3-2. Synthesis of Phenanthro- and Pyreno-annulated Chlorido and Acetonitrilo Species   Both [Pd(phC^N^phC)Cl]OTf and [Pd(pyC^N^pyC)Cl]OTf were found to have low solubility in many organic solvents other than DMF and DMSO, presumably due to intermolecular π-stacking inter-actions. The analogous acetonitrilo complexes have improved, but still limited, solubility in solvents such as acetonitrile, acetone, and dichloromethane. Solutions of the phenanthro-annulated species were pale   69 yellow in colour, whereas solutions of the pyreno-annulated species were more orange. The UV-vis ab-sorption spectra for these complexes in acetonitrile were collected, with absorption from [Pd(pyC^N^pyC)Cl]OTf trailing into the blue end of the spectrum, which could be relevant for photo-catalytic applications (Figure 3-1).  Figure 3-1. Overlaid and normalized UV-Vis absorption spectra for [Pd(phC^N^phC)Cl]OTf and [Pd(pyC^N^pyC)Cl]OTf in acetonitrile.  X-ray quality crystals of [Pd(phC^N^phC)Cl]OTf were grown by slow diffusion of diethyl ether into a saturated DMF/CH3CN solution. Interestingly, syntheses of [Pd(phC^N^phC)Cl]+ with [BF4]- as the counterion instead of [OTf]- resulted in the growth of modulated crystals which contained a periodically expanding and contracting unit cell, requiring an unusual and complicated solution and refinement pro-cess.123 Despite repeated attempts under a variety of conditions, X-ray quality crystals of [Pd(pyC^N^pyC)Cl]+ could not be grown with either tetrafluoroborate or triflate counterions- fine pow-ders were obtained in all cases. 200 300 400 500 600 7000.00.20.40.60.81.0Normalized AbsorbanceWavelength (nm)   [Pd(phC^N^phC)Cl]OTf   [Pd(pyC^N^pyC)Cl]OTf  70 The solid state structure of [Pd(phC^N^phC)Cl]+ revealed a Pd-pyridyl bond length of 2.048 Å, a Pd-Cl bond length of 2.295 Å, an average Pd-NHC bond length of 2.038 Å, and a bite angle of 172.0° for the pincer ligand, all of which are comparable to similar complexes with other NHC groups, though the Pd-pyridyl length is approximately 0.02 Å shorter than in analogous imidazol-2-ylidene and benzimid-azol-2-ylidene complexes.85,87,124–128 This may be due to increased π-backbonding into the π-acidic pyridyl moiety, as the phenanthro-annulated NHC has been shown to have increased π-charge density at the carbene center compared with benzannulated imidazol-2-ylidenes.118 In general, there is an increasing degree of aromaticity in the imidazol-2-ylidene moiety as the size of the annulated π-system increases, as seen by a downfield shift for the adjacent methylenic protons in the 1H NMR spectra due to increased ring current, from 5.64 ppm for the bridging methylenic protons in the non-annulated species, to 6.04, 6.63, and 6.80 ppm for benz-, phenanthro-, and pyreno-annulated species, respectively.  Figure 3-2. Solid state structure of [Pd(phC^N^phC)Cl]OTf. Triflate anion and acetonitrile within the unit cell are omitted.  The NHC moieties are twisted out of plane by approximately 45°, comparable to the value in similar lutidine-linked bis-NHC palladium complexes with simpler NHC moieties, therefore limiting π-stacking in solution and enabling solubility in some polar aprotic solvents. Within the unit cell, the phenanthrene   71 moieties of neighbouring molecules are aligned with an interplanar distance of 3.4 – 3.8 Å, which is consistent with the presence of a weak π-stacking interaction. 3.2.2 Electrochemical Characterization under N2 and CO2 [Pd(phC^N^phC)Cl]+ and [Pd(pyC^N^pyC)Cl]+ were electrochemically characterized by cyclic and square-wave voltammetry under both N2 and CO2 atmospheres, revealing two prominent irreversible reduction waves followed by a larger reduction feature which displays some reversibility for [Pd(phC^N^phC)Cl]+ at scan rates above 1 V/s, and is electrochemically reversible for [Pd(pyC^N^pyC)Cl]+. In both cases, these features are reversible in CVs of the proligands (Figure C-1). The irreversible waves were tested for reversibility by immediate post-reduction oxidation, but none was detected. The peak potentials for these waves were determined by square-wave voltammetry and are reported in Table 3-1. The first two reduction events were found to occur at potentials very similar to reduction of [Pd(bC^N^bC)Cl]+, indicating that the potential trend of less negative reduction potentials with increasing size of the annulated π-system did not extend beyond [Pd(bC^N^bC)Cl]+ (Figure 3-3). The incorporation of polyannulated moieties does, however, lead to new reduction features at approxi-mately −2.92 V and −2.57 V for the phenanthro- and pyreno-annulated complexes, respectively, which can be assigned as one electron reductions of the two polyaromatic moieties present in each complex, as the peak current is twice that of the first reduction event in both cases (Figure 3-4).    72  Figure 3-3. Overlaid SWVs of [Pd(C^N^C)Cl]+, [Pd(bC^N^bC)Cl]+, [Pd(phC^N^phC)Cl]+, and [Pd(pyC^N^pyC)Cl]+ at a frequency of 25 Hz with 5 mV potential steps and 25 mV amplitude.   Table 3-1. Peak Potentials for [Pd(phC^N^phC)Cl]+ and [Pd(pyC^N^pyC)Cl]+ Complex Epc1 (v) Epc2 (v) Epc3 (v) Epc4 (v) Epc5 (v) [Pd(phC^N^phC)Cl]+ −1.77 −2.18 −2.49 −2.83a −2.96 [Pd(pyC^N^pyC)Cl]+ −1.74 −2.20a −2.30a −2.56 (rev)  aPeak potentials determined by deconvolution.   -3.0 -2.5 -2.0 -1.5 -1.0-75-50-250 [Pd(C^N^C)Cl]+ [Pd(bC^N^bC)Cl]+ [Pd(phC^N^phC)Cl]+ [Pd(pyC^N^pyC)Cl]+Current Difference (µA)Potential (V vs. Fc0/+)  73       Figure 3-4. Cyclic voltammograms of [Pd(phC^N^phC)Cl]+ (left) and [Pd(phC^N^phC)Cl]+ (right) at 100 mV/s under N2 and CO2.  At the peak potential of the first reduction event in [Pd(phC^N^phC)Cl]+, a smaller current enhance-ment in the presence of CO2 is observed, compared to the current enhancements with [Pd(C^N^C)Cl]+ and [Pd(bC^N^bC)Cl]+, and [Pd(pyC^N^pyC)Cl]+ showed no current enhancement at all. This may be due to greater stabilization of the additional charge due to the presence of the extended π-systems. A large current enhancement was observed for each complex upon transfer of the 2nd electron, indicative of chemical reactivity between the reduced complex and CO2. A further current enhancement was ob-served upon reduction of the polyaromatic NHC groups, as well as a loss of electrochemical reversibility. This behaviour is consistent with an increased degree of CO2 activation due to increased electron donation from the reduced NHC groups. In addition, due to the size of the π-systems in these molecules, especially [Pd(pyC^N^pyC)Cl]+, it is conceivable that their electrochemical response could deviate from normal solution behaviour due to π-interactions between the graphitic surface of the glassy carbon electrode and the complexes. Thus, elec-trochemical experiments were also performed with a Pt electrode (without a Brønsted acid present), and these revealed the same characteristics and broad waves as with glassy carbon. In addition, with either -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-200-150-100-50050Current (µA)Potential (V vs. Fc0/+) N2 CO2 CO2 + 10 mM TFA[Pd(phC^N^phC)Cl]+-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-150-120-90-60-30030[Pd(pyC^N^pyC)Cl]+Current (µA)Potential (V vs. Fc0/+) N2 CO2 CO2 + 10 mM TFA  74 Pt or carbon electrodes and for both complexes, peak current increased linearly with the square root of the scan rate, indicating freely diffusing species in solution according to the Randles-Sevcik equation.29 Thus, the relative broadness of the reduction waves, especially evident with [Pd(pyC^N^pyC)Cl]+, are attributed solely to slower diffusion rates due to the increased size of the molecules (as well as some aggregation in solution). To provide further support for this, 1H NMR diffusion-ordered spectroscopy (DOSY) experiments were performed to compare the relative diffusion rates of [Pd(bC^N^bC)Cl]BF4, [Pd(phC^N^phC)Cl]OTf, and [Pd(pyC^N^pyC)Cl]OTf in DMSO-d6 as each of these complexes have characteristic, non-overlapping resonances for the diastereotopic protons of the methylene groups linking the pyridyl rings to the imidazol-2-ylidenes. These experiments gave diffusion rates of 1.63±0.10 × 10-6 cm2/s for [Pd(bC^N^bC)Cl]BF4, 1.31±0.04 × 10-6 cm2/s for [Pd(phC^N^phC)Cl]OTf, and 1.13±0.06 × 10-6 cm2/s for [Pd(pyC^N^pyC)Cl]OTf in DMSO-d6 (Figure A-33, Figure A-34). By comparing con-servative estimates of the hydrodynamic radii based on the geometry optimized DFT structures, de-creases in diffusion rate of 18% from [Pd(bC^N^bC)Cl]BF4 to [Pd(phC^N^phC)Cl]OTf and then 8% to [Pd(pyC^N^pyC)Cl]OTf are predicted according to the Stokes-Einstein equation.29 Decreases of 20% and then 14%, respectively, are observed, with this unaccounted-for decrease attributable to π-aggrega-tion interactions, especially in the case of the pyreno-annulated species. This is also consistent with the broadness of 1H NMR resonances for [Pd(phC^N^phC)Cl]OTf and especially [Pd(pyC^N^pyC)Cl]OTf, most prominent at high concentrations, in the high viscosity NMR solvent DMSO-d6.   75      Figure 3-5. Cyclic voltammograms of [Pd(phC^N^phC)(CH3CN)]2+ (left) and [Pd(pyC^N^pyC)(CH3CN)]2+ (right) at 100 mV/s under N2 and CO2.  The electrochemical properties of the dicationic solvento complexes [Pd(phC^N^phC)(CH3CN)]2+ and [Pd(pyC^N^pyC)(CH3CN)]2+ were also analyzed (Figure 3-5), revealing in both cases an anodic shift in the peak potential of the first cathodic wave, by +0.34 V and +0.36 V, respectively, relative to the monocationic chlorido complexes (Table 3-2). As observed previously with related complexes,117 the dicationic species were able to scavenge trace chloride ions in solution to replace the labile solvento ligand. Addition of tetrabutylammonium chloride to a solution of [Pd(phC^N^phC)(CH3CN)]2+ led to a loss of the cathodic wave at −1.43 V and growth at −1.77 V, matching the electrochemical features of [Pd(phC^N^phC)Cl]+. Addition of tetraoctylammonium bromide to a solution of [Pd(phC^N^phC)(CH3CN)]2+ led to loss of the cathodic wave at −1.43 V and growth of a new wave at −1.61 V, attributed to a bromido species. The reduction waves at −2.18 V and between −2.83 and −2.96 V are unchanged relative to the chlorido species for both the solvento and bromido species, and the wave at −2.49 V is no longer present (or cathodically shifted). Thus, the first and third reduction potentials for the phenanthro-annulated species are found to be sensitive to the ligand in the fourth coordination site. -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-200-160-120-80-40040Current (µA)Potential (V vs. Fc0/+) [Pd(phC^N^phC)(CH3CN)]2+, N2 [Pd(phC^N^phC)(CH3CN)]2+, CO2-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-120-80-40040Current (µA)Potential (V vs. Fc0/+) [Pd(pyC^N^pyC)(CH3CN)]2+, N2 [Pd(pyC^N^pyC)(CH3CN)]2+, CO2  76  Table 3-2. Peak Potentials vs. Fc0/+ of First Cathodic Wave for Solvento and Chlorido Complexesa Complex Epc1 (V) ∆(Epc1) (V) [Pd(phC^N^phC)Cl]+ −1.77 +0.34 [Pd(phC^N^phC)(CH3CN)]2+ −1.43 [Pd(pyC^N^pyC)Cl]+ −1.76 +0.36 [Pd(pyC^N^pyC)(CH3CN)]2+ −1.40 aRecorded from square-wave voltammograms with 25 Hz frequency, 5 mV potential steps, and 25 mV amplitude for 2 mM solutions in 0.10 M [n-Bu4N]PF6/DMF under N2 using a glassy carbon electrode.  3.2.3 Electrochemical Effects of Stabilizing Cations  Figure 3-6. Overlaid cyclic voltammograms of [Pd(phC^N^phC)Cl]+ at 100 mV/s under N2 and CO2 with and without 25 mM Mg(ClO4)2 present.  -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-100-75-50-25025Current (µA)Potential (V vs. Fc0/+)  N2  N2 + 25 mM Mg2+  CO2  CO2 + 25 mM Mg2+[Pd(phC^N^phC)Cl]+  77 Electrochemical characterization of [Pd(phC^N^phC)Cl]+ was also carried out in the presence of Mg2+ through the addition of 25 mM Mg(ClO4)2. Previous investigations have shown that the presence of alkali and alkali earth metal cations can assist in CO2 reduction by stabilizing the charge build-up on the oxygen atoms of partially activated CO2.95,97,117,129 In addition to this general CO2-stabilizing effect, the presence of coordinating cations may be particularly beneficial for electrocatalysts with a pincer motif where initial loss of the ligand in the fourth coordination site, often a halide, upon reduction is required for catalytic turnover. For example, it has been shown that palladium triphosphine pincer complexes require a labile solvento ligand in the fourth coordination site for CO2 reduction activity. Similarly, though there is CO2 reduction activity with a halide ligand present for lutidine-linked bis-NHC palladium pincer complexes, performance is improved when the halide is replaced by a solvento ligand.65,117 Thus, the presence of more strongly interacting cations in solution may have a dual function by also helping to abstract and/or passivate coordinating anions. Both of these effects can be inferred from a comparison of CVs taken under N2 and CO2 and with/without Mg2+ ions present (Figure 3-6). Without any CO2 in solution, the presence of Mg2+ ions led to increased current at the 2nd reduction wave, a peak which likely results from reduction of an electrogenerated species with the chlorido ligand dissociated. This assignment is also supported by cyclic voltammograms taken at high scan rates (1-5 V/s), where the 2nd reduction wave diminishes as the scan rate increases (attributed to a chloro-dissociated species), and the 3rd wave at −2.49 V becomes more prominent (attributed to a chloro-bound species). With CO2 but no proton source in solution, an attenuated current enhancement is observed with Mg2+, which can be explained by its stabi-lizing ability. With a proton source present, the activated CO2 can be protonated and reduced. 3.2.4 DFT Modelling & Investigations To gain greater insight into these complexes, DFT calculations were carried out to (1) give optimized solution state structures of the unreduced, one electron, two electron, and three electron reduced species,   78 (2) determine an approximate degree of activation for CO2 bound to the reduced species, and (3) calculate the energetics and activation of CO2 as it approaches a reduced polyaromatic complex.  A molecular orbital (MO) analysis of DFT(phC^N^phC) and DFT(pyC^N^pyC) shows a smaller HOMO/LUMO gap (∆HOMO-LUMO = 6.96 eV and 6.45 eV, respectively) for the polyaromatic species compared to [Pd(C^N^C)Cl]+ (∆HOMO-LUMO = 8.58 eV) and [Pd(bC^N^bC)Cl]+ (∆HOMO-LUMO = 8.42 eV). There are also additional lower lying unoccupied MOs available in DFT(phC^N^phC) and DFT(pyC^N^pyC). Additionally, the polyannulated NHCs were found to be stronger electron donors than analogous (benz)imidazol-2-ylidenes, yielding an atomic charge of +0.176 and +0.175 on Pd for DFT(phC^N^phC) and DFT(pyC^N^pyC) compared to +0.234 and +0.250 for model complexes of [Pd(C^N^C)Cl]+ and [Pd(bC^N^bC)Cl]+, respectively, based on natural bond orbital (NBO) analyses. In both model systems, the LUMO is localized on the lutidine linker and contains some Pd dxz character (Figure 3-7). At higher energies than the LUMO (approx. +0.75 eV) are a cluster of distinct MOs at very similar energies, providing information about where added electrons may reside on the reduced com-plexes. Within this cluster, there is an orbital with primarily Pd dx2-y2 character, and two orbitals with primarily phenanthrene/pyrene character (Figure 3-7), showing that the polyaromatic NHC ligands can potentially act as electron reservoirs, providing additional redox-activity to the pincer proligand. Each of these orbitals are localized in distinct and complementary areas of the molecule, providing potentially good charge diffusion and stabilization when populated.   79    DFT(pyC^N^pyC) LUMO  DFT(pyC^N^pyC) LUMO+1  DFT(pyC^N^pyC) LUMO+3  Figure 3-7. Calculated geometries of low-lying unoccupied molecular orbitals for DFT(pyC^N^pyC). LUMO+2 is similar in energy and geometry to LUMO+1, but also contains some pyridyl character.      The relative energies of these orbitals and the order of their population as electrons are added to the system will be perturbed as additional charge is added, especially when there are accompanying geometry changes in the complex. Thus, solution-state geometry optimized structures were calculated as additional electrons were added to the system.  The first electron added to each of DFT(phC^N^phC) and DFT(pyC^N^pyC) was found to be local-ized on the pyridyl group, with some Pd and NHC character, and resulted in contraction of the Pd-N bond (approx. 2.05 Å to 2.01 Å) and lengthening of the Pd-Cl bond (approx. 2.30 Å to 2.38 Å), as well as lengthening and contracting of bonds within the pyridyl group towards more distinct single bond/double bond character, intermediate to dihydropyridine (Figure 3-8). The weakening of the Pd-Cl bond is perti-nent for catalysis, where displacement of the ligand is necessary. Calculations of the free energy of chlo-rido dissociation were performed on DFT(phC^N^phC), DFT(pyC^N^pyC), and a model complex of [Pd(C^N^C)Cl]+, yielding similar results of +42 ± 1 kcal/mol for the non-reduced species, and −1 ± 1 kcal/mol for the one-electron reduced species. The dissociation energies were found to correlate with basicity of the ligand, where bromo, trifluoroacetate, and trifluoroethanolate have free energies of disso-ciation of −4, −11, and +8 kcal/mol, respectively, from the one-electron reduced species of [Pd(C^N^C)X]+. Given the square planar geometry of the complex, an associative mechanism for loss of the ligand in the fourth coordination site is likely, with a solvent or CO2 molecule coordinating to the   80 complex to form a five-coordinate intermediate, followed by dissociation of the fourth ligand. This is in contrast to M(bpy)(CO)3X-type catalysts, where halide loss will occur through a dissociative mechanism (Scheme 1-3). The second electron can be added in either a spin-aligned (singlet) or anti-aligned (triplet) manner. Both configurations were tested and were found to be almost identical in energy, with the singlet state being marginally more stable for DFT(phC^N^phC) and the triplet state slightly more stable for DFT(pyC^N^pyC) (by 1-3 kcal/mol). In the triplet state, the electron is localized on the polyaromatic moieties, with the SOMO in DFT(pyC^N^pyC) being 0.42 eV lower in energy due to more extensive π-conjugation. In the singlet state, the additional electron is localized across the Pd, NHC and pyridyl groups, resulting in a further lengthening of the Pd-Cl bond to 2.41 Å. At this point, the chloride has only a small energy barrier to dissociation which is calculated to be exergonic by approximately 50 kcal/mol. Dissociation of the halide lowers the energy of the empty dx2-y2 orbital below that of the polyaromatic moieties. The third electron added to the system for both complexes, even for halide-dissociated species, is localized on the polyaromatic moieties. The LUMO is also phenanthro- or pyreno-localized, which is consistent with the experimental results showing multi-electron transfer at the most negative reduction potentials. The characteristics of the modelled reduced species are consistent with the electrochemical data and provide additional insight, suggesting stability of the Pd-NHC bonds even in highly reduced species, weakening of the Pd-halide bond by the first two pyridyl/Pd-centered electron transfers, and additional redox-activity on the phenanthrene and pyrene moieties upon transfer of three and four elec-trons.     81     DFT(phC^N^phC) HOMO 1e– reduced, SOMO 2e– reduced, HOMO 3e– reduced, HOMO          DFT(pyC^N^pyC) HOMO 1e– reduced, SOMO 2e– reduced, HOMO 3e– reduced, HOMO      Figure 3-8. HOMO and SOMO geometries of lowest energy geometry optimized structures of reduced species of DFT(phC^N^phC) (top) and DFT(pyC^N^pyC) (bottom) as pristine, non-halide-dissociated species. Another redox-active annulated NHC ligand, naphthoquinoinimidazol-2-ylidene, has been previously reported, showing reversible reduction/oxidation in Ir(L)(CO)2Cl at −1.08 V, a much less negative po-tential than pyrene (−2.54 V) or phenanthrene (−2.82 V),110,130,131 where reduction of the redox-active group switched the NHC to a more electron-donating state.132 A similar effect would be expected upon reduction of the polyaromatic groups fused to the NHC, providing additional electron donation to a bound CO2 molecule. For this reason, calculations of the degree of CO2 activation as a function of the degree of reduction of the complex were investigated and compared to other known electrocatalysts. The bond angle of the bound CO2 at the energy minimum of the reaction coordinate was used as a convenient quantity to represent this, though an analysis of the bonding energy along the whole reaction coordinate is required for strict comparisons. These bond angles are dependent on the chosen computational param-eters such as functional and basis set, and are best interpreted as relative assessments of CO2 activation.  The results of these calculations show an increase in the degree of CO2 activation by species with reduced polyaromatic moieties, from a bond angle of approximately 128° in two electron reduced species to 124° in four electron reduced species (Table 3-3).   82 A comparison of the degree of CO2 activation as determined by bond angle, when modelled as bound to an active electrocatalyst species, to the proton sources used in CO2 reduction with the particular elec-trocatalyst shows a correlation between degree of activation and pKa of the proton source required; highly activated CO2 can be protonated by weak acids, and weak acids lead to less undesired H+ reduction in CO2 electroreduction experiments (Table 3-4). For example, Mn(bpy)(CO)3(X) requires the use of weak acids like 2,2,2-trifluoroethanol (TFE; pKa 23.5 in DMSO90) to reduce CO2, whereas reduction using Re(bpy)(CO)3(X) can proceed without an added proton source, due to the difference in CO2 activation capability, calculated to bond angles of 123.7° and 122.5° at their potential energy minima, respectively. Thus, it is reasonable that four electron reduced species of [Pd(phC^N^phC)Cl]+ and [Pd(pyC^N^pyC)Cl]+  may be able to reduce CO2 in the presence of a weak acid, such as trifluoroethanol. Accordingly, cyclic voltammograms of [Pd(pyC^N^pyC)Cl]+ and [Pd(pyC^N^pyC)(CH3CN)]2+ were taken under CO2, with addition of TFE resulting in increased current at the wave attributed to reduction of the pyrene moieties, consistent with these theoretical results (Figure 3-9).  Figure 3-9. Overlaid CVs of [Pd(pyC^N^pyC)Cl]OTf at 100 mV/s under N2, CO2, and CO2 + 200 mM 2,2,2-trifluoroethanol.    83 Table 3-3. Calculated Degree of CO2 Activation when Bound to Reduced Complexes Complex Degree of reduction CO2 bond angle (°) DFT(phC^N^phC) 2e− 127.6  3e− 125.8  4e− 123.8 DFT(pyC^N^pyC) 2e− 128.2  3e− 127.9  4e− 123.9  Table 3-4. Correlation between Calculated Degree of CO2 Activation at Bonding Potential Energy Min-imum and Required Proton Source19,20,65,117 Reduced complex CO2 bond angle (°) Proton source Re(bpy)(CO)3− 122.5 Adventitious H’s, H2O Mn(bpy)(CO)3− 123.7 Trifluoroethanol, methanol, water Pd(triphos)0 130.0 HBF4, H3PO4 Pd(C^N^C)0 128.0 Trifluoroacetic acid  The optimized structures for the highly reduced CO2-bound species of DFT(phC^N^phC) and DFT(pyC^N^pyC) were chemically reasonable with square planar geometry, conventional Pd-L bond lengths, and CO2 occupying the fourth coordination site. Conversely, optimizations of simpler cationic complexes such as [Pd(C^N^C)]+ and [Pd(bC^N^bC)]+ with three and four electrons added result in distortions towards a tetrahedral geometry, loss of planarity in the pyridyl moiety, and loss of symmetry in pincer coordination. An NBO charge analysis of the optimized structures revealed increased electron   84 density on the NHC moieties relative to the unreduced complex for the 3e− and 4e− reduced cases, indi-cating charge storage. In the 2e− reduced case, the additional charge is localized on the pyridyl group, bound CO2, and metal center; in the 3e− reduced case, most of the additional charge is located on the periphery of the polyaromatic groups; and in the 4e− reduced case, most of the additional charge is local-ized to the pyridyl group and metal center, as well. Lastly, the thermodynamics of CO2 coordination to one and two electron reduced DFT(phC^N^phC) were modelled as a function of distance from the metal center. Compared to [Pd(C^N^C)Cl]+, the singly-reduced species has a slightly more exergonic interaction, and the doubly-reduced species is almost equivalent (Figure 3-10). Due to computational expense, only the one and two electron reduced species of DFT(phC^N^phC) were modelled in this way, with only the optimized structures at the energy minima calculated for three and four electron reduced DFT(phC^N^phC) and DFT(pyC^N^pyC) (see above).  Figure 3-10. DFT-calculated energetics for the activation of CO2 as it approaches a reduced metal com-plex. Bond angles of CO2 are given at the energy minimum, unless otherwise stated.    85 3.2.5 Controlled Potential Electrolysis To survey the relative performance of the complexes, controlled potential electrolysis (CPE) experi-ments were performed at a variety of potentials with CO2-sparged 2 mM solutions in 0.10 M [n-Bu4N]PF6/DMF with 10 mM TFA present, unless otherwise specified. The headspace gas was then sampled and analyzed by GC to quantify the CO and H2 produced. Electrolysis at the first reduction potential of [Pd(phC^N^phC)Cl]+ led to 18% CO in the gas produced (Table 3-5). This ratio decreased to 12% at the second reduction potential, and then increased to 20% at more extreme potentials where the phenanthrene moieties could be reduced (even though the rate of competitive H+ reduction at the electrode is very high). The addition of Mg2+ improved selectivity for CO2 reduction during electrolysis at the first reduction potential, but selectivity at more negative poten-tials was either comparable or slightly worse. A longer experiment was also performed where 68 C were passed at −2.02 V and the headspace gas was monitored over six intervals. The ratio of CO produced was consistently between 29-32% over the first 56 C and 23% over the last 12 C, representing 5.5 turnovers of CO2 reduction to produce CO. Cyclic voltammograms taken throughout the experiment retain characteristic features of the complex (Figure 3-11). Additional TFA was added at intervals of approximately 8 C to maintain its concentration between 2 and 10 mM. The higher efficiency for CO production observed in this longer experiment is attributed to the smaller average concentration of acid present as well as achieving a steady-state concentration of the active, reduced species. The shorter experiments serve to screen relative performance at different reduction potentials.   86            Figure 3-11. Plot of current over time during electrolysis of [Pd(phC^N^phC)Cl]OTf at -2.00 V under CO2 with TFA present (left) and overlaid cyclic voltammograms taken at various points during the ex-periment (right, variable concentrations of TFA between 2 – 8 mM).  Table 3-5. Controlled Potential Electrolysis Results for [Pd(phC^N^phC)Cl]+ a Solution Potential (V) Charge Passed (C) Ratio CO Produced FE CO FE H2 [Pd(phC^N^phC)Cl]+ −2.02 3.7 18% 17% 79% [Pd(phC^N^phC)Cl]+ −2.32 3.7 12% 10% 68% [Pd(phC^N^phC)Cl]+ −2.97 3.8 20% 15% 61% [Pd(phC^N^phC)Cl]+ + Mg2+ −2.02 3.0 27% 26% 70% [Pd(phC^N^phC)Cl]+ + Mg2+ −2.32 3.7 14% 11% 65% [Pd(phC^N^phC)Cl]+ + Mg2+ −2.77 4.1 16% 14% 74% [Pd(phC^N^phC)Cl]+b −2.02 68 28% 27% 69% aPerformed with 2 mM solutions in 10 mL 0.10 M [n-Bu4N]PF6/DMF under CO2 using a glassy carbon rod working electrode. 25 mM Mg(ClO4)2 added where indicated. Potentials versus Fc0/+. b1.6 mM con-centration of the complex was used.  -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-80-40040Current (µA)Potential (V) N2 10 mM TFA, CO2 TFA, after 15 C, CO2 TFA, after 27 C, CO2 TFA, after 41 C, CO2 TFA, after 47 C, CO2 TFA, after 56 C, CO2  87 Electrolysis of [Pd(pyC^N^pyC)Cl]+ yielded similar results, with 12% CO produced at the first re-duction potential, and a decrease to 7% at the second reduction potential (Table 3-6). The addition of Mg2+ did not result in the same increase in selectivity at the first reduction potential observed with [Pd(phC^N^phC)Cl]+, though there was a small improvement during electrolysis at the second and third reduction potentials. Due to the more accessible potential for reduction into the redox-active pyreno-imidazol-2-ylidenes, and the effect this could have on the degree of CO2 activation, electrolysis was attempted at −2.77 V using 200 mM trifluoroethanol as the proton source. This resulted in selective but short-lived CO2 reduction, where the current diminished within approximately 15 seconds of electrolysis. After electrolysis, it was observed that a dark film had deposited on the working electrode. A potential step to +0.7 V removed the film and recovered the initial current which quickly diminished again upon reduction at −2.77 V. This process was repeated several times, and the headspace gas then sampled, revealing 87% CO in the gas produced, which is consistent with greater CO2 activation resulting from reduction of the high energy redox-active pyrene moieties. Given that the formation of this film with resulting current deterioration is not observed at these potentials using TFA as the acid source, it may be due to formation of an insoluble species by chemical reaction with trifluoroethanol or its conjugate base (additional discussion below; Figure 3-12).      88 Table 3-6. Controlled Potential Electrolysis Results for [Pd(pyC^N^pyC)Cl]+ a Solution Potential (V) Charge Passed (C) Ratio CO Produced FE CO FE H2 [Pd(pyC^N^pyC)Cl]+ −1.97 3.8 12% 12% 88% [Pd(pyC^N^pyC)Cl]+ −2.32 3.5 7% 6% 87% [Pd(pyC^N^pyC)Cl]+ + Mg2+ −2.02 4.2 11% 11% 89% [Pd(pyC^N^pyC)Cl]+ + Mg2+ −2.32 4.0 12% 12% 86% [Pd(pyC^N^pyC)Cl]+ + Mg2+ −2.62 4.0 13% 10% 67% [Pd(pyC^N^pyC)Cl]+ + TFE −2.77 1.5b 87% 33% 5% aPerformed with 2 mM solutions in 10 mL 0.10 M [n-Bu4N]PF6/DMF under CO2 using a glassy carbon rod working electrode. 25 mM Mg(ClO4)2 or 200 mM TFE added where indicated. Potentials versus Fc0/+. bCurrent diminished quickly as film formed on electrode. A potential step to +0.7 V removes the film and recovers a moderate current, which quickly diminishes at −2.77 V.  Electrolysis with [Pd(phC^N^phC)(CH3CN)]2+ revealed an initial ratio of 25% CO produced at −1.67 V, which is an improvement over 17% with the analogous [Pd(C^N^C)(CH3CN)]2+ species.117 This se-lectivity also seems to be short-lived, however, as the characteristic electrochemical features for the sol-vento complex were diminished after the first electrolysis experiment, which may be due to coordination of the generated trifluoroacetate anions. Subsequent experiments showed low selectivities for CO2 re-duction regardless of the potential applied and whether or not Mg2+ was added (Table 3-7). This is in contrast to [Pd(phC^N^phC)Cl]+ and also to the simpler complex [Pd(C^N^C)(CH3CN)]2+, which main-tained selectivity for CO2 throughout six times as much charge transfer.117    89 Table 3-7. Controlled Potential Electrolysis Results for [Pd(phC^N^phC)(CH3CN)]2+ a Solution Potential (V) Charge Passed (C) Ratio CO Produced FE CO FE H2 [Pd(phC^N^phC)(CH3CN)]2+ −1.67 3.5 25% 24% 72% [Pd(phC^N^phC)(CH3CN)]2+ b −1.97 3.5 14% 12% 78% [Pd(phC^N^phC)(CH3CN)]2+ −2.32 3.5 5% 4% 75% [Pd(phC^N^phC)(CH3CN)]2+ −1.67 3.1 5% 5% 94% [Pd(phC^N^phC)(CH3CN)]2+ + Mg2+ −1.62 3.5 7% 7% 90% [Pd(phC^N^phC)(CH3CN)]2+ + Mg2+ −1.97 3.8 7% 7% 87% [Pd(phC^N^phC)(CH3CN)]2+ + Mg2+ −2.17 3.8 6% 6% 85% [Pd(phC^N^phC)(CH3CN)]2+ + Mg2+ −2.32 3.8 2% 2% 80% aPerformed in sequence from top to bottom row with 2 mM solutions in 10 mL 0.10 M [n-Bu4N]PF6/DMF under CO2 using a glassy carbon rod working electrode. Potentials versus Fc0/+. 25 mM Mg(ClO4)2 added where indicated. bDistinctive electrochemical feature for acetonitrilo complex di-minished after CPE at −1.67 V.  Due to the deactivation of [Pd(phC^N^phC)(CH3CN)]2+ during electrolysis with TFA as the proton source, and given that its first reduction peak is at −1.43 V, with current onset at approximately −1.31 V, electrolysis was attempted with the strong acid HPF6 (pKa ~0 in DMSO90,133) as the proton source, given that its conjugate base is the non-coordinating anion PF6−. Electrolysis at −1.67 V and −1.47 V yielded 6% and 5% CO production, respectively, with H+ reduction predominant with this proton source (Table 3-8). Despite the poor selectivity for CO2 reduction, the CVs of the complex before and after electrolysis experiments were more consistent, suggesting that inhibition of the complex with the conjugate base of the proton source may be a relevant factor.   90 Table 3-8. CPE Results for [Pd(phC^N^phC)(CH3CN)]2+ with 10 mM HPF6 a Solution Potential (V) Charge Passed (C) Ratio CO Produced FE CO FE H2 [Pd(phC^N^phC)(CH3CN)]2+ −1.67 4.1 6% 6% 89% [Pd(phC^N^phC)(CH3CN)]2+ −1.47 3.0 5% 5% 91% [Pd(phC^N^phC)(CH3CN)]2+ −2.32 2.2 3% 2% 73% [Pd(phC^N^phC)(CH3CN)]2+ + Mg2+ −1.82 3.0 4% 4% 97% [Pd(phC^N^phC)(CH3CN)]2+ + Mg2+ −2.32 3.0 4% 3% 83% aPerformed with 2 mM solutions in 10 mL 0.10 M [n-Bu4N]PF6/DMF under CO2 using a glassy carbon rod working electrode. 25 mM Mg(ClO4)2 added where indicated. Potentials versus Fc0/+.  Finally, separate electrolysis experiments were carried out with [Pd(pyC^N^pyC)(CH3CN)]2+ using TFE and TFA as proton sources (Table 3-9). With TFE, similar behaviour as for [Pd(pyC^N^pyC)Cl]+ was observed, with formation of a film on the electrode surface and a quickly diminishing current, re-sulting in the reduction of CO2 to CO. Again, holding the working electrode at +0.7 V resulted in removal of the film and temporarily restored current. With TFA, selectivities for CO production were lower than with [Pd(phC^N^phC)Cl]+, [Pd(pyC^N^pyC)Cl]+, or [Pd(phC^N^phC)(CH3CN)]2+ at 4-12%, and a no-ticeable amount of a dark precipitate was formed. This insoluble black powder was isolated by centrifu-gation, dried, and analyzed by ATR-FTIR, revealing features similar to [Pd(pyC^N^pyC)Cl]+, but with a strong, broad stretch centered at 1854 cm−1, tentatively assigned to a carbonate or bicarbonate species (Figure 3-12). Given its complete insolubility, NMR and MS data were not obtained.   91  Figure 3-12. ATR-FTIR spectrum of insoluble precipitate formed during controlled potential electrolysis with [Pd(pyC^N^pyC)(CH3CN)](OTf)2 and [Pd(pyC^N^pyC)Cl]OTf.  Table 3-9. Controlled Potential Electrolysis Results for [Pd(pyC^N^pyC)(CH3CN)]2+ a Solution Potential (V) Charge Passed (C) Ratio CO Produced FE CO FE H2 [Pd(pyC^N^pyC)(CH3CN)]2+ + TFE −2.52 0.5b 100% 12% 0% [Pd(pyC^N^pyC)(CH3CN)]2+ + TFE −2.72 1.0b 100% 14% 0% [Pd(pyC^N^pyC)(CH3CN)]2+ −2.07 2.6 4% 4% 84% [Pd(pyC^N^pyC)(CH3CN)]2+ −2.47 2.5 6% 4% 73% [Pd(pyC^N^pyC)(CH3CN)]2+ + Mg2+ −2.07 3.1 2% 1% 74% [Pd(pyC^N^pyC)(CH3CN)]2+ + Mg2+ −2.47 3.2 12% 8% 59% [Pd(pyC^N^pyC)(CH3CN)]2+ + Mg2+ −2.72 3.2 9% 5% 49% aPerformed with 2 mM solutions in 10 mL 0.10 M [n-Bu4N]PF6/DMF under CO2 using a glassy carbon rod working electrode. 25 mM Mg(ClO4)2 or 500 mM TFE added where indicated. bCurrent diminished quickly as film formed on electrode. A potential step to +0.7 V removes the film and recovers a moderate current, which quickly diminishes at these potentials.   92 Colourimetric spot tests with chromotropic acid and methylquinaldinium93,94 were performed on the solutions after electrolysis to test for the presence of formic acid, formate, or formaldehyde; none of these species were detected. Additionally, two CPE experiments were performed with [Pd(pyC^N^pyC)(CH3CN)]2+ in acetonitrile with the resulting solution analyzed by 1H NMR in CD3CN with suppression of the acetonitrile solvent signal (1.96 ppm) which is well separated from signals for formate/formic acid (8.0 ppm) and formaldehyde (9.6 ppm). Again, none of these potential reduction products were detected. In some CPE experiments at more cathodic potentials (-2.3 V and below), it was observed that the overall faradaic yield for CO and H2 was less than unity. This unaccounted for charge is primarily attributed to direct reduction of CO2 at the electrode to produce radical anion species which can then react with the solvent or other radical anions (resulting in oxalate or carbonate formation), in addition to some degradation of the complex and perhaps also some leakage of oxidized species from the counter electrode compartment to the working electrode compartment in some cases. Finally, two experiments were performed to see if heterogeneous catalysis may be a significant con-tributor to the production of CO: (1) a mercury drop test was performed with [Pd(phC^N^phC)Cl]+ at -2.05 V (where drops of mercury are added and stirred during electrolysis to remove any potential heter-ogeneous species), revealing no difference in performance, and (2) after an electrolysis experiment with [Pd(phC^N^phC)Cl]+ the working electrode was transferred to a new electrolyte solution with 10 mM TFA but no dissolved complex. The passage of 5 C resulted in a ratio of only 1% CO produced. 3.3 Conclusions Phenanthro- and pyreno-annulated bis-NHC palladium pincer complexes [Pd(phC^N^phC)Cl]+ and [Pd(pyC^N^pyC)Cl]+ were synthesized and found to have similar reduction potentials to [Pd(bC^N^bC)Cl]+. These complexes are able to electrocatalytically reduce CO2 to CO in the presence of TFA, with [Pd(phC^N^phC)Cl]+ having improved selectivity for CO2 reduction at the first reduction   93 potential compared to [Pd(C^N^C)Cl]+ and [Pd(bC^N^bC)Cl]+, and [Pd(pyC^N^pyC)Cl]+ having di-minished relative selectivity, showing that electronic modifications to the NHC donors is an important factor affecting reactivity. The polyannulated dicationic solvento complexes were found to be more quickly deactivated during electrolysis than their non-annulated counterpart. The addition of polyaro-matic groups enabled additional redox-activity on the ligand at −2.96 V for [Pd(phC^N^phC)Cl]+ and −2.56 V for [Pd(pyC^N^pyC)Cl]+, allowing for increased electron donation from the ligand. The addition of the weak acid TFE results in reactivity with CO2 at these potentials, but controlled potential electro-lyses are short-lived and result in the formation of an insulating and insoluble species on the electrode. These systems were also computationally modeled in solution, providing support for assignments of the redox events, giving greater understanding of the chemical behaviour of the reduced species, and high-lighting the importance of anion dissociation energies within the pincer motif.  3.4 Experimental 3.4.1 General Unless otherwise specified, all reactions were performed under nitrogen using standard Schlenk tech-niques and solvents and reagents were used as received from commercial sources. Magnesium perchlo-rate (Alfa), 2,2,2-trifluoroethanol (Aldrich), trifluoroacetic acid (Aldrich), and 99.998% carbon dioxide (Praxair) were used as received for electrochemical experiments. N,N-dimethylformamide (EMD Milli-pore Omnisolv®) and distilled acetonitrile (Anachemia Accusolv™) were dried and stored over 15% m/v 4Å molecular sieves.134,135    94 1H NMR spectra were acquired using Bruker AV300 or AV400 spectrometers with chemical shifts referenced to residual solvent signals. Mass spectra were acquired using a Waters LC-MS ESI-MS, ex-cept for the acetonitrilo complexes which were acquired using a Waters Micromass LCT ESI-MS. IR spectra were collected using a PerkinElmer Frontier FT-IR Spectrometer with ATR attachment. Gaseous products were analyzed using an SRI Model 8610C gas chromatograph equipped with molecular-sieve columns, dual TCD and FID detectors, and methanizer. Crystallographic data was acquired using a Bruker X8 APEX II diffractometer with graphite monochromated Mo-Kα radiation. 3.4.2 Electrochemistry Electrochemical experiments were performed using a Metrohm Autolab PGSTAT12 or Pine AFCBP1 bipotentiostat in an air-tight three-electrode cell with a 7 mm2 glassy carbon working electrode (Bioana-lytical Systems, Inc.), Pt mesh counter electrode, and Ag wire pseudo-reference electrode in a 0.010 M AgNO3 acetonitrile solution separated from the bulk solution by a Vycor frit. Experiments were per-formed under N2 or CO2 using 2 mM concentrations of the complexes in 10 mL anhydrous electrolyte solution unless otherwise stated. Electrolyte solutions were 0.10 M triply recrystallized [n-Bu4N]PF6 in anhydrous N,N-dimethylformamide and sparged with nitrogen prior to use. For experiments with CO2, the solution was sparged with CO2 for 15 minutes resulting in a concentration of ~0.20 M.27 Controlled potential experiments used glassy carbon rod (Alfa Aesar, 5 mm diameter) working electrodes in a two compartment H-cell, where the counter electrode compartment was separated from the compartment containing the working and reference electrodes by fritted glass. All glassy carbon electrodes were cleaned by successive polishing with 1 μM, 0.3 μM, and 0.05 μM alumina paste, following by rinsing with water, sonication (5 min) in distilled water, and sonication (5 min) in methanol. Decamethylferro-cene was added at the end of electrochemical experiments as an internal standard, showing a reversible   95 redox couple at -404 mV vs. Ag/AgNO3 and -476 mV vs. ferrocene/ferrocenium.136 Cyclic voltammo-grams were collected at a scan rate of 100 mV/s and square-wave voltammograms at a frequency of 25 Hz with 5 mV steps and 25 mV amplitude unless otherwise stated. 3.4.3 Computational Methods DFT calculations were performed using Gaussian 09 (Revision D.01) using the long range and dis-persion-corrected ωB97xD hybrid functional without symmetry constraints.111 Calculations for reduced species were performed using unrestricted, open-shell wavefunctions. The D95(d) basis set was used for all atoms except transition metals, which employed the Stuttgart-Dresden-Bonn quasi-relativistic effec-tive-core potential and corresponding correlation-consistent triple-ζ basis set.112,113 Calculations were performed with the presence of a solvent reaction field produced by the conductor-like polarizable con-tinuum model (CPCM).137 Frequency calculations were performed on all geometry optimized structures to ensure that energy minima were achieved.  3.4.4 Synthesis The proligand precursor 9,10-phenanthrenequinone (95%) was purchased from Sigma-Aldrich with the remaining precursors synthesized according to literature procedures: 2,6-bis(bromomethyl)pyri-dine,138 1H-phenanthro[9,10-d]imidazole,118 pyrene-4,5-dione,139 and 9H-pyreno[4,5-d]imidazole.119 Alkylation of Polyaromatic Imidazoles 1-Butyl-1H-phenanthro[9,10-d]imidazole The compound was synthesized via a modified literature procedure.118 Sodium hydroxide (4.40 mmol, 176 mg) was added to a 10 mL solution of 1H-phenanthro[9,10-d]imidazole (3.99 mmol, 870 mg) in DMSO and stirred for 2 hours at room temperature, after which 1-iodobutane (4.5 mmol, 0.51 mL) was added and the solution was heated to 50 °C overnight. Water (15 mL) was added to the solution which   96 was extracted with diethyl ether (15 mL × 4). The organic extracts were washed with water (15 mL × 2) and then the solvent was removed, yielding a yellow-orange oil (899 mg). The crude product was purified by column chromatography over SiO2 using acetone as the eluent, collecting the second band. Upon removal of the solvent, the oil crystallized as a yellow-orange solid (813 mg, 74% yield). 1H NMR (300 MHz, (CD3)2CO)  0.98 (t, J = 7.4 Hz, 3 H) 1.48 (sxt, J = 7.5 Hz, 2 H) 2.02 (quin, J = 7.5 Hz, 2 H) 4.78 (t, J = 7.2 Hz, 2 H) 7.55 - 7.79 (m, 4 H) 8.12 (s, 1 H) 8.41 (dt, J = 8.0, 0.9 Hz, 1 H) 8.62 - 8.71 (m, 1 H) 8.81 (dt, J = 8.2, 0.7 Hz, 1 H) 8.93 (dt, J = 8.2, 0.8 Hz, 1 H) 9-Butyl-9H-pyreno[4,5-d]imidazole CAUTION: Many pyrene derivatives are carcinogenic. Exposure to these materials, especially contact with skin, should be avoided. The compound was synthesized via a modified literature procedure.119 Sodium hydroxide (3.90 mmol, 156 mg) was added to a 12 mL solution of 9H-pyreno[4,5-d]imidazole (3.55 mmol, 860 mg) in DMSO and stirred for 2 hours at room temperature, after which 1-iodobutane (4.0 mmol, 0.45 mL) was added and the solution was heated to 50°C overnight. Water (15 mL) was added to the solution which was extracted with diethyl ether (25 mL × 5). The organic extracts were washed with water (20 mL × 2), dried with MgSO4, and the solvent was then removed yielding an orange crystalline powder (589 mg, 56% yield) which was used without further purification. 1H NMR (300 MHz, (CD3)2SO)  0.89 (t, J = 7.3 Hz, 3 H) 1.38 (sxt, J = 7.3 Hz, 2 H) 1.93 (quin, J = 7.2 Hz, 2 H) 4.78 (t, J = 7.1 Hz, 2 H) 8.07 - 8.14 (m, 2 H) 8.17 (s, 2 H) 8.19 - 8.27 (m, 2 H) 8.36 (s, 1 H) 8.57 (d, J = 7.6 Hz, 1 H) 8.80 (dt, J = 7.5, 0.6 Hz, 1 H)   97 Synthesis of Proligands phC^N^phC • 2HBr A solution of 1-butyl-1H-phenanthro[9,10-d]imidazole (2.96 mmol, 813 mg) and 2,6-bis(bromome-thyl)pyridine (1.40 mmol, 372 mg) in 20 mL 1,4-dioxane was heated to reflux for 84 hours, during which time an off-white precipitate was formed. The precipitate was collected by centrifugation, washed with diethyl ether (10 mL × 2), and then dried under vacuum, yielding an off-white powder (732 mg, 64% yield). 1H NMR (300 MHz, (CD3)2SO)  0.97 (t, J = 7.3 Hz, 6 H) 1.43 (sxt, J = 7.3 Hz, 4 H) 1.74 (quin, J = 7.4 Hz, 4 H) 4.38 (t, J = 7.4 Hz, 4 H) 6.14 (s, 4 H) 7.23 (t, J = 7.8 Hz, 2 H) 7.55 (t, J = 7.8 Hz, 2 H) 7.66 (d, J = 8.4 Hz, 2 H) 7.71 - 7.85 (m, 6 H) 8.05 (d, J = 8.1 Hz, 2 H) 8.20 (t, J = 7.8 Hz, 1 H) 8.46 (d, J = 8.4 Hz, 2 H) 8.54 (d, J = 8.3 Hz, 2 H) 9.47 (s, 2 H). ESI-MS [M-Br]+ = 732.8 m/z. pyC^N^pyC • 2HBr CAUTION: Many pyrene derivatives are carcinogenic. Exposure to these materials, especially contact with skin, should be avoided. A solution of 9-butyl-9H-pyreno[4,5-d]imidazole (1.46 mmol, 436 mg) and 2,6-bis(bromome-thyl)pyridine (0.710 mmol, 188 mg) in 15 mL 1,4-dioxane was heated to reflux for 84 hours, during which a light-coloured precipitate was formed. The precipitate was collected by centrifugation, washed with diethyl ether (10 mL × 2), and then dried under vacuum, yielding an off-white powder (411 mg, 67% yield). 1H NMR (300 MHz, (CD3)2SO)  0.98 (t, J = 7.3 Hz, 6 H) 1.44 (sxt, J = 7.4 Hz, 4 H) 1.71 (quin, J = 7.4 Hz, 4 H) 4.25 (t, J = 7.5 Hz, 4 H) 6.23 (s, 4 H) 7.54 (t, J = 7.9 Hz, 2 H) 7.80 - 8.04 (m, 14 H) 8.20 (dt, J = 7.3, 0.8 Hz, 2 H) 8.29 (t, J = 7.8 Hz, 1 H) 9.58 (s, 2 H). ESI-MS [M-Br]+ = 780.9 m/z.   98 Synthesis of Monocationic Halido Complexes [Pd(phC^N^phC)Cl]OTf To a solution of the proligand phC^N^phC • 2HBr (0.369 mmol, 300 mg) in DMSO (12 mL) was added Ag2O (0.368 mmol, 85.3 mg) and 3Å molecular sieves. The reaction vessel was covered in foil and left to stir at 50 °C for 24 hours and then left to cool to room temperature. To the resulting mixture was added PdCl2(cod) (0.369 mmol, 105 mg) and then AgOTf (0.405 mmol, 104 mg). The mixture was left to stir for 48 hours, then centrifuged and filtered through Celite. The solvent was removed under vacuum at 55 °C and the residue taken up in acetonitrile (5 mL). Addition to Et2O (12 mL) yielded a light yellow precipitate, which was isolated by centrifugation and washed with Et2O (5 mL), MeOH (2 mL), and Et2O (5 mL), yielding the crude product (260 mg). Purification was achieved by successive washes with a minimum of MeOH (4 × 1 mL), yielding a light tan powder (160 mg, 46% yield). X-ray quality crystals were grown by slow diffusion of Et2O into a concentrated 1:1 DMF:CH3CH solution at -30 °C. 1H NMR (400 MHz, (CD3)2SO)  0.94 (t, J = 7.3 Hz, 6 H) 1.31 - 1.43 (m, 2 H) 1.55 - 1.68 (m, 2 H) 1.87 - 1.99 (m, 2 H) 2.00 - 2.12 (m, 2 H) 5.24 - 5.36 (m, 2 H) 5.48 - 5.63 (m, 2 H) 6.41 (d, J = 15.9 Hz, 2 H) 6.90 (d, J = 15.4 Hz, 2 H) 7.79 - 7.95 (m, 8 H) 8.16 (s, 3 H) 8.57 (d, J = 8.2 Hz, 2 H) 8.99 (dt, J = 8.4, 0.7 Hz, 2 H) 9.07 (d, J = 8.2 Hz, 2 H) 9.08 (d, J = 8.2 Hz, 2 H). ESI-MS [M-OTf]+ = 792.6 m/z. Anal. Calculated for C46H41ClF3N5O3PdS: C, 58.60; H, 4.38; N, 7.43. Found: C, 58.09; H, 4.41; N, 7.19. Averaged results for repeat elemental analysis are shown. For one attempt the C was within error of the calculated value, but for the other it was below. This is accounted for by the presence of a small amount (~7%) of bromido species present, as shown by ESI-MS at [MBr-OTf]+ = 838 m/z, as well as some in-tractable DMSO (3 mol% by NMR), giving calculated values of 58.36 %C, 4.38 %H, and 7.39 %N. [Pd(pyC^N^pyC)Cl]OTf CAUTION: Many pyrene derivatives are carcinogenic. Exposure to these materials, especially contact with skin, should be avoided.   99 To a solution of the proligand pyC^N^pyC • 2HBr  (0.319 mmol, 275 mg) in DMSO (12 mL) was added Ag2O (0.324 mmol, 75.1 mg) and 3Å molecular sieves. The reaction vessel was covered in foil and left to stir at 55 °C for 24 hours and then left to cool to room temperature. To the resulting mixture was added PdCl2(cod) (0.319 mmol, 99.1 mg) and then AgOTf (0.351 mmol, 90.2 mg). The mixture was left to stir for 56 hours, then centrifuged and filtered through Celite. The solvent was removed under vacuum at 60 °C and the dark residue taken up in acetonitrile (5 mL). Addition to Et2O (10 mL) yielded a yellow-orange precipitate, which was isolated by centrifugation and washed with Et2O (5 mL), MeOH (4 × 2 mL), and Et2O (5 mL), yielding a tan powder (215 mg, 68% yield). The solute from methanol washes were analyzed by NMR to track the degree of purification. 1H NMR (400 MHz, (CD3)2SO)  0.99 (t, J = 7.4 Hz, 6 H) 1.39 - 1.55 (m, 2 H) 1.64 - 1.79 (m, 2 H) 2.09 (s, 2 H) 2.11 - 2.24 (m, 2 H) 5.40 - 5.53 (m, 2 H) 5.64 - 5.81 (m, 2 H) 6.53 (d, J = 15.9 Hz, 1 H) 7.12 (d, J = 15.7 Hz, 2 H) 8.18 - 8.38 (m, 11 H) 8.47 (d, J = 7.8 Hz, 2 H) 8.50 (d, J = 7.5 Hz, 3 H) 8.88 (d, J = 8.4 Hz, 2 H) 9.33 (d, J = 8.2 Hz, 2 H). ESI-MS [M-OTf]+ = 840.6 m/z. Anal. Calculated for C50H41ClF3N5O3PdS: C, 60.61; H, 4.17; N, 7.07. Found: C, 57.76; H, 4.06; N, 7.21.  Synthesis of Dicationic Acetonitrilo Complexes [Pd(phC^N^phC)(CH3CN)](OTf)2 [Pd(phC^N^phC)Cl]OTf (0.050 mmol, 47 mg) was taken up in a solvent mixture of 20 mL 1:1 CH3CN:CH2Cl2 and the light yellow solution was reacted with AgOTf (0.11 mmol, 27 mg) at room temperature overnight in the dark, forming a fine white precipitate. The precipitate was removed by cen-trifugation and the solvent removed under reduced pressure, leaving a light yellow residue which was taken up in CH2Cl2 (3 mL), reprecipitated by addition of Et2O, washed with Et2O (2 × 3 mL), and then dried under vacuum, yielding an off-white powder (36 mg, 66% yield). 1H NMR (400 MHz, CD3CN)  0.98 (t, J = 7.4 Hz, 6 H) 1.37 - 1.51 (m, 2 H) 1.59 - 1.73 (m, 2 H) 1.92 - 2.07 (m, 4 H) 1.96 (s, 3 H) 5.00 (dt, J = 14.9, 8.1 Hz, 2 H) 5.23 (dt, J = 14.9, 6.2 Hz, 2 H) 6.29 (d, J = 15.7 Hz, 2 H) 6.75 (d, J = 15.9 Hz,   100 2 H) 7.81 - 8.01 (m, 10 H) 8.09 (t, J = 8.0 Hz, 1 H) 8.54 (dt, J = 8.2, 0.7 Hz, 2 H) 8.80 (dt, J = 8.3, 0.7 Hz, 2 H) 8.98 (d, J = 8.2 Hz, 2 H) 8.99 (d, J = 8.2 Hz, 2 H) ESI-MS [M-CH3CN-2(OTf)]2+  =  377.9 m/z, [M-CH3CN-OTf]+ = 905.6 m/z.  [Pd(pyC^N^pyC)(CH3CN)](OTf)2 CAUTION: Many pyrene derivatives are carcinogenic. Exposure to these materials, especially contact with skin, should be avoided. [Pd(pyC^N^pyC)Cl]OTf (0.082 mmol, 81 mg) was taken up in a solvent mixture of 20 mL 1:1:0.1 CH3CN:CH2Cl2:DMF. The dark orange solution was reacted with AgOTf (0.15 mmol, 38 mg) at room temperature for 4 hours in the dark, forming a light yellow precipitate. The precipitate was removed by centrifugation and the solution concentrated to ~1 mL under reduced pressure. Et2O (5 mL) was added, inducing precipitation of a sticky orange solid which was collected and triturated with CHCl3 (2 mL) and washed with Et2O (2 × 3 mL). The resulting light orange powder was dissolved in CH3CN (2 mL) and precipitated by addition of Et2O (5 mL). The powder was dried under a stream of N2 and then under vacuum, yielding a tan powder (75 mg, 80% yield). 1H NMR (400 MHz, CD3CN)  1.05 (t, J = 7.4 Hz, 6 H) 1.48 - 1.63 (m, 2 H) 1.73 - 1.85 (m, 2 H) 1.91 - 2.02 (m, 2 H) 1.96 (s, 3 H) 2.08 - 2.20 (m, 2 H) 5.10 - 5.22 (m, 2 H) 5.37 - 5.49 (m, 2 H) 6.45 (d, J = 15.9 Hz, 2 H) 6.99 (d, J = 15.7 Hz, 2 H) 8.08 (s, 4 H) 8.29 - 8.39 (m, 7 H) 8.47 (d, J = 7.7 Hz, 2 H) 8.50 (d, J = 7.8 Hz, 2 H) 8.87 (d, J = 8.0 Hz, 2 H) 9.16 (d, J = 8.0 Hz, 2 H) ESI-MS [M-CH3CN-2(OTf)]2+ = 402.7 m/z. Anal. Calculated for C52H43F6N6O6PdS2: C, 55.15; H, 3.83; N, 7.42; S, 5.60. Found: C, 52.49; H, 3.78; N, 7.40; S, 5.36.     101 Chapter 4: The Influence of Pyridyl para Substituents in Bis-NHC Palladium Pincer Complexes 4.1 Introduction The development of more efficient methods for the reduction of CO2 may enable its economical use as a source of carbon for value-added chemicals and hydrocarbon fuels, in principle allowing an anthro-pogenic carbon cycle where energy-dense hydrocarbon fuels can be used in a sustainable, carbon-neutral way.5,114,116 Approaching this challenge through the development of molecular electrocatalysts is prom-ising given the success seen in the design of highly efficient hydrogen reduction electrocatalysts140 and the existence of redox enzymes such as [NiFe] carbon monoxide dehydrogenase which can equilibrate CO2 and CO at extremely high rates under benign conditions.8 A variety of homogeneous molecular species which can facilitate the two electron reduction of CO2 to CO or HCOO- have been developed.76  Dihydrogen, H2, is often produced as a byproduct due to the inherent presence of H+ in the CO2 reduction half-reaction, thereby creating syngas which can be used in Fischer-Tropsch processes to produce liquid hydrocarbon fuels.11 Standard reduction potentials for CO2 and H+ reduction in DMF to form CO and H2, respectively, are given below.  CO2  +  2e–  +  2H+ → CO  +  H2O (E° = -0.73 V vs. Fc0/+; in DMF) 2H+  +  2e– → H2 (E° = -0.66 V vs. Fc0/+; in DMF)14  Continuing work has been performed to further investigate CO2 reduction with square-planar com-plexes containing tetradentate ligands18,46 and octahedral Lehn-type complexes typically containing a bipyridine (or related), halide, and carbon monoxide ligands.8,51,57,141 In Chapter 2, we began a further investigation of the pincer motif for CO2 reduction, first studied with cationic Pd triphosphine complexes   102 capable of producing CO with high faradaic efficiencies, by incorporating a neutral redox-active lutidyl-linked bis-NHC pincer ligand (C^N^C), also resulting in CO2 reduction electrocatalysis.117 In Chapter 3, modifications of the NHC groups for Pd(C^N^C) complexes were explored.142 With regard to the pincer motif, a tethered bimetallic triphosphine complex has been found to exhibit cooperative reactivity with CO2 resulting in high reaction rates on the order of the rate of [NiFe] CODH, though with a limited turnover number.67 The lutidyl-linked bis-NHC ligand platform has the modular capacity to be similarly extended and/or incorporate additional functionalities to enhance reactivity with CO2. In this chapter, through a combined experimental and computational approach, modifications of the para substituent of the bridging pyridyl donor are investigated to affect reactivity with CO2 by modula-tion of the electron density on the metal center and the ability of the monodentate trans ligand to disso-ciate. Computational modelling in Chapter 2 has supported the pyridyl moiety to be the site of ligand redox activity upon reduction by one electron, intimating its electrochemical significance.117 Similar para substituent modifications for the bridging donor group in a pincer ligand have previously been explored by Van Koten and co-workers and Chirik and co-workers in relation to Hammett para substituent con-stants (σp) with anionic 2,6-bis(dimethylamino)phenyl and neutral 2,6-bis(imino)pyridine ligands, re-spectively, showing that such modifications can have a strong effect on redox potentials and catalytic properties.143–146 Modification of the pyridyl para substituent has not yet been performed for C^N^C pincers, with the closest examples being pyridyl-linked bis-NHC (C-N-C) pincers with the addition of a para carboxylate for surface anchoring147 and reactivity of the para position in radical species.84 The work reported in this chapter is the first investigation of the effects of modifying the pyridyl para position for any C^N^C complex, especially in the context of electrochemical properties and reactivity with CO2.   103 4.2 Results & Discussion 4.2.1 Synthesis and Characterization A series of four complexes with varied para substituents were prepared, incorporating an electron-donating (ED) group (R = OMe; σp = -0.27), electron-neutral group (R = H; σp = 0.00), mildly electron-withdrawing (EW) group (R = Br; σp = +0.23), and moderately EW group (R = COOR; σp = +0.45). The complex [Pd(C^N^Cp-H)Cl]BF4 was prepared previously,117 with the other three complexes synthesized from the pyridine derivative chelidamic acid, containing a hydroxyl group in the para position and car-boxylic acid groups in the ortho positions.  The R = Br and R = OMe species were synthesized according to Scheme 4-1. For incorporation of the para-Br substituent, chelidamic acid in chloroform was reacted with an excess of phosphorus tribromide and bromine at reflux for 72 h, producing a para-Br acid bromide which was quenched with methanol to yield the para-Br methyl ester. The para-OMe substituent was installed in a similar manner by reaction with thionyl chloride and then methanol to form a para-hydroxyl methyl ester species, followed by deprotonation and methylation with potassium carbonate and methyl iodide to yield the para-OMe me-thyl ester, thereby avoiding complications with pyridine-pyridone tautomerism. The methyl esters were   104 then reduced to alcohols with sodium borohydride in tetrahydrofuran and methanol, followed by conver-sion to the respective 2,6-bis(bromomethyl)pyridine species by reaction with phosphorus tribromide and then 2 equivalents of 1-(n-butyl)imidazole to yield the dibromide proligand salts in high purity (Figure A-15, Figure A-18). Coordination to palladium proceeded through transmetallation from a silver carbene intermediate via reaction of the proligand with silver(I) oxide followed by addition of PdCl2(cod) and silver triflate, yielding [Pd(C^N^Cp-Br)Cl]OTf and [Pd(C^N^Cp-OMe)Cl]OTf in good yields. These two species were further functionalized by halide abstraction with silver triflate in the presence of acetonitrile, producing the dicationic acetonitrilo complexes [Pd(C^N^Cp-Br)(CH3CN)](OTf)2 and [Pd(C^N^Cp-OMe)(CH3CN)](OTf)2. Abstraction was complete within minutes at room temperature with R = OMe, but required heating for 2 hours in the case of R = Br, attributed to the trans effect of the electronically-modified pyridyl moiety affecting lability of the halide.  Scheme 4-1. Synthetic Route to [Pd(C^N^Cp-Br)Cl]OTf and [Pd(C^N^Cp-OMe)Cl]OTf     105 The R = COOR species was synthesized according to Scheme 4-2, where the alcohol groups of (4-bromopyridine-2,6-diyl)dimethanol were protected by reaction with sodium hydride and then chlorome-thyl methyl ether (MOM-Cl). The use of weaker bases such as potassium carbonate was also attempted, but led to a complex mixture of products. Additionally, protection of the alcohols by formation of methyl ethers was attempted, leading to species susceptible to controlled nucleophilic attack but difficult to deprotect, with standard procedures with trimethylsilyl iodide, boron tribromide, and even concentrated hydrobromic acid all failing. Only extended heating (18 h) of a concentrated hydrobromic acid solution at reflux led to deprotection of the methyl ethers. The MOM-protected p-Br species was lithiated using 2 equivalents of t-BuLi at -78 °C in diethyl ether, producing a deep red solution which was carboxylated by sparging with CO2(g), resulting in an immediate color change to pale yellow. The resulting carboxylate species was deprotected by stirring in 5 M HBr(aq) at room temperature overnight. It should be noted that a variety of lithiation procedures were attempted, and it was found that the use of diethyl ether as the solvent was required as the use of THF consistently resulted in an intractable mixture of species under otherwise identical conditions, perhaps due to the presence of three metalation directing groups. Addi-tionally, the reaction required t-BuLi for full conversion; the use of n-BuLi consistently resulted in only about 60% lithiation, indicating that the para-lithiated pyridine species is less stable than typical sp2 carbanions as lithium-halogen exchange reactions are equilibria determined by the stability of the car-banion species involved. Following deprotection, an excess of phosphorus tribromide was added to a suspension of the product in dichloromethane. No reaction was observed after stirring overnight at room temperature due to insol-ubility of 2,6-bis(hydroxymethyl)isonicotinic acid in the solvent, leading to the addition of THF for ad-ditional solubility, which was unsuccessful, and finally to the addition of N,N-dimethylformamide (DMF) wherein the reaction occurred. After 4 hours, methanol was added and the product was collected and purified by column chromatography, yielding a pure, pale yellow oil. Through characterization by   106 1H NMR and mass spectrometry (MS), the product was determined to be 4-hydroxybutyl-2,6-bis(bro-momethyl)isonicotinate and not the desired methyl ester species as a result of the incorporation of ring-opened tetrahydrofuran to the molecule. Given that the electronic properties are equivalent to the methyl ester, this species was carried forward and reacted with 2 equivalents of 1-(n-butyl)imidazole to yield the dibromide proligand salt, followed by coordination to palladium through transmetallation from a silver carbene species as described above. The reaction was sluggish, likely due to increased solubility of the in situ silver carbene species intermediate, and the crude product required additional purification via column chromatography, yielding [Pd(C^N^Cp-COOR)Cl]OTf in a low isolated yield but sufficient purity for electrochemical study.  Scheme 4-2. Synthetic Route to [Pd(C^N^Cp-COOR)Cl]OTf   All complexes were characterized by 1H NMR, MS, and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, with the IR spectra showing the presence of coordinated acetonitrile for [Pd(C^N^Cp-Br)(CH3CN)](OTf)2 and [Pd(C^N^Cp-OMe)(CH3CN)](OTf)2. Microanalysis was used to   107 determine the percentage of H, C, N, and S in the bulk samples of [Pd(C^N^Cp-Br)Cl]OTf and [Pd(C^N^Cp-OMe)Cl]OTf, matching the calculated values. Insufficient material was available for microa-nalysis in the case of [Pd(C^N^Cp-COOR)Cl]OTf. Additionally, the growth of X-ray quality crystals was attempted using numerous methods and solvent combinations for [Pd(C^N^Cp-Br)Cl]OTf and [Pd(C^N^Cp-OMe)Cl]OTf; the complexes repeatedly condensed as yellow gels and no X-ray quality crys-tals were obtained. The chemical shifts of the pyridyl meta-H atoms are sensitive probes of the electron density of the pyridyl moiety of the ligand, providing a strong linear correlation to the Hammett para-substituent con-stant σp (Figure 4-1). The chemical shifts of cationic chloride complexes vs. dicationic acetonitrilo com-plexes were also compared for each of R = H, Br, and OMe in CD3CN, showing a consistent effect in each case where the imidazol-2-ylidene, pyridyl, and pyridyl-CH2- protons were shifted downfield by 0.12, 0.07, and 0.05 ppm, respectively, due to decreased electron density on the metal center. The N-methylene group of the butyl chain was shifted upfield by 0.27 ppm with the resonance changing from a pair of doublets of triplets, due to chemical inequivalence and geminal coupling of the diastereotopic protons (2JH,H = 13-14 Hz) resulting from the presence of the adjacent chloride and the twisted confor-mation imposed by the C^N^C ligand. Finally, it was observed that in CD2Cl2 the acetonitrilo complexes displayed broadened signals at room temperature, indicating an increased rate of interconversion between conformers as compared to in CD3CN. All of the chloride complexes examined, regardless of solvent, displayed sharp signals and diastereotopic splitting indicating significantly slower fluxional pro-cesses.85,148   108  Figure 4-1. Correlation of Hammett σp constant to chemical shift of pyridyl meta-H atom from 1H NMR spectra of complexes taken in (CD3)2CO.  4.2.2 Reactivity of R = Br Species with Acid and Base An interesting effect was observed for as-synthesized [Pd(C^N^Cp-Br)Cl]OTf, where a color change from pale yellow to pink-red occurred upon addition of a base to the solution. It should be noted that pyridyl para-bromide groups can be reactive, warranting further investigation as to the stability of the product. For example, 4-bromopyridine slowly self-condenses at room temperature to form ionic oligo-mers.149 The color change was initially observed with the addition of potassium carbonate, but also oc-curred with triethylamine and aqueous sodium hydroxide. No solvent dependence was observed. The addition of an acid, such as trifluoroacetic acid or dilute hydrochloric acid, reversed the color change, leaving a pale yellow solution. The addition of base followed by acid, with concomitant color changes, could be carried out repeatedly. This color change in the presence of a base did not occur with any of the other complexes. Additionally, when base was added to a solution of the para-Br proligand, no color change occurred, ruling out the possibility of its cause being a minor impurity brought forward from previous steps and suggesting that the metal center may be involved.    109 To investigate further, UV-Vis spectra of an acetonitrile solution of [Pd(C^N^Cp-Br)Cl]OTf were col-lected before and after the addition of potassium carbonate, revealing growth of a broad absorption cen-tered at 508 nm. No isosbestic point was observed, indicating that the addition of base does not convert [Pd(C^N^Cp-Br)Cl]OTf into another species. Moreover, MS and 1H NMR spectroscopy did not reveal any new signals or changes upon the addition of an excess of base and then neutralization with an acid. Based on these data, the color change is not due to a reaction with the para-Br complex itself, but is attributed to a trace impurity formed during the coordination reaction to Pd, likely involving modification at the para-bromido site. 4.2.3 Electrochemical Behaviour under N2 and CO2 The series of synthesized complexes were electrochemically characterized by cyclic and square-wave voltammetry in N2 and CO2 sparged solutions. Previous work has shown that for the unsubstituted Pd(C^N^C)X+ pincer complexes (where R = H), two electrochemically irreversible one-electron reduc-tion events are typically observed. The first electron is suggested to be localized on the pyridyl moiety, thereby weakening the Pd-halide bond and enabling its dissociation, and the second localized on the metal center.117,142 The first reduction events of the pyridyl para-substituted complexes were electro-chemically irreversible with peak potentials determined by square-wave voltammetry (Figure 4-3 and Table 4-1). These data indicate a very strong electronic effect of the substituents on the reduction poten-tial, five times stronger than in a bis(amino)phenyl [Ni(NCN)X] pincer complex where the HOMO and LUMO span the complex (Figure 4-2), for example,143 and providing further evidence of the first reduc-tion potential being pyridyl-localized.   110  [Ni(NCN)Cl]  [Ni(NCN)Cl] HOMO  [Ni(NCN)Cl] LUMO Figure 4-2. Calculated HOMO and LUMO geometries of bis(dimethylamino)phenyl [Ni(NCN)Cl] pin-cer complex.  When plotted against the Hammett σp constants of the substituents (Figure 4-4), a strong linear cor-relation was observed (Epc1 = 0.581σp – 1.77; R2 = 0.990), quantifying the electronic sensitivity to the para substituent and providing an empirical means of predicting the first reduction potential of complexes with numerous other substituents, including R = NO2 (σp = 0.78; predicted Epc1 = -1.32 V), R = CF3 (σp = 0.54; predicted Epc1 = -1.46 V), t-Bu (σp = -0.20; predicted Epc1 = -1.89 V), and NMe2 (σp = -0.83; predicted Epc1 = -2.25 V). The use of substituents from one side of the Hammett σp constant spectrum to the other represents reduction potential tunability over an expansive range of 1 V.  Correlations with the Hammett σ- constant and individual Swain-Lupton constants F (field inductive parameter) and R (reso-nance parameter) were considered, with the reduction potentials correlating well to σ- (R2 = 0.97) but not as well as to σp (R2 = 0.99), poorly to F (R2 = 0.19) and moderately to R (R2 = 0.59) for the series.146,150,151 The second and later reduction events were less uniform across the series. For the R = OMe and H complexes, only one additional reduction event was observed in the potential range with equivalent peak current to the first reduction event, indicating another one-electron transfer. For the R = Br and COOR complexes, however, at least three additional reduction events occurred, with smaller currents than the first reduction event, with the most prominent listed in Table 4-1. This is attributed to the varying degrees   111 of ligand dissociation from the one-electron reduced species. With more electron-donating (ED) para substituents there is a stronger trans effect and increased charge on the metal center, thereby labilizing the chloride and resulting in a fast chemical step after reduction and full conversion to a new species able to be further reduced. With electron-withdrawing (EW) para substituents, dissociation of the monoden-tate ligand is hindered, resulting in an incomplete chemical step after reduction, and perhaps ion-pairing, and consequently multiple species undergoing further reduction at varying potentials. For the complexes studied here, the scan rates were varied across the range of 50 to 1000 mV/s and did not change the features in the cyclic voltammograms after the first reduction potential or their relative proportions in any significant measure.  Table 4-1. Peak Potentials of Complexesa versus Fc0/+ complex σp Epc1 (V) Epc2 (V) Epc3 (V) Epc4 (V) [Pd(C^N^Cp-OMe)Cl]+ -0.27 -1.91 -2.55 - - [Pd(C^N^Cp-H)Cl]+ 117 0.00 -1.79 -2.37 - - [Pd(C^N^Cp-Br)Cl]+ 0.23 -1.63 -2.11 -2.49 -2.72 [Pd(C^N^Cp-COOR)Cl]+ 0.45 -1.50 -2.13 -2.32 -2.51 aDetermined from square-wave voltammograms (25 Hz) for 2 mM solutions in 0.10 M [n-Bu4N]PF6/DMF under N2 using a glassy carbon working electrode.    112  Figure 4-3. Overlaid square-wave voltammograms (25 Hz) of 2 mM solutions of complexes in 0.10 M [n-Bu4N]PF6/DMF under N2 using a glassy carbon working electrode.   Figure 4-4. Linear correlation between first reduction potential of Pd(C^N^Cp-R)Cl+ complexes and Hammett σp substituent constant.  For each complex, the first reduction event was analyzed by cyclic voltammetry at scan rates of 50 to 2000 mV/s, undergoing immediate re-oxidation upon reduction and showing electrochemical irreversi--2.5 -2.0 -1.5 -1.0 -0.5-30-25-20-15-10-50  R = COOR  R = Br  R = H  R = OMeCurrent Difference (µA)Potential (V vs. Fc0/+)  113 bility in all cases except with the strongest EW substituent, R = COOR, where electrochemical reversi-bility was observed at higher scan rates (Figure 4-5). This behaviour is characteristic of an EC mecha-nism (see below), which in this case is attributed to ligand dissociation upon reduction which has been sufficiently hindered by a weaker electron donor in the trans position, allowing the reduced species to be re-oxidized before undergoing a chemical reaction. This emergence of reversibility when using a strong EW substituent is further evidence of ligand dissociation being enabled upon reduction. E: [Pd(C^N^C)X]+   +   e–    →    [Pd(C^N^C)X]0 C: [Pd(C^N^C)X]0    →    [Pd(C^N^C)]+  +  X–          Figure 4-5. Overlaid and normalized cyclic voltammograms of [Pd(C^N^Cp-COOR)Cl]OTf at varied scan rates (left) with [Pd(phC^N^phC)Cl]+ at 800 mV/s as a representative comparison (right).  -1.80 -1.60 -1.40 -1.20 -1.00 -0.80-1.5-1.0-0.50.00.5[Pd(C^N^Cp-COOR)Cl]+Current / (scan rate)1/2 (μA mV-1/2 s1/2)Potential (V vs. Fc0/+) 50 mV/s 200 mV/s 400 mV/s 800 mV/s 1000 mV/s-2.00 -1.80 -1.60 -1.40-3.0-2.0-1.00.0Current / (scan rate)1/2 (μA mV-1/2 s1/2)Potential (V vs. Fc0/+) 200 mV/s 800 mV/s[Pd(phC^N^phC)Cl]+  114              Figure 4-6. Cyclic voltammograms of (A) [Pd(C^N^Cp-COOR)Cl]OTf, (B) [Pd(C^N^Cp-Br)Cl]OTf, (C) [Pd(C^N^Cp-H)Cl]BF4, and (D) [Pd(C^N^Cp-OMe)Cl]OTf taken at 100 mV/s under N2, CO2, and CO2 with 10 mM trifluoroacetic acid present.  Under CO2, the complexes displayed differing behaviours (Figure 4-6). For the R = OMe and R = H species, significant current enhancements, suggestive of reactivity with CO2, were observed at potentials directly corresponding to each of the two reduction potentials of the complexes under N2, with the current enhancement at the second reduction potential with R = OMe especially prominent. In the R = Br case, a current enhancement was also observed at the first reduction potential and also at more cathodic poten-tials, though at different or shifted potentials when compared to the complex under N2. Finally, with   115 R = COOR, no current enhancement was observed at the first reduction potential, and only a weak en-hancement at more cathodic potentials, with peaks different or shifted relative to the complex under N2. Under CO2 with 10 mM trifluoroacetic acid (TFA) added, the current at the first reduction potential more than doubled for all complexes except for the complex where R = COOR, in which case only a small increase was observed (Figure 4-6). For the species with R = OMe and R = H, the current increase preceded the first reduction potential by 280 and 200 mV, respectively, and in the case of R = OMe only, the current increase is strongly localized at the first reduction potential. It should be noted that under these conditions TFA, without any complex present, has a peak potential of approximately -2.25 V, com-prising or contributing to the features observed at -2.25 V in Figure 4-6[A-C].117 Finally, the effect of adding the Lewis acid Mg2+ (as Mg(ClO4)2), which has been shown to affect electrocatalytic reactivity with CO2,45,51,117,142 was examined by cyclic voltammetry resulting in no sig-nificant changes for complexes with R = OMe, H, and COOR. For R = Br a simplification of the voltam-mogram was observed, where the second reduction event became more prominent and the third and fourth reduction events disappeared (Figure 4-7), attributed to assistance with halide abstraction and/or passivation in solution where ion-pairing may occur with the moderately weak EW group R = Br.   116  Figure 4-7. Cyclic voltammograms of [Pd(C^N^Cp-Br)Cl]OTf in a CO2 sparged solution with TFA pre-sent after a controlled potential electrolysis experiment, and then with 25 mM Mg(ClO4)2 added. 4.2.4 Density Functional Theory Modeling Unreduced and one-electron reduced N-methyl variants of the synthesized complexes were modeled by Density Functional Theory (DFT) together with models of complexes with additional para substitu-ents (such as NMe2, tBu, and CF3). The goal of these studies was to better understand the effects of the para substituent on various molecular properties including energy level and geometry of the frontier orbitals, natural population analysis (NPA) charge on the metal center, Pd-Cl bond length and dissocia-tion energy, interaction energy of CO2 as it approaches the reduced species, and energy of the nucleo-philic dz2 orbital of Pd.    117 In each case throughout the series, the LUMO was calculated to be localized primarily on the pyridyl group with some secondary Pd character. The HOMO geometry was likewise quite consistent across the series being localized on the NHC π-orbitals and Pd dxz orbital, with the exception of the complex with R = NMe2 where the HOMO is pyridyl-localized with some Pd and carbene character (Figure 4-8). It was found that the calculated LUMO energies for the unreduced complexes in the series exhibited a good linear correlation with the Hammett σp constant of the substituent (ELUMO = -0.0353σp – 0.0152; R2 = 0.926), showing the direct electronic effect of the para substituent upon the LUMO. Similarly, the NPA charge of Pd also showed a strong linear correlation (QNPA = 0.0098σp + 0.2346; R2 = 0.989), showing the direct influence of the para substituent on the electron density of the metal center (Figure 4-10). The Pd-Cl bond length was also tightly correlated with σp (d(Pd,Cl) = -0.084σp + 2.3306; R2 = 0.985), consistent with the trans-effect modulating the bond strength of the chlorido ligand according to the varied donor strength of the para-substituted pyridyl group (Figure 4-10). R = CF3 ELUMO = -0.0312  R = COOR ELUMO = -0.0395  R = Br ELUMO = -0.0216  R = H ELUMO = -0.0129                (HOMO)       (LUMO) R = t-Bu ELUMO = -0.0085  R = OMe ELUMO = 0.0000  R = NMe2 ELUMO = +0.0103                (HOMO)                 (LUMO) Figure 4-8. LUMO geometries and energies (Ha) for DFT-modelled complexes. The HOMO geometries were identical to that of R = H (shown) for all complexes except R = NMe2 (shown).    118 Given the strong correlation between σp and the first reduction potential (Figure 4-4), the relationship between the calculated LUMO energy and both the Pd NPA charge and the first reduction potential was analyzed, showing linear relationships with R2 = 0.974 and R2 = 0.972, respectively (Figure 4-11). When data from other previously studied complexes was also included, where R = H but the carbenes are benz- (weaker overall donor) and phenanthro-annulated (stronger π-donor) imidazol-2-ylidenes, the correlation between LUMO energy and first reduction potential remained good (R2 = 0.954) but the correlation with the NPA charge of Pd was eliminated (R2 = 0.107). This indicates that the LUMO energies can be used in a more general way, with varied NHC groups and/or para substituents, to predict the first reduction potential of the complex (Epc1 = -10.51ELUMO – 1.90). The NPA charges on Pd, however, are strongly influenced by the electron-donating ability of the NHC moiety which has only a tangential effect on the LUMO and first reduction potential (Figure 4-9).  Figure 4-9. Linear fits of calculated LUMO and NPA values to experimental first reduction potentials, including varied NHC groups imidazol-2-ylidene, benzimidazol-2-ylidine (benzC), and phenanthroim-idazol-2-ylidene (phenC) with R = H on the para position of the pyridyl (crossed entries, ×).     119     Figure 4-10. Calculated Pd-Cl bond length (left) and LUMO energy and Pd NPA charge (right) as a function of Hammett σp constant of para substituent.   Figure 4-11. Calculated LUMO energy and Pd NPA charge vs. first reduction potential.   Calculated Chloride Dissociation Energy Barriers Strong correlations across the series, as described above for the unreduced species, were not found for the reduced species with regard to SOMO energy or NPA charges due to significant geometry changes where complexes with more ED pyridyl para substituents (NMe2, OMe, and t-Bu) freely dissociated   120 chloride when reduced, and species with less ED substituents retained their Pd-Cl bonding with varying barriers to dissociation (Figure 4-12). Given that ligand dissociation is an important step in the putative electrocatalytic cycle for CO2 reduction and is an important factor in understanding reduction events following the first reduction, additional calculations were performed to investigate the energy barriers to halide dissociation where the energy of each of the model complexes, both unreduced and reduced by one electron, was monitored as a function of Pd-Cl distance from 2.3 to 3.0 Å.  Figure 4-12. Calculated Pd-Cl bond length in one-electron reduced model species as a function of Ham-mett σp constant.  For the unreduced model species, there was, unsurprisingly, an increasing energy penalty as the Pd-Cl distance increased to 3 Å, indicating that chloride dissociation will not occur under typical conditions, though the energy penalty is marginally smaller with stronger ED substituents (Figure 4-13). The one-electron reduced species provided more interesting results, where the modulation of the Pd-Cl bond strength due to the varied pyridyl para substituents resulted in no energy barrier for chloride dissociation for complexes with R = OMe and NMe2, a small barrier with the R = H and Br species (where dissociation with the R = H complex is more exogonic), and no dissociation in the cases when R = COOR and CF3.   121 Consequently, chloride dissociation for the two-electron reduced species with R = COOR and CF3 was modeled, showing a comparable energy barrier to the one-electron reduced species of R = H and Br in the case of R = COOR, and a minimal barrier in the case of R = CF3 (Figure 4-13). This is consistent with the behaviour of the synthesized complexes observed by cyclic voltammetry: firstly, current en-hancements in the presence of CO2 are only observed for the complexes with R = OMe, H, and Br, but not with R = COOR, given that loss or labilization of the ligand in the fourth coordination site is required for reactivity, and secondly, the existence of only a single additional reduction event after the first, and of equal current to the first event, for the R = OMe and H species indicates complete conversion to a single reduced species, whereas with the more EW groups R = Br and COOR the results are multifarious, consistent with sluggish and/or incomplete dissociation.          Figure 4-13. Calculated energies of unreduced (left) and reduced (right) complexes as a function of Pd-Cl distance incremented from 2.3 to 3.0 Å relative to the energy of the optimized structure where the Pd-Cl distance is 2.3 Å, with geometry relaxed at each step. Inset graph retains axes of parent and shows energies of unreduced species from a Pd-Cl distance of 2.85 to 2.95 Å.    122 Energetics of CO2 Interaction with Reduced Species Finally, the interaction of the reduced species with CO2 as it approaches the metal center was modeled, both in an acetonitrile solvent field and vacuum, with the energy monitored as a function of Pd-CO2 distance (Figure 4-14). In each case the CO2 molecule is brought closer to the complexes in 0.25 Å increments from a pseudo-axial position beginning at a distance of 3.25 Å. The interaction is described below with the solvent field present, though it is very similar in vacuum. As CO2 is brought closer to the metal center, its interaction with the dz2 orbital becomes stronger until at approximately 2.75 Å the carbon atom is aligned in the axial position above the square planar complex, with nearly equal bond angles of ~90° between each pincer donor atom, Pd, and the C of CO2. At a Pd-CO2 distance of 2.50 Å for the complexes with more strongly ED substituents (R = NMe2, OMe, and H) the chloride is displaced away from the CO2 molecule and out of the plane of the pincer ligand, forming a pseudo-trigonal bipyramidal geometry. For the complex with R = Br, this displacement occurs nearer to 2.25 Å, and for the R = COOR case it only occurs when forced to a Pd-CO2 distance of 1.75 Å. From this point, as the Pd-CO2 distance is decreased to smaller values, the chloride moves into the opposite axial position with ∠(pyridyl N, Pd, Cl) ≈ 90°, whereas the trans-coordinated CO2 has a bond angle of ∠(N, Pd, C) ≈ 105°, forming an octa-hedrally-distorted trigonal bipyramidal geometry.    123    Figure 4-14. Calculated energy profiles for approach of CO2 to reduced complex in vacuum, starting from axial position at 3.25 Å.  Table 4-2. Calculated CO2 Bond Angles at Pd-CO2 Distance of 2.0 Å Model Species in Vacuum in Solvent Fielda R = COOR 144.5° 141.8° R = Br 140.1° 137.5° R = H 139.9° 137.3° R = OMe 139.7° 137.0° R = NMe2 139.5° 136.7° aA conductor-like polarizable continuum model (CPCM) solvent field of acetonitrile was utilized.  The energy profile of CO2 coordination to each of the complexes in vacuum follows the trend in σp constants for the pyridyl para substituents, showing a small energy barrier to coordination of ~8 kcal/mol and minimal overall energy change for the complexes with strong EDGs R = NMe2 and OMe, a larger 1.75 2.00 2.25 2.50 2.75 3.00 3.25-15-10-505101520253035 R = COOR R = Br R = H R = OMe R = NMe2 R = COOR, 2e-Relative Energy (kcal/mol)Pd-CO2 Distance (Å)  124 barrier for the R = H and Br species of ~13 kcal/mol (with coordination being more endogonic for R = Br), and no favourable coordination at all for the R = COOR complex, with an energy barrier of ~33 kcal/mol. The degree of CO2 activation was monitored from the bond angle of the coordinated CO2 at a distance of 2 Å (Table 4-2). In vacuum, the bond angles for R = Br, H, OMe, and NMe2 are similar, except for the complex with R = COOR which shows a larger angle. With a solvent field present, the trend was the same with an increase in the degree of CO2 activation, due to stabilization of the negative charge which accumulates on the oxygen atoms. The interaction energy profiles for the species were more difficult to directly compare with the solvent field present due to variable solvation energies, though the trend in activation energy barrier is consistent (Figure 4-15). The differences observed in the energy profiles across the series are attributed mainly to the energetics of chloride dissociation, described earlier, and to a lesser extent the energy of the dz2 orbital (nucleophile) which the coordinating CO2 (electrophile) interacts with, calculated by Natural Bond Orbital (NBO) methods (Figure 4-16).    Figure 4-15. Calculated interaction energy of CO2 approaching reduced species in CPCM solvent field of acetonitrile.  1.75 2.00 2.25 2.50 2.75 3.00 3.25-30-20-10010203040 R = COOR R = Br R = H R = OMe R = NMe2 R = COOR, 2e-Relative Energy (kcal/mol)Pd-CO2 Distance (Å)  125  Figure 4-16. Calculated energies of dz2 orbitals by Natural Bond Orbital (NBO) methods.  4.2.5 Controlled Potential Electrolysis (CPE) Experiments The performance of the synthesized complexes for CO2 reduction was tested by CPE experiments with 2 mM solutions of the species in 0.10 M [n-Bu4N]PF6/DMF sparged with CO2 and with 10 mM TFA added. After electrolysis, the headspace gas from the airtight cell was analyzed by gas chromatography to quantify the H2 and CO produced. Each species displayed satisfactory stability throughout the elec-trolysis experiments: cyclic voltammograms after electrolysis maintained all of the features of the com-plex before electrolysis, no precipitates were formed, and there were no unanticipated color changes. The potentials chosen for electrolysis were determined with reference to the reduction potentials of the complex under CO2 and with TFA added (see Figure 4-6). With the potential set in a range where one-electron reduction of the complex occurs, the percentage of CO in the gaseous products was 26% for the complex with R = OMe, 8% for R = H,117 10% for R = Br, and no significant activity was seen for R = COOR near its first reduction potential (Table 4-3). Faradaic efficiencies for CO and H2 produc-tion are also reported, indicating nearly all of the charge transfer resulting in the formation of those   126 products. These results show the best performance for CO2 reduction with an ED para substituent present, consistent with more facile ligand dissociation, a more electron-rich metal center, and a higher energy dz2 orbital on Pd. It should be noted that when operating at the potentials used in these experiments, there is also direct reduction of H+ at the electrode, without complex present, accounting for some of the H2 produced.117 For the complex with R = COOR, electrolysis was performed at potentials more cathodic than the first reduction potential, -2.05 and -2.50 V, where current increases in the presence of CO2 were observed (see Figure 4-6A). Equal performance for CO2 reduction was observed in both cases with 22% CO de-tected in the headspace gas, a result comparable to that of R = OMe operating at -2.00 V. The recorded reduction potentials and computational data discussed above for energies of chloride dissociation and interaction with CO2 would suggest that the R = COOR complex is operating in a regime of two-electron reduction where ligand dissociation is facile (Figure 4-13) and the dz2 orbital is of high energy, interme-diate to that of the R = OMe and R = NMe2 complexes when reduced by one electron (Figure 4-16). These results raise the question of whether the lutidyl-linked bis-NHC pincer motif is most effective for reducing CO2 with a strong ED pyridyl para substituent (e.g. OMe or NMe2) when operating near the first reduction potential, or with a strong EW pyridyl para substituent (e.g. COOR or NO2) when oper-ating near, or more cathodic than, the second reduction potential.  Table 4-3. Results of CPE Experimentsa Species Potential (V) Charge Passed (C) FE CO FE H2 Ratio CO in Produced Gas R = OMe -2.00 V 6.0 26% 74% 26% R = H -1.75 V 13 9% 93% 8% R = Br -1.80 V 6.0 11% 94% 10%   127 Species Potential (V) Charge Passed (C) FE CO FE H2 Ratio CO in Produced Gas R = COOR -2.05 V 3.0 22% 74% 23% R = COOR -2.50 V 3.0 22% 75% 23% aPerformed with 2 mM solutions in 10 mL 0.10 M [n-Bu4]PF6/DMF under CO2, except for the complex with R = COOR where a 3 mL solution was used. Potentials reported vs. Fc0/+ and  adjusted for IR drop with higher surface area electrode, including those previously reported for R = H.117  The addition of Mg2+ was also tested as it has been shown to affect reactivity with CO2 with this class of complexes and others by acting as a Lewis acid to stabilize the coordinated CO2 adduct, assisting its activation.51,117,142 Here, the only observed improvement in CO production was for the R = H species, reported previously (Table 4-4).117 Similar but slightly decreased performance for CO production was observed in the cases of the R = OMe complex at -2.00 V and the R = Br complex at -1.80 V, with 21% and 8% CO detected, respectively, in the gaseous products. For the R = Br species, electrolysis was also performed at -2.25 V due to a reduction feature becoming more prominent at that potential with Mg2+ present (Figure 4-7), resulting in 14% CO in the gaseous products. Finally, the presence of the Lewis acid Mg2+ diminished performance in the case of the complex with R = COOR, from 22% to 7% CO in the headspace gas.  Table 4-4. Results of CPE Experiments with Mg2+ Addeda Species Potential (V) Charge Passed (C) FE CO FE H2 Ratio CO in Produced Gas R = OMe + Mg2+ -2.00 V 6.0 21% 79% 21% R = H + Mg2+ b -1.90 V 7.0 26% 78% 25%   128 Species Potential (V) Charge Passed (C) FE CO FE H2 Ratio CO in Produced Gas R = Br + Mg2+ -1.80 V 6.0 8% 90% 8% R = Br + Mg2+ -2.25 V 7.6 13% 80% 14% R = COOR + Mg2+ -2.05 V 2.5 7% 95% 7% aPerformed under CO2 with 2 mM solutions in 10 mL 0.10 M [n-Bu4]PF6/DMF with 25 mM Mg(ClO4)2, except for the R = COOR complex where a 3 mL solution was used. Potentials reported vs. Fc0/+ and adjusted for IR drop with higher surface area electrode, including those previously reported for the R = H species.117 bFor the R = H species, 65 mM Mg(ClO4)2 was used.  In totality, the CPE results give a more intricate picture than that of molecular properties tuned as a function of the pyridyl para substituent. Activity for reduction of CO2 near the first reduction potential does follow the general trend that with the most electron-donating para substituent, where facile disso-ciation of the trans ligand occurs and the metal center is more electron rich, more CO is produced. With the most electron-withdrawing para substituent there is little to no activity, though there seems to be little difference between the R = H and R = Br complexes. There is a trade-off between the ease of ligand dissociation and nucleophilicity of the metal center on the one hand, and the reduction potential (and hence overpotential), on the other, where the ligand must be labile enough to easily dissociate upon reduction of the complex, but after that point a stronger ED para substituent and trans effect may not be necessary because the metal center is already sufficiently electron rich to react with CO2. In addition to these factors, the presence of coordinating anions (conjugate base from proton source) and water gener-ated during the reaction will presumably affect competing reduction of H+ to differing degrees than re-duction of CO2, making prediction of overall performance difficult amidst an interplay of various factors.   129 4.3 Conclusions The electronic effect of modifying the pyridyl para-position of lutidyl-linked bis-NHC Pd pincer com-plexes was investigated, exhibiting a strong and consistent effect on the first reduction potential of the complex in relation to the Hammett σp value of the para substituent, consistent with DFT analysis of the pyridyl-localized nature of the LUMO for these complexes. The presence of a sufficiently strong EW group, R = COOR, resulted in electrochemical reversibility of the first reduction potential at high scan rates supporting the assignment of an EC event where ligand dissociation occurs upon reduction. The effect of the para substituents on electronic properties of the metal center and ligand dissociation in the reduced species was quantified by DFT modelling, showing the energy barrier to dissociation to be strongly dependent on the σp value of the substituent. Dissociation was found to be spontaneous in the singly-reduced species with ED groups and significantly hindered with moderately strong EW groups, having implications for initial reactivity with CO2 and continued activity where coordinating anions (con-jugate base), water, and CO are produced during the catalytic cycle. The presence of an ED group, R = OMe, was found to improve CO2 reduction relative to R = H when operating in a regime of initial one-electron reduction of the complex, but the presence of a moderately strong EW group, R = COOR, also improved CO2 reduction performance relative to R = H when operating in a regime of two-electron re-duction, and at similar potentials, raising the question of whether this redox-active pincer framework is most effective with a strongly ED or strongly EW para substituent. In either case, the electrochemical properties of the C^N^C pincer motif have been better understood, providing an extendable platform where reduction potentials can be predicted and tuned over a broad range and additional functionalities can be incorporated into the pyridyl, NHC, and/or NHC N-substituents.   130 4.4 Experimental 4.4.1 General Unless otherwise specified, all reactions were performed under nitrogen using standard Schlenk tech-niques and solvents and reagents were used as received from commercial sources. N,N-dimethylforma-mide (EMD Millipore Omnisolv®), distilled acetonitrile (Anachemia Accusolv™), dimethyl sulfoxide (ACS grade, VWR), and chloroform (HPLC grade, Fisher Chemical) were dried and stored over 15% m/v 4Å molecular sieves.134,135 Tetrahydrofuran was distilled from sodium and benzophenone. Diethyl ether for reaction solvent use was collected from a solvent purification system. Magnesium perchlorate (Alfa Aesar), trifluoroacetic acid (Aldrich), and 99.998% carbon dioxide (Praxair) were used as received for electrochemical experiments.  1H NMR spectra were acquired using Bruker AV300 or AV400 spectrometers with chemical shifts referenced to residual solvent signals. Mass spectra were acquired using a Waters LC-MS ESI-MS, ex-cept for the acetonitrilo complexes which were acquired using a Waters Micromass LCT ESI-MS. IR spectra were collected using a PerkinElmer Frontier FT-IR Spectrometer with ATR attachment. Gaseous products were analyzed using an SRI Model 8610C gas chromatograph equipped with molecular-sieve columns, dual TCD and FID detectors, and methanizer. 4.4.2 Electrochemistry Electrochemical experiments were performed using a Metrohm Autolab PGSTAT12 or Pine AFCBP1 potentiostat in an air-tight three-electrode cell with a 7 mm2 glassy carbon working electrode (Bioana-lytical Systems, Inc.), Pt mesh counter electrode, and Ag wire pseudo-reference electrode in a 0.010 M AgNO3 acetonitrile solution separated from the bulk solution by a Vycor frit. Experiments were per-formed under N2 or CO2 using 2 mM concentrations of the complexes in 10 mL anhydrous electrolyte   131 solution unless otherwise stated. Electrolyte solutions were 0.10 M triply recrystallized [n-Bu4N]PF6 in anhydrous N,N-dimethylformamide and sparged with nitrogen prior to use. For experiments with CO2, the solution was sparged with CO2 for 15 minutes resulting in a concentration of ~0.20 M.27 Controlled potential electrolysis experiments used a glassy carbon rod (Alfa Aesar, 5 mm diameter) working elec-trode in a two compartment H-cell, where the counter electrode compartment was separated from the compartment containing the working and reference electrodes by fritted glass. All glassy carbon elec-trodes were cleaned by successive polishing with 1 μM, 0.3 μM, and 0.05 μM alumina paste, following by rinsing with water, sonication (5 min) in distilled water, and sonication (5 min) in methanol. Deca-methylferrocene was added at the end of electrochemical experiments as an internal standard, showing a reversible redox couple at -404 mV vs. Ag/AgNO3 and -476 mV vs. ferrocene/ferrocenium.136 Cyclic voltammograms were collected at a scan rate of 100 mV/s and square-wave voltammograms at a fre-quency of 25 Hz with 5 mV steps and 25 mV amplitude unless otherwise stated. 4.4.3 Computational Methods DFT calculations were performed using Gaussian 09 (Revision D.01) using the long range and dis-persion-corrected ωB97xD hybrid functional without symmetry constraints.111 Calculations for reduced species were performed using unrestricted, open-shell wavefunctions. The D95(d) basis set was used for all atoms except palladium, which employed the Stuttgart-Dresden-Bonn quasi-relativistic effective-core potential and corresponding correlation-consistent triple-ζ basis set, and Br, which used the 6-31G(d) basis set.112,113 Calculations were performed with the presence of a solvent reaction field produced by the conductor-like polarizable continuum model (CPCM) unless otherwise stated.137 Frequency calculations were performed on all geometry optimized structures to ensure that energy minima were achieved.    132 4.4.4 Synthesis Chelidamic acid monohydrate (98%, Alfa Aesar), 1-(n-butyl)imidazole (99%, Alfa Aesar), di-chloro(1,5-cyclooctadiene)palladium(II) (99%, Strem), tert-butyllithium (1.7 M in pentane, Aldrich), io-domethane (99%, copper stabilized, Sigma-Aldrich), chloromethyl methyl ether (tech. grade, Aldrich), and silver trifluoromethanesulfonate (99%, Aldrich) were used as received for synthesis. [Pd(C^N^C)Cl]BF4 was previously synthesized.117 Synthesis of para R = OMe Precursors Dimethyl 4-hydroxypyridine-2,6-dicarboxylate Synthesized according to modified literature procedures.152 Thionyl chloride (5.0 mL, 69 mmol) was added to methanol (20 mL) cooled to 0 °C by an ice bath. Chelidamic acid monohydrate (2.00 g, 10 mmol) was added to the stirring solution in portions and left to stir overnight before being heated to reflux for 2 h. The solvent was removed under reduced pressure and the residue then washed with methanol and evaporated to dryness again before being taken up in de-ionized water (10 mL) and neutralized by addi-tion of sodium bicarbonate solution. A light precipitate formed which was collected by filtration, dried, and then recrystallized from ethanol as a white microcrystalline powder (1.89 g, 90% yield). 1H NMR (300 MHz, CD3OD):  3.95 (s, 6 H), 7.47 (s, 2 H). Dimethyl 4-methoxypyridine-2,6-dicarboxylate Synthesized according to modified literature procedures.153,154 Dimethyl 4-hydroxypyridine-2,6-di-carboxylate (1.89 g, 8.25 mmol) was suspended in acetonitrile (60 mL) and stirred vigorously, followed by addition of potassium carbonate (1.86 g, 13.4 mmol) and then, after 5 min, iodomethane (1.9 mL, 31 mmol) dropwise [CAUTION: Inhalation, ingestion, or skin absorption of iodomethane can be fatal]. The mixture was heated at reflux for 14 h and then cooled to room temperature, diluted with water to dissolve the white precipitate formed, and extracted with dichloromethane (25 mL × 5). The organic extracts were   133 dried with magnesium sulfate and filtered, with the solvent removed by rotary evaporation yielding a white powder (732 mg, 36% yield). 1H NMR (300 MHz, CDCl3)  3.99 (s, 3 H), 4.02 (s, 6 H), 7.82 (s, 2 H). (4-Methoxypyridine-2,6-diyl)dimethanol Synthesized according to modified literature procedures.153,154 Sodium borohydride (1.2 g, 32 mmol) was added to a stirring solution of dimethyl 4-methoxypyridine-2,6-dicarboxylate (732 mg, 3.24 mmol) in tetrahydrofuran (40 mL). The solution was heated to reflux, resulting in the appearance of a bright yellow colour. After 30 min, methanol (15 mL) was carefully added over a period of 45 min, resulting in effervescence and disappearance of the bright yellow colour. The solution was left to stir for 2 h before being cooled to room temperature. A solution of saturated ammonium chloride (10 mL) and water (10 mL) were added and the mixture left to stir for 1 h before removal of the organic solvents by rotary evaporation and then extraction of the remainder with ethyl acetate (20 mL × 4). The organic extracts were dried with magnesium sulfate and filtered, followed by removal of the solvent by rotary evaporation yielding a white powder which was further dried under vacuum (393 mg, 71% yield). 1H NMR (300 MHz, CD3OD)  3.90 (s, 3 H), 4.62 (s, 4 H), 6.98 (s, 2 H). 2,6-Bis(bromomethyl)-4-methoxypyridine Synthesized according to modified literature procedures.153,154 A solution of phosphorus tribromide (1.75 mL, 18.5 mmol) in chloroform (20 mL) was slowly dropped into a stirring solution of (4-methox-ypyridine-2,6-diyl)dimethanol (393 mg, 2.32 mmol) in chloroform (70 mL) at room temperature. The reaction mixture was heated to reflux for 18 h, cooled, neutralized by the addition of 1 M sodium bicar-bonate solution (100 mL), and left to stir for 3 h. The organic layer was collected, dried with magnesium sulfate and filtered, and the solvent removed by rotary evaporation yielding an off-white crystalline solid (484 mg, 71% yield). 1H NMR (300 MHz, CDCl3)  ppm 3.91 (s, 3 H), 4.52 (s, 4 H), 6.92 (s, 2 H).   134 Synthesis of para R = Br Precursors Dimethyl 4-bromopyridine-2,6-dicarboxylate Synthesized according to modified literature procedures.155 Phosphorus tribromide (2.8 mL, 30 mmol) and bromine (1.5 mL, 30 mmol) were slowly added to a vigorously stirred suspension of chelidamic acid monohydrate (1.5 g, 7.5 mmol) in chloroform (15 mL) cooled to 0 °C in an ice bath. The reaction mixture was then heated to reflux for 72 h, resulting in a colour change from orange to cherry red and full disso-lution of the suspended solids after 48 h, and then cooled to 0 °C with an ice bath before slow addition of anhydrous methanol (10 mL). The solution was then left to warm to room temperature and stir for 2 h, after which the solvent was removed by rotary evaporation and the residue taken up in water (15 mL), extracted with ethyl acetate (30 mL × 3), neutralized with sodium bicarbonate, and extracted again (30 mL × 2). The organic extracts were dried with magnesium sulfate, filtered, and concentrated to dryness leaving an off-white powder which was further purified by flash chromatography (SiO2, hexanes/ethyl acetate; first band eluted) yielding a white powder (1.48 g, 72% yield). 1H NMR (400 MHz, (CD3)2CO)  3.97 (s, 6 H), 8.42 (s, 2 H). (4-Bromopyridine-2,6-diyl)dimethanol Synthesized according to modified literature procedures.156 Sodium borohydride (1.02 g, 27.0 mmol) was added to a stirring solution of dimethyl 4-bromopyridine-2,6-dicarboxylate (1.48 g, 5.40 mmol) in tetrahydrofuran (80 mL). The solution was heated to reflux and then methanol (15 mL) was carefully added over 45 min, resulting in effervescence. The solution was left to stir for 2 h before being cooled to room temperature and quenched by addition of saturated ammonium chloride (20 mL) and water (30 mL). The resulting solution was left to stir for 1 h before removal of organic solvents by rotary evapora-tion and extraction with ethyl acetate (30 mL × 4). The organic extracts were dried with magnesium sulfate, filtered, and the solvent removed by rotary evaporation yielding a pale yellow powder which was further purified by flash chromatography (SiO2, ethyl acetate) to remove a debrominated minor product.   135 Upon removal of the solvent, an off-white powder was obtained (0.85 g, 72% yield). 1H NMR (400 MHz, CD3OD)  4.65 (s, 4 H), 7.61 (s, 2 H). 2,6-Bis(bromomethyl)-4-bromopyridine Synthesized according to modified literature procedures.156 Phosphorus tribromide (0.38 mL, 4.0 mmol) was added dropwise to a stirring solution of (4-bromopyridine-2,6-diyl)dimethanol (373 mg, 1.71 mmol) in chloroform (40 mL) at 0 °C. The reaction mixture was heated to reflux for 18 h, cooled to room temperature, quenched with water (25 mL) and saturated sodium bicarbonate solution (5 mL), and left to stir for 2 h. The organic layer was then separated and the aqueous layer was extracted with chloroform (30 mL × 3). The combined organic extracts were washed with brine, dried with magnesium sulfate, filtered, and concentrated to dryness by rotary evaporation yielding the crude product which was further purified by flash chromatography (SiO2, ethyl acetate) and then dried under vacuum, yielding an off-white powder (292 mg, 50% yield). 1H NMR (400 MHz, (CD3)2CO)  4.63 (s, 4 H), 7.77 (s, 2 H). Synthesis of para R = COOR Precursors 4-Bromo-2,6-bis((methoxymethoxy)methyl)pyridine  Synthesized according to related literature procedures.157,158 To a solution of (4-bromopyridine-2,6-diyl)dimethanol (600 mg, 2.75 mmol) in N,N-dimethylformamide (5 mL) cooled to 0 °C in an ice bath was added sodium hydride (400 mg 60% w/w mineral oil dispersion, 10.0 mmol) washed with anhydrous diethyl ether (4 mL × 3) and then dried in vacuo prior to addition. Bubbles were evolved and the solution colour changed to yellow-orange. After 5 min, chloromethyl methyl ether (1.04 mL, 13.8 mmol) was added via syringe, resulting in a decreased colour intensity of the solution and formation of a precipitate. The stirring reaction mixture was left to warm to room temperature, filtered, and the solvent removed by vacuum distillation at 40 °C leaving a pale yellow oil (695 mg, 83% yield). Debrominated para-H species   136 was detected by NMR as a minor component- the product was used without further purification. 1H NMR (400 MHz, (CD3)2CO)  3.37 (s, 6 H), 4.63 (t, J=0.7 Hz, 4 H), 4.76 (s, 4 H), 7.58 (t, J=0.7 Hz, 2 H). 4-Hydroxybutyl 2,6-bis(bromomethyl)isonicotinate The MOM-protected 4-bromopyridine (vide infra; 651 mg, 1.70 mmol) was dissolved in anhydrous diethyl ether (70 mL) and cooled to -78 °C in a CO2(s)/acetone slurry. To this stirring solution was added 1.7 M tert-butyllithium in pentane (2.30 mL, 3.91 mmol) over 5 minutes, resulting in a dark red solution. After 1 h, the solution was sparged with carbon dioxide for 5 minutes, resulting in a colour change from dark red to pale yellow. The reaction mixture was warmed to room temperature and left to stir for 2 h before removal of the solvent by rotary evaporation. 1H NMR (400 MHz, (CD3)2CO)  3.37 (s, 6 H), 4.66 (s, 4 H), 4.75 (s, 4 H), 7.89 (s, 2 H). The residue was then taken up in water (5 mL) and concentrated hydrobromic acid (5 mL) was added. The solution was left to stir at room temperature overnight. (Various milder deprotection schemes were attempted without success.) The aqueous solution was neutralized to pH 3 and extracted repeatedly with ethyl acetate (30 mL × 10) with the pH being regularly monitored. The organic extracts were dried with magnesium sulfate, filtered, and concentrated to dryness under vacuum. The resulting off-white solid was used without further purification and dichloromethane (20 mL) was added. The suspension was cooled to 0 °C and phosphorus tribromide (1.00 mL, 10.6 mmol) added dropwise. The mixture was then left to stir and warm to room temperature overnight. No reaction or dissolution of the starting material was observed, leading to addition of tetrahydrofuran (10 mL), but dissolution still did not occur. Finally, N,N-dimethylformamide (4 mL) was added, dissolving the starting material. The reaction mixture was then stirred at room temperature for 4 h, quenched with methanol (20 mL), and left to stir overnight. The solvent was removed by rotary evaporation, the residue taken up in water (5 mL), neutralized with so-dium bicarbonate, and then extracted with ethyl acetate (30 mL × 3). The organic extracts were dried with magnesium sulfate, filtered, and the solvent removed in vacuo, leaving an orange oil. The crude   137 product was purified by flash column chromatography (SiO2, 8:1:1 hexanes:ethyl acetate:dichloro-methane), yielding a pale yellow oil (84 mg, 13% yield). 1H NMR (400 MHz, (CD3)2CO)  1.94 - 2.03 (m, 2 H), 2.03 - 2.12 (m, 2 H), 3.61 (t, J=6.5 Hz, 2 H), 4.45 (t, J=6.3 Hz, 2 H), 4.76 (s, 4 H), 8.02 (s, 2 H). ESI-MS: m/z 443.8 ([M-OH+HBr]+), 309.9 ([M-C4H8O+H]+).  Synthesis of Proligands 1,1'-((4-methoxypyridine-2,6-diyl)bis(methylene))bis(3-butyl-1H-imidazol-3-ium) dibromide, C^N^Cp-OMe ∙ 2HBr To a solution of 2,6-bis(bromomethyl)-4-methoxypyridine (484 mg, 1.64 mmol) in tetrahydrofuran (15 mL) was added 1-(n-butyl)imidazole (0.46 mL, 3.5 mmol). The reaction solution was stirred at room temperature overnight and then heated to reflux for 3 h, resulting in the formation of a precipitate which was collected by centrifugation, washed with diethyl ether twice, and dried under vacuum as a hygro-scopic fine white powder (615 mg, 69% yield). 1H NMR (300 MHz, (CD3)2SO)  0.91 (t, J=7.4 Hz, 6 H), 1.27 (sxt, J=7.4 Hz, 4 H), 1.78 (quin, J=7.3 Hz, 4 H), 3.88 (s, 3 H), 4.23 (t, J=7.2 Hz, 4 H), 5.48 (s, 4 H), 7.14 (s, 2 H), 7.74 (t, J=1.8 Hz, 2 H), 7.84 (t, J=1.8 Hz, 2 H), 9.35 (s, 2 H). ESI-MS: m/z 462.6 ([M-Br]+). 1,1'-((4-bromopyridine-2,6-diyl)bis(methylene))bis(3-butyl-1H-imidazol-3-ium) dibromide, C^N^Cp-Br ∙ 2HBr 1-(n-Butyl)imidazole (0.25 mL, 1.9 mmol) was added to a solution of 2,6-bis(bromomethyl)-4-bro-mopyridine (292 mg, 0.848 mmol) in tetrahydrofuran (10 mL), with the reaction solution stirred at room temperature overnight and then heated to reflux for 3 h, resulting in the formation of a precipitate which was collected by centrifugation, washed with diethyl ether twice, and dried under vacuum as a hygro-scopic fine white powder (414 mg, 82% yield). 1H NMR (400 MHz, CD2Cl2)  0.97 (t, J=7.4 Hz, 6 H),   138 1.40 (sxt, J=7.5 Hz, 4 H), 1.90 (quin, J=7.5 Hz, 4 H), 4.39 (t, J=7.4 Hz, 4 H), 5.76 (s, 4 H), 7.23 (t, J=1.5 Hz, 2 H), 7.92 (s, 2 H), 7.97 (t, J=1.5 Hz, 2 H), 10.79 (s, 2 H). ESI-MS: m/z 512.5 ([M-Br]+).  1,1'-((4-((4-hydroxybutoxy)carbonyl)pyridine-2,6-diyl)bis(methylene))bis(3-butyl-1H-imidazol-3-ium) dibromide, C^N^Cp-COOR ∙ 2HBr 1-(n-Butyl)imidazole (70 µL, 0.53 mmol) was added to a solution of 4-hydroxybutyl 2,6-bis(bromo-methyl)isonicotinate (84 mg, 0.22 mmol) in tetrahydrofuran (5 mL) and left to stir at room temperature overnight before being heated to reflux for 4 h,  resulting in the deposition of a viscous, pale yellow oil on the glass surface. The solvent was removed by rotary evaporation with the residue washed with pen-tane and then taken up in dichloromethane (2 mL), filtered, and dropped into pentane (3 mL) resulting in the formation of a white precipitate which conglomerated as a viscous oil. The oily residue was triturated with anhydrous diethyl ether and then dried under vacuum, resulting in bubbling and solidification of the residue to yield a hygroscopic white powder (126 mg, 91% yield). 1H NMR (400 MHz, (CD3)2SO)  0.91 (t, J=7.3 Hz, 6 H), 1.27 (sxt, J=7.4 Hz, 4 H), 1.78 (quin, J=7.3 Hz, 4 H), 1.84 - 1.91 (m, 2 H), 1.91 - 2.02 (m, 2 H), 3.62 (t, J=6.5 Hz, 2 H), 4.20 (t, J=7.2 Hz, 4 H), 4.35 - 4.44 (m, 2 H), 5.63 (s, 4 H), 7.70 (t, J=1.7 Hz, 2 H), 7.82 (t, J=1.7 Hz, 2 H), 7.97 (s, 2 H), 9.22 (t, J=1.7 Hz, 2 H). ESI-MS: m/z 612.1 ([M-OH]+), 488.0 ([M-(butylimidazole)-OH+H]+), 408.1 ([M-(butylimidazole)-OH-Br]+). Synthesis of Palladium Halido Complexes [Pd(C^N^Cp-OMe)Cl]OTf To a solution of proligand C^N^Cp-OMe ∙ 2HBr (250 mg, 0.460 mmol) in dimethyl sulfoxide (8 mL) was added silver(I) oxide (106 mg, 0.460 mmol) and 4 Å molecular sieves. The reaction mixture was heated to 55 °C, covered in foil, and left to gently stir for 24 h, resulting in the formation of a light brown solid. The mixture was cooled to room temperature before addition of silver trifluoromethanesulfonate (125 mg, 0.486 mmol) and then dichloro(1,5-cyclooctadiene)palladium(II) (131 mg, 0.460 mmol), fol-lowed by stirring at 30 °C for 48 h. After centrifugation, the yellow supernatant solution was concentrated   139 by vacuum distillation at 55 °C. The oily residue was taken up in dichloromethane (2 mL), filtered through Celite, and then dropped into diethyl ether (12 mL), yielding a pale yellow precipitate which conglom-erated upon centrifugation to form a golden yellow oil. The oil was triturated with diethyl ether, then taken up dichloromethane (2 mL) and washed with water (4 mL × 2), dried with MgSO4, filtered, and concentrated to dryness under vacuum, yielding a pale yellow powder (182 mg, 59% yield). 1H NMR (400 MHz, CD3CN)  0.93 (t, J=7.4 Hz, 6 H), 1.31 (sxt, J=7.4 Hz, 4 H), 1.83 (quin, J=7.4 Hz, 4 H), 3.95 (s, 3 H), 4.18 (dt, J=13.3, 7.1 Hz, 4 H), 4.69 (dt, J=13.3, 7.6 Hz, 2 H), 5.23 (d, J=15.1 Hz, 2 H), 5.51 (d, J=15.1 Hz, 2 H), 7.09 (d, J=1.7 Hz, 2 H), 7.22 (s, 2 H), 7.28 (d, J=1.7 Hz, 2 H). ESI-MS: m/z 524.5 ([M-OTf]+). Anal. Calcd for C23H31ClF3N5O4PdS∙H2O: C, 40.01; H, 4.82; N, 10.14; S, 4.64. Found: C, 40.05; H, 4.69; N, 9.96; S, 4.57. [Pd(C^N^Cp-Br)Cl]OTf To a solution of proligand C^N^Cp-Br ∙ 2HBr (251 mg, 0.422 mmol) in dimethyl sulfoxide (7 mL) was added silver(I) oxide (100 mg, 0.432 mmol) and 4 Å molecular sieves. The reaction mixture was heated to 55 °C, covered in foil, and left to gently stir for 24 h, resulting in the formation of a light brown solid. The mixture was cooled to room temperature before subsequent additions of silver trifluoromethanesul-fonate (114 mg, 0.443 mmol) and dichloro(1,5-cyclooctadiene)palladium(II) (121 mg, 0.422 mmol), fol-lowed by stirring at 30 °C for 48 h. After centrifugation, the red supernatant solution was concentrated by vacuum distillation at 55 °C, leaving a wet solid. The residue was taken up in dichloromethane (4 mL), washed with water (4 mL × 2), dried with magnesium sulfate, filtered, and the solvent removed, yielding an orange solid. The crude solid was dissolved in dichloromethane (1 mL) and dropped into diethyl ether (10 mL), forming a light precipitate which was collected by centrifugation as a sticky orange solid. Further drying under vacuum at 60 °C for 24 h resulted in a yellow-orange powder (218 mg, 72% yield). 1H NMR (400 MHz, CD3CN)  0.93 (t, J=7.3 Hz, 6 H), 1.31 (sxt, J=7.4 Hz, 4 H), 1.83 (quin, J=7.5 Hz, 4 H), 4.18 (dt, J=13.5, 7.1 Hz, 2 H), 4.67 (dt, J=13.5, 7.7 Hz, 2 H), 5.29 (d, J=15.2 Hz, 2 H),   140 5.54 (d, J=15.2 Hz, 2 H), 7.10 (d, J=1.7 Hz, 2 H), 7.29 (d, J=1.9 Hz, 2 H), 7.92 (s, 2 H). ESI-MS: m/z 571.9 ([M-OTf]+). Anal. Calcd for C22H28BrClF3N5O3PdS: C, 36.63; H, 3.91; N, 9.71; S, 4.44. Found: C, 36.89; H, 4.10; N, 9.45; S, 4.06. [Pd(C^N^Cp-COOR)Cl]OTf To a solution of proligand C^N^Cp-COOR ∙ 2HBr (117 mg, 0.186 mmol) in dimethyl sulfoxide (5 mL) was added silver(I) oxide (48 mg, 0.21 mmol) and 4 Å molecular sieves. The reaction mixture was heated to 55 °C, covered in foil, and left to gently stir for 36 h. After cooling to room temperature, silver trifluo-romethanesulfonate (52 mg, 0.202 mmol) and dichloro(1,5-cyclooctadiene)palladium(II) (53.0 mg, 0.186 mmol) were added, followed by stirring at 30 °C in darkness for 72 h. Afterwards, the mixture was centrifuged and the solvent removed from the supernatant solution by vacuum distillation at 55 °C. The residue was taken up in dichloromethane (1.5 mL) and washed with water (3 mL × 2), then dried with magnesium sulfate, filtered, and dropped into diethyl ether resulting in the formation of an off-white precipitate which conglomerated to form a viscous oil. The oily residue was washed and triturated with diethyl ether then dried under vacuum, leading to bubbling and solidification. A pale yellow solid was obtained (84 mg) which required further purification by flash chromatography (SiO2, 80:12:8 CH3CN:H2O:KNO3(aq)). The first fraction collected was purified through another column (SiO2, 90:6:4 CH3CN:H2O:KNO3(aq)), the solvent removed under vacuum at 50 °C, and the residue taken up in ace-tone (2 mL) and then saturated with potassium trifluoromethanesulfonate. After stirring at room temper-ature for 1 h, the solvent was removed by rotary evaporation and the residue extracted with dichloro-methane which was then washed with water, dried with magnesium sulfate, filtered, and concentrated to dryness under vacuum. A pale yellow powder was obtained (4.0 mg, 3% yield). 1H NMR (400 MHz, (CD3)2CO)  0.93 (t, J=7.5 Hz, 6 H), 1.30 - 1.39 (m, 4 H), 1.83 - 1.92 (m, 4 H), 1.92 - 1.98 (m, 2 H), 2.03 - 2.09 (m, 2 H), 3.68 (t, J=6.1 Hz, 2 H), 4.31 (dt, J=14.2, 7.5 Hz, 2 H), 4.42 (s, 1 H), 4.46 (t, J=6.0   141 Hz, 2 H), 4.70 (dt, J=13.6, 7.8 Hz, 2 H), 5.85 (d, J=15.2 Hz, 2 H), 5.96 (d, J=15.2 Hz, 2 H), 7.38 (d, J=1.7 Hz, 2 H), 7.61 (d, J=1.7 Hz, 2 H), 8.42 (s, 2 H). ESI-MS: m/z 594.1 ([M-OH]+). Synthesis of Dicationic Acetonitrilo Complexes [Pd(C^N^Cp-OMe)(CH3CN)](OTf)2 Silver trifluoromethanesulfonate (55 mg, 0.21 mmol) was added to a solution of [Pd(C^N^Cp-OMe)Cl]OTf (68.4 mg, 0.106 mmol) in acetonitrile (4 mL) resulting in the formation of a white precipitate within seconds. The reaction mixture was sonicated and then left to stir at room temperature in the dark for 2 h before being filtered through Celite, concentrated in vacuo, and precipitated by dropwise addition to diethyl ether (10 mL). The pale yellow precipitate conglomerated into a viscous orange oil upon cen-trifugation and was triturated with anhydrous diethyl ether, taken up in a minimum of dichloromethane and re-precipitated in anhydrous diethyl ether. The product was further triturated with pentane, then dried under vacuum leading to foaming and solidification as an off-white solid (38.3 mg, 44% yield). 1H NMR (400 MHz, CD3CN)  0.94 (t, J=7.4 Hz, 6 H), 1.25 - 1.40 (m, 4 H), 1.78 - 1.90 (m, 4 H), 1.96 (s, 3 H), 3.98 (s, 3 H), 4.10 - 4.22 (m, 4 H), 5.32 (d, J=15.4 Hz, 2 H), 5.50 (d, J=15.3 Hz, 2 H), 7.21 (d, J=1.9 Hz, 2 H), 7.28 (s, 2 H), 7.40 (d, J=2.1 Hz, 2 H). ESI-MS: m/z 242.6 ([M-CH3CN-2(OTf)]2+), 263.0 ([M-2(OTf)]2+), 636.1 ([M-CH3CN-OTf]+). [Pd(C^N^Cp-Br)(CH3CN)](OTf)2 Silver trifluoromethanesulfonate (27 mg, 0.10 mmol) was added to a solution of [Pd(C^N^Cp-Br)Cl]OTf (50.4 mg, 0.069 mmol) in acetonitrile (3 mL) leading to some cloudiness after minutes of stirring. After 2 h, the reaction mixture was heated to 60 °C for 2 h which resulted in the formation of a distinct white precipitate. The reaction mixture was then centrifuged to remove silver salts with the su-pernatant solution concentrated to dryness by rotary evaporation at 50 °C. The resulting residue was washed by sonication in diethyl ether and then dried under vacuum to yield a pale yellow powder (43.0 mg, 70% yield). 1H NMR (400 MHz, CD3CN)  0.94 (t, J=7.4 Hz, 6 H), 1.28 - 1.41 (m, 4 H), 1.75 - 1.90   142 (m, 4 H), 1.96 (s, 3 H), 4.15 (td, J=7.4, 1.9 Hz, 4 H), 5.39 (d, J=15.5 Hz, 2 H), 5.54 (d, J=15.5 Hz, 2 H), 7.21 (d, J=2.1 Hz, 2 H), 7.41 (d, J=1.9 Hz, 2 H), 7.99 (s, 2 H), ESI-MS: m/z 267.4 ([M-CH3CN-2(OTf)]2+), 288.0 ([M-2(OTf)]2+), 685.8 ([M-CH3CN-OTf]+).     143 Chapter 5: Traversing Group 10 – A Comparison of Nickel, Palladium, and Platinum Analogues 5.1 Introduction Activity for electrocatalytic reduction of CO2 has been observed in a number of varied lutidyl-linked bis-NHC (C^N^C) complexes with Pd as the metal center.117,142 With the desire to better understand the system, the other group 10 metals, Ni and Pt, are herein investigated to see how the electrochemical properties of the complexes change and if electrocatalytic CO2 reduction can be achieved or improved. It is desirable to use more earth-abundant metals for catalysis where possible, due to lower costs and generally lower toxicities. The market prices of metallic nickel, palladium, and platinum at the time of writing are 0.81 CAD/mol, 3214 CAD/mol, and 8389 CAD/mol, respectively, illustrating the vast differ-ences in cost between base and precious metals. Additionally, due to their widespread use in the auto-motive and chemical industries, and production limited to only a few specific geographical areas due to low natural abundances, the market prices of Pd and Pt can be subject to significant volatility.159  The market price of the metal is only one factor to consider in the overall picture of sustainability and lifetime cost, however. A 4000-fold difference in cost for the metal may be counteracted by the require-ment of higher catalyst loadings, differences in performance and selectivity, ease of synthesis, the re-quirement of more specific and/or more expensive ligands, and the ability to extract and recycle the catalyst, among other factors. In general, second- and third-row metals are more electronegative, react with simpler, more well-defined two-electron reaction manifolds and are easier to study as they are typ-ically less substitutionally labile and avoid open-shell electronic configurations, which can limit the use of NMR spectroscopy.159   144 The use of nickel as the metal center has resulted in catalytic CO2 reduction ability in a number of cases, including Ni(cyclam)2+ and related Ni tetraazomacrocyclic complexes,8 tetradentate Ni pyridine-NHC,17 and anaerobic [NiFe] CO dehydrogenase enzymes.1 On the other hand, DuBois et al. investigated Ni and Pt pincer complexes with a triphosphine ligand and found CO2 reduction activity with only Pd.26 In other catalytic systems where M-C bond formation and breaking is critical, such as in C-C cross-coupling reactions, the use of Ni has led to moderate success, though often requiring longer reaction times and higher catalyst loadings than Pd-based catalysts.160 The replacement of a third-row metal with a more earth-abundant first-row metal while retaining elec-trocatalytic activity for CO2 reduction has recently been demonstrated for Re and Mn in M(bpy)(CO)3X (where M = Re, Mn) systems. In this case, the change in metal center resulted in a decreased overpotential for CO production, though with the added requirement of the presence of a weak acid. The mechanistic pathways were computationally modelled and found to be very similar, though with differing activation energies between steps.20,49 The lutidyl-linked bis-(N-butylbenzimidazol-2-ylidene) ligand, bC^N^bC, was chosen for these stud-ies as its use has previously shown improved activity for CO2 reduction relative to C^N^C,117 with addi-tional data for Pd therefore useful in addition to a comparison of the bis-NHC lutidyl pincer framework in Ni and Pt.    145 5.2 Results & Discussion 5.2.1 Synthesis of [Pd(bC^N^bC)Cl]OTf and [Pd(bC^N^bC)CH3CN](OTf)2 Following previously established synthetic protocols, [Pd(bC^N^bC)Cl]OTf ([Pd]) was synthesized in good yields and high purity according to previously established synthetic protocols through reaction of the bC^N^bC ∙ 2HBr proligand with silver(I) oxide to form a silver carbene intermediate, followed by transmetallation to palladium upon addition of dichloro(1,5-cyclooctadiene)palladium(II), and abstrac-tion of a chloride using silver triflate (AgOTf), yielding an off-white powder in 69% yield.117  The dicationic acetonitrilo species [Pd(bC^N^bC)CH3CN](OTf)2 ([Pd]-sol) was formed in good yields and purity by addition of 1.05 equivalents of silver triflate to a concentrated acetonitrile solution of the chlorido species at room temperature, resulting in halide abstraction and the rapid precipitation of silver chloride. The chlorido species was moderately hygroscopic and the acetonitrilo species highly hy-groscopic. 5.2.2 Synthesis of [Ni(bC^N^bC)Cl]OTf Initial attempts to synthesize [Ni(bC^N^bC)Cl]OTf ([Ni]) followed the original procedure of Doi et al., who synthesized the bis(imidazol-2-ylidene) complex and studied its high catalytic activity for the Suzuki-Miyaura cross-coupling reaction of aryl halides and boronic acids,161 with the modification of incorporating benzimidazol-2-ylidene donors. The proligand was reacted with silver(I) oxide to form the silver carbene species, which was isolated in good purity, followed by slow addition to a suspension of dichloro(dimethoxyethane)nickel(II) in CH2Cl2. 1H NMR analysis of the product indicated approxi-mately 5 distinct chemical environments for the methylene protons bridging the pyridyl and benzimid-azole-2-ylidene moieties (Figure 5-1), which is characteristically a doublet of doublets with J = ~15 Hz when the carbenes are coordinated to the metal center in the intended complex, making the two methylene   146 protons diastereotopic. The spectrum is interpreted to represent an oligomeric or macrocyclic structure caused by slow addition of the silver carbene species to a suspension of NiCl2(DME), where one triden-tate pincer ligand binds to two nickel atoms which may or may not already be bound by another carbene. Additionally, TLC showed four distinct spots for the crude product, supporting the presence of multiple species in solution. 10 9 8 7 6 5 4 3 2 1 0Chemical Shift (ppm)  Figure 5-1. 1H NMR spectrum of isolated impurities from column chromatography of as-synthesized [Ni(bC^N^bC)Cl]OTf wherein NiCl2(DME) was slowly added to the silver carbene proligand species.  The synthetic route to [Ni] was therefore modified to avoid slow addition of the silver carbene species to the nickel precursor. The silver carbene species was generated in situ and the nickel precursor was added in one quantity, resulting in formation of the monometallic pincer species with improved purity. The additional step of washing a dichloromethane solution of the isolated product with distilled water removed minor impurities present, resulting in a yellow solid in 85% yield with sufficient purity for electrochemical analysis. The identity and purity of the product was determined by 1H NMR, MS, and   147 elemental analysis. Additionally, X-ray quality crystals were grown by slow evaporation of an acetone solution in a desiccator. 5.2.3 Synthesis of [Ni(bC^N^bC)CH3CN](OTf)2 Synthesis of the halide-free acetonitrilo complex [Ni(bC^N^bC)CH3CN](OTf)2 ([Ni]-sol) was ini-tially approached in a similar manner as for [Pd] by addition of a slight excess of silver triflate to an acetonitrile solution of the chlorido complex. As it has been previously observed that transmetallation from Ni back to Ag is a possibility (unpublished results), the silver triflate was dissolved in acetonitrile and slowly added to the stirring solution in order to avoid even temporary high local concentrations. A white precipitate was promptly formed upon addition. The crude product was collected in good yield and characterized by 1H NMR and MS, indicating complete consumption of the chlorido complex but for-mation of a major and multiple minor products with MS peaks at 254.0 ([M-CH3CN-2(OTf)]2+), 274.6 ([M-2(OTf)]2+), and 658.6 m/z ([M-CH3CN-OTf]+), indicating the presence of the desired [Ni(bC^N^bC)(CH3CN)](OTf)2 species, but also 558.6 m/z, unambiguously indicating a [Ag(bC^N^bC)]+ species. The experiment was repeated with a substoichiometric amount of silver triflate (0.9 equiv.) in dilute solution, though the formation of a silver pincer complex as a minor product was not suppressed and a pure product could not be obtained. It should be noted that washing a dichloro-methane solution of the crude product with water resulted in conversion of the [Ni] acetonitrilo complex back to the chlorido complex. A suitable synthetic route to the acetonitrilo complex was not found using silver-based halide abstraction reagents.  Alternative synthetic pathways were explored to avoid the presence of any coordinated halide. Given the ability to synthesize analogous Pd complexes by protonolysis of the proligand with Pd(OAc)2 at high temperatures, protonolysis routes to Ni were investigated. The proligand anions were exchanged for tri-fluoromethanesulfonate groups by reaction with two equivalents of silver triflate, then dissolved in   148 DMSO along with either Ni(acac)2 or Ni(OAc)2 in a microwave vial, degassed, and then heated in a microwave at increasing temperatures. Given the blue-green colour of the nickel precursors and the yel-low colour of the product, reaction progress could be visually monitored. Heating at 140 °C led to a colour change to yellow over 60 minutes, which an additional 10 minutes at 160 °C intensified. No evi-dence of the desired product was detected by 1H NMR or MS, however, and some unreacted proligand was present. More forcing conditions were also attempted, stirring at 175 °C for 150 minutes, which also resulted in a yellow solution. Aliquots were removed after 60 and 105 minutes and analyzed by 1H NMR, revealing consumption of the starting material and at least two sets of overlapping resonances analogous to the proligand or a pincer complex, though intractably mixed. The protonolysis approach to [Ni(bC^N^bC)CH3CN](OTf)2 was therefore abandoned.  In order to obtain a solvento complex, abstraction of the chloride from [Ni(bC^N^bC)Cl]OTf  was attempted again using thallium(I) triflate (TlOTf) as an alternative halide abstraction agent. A slightly substoichiometric amount of TlOTf was dissolved in acetonitrile and slowly added to an acetonitrile solution of the [Ni] chlorido complex, resulting in the immediate formation of a white precipitate. After 2 hr, the reaction mixture was filtered, the solvent removed by rotary evaporation, and the residue washed with diethyl ether and then dried under vacuum, leaving a yellow solid in nearly quantitative yield. The compound identity was confirmed by 1H NMR, MS, and ATR-FTIR. Given the insolubility of TlOTf in the diethyl ether wash (relative to AgOTf), a slight excess or even stoichiometric amount of TlOTf could carry through to the product as a redox active contaminant, which was observed in the cyclic voltammo-grams of some impure samples. Contaminating Tl, if present, could be easily detected by mass spectrom-etry.   149 5.2.4 Synthesis of [Pt(bC^N^bC)Br]OTf and [Pt(bC^N^bC)CH3CN](OTf)2 The coordination of C^N^C ligands to Pt was reported once previously by Serra et al., where it was described as “difficult”.162 Thus, to produce the complex [Pt(bC^N^bC)Br]OTf ([Pt]), the synthetic method of Serra et al. was initially attempted where the proligand (C^N^C and bC^N^bC), silver(I) oxide, and platinum(II) bromide were heated together in DMSO at 100-150 °C for 12 h. Our initial at-tempts utilized PtBr2(cod) as the platinum source, and despite numerous attempts at varied temperatures and concentrations, a complex mixture of products resulted. Finally, the method was repeated using PtBr2, leading to the formation of [Pt(C^N^C)Br]Br and [Pt(bC^N^bC)Br]Br in greatly improved but still insufficient purity, where broad, complicated signals in the regions of 0.0-0.5 (bC^N^bC complex only), 0.6-1.5, 5.3-6.5, and 6.8-8.0 ppm in the 1H NMR spectra were still present after workup procedures (Figure 5-2). The procedure was therefore modified to operate at a lower temperature, as in the synthetic procedure for [Pd]. The proligand and silver(I) oxide were reacted first at 55 °C, then PtBr2 and AgOTf were added and the mixture was left to stir at 50 °C. The reaction was monitored by 1H NMR, indicating that after 20 hours nearly complete product formation was observed. In addition to a characteristic doublet of doublets for the methylene group bridging the pyridyl to imidazol-2-ylidene, approximately seven additional sets of doublets at lower intensities were detected and attributed to a minor product, consistent with formation of macrocyclic or oligomeric spe-cies as also seen with [Ni] (Figure 5-2). It should be noted that the competitive formation of macrocycles or oligomers against the desired monometallic species may have been aided by the presence of the bi-dentate 1,5-cyclooctadiene ligand when PtBr2(cod) was utilized, dissociating and acting as a bridge be-tween two Pt metal centers.   150 3-aliquot2-20hr-rxn.001.1r.espACETONITRILE-d37.0 6.5 6.0 5.5Chemical Shift (ppm)  Figure 5-2. 1H NMR spectrum from 5.0 to 7.5 ppm of as-synthesized [Pt(bC^N^bC)Br]OTf before pu-rification by column chromatography.  Thin layer chromatography of the crude product indicated at least five different spots with adequate separation, representing the presence of at least five different species. Purification of the crude product mixture was achieved by column chromatography on silica gel with a 85:10:5 CH3CN:H2O:KNO3(aq) solution as the eluent, where the first fraction eluted was the desired monometallic product. After collec-tion, the fraction underwent counterion metathesis with KOTf to give [Pt(bC^N^bC)Br]OTf in 27% iso-lated yield and sufficient purity for electrochemical experiments. The acetonitrilo complex was synthe-sized in nearly quantitative yield following the same method as for [Pd]-sol. Synthesis of both [Pt] and [Pt]-sol was also attempted by a protonolysis reaction between the proligand and Pt(acac)2 at high temperatures in a microwave reactor. Intractable mixtures were obtained. In summary, the best synthetic routes to Ni, Pd, and Pt complexes were by formation of a silver car-bene proligand species and transmetallation to the group 10 metal. The dicationic acetonitrilo species were then formed by halide abstraction using AgOTf in the cases of Pd and Pt, or TlOTf in the case of Ni (Scheme 5-1).   151 Scheme 5-1. General Synthetic Route to M(bC^N^bC) Complexes  (A): For M = Ni, NiCl2(DME) was used with CH3CN as the solvent. For M = Pd and Pt, PdCl2(cod) and PtBr2 were used with DMSO as the solvent. (B): For M = Ni, TlOTf was used at room temperature, whereas AgOTf was used in the cases of M = Pd and Pt, and at 40 °C for Pt. 5.2.5 Characterization of Complexes Each of the complexes were characterized by 1H NMR (Figure A-25 to Figure A-32), ESI- and ESCI-MS, and ATR-FTIR. The halido complexes also characterized by microanalysis. The 1H NMR chemical shifts of the complexes were very similar across the series, indicative of the isostructural and isoelec-tronic nature of the complexes, but there were some notable minor variations (Figure 5-3). The largest difference was for the bridging methylene groups which in each case appear as two distinct doublets as a result of hydrogen atoms being diastereotopic due to the twist of the ligand. In CD2Cl2, the chemical shifts of these doublets were 6.04 and 6.45 ppm for [Ni], 5.78 and 5.94 ppm for [Pd], and 5.60 and 5.78 ppm for [Pt], where one hydrogen in the bridging methylene points towards the metal center and the other points away. The chemical shift is strongly correlated with the twist of the pyridyl out of the coor-dination plane of the complex, where a larger twist angle from coplanarity (where the pyridyl group is flat in plane) places the inward-pointing hydrogen in a more direct axial orientation to the metal center leading to stronger electron shielding. DFT calculations show an out-of-plane twist angle of 46.0°, 42.2°, and 41.7° for [Ni], [Pd], and [Pt], respectively, closely matching the available solid state XRD data of   152 46.3° for [Ni] and 41.7° for [Pd], which result from the differing radii of the metal atoms. The donor groups in the lutidyl-linked bis-NHC pincer ligand can become more planar to chelate larger metals and more twisted to chelate smaller metals, an interesting feature adding to the versatility of the ligand motif. The other notable chemical shift across the series is for the pyridyl para-H at 7.98 ppm in [Ni] and 8.09 ppm in both [Pd] and [Pt], with the difference likely related to the twist angle of the pyridyl combined with varied π-back bonding from the metal center.  Figure 5-3. Stack plot of 1H NMR spectra from 3.5 to 9.0 ppm for [Ni], [Pd], and [Pt] in CD3CN.  For [Pt], 195Pt NMR analysis was also performed. 195Pt NMR chemical shifts span a range of 13,000 ppm and are sensitive to numerous properties such as oxidation state and electron donation from ligands, with stronger electron donation from ligands leading to more negative chemical shifts within an oxidation state and geometry type.163 NMR spectra for a few examples of other Pt pincer complexes have been collected, allowing a comparison of chemical shifts to indicate the degree of electron donation from the ligand set. For example, the pincer complex [Pt(tpy)Br]PF6 has a 195Pt chemical shift of -2765 ppm164 and [PtCl(NCN{CH=NC6H4R’-4’}-4)] (where (NCN{CH=NC6H4R’-4’}-4) is a 4,4’-substituted dime-thylamino(benzylidene) aniline ligand) shows a 195Pt shift of -3090 to -3125 ppm depending on the spe-cific substituents on the ligand.165 The 195Pt resonance for [Pt(bC^N^bC)Br]OTf has a chemical shift   153 of -3665 ppm, indicating more electron shielding than the previously mentioned examples and suggesting a more electron rich metal center due to the strongly electron donating N-heterocyclic carbene donors (Figure A-31). To our knowledge, this is the first reported 195Pt chemical shift for a bis-NHC pincer complex. Single-crystals suitable for X-ray diffraction were grown for [Ni(bC^N^bC)Cl]OTf by slow evapora-tion of a saturated acetone solution and for [Pd(bC^N^bC)Cl]BF4 by diffusion of diethyl ether into an acetonitrile solution. Suitable crystals of [Pt(bC^N^bC)Br]OTf were not obtained. In each case, the solid state structures were as expected, with the pincer ligand binding to form a monometallic square planar complex (Figure 5-4). Both structures are essentially identical, with the exception of increased M-L bond lengths (~0.12 – 0.15 Å) and a smaller bite angle (174.1° vs. 176.2°) in [Pd] due to the larger atomic radius of Pd. All bond lengths are typical, with Pd-pyridyl, Pd-NHC, and Pd-Cl bond lengths of 2.08 Å, 2.03 Å, and 2.31 Å, respectively, and Ni-pyridyl, Ni-NHC, and Ni-Cl bond lengths of 1.92 Å, 1.91 Å, and 2.17 Å, respectively. The solid state structural data for [Ni] and [Pd], combined with analogous 1H NMR spectra, indicates that all three complexes are indeed isostructural and valence isoelectronic.      Figure 5-4. Solid state structures of [Ni(bC^N^bC)Cl]OTf (left) and [Pd(bC^N^bC)Cl]BF4 (right), with counterions and solvent molecules omitted.    154 5.2.6 Fluxionality of [Ni(bC^N^bC)Cl]OTf and [Ni(bC^N^bC)CH3CN](OTf)2 One distinctive feature of the lutidyl-linked bis-NHC ligand is the ability to interconvert between two conformers (Figure 5-5), resulting in a coalescence of 1H NMR signals for the bridging methylene and imidazolylidene N-methylene groups. Such fluxionality has been previously noticed and investigated for [Pd] complexes with nucleophilic halide counterions, particularly, where the effects of fluxionality can be observed at room temperature in 1H NMR spectra.127 For lutidyl-linked bis-NHC Pd complexes with non-coordinating anions, the rate of conformer interconversion is very low except at high temperatures (>100 °C) and the 1H NMR spectra at room temperature are not noticeably affected, whereas for [Ni] some minor signal broadening was observed at room temperature in CD2Cl2 (Figure A-25) and the res-onances for the diastereotopic protons on the methylene groups indicated above are nearly coalesced at room temperature in CD3CN (Figure 5-3). In [Ni]-sol, the 1H NMR signals were partially and completely coalesced in CD2Cl2 and CD3CN, respectively. The analogous [Pd]-sol and [Pt]-sol complexes do not show any line broadening even in CD3CN, indicating much slower rates of interconversion (Figure A-29 and Figure A-32, respectively). In general, the rate of interconversion for [Pt] seems similar to that of [Pd] with no signal broadening observed at room temperature in CD3CN or (CD3)2SO. No further variable temperature experiments were attempted.                                                    Figure 5-5. Interconversion between conformers in [M(bC^N^bC)X]+ species.   155 Additional variable temperature NMR experiments were performed for [Ni] and [Ni]-sol in CD3CN, however (Figure 5-6), and compared to data collected from a sample of [Pd(bC^N^bC)Cl]BF4 in (CD3)2SO (Figure A-35). The activation energy barrier for conformer interconversion was first estimated through analysis of the coalescence temperature,125,148 yielding ∆G‡ = 84.3, 59.6, and 50.9 kJ/mol for [Pd(bC^N^bC)Cl]BF4 in (CD3)2SO, and [Ni(bC^N^bC)Cl]OTf and [Ni(bC^N^bC)CH3CN](OTf)2 in CD3CN, respectively. Simulation of the fluxional process was then performed using Bruker TopSpin 3.5, yielding interconversion rates as a function of temperature, followed by an Eyring analysis to yield ∆H‡ = 55.7, 29.4, and 38.6 kJ/mol and ∆S‡ = -66, -99, and -40 J/(mol K) for [Pd(bC^N^bC)Cl]BF4, [Ni(bC^N^bC)Cl]OTf, and [Ni(bC^N^bC)CH3CN](OTf)2, respectively. These enthalpy and entropy data give ∆G‡ = 83.8, 59.7, and 48.7 kJ/mol for the respective complexes at the coalescence temperatures, consistent with the estimated values from the coalescence temperature technique. The negative entropy of activation values in each case indicate an associative mechanism for inter-conversion, where the solvent (or counterion) comes together with the metal complex to form the transi-tion state between conformers, which is consistent with previous studies on analogous [Pd(C^N^C)X]+ complexes where it was found that the presence of a nucleophilic outer sphere halide anion significantly increased the rate of conformer interconversion and consistent with the higher interconversion rates ob-served in CD3CN relative to CD2Cl2.85,148 The faster rate of interconversion for [Ni] is attributed to the smaller atomic radius of Ni lowering the energy of the transition state where both bridging methylene groups move to the coordination plane of the complex148 resulting in the outer NHC groups pushed in-wards towards the metal center.    156     Figure 5-6. Overlaid 1H NMR spectra of [Ni(bC^N^bC)Cl]OTf (left) and [Ni(bC^N^bC)CH3CN](OTf)2 (right) at varied temperatures in CD3CN.  5.2.7 Electrochemical Characterization Each of the complexes were electrochemically characterized by cyclic and square-wave voltammetry in DMF under N2, CO2, and CO2 with 10 mM trifluoroacetic acid present (Table 5-1, Figure 5-7). For [Pd], two irreversible reduction events at -1.72 and -2.20 V were observed, as previously reported,117 assigned to a pyridyl-localized reduction and then Pd-localized reduction, respectively. Under CO2, a significant current enhancement was observed at the first cathodic wave, as well as a slight cathodic shift in the peak potential of the second cathodic wave.   157      Figure 5-7. Overlaid cyclic voltammograms (left) and square-wave voltammograms (right) of [Ni(bC^N^bC)Cl]OTf (A), [Pd(bC^N^bC)Cl]OTf (B), and [Pt(bC^N^bC)Br]OTf (C) under N2, CO2, and CO2 with 10 mM trifluoroacetic acid present.  -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-150-125-100-75-50-25025 [Ni], N2 [Ni], CO2 [Ni], CO2 + TFACurrent (µA)Potential (V vs. Fc0/+)-2.5 -2.0 -1.5 -1.0 -0.5 0.0-70-60-50-40-30-20-100Current Difference (µA)Potential (V vs. Fc0/+) [Ni], N2 [Ni], CO2 [Ni], CO2 + TFA-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-80-60-40-20020Current (µA)Potential (V vs. Fc0/+) [Pd], N2 [Pd], CO2 [Pd], CO2 + TFA-2.5 -2.0 -1.5 -1.0 -0.5 0.0-60-50-40-30-20-100Current Difference (µA)Potential (V vs. Fc0/+) [Pd], N2 [Pd], CO2 [Pd], CO2 + TFA-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-100-80-60-40-20020Current (µA)Potential (V vs. Fc0/+) [Pt], N2 [Pt], CO2 [Pt], CO2 + TFA-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0-50-40-30-20-100Current Difference (µA)Potential (V vs. Fc0/+) [Pt], N2 [Pt], CO2 [Pt], CO2 + TFAA B C   158 The behaviour of the isoelectronic species [Ni] was different, with three distinct reduction events at -1.48, -1.89, and -2.31 V. The first cathodic event is quasi-reversible with the peak potential of the oxidative return wave, but not the current ratio, strongly dependent on scan rate and therefore suggestive of slow electron transfer kinetics for oxidation. The three distinct reduction events for the complex are difficult to assign given that the related complex [Ni(C^N^C)Cl]OTf shows only two distinct reduction events (Figure 5-8, although there is a small feature at approx. -2.05 V) and similar molecular orbitals are indicated for both [Ni] and [Pd] by DFT calculations. Nickel, being a first-row metal, is more capable of having high-spin electronic configurations, and, with weaker benzimidazol-2-ylidene NHCs as op-posed to stronger imidazol-2-ylidenes, a high spin electronic configuration may be a factor. Under CO2, no current enhancement is observed at the first reduction event, a slight anodic shift and current enhance-ment is observed at the second, and a larger anodic shift and current enhancement is observed at the third, indicative of some reactivity with CO2.  Figure 5-8. Overlaid cyclic voltammograms of [Ni(C^N^C)Cl]OTf taken at a scan rate of 100 mV/s under N2 and CO2.  -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-75-50-25025 [Ni(C^N^C)Cl]OTf, N2 [Ni(C^N^C)Cl]OTf, CO2Current (µA)Potential (V vs. Fc0/+)  159 For [Pt], one distinct reduction event was observed at -1.92 V, followed by additional irreversible events at -2.20 and -2.42 V with diminishing currents, and then multiple, overlapping waves from -2.7 to -2.9 V. When exposed to CO2, the electrochemical behaviour changes significantly with a small current enhancement at the first reduction event at -1.92 V, a large current enhancement and anodic shift for the second reduction event, and a removal of the additional distinct reduction events at more cathodic poten-tials. These results are again indicative of the complex reacting with CO2 upon reduction. In summary, for the halido complexes, the largest changes in electrochemical behaviour upon addition of CO2 were for [Pd] > [Pt] > [Ni].  Table 5-1. Peak Potentials of Halido Complexesa versus Fc0/+ Complex 1st Reduction (V) 2nd Reduction (V) 3rd Reduction (V) [Ni] -1.48 (-1.48) -1.89 (-1.88) -2.31 (-2.22) [Pd]  -1.72 (-1.72) -2.20 (-2.27) -2.50 (-2.54) [Pt]  -1.92 (-1.92) -2.20 (-2.13) -2.42 (-2.80, broad) aDetermined from square-wave voltammograms (25 Hz frequency, 5 mV potential step, and 25 mV am-plitude) for 2 mM solutions of respective complexes in 0.10 M [n-Bu4N]PF6/DMF under N2 and CO2 using a glassy carbon working electrode. Numbers in parentheses represent peak potentials under CO2, with italics indicating a small current enhancement, and bond and italics indicating a large current en-hancement.  The electrochemical behaviour of the solvento complexes was also recorded (Figure 5-9). In each case the voltammograms were similar to those of the halido complexes, but with an additional reduction event approximately 300 mV less negative than the first reduction of the halido complex. It is likely that   160 the dicationic complexes are prodigious scavengers of coordinating anions from the solvent and electro-lyte, as a cathodic wave at the same potentials as the first reduction in the halido complexes is present in each case, though there is no evidence of the halide being bound for the solvento species in 1H NMR and MS characterization. After sparging with CO2, the first reduction event for the [Ni]-sol complex disap-pears and the voltammograms become more comparable to [Ni]. For [Pd]-sol, there is no current en-hancement in the presence of CO2 at the first reduction event, but there is an anodic shift of 39 mV, characteristic of some interaction with CO2.68 Finally, for [Pt]-sol, there is no anodic shift but a small current enhancement at the first cathodic wave, followed by behaviour as previously described for [Pt].  Table 5-2. Peak Potentials of Solvento Complexesa versus Fc0/+ Complex 1st Reduction (V) 2nd Reduction (V) 3rd Reduction (V) [Ni]-sol -1.12 (-1.26) -1.48 (-1.55) -1.78, -1.89, -2.29 (-1.78, -2.21) [Pd]-sol -1.34 (-1.30) -1.72 (-1.72) -2.20 (-2.27) [Pt]-sol -1.66 (-1.67) -1.90 (-1.90) -2.20 (-2.12) aDetermined from square-wave voltammograms (25 Hz frequency, 5 mV potential step, and 25 mV am-plitude) for 2 mM solutions of respective complexes in 0.10 M [n-Bu4N]PF6/DMF under N2 and CO2 using a glassy carbon working electrode. Numbers in parentheses represent peak potentials under CO2, with italics indicating a small current enhancement, and bond and italics indicating a large current en-hancement.     161       Figure 5-9. Overlaid cyclic voltammograms (left) and square-wave voltammograms (right) of [Ni(bC^N^bC)CH3CN](OTf)2 (A), [Pd(bC^N^bC)CH3CN](OTf)2 (B), and [Pt(bC^N^bC)CH3CN](OTf)2 (C) under N2, CO2, and CO2 with 10 mM TFA present. -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-80-60-40-20020 [Ni]-sol, N2 [Ni]-sol, CO2 [Ni]-sol, CO2 + TFACurrent (µA)Potential (V vs. Fc0/+)-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0-40-30-20-100Current Difference (µA)Potential (V vs. Fc0/+) [Ni]-sol, N2 [Ni]-sol, CO2 [Ni]-sol, CO2 + TFA-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-120-100-80-60-40-20020Current (µA)Potential (V vs. Fc0/+) [Pd]-sol, N2 [Pd]-sol, CO2 [Pd]-sol, CO2 + TFA-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0-70-60-50-40-30-20-100Current Difference (µA)Potential (V vs. Fc0/+) [Pd]-sol, N2 [Pd]-sol, CO2 [Pd]-sol, CO2 + TFA-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-100-80-60-40-20020Current (µA)Potential (V vs. Fc0/+) [Pt]-sol, N2 [Pt]-sol, CO2 [Pt]-sol, CO2 + TFA-2.5 -2.0 -1.5 -1.0 -0.5-50-40-30-20-100Current Difference (µA)Potential (V vs. Fc0/+) [Pt]-sol, N2 [Pt]-sol, CO2 [Pt]-sol, CO2 + TFAC A B   162 5.2.8 DFT Modelling DFT calculations were performed to gain more detailed information about the molecular and elec-tronic structure of the complexes, yielding optimized solution state structures at potential energy minima in excellent agreement with the experimental solid state structures. The DFT-modelled structures were simplified by substituting methyl groups for the n-butyl substituents on the ligand. In all cases the HOMO and LUMO were isolobal across the series, with the HOMO spanning the metal dxz* orbital and the π-system of the benzimidazol-2-ylidenes, and the LUMO localized on the pyridyl with some metal dyz* contribution. For [Ni] and [Pd], the LUMO+1 was primarily dx2-y2* in character, but for [Pt] the LUMO+1 spanned the π-system of the benzimidazol-2-ylidenes and bridging pyridyl (Figure 5-10). The analogous dx2-y2* orbital was calculated to be the LUMO+4 at a significantly higher energy in the case of [Pt].  [Ni] HOMO  [Pd] HOMO  [Pt] HOMO   [Ni] LUMO  [Pd] LUMO  [Pt] LUMO   [Ni] LUMO+1  [Pd] LUMO+1  [Pt] LUMO+1  Figure 5-10. Calculated HOMO, LUMO, and LUMO+1 geometries for model complexes of [Ni(bC^N^bC)Cl]+, [Pd(bC^N^bC)Cl]+, and [Pt(bC^N^bC)Cl]+.    163 The one-electron reduced species were also modelled, with each of the reduced species remaining analogous to one another. The NPA charges of the metal center and NBO dz2 energy levels are recorded for the reduced and unreduced species in Table 5-3. For each complex, reduction resulted in a slightly less positive charge on the metal center with most of the additional charge localized on the pyridyl and an increase in the energy level of the dz2 orbital. In the series, the dz2 orbital energies followed the trend Pt > Ni > Pd for the unreduced and reduced species. To corroborate these results, the energies of the dz2-containing MOs were also calculated, yielding the same trend. Table 5-3. Calculated NPA Charges and NBO dz2 Orbital Energies Metal Center Degree of Re-duction NPA Charge on M Energy of NBO dz2 orbital (eV) Energy of dz2-con-taining MO (eV) Ni 0e- +0.291 -9.10 -9.34  1e- +0.241 -8.42 -8.72 Pd 0e- +0.250 -9.49 -9.46  1e- +0.228 -9.08 -9.04 Pt 0e- +0.193 -7.73 -9.16  1e- +0.128 -7.30 -8.61  Finally, the interactions of the one-electron reduced species with CO2 were modelled, where the dis-tance between CO2 and the metal center was incrementally decreased from 3.25 to 1.75 Å with the system optimized at each point and the overall energy of the system calculated (Figure 5-11). Each complex showed some activation of CO2, to bond angles of 138.2°, 137.6°, and 131.6° for [Ni], [Pd], and [Pt], respectively. The activation of CO2 with [Pd] had an activation energy barrier of ~7 kcal/mol and was exogonic by ~25 kcal/mol, whereas the activation of CO2 in the cases of [Ni] and [Pt] was only exogonic   164 by ~2 kcal/mol with activation energy barriers of ~4 and ~12 kcal/mol, respectively. These results corre-late with the experimental electrochemical data under CO2, where only [Pd] showed a significant current enhancement upon reduction by one electron. It should be noted that these calculations do not reflect performance of these complexes for electrocatalytic CO2 reduction, but simply the first step of that cata-lytic cycle.  Figure 5-11. Calculated energy profiles for the interaction of CO2 approaching one-electron reduced complexes from axial position at an initial distance of 3.25 Å from the metal center.  5.2.9 Controlled Potential Electrolysis Controlled potential electrolysis experiments were performed for each of the complexes where 2 mM solutions of the species in 0.10 M [n-Bu4N]PF6/DMF were prepared in an air-tight H-cell, sparged with CO2, and then 10 mM TFA added. The working electrode was held at potentials where a current enhance-ment under CO2 was observed and the passage of charge recorded, followed by analysis of the headspace 1.75 2.00 2.25 2.50 2.75 3.00 3.25-30-25-20-15-10-505101520  M = Ni  M = Pd  M = PtRelative Energy (kcal/mol)M-CO2 Distance (Å)130140150160170180CO2 Bond Angle (°)  165 gas by gas chromatography to determine the production of H2 and CO. The results are summarized in Table 5-4. For [Ni], electrolysis was performed at -1.90 and -2.25 V, yielding little CO in the headspace gas (2% and 4%, respectively) and faradaic efficiencies for combined H2 and CO production under 60%. The addition of Mg2+ did not modify the electrochemical behaviour and resulted in the same performance as the complex without Mg2+ at -2.25 V. No precipitates were formed. In the case of [Ni]-sol, the overall faradaic efficiency for gas production increased to nearly unity, but yielded similarly low ratios of CO in the produced gas, 4% and 2%, when operating at -2.00 and -2.35 V, respectively. Given that no decom-position was observed and no precipitate formed (indicative of carbonate formation from the dispropor-tionation of CO2), it is possible that the reduced species of [Ni] lacks a strong driving force to consume CO2 or H+ and exists in an equilibrium where some of the transferred charge has resulted in a reduced [Ni] species but not product formation by the end of the electrolysis experiment. Another possibility is the production of a non-gaseous product such as formate, but none was detected by a chromotropic acid spot test.93 As a first row transition metal, stable states with unpaired electrons are more likely.  For [Pd], more controlled potential electrolysis experiments were performed than was done pre-viously.117 At -2.10 and -2.40 V the faradaic efficiency of CO production was 29% and 31%, respectively, with the remaining charge accounted for in H2 production. The complex displayed excellent stability. For [Pd]-sol, the overall faradaic efficiency dropped from approximately 100% to 50% at -2.25 V, though the ratio of CO in the produced gas remained nearly identical at 28%. With Mg2+ added, CO production was improved to 34% and 36% faradaic efficiency at -2.25 and -2.50 V, representing 45% and 57% of the gas produced.  And for [Pt], electrolysis was performed at -2.25 V without and with Mg2+ added, giving similar results 4% and 7% CO produced, respectively, with high overall faradaic efficiencies near 100%. At -  166 2.50 V with Mg2+ present, a large increase to 21% efficiency for CO production was observed, represent-ing 31% of the gas produced. This result was not repeated with [Pt]-sol, even at -2.50 V with Mg2+ present, with 7% being the highest faradaic efficiency for CO production for [Pt]-sol at -2.25 V. The complexes appeared to be reasonably stable throughout electrolysis experiments, with distinctive features in the voltammograms and 1H NMR spectra consistent before and after electrolysis. For [Pt]-sol some dark residue was observed on the working electrode after electrolysis at -2.50 V with Mg2+ present. Finally, electrolysis experiments for [Pd] were repeated but with 3Å molecular sieves present to see if the removal of H2O from solution as it is produced alongside CO could lead to improved rates of CO2 reduction relative to the competitive reaction of H+ reduction, either by helping to drive an equilibrium or by removing a species which could potentially bind to the complex to retard electrocatalysis. A fara-daic efficiency of 40% for CO production was achieved at -2.25 V, with the remaining charge resulting in H+ reduction. At -2.25 V with Mg2+ added, that efficiency was improved to 47% with CO and H2 produced in equal measure. Under the same conditions at -2.50 V, CO was produced with equal faradaic efficiency as at -2.25 V, but out-produced H2 composing 58% of the total produced gas.     167 Table 5-4. Results from CPE Experimentsa Species Potential (V) Charge (C) FE CO FE H2 Ratio CO in Gas [Ni(bC^N^bC)Cl]OTf -1.90 3.8 1% 51% 2% [Ni(bC^N^bC)Cl]OTf -2.25 5.4 2% 55% 4% [Ni(bC^N^bC)Cl]OTf + Mg2+ -2.25 6.0 2% 56% 3% [Ni(bC^N^bC)(CH3CN)](OTf)2 -2.00 4.7 4% 91% 4% [Ni(bC^N^bC)(CH3CN)](OTf)2 -2.35 5.7 2% 96% 2% [Pd(bC^N^bC)Cl]OTf -2.10 5.1 29% 71% 29% [Pd(bC^N^bC)Cl]OTf -2.40 5.0 31% 69% 31% [Pd(bC^N^bC)(CH3CN)](OTf)2  -2.25 6.0 14% 35% 28% [Pd(bC^N^bC)(CH3CN)](OTf)2 + Mg2+ -2.25 5.0 34% 41% 45% [Pd(bC^N^bC)(CH3CN)](OTf)2 + Mg2+ -2.50 5.0 36% 28% 57% [Pt(bC^N^bC)Br]OTf -2.25 6.0 4% 94% 4% [Pt(bC^N^bC)Br]OTf + Mg2+ -2.25 6.0 7% 92% 7% [Pt(bC^N^bC)Br]OTf + Mg2+ -2.50 6.0 21% 46% 31% [Pt(bC^N^bC)(CH3CN)](OTf)2 -2.05 3.1 0% 73% 0% [Pt(bC^N^bC)(CH3CN)](OTf)2 -2.25 4.0 7% 59% 10% [Pt(bC^N^bC)(CH3CN)](OTf)2 -2.50 4.1 4% 72% 5% [Pt(bC^N^bC)(CH3CN)](OTf)2 + Mg2+ -2.50 4.0 2% 84% 2% [Pd(bC^N^bC)Cl]OTfb -2.25 6.0 40% 61% 40% [Pd(bC^N^bC)Cl]OTfb + Mg2+ -2.25 6.0 47% 47% 50% [Pd(bC^N^bC)Cl]OTfb + Mg2+ -2.50 3.5 46% 33% 58% aPerformed with 2 mM solutions in 10 mL 0.10 M [n-Bu4]PF6/DMF under CO2 using a glassy carbon rod electrode. 25 mM Mg(ClO4)2 used where indicated. Potentials reported vs. Fc0/+ and adjusted for IR drop when using higher surface area electrode. b3Å molecular sieves present.    168 5.3 Discussion The CPE results indicate superior performance for electrocatalytic reduction of CO2 with [Pd] relative to [Ni] and [Pt], with only trace CO produced with [Ni] and [Pt], except for [Pt] operating at -2.50 V with the Lewis acid Mg2+ present. These results correlate well with the cyclic and square-wave voltammogram data where only [Pd] exhibits a significant current enhancement at the first reduction potential with CO2 and the proton source TFA present (Figure 5-7). The effect of inhibition by CO was tested for [Ni] and [Pd], where the differential ability of [Ni] to more strongly bond to CO to form a CO complex would limit any electrocatalytic turnover. The synthesis of CO complexes were attempted for [Ni] and [Pd] by sparging acetonitrile solutions of [Ni(bC^N^bC)(CH3CN)](OTf)2 and [Pd(bC^N^bC)(CH3CN)](OTf)2 with CO and then stirring them un-der 1 atm CO at room temperature. The solutions were then worked up with the complex isolated and characterized by 1H NMR, MS, and ATR-FTIR. No evidence of CO binding was detected, suggesting that product inhibition during CO2 reduction is not a distinguishing characteristic between these com-plexes. Calculated properties of the complexes were also considered (Table 5-3), including charge on the metal center, dz2 energy levels, and SOMO energies in the reduced species, but no clear trend relating electrocatalytic performance to properties or energy levels was discerned. It may be speculated that the dz2 orbital in [Ni] does not have sufficient axial spatial extent to overcome the minor steric hindrance of the pincer ligand upon a CO2 molecule, where the planar pyridyl and benzimidazol-2-ylidene donor groups, and their attached methylene linkers and alkyl substituents, extend out of the plane of the mole-cule due to a twist of approximately 45° (Figure 5-12). Related to this, the twist of the pyridyl and benzimidazol-2-ylidene donors may have a subtle effect on modulating the degree of π*-backdonation   169 from the metal as both pyridine and unsaturated NHCs are similarly π-acidic.79 As the donor is increas-ingly twisted out of the plane towards orthogonality, the π-overlap decreases and there is less π*-back-donation from the metal center.       Figure 5-12. Modeled [Ni] complex viewed side-on (left) and front-on (right), showing the twisted ori-entation of the pincer ligand.  Given the presence of two oxygen atoms in CO2 and the production of water alongside CO, another difference between the metal centers may lie in their oxophilicity, with Ni being the most oxophilic fol-lowed by Pt and then Pd.166 Similarly, the Ni-O, Pd-O, and Pt-O bond dissociation energies (BDEs) are 392±38, 234±29, and 347±34 kJ/mol, respectively, representing an anomalously low Pd-O bond strength among the Group 10 elements and late transition metals in general, with the exception of Ag (BDE = 213±84 kJ/mol).167 This may contribute to an effect where transitory end-on η1-O coordination of CO2 is more stabilized for Ni > Pt > Pd, leading to inhibited performance for CO2 reduction where η1-C coordi-nation is required.25 It may also be the case that the hardness of Ni does not allow for the forming and breaking of bonds with C (and O in off-cycle interactions) with the modest energies differences required for efficient catalytic turnover (Scheme 5-2).    170 Scheme 5-2. Putative Electrocatalytic Cycle for Reduction of CO2 with Pd(C^N^C) Complexes  5.4 Conclusions An isoelectronic and isostructural series of [M(bC^N^bC)X]OTf and [M(bC^N^bC)CH3CN](OTf)2 (where M = Ni, Pd, and Pt) complexes were synthesized, characterized, and investigated for their elec-trochemical reactivity with CO2. Controlled potential electrolysis experiments demonstrated the superior ability of [Pd] to reduce CO2 to CO in faradaic efficiencies up to 47% in the presence of trifluoroacetic acid, compared to [Pt] and [Ni] which showed only marginal production of CO, giving the trend [Pd] >> [Pt] > [Ni] for this series. This result is consistent with the calculated energy profiles of the coordination of CO2 to the one-electron reduced species, where only the Pd species showed a significantly exogonic activation of CO2, but does not straightforwardly correlate to metal center charge, MO geometry, or inhibition from product formation.   171 5.5 Experimental 5.5.1 General Unless otherwise specified, all reactions were performed under nitrogen using standard Schlenk tech-niques and solvents and reagents were used as received from commercial sources. N,N-Dimethylforma-mide (EMD Millipore Omnisolv®), distilled acetonitrile (Anachemia Accusolv™), and dimethyl sulfox-ide (ACS grade, VWR) were dried with and stored over 15% m/v 4Å molecular sieves.134,135 Magnesium perchlorate (Alfa Aesar), trifluoroacetic acid (Aldrich), and 99.998% carbon dioxide (Praxair) were used as received for electrochemical experiments. 1H NMR spectra were acquired using Bruker AV300 or AV400 spectrometers with chemical shifts referenced to residual solvent signals. Mass spectra were acquired using a Waters LC-MS ESI-MS, ex-cept for the acetonitrilo complexes which were acquired using a Waters Micromass LCT ESI-MS. IR spectra were collected using a PerkinElmer Frontier FT-IR Spectrometer with ATR attachment. Gaseous products were analyzed using an SRI Model 8610C gas chromatograph equipped with molecular-sieve columns, dual TCD and FID detectors, and methanizer. Crystallographic data was acquired using a Bruker X8 APEX II diffractometer with graphite monochromated Mo-Kα radiation. 5.5.2 Electrochemistry Electrochemical experiments were performed using a Metrohm Autolab PGSTAT12 or Pine AFCBP1 potentiostat in an air-tight three-electrode cell with a 7 mm2 glassy carbon working electrode (Bioana-lytical Systems, Inc.), Pt mesh counter electrode, and Ag wire pseudo-reference electrode in a 0.010 M AgNO3 acetonitrile solution separated from the bulk solution by a Vycor frit. Experiments were per-formed under N2 or CO2 using 2 mM concentrations of the complexes in 10 mL anhydrous electrolyte solution unless otherwise stated. Electrolyte solutions were 0.10 M triply recrystallized [n-Bu4N]PF6 in   172 anhydrous N,N-dimethylformamide and sparged with nitrogen prior to use. For experiments with CO2, the solution was sparged with CO2 for 15 minutes resulting in a concentration of ~0.20 M.27 Controlled potential experiments used glassy carbon rod (Alfa Aesar, 5 mm diameter) working electrodes in a two compartment H-cell, where the counter electrode compartment was separated from the compartment containing the working and reference electrodes by fritted glass. IR drop was compensated by correlation of the peak potentials between cyclic voltammograms at 100 mV/s with a 7 mm2 glassy carbon electrode and glassy carbon rod electrode. All glassy carbon electrodes were cleaned by successive polishing with 1 μM, 0.3 μM, and 0.05 μM alumina paste, following by rinsing with water, sonication (5 min) in distilled water, and sonication (5 min) in methanol. Decamethylferrocene was added at the end of electrochemical experiments as an internal standard, showing a reversible redox couple at -404 mV vs. Ag/AgNO3 and -476 mV vs. ferrocene/ferrocenium.136 Cyclic voltammograms were collected at a scan rate of 100 mV/s and square-wave voltammograms at a frequency of 25 Hz with 5 mV steps and 25 mV amplitude unless otherwise stated. 5.5.3 Computational Methods DFT calculations were performed using Gaussian 09 (Revision D.01) using the long range and dis-persion-corrected ωB97xD hybrid functional without symmetry constraints.111 Calculations for reduced species were performed using unrestricted, open-shell wavefunctions. The D95(d) basis set was used for all atoms except palladium, which employed the Stuttgart-Dresden-Bonn quasi-relativistic effective-core potential and corresponding correlation-consistent triple-ζ basis set.112,113 Calculations were performed with the presence of a solvent reaction field of acetonitrile produced by the conductor-like polarizable continuum model (C-PCM),137 except where indicated. Frequency calculations were performed on all geometry optimized structures to ensure that energy minima were achieved.   173 5.5.4 Synthesis Platinum(II) bromide (98%, Aldrich), dichloro(1,4-cyclooctadiene)palladium(II) (99%, Strem), di-chloro(dimethoxyethane)nickel(II) (98%, Aldrich), silver(I) triflate (99%, Aldrich), thallium(I) triflate (97%, Aldrich), benzimidazole (99%, Alfa Aesar), and 1-iodobutane (99%, Aldrich) were used as re-ceived for synthesis. 1-(n-Butyl)benzimidazole,168 2,6-bis(bromomethyl)pyridine,138 and 1,1'-(pyridine-2,6-diylbis(methylene))bis(3-butylimidazolium) dibromide (C^N^C ∙ 2HBr)127 were synthesized accord-ing to literature procedures.  Synthesis of Proligand 1,1'-(pyridine-2,6-diylbis(methylene))bis(3-butylbenzo[d]imidazolium) dibromide, bC^N^bC ∙ 2HBr To a solution of 1-(n-butyl)benzimidazole (450 mg, 2.58 mmol) in anhydrous 1,4-dioxane (40 mL) heated to reflux was added a solution of 2,6-bis(bromomethyl)pyridine (326 mg, 1.23 mmol) in 1,4-dioxane (10 mL) slowly over 4 h. The resulting solution was refluxed for 18 h, leading to the formation of an oily, tan-colored residue which solidified upon cooling. The precipitate was triturated with diethyl ether (2 × 10 mL), taken up in dichloromethane (5 mL), and re-precipitated by dropwise addition to diethyl ether (10 mL). The white precipitate was then isolated by centrifugation and dried under vacuum, yielding a hygroscopic fine white powder (660 mg, 88% yield). 1H NMR (400 MHz, (CD3)2SO)  0.93 (t, J=7.3 Hz, 6 H), 1.34 (sxt, J=7.5 Hz, 4 H), 1.84 (quin, J=7.3 Hz, 4 H), 4.46 (t, J=7.3 Hz, 3 H), 5.82 (s, 4 H), 7.39 - 7.47 (m, 2 H), 7.57 (d, J=8.4 Hz, 2 H), 7.60 - 7.69 (m, 4 H), 8.01 (t, J=7.9 Hz, 1 H), 8.05 (d, J=8.4 Hz, 2 H), 9.89 (s, 2 H). ESI-MS: m/z 534.3 ([M-Br]+).   174 Synthesis of Monocationic Halido Complexes [Ni(bC^N^bC)Cl]OTf To a solution of proligand bC^N^bC ∙ 2HBr (150 mg, 0.245 mmol) in dry acetonitrile (15 mL) was added silver(I) oxide (57.0 mg, 0.246 mmol) and 3 Å molecular sieves. The reaction mixture was covered in foil, heated to 55 °C, and left to gently stir for 18 h, resulting in the formation of a tan-colored solid. The mixture was cooled to room temperature and then silver triflate (63.0 mg, 0.245 mmol) was added, followed by dichloro(dimethoxyethane)nickel(II) (53.7 mg, 0.244 mmol). After stirring at 25 °C for 48 h, the mixture was centrifuged to remove the silver salts and sieves. The yellow supernatant solution was filtered through Celite, concentrated to dryness in vacuo, taken up in dichloromethane (5 mL), washed with water (2 × 5 mL), dried with magnesium sulfate, and then filtered through Celite into diethyl ether (10 mL) resulting in the formation of a yellow precipitate. The precipitate was isolated by centrifugation and dried in vacuo, leaving a yellow microcrystalline powder (144 mg, 85% yield). X-ray quality single crystals were grown by slow evaporation from a concentrated acetone solution. 1H NMR (400 MHz, CD2Cl2)  1.05 (t, J=7.4 Hz, 6 H), 1.40 - 1.51 (m, 2 H), 1.55 - 1.69 (m, 2 H), 1.96 - 2.16 (m, 4 H), 4.42 - 4.59 (m, 2 H), 4.90 - 5.07 (m, 2 H), 5.99 (d, J=15.4 Hz, 2 H), 6.41 (d, J=15.4 Hz, 2 H), 7.37 - 7.52 (m, 6 H), 7.80 - 7.88 (m, 4 H), 7.91 - 7.97 (m, 1 H). ESI-MS: m/z 544.1 ([M-OTf]+). Anal. Calcd for C30H33ClF3N5NiO3S ∙ 0.02C15H24O (BHT): C, 52.05; H, 4.83; N, 10.02; S, 4.58. Found: C, 52.37; H, 4.93; N, 9.76; S, 4.64. [Ni(C^N^C)Cl]OTf Synthesized as above but with the proligand C^N^C ∙ 2HBr. A yellow microcrystalline solid was isolated (77.4 mg, 45% yield). 1H NMR (400 MHz, CD2Cl2)  0.99 (t, J=7.4 Hz, 6 H), 1.32 - 1.49 (m, 4 H), 1.85 - 1.98 (m, 4 H), 4.08 - 4.22 (m, 2 H), 4.51 - 4.65 (m, 2 H), 5.53 (d, J=15.1 Hz, 2 H), 6.25 (d, J=14.9 Hz, 2 H), 6.85 (d, J=1.9 Hz, 2 H), 7.33 (d, J=1.9 Hz, 2 H), 7.70 (d, J=7.8 Hz, 2 H), 7.91 (t, J=7.7 Hz, 1 H). ESI-MS: m/z 444.5 ([M-OTf]+).   175 [Pd(bC^N^bC)Cl]OTf To a solution of proligand bC^N^bC ∙ 2HBr (106 mg, 0.172 mmol) in dimethyl sulfoxide (6 mL) was added silver(I) oxide (41.8 mg, 0.180 mmol) and 4 Å molecular sieves. The reaction mixture was covered in foil, heated to 55 °C, and left to gently stir for 18 h, resulting in the formation of a tan-colored solid. The mixture was cooled to room temperature and then silver triflate (45.0 mg, 0.175 mmol) was added, followed by dichloro(1,4-cyclooctadiene)palladium(II) (49.1 mg, 0.172 mmol). After stirring at 25 °C for 72 h, the mixture was centrifuged to remove the silver salts and sieves. The solvent was removed by vacuum distillation at 50 °C and the residue taken up in dichloromethane (3 mL), washed with water (2 × 3 mL), dried with magnesium sulfate, and then filtered through Celite into diethyl ether (10 mL) re-sulting in the formation of a pale yellow precipitate. The precipitate was sonicated in diethyl ether, then isolated and dried in vacuo to yield a fine, off-white powder (88 mg, 69% yield). 1H NMR (400 MHz, CD2Cl2)  1.01 (t, J=7.4 Hz, 6 H), 1.37 - 1.57 (m, 4 H), 1.98 (quin, J=7.6 Hz, 4 H), 4.59 (dt, J=14.1, 7.0 Hz, 2 H), 5.04 (dt, J=14.0, 7.9 Hz, 2 H), 5.78 (d, J=15.4 Hz, 2 H), 5.94 (d, J=15.4 Hz, 2 H), 7.42 - 7.53 (m, 4 H), 7.53 - 7.58 (m, 2 H), 7.85 - 7.92 (m, 2 H), 7.99 - 8.05 (m, 2 H), 8.05 - 8.12 (m, 1 H). ESI-MS: m/z 594.0 ([M-OTf]+). Anal. Calcd for C30H33ClF3N5PdO3S: C, 48.53; H, 4.48; N, 9.43; S, 4.32. Found: C, 48.74; H, 4.48; N, 9.10; S, 4.69. [Pt(bC^N^bC)Br]OTf To a solution of proligand bC^N^bC ∙ 2HBr (135 mg, 0.220 mmol) in dimethyl sulfoxide (6 mL) was added silver(I) oxide (53.5 mg, 0.231 mmol) and 4 Å molecular sieves. The reaction mixture was covered in foil, heated to 55 °C, and left to gently stir for 18 h, resulting in the formation of a tan-colored solid. The mixture was cooled to room temperature and then platinum(II) bromide (78.1 mg, 0.220 mmol), silver triflate (57.0 mg, 0.222 mmol), and additional dimethyl sulfoxide (6 mL) were added. The mixture was warmed to 40 °C and left to stir for 72 h, followed by centrifugation to remove the silver salts and sieves, filtration through Celite, and removal of the solvent by vacuum distillation at 50 °C. The residue   176 was taken up in dichloromethane (2 mL), washed with water (2 × 2 mL), dried with magnesium sulfate, and then filtered through Celite into diethyl ether (5 mL) resulting in the formation of a sticky orange precipitate. The precipitate was taken up in a minimum of dichloromethane and again dropped into di-ethyl ether (5 mL) with this process repeated twice, resulting in an off-white powder which was dried in vacuo to yield the crude product (110 mg). The crude product was purified by column chromatography (SiO2, 85:10:5 CH3CN:H2O:KNO3(aq)) with the first fraction collected. The solvent was removed in vacuo with mild heating and the residue taken up in a saturated methanol solution of potassium triflate. After stirring at room temperature for 2 h, the solvent was removed by rotary evaporation and the residue extracted with dichloromethane (2 mL), filtered, washed with water (2 × 2 mL), dried with magnesium sulfate, filtered, and concentrated to dryness in vacuo, yielding a pale tan powder (52.9 mg, 27% yield). 1H NMR (400 MHz, CD2Cl2)  0.99 (t, J=7.4 Hz, 6 H), 1.37 - 1.51 (m, 4 H), 2.00 (quin, J=7.6 Hz, 4 H), 4.59 (dt, J=13.9, 7.2 Hz, 2 H), 4.99 (dt, J=14.0, 8.0 Hz, 2 H), 5.60 (d, J=15.2 Hz, 2 H), 5.77 (d, J=15.2 Hz, 2 H), 7.43 - 7.53 (m, 4 H), 7.54 - 7.60 (m, 2 H), 7.85 (m, J=7.3 Hz, 2 H), 7.97 (d, J=7.8 Hz, 2 H), 8.09 (t, J=7.7 Hz, 1 H). 195Pt NMR (300 MHz, CD3CN)  ESI-MS: m/z 726.0 ([M-OTf]+). Anal. Calcd for C30H33BrF3N5PtO3S ∙ 0.05C23H48 (grease) ∙ 0.05 C15H24O (BHT): C, 42.43; H, 4.09; N, 7.76; S, 3.55. Found: C, 42.59; H, 4.28; N, 7.49; S, 3.23. Synthesis of Dicationic Acetonitrilo Complexes [Ni(bC^N^bC)(CH3CN)](OTf)2 Thallium triflate (8.2 mg, 0.0232 mmol) [CAUTION: Thallium compounds are extremely toxic and must be handled carefully] in acetonitrile (1.5 mL) was slowly added to a stirring solution of [Ni(bC^N^bC)Cl]OTf (16.2 mg, 0.0230 mmol) in acetonitrile (1.5 mL), resulting in the immediate for-mation of a white precipitate. The reaction mixture was left to stir at room temperature for 2 h, filtered, and concentrated to an oily yellow residue in vacuo. The oil was covered with diethyl ether and sonicated for 2 minutes and the ether decanted, followed by further drying in vacuo which resulted in foaming and   177 solidification of the yellow oil into a hygroscopic yellow solid (19.7 mg, 99% yield). 1H NMR (400 MHz, CD3CN)  0.99 (t, J=7.4 Hz, 6 H), 1.51 (sxt, J=7.5 Hz, 4 H), 1.96 (s, 3 H), 2.02 (quin, J=7.5 Hz, 4 H), 4.52 (t, J=7.4 Hz, 4 H), 6.17 (br. s., 4 H), 7.46 - 7.56 (m, 4 H), 7.71 (d, J=7.9 Hz, 2 H), 7.77 (d, J=7.8 Hz, 2 H), 7.89 (d, J=7.9 Hz, 2 H), 7.98 (t, J=7.7 Hz, 1 H). ESI-MS: m/z 658.0 ([M-OTf-CH3CN]+), 275.2 ([M-2(OTf)]2+), 254.7 ([M-2(OTf)-CH3CN]2+).  [Pd(bC^N^bC)(CH3CN)](OTf)2 A solution of silver triflate (9.2 mg, 0.0358 mmol) in acetonitrile (1 mL) was slowly added to a solu-tion of [Pd(bC^N^bC)Cl]OTf (25.2 mg, 0.0343 mmol) in acetonitrile (2 mL), resulting immediately in the formation of a white precipitate. The reaction mixture was left to stir behind foil at room temperature for 2 h before being centrifuged, decanted, concentrated by rotary evaporation, and filtered into diethyl ether (3 mL) where an off-white powder precipitated. The precipitate was collected by centrifugation, taken up in a minimum of dichloromethane and re-precipitated by addition of diethyl ether, isolated, and then washed again with diethyl ether before being dried in vacuo, leaving an off-white powder (26.0 mg, 85% yield). 1H NMR (400 MHz, CD3CN)  0.97 (t, J=7.4 Hz, 6 H), 1.33 - 1.55 (m, 4 H), 1.81 - 1.91 (m, 2 H), 1.96 (s, 1 H), 1.97 - 2.08 (m, 2 H), 4.41 - 4.52 (m, 2 H), 4.55 - 4.65 (m, 2 H), 5.75 (d, J=15.7 Hz, 2 H), 5.93 (d, J=15.7 Hz, 2 H), 7.50 - 7.60 (m, 4 H), 7.74 - 7.79 (m, 2 H), 7.89 - 7.94 (m, 4 H), 8.11 (t, J=7.6 Hz, 1 H). ESI-MS: m/z 706.1 ([M-OTf-CH3CN]+), 298.8 ([M-2(OTf)]2+), 278.6 ([M-2(OTf)-CH3CN]2+). [Pt(bC^N^bC)(CH3CN)](OTf)2 A solution of silver triflate (6.9 mg, 0.0269 mmol) in acetonitrile (0.5 mL) was slowly added to a solution of [Pt(bC^N^bC)Br]OTf (17.3 mg, 0.0198 mmol) in 1:1 acetonitrile/dichloromethane (2 mL). A small amount of a fine white precipitate was observed after 10 minutes. The reaction mixture was heated to 30 °C and left to stir behind foil for 3 h, after which a greater quantity of fine white precipitate was suspended in solution. The mixture was further heated to 40 °C for an additional 3 h during which   178 time more precipitate was formed and the precipitate aggregated, cleanly settling out of solution. The mixture was then centrifuged, decanted, concentrated by rotary evaporation, and filtered into diethyl ether (2 mL) where pale tan precipitated formed. The precipitate was collected by centrifugation, taken up in a minimum of dichloromethane and re-precipitated by addition of diethyl ether, isolated, and then washed again with diethyl ether before being dried in vacuo, leaving a pale tan powder (19.5 mg, quant. yield). 1H NMR (400 MHz, CD3CN)  0.95 (t, J=7.4 Hz, 6 H), 1.32 - 1.50 (m, 4 H), 1.83 - 1.91 (m, 2 H), 1.97 - 2.05 (m, 2 H), 4.40 - 4.50 (m, 2 H), 4.50 - 4.60 (m, 2 H), 5.53 (d, J=15.7 Hz, 2 H), 5.83 (d, J=15.7 Hz, 2 H), 7.56 (quind, J=7.6, 7.6, 7.6, 7.6, 1.3 Hz, 4 H), 7.76 - 7.81 (m, 2 H), 7.88 - 7.95 (m, 4 H), 8.11 (t, J=7.9 Hz, 1 H). ESI-MS: m/z 795.2 ([M-OTf-CH3CN]+), 343.6 ([M-2(OTf)]2+), 323.1 ([M-2(OTf)-CH3CN]2+).     179 Chapter 6: Conclusions and Future Work 6.1 Conclusions A series of four bis-NHC palladium pincer complexes, with pyridine- or lutidine-linked imidazol-2-ylidene or benzimidazol-2-ylidene NHCs, were electrochemically characterized with the lutidine-linked complexes [Pd(C^N^C)Cl]+ and [Pd(bC^N^bC)Cl]+ established as homogeneous electrocatalysts for the reduction of CO2 to CO in the presence of strong acids such as TFA. The one-electron reductions of [Pd(C^N^C)Cl]+ and [Pd(bC^N^bC)Cl]+ were shown to be chemically reversible and, due to charge de-localization onto the pyridyl moiety, avoid decomposition into inert bimetallic species thereby mitigating the primary deactivation pathway limiting the TON of palladium triphosphine electrocatalysts. Computational studies of the initial interaction of CO2 with reduced species of Re(bpy)(CO)3Cl, [Pd(triphosphine)(CH3CN)]2+, and DFT(C^N^C) correctly predicted the proton source requirements of the electrocatalytic systems by correlation to the degree of CO2 activation and gave insight into charge transfer dynamics of the reduced species with CO2. The reduced species of DFT(C^N^C) exhibited charge transfer from the redox-active ligand to CO2, a characteristic important to the reactivity of Re(bpy)(CO)3Cl and other related homogeneous CO2 reduction electrocatalysts. Selectivity for reduction of CO2 over H+ with [Pd(C^N^C)Cl]+ was improved by up to a factor of 3 (to faradaic efficiencies up to 47%) by the addition of the CO2-adduct-stabilizing cations K+, Mg2+, and [BMIM]+, as well as by anod-ically shifting the first reduction potential of the complex by synthesis of the dicationic species [Pd(C^N^C)(CH3CN)]2+. This work has demonstrated the capacity of lutidine-linked bis-NHC pincer complexes for CO2 reduction electrocatalysis, thus expanding the palladium pincer motif of CO2 reduc-tion electrocatalysts to include strongly electron donating redox-active ligands. Phenanthro- and pyreno-annulated bis-NHC palladium pincer complexes [Pd(phC^N^phC)Cl]OTf and [Pd(pyC^N^pyC)Cl]OTf were synthesized and found to have similar reduction potentials to   180 [Pd(bC^N^bC)Cl]+. Upon reduction, these complexes are able to reduce CO2 to CO in the presence of TFA. [Pd(phC^N^phC)Cl]OTf demonstrated appreciably improved selectivity for CO2 reduction at the first reduction potential in comparison to [Pd(C^N^C)Cl]+ and [Pd(bC^N^bC)Cl]+, whereas [Pd(pyC^N^pyC)Cl]OTf displayed lower relative selectivity, showing that electronic modifications to the NHC donors can drastically affect reactivity. The solvento complexes were found to be more quickly deactivated during electrolysis than their non-annulated counterparts. The addition of polyaromatic groups enabled additional redox-activity on the ligand at −2.96 V for [Pd(phC^N^phC)Cl]+ and −2.56 V for [Pd(pyC^N^pyC)Cl]+, allowing for increased electron donation from the ligand. The addition of the weak acid TFE results in reactivity with CO2 at these potentials, but controlled potential electrolyses are short-lived and result in the formation of an insulating and insoluble species on the electrode. These systems were also computationally modelled, providing support for assignments of the redox events, giving greater understanding of the chemical behaviour of the reduced species, and highlighting the im-portance of anion dissociation energies within the pincer motif. The electronic effect of modifying the pyridyl para position of lutidyl-linked bis-NHC Pd pincer com-plexes was also investigated, exhibiting a strong and consistent effect on the first reduction potential of the complex in relation to the Hammett σp value of the para substituent, consistent with DFT analysis of the pyridyl-localized nature of the LUMO. The presence of a sufficiently strong EW group, R = COOR, resulted in electrochemical reversibility of the first reduction potential at high scan rates supporting the assignment of an EC event where ligand dissociation occurs upon reduction. The effect of the para sub-stituents on electronic properties of the metal center and ligand dissociation in the reduced species was quantified by DFT modelling, showing the energy barrier to dissociation to be strongly dependent on the σp value of the substituent. Dissociation was found to be spontaneous in the singly-reduced species with ED groups and significantly hindered with moderately strong EW groups, having implications for initial reactivity with CO2 and continued activity where coordinating anions (conjugate base), water, and CO   181 are produced during the catalytic cycle. The presence of an ED group, R = OMe, was found to improve CO2 reduction relative to R = H when operating in a regime of initial one-electron reduction of the com-plex, but the presence of a moderately strong EW group, R = COOR, also improved CO2 reduction per-formance relative to R = H when operating in a regime of two-electron reduction, which occurs at similar potentials. This raises the question of whether this redox-active pincer framework is most effective with a strongly ED or strongly EW para substituent. In either case, the electrochemical properties of the C^N^C pincer motif have been better understood, providing an extendable platform where reduction potentials can be predicted and tuned over a broad range and additional functionalities can be incorpo-rated into the pyridyl, NHC, and/or NHC N-substituents. Finally, a valence isoelectronic and isostructural series of [M(bC^N^bC)X]OTf and [M(bC^N^bC)CH3CN](OTf)2 (where M = Ni, Pd, and Pt) were synthesized, characterized, and investi-gated for their electrochemical reactivity with CO2. Controlled potential electrolysis experiments demon-strated the superior ability of [Pd] to reduce CO2 to CO in faradaic efficiencies up to 58%, compared to [Pt] and [Ni] which showed only marginal production of CO, giving the trend [Pd] >> [Pt] > [Ni] for this series.   Figure 6-1. General design of lutidyl-linked bis-NHC pincer complexes, showing modular components (pyridyl, NHC groups, metal center, monodentate ligand, and NHC N-substituents).    182 Overall, the work presented in this thesis has explored the electrochemistry of lutidyl-linked bis-NHC pincer complexes and established these as homogeneous electrocatalysts for the reduction of CO2 to CO, thus refreshing the pincer motif with an extensible, redox-active ligand platform. Three key components of the modular design for these species were investigated, namely modification of the NHC groups, the pyridyl, and the metal center, thereby laying the groundwork for how these components affect the elec-trochemical properties of the complex and providing synthetic pathways for future modifications. The combination of optimized features, such as benz- or phenanthro-annulated NHCs with a para-OMe pyridyl substituent, for example, would likely yield further improved performance, with additional im-provements possible through modification of other components in the system (Figure 6-1). 6.2 Future Work With a foundation laid for a new class of CO2 reducing electrocatalysts bearing redox active pincer ligands, the extension to homo- and hetero-bimetallic systems through a para-pyridyl bridge or addition of ligands as imidazole N-substituents, and investigation of potential cooperative effects, should be pur-sued, with preliminary work begun on pyridyl-bridged species with alkyl ether and ferrocenyl bridges (Figure 6-2).  Figure 6-2. Prospective bimetallic complexes through modification of the NHC N-substituents to include additional donors (left), and addition of a pyridyl-linked bridge (center and right).    183 Although the use of Ni and Pt as metal centers led to inferior activity relative to Pd, the incorporation of Group 9 metals in the +1 oxidation state, Co(I), Rh(I), and Ir(I) as isoelectronic d8 complexes should modulate ligand-metal bonding and increase the energy levels of the d-orbitals relative to the Group 10 species, which may potentially lead to improved activity for CO2 reduction. Preliminary computational modelling has suggested that an isoelectronic Rh species may have dxz, dyz, and dz2 orbitals which are higher in energy by 2-3 eV and activate CO2 to a greater extent than Pd (Figure 6-3).  Figure 6-3. Preliminary DFT-modelling (ωB97xD/D95(d)/SDD, CPCM with CH3CN solvent) of inter-action between CO2 and isoelectronic one-electron reduced species [Rh(C^N^C)Cl]- and [Pd(C^N^C)Cl]0. Dashed lines represent the CO2 bond angle at each distance increment.  Following the work of incorporating strongly reducing electron reservoirs into the NHC moiety of the ligand, the incorporation of moderately reducing electron reservoirs may also be pursued, such as bio-logically-inspired quinone-annulated NHCs which are able store multiple redox equivalents between po-tentials of approximately -0.5 V to -1.7 V vs Fc0/+.169,170 Preliminary calculations support this assessment, where the LUMO and LUMO+1 of a model complex with quinone-annulated NHCs are quinone-local-ized, followed by the LUMO+3 which is pyridyl-localized (Figure 6-4). Besides the storage of redox 1.75 2.00 2.25 2.50 2.75 3.00 3.25 3.50-10-505101520 137.3°131.9°  Pd   RhRelative Energy (kcal/mol)M-CO2 Distance (Å)120130140150160170180CO2 Bond Angle (°)  184 equivalents, quinoidal reduction may also enable intramolecular proton transfer from a local hydroqui-none proton source.  molecular geometry  HOMO  LUMO  LUMO+1  LUMO+2  LUMO+3 Figure 6-4. Calculated solution-state molecular and MO geometries for a naphthoquinone-annulated NHC species.  Relatedly, the pincer architecture allows modification of the imidazolylidene N-substituent, where, in the context of CO2 reduction, only n-butyl chains have to this point been used (improving solubility and making the species less hygroscopic and therefore easier to handle). These substituents may be modified to include precisely positioned proton donors/transfer sites for protonation of the activated M-CO2 inter-mediate, thereby assisting catalysis. 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C., Ed.; Vol. C.; Kluwer Academic Publishers: Boston, 1992.      193 Appendix A:  NMR Data  Figure A-1. 1H NMR spectrum of [Pd(C^N^C)Cl]BF4 in (CD3)2SO. Residual solventa and waterb indi-cated.  Figure A-2. 1H NMR spectrum of [Pd(bC^N^bC)Cl]BF4 in (CD3)2SO. Residual solventa and waterb indicated.  a b a b Frequency (MHz):  400.13 Temperature (K): 298.1 Frequency (MHz):  400.13 Temperature (K): 298.1   194  Figure A-3. 1H NMR spectrum of [Pd(C-N-C)Br]Br in (CD3)2SO. Residual solventa and waterb indi-cated.   Figure A-4. 1H NMR spectrum of [Pd(bC-N-bC)Br]Br in CD2Cl2. Residual solventa and waterb indi-cated.   a b a b Frequency (MHz):  300.13 Temperature (K): 297.7 Frequency (MHz):  300.13 Temperature (K): 297.3    195  Figure A-5. 1H NMR spectrum of [Pd(C^N^C)(CH3CN)](BF4)2 in (CD3)2SO. Residual solventa and wa-terb indicated.   Figure A-6. 1H NMR spectrum of [Pd(C-N-C)(CH3CN)]BF4 in CD3OD. Residual solventa and waterb indicated. a b a b Frequency (MHz):  400.13 Temperature (K): 298.1 Frequency (MHz):  300.13 Temperature (K): 297.7   196  Figure A-7. 1H NMR spectrum of 1-butyl-1H-phenanthro[9,10-d]imidazole in (CD3)2CO. Residual sol-venta and water signalsb indicated.  Figure A-8. 1H NMR spectrum of 9-butyl-9H-pyreno[4,5-d]imidazole in (CD3)2SO. Residual solventa, waterb, and acetonitrilec signals are indicated.  b b a c a Frequency (MHz):  300.13 Temperature (K): 298.2 Frequency (MHz):  300.13 Temperature (K): 297.8   197  Figure A-9. 1H NMR spectrum of phC^N^phC ∙ 2HBr in (CD3)2SO. Residual solventa and waterb signals are indicated.  Figure A-10. 1H NMR spectrum of pyC^N^pyC ∙ 2HBr in (CD3)2SO. Residual solventa and waterb sig-nals are indicated. a b a b Frequency (MHz):  300.13 Temperature (K): 298.2 Frequency (MHz):  300.13 Temperature (K): 298.9   198  Figure A-11. 1H NMR spectrum of [Pd(phC^N^phC)Cl]OTf in (CD3)2SO. Residual solventa and waterb signals indicated. Singlet at 1.15 ppmc is due to an impurity in as-received NMR solvent.  Figure A-12. 1H NMR spectrum of [Pd(pyC^N^pyC)Cl]OTf in (CD3)2SO. Residual solventa and waterb signals indicated. Singlet at 1.15 ppmc is due to an impurity in as-received NMR solvent.  a b a b c c Frequency (MHz):  400.20 Temperature (K): 298.9 Frequency (MHz):  400.20 Temperature (K): 298.7   199  Figure A-13. 1H NMR spectrum of [Pd(phC^N^phC)(CH3CN)](OTf)2 in CD3CN. Inset: 1H resonance for CH3CN at 1.96 ppm. Residual solventa and waterb signals indicated.  Figure A-14. 1H NMR spectrum of [Pd(pyC^N^pyC)(CH3CN)](OTf)2 in CD3CN. Inset: 1H resonance for CH3CN at 1.96 ppm. Residual solventa and waterb signals indicated. a a b a a b Frequency (MHz):  400.20 Temperature (K): 298.8 Frequency (MHz):  400.20 Temperature (K): 299.4   200  Figure A-15. 1H NMR spectrum of C^N^Cp-OMe ∙ 2HBr in (CD3)2SO. Residual solventa and waterb indi-cated.   Figure A-16. 1H NMR spectrum of [Pd(C^N^Cp-OMe)Cl]OTf in CD3CN. Residual solventa and waterb indicated.  a b b a Frequency (MHz):  300.13 Temperature (K): 298.2 Frequency (MHz):  400.20 Temperature (K): 298.0   201  Figure A-17. 1H NMR spectrum of [Pd(C^N^Cp-OMe)(CH3CN)](OTf)2 in CD3CN. Residual solventa and waterb indicated.    Figure A-18. 1H NMR spectrum of C^N^Cp-Br ∙ 2HBr in CD2Cl2. Residual solventa and waterb indicated.  a b b a Frequency (MHz):  400.20 Temperature (K): 298.2 Frequency (MHz):  400.20 Temperature (K): 298.5   202  Figure A-19. 1H NMR spectrum of [Pd(C^N^Cp-Br)Cl]OTf in CD3CN. Residual solventa and waterb in-dicated.  Figure A-20. 1H NMR spectrum of [Pd(C^N^Cp-Br)(CH3CN)](OTf)2 in CD3CN. Residual solventa and waterb indicated.  b a b a Frequency (MHz):  400.20 Temperature (K): 298.4 Frequency (MHz):  400.20 Temperature (K): 298.3   203  Figure A-21. 1H NMR spectrum of 4-hydroxybutyl 2,6-bis(bromomethyl)isonicotinate in (CD3)2CO. Residual solventa and waterb indicated.   Figure A-22. 1H NMR spectrum of C^N^Cp-COOR ∙ 2HBr in (CD3)2SO. Residual solventa, waterb, and diethyl etherc indicated.102 a b a b c c Frequency (MHz):  400.20 Temperature (K): 299.0 Frequency (MHz):  400.20 Temperature (K): 298.4   204  Figure A-23. 1H NMR spectrum of [Pd(C^N^Cp-COOR)Cl]OTf in (CD3)2CO. Residual solventa and sili-cone greaseb indicated. Water signal suppressed by W5 pulse sequence.171  Figure A-24. 1H NMR spectrum of bC^N^bC ∙ 2HBr in (CD3)2SO. Residual solventa and waterb indi-cated. a b a b Frequency (MHz):  400.20 Temperature (K): 298.5 Frequency (MHz):  400.20 Temperature (K): 298.0   205  Figure A-25. 1H NMR spectrum of [Ni(bC^N^bC)Cl]OTf in CD2Cl2. Residual solventa, waterb, and BHTc indicated.  Figure A-26. 1H NMR spectrum of [Ni(bC^N^bC)(CH3CN)](OTf)2 in CD3CN. Residual solventa, wa-terb, and BHTc indicated.   a b c c c c a b c c c c Frequency (MHz):  400.20 Temperature (K): 298.2 Frequency (MHz):  400.20 Temperature (K): 298.5   206  Figure A-27. 1H NMR spectrum of [Ni(C^N^C)Cl]OTf in CD2Cl2. Residual solventa and waterb indi-cated.  Figure A-28. 1H NMR spectrum of [Pd(bC^N^bC)Cl]OTf in CD2Cl2. Residual solventa and waterb in-dicated. a b a b Frequency (MHz):  400.20 Temperature (K): 298.2 Frequency (MHz):  400.20 Temperature (K): 298.7   207  Figure A-29. 1H NMR spectrum of [Pd(bC^N^bC)(CH3CN)](OTf)2 in CD3CN. Residual solventa and waterb indicated.  Figure A-30. 1H NMR spectrum of [Pt(bC^N^bC)Br]OTf in CD3CN. Residual solventa and waterb indi-cated.   a b a b Frequency (MHz):  400.20 Temperature (K): 298.0 Frequency (MHz):  400.20 Temperature (K): 298.1   208  Figure A-31. 195Pt NMR spectrum of [Pt(bC^N^bC)Br]OTf in CD3CN.  Figure A-32. 1H NMR spectrum of [Pt(bC^N^bC)(CH3CN)](OTf)2 in CD3CN. Residual solventa and waterb indicated. a b Frequency (MHz):  64.52 Temperature (K): 298.2 Frequency (MHz):  400.20 Temperature (K): 298.5   209 DOSY and Variable Temperature NMR Data  Figure A-33. Stacked 1H NMR spectra of [Pd(phC^N^phC)Cl]OTf, [Pd(pyC^N^pyC)Cl]OTf, and [Pd(bC^N^bC)Cl]BF4 in (CD3)2SO as magnetic gradient strength is varied.   Exponential Fitting, I = I0*exp(-D*Q): [Pd(bC^N^bC)Cl]BF4 I0=8971; D=1.66E-6 cm2/s; Error: 1.8E-8 I0=9130; D=1.61E-6 cm2/s; Error: 1.0E-8 [Pd(phC^N^phC)Cl]OTf I0=5686; D=1.30E-6 cm2/s; Error: 7.2E-9 I0=6118; D=1.31E-6 cm2/s; Error: 1.2E-8 [Pd(pyC^N^pyC)Cl]OTf I0=4955;  D=1.10E-6 cm2/s; Error: 4.1E-9 I0=4629; D=1.16E-6 cm2/s; Error: 6.2E-9 Figure A-34. Plot of monoexponentially fit DOSY data, yielding diffusion rates.  0 10 20 30 40 500200040006000800010000 [Pd(bC^N^bC)Cl]BF4        (  fit) [Pd(phC^N^phC)Cl]OTf    (  fit) [Pd(pyC^N^pyC)Cl]OTf    (  fit)Integration UnitsG/cm  210  Figure A-35. VT 1H NMR spectrum of [Pd(bC^N^bC)Cl]BF4 in (CD3)2SO.     211 Appendix B:  FTIR Data  Figure B-1. ATR-FTIR spectra of free CH3CN, [Pd(C^N^C)Cl]BF4, and [Pd(C^N^C)(CH3CN)](BF4)2, revealing a shifted nitrile stretch at 2342 cm-1.  Figure B-2. ATR-FTIR spectrum of [Pd(phC^N^phC)Cl]OTf.  7501000125015001750200022502500275030003250350020406080100Transmittance (%)Wavenumber (cm-1) Acetonitrile [Pd(C^N^C)Cl]BF4 [Pd(C^N^C)(CH3CN)]BF42000220024002600  212  Figure B-3. ATR-FTIR spectrum of [Pd(phC^N^phC)(CH3CN)](OTf)2.   Figure B-4. ATR-FTIR spectrum of [Pd(pyC^N^pyC)Cl]OTf.    213  Figure B-5. ATR-FTIR spectrum of [Pd(pyC^N^pyC)(CH3CN)](OTf)2.   Figure B-6. ATR-FTIR spectrum of [Pd(C^N^Cp-OMe)Cl]OTf.      214  Figure B-7. ATR-FTIR spectrum of [Pd(C^N^Cp-OMe)(CH3CN)](OTf)2.    Figure B-8. ATR-FTIR spectrum of [Pd(C^N^Cp-Br)Cl]OTf.    215  Figure B-9. ATR-FTIR spectrum of [Pd(C^N^Cp-Br)(CH3CN)](OTf)2.  Figure B-10. ATR-FTIR spectrum of [Ni(bC^N^bC)Cl]OTf.  75010001250150017502000225025002750300032503500405060708090100Transmittance (%)Wavenumber (cm-1)  216  Figure B-11. ATR-FTIR spectrum of [Ni(bC^N^bC)(CH3CN)](OTf)2.   Figure B-12. ATR-FTIR spectrum of [Pd(bC^N^bC)Cl]OTf.  75010001250150017502000225025002750300032503500102030405060708090100Transmittance (%)Wavenumber (cm-1)200022502500750100012501500175020002250250027503000325035005060708090100Transmittance (%)Wavenumber (cm-1)  217  Figure B-13. ATR-FTIR spectrum of [Pd(bC^N^bC)(CH3CN)](OTf)2.  Figure B-14. ATR-FTIR spectrum of [Pt(bC^N^bC)Br]OTf. 7501000125015001750200022502500275030003250350060708090100Transmittance (%)Wavenumber (cm-1)2100220023002400250075010001250150017502000225025002750300032503500708090100Transmittance (%)Wavenumber (cm-1)  218  Figure B-15. ATR-FTIR spectrum of [Pt(bC^N^bC)(CH3CN)](OTf)2.     7501000125015001750200022502500275030003250350060708090100Transmittance (%)Wavenumber (cm-1)21002200230024002500  219 Appendix C:  Electrochemical Data Rotating Disk Electrode Voltammetry Levich equation:  𝐼𝐿 = 0.62 𝑛 𝐹 𝐴 𝐷2/3 𝜔1/2 𝜈−1/6 𝐶 where 𝐼𝐿 is the limiting current (A), n is the number of electrons transferred in the half-reaction (mol-1), F is Faraday’s constant (C/mol), A is the elec-trode area (cm2), D is the diffusion coefficient (cm2/s), ω is the angular rotation rate of the electrode (rad/s), ν is the kinematic viscosity of the so-lution (cm2/s), and C is concentration of the species investigated (mol/cm3)  Concentration:   2 mM of [Pd(C^N^C)Cl]BF4 Kinematic viscosity:  0.008971 cm2/s for 0.1M Et4NClO4 in DMF172 Scan rate:    20 mV/s  Epc1 = -1.77 V   Limiting Current (A) RPM ω1/2 0.000094 300 5.60 0.000129 600 7.93 0.000159 900 9.71 0.000185 1200 11.21 0.000200 1500 12.53 0.000216 1800 13.73 Epa1 = +0.60 V after exhaustive one-electron reduction   Limiting Current (A) RPM ω1/2 0.000122 300 5.60 0.000177 600 7.93 0.000217 900 9.71 0.000250 1200 11.21 0.000279 1500 12.53 0.000305 1800 13.73 y = 1.521E-05x + 9.863E-06R² = 9.965E-015.0E-051.0E-041.5E-042.0E-042.5E-040 5 10 15Limiting Current (A)(angular rotation rate)1/2y = 2.242E-05x - 1.829E-06R² = 9.997E-015.0E-051.5E-042.5E-043.5E-040 5 10 15Limiting Current (A)(angular rotation rate)1/2  220 Plots of Peak Current vs. (Scan Rate)1/2 According to the Randles-Sevcik equation, the peak current of a freely diffusing electroactive species in a redox event will be proportional to the square root of the scan rate. Linear plots of background-corrected peak current vs. the square root of the scan rate provide evidence for a freely diffusing species in solution.28 These plots are provided for [Pd(C^N^C)Cl]BF4, [Pd(bC^N^bC)Cl]BF4, [Pd(C-N-C)Br]Br, [Pd(phC^N^phC)Cl]OTf, [Pd(pyC^N^pyC)Cl]OTf, and [Pd(C^N^Cp-R)Cl]OTf (R = OMe, Br, COOR). In each case, the plots are linear. For all plots below, the y-axes represent peak current (μA) and the x-axes represent the square root of the scan rate (mV1/2 s-1/2). [Pd(bC^N^bC)Cl]BF4: Epc1 (N2) [Pd(bC^N^bC)Cl]BF4: Epc1 (CO2)   [Pd(bC^N^bC)Cl]BF4: Epc2 (CO2) [Pd(bC^N^bC)Cl]BF4: Epa1 (CO2)   [Pd(C^N^C)Cl]BF4: Epc1 (N2) [Pd(C^N^C)Cl]BF4: Epc2 (N2)   R² = 0.9999702040605 12.5 20 27.5R² = 0.999380204060800 10 20 30 40R² = 0.9985702040600 10 20 30 40R² = 0.9648505101520250 10 20 30 40R² = 0.999520204060800 10 20 30R² = 0.999870204060800 10 20 30  221 [Pd(C^N^C)Cl]BF4: Epa1 (N2) [Pd(C^N^C)Cl]BF4: Epc1 (CO2)   [Pd(C^N^C)Cl]BF4: Epc2 (CO2) [Pd(C-N-C)Br]Br: Epc1 (1.3 mM; N2)   [Pd(C-N-C)Br]Br: Epc2 (1.3 mM; N2) [Pd(C-N-C)Br]Br: Epa1 (1.3 mM; N2)   [Pd(bC-N-bC)Br]Br: Epc1 (N2) [Pd(bC-N-bC)Br]Br: Epc2 (N2)      R² = 0.995720510152025300 10 20 30R² = 0.9993502550751000 10 20 30R² = 0.994840204060801000 10 20 30R² = 0.99979010203040500 10 20 30R² = 0.998820102030400 10 20 30R² = 0.99997010203040500 10 20 30R² = 0.999920102030400 10 20 30 40R² = 0.9998601020304050600 10 20 30 40  222 [Pd(bC-N-bC)Br]Br: Epc1 (CO2) [Pd(bC-N-bC)Br]Br: Epc2 (CO2)   [Pd(pyC^N^pyC)Cl]OTf: Epc1 (N2) [Pd(pyC^N^pyC)Cl]OTf: Epc2 (N2)   [Pd(pyC^N^pyC)Cl]OTf: Epc3 (N2) [Pd(pyC^N^pyC)Cl]OTf: Epc3 (CO2)   [Pd(phC^N^phC)(CH3CN)](OTf)2: Epc1 (N2) [Pd(phC^N^phC)(CH3CN)](OTf)2: Epc2 (N2)     R² = 0.997830102030400 10 20 30 40R² = 0.9990002040600 10 20 30 40R² = 0.999110204060800 10 20 30 40R² = 0.9997102550751001250 10 20 30 40R² = 0.999020501001502000 10 20 30 40R² = 0.99939040801201600 10 20 30R² = 0.999420153045600 10 20 30 40R² = 0.999410204060800 10 20 30 40  223 [Pd(phC^N^phC)(CH3CN)](OTf)2: Epc3 (N2) [Pd(phC^N^phC)(CH3CN)](OTf)2: Epc4 (N2)   [Pd(phC^N^phC)(CH3CN)](OTf)2: Epc2 (CO2) [Pd(phC^N^phC)(CH3CN)](OTf)2: Epc3 (CO2)   [Pd(C^N^Cp-OMe)Cl]OTf, Epc1 (N2) [Pd(C^N^Cp-OMe)Cl]OTf, Epc2 (N2)   [Pd(C^N^Cp-Br)Cl]OTf, Epc1 (N2) [Pd(C^N^Cp-COOR)Cl]OTf, Epc1 (N2)    R² = 0.99829015304560750 10 20 30 40R² = 0.997510501001502000 10 20 30 40R² = 0.998970204060801000 10 20 30 40R² = 0.99893045901351800 10 20 30 40R² = 0.998850204060800 10 20 30 40R² = 0.999660102030400 10 20 30 40R² = 0.999560204060801000 10 20 30 40R² = 0.999300102030400 10 20 30 40  224 Background Current from Brønsted Acids Using a 7 mm2 glassy carbon electrode, the steady-state current of a 5 mL CO2-saturated solution of 10 mM TFA in 0.10 M TBAH/DMF was recorded during 30 s controlled potential electrolysis experi-ments with and without the presence of 2 mM [Pd(C^N^C)Cl]BF4 to determine the amount of current passed by direct reduction of H+ at the electrode. The solution was left to sit for 10 minutes after sparging with CO2 before experiments were performed. The rate of stirring was kept approximately constant be-tween experiments.  Table C-1. Steady-state currents of a 5 mL CO2-saturated solution of 10 mM TFA in 0.10 M [n-Bu4N]PF6/DMF with and without 2 mM [Pd(C^N^C)Cl]BF4 present, and resulting proportions of current with [Pd(C^N^C)Cl]BF4 present resulting from direct reaction of H+ and/or CO2 at the glassy carbon electrode. Stirred or Un-stirred Concentration of Complex Current (μA) -1.50 V -1.75 V -1.85 V -2.00 V -2.30 V Stirred 0 mM 5.2 27 40 61 122 Stirred 2 mM 16 111 143 173 252 Background current proportion: 33% 24% 28% 35% 48% Unstirred 0 mM 2.9 10 12 15 27 Unstirred 2 mM 4.1 22 25 29 33 Background current proportion: 71% 45% 47% 53% 83%     225 Table C-2. Preparative controlled potential electrolysis results from alkali and organic cations in CO2-saturated 0.10 M [n-Bu4N]PF6/DMF solutions.a Solution Acid Potential (V) Charge Passed (C) FE H2 FE CO 50 mM K+ 10 mM TFA -2.35 5.7 88% 6% 50 mM K+ 10 mM TFA -2.05 7.3 95% 1% 50 mM K+  10 mM TFA -1.80 6.0 100% 0% 70 mM K+ 100 mM GAA -2.35 7.9 74% 23% 70 mM K+ 100 mM GAA -1.80 7.2 6% 0% 50 mM K+ in CH3CN 100 mM GAA -2.35 6.2 96% 7% 65 mM Mg2+ 10 mM TFA -2.35 6.0 100% 0% 65 mM Mg2+ 10 mM TFA -1.95 6.0 100% 0% 30% [BMIM][PF6] 10 mM TFA -2.35 2.6 95% 6% 30% [BMIM][PF6] 10 mM TFA -1.95 5.7 100% 1% aThe 80 mL electrochemical cell employed a reticulated vitreous carbon working electrode in a com-partment containing 10 mL solution which was separated by fritted glass from the platinum mesh counter electrode compartment, which contained 5 mL solution. Hydrogen and carbon monoxide production are given in terms of faradaic efficiency. All potentials reported vs. Fc0/+.  TOF and Overpotential Calculations TOF and overpotential calculations were determined following the method of Costentin and Savéant using the following equations:20,46  𝐸𝐶𝑂2/CO,DMF,HA° = −0.776 −𝑅𝑇ln10𝐹𝑝𝐾𝑎,HA,DMF  V vs. Fc0/+ (1)   226 𝑘cat =𝑖2 (1 + exp [𝐹𝑅𝑇 (Eapplied − Ecat° )])𝐹2𝐴2𝐷[catalyst]2  (2) 𝑘cat = 2𝑘[CO2]° (3) TOF =𝑘cat1 + exp [𝐹𝑅𝑇 (Eapplied − Ecat° )] (4)  Using pKa(TFA) ~ 5 (in DMF), the standard potential for reduction of CO2 to CO in DMF is deter-mined to be 𝐸𝐶𝑂2/CO,DMF,TFA°  = ˗1.24 V vs. Fc0/+ using equation 1. Thus, the overpotential for a complex like [Pd(C^N^C)(CH3CN)](BF4)2 is 0.45 V when the peak potential for CO2 reduction is used, and 0.33 V using the half-wave potential. 𝑖 = 5 × 10−3A 𝐷 =  5 × 10−6 cm2∙s−1 𝐴 = 2.5 cm2 𝐹 𝑅𝑇⁄ = 38.94 V−1 𝐸applied = −1.90 V [CO2]° = 0.18 M [catalyst] =  2.0 × 10−6 mol∙cm−3  Using the values listed above in equations 2-4 for [Pd(C^N^C)Cl]BF4 with K+ and assuming a 50% selectivity for CO2 over H+  by the catalyst, kcat = TOF = 11 s-1, yielding the rate constant k = 30 M-1 s-1 for the generalized chemical equation below.46 These values should only be used as an order of magnitude estimate. [M]n-2  +  CO2  +  2AH          𝑘      →        [M]CO  +  H2O  +  2A-    227       Figure C-1. Cyclic voltammograms of proligands phC^N^phC • 2HBr (left) and pyC^N^pyC • 2HBr (right) at 200 mV/s with added decamethylferrocene as an internal standard.   -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-160-120-80-4004080Current (µA)Potential (V vs. Fc0/+) phC^N^phC · 2HBr decamethylferrocene-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-160-120-80-4004080Current (µA)Potential (V vs. Fc0/+) pyC^N^pyC · 2HBr decamethylferrocene  228 Appendix D:  Crystallography Data [Pd(C^N^C)(CH3CN)](BF4)2  (see Figure 2-13) All measurements were made on a Bruker X8 APEX II diffractometer with graphite monochromated Mo-Kα radiation (l = 0.71073 Å). The data were collected at a temperature of -173.0 + 2 °C to a maximum 2 value of 58.3°. Data were collected in a series of and  scans in 0.50° oscillations with 10.0-second exposures. The crystal-to-detector distance was 49.73 mm. Of the 26959 reflections that were collected, 7767 were unique (Rint = 0.061); equivalent reflections were merged.  Data were collected and integrated using the Bruker SAINT software package. The linear absorption coefficient, , for Mo-K radiation is 7.91 cm-1. Data were corrected for absorption effects using the multi-scan technique (SADABS), with minimum and maximum transmission coefficients of 0.799 and 0.895, respectively. The data were corrected for Lorentz and polarization effects. The structure was solved by direct methods.173 The material crystallized with significant disorder of one N-butyl group, and both BF4 anions. In addition, the material crystallized with one half molecule of solvent CH2Cl2, disordered about an inversion center, as well as a small fraction of water.  All non-hydrogen atoms were refined anisotropically, while constraints and restraints were employed when mod-elling the disorder to maintain reasonable geometries. All hydrogen atoms were placed in calculated positions. All refinements were performed using SHELXL-2012 via the OLEX2 interface.174,175 Specific atomic coordinates and structural details are available in the published CIF file.117      229 Table D-1. Crystal and Refinement Data for [Pd(C^N^C)(CH3CN)](BF4)2 Crystal Data Structure Solution and Refinement  Emp. Formula C23.5H33.75N6PdB2F8O0.2Cl Solution Direct Methods (SIR97) FW 718.9 Refinement Full-matrix least-squares on F2 Crystal System triclinic Function Minimized Σ w (Fo2 – Fc2)2   Lattice Type primitive Least Squares Weights w = (σ2(Fo2)+(0.0505P)2 + 1.4029P)-1 Lattice Parameters a = 8.3763(12) Å Anomalous Dispersion All non-hydrogen atoms  b = 11.824(2) Å Observations (I>0.00s(I)) 7767  c = 16.462(3) Å No. Variables 468  α = 76.396(6)° Reflection/Parameter Ratio 16.53  β = 85.371(9)° R1; wR2 0.067; 0.111  γ = 69.781(8)° Goodness of Fit 1.04  V = 1487.1(4) Å3   Space Group P -1  (#2) Observations (I>2.00s(I)) 6141 Z value 2 R1; wR2 0.047; 0.101 Dcalc 1.606 g/cm3 Max Shift/Error in Final Cycle 0.00 F000 727.00 Max peak in Final Diff. Map 0.87 e-/Å3   Min peak in Final Diff. Map -1.04 e-/Å3  [Pd(phC^N^phC)Cl]OTf  (see Figure 3-2) A colourless prism crystal of C46H41PdN5ClSO3F3 • CH3CN having approximate dimensions of 0.12 × 0.16 × 0.29 mm was mounted on a glass fiber. The data were collected at a temperature of -173.0 ± 0.1 °C to a maximum 2 value of 60.2°. Data were collected in a series of and  scans in 0.50° oscillations with 10.0-second exposures. The crystal-to-detector distance was 49.81 mm. The material crystallizes as a two-component racemic twin with components one and two related by a 180.0° rotation about the (0 1 -1) reciprocal crystal axis.  Data were integrated for both twin compo-  230 nents, including both overlapped and non-overlapped reflections.  In total, 65884 reflections were inte-grated (24745 from component one only, 24119 from component two only, 17020 overlapped).  Data were collected and integrated using the Bruker SAINT software packages. The linear absorption coeffi-cient, , for Mo-K radiation is 6.12 cm-1. Data were corrected for absorption effects using the multi-scan technique (TWINABS), with minimum and maximum transmission coefficients of 0.820 and 0.929, respectively.  The data were corrected for Lorentz and polarization effects. The structure was solved by direct methods173 using non-overlapped data from the major twin com-ponent. Subsequent refinements were carried out using a HKLF 5 format data set, containing data from both twin components. The material also crystallizes with one molecule of acetonitrile in the asymmetric unit.  Finally, one butyl chain is disordered and was modeled in two orientations. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were included in calculated positions but not refined. The final cycle of full-matrix least-squares refinement on F2 was based on 20995 reflections and 582 variable parameters and converged (largest parameter shift was 0.00 times its esd) with unweighted and weighted agreement factors of:  R1 (I>2.00(I)) =  ||Fo| - |Fc|| /  |Fo| = 0.031 wR2 (all data) = [  ( w (Fo2 - Fc2)2 )/  w(Fo2) 2]1/2 = 0.078 The standard deviation of an observation of unit weight was 1.03. The weighting scheme was based on counting statistics. The maximum and minimum peaks on the final difference Fourier map corre-sponded to 0.48 and -0.42 e-/Å3, respectively.  Neutral atom scattering factors were taken from Cromer and Waber.176 Anomalous dispersion effects were included in Fcalc;177 the values for f' and f" were those of Creagh and McAuley.178 The values for the mass attenuation coefficients are those of Creagh and Hubbell.178 All refinements were performed using the XL via the OLEX2 interface.174,175   231 Specific atomic coordinates and structural details are available in the published CIF file.142  Table D-2. Crystal and Refinement Data for [Pd(phC^N^phC)Cl]OTf Crystal Data Structure Solution and Refinement  Emp. Formula C48H44PdN6ClSO3F3 Solution Direct Methods (SHELXT) FW 983.80 Refinement Full-matrix least-squares on F2 Crystal System Monoclinic Function Minimized Σ w (Fo2 – Fc2)2   Lattice Type C-centered Least Squares Weights w = (σ2(Fo2)+(0.0410P)2 + 1.7078P)-1 Lattice Parameters a = 27.8478(13) Å Anomalous Dispersion All non-hydrogen atoms  b = 9.8610(5) Å Observations (I>0.00s(I)) 20995  c = 20.2588(10) Å No. Variables 582  α = 90° Reflection/Parameter Ratio 36.07  β = 130.012(2) ° R1; wR2 (all data) 0.036; 0.078  γ = 90° Goodness of Fit 1.03  V = 4260.9(4) Å3   Space Group Cc (#9) Observations (I>2.00s(I)) 19678 Z value 4 R1; wR2 0.031; 0.075 Dcalc 1.534 g/cm3 Max Shift/Error in Final Cycle 0.00 F000 2016.00 Max peak in Final Diff. Map 0.48 e-/Å3   Min peak in Final Diff. Map -0.42 e-/Å3      232 Appendix E:  DFT Input Code and Calculated Geometries A typical example of the input used for geometry optimization of reduced species in Gaussian 09 is given below for input files used in sequence: %chk=C^N^C-geom-Pd2-4=OMe-cpcm.chk # opt freq=noraman wb97xd/genecp scrf=(cpcm,solvent=acetonitrile) nosymm pop=npa   Pd(C^N^C,R=OMe)Cl+, unreduced  1 1  Pd                 0.01396400   -0.93000100   -0.01055700  Cl                 0.03649900   -3.66522600   -0.17917200  N                 -3.12872100   -1.14897800   -0.59079400  N                 -2.59293900    0.45096300    0.72372000  N                  3.13404000   -1.22866800    0.61093000  N                  2.66279000    0.40098300   -0.69233500  N                  0.04890800    1.51416400    0.01260900  C                  3.02499400   -2.39696400    1.47292000  C                 -3.07313500   -2.31556900   -1.46032000  C                  4.30454600   -0.50719700    0.44301500  C                  4.00559200    0.53047900   -0.38213500  C                  2.10020900   -0.68015000   -0.07699100  C                 -4.27455600   -0.39443900   -0.40088800  C                 -2.06961500   -0.64039900    0.09037200  C                 -3.93440200    0.62474700    0.43100100  C                 -1.80745300    1.35629800    1.55159000  C                  1.91127600    1.33393700   -1.52151800  C                  0.06497600    4.29266100    0.00794500  C                  1.00889400    3.57418400   -0.73960900  C                 -0.88477600    3.58827400    0.76119800  C                  0.95548600    2.18622100   -0.70761500  C                 -0.84580000    2.19708000    0.73246700  H                  3.11837100   -2.10001100    2.52193600  H                  2.05172200   -2.86009200    1.29760400  H                 -3.21205600   -2.01281000   -2.50259100  H                 -2.09813500   -2.79120800   -1.33374800  H                  5.23588700   -0.79107300    0.91129400  H                  4.62570400    1.32263200   -0.77612900  H                 -5.21936300   -0.64476700   -0.86127200  H                 -4.52416100    1.43220200    0.84020000  H                 -2.49027800    1.99199500    2.11763800  H                 -1.22569600    0.75856500    2.25972800  H                  2.61660600    1.96213900   -2.06790700  H                  1.32950700    0.75787100   -2.24755300  H                  1.75593500    4.10024400   -1.32589600   233  H                 -1.63230900    4.11034900    1.35301900  H                 -3.86156100   -3.02087300   -1.18260200  H                  3.81669100   -3.11081500    1.22894600  O                  0.12070600    5.63718200   -0.03644300  C                 -0.91061027    6.20151254    0.77769322  H                 -1.86610349    5.89489891    0.40631632  H                 -0.79445297    5.86288960    1.78602947  H                 -0.84295823    7.26901028    0.74991247  H C N O Cl 0 D95(d) **** @./basis/sdd-pd.gbs   ///// (end of file) ///// %oldchk=C^N^C-geom-Pd2-4=OMe-cpcm.chk %chk=C^N^C-geom-Pd1-4=OMe-cpcm.chk # opt freq=noraman wb97xd/genecp scrf=(cpcm,solvent=acetonitrile) pop=npa geom=check guess=read  Pd(C^N^C,R=OMe)Cl+, 1e reduced  0 2  H C N O Cl 0 D95(d) **** @./basis/sdd-pd.gbs   ///// (end of file) /////   A typical example of the input used for modelling the approach of CO2 to a reduced complex is below: %chk=C^N^C-geom-Pd1-4=OMe2-cpcm-scan.chk # opt=modredundant wb97xd/genecp scrf=(cpcm,solvent=acetonitrile) nosymm  CO2 Scan from 3.25A to 1.75A - 0.25A/6steps  0 2  Cl                 0.03649900   -3.66522600   -0.17917200  N                 -3.12872100   -1.14897800   -0.59079400  N                 -2.59293900    0.45096300    0.72372000  N                  3.13404000   -1.22866800    0.61093000   234  N                  2.66279000    0.40098300   -0.69233500  N                  0.04890800    1.51416400    0.01260900  C                  3.02499400   -2.39696400    1.47292000  C                 -3.07313500   -2.31556900   -1.46032000  C                  4.30454600   -0.50719700    0.44301500  C                  4.00559200    0.53047900   -0.38213500  C                  2.10020900   -0.68015000   -0.07699100  C                 -4.27455600   -0.39443900   -0.40088800  C                 -2.06961500   -0.64039900    0.09037200  C                 -3.93440200    0.62474700    0.43100100  C                 -1.80745300    1.35629800    1.55159000  C                  1.91127600    1.33393700   -1.52151800  C                  0.06497600    4.29266100    0.00794500  C                  1.00889400    3.57418400   -0.73960900  C                 -0.88477600    3.58827400    0.76119800  C                  0.95548600    2.18622100   -0.70761500  C                 -0.84580000    2.19708000    0.73246700  H                  3.11837100   -2.10001100    2.52193600  H                  2.05172200   -2.86009200    1.29760400  H                 -3.21205600   -2.01281000   -2.50259100  H                 -2.09813500   -2.79120800   -1.33374800  H                  5.23588700   -0.79107300    0.91129400  H                  4.62570400    1.32263200   -0.77612900  H                 -5.21936300   -0.64476700   -0.86127200  H                 -4.52416100    1.43220200    0.84020000  H                 -2.49027800    1.99199500    2.11763800  H                 -1.22569600    0.75856500    2.25972800  H                  2.61660600    1.96213900   -2.06790700  H                  1.32950700    0.75787100   -2.24755300  H                  1.75593500    4.10024400   -1.32589600  H                 -1.63230900    4.11034900    1.35301900  H                 -3.86156100   -3.02087300   -1.18260200  H                  3.81669100   -3.11081500    1.22894600  O                  0.12070600    5.63718200   -0.03644300  Pd                 0.01396400   -0.93000100   -0.01055700  C                  0.13970997   -1.52649542    3.18175914  O                  0.71205416   -0.50395513    3.38896311  O                 -0.42930209   -2.56406677    3.05617662  C                 -0.91061027    6.20151254    0.77769322  H                 -1.86584757    5.88818972    0.41129108  H                 -0.78957276    5.86947750    1.78764503  H                 -0.84809436    7.26913157    0.74332214  B 39 40 S 6 -0.250000  H C N O Cl 0 D95(d) **** @./basis/sdd-pd.gbs    235  ///// (end of file) /////   DFT-Optimized Solution-State Molecular Geometries [Pd(C^N^C)Cl]+ Pd      -0.000950000     -0.670832000     -0.000725000 Cl      -0.003180000     -3.001335000     -0.002252000 N       -3.048683000     -1.129663000     -0.685639000 N       -2.621811000      0.374048000      0.782504000 N        3.045246000     -1.136749000      0.685384000 N        2.622506000      0.369485000     -0.781389000 N        0.001462000      1.406674000      0.000883000 C        2.913634000     -2.165710000      1.710593000 C       -2.919844000     -2.155638000     -1.714177000 C        4.260421000     -0.585724000      0.312288000 C        3.994590000      0.373925000     -0.613834000 C        2.032220000     -0.553909000      0.012521000 C       -4.262578000     -0.577421000     -0.310163000 C       -2.033927000     -0.549232000     -0.013312000 C       -3.994131000      0.380556000      0.616936000 C       -1.838065000      1.277804000      1.615111000 C        1.841003000      1.276078000     -1.613022000 C        0.004952000      4.184660000      0.003437000 C        0.904939000      3.475168000     -0.788605000 C       -0.896788000      3.475959000      0.794180000 C        0.878871000      2.084058000     -0.767465000 C       -0.874214000      2.084799000      0.770511000 H        3.411772000     -1.830183000      2.623196000 H        1.855478000     -2.333688000      1.904671000 H       -3.411399000     -1.813067000     -2.627754000 H       -1.862037000     -2.330429000     -1.904092000 H        5.196982000     -0.916139000      0.736779000 H        4.650779000      1.036138000     -1.158907000 H       -5.200128000     -0.905753000     -0.734093000 H       -4.648634000      1.043004000      1.163750000 H       -2.517533000      1.943657000      2.146629000 H       -1.281714000      0.694778000      2.355576000 H        2.522064000      1.941047000     -2.143611000 H        1.283503000      0.695206000     -2.354299000 H        0.006360000      5.270214000      0.004495000 H        1.624353000      3.990372000     -1.416159000 H       -1.614835000      3.991863000      1.422721000 H       -3.384185000     -3.083910000     -1.372852000 H        3.369623000     -3.096054000      1.363655000   236 [Pd(C^N^C)Cl] (1e- reduced) Pd      -0.000428000     -0.684598000      0.001283000 Cl      -0.001733000     -3.052172000      0.002427000 N       -3.050885000     -1.127312000     -0.672594000 N       -2.629607000      0.365088000      0.809957000 N        3.050223000     -1.129984000      0.673920000 N        2.629318000      0.362044000     -0.809097000 N        0.000252000      1.354201000      0.000351000 C        2.913867000     -2.139454000      1.715477000 C       -2.914922000     -2.139098000     -1.711963000 C        4.265726000     -0.577322000      0.304993000 C        3.998388000      0.372658000     -0.632204000 C        2.035119000     -0.553997000     -0.011184000 C       -4.266081000     -0.573001000     -0.305114000 C       -2.035776000     -0.552225000      0.013242000 C       -3.998462000      0.377485000      0.631488000 C       -1.834749000      1.282134000      1.633795000 C        1.834934000      1.279260000     -1.633238000 C        0.002385000      4.210039000      0.000292000 C        0.919137000      3.458772000     -0.785280000 C       -0.915657000      3.460171000      0.785705000 C        0.895802000      2.084540000     -0.779863000 C       -0.894398000      2.085911000      0.780320000 H        3.361484000     -1.769074000      2.641472000 H        1.854598000     -2.340887000      1.869191000 H       -3.370144000     -1.773936000     -2.636278000 H       -1.855473000     -2.334891000     -1.871545000 H        5.201275000     -0.900990000      0.737232000 H        4.654999000      1.032998000     -1.179466000 H       -5.201638000     -0.895806000     -0.737984000 H       -4.654772000      1.039221000      1.177419000 H       -2.516370000      1.936743000      2.178967000 H       -1.278801000      0.675023000      2.360117000 H        2.516949000      1.932819000     -2.179175000 H        1.278039000      0.672185000     -2.358860000 H        0.003238000      5.294916000      0.000319000 H        1.650751000      3.962665000     -1.413367000 H       -1.646644000      3.965196000      1.413614000 H       -3.409164000     -3.063480000     -1.402014000 H        3.415342000     -3.061786000      1.411115000      237 [Pd(bC^N^bC)Cl]+ N        3.007707347     -1.171964144      0.772147051 N        2.626548918      0.328401067     -0.748592382 N       -3.008050614     -1.171982030     -0.771767928 N       -2.626363838      0.328620951      0.748592340 N        0.000082598      1.364571565     -0.000337259 C       -2.835020851     -2.168310070     -1.818680508 C        2.834327353     -2.168063235      1.819219157 C       -4.244491629     -0.622161195     -0.438886185 C       -2.031554037     -0.589523771     -0.049867775 C        2.031464166     -0.589606349      0.049820914 C        4.001683771      0.352571334     -0.536155695 C        1.852467454      1.226960752     -1.594246340 C       -1.852021881      1.227224378      1.593945031 C        0.000278905      4.139271782     -0.000321671 C       -0.908009130      3.431251356      0.783744918 C        0.908513044      3.431136101     -0.784343682 C       -0.882837612      2.040311460      0.762049471 C        0.883159040      2.040200782     -0.762641451 H       -3.250108792     -1.783655885     -2.754438126 H       -1.771882352     -2.370238435     -1.939106660 H        3.249216499     -1.783253345      2.755001451 H        1.771143425     -2.369876917      1.939426632 H        2.535602235      1.890384168     -2.123369501 H        1.305247552      0.643048774     -2.340629010 H       -2.534987977      1.890750067      2.123156529 H       -1.304677273      0.643352252      2.340268408 H        0.000360069      5.224828869     -0.000307318 H       -1.632626020      3.947016311      1.404810965 H        1.633264764      3.946805233     -1.405331824 H        3.348970321     -3.092202333      1.542896985 H       -3.349670136     -3.092351095     -1.542041739 Pd      -0.000033466     -0.710278606     -0.000413138 Cl      -0.000102863     -3.040201788     -0.000410807 C       -4.001556022      0.352849722      0.536536850 C       -5.035724326      1.110827000      1.088911404 C       -6.322399741      0.851100800      0.618775376 C       -5.531455281     -0.886060556     -0.911060852 C       -6.567086432     -0.130435640     -0.363712274 C        4.244276328     -0.622280869      0.439514204 C        5.531087556     -0.886168513      0.912113868 C        6.566921117     -0.130698300      0.364939241 C        5.036057745      1.110396186     -1.088355975 C        6.322578338      0.850680384     -0.617792377 H        5.719007975     -1.642377847      1.668483325 H        7.584826523     -0.300444645      0.703643353 H        7.156980949      1.418594572     -1.019021526 H        4.859749406      1.868204984     -1.845782865 H       -5.719642036     -1.642397315     -1.667236546   238 H       -7.585102595     -0.300191290     -0.702079613 H       -7.156652542      1.419125713      1.020158066 H       -4.859150001      1.868751301      1.846160386  [Pd(bC^N^bC)Cl] (1e- reduced) N       -1.064783531      2.918097578      1.177700748 N        0.505668656      2.697560583     -0.305848723 N        1.078849946     -2.918305089      1.176041269 N       -0.496917344     -2.698232379     -0.301916546 N        0.002285424     -0.000613922     -1.364905328 C        2.123963688     -2.629613537      2.144939343 C       -2.105735707      2.629536081      2.151097262 C        0.845466766     -4.188365884      0.656871937 C        0.261527990     -2.018866520      0.584620583 C       -0.249052871      2.018649351      0.584196326 C        0.176535812      4.046299583     -0.305548978 C        1.422543309      1.987694831     -1.211750965 C       -1.417355604     -1.988812438     -1.204625974 C       -0.003284583     -0.001663276     -4.153792019 C       -0.666239448     -1.019335264     -3.468580635 C        0.662464261      1.016476056     -3.472010471 C       -0.692901340     -0.951248340     -2.015014115 C        0.694916982      0.949468156     -2.018501954 H        3.089783654     -2.953084745      1.745761708 H        2.140112363     -1.556912784      2.332500265 H       -3.073617220      2.950870361      1.755176036 H       -2.119543804      1.557100367      2.340420530 H        1.875393370      2.721414962     -1.880532755 H        2.225477898      1.562485666     -0.597594692 H       -1.872948545     -2.722940141     -1.871095387 H       -2.217798837     -1.563221021     -0.587480337 H       -0.005583472     -0.002066951     -5.243699467 H       -1.183798373     -1.822572389     -3.983220221 H        1.177822697      1.819455493     -3.989257546 H       -1.901280187      3.157584611      3.086742192 H        1.922443980     -3.155815455      3.082263601 Pd       0.006641937      0.000037118      0.698671510 Cl       0.013232093      0.001211041      3.052890358 C       -0.168703876     -4.047191019     -0.301457893 C       -0.643934722     -5.141863248     -1.027448314 C       -0.060071182     -6.378956088     -0.755858171 C        1.432117908     -5.425223055      0.931813073 C        0.960337039     -6.519464343      0.207462338 C       -0.834159296      4.187833404      0.656408039 C       -1.420661919      5.424556556      0.932165066 C       -0.952214787      6.518399321      0.204994206 C        0.648448372      5.140556530     -1.034307266   239 C        0.064852417      6.377571231     -0.761783443 H       -2.203582014      5.532791242      1.677194503 H       -1.380821821      7.500075136      0.385670253 H        0.402430583      7.253996126     -1.307710443 H        1.430297041      5.042419879     -1.781571223 H        2.217531439     -5.533250780      1.674231460 H        1.388895684     -7.501276315      0.387578484 H       -0.400225965     -7.255705051     -1.299613211 H       -1.428366809     -5.043953889     -1.772030390  [Pd(C-N-C)Cl]+ Pd       0.000000000      0.000000000      0.708457000 Cl       0.000000000      0.000000000      3.039259000 N        0.000000000      0.000000000     -1.271246000 N        0.000000000      3.224218000      0.876184000 N        0.000256000      2.252373000     -1.038328000 N        0.000000000     -3.224218000      0.876184000 N       -0.000256000     -2.252373000     -1.038328000 C       -0.000093000     -1.167301000     -1.924122000 C        0.000093000      1.167301000     -1.924122000 C       -0.000131000     -1.220536000     -3.310459000 C        0.000131000      1.220536000     -3.310459000 C        0.000000000      0.000000000     -3.991725000 C        0.000483000      3.612707000     -1.305170000 C        0.000294000      4.220911000     -0.094228000 C       -0.000294000     -4.220911000     -0.094228000 C       -0.000483000     -3.612707000     -1.305170000 C       -0.000208000      3.501449000      2.309700000 C        0.000208000     -3.501449000      2.309700000 C        0.000236000      2.007910000      0.320183000 C       -0.000236000     -2.007910000      0.320183000 H       -0.000243000     -2.162824000     -3.844931000 H        0.000243000      2.162824000     -3.844931000 H        0.000000000      0.000000000     -5.076874000 H        0.000652000      4.029291000     -2.300498000 H        0.000284000      5.269969000      0.162017000 H       -0.000284000     -5.269969000      0.162017000 H       -0.000652000     -4.029291000     -2.300498000 H       -0.001333000      2.551853000      2.844692000 H        0.893540000      4.074282000      2.566821000 H       -0.892950000      4.076105000      2.566245000 H        0.892950000     -4.076105000      2.566245000 H        0.001333000     -2.551853000      2.844692000 H       -0.893540000     -4.074282000      2.566821000    240 [Pd(C^N^Cp-NMe2)Cl]+ Pd       0.012643000     -0.650729000      0.002471000 Cl      -0.027251000     -2.988037000      0.028721000 N       -3.008921000     -1.072265000     -0.799059000 N       -2.625366000      0.451627000      0.660028000 N        3.022919000     -1.152874000      0.808223000 N        2.682796000      0.360058000     -0.672758000 N        0.044206000      1.419025000     -0.010922000 C        2.836450000     -2.180795000      1.825665000 C       -2.851103000     -2.121738000     -1.799151000 C        4.259766000     -0.614726000      0.491582000 C        4.045562000      0.348973000     -0.443639000 C        2.046320000     -0.557819000      0.091787000 C       -4.229624000     -0.493873000     -0.491290000 C       -2.015936000     -0.495666000     -0.090187000 C       -3.987912000      0.476699000      0.430022000 C       -1.870371000      1.360301000      1.514260000 C        1.953469000      1.281249000     -1.536096000 C        0.097242000      4.252469000     -0.017309000 C        1.026116000      3.477073000     -0.762755000 C       -0.860524000      3.515615000      0.730569000 C        0.964539000      2.099731000     -0.732506000 C       -0.850812000      2.136630000      0.706603000 H        3.311674000     -1.857226000      2.754753000 H        1.769464000     -2.327563000      1.985356000 H       -3.330630000     -1.806939000     -2.728987000 H       -1.788455000     -2.289303000     -1.967215000 H        5.172941000     -0.956162000      0.956400000 H        4.732244000      1.006236000     -0.956221000 H       -5.152122000     -0.814546000     -0.952472000 H       -4.655682000      1.161034000      0.932039000 H       -2.568016000      2.043450000      1.998456000 H       -1.362607000      0.782208000      2.292622000 H        2.670298000      1.934966000     -2.032714000 H        1.422992000      0.709527000     -2.303893000 H        1.796805000      3.946879000     -1.361431000 H       -1.612683000      4.017268000      1.326937000 H       -3.310712000     -3.047251000     -1.443932000 H        3.284203000     -3.120176000      1.492319000 N        0.122350000      5.601459000     -0.020139000 C       -0.846408000      6.352537000      0.770039000 H       -0.744128000      6.131825000      1.839402000 H       -0.675661000      7.419166000      0.626214000 H       -1.873196000      6.128647000      0.457408000 C        1.127915000      6.312868000     -0.801083000 H        1.034902000      6.085337000     -1.869692000 H        0.988757000      7.385779000     -0.670639000 H        2.141991000      6.058918000     -0.470009000    241 [Pd(C^N^Cp-NMe2)Cl] (1e- reduced) Pd       0.002909000     -0.905214000      0.018502000 Cl      -0.075876000     -3.667037000      0.181682000 N       -3.101371000     -1.153697000     -0.708421000 N       -2.639940000      0.484015000      0.588660000 N        3.119195000     -1.218302000      0.701708000 N        2.672241000      0.389881000     -0.636256000 N        0.026557000      1.510525000     -0.022957000 C        3.000681000     -2.372793000      1.580733000 C       -2.989167000     -2.339117000     -1.545960000 C        4.293771000     -0.502549000      0.539574000 C        4.009839000      0.521489000     -0.307522000 C        2.096464000     -0.679684000     -0.012194000 C       -4.260880000     -0.404978000     -0.591564000 C       -2.080244000     -0.617477000      0.008507000 C       -3.967758000      0.637414000      0.229923000 C       -1.899598000      1.410099000      1.436373000 C        1.937398000      1.316730000     -1.488455000 C        0.105343000      4.335655000     -0.006023000 C        1.058580000      3.562191000     -0.721627000 C       -0.893702000      3.607917000      0.694385000 C        0.969231000      2.179416000     -0.700507000 C       -0.881168000      2.222468000      0.657857000 H        3.103237000     -2.060972000      2.624649000 H        2.021356000     -2.828604000      1.418701000 H       -3.050182000     -2.060514000     -2.602419000 H       -2.028522000     -2.814519000     -1.336419000 H        5.217107000     -0.780708000      1.026807000 H        4.637639000      1.306042000     -0.704675000 H       -5.181529000     -0.675096000     -1.088311000 H       -4.583782000      1.448215000      0.591159000 H       -2.612665000      2.066157000      1.938753000 H       -1.372024000      0.829622000      2.199662000 H        2.655992000      1.936142000     -2.028016000 H        1.368563000      0.733236000     -2.218827000 H        1.855574000      4.031046000     -1.287354000 H       -1.666753000      4.113976000      1.261178000 H       -3.798459000     -3.036021000     -1.310291000 H        3.783962000     -3.098584000      1.344113000 N        0.145691000      5.693193000      0.006967000 C       -0.862244000      6.444540000      0.740829000 H       -0.840705000      6.205504000      1.811703000 H       -0.666939000      7.511501000      0.630112000 H       -1.870713000      6.244207000      0.356976000 C        1.186055000      6.398449000     -0.727369000 H        1.133298000      6.186454000     -1.802968000 H        1.058352000      7.472341000     -0.588704000 H        2.185632000      6.127309000     -0.364902000    242 [Pd(C^N^Cp-OMe)Cl]+ Pd       0.009468000     -0.688237000     -0.001233000 Cl       0.004321000     -3.020732000     -0.034546000 N       -3.038374000     -1.127191000     -0.699166000 N       -2.610395000      0.365879000      0.779207000 N        3.049372000     -1.163845000      0.696098000 N        2.640145000      0.361903000     -0.754222000 N        0.016462000      1.386471000      0.019690000 C        2.908721000     -2.206173000      1.706461000 C       -2.910870000     -2.149000000     -1.731947000 C        4.267964000     -0.607745000      0.342142000 C        4.010578000      0.364042000     -0.573514000 C        2.042214000     -0.572326000      0.021467000 C       -4.250826000     -0.568130000     -0.328958000 C       -2.023321000     -0.557576000     -0.017685000 C       -3.981512000      0.383190000      0.604540000 C       -1.828020000      1.262385000      1.620445000 C        1.869159000      1.279889000     -1.583149000 C        0.017559000      4.180326000      0.036052000 C        0.928415000      3.458874000     -0.755438000 C       -0.891810000      3.457658000      0.822343000 C        0.899534000      2.079044000     -0.738061000 C       -0.857696000      2.070109000      0.782401000 H        3.404146000     -1.885362000      2.625842000 H        1.849053000     -2.372458000      1.893660000 H       -3.398304000     -1.800033000     -2.645331000 H       -1.853264000     -2.327425000     -1.919589000 H        5.200564000     -0.943677000      0.770990000 H        4.671760000      1.033977000     -1.102896000 H       -5.187983000     -0.887686000     -0.760379000 H       -4.634718000      1.047571000      1.150558000 H       -2.508451000      1.924242000      2.155544000 H       -1.273100000      0.673329000      2.357082000 H        2.557505000      1.948929000     -2.098959000 H        1.317752000      0.708937000     -2.336528000 H        1.647063000      3.992045000     -1.368666000 H       -1.622352000      3.948398000      1.454279000 H       -3.380159000     -3.076943000     -1.396478000 H        3.362324000     -3.133743000      1.349049000 O        0.092821000      5.508066000     -0.025571000 C       -0.804314000      6.282174000      0.775089000 H       -1.844824000      6.080584000      0.499677000 H       -0.647033000      6.078877000      1.839544000 H       -0.565349000      7.323624000      0.562165000      243 [Pd(C^N^Cp-OMe)Cl] (1e- reduced) Pd       0.013007000     -0.952870000     -0.015380000 Cl       0.049041000     -3.690377000     -0.194048000 N       -3.127250000     -1.187284000     -0.601275000 N       -2.603506000      0.403997000      0.728634000 N        3.135602000     -1.234897000      0.603657000 N        2.652280000      0.402549000     -0.685442000 N        0.030141000      1.488909000      0.029848000 C        3.035784000     -2.412547000      1.453926000 C       -3.062033000     -2.344678000     -1.482325000 C        4.300378000     -0.502545000      0.443234000 C        3.993650000      0.540071000     -0.372822000 C        2.097913000     -0.688549000     -0.080335000 C       -4.278229000     -0.442014000     -0.405903000 C       -2.072399000     -0.677902000      0.085846000 C       -3.945715000      0.571641000      0.435841000 C       -1.824218000      1.307223000      1.564751000 C        1.894056000      1.336983000     -1.507142000 C        0.023327000      4.274409000      0.048971000 C        0.972201000      3.560761000     -0.705453000 C       -0.920016000      3.553702000      0.796924000 C        0.932266000      2.175415000     -0.686571000 C       -0.866622000      2.160577000      0.752826000 H        3.132136000     -2.126143000      2.505590000 H        2.064019000     -2.878556000      1.277818000 H       -3.195562000     -2.031691000     -2.522294000 H       -2.086035000     -2.817996000     -1.354511000 H        5.233562000     -0.782989000      0.909920000 H        4.607512000      1.340863000     -0.759075000 H       -5.220850000     -0.694251000     -0.869733000 H       -4.541330000      1.371312000      0.851811000 H       -2.511621000      1.933907000      2.135335000 H       -1.239813000      0.707020000      2.268610000 H        2.594991000      1.974931000     -2.047954000 H        1.317149000      0.762380000     -2.238225000 H        1.713962000      4.098692000     -1.287818000 H       -1.677230000      4.043865000      1.398474000 H       -3.849587000     -3.055573000     -1.216726000 H        3.829779000     -3.120137000      1.199304000 O        0.098367000      5.611647000     -0.008537000 C       -0.840161000      6.374285000      0.747690000 H       -1.866430000      6.167705000      0.423909000 H       -0.737128000      6.170821000      1.819433000 H       -0.601165000      7.419571000      0.551226000      244 [Pd(C^N^Cp-t-Bu)Cl]+ Pd      -0.000249000     -0.673080000      0.006489000 Cl      -0.014507000     -3.005033000      0.030427000 N       -3.045507000     -1.122012000     -0.689761000 N       -2.621416000      0.396519000      0.763937000 N        3.044233000     -1.143539000      0.700462000 N        2.628893000      0.359370000     -0.771727000 N        0.010022000      1.400358000     -0.009019000 C        2.908270000     -2.168017000      1.729531000 C       -2.914889000     -2.163011000     -1.702815000 C        4.261661000     -0.596323000      0.329312000 C        4.000380000      0.361012000     -0.600407000 C        2.033909000     -0.560565000      0.023032000 C       -4.258911000     -0.557338000     -0.331827000 C       -2.032548000     -0.540272000     -0.015309000 C       -3.991994000      0.409957000      0.585972000 C       -1.840508000      1.306929000      1.592493000 C        1.854225000      1.266109000     -1.609980000 C        0.027563000      4.215822000     -0.026545000 C        0.918444000      3.467203000     -0.799975000 C       -0.876171000      3.483436000      0.756978000 C        0.888237000      2.076795000     -0.771462000 C       -0.864468000      2.097524000      0.746964000 H        3.401295000     -1.827907000      2.643256000 H        1.849340000     -2.336450000      1.918813000 H       -3.411965000     -1.837249000     -2.619544000 H       -1.856913000     -2.334839000     -1.894481000 H        5.196285000     -0.927433000      0.757497000 H        4.659380000      1.020457000     -1.145461000 H       -5.194974000     -0.884550000     -0.759879000 H       -4.646787000      1.083555000      1.118653000 H       -2.522348000      1.982145000      2.109174000 H       -1.295545000      0.729790000      2.345904000 H        2.540692000      1.927520000     -2.138166000 H        1.300195000      0.684311000     -2.353189000 H        1.652959000      3.950363000     -1.433977000 H       -1.606399000      3.984270000      1.384740000 H       -3.372434000     -3.088728000     -1.345657000 H        3.367208000     -3.099388000      1.389247000 C        0.009565000      5.741980000     -0.010655000 C       -1.378770000      6.225931000     -0.479308000 C        0.273414000      6.225703000      1.430549000 C        1.077173000      6.341836000     -0.936499000 H       -1.590037000      5.880513000     -1.497577000 H       -2.180544000      5.871736000      0.177089000 H       -1.404195000      7.321336000     -0.476195000 H        1.250178000      5.880171000      1.787772000 H        0.266767000      7.321100000      1.456347000 H       -0.491294000      5.871608000      2.130011000   245 H        1.024037000      7.434287000     -0.883648000 H        2.089880000      6.045830000     -0.639212000 H        0.919393000      6.054044000     -1.982380000  [Pd(C^N^Cp-t-Bu)Cl] (1e- reduced) Pd      -0.007680000     -0.954948000      0.017992000 Cl      -0.054449000     -3.691715000      0.196986000 N       -3.136149000     -1.221506000     -0.583150000 N       -2.635847000      0.416449000      0.698826000 N        3.135867000     -1.207599000      0.584435000 N        2.613439000      0.384738000     -0.744823000 N       -0.013653000      1.485656000     -0.031649000 C        3.068563000     -2.361104000      1.470398000 C       -3.048358000     -2.402824000     -1.429628000 C        4.291164000     -0.472528000      0.376812000 C        3.959405000      0.541824000     -0.464553000 C        2.079245000     -0.690964000     -0.094722000 C       -4.295461000     -0.481553000     -0.418582000 C       -2.091457000     -0.679619000      0.093884000 C       -3.977847000      0.561387000      0.393003000 C       -1.867810000      1.347616000      1.515418000 C        1.832784000      1.293757000     -1.574648000 C        0.028632000      4.293986000     -0.029172000 C        0.945273000      3.545806000     -0.776542000 C       -0.915957000      3.567406000      0.714279000 C        0.887444000      2.151420000     -0.754472000 C       -0.900999000      2.177181000      0.691858000 H        3.207757000     -2.044615000      2.508574000 H        2.089886000     -2.830387000      1.348362000 H       -3.150853000     -2.120039000     -2.481675000 H       -2.077646000     -2.872915000     -1.258584000 H        5.235733000     -0.731483000      0.832926000 H        4.557872000      1.335425000     -0.888002000 H       -5.232791000     -0.757358000     -0.879688000 H       -4.584436000      1.367124000      0.780480000 H       -2.562994000      1.992604000      2.055704000 H       -1.295863000      0.770093000      2.248229000 H        2.520000000      1.918372000     -2.147975000 H        1.242613000      0.696770000     -2.276530000 H        1.708097000      4.028758000     -1.377969000 H       -1.665733000      4.073276000      1.316005000 H       -3.843975000     -3.105511000     -1.166565000 H        3.851702000     -3.076731000      1.204506000 C        0.024925000      5.822524000      0.002266000 C       -1.331192000      6.330806000     -0.529200000 C        0.221234000      6.293423000      1.458157000 C        1.144516000      6.422603000     -0.860246000   246 H       -1.494459000      6.002164000     -1.562011000 H       -2.169126000      5.974208000      0.079389000 H       -1.347489000      7.426660000     -0.511360000 H        1.173829000      5.930874000      1.861459000 H        0.228058000      7.388875000      1.495349000 H       -0.582267000      5.942651000      2.114653000 H        1.103447000      7.515289000     -0.794890000 H        2.136669000      6.107213000     -0.516760000 H        1.036559000      6.150252000     -1.916557000  [Pd(C^N^Cp-H)Cl]+ Pd      -0.000950000     -0.670832000     -0.000725000 Cl      -0.003180000     -3.001335000     -0.002252000 N       -3.048683000     -1.129663000     -0.685639000 N       -2.621811000      0.374048000      0.782504000 N        3.045246000     -1.136749000      0.685384000 N        2.622506000      0.369485000     -0.781389000 N        0.001462000      1.406674000      0.000883000 C        2.913634000     -2.165710000      1.710593000 C       -2.919844000     -2.155638000     -1.714177000 C        4.260421000     -0.585724000      0.312288000 C        3.994590000      0.373925000     -0.613834000 C        2.032220000     -0.553909000      0.012521000 C       -4.262578000     -0.577421000     -0.310163000 C       -2.033927000     -0.549232000     -0.013312000 C       -3.994131000      0.380556000      0.616936000 C       -1.838065000      1.277804000      1.615111000 C        1.841003000      1.276078000     -1.613022000 C        0.004952000      4.184660000      0.003437000 C        0.904939000      3.475168000     -0.788605000 C       -0.896788000      3.475959000      0.794180000 C        0.878871000      2.084058000     -0.767465000 C       -0.874214000      2.084799000      0.770511000 H        3.411772000     -1.830183000      2.623196000 H        1.855478000     -2.333688000      1.904671000 H       -3.411399000     -1.813067000     -2.627754000 H       -1.862037000     -2.330429000     -1.904092000 H        5.196982000     -0.916139000      0.736779000 H        4.650779000      1.036138000     -1.158907000 H       -5.200128000     -0.905753000     -0.734093000 H       -4.648634000      1.043004000      1.163750000 H       -2.517533000      1.943657000      2.146629000 H       -1.281714000      0.694778000      2.355576000 H        2.522064000      1.941047000     -2.143611000 H        1.283503000      0.695206000     -2.354299000 H        0.006360000      5.270214000      0.004495000 H        1.624353000      3.990372000     -1.416159000   247 H       -1.614835000      3.991863000      1.422721000 H       -3.384185000     -3.083910000     -1.372852000 H        3.369623000     -3.096054000      1.363655000  [Pd(C^N^Cp-H)Cl] (1e- reduced) Pd      -0.000428000     -0.684598000      0.001283000 Cl      -0.001733000     -3.052172000      0.002427000 N       -3.050885000     -1.127312000     -0.672594000 N       -2.629607000      0.365088000      0.809957000 N        3.050223000     -1.129984000      0.673920000 N        2.629318000      0.362044000     -0.809097000 N        0.000252000      1.354201000      0.000351000 C        2.913867000     -2.139454000      1.715477000 C       -2.914922000     -2.139098000     -1.711963000 C        4.265726000     -0.577322000      0.304993000 C        3.998388000      0.372658000     -0.632204000 C        2.035119000     -0.553997000     -0.011184000 C       -4.266081000     -0.573001000     -0.305114000 C       -2.035776000     -0.552225000      0.013242000 C       -3.998462000      0.377485000      0.631488000 C       -1.834749000      1.282134000      1.633795000 C        1.834934000      1.279260000     -1.633238000 C        0.002385000      4.210039000      0.000292000 C        0.919137000      3.458772000     -0.785280000 C       -0.915657000      3.460171000      0.785705000 C        0.895802000      2.084540000     -0.779863000 C       -0.894398000      2.085911000      0.780320000 H        3.361484000     -1.769074000      2.641472000 H        1.854598000     -2.340887000      1.869191000 H       -3.370144000     -1.773936000     -2.636278000 H       -1.855473000     -2.334891000     -1.871545000 H        5.201275000     -0.900990000      0.737232000 H        4.654999000      1.032998000     -1.179466000 H       -5.201638000     -0.895806000     -0.737984000 H       -4.654772000      1.039221000      1.177419000 H       -2.516370000      1.936743000      2.178967000 H       -1.278801000      0.675023000      2.360117000 H        2.516949000      1.932819000     -2.179175000 H        1.278039000      0.672185000     -2.358860000 H        0.003238000      5.294916000      0.000319000 H        1.650751000      3.962665000     -1.413367000 H       -1.646644000      3.965196000      1.413614000 H       -3.409164000     -3.063480000     -1.402014000 H        3.415342000     -3.061786000      1.411115000     248 [Pd(C^N^Cp-Br)Cl]+ N       -3.048288000     -1.131018000     -0.678590000 N       -2.620190000      0.376719000      0.784820000 N        3.047777000     -1.132090000      0.680222000 N        2.620491000      0.375173000     -0.783923000 N        0.000422000      1.406563000      0.000373000 C        2.920092000     -2.163451000      1.703726000 C       -2.921129000     -2.162774000     -1.701758000 C        4.261234000     -0.576533000      0.307987000 C        3.992789000      0.383868000     -0.616285000 C        2.032992000     -0.551853000      0.008536000 C       -4.261491000     -0.574960000     -0.306288000 C       -2.033169000     -0.550836000     -0.007379000 C       -3.992526000      0.385749000      0.617518000 C       -1.837690000      1.281413000      1.616121000 C        1.838389000      1.280033000     -1.615409000 C        0.001269000      4.174952000      0.000168000 C        0.905100000      3.475138000     -0.794996000 C       -0.903012000      3.475807000      0.795404000 C        0.875233000      2.085401000     -0.768139000 C       -0.873997000      2.086043000      0.768754000 H        3.420039000     -1.828783000      2.615602000 H        1.862732000     -2.333398000      1.900241000 H       -3.421053000     -1.828229000     -2.613692000 H       -1.863851000     -2.333187000     -1.898327000 H        5.198818000     -0.904892000      0.731768000 H        4.646855000      1.049095000     -1.160199000 H       -5.199262000     -0.903142000     -0.729794000 H       -4.646260000      1.051460000      1.161237000 H       -2.516770000      1.946940000      2.148268000 H       -1.277383000      0.701415000      2.355842000 H        2.517750000      1.945028000     -2.147863000 H        1.277666000      0.700091000     -2.354857000 H        1.623391000      3.991665000     -1.422080000 H       -1.620990000      3.992873000      1.422401000 H       -3.378085000     -3.091232000     -1.351209000 H        3.376736000     -3.092207000      1.353554000 Pd      -0.000104000     -0.671793000      0.000576000 Cl      -0.000603000     -2.999658000      0.000777000 Br       0.001874000      6.052683000      0.000054000      249 [Pd(C^N^Cp-Br)Cl] (1e- reduced) N       -3.051279000     -1.124716000     -0.670519000 N       -2.627237000      0.368298000      0.810202000 N        3.050339000     -1.128035000      0.672127000 N        2.628163000      0.365675000     -0.808435000 N        0.001497000      1.358725000      0.001249000 C        2.915678000     -2.141751000      1.710099000 C       -2.917805000     -2.138103000     -1.708974000 C        4.265225000     -0.572933000      0.304556000 C        3.997375000      0.378492000     -0.630688000 C        2.035084000     -0.553056000     -0.012260000 C       -4.265603000     -0.569054000     -0.301923000 C       -2.035253000     -0.550532000      0.013350000 C       -3.996561000      0.381880000      0.633486000 C       -1.833558000      1.282986000      1.635132000 C        1.835655000      1.281197000     -1.633468000 C        0.000987000      4.188260000     -0.000805000 C        0.924730000      3.458474000     -0.793644000 C       -0.922611000      3.459355000      0.792896000 C        0.894879000      2.086220000     -0.779645000 C       -0.892380000      2.087065000      0.780908000 H        3.372554000     -1.777975000      2.634053000 H        1.856520000     -2.337718000      1.871125000 H       -3.375322000     -1.773819000     -2.632410000 H       -1.858834000     -2.334298000     -1.870939000 H        5.201027000     -0.896033000      0.736608000 H        4.653197000      1.040855000     -1.176380000 H       -5.201855000     -0.891296000     -0.733642000 H       -4.651619000      1.044583000      1.179684000 H       -2.514169000      1.937231000      2.181551000 H       -1.274035000      0.678773000      2.360117000 H        2.517036000      1.934749000     -2.179760000 H        1.275677000      0.677567000     -2.358605000 H        1.651314000      3.969217000     -1.419346000 H       -1.649362000      3.970792000      1.417842000 H       -3.411671000     -3.061661000     -1.396206000 H        3.409508000     -3.065356000      1.397389000 Pd      -0.000192000     -0.683285000      0.000486000 Cl      -0.001905000     -3.046372000      0.000262000 Br       0.000604000      6.083674000     -0.002315000      250 [Pd(C^N^Cp-COOR)Cl]+ N       -3.045969000     -1.115213000     -0.686949000 N       -2.616721000      0.374185000      0.794828000 N        3.049167000     -1.125653000      0.678924000 N        2.625017000      0.394273000     -0.773181000 N        0.003326000      1.418305000      0.015627000 C        2.918541000     -2.165861000      1.693064000 C       -2.919337000     -2.132294000     -1.724703000 C        4.263421000     -0.567147000      0.313896000 C        3.996937000      0.401112000     -0.602841000 C        2.036146000     -0.539358000      0.009871000 C       -4.259186000     -0.566552000     -0.303943000 C       -2.030215000     -0.541434000     -0.011152000 C       -3.989393000      0.382637000      0.631617000 C       -1.831604000      1.270662000      1.633308000 C        1.843939000      1.305245000     -1.599340000 C       -0.012005000      4.192358000      0.026303000 C        0.893521000      3.493571000     -0.768754000 C       -0.906289000      3.476385000      0.817706000 C        0.876027000      2.102602000     -0.750393000 C       -0.874693000      2.086972000      0.790348000 H        3.418753000     -1.840635000      2.608207000 H        1.860663000     -2.334993000      1.887612000 H       -3.413610000     -1.781902000     -2.633780000 H       -1.862005000     -2.304864000     -1.919128000 H        5.200085000     -0.899074000      0.736937000 H        4.652259000      1.070957000     -1.139534000 H       -5.197507000     -0.891112000     -0.729025000 H       -4.642745000      1.039922000      1.185957000 H       -2.508917000      1.929842000      2.175574000 H       -1.268105000      0.682110000      2.363845000 H        2.524399000      1.977326000     -2.121575000 H        1.289981000      0.730808000     -2.348185000 H        1.606633000      4.018700000     -1.394204000 H       -1.622111000      3.997825000      1.444443000 H       -3.382335000     -3.063569000     -1.389932000 H        3.373050000     -3.092585000      1.334836000 Pd       0.003680000     -0.659510000     -0.003946000 Cl       0.006963000     -2.986950000     -0.035747000 C       -0.062858000      5.696144000      0.060769000 O       -0.852190000      6.315065000      0.745123000 O        0.845629000      6.241903000     -0.738067000 C        0.873100000      7.678840000     -0.781731000 H        1.074339000      8.083292000      0.213205000 H        1.678876000      7.933711000     -1.468900000 H       -0.081164000      8.061750000     -1.152042000      251 [Pd(C^N^Cp-COOR)Cl] (1e- reduced) N       -3.049168000     -1.116486000     -0.680228000 N       -2.623849000      0.368860000      0.807597000 N        3.051197000     -1.116501000      0.673698000 N        2.625776000      0.389739000     -0.792982000 N       -0.000857000      1.379173000      0.011033000 C        2.918859000     -2.144112000      1.698643000 C       -2.916916000     -2.125637000     -1.723317000 C        4.264906000     -0.556103000      0.310204000 C        3.995335000      0.403052000     -0.616444000 C        2.035732000     -0.537513000     -0.004755000 C       -4.263578000     -0.566317000     -0.303733000 C       -2.033167000     -0.543904000      0.002956000 C       -3.994075000      0.379578000      0.636460000 C       -1.833420000      1.276112000      1.639835000 C        1.834911000      1.305497000     -1.615159000 C       -0.011471000      4.227622000      0.024289000 C        0.909303000      3.474697000     -0.771653000 C       -0.923064000      3.459403000      0.814216000 C        0.882726000      2.104809000     -0.759866000 C       -0.888569000      2.090754000      0.791379000 H        3.391235000     -1.798103000      2.621437000 H        1.859927000     -2.331463000      1.870687000 H       -3.379685000     -1.758843000     -2.642996000 H       -1.858134000     -2.317884000     -1.890839000 H        5.201404000     -0.880829000      0.739419000 H        4.649742000      1.071399000     -1.156425000 H       -5.200456000     -0.888104000     -0.734325000 H       -4.648817000      1.037079000      1.189203000 H       -2.514570000      1.929223000      2.186391000 H       -1.274137000      0.674317000      2.364969000 H        2.515825000      1.968619000     -2.149864000 H        1.281328000      0.711876000     -2.351300000 H        1.635645000      3.984009000     -1.397023000 H       -1.648999000      3.966783000      1.442715000 H       -3.406687000     -3.051561000     -1.411440000 H        3.398939000     -3.067829000      1.365828000 Pd       0.001840000     -0.670512000     -0.003598000 Cl       0.007279000     -3.026087000     -0.026584000 C       -0.056614000      5.667908000      0.059501000 O       -0.840009000      6.344649000      0.735571000 O        0.874571000      6.248838000     -0.748893000 C        0.880143000      7.673447000     -0.767243000 H        1.076100000      8.081697000      0.229405000 H        1.683032000      7.958932000     -1.449575000 H       -0.074019000      8.067515000     -1.132246000      252 [Pd(C^N^Cp-COOR)Cl]- (2e- reduced) N       -3.080625000     -1.246069000     -0.641443000 N       -2.694370000      0.330124000      0.760314000 N        3.022283000     -0.976180000      0.659247000 N        2.559453000      0.421081000     -0.901024000 N       -0.215233000      1.347237000     -0.252609000 C        2.914525000     -1.913259000      1.768401000 C       -2.914502000     -2.306221000     -1.625475000 C        4.225667000     -0.438259000      0.234531000 C        3.930803000      0.451666000     -0.752578000 C        1.986437000     -0.448145000     -0.039210000 C       -4.304378000     -0.668026000     -0.341644000 C       -2.078935000     -0.629222000      0.031540000 C       -4.057462000      0.335872000      0.544069000 C       -1.910085000      1.283491000      1.547595000 C        1.741235000      1.305868000     -1.743828000 C        0.029810000      4.246038000      0.046210000 C        0.929541000      3.446924000     -0.780997000 C       -0.917389000      3.434851000      0.809136000 C        0.797360000      2.100383000     -0.890716000 C       -0.987321000      2.087437000      0.672234000 H        3.357734000     -1.471079000      2.664970000 H        1.860689000     -2.128221000      1.940557000 H       -3.160332000     -1.929835000     -2.622687000 H       -1.878319000     -2.643773000     -1.598373000 H        5.172398000     -0.726558000      0.667814000 H        4.570645000      1.086458000     -1.348109000 H       -5.230224000     -1.017930000     -0.774483000 H       -4.726640000      1.026999000      1.035417000 H       -2.596601000      1.937120000      2.089300000 H       -1.336261000      0.695733000      2.278308000 H        2.414078000      1.962350000     -2.298436000 H        1.196389000      0.672621000     -2.456866000 H        1.721062000      3.934951000     -1.344794000 H       -1.583812000      3.922919000      1.517393000 H       -3.573616000     -3.143723000     -1.381823000 H        3.434878000     -2.843749000      1.526064000 Pd      -0.031539000     -0.671089000     -0.012081000 Cl       0.094726000     -3.068752000      0.088542000 C        0.056572000      5.635877000      0.159500000 O       -0.680391000      6.374119000      0.870637000 O        1.045085000      6.240013000     -0.635607000 C        1.087084000      7.652028000     -0.593573000 H        1.283392000      8.030312000      0.417221000 H        1.907479000      7.950467000     -1.254065000 H        0.154199000      8.104951000     -0.953632000      253 [Pd(C^N^Cp-CF3)Cl]+ Pd      -0.002991000     -0.665637000      0.002680000 Cl      -0.001135000     -2.991616000      0.020791000 N       -3.048584000     -1.132851000     -0.679713000 N       -2.624887000      0.381231000      0.778503000 N        3.046968000     -1.116599000      0.683964000 N        2.614428000      0.378123000     -0.791391000 N       -0.005668000      1.414089000     -0.008092000 C        2.922609000     -2.139566000      1.716328000 C       -2.917927000     -2.168923000     -1.698199000 C        4.258739000     -0.561750000      0.305108000 C        3.987031000      0.390923000     -0.626199000 C        2.030217000     -0.543520000      0.009670000 C       -4.263041000     -0.576589000     -0.311790000 C       -2.035716000     -0.548867000     -0.008881000 C       -3.997017000      0.388089000      0.608714000 C       -1.844419000      1.288871000      1.608235000 C        1.827988000      1.275033000     -1.627283000 C       -0.001690000      4.179700000     -0.015165000 C        0.896710000      3.477049000     -0.808833000 C       -0.902819000      3.482871000      0.782452000 C        0.868586000      2.086076000     -0.781571000 C       -0.880089000      2.092406000      0.760972000 H        3.417979000     -1.793666000      2.626487000 H        1.865704000     -2.313933000      1.911464000 H       -3.418766000     -1.840063000     -2.611666000 H       -1.860092000     -2.336786000     -1.894012000 H        5.197655000     -0.885036000      0.729831000 H        4.638851000      1.053002000     -1.176598000 H       -5.199679000     -0.907546000     -0.735628000 H       -4.652545000      1.055178000      1.148555000 H       -2.524689000      1.955968000      2.136903000 H       -1.285326000      0.712113000      2.351402000 H        2.503844000      1.936817000     -2.168099000 H        1.264345000      0.688678000     -2.359404000 H        1.612188000      3.989740000     -1.442389000 H       -1.621468000      4.000268000      1.408994000 H       -3.371907000     -3.097194000     -1.343399000 H        3.385820000     -3.068544000      1.375612000 C        0.024719000      5.694484000      0.019529000 F       -1.205165000      6.204804000      0.174250000 F        0.542250000      6.208893000     -1.103600000 F        0.773366000      6.132391000      1.046916000      254 [Pd(C^N^Cp-CF3)Cl] (1e- reduced) Pd      -0.003705000     -0.676903000     -0.001989000 Cl       0.001805000     -3.036077000      0.019936000 N       -3.053465000     -1.135636000     -0.670502000 N       -2.632168000      0.371200000      0.796782000 N        3.047746000     -1.109492000      0.676302000 N        2.620784000      0.373469000     -0.813482000 N       -0.011294000      1.368195000     -0.018998000 C        2.916462000     -2.116142000      1.721769000 C       -2.918259000     -2.159216000     -1.698902000 C        4.261133000     -0.555120000      0.302690000 C        3.990380000      0.389363000     -0.638653000 C        2.030980000     -0.540908000     -0.010079000 C       -4.269150000     -0.581610000     -0.303810000 C       -2.038830000     -0.552179000      0.006288000 C       -4.002049000      0.378004000      0.623148000 C       -1.842206000      1.291571000      1.616790000 C        1.827393000      1.279987000     -1.645818000 C       -0.026161000      4.208356000     -0.047924000 C        0.907974000      3.459306000     -0.819804000 C       -0.932600000      3.464132000      0.763767000 C        0.881496000      2.089632000     -0.798654000 C       -0.900465000      2.095030000      0.758487000 H        3.372478000     -1.743911000      2.642706000 H        1.858000000     -2.314189000      1.884583000 H       -3.384002000     -1.808198000     -2.623266000 H       -1.858852000     -2.349178000     -1.865243000 H        5.198061000     -0.873340000      0.735860000 H        4.644174000      1.048929000     -1.190096000 H       -5.204959000     -0.910809000     -0.731156000 H       -4.658552000      1.042574000      1.165291000 H       -2.524726000      1.948554000      2.157269000 H       -1.281730000      0.696662000      2.347501000 H        2.507487000      1.933160000     -2.193953000 H        1.270509000      0.673252000     -2.369748000 H        1.638670000      3.961519000     -1.447534000 H       -1.660966000      3.971012000      1.390829000 H       -3.403210000     -3.082872000     -1.372936000 H        3.413177000     -3.040263000      1.415436000 C        0.038011000      5.677771000      0.033553000 F       -1.179501000      6.249638000      0.213834000 F        0.578134000      6.242577000     -1.071949000 F        0.796374000      6.150262000      1.077123000      255 [Pd(C^N^Cp-CF3)Cl]- (2e- reduced) Pd      -0.034242000     -0.670906000     -0.034605000 Cl       0.131457000     -3.092596000      0.064487000 N       -3.106709000     -1.259335000     -0.626253000 N       -2.696440000      0.328827000      0.754557000 N        3.011063000     -0.948471000      0.687449000 N        2.571965000      0.424142000     -0.900142000 N       -0.290277000      1.338753000     -0.400377000 C        2.882624000     -1.863523000      1.812041000 C       -2.948843000     -2.329828000     -1.599848000 C        4.222208000     -0.426954000      0.264601000 C        3.942148000      0.447150000     -0.741296000 C        1.984320000     -0.424792000     -0.029593000 C       -4.319979000     -0.649395000     -0.345496000 C       -2.094450000     -0.651845000      0.040933000 C       -4.057346000      0.363216000      0.526250000 C       -1.881772000      1.283682000      1.509548000 C        1.765083000      1.291271000     -1.774366000 C       -0.063935000      4.236481000     -0.138726000 C        0.908462000      3.434826000     -0.860787000 C       -0.930007000      3.433965000      0.712436000 C        0.790884000      2.085732000     -0.963869000 C       -0.999833000      2.084004000      0.595090000 H        3.316695000     -1.408363000      2.706766000 H        1.824491000     -2.066691000      1.972601000 H       -3.066454000     -1.934591000     -2.613476000 H       -1.953829000     -2.760981000     -1.480881000 H        5.162820000     -0.712121000      0.713253000 H        4.591360000      1.066705000     -1.342882000 H       -5.250837000     -0.989756000     -0.775349000 H       -4.715729000      1.073791000      1.004469000 H       -2.544575000      1.934288000      2.084336000 H       -1.276965000      0.686301000      2.208837000 H        2.446720000      1.945059000     -2.321925000 H        1.246166000      0.640452000     -2.492975000 H        1.735394000      3.917847000     -1.380028000 H       -1.562184000      3.917328000      1.456395000 H       -3.702980000     -3.101503000     -1.423900000 H        3.398313000     -2.802896000      1.595190000 C        0.277487000      5.575745000      0.253990000 F       -0.782842000      6.314385000      0.706224000 F        0.875487000      6.312642000     -0.733381000 F        1.209462000      5.736062000      1.326150000        256 [Ni(bC^N^bC)Cl]+ N       -1.116328316      2.782136416      1.097091934 N        0.533257983      2.618045076     -0.299824827 N        1.116328332     -2.782136432      1.097091940 N       -0.533257962     -2.618045060     -0.299824817 N        0.000000027      0.000000005     -1.341315717 C        2.211314654     -2.463909968      2.000990510 C       -2.211314633      2.463909931      2.000990505 C        0.852232191     -4.072907600      0.643761084 C        0.272276687     -1.904607769      0.521362270 C       -0.272276651      1.904607769      0.521362264 C        0.208327385      3.970668645     -0.266104328 C        1.463248665      1.948882289     -1.198929556 C       -1.463248639     -1.948882257     -1.198929540 C        0.000000010      0.000000008     -4.117756668 C       -0.743554165     -0.942024511     -3.409676492 C        0.743554186      0.942024527     -3.409676498 C       -0.715833723     -0.919327490     -2.019042307 C        0.715833760      0.919327506     -2.019042313 H        3.148021230     -2.834841052      1.575684469 H        2.262822645     -1.383854350      2.126262402 H       -3.148021214      2.834840994      1.575684463 H       -2.262822603      1.383854313      2.126262397 H        1.924101474      2.686878463     -1.854536896 H        2.257851189      1.469657271     -0.618574305 H       -1.924101463     -2.686878431     -1.854536881 H       -2.257851152     -1.469657229     -0.618574289 H        0.000000002      0.000000010     -5.203347998 H       -1.331388347     -1.694067838     -3.925051349 H        1.331388357      1.694067859     -3.925051359 H       -2.041085921      2.932350464      2.974104465 H        2.041085932     -2.932350501      2.974104470 Ni       0.000000043      0.000000005      0.598585529 Cl       0.000000027      0.000000006      2.801600852 C       -0.208327393     -3.970668639     -0.266104318 C       -0.724743517     -5.090978442     -0.919311255 C       -0.130387601     -6.317464475     -0.624118056 C        1.448384446     -5.299082227      0.942552720 C        0.936742475     -6.420686636      0.291516619 C       -0.852232202      4.072907589      0.643761073 C       -1.448384478      5.299082227      0.942552709 C       -0.936742533      6.420686636      0.291516607 C        0.724743486      5.090978458     -0.919311265 C        0.130387544      6.317464475     -0.624118066 H       -2.268457684      5.379294914      1.649914347 H       -1.370173880      7.395998565      0.492984381 H        0.500835228      7.215082812     -1.110691848 H        1.546610476      5.023638394     -1.625754045 H        2.268457647     -5.379294967      1.649914357   257 H        1.370173806     -7.395998565      0.492984394 H       -0.500835305     -7.215082812     -1.110691832 H       -1.546610507     -5.023638362     -1.625754029  [Ni(bC^N^bC)Cl] (1e- reduced) N       -1.088744692      2.795010717      1.113730775 N        0.581557780      2.621393154     -0.260131360 N        1.088744692     -2.795010717      1.113730775 N       -0.581557780     -2.621393154     -0.260131359 N        0.000000002      0.000000000     -1.246395448 C        2.190014798     -2.480367793      2.008597968 C       -2.190014798      2.480367793      2.008597962 C        0.845654079     -4.075560688      0.626341071 C        0.221354624     -1.911524545      0.566223070 C       -0.221354624      1.911524539      0.566223070 C        0.227153261      3.964446341     -0.270290272 C        1.503985597      1.924124255     -1.159689867 C       -1.503985597     -1.924124255     -1.159689862 C       -0.000000001      0.000000000     -4.103510137 C       -0.742511686     -0.952594011     -3.354270031 C        0.742511686      0.952594011     -3.354270031 C       -0.736246286     -0.927323628     -1.980015903 C        0.736246286      0.927323628     -1.980015903 H        3.130636968     -2.812297631      1.558829771 H        2.215126534     -1.403710991      2.168749763 H       -3.130636974      2.812297636      1.558829771 H       -2.215126540      1.403710991      2.168749758 H        2.002243628      2.654703174     -1.798074515 H        2.264371923      1.428585936     -0.541657783 H       -2.002243628     -2.654703174     -1.798074515 H       -2.264371923     -1.428585936     -0.541657783 H       -0.000000002      0.000000000     -5.188473524 H       -1.332992918     -1.715019172     -3.857843637 H        1.332992918      1.715019177     -3.857843642 H       -2.050213668      2.982813389      2.970131614 H        2.050213668     -2.982813389      2.970131614 Ni       0.000000002      0.000000000      0.659575524 Cl       0.000000010      0.000000001      2.893983553 C       -0.227153261     -3.964446341     -0.270290272 C       -0.726286715     -5.075518317     -0.952357268 C       -0.107039687     -6.300723265     -0.701262997 C        1.465849270     -5.299395817      0.881979753 C        0.969611405     -6.411837945      0.201528822 C       -0.845654079      4.075560688      0.626341071 C       -1.465849270      5.299395817      0.881979753 C       -0.969611405      6.411837945      0.201528824   258 C        0.726286715      5.075518317     -0.952357268 C        0.107039687      6.300723318     -0.701262992 H       -2.294761699      5.385387173      1.578687766 H       -1.423357580      7.384418897      0.369988344 H        0.464725900      7.189731835     -1.213033870 H        1.554959300      4.999671951     -1.650258243 H        2.294761699     -5.385387173      1.578687766 H        1.423357580     -7.384418897      0.369988342 H       -0.464725900     -7.189731835     -1.213033876 H       -1.554959300     -4.999671951     -1.650258243  [Pt(bC^N^bC)Cl]+ N       -1.042175454      2.921074750      1.195701336 N        0.503506998      2.695358820     -0.312757672 N        1.042162510     -2.921062431      1.195694447 N       -0.503525210     -2.695371001     -0.312763473 N       -0.000017485     -0.000006193     -1.338482016 C        2.070934746     -2.647506469      2.188402242 C       -2.070946330      2.647535849      2.188415053 C        0.828523690     -4.186199951      0.652709466 C        0.231308860     -2.019517360      0.604690424 C       -0.231341515      2.019515328      0.604692446 C        0.161536464      4.043297577     -0.327395960 C        1.412579737      2.004999689     -1.216256576 C       -1.412601417     -2.005021708     -1.216266826 C       -0.000004229     -0.000007764     -4.117472965 C       -0.688645649     -0.980618355     -3.408083208 C        0.688632732      0.980601993     -3.408077604 C       -0.673447059     -0.955265441     -2.018075151 C        0.673421632      0.955250344     -2.018069547 H        3.034050653     -3.006432453      1.814956957 H        2.120263559     -1.573633415      2.360965985 H       -3.034059379      3.006474311      1.814974272 H       -2.120286303      1.573663615      2.360981855 H        1.862296148      2.733478796     -1.889715436 H        2.215283203      1.536350157     -0.638369439 H       -1.862306499     -2.733505260     -1.889728401 H       -2.215312646     -1.536380310     -0.638383822 H        0.000000697     -0.000008194     -5.202902325 H       -1.234584963     -1.763688958     -3.923192936 H        1.234579089      1.763670448     -3.923183136 H       -1.829537325      3.154959325      3.126109000 H        1.829538267     -3.154931247      3.126098697 Pt      -0.000038994     -0.000004287      0.719374940 Cl      -0.000038310     -0.000004327      3.067057428 C       -0.161534028     -4.043304424     -0.327401634   259 C       -0.610582908     -5.129604328     -1.080012262 C       -0.022043447     -6.364679309     -0.810031734 C        1.418584634     -5.420959738      0.927014075 C        0.974922455     -6.508898948      0.176367420 C       -0.828517832      4.186208841      0.652716101 C       -1.418557090      5.420978365      0.927023458 C       -0.974877512      6.508910961      0.176377457 C        0.610603181      5.129590892     -1.080005558 C        0.022084640      6.364675499     -0.810023204 H       -2.185640492      5.532140513      1.687573240 H       -1.407427630      7.489152282      0.354366486 H        0.340622057      7.236754261     -1.373711176 H        1.378143988      5.029932719     -1.841551913 H        2.185671830     -5.532109556      1.687561837 H        1.407490020     -7.489133020      0.354354155 H       -0.340566435     -7.236762728     -1.373720659 H       -1.378125647     -5.029958088     -1.841558232  [Pt(bC^N^bC)Cl] (1e- reduced) N       -1.023231623      2.922749803      1.200642201 N        0.527764896      2.702242765     -0.305534040 N        1.023332902     -2.922797667      1.200618155 N       -0.527640079     -2.702204690     -0.305557599 N        0.000127121     -0.000010419     -1.296717278 C        2.053581886     -2.643961738      2.188204737 C       -2.053505050      2.643808341      2.188171706 C        0.820441900     -4.185873464      0.653770122 C        0.202770996     -2.020413570      0.609871250 C       -0.202669089      2.020404007      0.609837869 C        0.173437475      4.044209222     -0.324484620 C        1.429003700      2.002776113     -1.225741485 C       -1.428846577     -2.002719068     -1.225770781 C       -0.000049070      0.000143747     -4.149439595 C       -0.679068335     -0.997774509     -3.400763195 C        0.679052819      0.997987630     -3.400744663 C       -0.680151338     -0.976859805     -2.027178296 C        0.680303911      0.976931545     -2.027160161 H        3.023277515     -2.973298861      1.803086771 H        2.076565472     -1.572264565      2.380050456 H       -3.023295464      2.972583922      1.802807667 H       -2.076047413      1.572162767      2.380365173 H        1.892151678      2.736775163     -1.885914657 H        2.218422906      1.532803590     -0.624772415 H       -1.891998296     -2.736707333     -1.885953853 H       -2.218269477     -1.532744016     -0.624807055 H       -0.000118766      0.000201980     -5.234184220 H       -1.221837745     -1.793529358     -3.906116402   260 H        1.221758744      1.793796365     -3.906081239 H       -1.833765758      3.172417793      3.120069140 H        1.833482288     -3.172166619      3.120248309 Pt       0.000112198      0.000013820      0.735160562 Cl       0.000166063      0.000068771      3.117493274 C       -0.173425396     -4.044201295     -0.324474526 C       -0.613302414     -5.131981239     -1.081168435 C       -0.015600946     -6.365090903     -0.818961823 C        1.419861983     -5.417922631      0.920263488 C        0.983742136     -6.506831030      0.165091826 C       -0.820453737      4.185824944      0.653747833 C       -1.419997088      5.417825686      0.920182482 C       -0.983963829      6.506749272      0.164981483 C        0.613233811      5.132006142     -1.081199921 C        0.015415376      6.365070266     -0.819043766 H       -2.190048041      5.526726765      1.678382183 H       -1.424898417      7.484420750      0.337598267 H        0.327986639      7.236621755     -1.387276053 H        1.381931336      5.032765505     -1.841841310 H        2.189881388     -5.526872554      1.678488034 H        1.424582837     -7.484537327      0.337750942 H       -0.328234026     -7.236630328     -1.387178546 H       -1.381971987     -5.032695464     -1.841832113  

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