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The synthesis and reactivity of tantalum diamidophosphine complexes featuring an activated alkyne unit Parker, Kyle Daniel James 2014

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THE SYNTHESIS AND REACTIVITY OF TANTALUM DIAMIDOPHOSPHINE COMPLEXES FEATURING AN ACTIVATED ALKYNE UNIT  by Kyle Daniel James Parker  B.Sc.(H), McMaster University, 2006  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)  June 2014  © Kyle Daniel James Parker, 2014   ii Abstract  A series of tantalum complexes supported by the diamidophosphine ligand [PhNPN*] were synthesized using Ta(alkyne)Cl3(DME), Ta(V) reagents that feature a reduced alkyne unit: [PhNPN*]Ta(3-hexyne)X, and [PhNPN*]Ta(BTA)X (X = Cl, H, alkyl, N3; BTA = bis(trimethylsilyl)acetylene). For these complexes, the bonding and reactivity at tantalum is best understood as a combination of both the high-valent ‘Ta(V)-alkenediyl’ and low-valent ‘Ta(III)-alkyne’ structural formalisms. The synthesis and reactivity of a series of Ta imide complexes, generated via the displacement of the alkyne ligand with an aryl azide from corresponding [PhNPN*]Ta alkyne complex is reported. In addition, the synthesis and attempted synthesis of Ta alkyne azide and nitride complexes are discussed.  The further reactivity of the Ta imide complexes with aryl azides, and the synthesis of a triazenide moiety is presented. The reactivity of [PhNPN*]Ta alkyne monohydrides with a variety of small molecules was explored. These monohydride complexes combine with 2,6-dimethylphenyl isocyanide and phenylacetylene to form five-membered tantallacyclic products by coupling with the Ta-bound alkyne ligand. A kinetic study of the thermal rearrangement of a Ta alkyne phenylvinyl complex to the corresponding tantallacycle is included. The synthesis of formate and methylene diolate moieties via the reaction of carbon dioxide with multiple equivalents of Ta monohydride was also explored. The hydrogenolysis of [PhNPN*]TaMe3, and several [PhNPN*]Ta alkyne alkyl and hydride complexes were investigated.  The motivation for this work came from the remarkable reactivity observed by a previously reported Ta tetrahydride, ([NPNSi]Ta)2(µ-H)4  (1.71) with various small   iii molecules, including N2. An analogous tetrahydride complex, ([PhNPN*]Ta)2(µ-H)4, (5.4) was synthesized via the high-pressure hydrogenolysis of the [PhNPN*]Ta complexes. Unfortunately, the inertness of 5.4 with respect to N2 means that comparisons to the reactivity observed with 1.71 could not be made. The synthesis and structure of the Ta alkene hydride intermediates formed via low-pressure hydrogenolysis of the Ta alkyne complexes is presented.  Possible mechanisms for the formation of these intermediates are discussed.   The synthesis and proposed structure of a cationic [PhNPN*]Ta imide complex is presented, and potential catalytic applications of this complex are discussed.  Newly synthesized compounds were structurally characterized by a combination of NMR spectroscopy and X-ray crystallographic studies.    iv Preface I, in consultation with my supervisor Professor Michael Fryzuk, designed all experiments described herein.  Furthermore, I wrote all sections of each Chapter included in this thesis. I also synthesized, spectroscopically characterized, and analyzed all experimental data for all of the new compounds described in Chapters 2, 3, 4 and 6.   A portion of the experimental work described in Chapter 5 was performed by Dr. Dominik Nied: specifically, the synthesis and spectroscopic characterization of compound 5.3 (Section 5.2.1) and the synthesis of compound 5.4 from compound 5.3 (a portion of Section 5.2.2).  In addition, the samples used for single crystal X-ray diffraction analysis of compounds 5.3 and 5.4 were prepared by Dr. Dominik Nied, and processed by Dr. Nathan Halcovitch.  I conducted all other experimental work reported in Chapter 5, including the alternative synthesis of 5.4 from compounds 2.4 – 2.8.  In addition, I wrote all sections of Chapter 5, including the analysis and discussion of all results described therein.     v Table of Contents  Abstract .......................................................................................................................................... ii	  Preface ........................................................................................................................................... iv	  Table of Contents ...........................................................................................................................v	  List of Tables ................................................................................................................................ ix	  List of Figures ............................................................................................................................... xi	  List of Schemes ............................................................................................................................. xv	  List of Symbols ......................................................................................................................... xxiv	  List of Abbreviations ............................................................................................................... xxvi	  Acknowledgements .................................................................................................................. xxix	  Chapter  1: Introduction ...............................................................................................................1	  1.1	   Group 5 Metal–Alkyne Complexes ................................................................................... 1	  1.1.1	   Overview of Alkyne Ligands and Metal–Alkyne Complexes .................................... 1	  1.1.2	   Bonding in Transition Metal–Alkyne Complexes ...................................................... 5	  1.1.3	   The Synthesis of Tantalum and Niobium Alkyne Complexes .................................... 7	  1.1.4	   The Reactivity of Tantalum and Niobium Alkyne Complexes ................................ 22	  1.2	   Tantalum Amidophosphine Complexes, and Ligand Design in the Fryzuk Group ......... 29	  1.3	   Scope of Thesis ................................................................................................................ 37	  Chapter  2: Synthesis of Tantalum Complexes of [PhNPN*], a Diamidophosphine Ligand Featuring an ortho-Phenylene Backbone ...................................................................................39	  2.1	   Introduction ...................................................................................................................... 39	  2.2	   Results and Discussion .................................................................................................... 41	    vi 2.2.1	   Attempted Synthesis of [PhNPN*]TaCl3 ................................................................... 41	  2.2.2	   Attempted Synthesis of [PhNPN*]TaR3 (R = Alkyl, Amido) Complexes ................ 42	  2.2.3	   Synthesis of [PhNPN*]Ta Complexes Featuring an Activated Alkyne Unit ............. 43	  2.2.4	   Synthesis of [PhNPN*]Ta Monohydride Complexes ................................................ 51	  2.2.5	   Synthesis of [PhNPN*]Ta Alkyl Complexes ............................................................. 55	  2.2.6	   Thermal Decomposition of [PhNPN*]Ta Alkyl Complexes ..................................... 64	  2.3	   Conclusions ...................................................................................................................... 65	  Chapter  3: Synthesis and Reactivity of [PhNPN*]Ta Azide and Imide Complexes ..............67	  3.1	   Introduction ...................................................................................................................... 67	  3.1.1	   Transition Metal Azide and Nitride Complexes ....................................................... 68	  3.2	   Results and Discussion .................................................................................................... 70	  3.2.1	   Synthesis of Tantalum Azide Complexes, and the Attempted Synthesis of a Tantalum Nitride ................................................................................................................... 70	  3.2.2	   Reactions of Tantalum Alkyne Complexes with Organoazides: Synthesis of Tantalum Imides ................................................................................................................... 73	  3.2.3	   Reactions of Tantalum Benzyl Imides with Organoazides: Attempted Synthesis of Tetraazadiene and Triazenide Complexes ............................................................................ 89	  3.3	   Conclusions ...................................................................................................................... 96	  Chapter  4: Reactions of Tantalum Alkyne–Hydride Complexes with Small Molecules ......98	  4.1	   Introduction ...................................................................................................................... 98	  4.2	   Results and Discussion .................................................................................................... 99	  4.2.1	   Reactions of Complexes 2.4 & 2.5 with a Terminal Alkyne .................................... 99	  4.2.2	   Structural Rearrangement of Alkyne-phenylvinyl Complexes 4.1 & 4.3 ............... 101	    vii 4.2.3	   Kinetic Study of the Rearrangement of Complex 4.1 ............................................. 110	  4.2.4	   Reactions of Complexes 2.4 & 2.5 with an Aryl Isocyanide .................................. 112	  4.2.5	   Reactions of Complex 2.4 with Carbon Dioxide .................................................... 120	  4.3	   Conclusions .................................................................................................................... 127	  Chapter  5: The Hydrogenolysis Chemistry of Tantalum Alkyne Complexes, and the Synthesis of a Dinuclear Tantalum Tetrahydride ..................................................................129	  5.1	   Introduction .................................................................................................................... 129	  5.2	   Results and Discussion .................................................................................................. 134	  5.2.1	   The Synthesis and Structure of [PhNPN*]TaMe3 .................................................... 134	  5.2.2	   The Synthesis and Structure of ([PhNPN*]Ta)2(µ-H)4  (5.4) ................................... 137	  5.2.3	   The Synthesis of Tantalum Alkene Hydrides from Tantalum Alkyne Alkyl Complexes ........................................................................................................................... 145	  5.2.4	   The Synthesis of ([PhNPN*]Ta)2(µ-H)4 from Tantalum Alkene Hydrides (and Tantalum Alkyne Benzyl Complexes) ................................................................................ 163	  5.2.5	   The Hydrogenolysis Chemistry of [PhNPN*]Ta(alkyne)H Complexes .................. 165	  5.3	   Conclusions .................................................................................................................... 170	  Chapter  6: Thesis Synopsis and Future Directions ...............................................................172	  6.1	   Thesis Synopsis .............................................................................................................. 172	  6.2	   Future Directions ........................................................................................................... 176	  6.2.1	   Synthesis and Potential Reactivity of a Cationic Tantalum Imide ......................... 176	  6.2.2	   Expansion to [PhNPN*] Niobium Complexes ......................................................... 183	  6.2.3	   Investigating the Reactivity of ([PhNPN*]Ta)2(µ-H)4 (5.4) ..................................... 189	  6.3	   Final Conclusions .......................................................................................................... 192	    viii Chapter  7: Experimental Details .............................................................................................194	  7.1	   General Procedures ........................................................................................................ 194	  7.1.1	   Laboratory Equipment and Procedures ................................................................... 194	  7.1.2	   Solvents ................................................................................................................... 194	  7.1.3	   Starting Materials and Reagents ............................................................................. 194	  7.1.4	   Instrumentation and Methods of Analysis .............................................................. 195	  7.2	   Synthesis of Compounds ............................................................................................... 197	  7.2.1	   Complexes Pertaining to Chapter 2 ........................................................................ 197	  7.2.2	   Complexes Pertaining to Chapter 3 ........................................................................ 205	  7.2.3	   Complexes Pertaining to Chapter 4 ........................................................................ 212	  7.2.4	   Complexes Pertaining to Chapter 5 ........................................................................ 220	  7.2.5	   Complexes Pertaining to Chapter 6 ........................................................................ 225	  Bibliography ...............................................................................................................................227	  Appendices ..................................................................................................................................248	  Appendix A : Variable-temperature NMR Study of [PhNPN*]Ta(3-hexyne)Cl (2.2) ............ 248	  Appendix B : Kinetic Study of the Thermal Rearrangement of 4.1 to 4.2 ............................. 251	  Appendix C : X-ray Crystallographic Structure and Refinement Data .................................. 255	     ix List of Tables  Table 2.1: Selected bond lengths (Å) and angles (°) for 2.2 ........................................................ 46	  Table 2.2: Selected bond lengths (Å) and angles (°) for 2.3 ........................................................ 49	  Table 2.3: Selected bond lengths (Å) and angles (°) for 2.4 ........................................................ 54	  Table 2.4: Selected bond lengths (Å) and angles (°) for 2.6 ........................................................ 58	  Table 2.5: Selected bond lengths (Å) and angles (°) for 2.7 ........................................................ 59	  Table 2.6: Selected bond lengths (Å) and angles (°) for 2.8. ....................................................... 63	  Table 3.1: Selected bond lengths (Å) and angles (°) for 3.3 ........................................................ 76	  Table 3.2: Selected bond lengths (Å) and angles (°) for 3.5 ........................................................ 81	  Table 3.3: Selected NMR data for the benzyl moiety in complexes 2.6, 2.7, 3.5 and 3.6 ........... 83	  Table 3.4: Selected bond lengths (Å) and angles (°) for 3.7 ........................................................ 93	  Table 4.1: Selected bond lengths (Å) and angles (°) for 4.2. ..................................................... 104	  Table 4.2: 1H and 13C{1H} NMR spectral assignments for 4.2, and a schematic representation of the core of 4.2; this depiction is only meant to indicate connectivity, and does not accurately reflect the bonding in the tantallacycle. ...................................................................................... 108	  Table 4.3: Selected 1H and 13C NMR spectra assignments for 4.4, and a schematic representation of the core of 4.4; this depiction is only meant to indicate connectivity, and does not accurately reflect the bonding in the tantallacycle. ...................................................................................... 110	  Table 4.4: Selected bond lengths (Å) and angles (°) for 4.6 ...................................................... 115	  Table 5.1: Selected bond lengths (Å) and angles (°) for 5.3 ...................................................... 136	  Table 5.2: Selected bond lengths (Å) and angles (°) for 5.4 ...................................................... 140	  Table 5.3: Selected bond lengths (Å) and angles (°) for 5.5 ...................................................... 146	    x Table 5.4: 1H and 13C NMR spectral assignments for the 1-hexene ligand of complex 5.6. ..... 158	  Table 5.5: Summary of the experimental conditions for the formation of the [PhNPN*]Ta alkene hydrides 5.5 or 5.6, via the hydrogenolysis of the alkyne alkyl and hydride complexes 2.4 – 2.8.  All reactions were conducted in 200 mL thick-walled Kontes-sealed glass reactors charged with 1 or 4 atm of H2 and 32 mM (0.11 mmol in 3.5 mL C6D6) solutions of the relevant Ta complex...................................................................................................................................................... 167	     xi List of Figures  Figure 1.1: Possible bonding interactions between the d-orbitals of a transition metal and the π-bonding (I, III) and π*-antibonding (II, IV) orbitals of an alkyne ligand, according to the Dewar-Chatt-Duncanson model. ................................................................................................................. 5	  Figure 1.2: Two possible resonance structures for Ta–alkyne complexes; [M] = TaX3, X = monoanionic ligand (e.g. halide, hydride, alkyl, amide). ............................................................... 6	  Figure 1.3: Examples of amidophosphine ligand sets synthesized by the Fryzuk group. ........... 30	  Figure 2.1: ORTEP drawing of the solid-state molecular structure of 2.2 (ellipsoids at 50% probability).  All hydrogen atoms and the mesityl group at N02 (except for Cipso) have been omitted for clarity. ........................................................................................................................ 46	  Figure 2.2: Two possible resonance structures for [PhNPN*]Ta(alkyne)Cl complexes. ............. 47	  Figure 2.3: ORTEP drawing of the solid-state molecular structure of 2.3 (ellipsoids at 50% probability).  All hydrogen atoms and the mesityl group at N02 (except for Cipso) have been omitted for clarity. ........................................................................................................................ 49	  Figure 2.4: ORTEP drawing of the solid-state molecular structure of 2.4 (ellipsoids at 50% probability).  All hydrogen atoms (except for H99, which was located from the difference map and refined isotropically), and the mesityl group at N01 (except for Cipso) have been omitted for clarity. ........................................................................................................................................... 54	  Figure 2.5: ORTEP drawing of the solid-state molecular structure of 2.6 (ellipsoids at 50% probability).  All hydrogen atoms and the mesityl group at N02 (except for Cipso) have been omitted for clarity.  The phenyl ring of the benzyl arm appears in the difference map as   xii disordered over two discrete positions; although only one orientation is displayed, both orientations were located and modeled anisotropically. ............................................................... 58	  Figure 2.6: ORTEP drawing of the solid-state molecular structure of 2.7 (ellipsoids at 50% probability).  All hydrogen atoms and the mesityl group at N01 (except for Cipso) have been omitted for clarity. ........................................................................................................................ 59	  Figure 2.7: ORTEP drawing of the solid-state molecular structure of 2.8 (ellipsoids at 50% probability).  All hydrogen atoms and the mesityl group at N02 have been omitted for clarity. . 63	  Figure 3.1: Solid-state molecular structure of 3.1.  All hydrogen atoms have been omitted for clarity. Only one of the three molecules present in the asymmetric unit is shown.  Poor quality data did not allow for the determination of metrical parameters. ................................................. 72	  Figure 3.2: ORTEP diagram of the solid-state molecular structure of 3.3 (ellipsoids at 50% probability). Half of the molecule is generated by the symmetry operation x, -y–½ , z.  All hydrogen atoms and the mesityl group at N01 (except for Cipso), as well as the disorder that affects the p-isopropylphenyl group and sections of the [PhNPN*] ligand have been omitted for clarity. ........................................................................................................................................... 75	  Figure 3.3: The relative ligand orientations of various [PhNPN*]Ta alkyne complexes. ............ 79	  Figure 3.4: ORTEP diagram of the solid-state molecular structure of 3.5 (ellipsoids at 50% probability).  All hydrogen atoms and the mesityl group at N01 (except for Cipso) have been omitted for clarity. ........................................................................................................................ 81	  Figure 3.5: ORTEP diagram of the solid-state molecular structure of 3.7 (ellipsoids at 50% probability).  All hydrogen atoms and the mesityl group at N02 (except for Cipso) have been omitted for clarity. ........................................................................................................................ 92	    xiii Figure 4.1: ORTEP depiction of the solid-state molecular structure of 4.2 (ellipsoids at 50% probability).  All hydrogen atoms (except H1 and H2) have been omitted for clarity; H1 and H2 were located from the difference map and refined isotropically. ............................................... 103	  Figure 4.2: Truncated ORTEP depiction of the core of 4.2. ...................................................... 104	  Figure 4.3: Resonance structures for the tantalum–η4-butadienyl moiety in 4.2 and 4.4. ......... 106	  Figure 4.4: Truncated (δ 0 – 5 region) 1H NMR spectrum (300 MHz, 298 K) of 4.2 in C6D6.  Some residual pentane (*) is also present. .................................................................................. 107	  Figure 4.5: Eyring plot of ln(kobs/T) versus 1/T for the rearrangement of 4.1 to 4.2. ................ 111	  Figure 4.6: ORTEP depiction of the solid-state molecular structure of 4.6 (ellipsoids at 40% probability).  All hydrogen atoms (except H3) have been omitted for clarity; H3 was located from the difference map and refined isotropically. ..................................................................... 114	  Figure 4.7: Truncated ORTEP depiction of the core of 4.6. ...................................................... 115	  Figure 4.8: Possible coordination modes of 1-aza-1,3-butadiene (X = NR, Y = CR) and 1,4-diazabutadiene (X = Y = NR) moieties to a metal centre (‘M’). ................................................ 116	  Figure 4.9: Resonance structures for the tantalum–AD moiety in 4.5 & 4.6. ........................... 117	  Figure 4.10: Possible synthetic routes to η2–imine, η2–iminoacyl/iminoformyl, and 1,3-diazabutadiene/ 1-aza-1,3-butadiene moieties. ........................................................................... 119	  Figure 4.11: Two possible formate binding modes for 4.7 – monodentate (η1-O-O(O)CH, left) and bidentate (η2-O, O-O(O)CH, right). ..................................................................................... 123	  Figure 4.12: Bidentate ligand coordination in generalized Ta formate (left) and Ta amidate (right) complexes ........................................................................................................................ 124	  Figure 4.13: Proposed structure of the dinuclear Ta methylene diolate complex 4.8. .............. 126	    xiv Figure 5.1: ORTEP diagram of the solid-state molecular structure of 5.3 (ellipsoids at 50% probability).  All hydrogen atoms and the mesityl group at N01 (except for Cipso) have been omitted for clarity. ...................................................................................................................... 136	  Figure 5.2: ORTEP diagram of the solid-state molecular structure of 5.4 (ellipsoids at 30% probability).  All hydrogen atoms and the mesityl groups at N01 and N02 (except for Cipso) have been omitted for clarity.  Half of the molecule is generated by the symmetry operation:  2 - x, y, ½ - z.  The bridging hydride moieties could not be located in the difference map. ................... 139	  Figure 5.3: ORTEP diagram of the solid-state molecular structure of 5.5 (ellipsoids at 50% probability).  All hydrogen atoms (except for H39, H43 and H100) and the mesityl group at N02 (except for Cipso) have been omitted for clarity; H39, H43 and H100 were located from the difference map and refined isotropically. ................................................................................... 145	  Figure 5.4: Truncated (δ 0 – 3 region) 1H NMR spectrum (400 MHz, 298 K) of 5.6 in C6D6.  Some residual pentane (*) and polydimethylsiloxane grease (#) is also present. Assignments for C atoms are labeled in red. .......................................................................................................... 158	  Figure 5.5: i) 2H NMR spectrum (61 MHz, 293 K, C6H6) of 5.6–dn. ii) 1H NMR spectrum (400 MHz, 293 K, C6D6) of 5.6–dn. iii) 1H NMR spectrum (400 MHz, 293 K, C6D6) of 5.6. Some residual pentane (*) and polydimethylsiloxane grease (#) is also present. ................................. 159	     xv List of Schemes  Scheme 1.1: Some examples of late transition metal-alkyne complexes. ...................................... 2	  Scheme 1.2: Synthesis of Ti-alkyne complexes from Ti(II) metallocene precursors: 1.4 (Fachinetti9,10), 1.5 (Bercaw12). ....................................................................................................... 3	  Scheme 1.3: Synthesis of Ti and Zr alkyne complexes via the reduction of M(IV) metallocene precursors by Rosenthal and coworkers. ........................................................................................ 4	  Scheme 1.4: Synthesis of bimetallic alkyne complexes from the low-valent Group 5 chlorides M2Cl6(THT). ................................................................................................................................... 8	  Scheme 1.5: Synthesis of an alkyne-bridged bimetallic complex, via the addition of a sterically bulky alkyne to Ta2Cl6(THT)3. ....................................................................................................... 9	  Scheme 1.6: Synthesis of the mononuclear Ta and Nb alkyne trichloride complexes 1.15 and 1.16, via reduction of the corresponding metal pentahalide. ........................................................ 10	  Scheme 1.7: Examples of the use of 1.15 and 1.16 to synthesize alkyne complexes with various ancillary ligands: 1.17 and 1.18 (Etienne33-39), 1.19 (Otero40), 1.20 (Hill42). ............................... 11	  Scheme 1.8: Synthetic routes to monocyclopentadienyl Ta and Nb alkyne complexes. ............. 13	  Scheme 1.9: Synthetic routes to metallocene and ansa-metallocene Ta and Nb alkyne complexes. .................................................................................................................................... 14	  Scheme 1.10: Synthetic routes to niobocene alkyne and diyne complexes, via the stable Nb(III) complex 1.27. ................................................................................................................................ 15	  Scheme 1.11: Synthesis of the Ta alkyne complex 1.31 via reduction of the Ta trichloride precursor 1.30. .............................................................................................................................. 16	    xvi Scheme 1.12: Synthetic route to Ta(V) alkyne complexes, via the stable Ta(III) siloxide complex 1.32................................................................................................................................................ 16	  Scheme 1.13: Synthesis of Ta alkyne complexes (1.38 – 1.41) via the reduction of the corresponding Ta aryloxide or aryl imide chlorides.  In some cases, these complexes go on to form Ta butadienediyl or η6-arene complexes when treated with additional equivalents of alkyne........................................................................................................................................................ 18	  Scheme 1.14: Synthesis of Ta and Nb alkyne complexes via the reductive elimination of H2. .. 19	  Scheme 1.15: Synthesis of Ta and Nb alkene hydride complexes (adapted from ref. 63, 67–69)........................................................................................................................................................ 20	  Scheme 1.16: Synthesis of an alkyne complex (1.49) from a stable Nb(III) species (1.48), generated via the reductive hydrogenolysis of a Nb dimethyl precursor. ..................................... 21	  Scheme 1.17: Synthesis of a Ta alkyne complex via the reductive elimination of TMSCl. ........ 22	  Scheme 1.18: The cyclotrimerization and polymerization of alkynes, catalyzed by the low-valent Group 5 chloride complexes M2Cl6(THT)3; [M] = TaCl3 or NbCl3. ............................................ 23	  Scheme 1.19: Formation of the metallacycle 1.53 via the insertion of coordinated alkyne ligands into the Ta-Cacetylide bond. .............................................................................................................. 24	  Scheme 1.20: Examples of the synthesis of Ta butadienediyl complexes via the insertion of a second alkyne unit into a Ta–Calkyne bond. .................................................................................... 25	  Scheme 1.21: Reaction of an equivalent of alkyne or nitrile with a Ta butadienediyl complex to generate the corresponding Ta(η6-arene) or Ta(η2-pyridine) species. .......................................... 26	  Scheme 1.22: Examples of the formation of 5-membered tantallacycles via the insertion of an equivalent of ketone or aldehyde into a Ta–Calkyne bond. ............................................................. 27	    xvii Scheme 1.23: Examples of tantallacycle formation via the coupling of a Ta alkyne moiety with various ligands: 1.65 (Curtis and Real87), 1.67 (Etienne and Templeton34), 1.69 (Herberich64). . 28	  Scheme 1.24: Synthesis of the dinuclear [NPNSi]Ta dinitrogen complex 1.72 from the Ta tetrahydride 1.71. .......................................................................................................................... 31	  Scheme 1.25: Potential catalytic cycle for the generation of amine and hydrazine products via the coupling of electrophilic substrates (E–H) with molecular N2, mediated by complexes 1.71 and 1.72. ........................................................................................................................................ 33	  Scheme 1.26: Examples of [NPNSi] ligand degradation and rearrangement, as a result of the reaction of complex 1.72 with electrophiles. ................................................................................ 34	  Scheme 1.27: Synthetic route to [PhNPN*]Li2S2 (adapted from ref 109). ................................... 35	  Scheme 1.28: Synthesis of a Zr-dinitrogen complex supported by the [PhNPN*] ligand set, and subsequent reactivity of the coordinated N2-unit with electrophiles. ........................................... 36	  Scheme 2.1: Examples of coordination complexes of the [NPNSi] ligand set synthesized via salt metathesis. ..................................................................................................................................... 39	  Scheme 2.2: Some examples of Group 4 complexes of the [PhNPN*] ligand set synthesized via a protonolysis reaction. .................................................................................................................... 40	  Scheme 2.3: The preparation of Ta(alkyne)Cl3(DME) complexes from TaCl5 (top), and their use in the synthesis of Ta alkyne chloride complexes of the [PhNPN*] ligand set (bottom). ............. 43	  Scheme 2.4: i) The conversion of a Ta(alkyne) complex to the corresponding diiodide, (by Cummins and coworkers) and ii) the proposed conversion of [PhNPN*]Ta(alkyne)Cl complexes to their corresponding Ta trihalide, via the displacement of an alkyne ligand with I2. ................ 50	  Scheme 2.5: Synthesis of the [PhNPN*]Ta alkyne hydride complexes 2.4 and 2.5. .................... 51	  Scheme 2.6: Synthesis of the [PhNPN*]Ta alkyne benzyl complexes 2.6 and 2.7. ...................... 55	    xviii Scheme 2.7: Two routes for the preparation of [PhNPN*]Ta(BTA)Et (2.8). ............................... 61	  Scheme 2.8: Formation of the cyclometallated complex [PhNPNC*]Ta(BTA) (2.9), the proposed result of the thermal decomposition of the [PhNPN*]Ta(BTA) alkyl complexes 2.7 and 2.8. ..... 65	  Scheme 3.1: Two possible resonance structures for [PhNPN*]Ta alkyne complexes. ................. 67	  Scheme 3.2: Possible synthetic route to a Ta(V) nitride, via extrusion of N2 from a Ta(III)-alkyne azide complex. ................................................................................................................... 69	  Scheme 3.3: Synthesis of a Nb(V) nitride via the extrusion of N2 from a Nb(III) azide complex (adapted from ref. 179). ................................................................................................................ 70	  Scheme 3.4: Synthesis of the [PhNPN*]Ta alkyne azide complexes 3.1 and 3.2. ........................ 71	  Scheme 3.5: Synthesis of the [PhNPN*]Ta imido chloride complex 3.3. ..................................... 77	  Scheme 3.6: Two routes for the preparation of [PhNPN*]Ta(NAr)N3 (3.4). ............................... 78	  Scheme 3.7: Synthesis of the [PhNPN*]Ta imido benzyl complex, 3.5, from the [PhNPN*]Ta(BTA) benzyl complex, 2.7. ...................................................................................... 80	  Scheme 3.8: Synthesis of the [PhNPN*]Ta imido benzyl complex, 3.6, from the [PhNPN*]Ta(3-hexyne) benzyl complex, 2.6. ....................................................................................................... 82	  Scheme 3.9: The synthesis of [PhNPN*]Ta imido benzyl complexes (3.5 and 3.6) from [PhNPN*]Ta alkyne benzyl complexes (2.6 and 2.7) and an organoazide. ................................... 85	  Scheme 3.10: Proposed mechanism for the formation of the Ta imide complexes 3.3 – 3.6 from their Ta alkyne precursors ([Ta] = [PhNPN*]TaX, X = Bn or Cl), adapted from Bergman and coworkers (ref 189). ...................................................................................................................... 86	  Scheme 3.11: Possible dissociative mechanisms for the reaction of an organoazide with the Ta alkyne complexes 2.2, 2.3, 2.7 (top) or 2.6 (bottom), where the relative position of the vacant coordination site is maintained. .................................................................................................... 87	    xix Scheme 3.12: Dissociative mechanism for the reaction of an organoazide with complexes 2.6 or 2.7 that proceeds via a common 4-coordinate intermediate. ........................................................ 88	  Scheme 3.13: Proposed associative mechanisms for the reaction of an organoazide with the Ta alkyne complexes 2.2, 2.3, 2.7 (top) or 2.6 (bottom). ................................................................... 89	  Scheme 3.14: Synthesis of a hafnium triazenide moiety, and the subsequent formation of a hafnium amide complex via thermal extrusion of N2 (adapted from ref. 210). ............................ 90	  Scheme 3.15: Formation of a metal-tetraazadiene moiety, via the combination of an organoazide and a metal imide. ......................................................................................................................... 90	  Scheme 3.16: The reaction of [PhNPN*]Ta(NAr)Bn with an organoazide, and the structures of three plausible products (I – III). .................................................................................................. 91	  Scheme 3.17: Three possible resonance structures that describe the bonding interaction between Ta and a triazenide ligand (adapted from ref. 208). ...................................................................... 94	  Scheme 3.18: The reaction of 3.5 and an organoazide, which affords 3.7 (~60%) and several unidentified products. ................................................................................................................... 96	  Scheme 4.1: Synthesis of the [PhNPN*]Ta alkyne phenylvinyl complexes 4.1 and 4.3, via the reaction of 2.4 or 2.5 with phenylacetylene. ............................................................................... 100	  Scheme 4.2: Rearrangement of the [PhNPN*]Ta alkyne phenylvinyl complexes 4.1 and 4.3 to the tantallacyclic products 4.2 and 4.4. ............................................................................................. 102	  Scheme 4.3: Possible mechanism for the formation of 4.2 and 4.4. .......................................... 107	  Scheme 4.4: Probable synthetic route for the tantallacyclic complexes 4.5 and 4.6, via the reaction of 2.4 or 2.5 with an aryl isocyanide. ............................................................................ 113	  Scheme 4.5: The reduction of CO2 to formaldehyde by an early metal hydride complex (e.g. Schwartz’s reagent, [Zr] = Cp2ZrCl).  Adapted from ref. 274. ................................................... 120	    xx Scheme 4.6: Synthesis of a dinuclear Ta methylene diolate complex by Cummins and coworkers (adapted from ref. 41). ................................................................................................................ 121	  Scheme 4.7: Synthesis of the dinuclear Ta methylene diolate complex ([NPNSi]Ta)2(µ-OCH2O)4(µ-H)2  (adapted from ref. 220) .................................................................................... 121	  Scheme 4.8: Synthesis of the [PhNPN*]Ta hexyne formate complex 4.7. ................................. 122	  Scheme 4.9: Two routes for the synthesis of the dinuclear Ta methylene diolate complex 4.8. 125	  Scheme 5.1: The hydrogenolysis of a metal–alkyl bond to generate a metal hydride functionality.  As an example of σ-bond metathesis, this reaction proceeds via a concerted 4-centred transition state. ............................................................................................................... 129	  Scheme 5.2: The relative rates of σ-bond metathesis of scandium alkyl complexes ([Sc] = Cp*2Sc) with various hydrocarbons: R’ = sp carbon > R’ = sp2 carbon > R’ = sp3 carbon. ...... 130	  Scheme 5.3: The possible synthesis of a [PhNPN*] Ta hydride complex via the hydrogenolysis of the corresponding Ta alkyne complex. ....................................................................................... 131	  Scheme 5.4: Previous examples from the Fryzuk group of the synthesis of the dinuclear Ta tetrahydride complexes via the hydrogenolysis of the corresponding Ta alkyl complexes. Adapted from refs. 90 and 138. .................................................................................................. 132	  Scheme 5.5: Synthesis of the Ta dinitrogen complex 1.72 from the dinuclear tetrahydride 1.71...................................................................................................................................................... 133	  Scheme 5.6: Synthesis of the [PhNPN*]Ta trimethyl complex 5.3. ........................................... 135	  Scheme 5.7: Synthesis of the dinuclear [PhNPN*]Ta tetrahydride complex 5.4 from complex 5.3; performed by Dr. Dominik Nied. ................................................................................................ 137	  Scheme 5.8: Synthesis of complex 5.4 from the Ta alkyne alkyl complexes 2.6 – 2.8, via the intermediacy of complexes 5.6 and 5.5; performed by K. Parker. ............................................. 138	    xxi Scheme 5.9: Relative orientations of the [PhNPN*] ligands in complex 5.4, as reflected by the low (263 K) and high (363 K) temperature 1H NMR spectra. .................................................... 142	  Scheme 5.10: Examples of the reactivity of Ta complexes featuring variations of the [NPNSi] ligand set. .................................................................................................................................... 144	  Scheme 5.11: Synthesis of the [PhNPN*]Ta alkene hydride complex 5.5. ................................ 146	  Scheme 5.12: Two possible resonance structures for [PhNPN*]Ta alkene hydride complexes. 147	  Scheme 5.13: Simplified mechanism for the hydrogenolysis of alkynes by Wilkinson’s catalyst (P = PPh3): i) oxidative addition of H2; ii) alkyne coordination; iii) migratory insertion; iv) reductive elimination; v) dissociation of cis-alkene. .................................................................. 149	  Scheme 5.14: Synthesis of the Ta alkene hydride complex 5.5 from complex 2.7, which does not proceed via the intermediacy of the Ta alkyne hydride complex 2.5. ........................................ 150	  Scheme 5.15: Possible mechanism for the formation of 5.5 ([Ta] = [PhNPN*]Ta); the trans-geometry of the alkene ligand is achieved via free rotation around the bond labeled in red. ..... 152	  Scheme 5.16: Synthesis of the Ta 1-hexene hydride complex 5.6 from complex 2.6, which may proceed via the intermediacy of a Ta 3-hexene hydride complex. ............................................. 153	  Scheme 5.17: The putative result of the hydrogenolysis of the benzyl group in complex 2.6: an isomer of complex 2.4 with the position of the alkyne and hydride ligands interchanged. ........ 154	  Scheme 5.18: One iteration of the proposed ‘chain walking’ mechanism for the formation of the 1-hexene ligand in complex 5.6 from the putative 3-hexene isomer. ......................................... 156	  Scheme 5.19: H/D exchange reactions between 5.6 & 5.6–dn, and 5.4 & 5.4–d4. .................... 160	  Scheme 5.20: Formation of the Ta alkyne monodeuteride complex 2.4–d1. ............................. 160	  Scheme 5.21: One iteration of the proposed ‘chain walking’ mechanism for H/D exchange in the 1-hexene ligand in complex 5.6. ................................................................................................. 162	    xxii Scheme 5.22: Formation of the tetrahydride complex 5.4 from the Ta alkyne benzyl complexes 2.6 and 2.7. .................................................................................................................................. 163	  Scheme 5.23: Possible route for the formation of the tetrahydride complex 5.4 from the Ta alkene hydride complexes 5.5 and 5.6. ....................................................................................... 164	  Scheme 5.24: Formation of the tetrahydride complex 5.4 from the Ta alkyne hydride complexes 2.4 and 2.5. .................................................................................................................................. 165	  Scheme 5.25: Possible mechanism for the formation of the Ta alkene hydride complexes (5.5 or 5.6) from the Ta alkyne alkyl (2.6 – 2.8) and alkyne hydride (2.4 and 2.5) complexes, which proceeds via a common Ta vinyl hydride intermediate ([Ta] = [PhNPN*]Ta).  Adapted from Scheme 5.15; the trans-geometry of the alkene ligand is achieved via free rotation around the bond labeled in red. ..................................................................................................................... 167	  Scheme 5.26: The σ-bond metathesis reaction between H2 and the Ta–H moiety of complex 2.4 or 2.5 ([Ta] = [PhNPN*]Ta(alkyne)). .......................................................................................... 169	  Scheme 6.1: The various Ta imide complexes reported in Chapter 3, and the synthesis of a Ta triazenide imide complex via azide insertion into a Ta–C bond. ................................................ 177	  Scheme 6.2: Examples of the reactivity of cationic Ta imides (6.1) with H2 and phenylacetylene to generate the amide hydride (6.2) and amide acetylide (6.3) products, by Bercaw and coworkers. ................................................................................................................................... 178	  Scheme 6.3: An example of the hydroamination of alkynes and aryl amines using neutral and cationic Ta imides, by Bergman and coworkers. ........................................................................ 179	  Scheme 6.4: Lewis acid-mediated rearrangement of the Ta benzyl imide complex 3.5 to its structural isomer 3.6. ................................................................................................................... 180	    xxiii Scheme 6.5: Possible mechanism for the structural rearrangement of the Ta benzyl imide complex 3.5 to its structural isomer 3.6.  (Adapted from ref. 323.) ........................................... 181	  Scheme 6.6: Synthesis of the Ta imide cation 6.6 via benzyl group abstraction by [CPh3][B(C6F5)4]. ........................................................................................................................ 182	  Scheme 6.7: Generation of the Nb dinitrogen complex 6.8 via reduction of N2 by the dinuclear Nb(IV) tetrahydride 6.7, as reported by Kawaguchi and coworkers. ......................................... 184	  Scheme 6.8: Synthesis of low-valent Nb amidophosphine complexes from NbCl3(DME). ...... 186	  Scheme 6.9: The generation of Nb dinitrogen complexes via the reaction of low-valent Nb amidophosphine complexes with N2. .......................................................................................... 187	  Scheme 6.10: The hydrogenolysis chemistry of some low-valent Nb complexes supported by the [P2N2] ligand set. ......................................................................................................................... 188	  Scheme 6.11: Potential synthesis and reactivity of a low-valent [PhNPN*]Nb complex, synthesized from NbCl3(DME). .................................................................................................. 189	  Scheme 6.12: Reactions of the Ta tetrahydride 1.71 with a variety of small molecule substrates...................................................................................................................................................... 191	  Scheme 6.13: The divergent reactivity displayed by two structurally similar zirconocene dinitrogen complexes with H2. .................................................................................................... 193	     xxiv List of Symbols Å  -  Ångström = 10-10 meters α  -  refers to carbon atom adjacent to the metal centre, or the hydrogen atoms attached to this carbon Ar  -  aryl group β  -   refers to 2nd carbon atom from the metal centre, or the hydrogen atoms attached to this carbon 11B  -  boron-11 cm-1  -  reciprocal centimeters (wavenumbers) °C  -  degrees Celsius Cn, Cs, C2v, C2h -  Schönflies point group notation for molecular symmetry δ  -  chemical shift in NMR spectroscopy, measured in parts per million (ppm) Δ  -   heat D  -  deuterium atom (2H isotope) dn  -  n-deuterated; X-dn indicates n 1H atoms are replaced by 2H in compound X 19F  -  fluorine-19 ΔGrot  -  free energy of rotation 1H  -  hydrogen ΔH‡  -  enthalpy of activation ΔS‡  -  entropy of activation   xxv Hz  -   Hertz (seconds-1) hν  - light (energy); denotes a photochemical process or reaction {1H}  -  proton decoupled ηn  -  denotes number of atoms (n) of a ligand bound to a metal centre I  -  nuclear angular momentum quantum number (spin) J  -  Joule (unit of energy); kJ = 103 J nJAB  -  n-bond scalar coupling constant between nuclei A and B, in NMR spectroscopy K  -  Kelvin (unit of temperature) k  -  rate constant λ  -  wavelength, typically measured in nm M  -  molarity, measured in moles/liter (mol/L) µ  -  defines atom(s) bridging multiple metal centres nm  -  nanometer = 10-9 meters ν  -  stretch (with reference to vibrational spectroscopy) 31P  -  phosphorus-31 σv  - vertical reflection plane t½  - half-life T1  - decay constant of spin magnetization in NMR spectroscopy    xxvi List of Abbreviations Anal.  - analysis APT  - Attached Proton Test NMR experiment atm  - atmosphere, unit of pressure (1 atm = 101.3 kPa) b  - broad; bs = broad singlet, bd = broad doublet, etc. Bn  - benzyl group, -CH2Ph BTA  - bis(trimethylsilyl)acetylene, (TMS)C≡C(TMS) BTE  - bis(trimethylsilyl)ethylene, (TMS)HC=C(TMS)H calc.  - calculated COSY  - homonuclear Correlation Spectroscopy NMR experiment Cp  - cyclopentadienyl ring, C5H5 ; Cp* = C5Me5 Cy  - cyclohexyl group, C6H11 d, dd  - doublet, doublet of doublets DME  - dimethoxyethane, MeOCH2CH2OMe EA  - elemental analysis EI–MS  - Electron-ionization mass spectrometry Et  - ethyl group, -CH2CH3 Et2O  - diethyl ether, (CH3CH2)2O FT–IR  - Fourier transform infrared spectroscopy h  - hour HMBC - Heteronuclear Multiple-Bond Correlation NMR experiment HOMO - highest occupied molecular orbital   xxvii HSQC  - Heteronuclear Singlet-Quantum Correlation NMR experiment iBu  - iso-butyl group, -CH2CHMe2 iPr  - iso-propyl group, -CHMe2 LUMO - lowest unoccupied molecular orbital m  - multiplet Me  - methyl group, -CH3 Mes  - mesityl group, 2,4,6-trimethylphenyl mL  - milliliter, 10-3 L nBu  - n-butyl group, -(CH2)3CH3 NMR  - Nuclear Magnetic Resonance spectroscopy nPr  - n-propyl group, -CH2CH2CH3 ORTEP - Oakridge Thermal Ellipsoid Plot Ph  - phenyl group, C6H5 PIPP  - para-isopropylphenyl group, 4-iPrC6H3 Pn  - pentyl group, -(CH2)4CH3 ppm  - parts per million py  - pyridine, C5H5N q  - quartet ref.  - reference rt  - room temperature s  - singlet, or time in seconds sept  - septet   xxviii Tol  - toluene, C6H5CH3 t  - triplet, or time tBu  - tert-butyl group, -CMe3 THF  - tetrahydrofuran, C4H8O THT  - tetrahydrothiophene, C4H8S TMS  - trimethylsilyl group, -SiMe3 UV-Vis - Ultraviolet-visible spectroscopy VT  - variable temperature xs  - excess     xxix Acknowledgements I would like to thank Mike Fryzuk, first and foremost, for all the opportunities he afforded me during my time in his laboratory.  Thanks also goes to all the Fryzuk group members I’ve had the pleasure of working closely with: Howie, Bryan, Rosa, Gab, Rui, Nathan, J.M., Ting, Truman, Lee, Fraser, Alyssa, Hiko, Amanda, Nick and Tiko.   During my time in Mike’s group I’ve been lucky to work with, and learn from, some excellent post-doctoral fellows and visiting scholars: thank you to Ohki, M.J., Owen, Patri, Joachim, Dominik and Thomas.   The work in this thesis would never have been possible if not for the UBC Chemistry Department’s excellent support staff: Dr. Brian Patrick and Ms. Anita Lam for X-ray crystallography; Dr. Maria Ezhova and Dr. Paul Xia (UBC), and Dr. Andrew Lewis (Simon Fraser University) for NMR; Mr. Marshall Lapawa for mass spectrometry.  A special thanks to Messrs. Milan Coschizza and Ken Love for keeping our glove box, vacuum pumps, and anything else that has a motor or an electrical circuit running smoothly over the years.  I also owe a huge debt of gratitude to Ms. Judy Wrinskelle, Ms. Sheri Harbour and the rest of the Chemistry Main Office staff for helping me navigate the various administrative hurdles that always accompany the completion of a PhD. Thanks also to those who helped me with the writing and editing process: Professor Michael Fryzuk, Professor Laurel Schafer, Dr. Nathan Halcovitch and Mr. Lee Wence. Your helpful insights and comments improved the quality of this document immeasurably. To my family and friends: Mom, Dad, Mallary, Audrie, Colleen, Catherine, Joey, Mike, Ty, Gab, Nathan, MT and Watson.  Thank you for all your love and support.    1 Chapter  1: Introduction  1.1 Group 5 Metal–Alkyne Complexes 1.1.1 Overview of Alkyne Ligands and Metal–Alkyne Complexes Alkynes (RC≡CR), along with alkenes (R2C=CR2), belong to a class of unsaturated hydrocarbon ligands that engage in bonding interactions with a wide range of transition metals.1-3  While in most cases an alkyne ligand binds to a single metal centre, some examples are known where an alkyne bridges two metal centres; Scheme 1.1 depicts three examples of transition metal–alkyne complexes that illustrate these different binding modes.  Complex 1.1, K[PtCl3(RC≡CR)], was synthesized4 via a substitution reaction with the well-known alkene complex Zeise’s salt,5 (K[PtCl3(H2C=CH2)]), and illustrates the structural similarities between metal-alkyne and metal-alkene complexes.  In both cases, the ligand binds ‘side-on’ to the metal centre through the C–C multiple bond; the specific details of transition metal-alkyne bonds will be discussed in Section 1.1.2.  Complex 1.2, Co2(CO)6(RC≡CR), is used as a reagent in the Pauson-Khand reaction, which generates cyclopentenones from an alkyne, an alkene and CO.6,7   In complex 1.3, alkyne moieties are coordinated to the iron centre as both bridging and terminal ligands.8     2  Scheme 1.1: Some examples of late transition metal-alkyne complexes.  Each of the late (Group 8 – 10) transition metal examples discussed above (1.1 – 1.3) were synthesized by the direct addition of an alkyne to an electron-rich, low- or zero-valent (i.e. Pt(II), Co(0), Fe(0)) metal centre.  Similarly, some of the first examples9-12 of early (Group 4 or 5) transition metal–alkyne complexes employed a comparable synthetic methodology, as shown in Scheme 1.2; adding an equivalent of alkyne to formally Ti(II) metallocenes affords the alkyne complexes such as 1.4 and 1.5.  FePtClClClPtClClCl CoCo 1.2R = alkyl, PhRR (CO)3(CO)3Co2(CO)8 RC CRRC CR RR1.1R = alkylKK - 2 CO C C FeFe RR C CRR C C RRRRRC CR- toluene,2 ethene 1.3R = TMS- ethene  3  Scheme 1.2: Synthesis of Ti-alkyne complexes from Ti(II) metallocene precursors: 1.4 (Fachinetti9,10), 1.5 (Bercaw12).  However, most early metal-alkyne complexes are generated via the reduction of an electron-poor, high-valent (typically d0) metal complex in the presence of an alkyne.  Rosenthal and Burlakov,1,13 informed by initial studies by Vol’pin and Shur,14,15 have developed an extensive body of work surrounding the chemistry of titanocene16 (1.6) and zirconocene17 (1.7) alkyne complexes (Scheme 1.3). In many cases, the reactivity of these complexes suggests that they can be regarded as synthons for ‘Cp2M’ (i.e. formally M(II), d2) metallocenes.1,13,18,19  Ti 1.4COCO 1.5Ti RC CRPhC CPhR = Me, Ph Ti CO PhPhTi RR- ethene  4  Scheme 1.3: Synthesis of Ti and Zr alkyne complexes via the reduction of M(IV) metallocene precursors by Rosenthal and coworkers.  The work presented in this thesis deals exclusively with alkyne complexes of tantalum, a Group 5 metal; the following section will discuss the bonding and structure of Group 5 metal–alkyne complexes, with a focus on the heavier analogues Nb and Ta.  Subsequent sections will provide a brief survey of the many synthetic routes to Nb and Ta alkyne complexes, as well as an overview of common reactivity patterns for these complexes.  Ti ClCl RC CR' Ti RR'Zr ClCl Zr L R'RRC CR'L, - MgCl2- MgCl2+ Mg+ Mg 1.7: R = TMSR' = TMS, tBuL = THF, py1.6: R = TMS R' = TMS, Ph, tBu  5 1.1.2 Bonding in Transition Metal–Alkyne Complexes The bond between an alkyne ligand and a transition metal can be described using the Dewar-Chatt-Duncanson model,20-22 as depicted in Figure 1.1; the filled π orbitals from one of the C–C multiple bonds can form a σ-bonding interaction with an empty orbital on the metal centre (I). The overall interaction is synergistic, with π-backdonation from filled metal d-orbitals to empty π* orbitals of the alkyne (II).  The C–C multiple bond of the alkyne perpendicular to the MCC plane can also engage in π-bonding (III) and δ-backbonding (IV) interactions, also shown in Figure 1.1.   Figure 1.1: Possible bonding interactions between the d-orbitals of a transition metal and the π-bonding (I, III) and π*-antibonding (II, IV) orbitals of an alkyne ligand, according to the Dewar-Chatt-Duncanson model.  Alkynes are known to act as either two- or four-electron donors,23 mediated in part by the electrophilicity of the metal centre; the bond between a metal and an alkyne ligand can be described by the two structural formalisms that reflect these extremes.  In the case of Group 5 (M = Ta or Nb) alkyne complexes, one possibility is that of a dative bond between M(III) and a neutral (two- or four-electron donating) alkyne ligand.  Alternatively, the interaction can be M CC M CC M C Mσ - bond π - backbond π - bond δ - backbondC CC(I) (II) (III) (IV)zy zx  6 viewed as the donation of four electrons via two formal covalent bonds between M(V) and an ‘alkenediyl’ dianion, which results in a ‘metallacyclopropene’ structure.  These two scenarios are depicted in Figure 1.2.    Figure 1.2: Two possible resonance structures for Ta–alkyne complexes; [M] = TaX3, X = monoanionic ligand (e.g. halide, hydride, alkyl, amide).  For a given metal–alkyne complex, there are several pieces of structural and spectroscopic data that can help determine which of these two bonding models is a more appropriate description.  In the context of the Dewar-Chatt-Duncanson model (Figure 1.1), the concomitant donation of electrons from alkyne π-bonding orbitals (I and III), and acceptance of electrons into the π*-antibonding orbitals (II and IV) generally results in a reduction in the triple bond character of the alkyne unit.  This leads to elongation of the C–C bond, a metric that can be probed by single-crystal X-ray diffraction studies. Other metrical parameters, such as the angle formed between the C–C bond and the alkyne substituent R (𝛳C–C–R), are useful indicators of the hybridization of the alkyne carbon atoms; bond angles that deviate significantly from 180° suggest sp2 hybridization, which is indicative of an alkenediyl-type ligand.  Vibrational spectroscopy can also be a useful tool for probing the strength of the metal-alkyne interaction.24  [M]RRRR M(III) + M(V) +'alkenediyl' ligandalkyne ligand[M]  7 As with a crystallographically determined elongation of the C–C bond, a decrease in the alkyne stretching frequency (νC–C) also correlates with a weaker C–C bond; a weakened C–C bond is indicative of an increased degree of electron donation and back-donation between the metal centre and the alkyne ligand.  In addition, 13C NMR experiments can be useful in elucidating the nature of the metal-alkyne interaction.23,24  The quaternary carbons of an alkyne strongly bound to a metal centre as a four-electron donor generally gives rise to resonances that are significantly downfield (> δ 180).  In contrast, alkyne ligands that interact with the metal centre as a two-electron donor feature 13C NMR resonances similar to those of the free alkyne (~ δ 120).  In most Group 5 metal alkyne complexes, the structural and spectroscopic data indicate that the M(V)–alkenediyl formalism is the more apt bonding description.  Nevertheless, the term ‘metal-alkyne’ complex will be used throughout this thesis to refer to all Ta and Nb complexes that feature an alkyne ligand; this nomenclature is chosen for simplicity and brevity, despite (and acknowledging) the fact that the spectroscopic and crystallographic data usually points more towards a metallacyclopropene-type structure.  1.1.3 The Synthesis of Tantalum and Niobium Alkyne Complexes While there are a variety of methods for the preparation of Nb(V) and Ta(V) alkyne complexes, the common feature of all synthetic routes is the addition of an alkyne to a M(III) metal centre, although the source of (or means of generating) this low-valent precursor varies. One of the first studies of Group 5 alkyne complexes was undertaken by Cotton and coworkers, using the formally M(III) starting material M2Cl6(THT)3 (1.8: M = Ta,25-27 1.9: M = Nb;28 THT = tetrahydrothiophene, C4H8S). Equimolar mixtures of the trivalent metal precursors and alkyne substrates react smoothly to generate dimeric M(V) complexes, the structures of which vary   8 considerably based on the relative steric bulk of the alkyne (RC≡CR’). As shown in Scheme 1.4, most disubstituted alkynes such as diphenylacetylene (R = R’ = Ph) or tert-butylmethylacetylene (R = CMe3, R’ = Me) give rise to chloride bridged bimetallic complexes [MCl2(L)(RC≡CR’)]2(µ-Cl)2 (1.10 – 1.12).    Scheme 1.4: Synthesis of bimetallic alkyne complexes from the low-valent Group 5 chlorides M2Cl6(THT).  In contrast, treating Ta2Cl6(THT)3 (1.8) with the bulkier di-tert-butylacetylene (R, R’ = CMe3) affords the alkyne bridged dimer, [TaCl3(L)]2(µ-η2:η2-RC≡CR’) (1.15, Scheme 1.5).26  This unusual bonding motif features a short Ta–Ta distance (2.677(1) Å), which the authors attribute to a metal-metal double bond between two Ta(III) centres.  Nevertheless, the elongated C–C distance (1.351(21) Å) suggests that the alkyne moiety can be considered strongly bound as a two-electron donor to each metal centre (i.e. an overall four-electron donor), similar to complexes 1.10 – 1.12.  ClClRC CR'M2Cl6(L)3 MClL Cl RR'MLClClR'R1.10: M = Ta, R = R' = Ph1.11: M = Nb, R = R' = Ph1.12: M = Ta, R = tBu, R' = Me1.8: M = Ta1.9: M = NbL = THT - L  9  Scheme 1.5: Synthesis of an alkyne-bridged bimetallic complex, via the addition of a sterically bulky alkyne to Ta2Cl6(THT)3.   In 1990, Pedersen and Roskamp reported29 the synthesis of a series of niobium alkyne trihalides, Nb(RC≡CR’)X3(DME) (1.15), mononuclear analogues of complexes 1.10 – 1.12.  The synthetic route was a simple one: a variety of Nb alkyne complexes could easily be prepared by treating NbX3(DME) (1.14: X = Cl or Br), formally Nb(III) precursors reported30 by the same workers only a few years prior, with an equivalent of the desired alkyne (Scheme 1.6).  Over a decade later, Oshiki and coworkers reported31,32 the synthesis of the tantalum analogues of 1.15, Ta(RC≡CR’)Cl3(DME) (1.16) (Scheme 1.6), generated by the in situ reduction of TaCl5.   ClClRC CRTa2Cl6(L)3 Ta L ClClTaLClCl 1.13: R = tBuL = THT - L RR1.8  10  Scheme 1.6: Synthesis of the mononuclear Ta and Nb alkyne trichloride complexes 1.15 and 1.16, via reduction of the corresponding metal pentahalide.  The ease with which these compounds can be synthesized, as well as the variety of different alkyne substrates supported, make them ideal starting materials for Group 5 alkyne complexes; indeed, there are numerous examples in the literature33-42 where 1.15 and 1.16 act as reagents for the installation of Nb(RC≡CR’) or Ta(RC≡CR’) units into a wide variety of ligand sets; some representative examples are shown in Scheme 1.7.  Ta ClCl ClR R'OODME/toluene1. Zn (xs)RC CR'2.TaCl5 - ZnCl2 1.16R, R' = Ph, Pn, tBu, Et, Me, TMS, H, COOMe, CONMe2DMEBu3SnHNbX5 - Bu3SnX, H2 1.15R, R' = Ph, tBu, nPr, Et, Me, TMS, HNbX3(DME)1.14X = Cl or Br RC CR' Nb XX XR R'OO  11  Scheme 1.7: Examples of the use of 1.15 and 1.16 to synthesize alkyne complexes with various ancillary ligands: 1.17 and 1.18 (Etienne33-39), 1.19 (Otero40), 1.20 (Hill42).  Prior to the advent of these conveniently accessible Nb (1.15) and Ta (1.16) alkyne trihalide starting materials, the syntheses of most Group 5 alkyne complexes began with the reduction of a high-valent M(V) or M(IV) organometallic precursor that already contained a NN MeMeNN MeMeBHNNMeMe NN MeMeNN MeMeCHH2COLi NNMe N NMeBHNNMe S SS [bdmpze]NbLi[bdmpze] 1.19R, R' = Ph, Et, Me, TMS- LiCl[Tp']MCl21.17M = Nb; R, R' = Ph, Pr,Et, Me1.18M = Ta; R, R' = Ph, Me- KClK K[Tp']Na [HB(mt)3]TaCl21.20R, R' = Ph, Me- NaClNa[HB(mt)3]R R'Cl2R R'R R'1.15 or 1.161.151.16  12 stabilizing ancillary ligand, such as a cyclopentadienyl (Cp = C5H5) unit.  Although there are some examples of M(III) compounds that are stable enough to be isolated and characterized (e.g. 1.21, 1.27, 1.32, vide infra), in most cases the reduced M(III) species was generated in situ and immediately treated with an alkyne substrate to afford the desired M(V) alkyne organometallic complex. The most common method for the reduction of a high-valent M(V) or M(IV) precursor cited in the literature is via the use of an external reducing agent, such as Zn, Mg, Al, or Na/Hg amalgam.  One of the first examples of this route was the synthesis of a variety of monocyclopentadienyl Ta and Nb alkyne chloride complexes by Curtis and coworkers (Scheme 1.8).43-45  Treating a solution of Cp’MCl4 (Cp’ = C5H5– or C5H4Me–) with aluminum powder in the presence of the desired alkyne afforded Cp’M(RC≡CR)Cl2 (1.22).  If the reduction was carried out in the presence of carbon monoxide (CO) instead of an alkyne substrate, the stable M(III) dimer [Cp’M(CO)2Cl]2(µ-Cl)2 (1.21) could be isolated; the addition of alkyne to a solution of 1.21 also afforded 1.22.44  Gomez and coworkers also employed this method to prepare a Nb analogue, (CpSi)Nb(BTA)Cl2, from (CpSi)NbCl4 (CpSi = C5H4(SiMe3)).46    13  Scheme 1.8: Synthetic routes to monocyclopentadienyl Ta and Nb alkyne complexes.  A variety of bis(cyclopentadienyl) alkyne complexes have also been synthesized via the reduction of the corresponding Ta(IV) or Nb(IV) dichloride complex with Na/Hg amalgam, as shown in Scheme 1.9; for example, Green and coworkers47 used this methodology to prepare (CpiPr)2Ta(MeC≡CMe)Cl (1.23) from (CpiPr)2TaCl2 (CpiPr = C5H4iPr).  Otero and Royo synthesized Cp2Nb(PhC≡CPh)Cl (1.24),48 and the ansa-niobocene alkyne complex, (SiMe2(Cp)2)Nb(RC≡CR)Cl (1.25)49 in a similar fashion. MCl ClClCl R Al, R'C CR' MClCl RR' R'MOC ClClOC M COCORRAl, CO (xs) R'C CR'M = Ta, Nb; R = H, Me 1.22: M = Ta, Nb; R' = Ph, 4-MeC6H4Cl Cl1.21  14  Scheme 1.9: Synthetic routes to metallocene and ansa-metallocene Ta and Nb alkyne complexes.  Otero and Royo also showed that, in the case of (CpSi)2NbCl2 (1.26), the use of one equivalent of reducing agent afforded the isolable Nb(III) monochloride, (CpSi)2NbCl (1.27), as shown in Scheme 1.10.  While the addition of an equivalent of alkyne generated the desired alkyne complex (CpSi)2M(RC≡CR)Cl (1.28), treating 1.27 with an equivalent of a diyne, such as 1,4-diphenylbuta-1,3-diyne, afforded the related Cp’2M(RC≡C–C≡CR)Cl complex (1.29).50  M ClClRR Na/HgR'C CR' 1.23: M = Ta; R = iPr; R' = Me1.24: M = Nb; R = H; R' = PhM ClRR R'R'- NaClNa/HgR'C CR' Nb ClMe2Si R'R'- NaClNb ClClMe2Si 1.25: R' = Me or Ph  15  Scheme 1.10: Synthetic routes to niobocene alkyne and diyne complexes, via the stable Nb(III) complex 1.27.  Although the majority of work in this field has focused on cyclopentadienyl-containing systems like the ones discussed above, alkyne moieties have been installed via a similar reduction protocol in Ta and Nb complexes featuring a variety of ancillary ligand sets.  For example, McConville and coworkers51 investigated the synthesis of Ta alkyne complexes featuring a pyridine-diamine pincer ligand, BDPP, as shown in Scheme 1.11.  The reduction of [BDPP]TaCl3 (1.30) with Na/Hg in the presence of an excess of alkyne afforded the Ta alkyne chloride complex, [BDPP]Ta(RC≡CR’)Cl (1.31).  Nb ClClTMSTMS Na/Hg- NaClNb ClTMSTMS R'R(CpSi)2NbCl1.271.28: R, R' = Ph or H1.26 Nb ClTMSTMS RC1.29: R = Ph or TMS CRRC CR RC C C CR  16  Scheme 1.11: Synthesis of the Ta alkyne complex 1.31 via reduction of the Ta trichloride precursor 1.30.  In addition, Wolczanski and coworkers reported52 the synthesis of the stable Ta(III) complex Ta(silox)3 (1.32, silox = tris(tert-butyl)siloxide) via the reduction of Ta(silox)3Cl2 with Na/Hg; as shown in Scheme 1.12, complex 1.32 readily combined with a variety of alkynes53,54 to form Ta(silox)3(RC≡CR) (1.33).    Scheme 1.12: Synthetic route to Ta(V) alkyne complexes, via the stable Ta(III) siloxide complex 1.32. N NArNArTa ClCl Cl N NArNArTa Cl RR- 2 NaClNa/HgRC CR 1.31R = Ph, nPr, Et1.30Ar = 2,6-iPr2C6H3(silox)3TaCl2 - 2 NaClNa/Hg RC CRTa OSi(tBu)3(tBu)3SiO(tBu)3SiO(silox)3Ta RR1.33R = H, Me, CF3 1.32  17  Similarly, Wigley and coworkers have investigated the synthesis and reactivity of Ta alkyne complexes featuring 2,6-diisopropylphenoxide (DIPP)55-58 and 2,6-diiisopropylphenylimide (DIPN)59 ligands; the steric bulk of these ligand plays a crucial role in determining the reactivity of these complexes with alkyne substrates. For example, the reduction (with Na/Hg) of the Ta dichlorides Ta(DIPP)3Cl2 (1.34) and Ta(DIPN)(DIPP)Cl2 (1.35), or the less sterically crowded Ta trichlorides Ta(DIPP)2Cl3 (1.36) and Ta(DIPN)Cl3 (1.37), in the presence of certain alkynes afforded the expected Ta(RC≡CR’) complexes (1.38 – 1.41), as shown in Scheme 1.13.  However, in some cases the addition of a second or third equivalent of alkyne can lead to the formation of the butadienediyl tantallacycles (1.42), or Ta-(η6-arene) products (1.43), depending on both the steric bulk at the Ta centre, and the size of the alkyne substituents.  The reactivity patterns of these Ta(DIPP) and Ta(DIPN) complexes will be addressed in more detail in Section 1.3.2.   18  Scheme 1.13: Synthesis of Ta alkyne complexes (1.38 – 1.41) via the reduction of the corresponding Ta aryloxide or aryl imide chlorides.  In some cases, these complexes go on to form Ta butadienediyl or η6-arene complexes when treated with additional equivalents of alkyne.  In addition to the use of an external reducing agent, a second method for synthesizing Ta and Nb alkyne complexes involves a Ta(V) or Nb(V) precursor complex undergoing reductive [Ta][Ta]- 2 NaClNa/HgNArLTaLCl ClX NArXTaLL- 2 NaClNa/Hg RR'ClClTaX OArOAr X TaArOArO RR'RC CR'RC CR'1.34: X = OAr1.36: X = Cl1.35: X = OAr1.37: X = Cl 1.38: X = OAr;R, R' = Ph, Me, TMS1.39: X = ClR, R' = TMS1.40: X = OAr; R, R' = Et, nPr1.41: X = ClR, R' = Ph, Et, Me, TMSRR' RC CR' [Ta]R R'R'R R R'RR'RR'or1.38 - 1.41 1.42 1.43(xs)L = pyAr = 2,6-diisopropylphenyl  19 elimination.   For example, one of the earliest methods for the synthesis of Group 5 alkyne complexes, initiated by Schwartz60-62 (1.44) and expanded upon by Herberich63,64 (1.45), proceeded via the thermally-induced reductive elimination of H2 from the metallocene trihydride complexes Cp2MH3, in the presence of an equivalent of dialkylacetylene (Scheme 1.14). Rothwell65 and coworkers have also reported the synthesis of a Ta alkyne complex via a similar H2 reductive elimination step: the addition of 3-hexyne to the Ta aryloxide dihydride complex Ta(OC6H3Ph2-2,6)2(H)2Cl(PMe3)2 resulted in the formation of Ta(OC6H3Ph2-2,6)2(3-hexyne)Cl(PMe3) (1.46, Scheme 1.14).    Scheme 1.14: Synthesis of Ta and Nb alkyne complexes via the reductive elimination of H2.  In complexes 1.44 - 1.46, the addition of an equivalent of alkyne led to the displacement of H2 from the metal centre, rather than insertion into the M–H bond.  However, further OArOArTaHH LLCl LClTaArOArO EtEt- H2, LEtC CEtL = PMe3, Ar = 2,6-Ph2C6H4 1.46MH3RR R'C CR' 1.44: R = Me; R' = tBu, nBn, nPr, iPr1.45: R = H; R' = Ph, TMS, MeM HRR R'R'- H2Δ,M = Ta, Nb  20 investigations by Herberich63 (and later Otero66,67) extended this work to include a variety of alkynes featuring both electron-donating and electron-withdrawing groups, and found that the reactions of alkynes with Group 5 metallocene trihydrides with fall into two classes.  Alkynes that featured electron-donating groups led to the displacement of H2, and the formation of metallocene alkyne hydride complexes (such as 1.44 or 1.45).  In contrast, alkynes that contained electron-withdrawing substituents ultimately afforded the metallocene alkene hydride complexes (1.47 and 1.48, Scheme 1.15).  Given the trans-stereochemistry of the alkene moiety, the formation of these complexes was thought to occur via a stereoselective insertion step that results in a trans-alkenyl dihydride intermediate (1.49), although the mechanism by which this would occur is unclear.68,69  Scheme 1.15: Synthesis of Ta and Nb alkene hydride complexes (adapted from ref. 63, 67–69). MH3RR R'C CR'1.47: M = Ta, R' = Ph, CO2Me, CO2TMS; 1.48: M = Nb, R' = CO2Me, CO2tBuM HRR R'R'Δ,M = Ta, R = H M = Nb, R = TMS HH M HRR R'R' HH1.49  21  Similar to complexes 1.44 - 1.46, Schrock and coworkers reported70 the synthesis of a niobium alkyne chloride complex that is also believed to proceed via a reductive elimination step (Scheme 1.16). The hydrogenolysis of Cp*NbMe2Cl2 afforded the Nb(III) dimer [Cp*NbCl2]2 (1.48), likely via the loss of H2 from the putative Nb(V) dihydride, Cp*Nb(H)2Cl2 (1.47); treating complex 1.48 with 3-hexyne generated the mononuclear Nb alkyne complex Cp*Nb(3-hexyne)Cl2 (1.49). In contrast to the Schwartz/Herberich or Rothwell systems (1.44 – 1.46), where the low-valent M(III) intermediate is generated in situ, complex 1.47 is stable enough to be isolated, similar to complexes 1.27 (Otero) and 1.32 (Wolczanski).  Scheme 1.16: Synthesis of an alkyne complex (1.49) from a stable Nb(III) species (1.48), generated via the reductive hydrogenolysis of a Nb dimethyl precursor.    Tilley and coworkers reported71 the synthesis of a series of Ta analogues to 1.49 generated via a related reductive elimination route.  Heating a benzene solution of NbCl MeMeCl 1.49 2 H2- 2 MeH NbCl HHCl - H21/2 [Cp*NbCl2]2NbClCl EtEt 1.471.483-hexyne  22 Cp*Ta(TMS)Cl3 and the desired alkyne resulted in the formation of Cp*Ta(RC≡CR’)Cl2 (1.50) over the course of several days (Scheme 1.17).  The reaction is believed to proceed via the reductive elimination of TMSCl, induced by alkyne coordination to the Ta centre.  Scheme 1.17: Synthesis of a Ta alkyne complex via the reductive elimination of TMSCl.  1.1.4 The Reactivity of Tantalum and Niobium Alkyne Complexes Early investigations by Cotton and coworkers into the reactivity of the low-valent Group 5 chlorides M2Cl6(THT)3 (M = Ta, 1.8; M = Nb, 1.9) with alkyne substrates revealed that, depending on the size of the substituents on the alkyne used, these complexes also functioned as effective polymerization and oligomerization catalysts.72  Whereas the addition of an excess of diphenylacetylene or di-tert-butylacetylene to solutions of 1.8 or 1.9 led to the formation of discrete metal-alkyne complexes (1.10 – 1.12, Scheme 1.4), terminal alkynes (RC≡CH) and less bulky internal alkynes generated a mixture of cyclotrimers and poly(alkyne); the authors hypothesized that the formation of the cyclotrimer product proceeded via the insertion of an alkyne unit into the M-Calkyne bond of a M(RC≡CR’)Cl3 intermediate (similar to 1.15 or 1.16), as shown in Scheme 1.18. TaCl TMSClCl - TMSClRC CR' TaClCl R'R1.50R = R' = Ph, Me, TMSR = Ph, R' = HΔ,  23  Scheme 1.18: The cyclotrimerization and polymerization of alkynes, catalyzed by the low-valent Group 5 chloride complexes M2Cl6(THT)3; [M] = TaCl3 or NbCl3.  This proposed mechanism is supported by the observation that Nb(RC≡CR’)Cl3(DME)29 (1.15) and Ta(RC≡CR’)Cl3(DME)73 (1.16) have also been shown to be effective catalysts for the cyclotrimerization of terminal alkynes.   With regards to alkyne polymerization by a Ta alkyne complex, investigations by McConville and coworkers provide evidence for a possible mechanistic pathway.74  As shown in Scheme 1.19, treating the [BDPP]Ta(RC≡CR) acetylide complexes 1.51 with excess phenylacetylene afforded the metallacyclic product 1.53.  The authors speculated that 1.53 was formed by the insertion of the coordinated alkyne unit into the Ta–C≡CPh bond (1.52), followed by the coordination and insertion of a phenylacetylene unit (coloured in red) into the newly formed Ta-‘alkynevinyl’ bond.  This hypothesis is similar to the well-known Cossee-Arlman mechanism75,76 for the growth of polymer chains catalyzed by Group 4 Ziegler-Natta type77-79 complexes. RC CR'M2Cl6(THT)31.8: M = Ta1.9: M = Nb R3R'3cyclotrimers CR R'C n+(xs)[M]R H [M] HRHRRC CHRC CH [M]H R HRHRRC CH poly(alkyne)  24  Scheme 1.19: Formation of the metallacycle 1.53 via the insertion of coordinated alkyne ligands into the Ta-Cacetylide bond.   In addition to the simple Group 5 alkyne halides studied by Cotton, the Ta aryloxide and arylimide chloride (1.34 – 1.37) or alkyne (1.38 – 1.41) complexes reported by Wigley and coworkers55-57,80,81 also engage in similar (although not catalytic) alkyne coupling and cyclotrimerization reactions, via the insertion of additional equivalents of alkyne into Ta(RC≡CR) bond.  Some representative examples of these alkyne coupling reactions are shown in Scheme 1.20.  For example, (DIPP)3Ta(PhC≡CPh) (1.38-Ph) reacted with a single equivalent of sterically small alkynes (such as 3-hexyne) to generate a Ta(butadienediyl) metallacyclic complex (1.54).  In addition, the reduction of (DIPP)3TaCl2 (1.34), (DIPP)2TaCl3 (1.36) or (DIPN)TaCl3L2 (1.37), in the presence of excess amounts of certain alkynes resulted in the PhC CH1.51R = Ph, nBu, TMS(derived from 1.31 ) TanPr nPr[BDPP] RPh HTa nPrnPr[BDPP] RHPh 1.521.53N NArNArTa C nPrnPrCR  25 formation of similar Ta(butadienediyl) complexes (1.55 – 1.57), which are believed to proceed by first forming the corresponding Ta alkyne complex (1.38, 1.39 or 1.41, respectively).   Scheme 1.20: Examples of the synthesis of Ta butadienediyl complexes via the insertion of a second alkyne unit into a Ta–Calkyne bond.  1.54(ArO)3TaCl21.34 - 2 NaClNa/HgEtC CEtxs Ta EtEt EtEt(ArO)3Ta PhPh1.38 - Ph Ta PhPh Et-EtEtC CEt (ArO)3(ArO)31.571.36 - 2 NaClNa/HgPhC CPhxs(ArO)2TaCl3 Ta PhPh PhPhCl(ArO)2 1.56ArNLTaLCl ClCl ArNClTaLL- 2 NaClNa/HgtBuC CH1.37L = py tButBu21.55  26 Furthermore, some Ta(butadienediyl) complexes (1.58) underwent an additional insertion reaction with an equivalent of alkyne or an organonitrile (RCN) to generate Ta(η6-arene) (1.59) or Ta(η2-pyridine) (1.60) complexes,82 as illustrated in Scheme 1.21.  Hydrolysis of 1.59 or 1.60 afforded the organic C6R6 or NC5R5 product, making these Ta complexes potential reagents for the stoichiometric synthesis of substituted benzene and pyridine rings.    Scheme 1.21: Reaction of an equivalent of alkyne or nitrile with a Ta butadienediyl complex to generate the corresponding Ta(η6-arene) or Ta(η2-pyridine) species.  In addition to alkynes and organonitriles, both Wigley58 (1.61) and Oshiki32 (1.62) have reported the synthesis of oxygen-containing tantallacycles similar to 1.54 – 1.57, formed via the insertion of ketones or aldehydes into the Ta(RC≡CR) bond (Scheme 1.22); Oshiki and coworkers32 have used this as means of stoichiometrically generating allylic alcohols.  tBuC N Ta tBu tButBuCl(ArO)21.58 tBuC CHTatBu tBuCl(ArO)2 Ta NtBu tButBuCl(ArO)2 1.591.60  27  Scheme 1.22: Examples of the formation of 5-membered tantallacycles via the insertion of an equivalent of ketone or aldehyde into a Ta–Calkyne bond.  The metallacyclic examples discussed above (1.54 – 1.62) were generated by the insertion of a substrate, such as an alkyne, nitrile or ketone, into a metal–Calkyne bond.  In addition, there is a wide body of related work that proceeds first via the insertion of a substrate, such as carbon monoxide,34,83-86 an organic isocyanide,46,83-87 or an alkyne64 into a M–Calkyl bond; the resulting iminoacyl, acyl or vinyl moiety then undergoes a coupling reaction with the metal-bound alkyne ligand to afford a metallacyclic product. Similar transformations are also known to occur via insertion into M–H bonds as well.  Scheme 1.23 illustrates some representative examples of these reactions. The structural and spectroscopic features of these complexes are addressed in detail in Chapter 4. 1.61Ta RR1.38 Ta RRO R'(ArO)3(ArO)3 O R'Ph R' = Ph or MePhR = Ph or TMS 1.62Ta RR1.16-pypy2Cl3 O R'H Ta RRO R'py2Cl3NaOH (aq.)OHRR R' = (CH2)2PhR = Et, C5H11 R' H  28  Scheme 1.23: Examples of tantallacycle formation via the coupling of a Ta alkyne moiety with various ligands: 1.65 (Curtis and Real87), 1.67 (Etienne and Templeton34), 1.69 (Herberich64). 1.63 R = Ph, 4-MeC6H4(derived from 1.22 ) [Ta]C R R1.64NtBuMe[Ta] N1.65 C tBuMeR R[Ta]Me R R 90oCC NtBu1.66R = Me, Et(derived from 1.17 ) [Nb] O1.67C MeR Ph[Nb] RPhMe C O1.45NbH PhPhCp2 NbC PhPhCp2C RR'HNb CC RPh PhR'H 1.68 R = R' = CO2MeR = CO2Me, R' = HCp2 1.69RC CR'[Nb] = [Tp']Nb(OMe)[Ta] = CpTaMe  29 1.2 Tantalum Amidophosphine Complexes, and Ligand Design in the Fryzuk Group The preceding section has attempted to provide a brief overview of the synthesis and reactivity of tantalum and niobium alkyne complexes featuring a number of ancillary ligands, including aryloxides, arylimides and cyclopentadienyl groups.  Of particular interest was the variety of novel transformations that metal-coordinated alkyne units have been shown to undergo with small molecule substrates.  The work described in this thesis extends these investigations to a series of Ta alkyne complexes supported instead by a diamidophosphine ligand set.  Previous work in the Fryzuk group has led to the developments of a family of structurally related amidophosphine ligand sets: the tridentate amidodiphosphine [PNP], the macrocyclic [P2N2], and the tridentate diamidophosphine [NPNSi]  (shown in Figure 1.3).  These ligands have been used to synthesize complexes of a wide variety of early and late transition metals from across the periodic table.88,89  Tantalum complexes of [NPNSi] are of particular relevance to the work presented in this thesis, specifically the dinuclear Ta(IV) tetrahydride, ([NPNSi]Ta)2(µ-H)4 (1.71), which is synthesized via the reduction (by H2) of the trimethyl compound [NPNSi]TaMe3 (1.70), as shown in Scheme 1.24.90,91    30  Figure 1.3: Examples of amidophosphine ligand sets synthesized by the Fryzuk group.  Complex 1.71 readily combined with N2 at ambient pressure and room temperature to generate the dinuclear Ta dinitrogen complex, ([NPNSi]Ta)2(η1:η2-µ-N2)(µ-H)2 ( 1.72).90,91  The facile nature of this transformation was surprising, as N2 is typically a poor ligand for organometallic complexes. Dinitrogen is a nonpolar molecule with a large HOMO – LUMO gap, and is both a weaker σ-donor and π-acceptor compared to its isoelectronic analogue carbon monoxide (CO).92  The combination of these factors means that N2 is considered to be   SiN PM RSiNRR •••• Me2Me2Si N SiP PMR RSi N Si•••• Me2Me2Me2Me2Si N SiP PMR RR R•• Me2Me2[PNP]M [P2N2]M [NPNSi]M  31    Scheme 1.24: Synthesis of the dinuclear [NPNSi]Ta dinitrogen complex 1.72 from the Ta tetrahydride 1.71.  chemically unreactive under most conditions.  Indeed, in modern air-sensitive chemistry many experimental techniques employ N2 as an inert working gas for Schlenk lines and glove-box workstations.  Despite these challenges, the field of organometallic dinitrogen chemistry has developed considerably over the last 50 years, and there are now dozens of examples of complexes featuring N2 ligands on metals from across the periodic table; interested readers are invited to consult these89,92-95 review articles.  In the case of early transition metals, the synthesis TaNN PhMe2SiMe2SiPhPh MeMeP Me PhTa TaNN PNNPh PhMe2SiMe2Si SiMe2SiMe2PhPh PhHHP HHH2- 6 CH42 1.70 1.71Ta TaHHNN [NPNSi][NPNSi] 1.72 N2- H2  32 of dinitrogen complexes generally proceeds via the reduction of the N2 ligand by a low-valent metal centre; this redox process typically requires the use of a strong reductant, such as an alkali metal (Li, Na, K).  Previous work from the Fryzuk group illustrates this point; a number of Ti,96 Zr,96-102 Nb103 and Ta101 dinitrogen complexes have been synthesized via the reduction of the relevant amidophosphine ([PNP], [P2N2] or [NPNSi]) metal halide with potassium graphite (KC8) or Na/Hg amalgam. In contrast, complex 1.72 is generated via the reduction of N2 by the Ta(IV) tetrahydride 1.71; the electrons that reduce the N2 unit are provided by complex 1.71 in the absence of an external reducing agent.  This point is crucial, as it is then possible to envision a catalytic cycle (Scheme 1.25) whereby an electrophilic substrate (E–H) combines with 1.72 to form amine or hydrazine products featuring new E–N bonds, and regenerate the Ta tetrahydride complex 1.71. Running this hypothetical catalytic system under an atmosphere of N2 would allow for the regeneration of complex 1.72.        33  Scheme 1.25: Potential catalytic cycle for the generation of amine and hydrazine products via the coupling of electrophilic substrates (E–H) with molecular N2, mediated by complexes 1.71 and 1.72.  A number of examples have been reported where complex 1.72 reacted with electrophilic substrates to generate products with new E–N bonds (1.73), as shown in Scheme 1.26. In several cases (E–H = Al(iBu)2H,104 9-borabicyclo[3.3.1]nonane (9-BBN)105,106) these complexes were thermally unstable, and engaged in side-reactions with [NPNSi] that led to the degradation or rearrangement of the ligand, and the overall decomposition of the [NPNSi]Ta complex (i.e. complexes 1.74 and 1.75).  In the case of E–H = RSiH3 (R = nBu, Ph), the addition of several equivalents of silane to 1.72 led to similar decomposition processes.107,108  1.71 Ta TaHHNN [NPNSi][NPNSi] 1.72N2- H2Ta TaHHHH [NPNSi][NPNSi] E HNE EE N NEE EEor E = H, SiR3, AlR2, BR2  34  Scheme 1.26: Examples of [NPNSi] ligand degradation and rearrangement, as a result of the reaction of complex 1.72 with electrophiles. Ta Ta[NPNSi]NNPh SiSi PhPh HHP NNEHMe2Me2 Ta Ta[NPNSi]NNPh SiSiPh P NNR2BMe2 Me2Ta Ta[NPNSi]NPhSiSi PhP NNMe2 Me2 H HAl iBuNPh - H2, benzene- H2,([NPNSi]Ta)2(µ-N2)(µ-H2)1.721.75E = BR2E = Al(iBu)2= Al(iBu)2H, 9-BBN1.741.73E Hisobutene  35  The examples of complexes 1.74 and 1.75 illustrate that both the flexibility of the –CH2SiMe2– linker, and the labile Si–N bond may be undesirable features of the [NPNSi] ligand set.  In an effort to address these issues, Dr. Erin MacLachlan, a former doctoral student in the Fryzuk group, investigated the coordination chemistry of a modified tridentate diamidophosphine ligand set, [PhNPN*], which features a more robust, rigid ortho-phenylene backbone. The lithium salt [PhNPN*]Li2S2 (1.76, S = dioxane, THF) is easily synthesized on a multi-gram scale, as shown in  Scheme 1.27.109  Scheme 1.27: Synthetic route to [PhNPN*]Li2S2 (adapted from ref 109).  NHMes BrNLiMes LiNMes P N MesPhMesNH2(Pd cat.) 2 nBuLiLiLiS[PhNPN*]Li2(S)2NHMesBr S1.76NBSPhPCl2Mes = 2,4,6-trimethylphenyl1/21/2  36 Dr. MacLachlan was able to synthesize a variety of Group 4 (Zr and Hf) complexes featuring the [PhNPN*] ligand set.109,110 In addition, the reduction of [PhNPN*]ZrCl2 (1.77) with KC8 in the presence of N2 afforded the Zr–dinitrogen complex, ([PhNPN*]Zr)2(µ-η2:η2-N2) (1.78),97 as shown in Scheme 1.28.  Scheme 1.28: Synthesis of a Zr-dinitrogen complex supported by the [PhNPN*] ligand set, and subsequent reactivity of the coordinated N2-unit with electrophiles. The dinitrogen unit in complex 1.78 reacts cleanly with electrophilic reagents such as H2 and PhSiH3 (Scheme 1.28) to form products with new E–N (E = H; 1.79; E = Si, 1.80) bonds; Zr NPPh NNZrN P PhN N LL2  [PhNPN*]ZrCl2 N2, KC8THF1.77 1.78ZrNZr N1.79HH Zr ZrHN NSiH2PhLL = THF, PMe3, PMe2Ph or py 1.80[PhNPN*][PhNPN*] [PhNPN*] [PhNPN*]H2 PhSiH3L  37 these complexes are thermally stable in solution and the solid-state,97,110 and do not engage in the kinds of deleterious ligand-based reactivity observed in the [NPNSi]Ta system (1.72). 1.3 Scope of Thesis The work done by Dr. Erin MacLachlan with Group 4 complexes of [PhNPN*] indicates that this ortho-phenylene bridged diamidophosphine ligand set may indeed be an improvement over [NPNSi] in terms of robustness and chemical stability; however, its coordination chemistry with other transition metals remains unexplored.  In particular, the example of the dinuclear Ta(IV) tetrahydride (1.71), and its facile conversion to the Ta nitrogen complex (1.72) make tantalum hydride complexes of the [PhNPN*] ligand set attractive synthetic targets.  Thus, the aim of this thesis is to extend the coordination chemistry of [PhNPN*] to Group 5 transition metals.  In particular, the focus will be on the synthesis and reactivity of new [PhNPN*]Ta complexes.   Chapter 2 of this thesis focuses on the attempted synthesis of several [PhNPN*]Ta organometallic and coordination complexes.  In addition, the synthesis of two families of [PhNPN*]Ta alkyne complexes is presented: [PhNPN*]Ta(3-hexyne)X, and [PhNPN*]Ta(BTA)X (X = Cl, H, alkyl; BTA = bis(trimethylsilyl)acetylene).  The structure and bonding in these complexes is discussed in terms of the low valent tantalum(III)–alkyne and high valent tantalum(V)–‘alkenediyl’ formalisms. Chapter 3 focuses on the synthesis and reactivity of a series of Ta imide complexes, generated via the reaction of the corresponding [PhNPN*]Ta alkyne complex with an aryl azide. In addition, the synthesis and attempted synthesis of [PhNPN*]Ta alkyne azide, and [PhNPN*]Ta   38 nitride complexes are discussed.  The systems presented in this chapter are best understood by invoking the low valent Ta(III)–alkyne structural formalism. Chapter 4 of this thesis deals with the reactivity of the two [PhNPN*]Ta alkyne monohydride derivatives with a variety of small molecules.  The reactions of these monohydride complexes with 2,6-dimethylphenyl isocyanide and phenylacetylene, and the resulting five-membered tantallacyclic products are discussed.  Additionally, the reactions of carbon dioxide with multiple equivalents of a Ta monohydride are explored.  A kinetic study of the thermal rearrangement of a Ta alkyne phenylvinyl complex to the corresponding tantallacycle is also reported. Chapter 5 investigates the reactivity of [PhNPN*]TaMe3, and several [PhNPN*]Ta alkyne alkyl and alkyne hydride complexes with H2.  The synthesis and structure of the Ta alkene hydrides formed via the low-pressure hydrogenolysis of the Ta alkyne complexes are reported, and possible mechanisms for the formation of these intermediates are discussed.  An N2-inert Ta(IV) tetrahydride complex, ([PhNPN*]Ta)2(µ-H)4, synthesized via the high-pressure hydrogenolysis of either [PhNPN*]TaMe3 or the Ta alkyne complexes is also presented.   Chapter 6 outlines the synthesis and proposed structure of a cationic [PhNPN*]Ta imide complex; potential catalytic applications of this complex are discussed. Areas for future investigation, including potential routes for the synthesis of niobium complexes of [PhNPN*], and substrate suggestions for reactivity studies with ([PhNPN*]Ta)2(µ-H)4 are also presented.     39 Chapter  2: Synthesis of Tantalum Complexes of [PhNPN*], a Diamidophosphine Ligand Featuring an ortho-Phenylene Backbone  2.1 Introduction In recent years, the Fryzuk group has had considerable success studying the chemistry of a wide variety of early transition metal (group 3 – 5) diamidophosphine complexes, where diamidophosphine is a generic descriptor for a tridentate dianionic ligand containing one phosphine (PR3) and two amido (NR2-) donors.91,96,106,108,109,111-118  The synthesis of many of these compounds is accomplished via a salt metathesis reaction between the appropriate metal halide and an alkali metal salt of the corresponding diamidophosphine ligand.  The formation of the alkali metal halide provides a strong thermodynamic driving force, in addition to being an easily removed reaction byproduct.  Several examples of this method with the [NPNSi] ligand set are shown in Scheme 2.1  Scheme 2.1: Examples of coordination complexes of the [NPNSi] ligand set synthesized via salt metathesis. S [M]NN PhMe2SiMe2SiPhPh PLi NPh Si PhP NSi PhLiMe2Me2 S [M] = TaMe3, ZrCl2, TiCl2, YCl, SmCl, HoCl, YbCl, LuCl- 2 S- 2 LiClS = THF, 1,4-dioxane toluene, Et2O or THF-78oC to rt+    [M]Cl2[NPNSi]Li2 S2  40  A second method for introducing a ligand system onto early transition metal precursors is via a protonolysis reaction between a metal amide or alkyl compound and the protonated form of the ligand, assuming that acidic protons on the ligand precursor are accessible; the resulting organic byproduct can easily be removed via vacuum evaporation.  Reagents such as Y(CH2TMS)3(THF)3, Zr(Bn)4 or Ta(NMe2)5 can be purchased from a commercial supplier, or easily synthesized from the appropriate metal halide.  Examples of this route in synthesizing group 4 complexes of [PhNPN*] are shown in Scheme 2.2  Scheme 2.2: Some examples of Group 4 complexes of the [PhNPN*] ligand set synthesized via a protonolysis reaction.  Due to the remarkable reactivity observed with the [PhNPNSi]Ta system – and the resultant issues with ligand degradation and deleterious side reactions (as discussed in Chapter 1) – the synthesis of tantalum complexes of the more rigid and robust [PhNPN*] backbone was of great interest.  Towards that end, this chapter discusses the synthesis and attempted synthesis of several [PhNPN*]Ta organometallic and coordination complexes.   P HNPhNH - 2 HXtoluene MN P PhNM = Zr, Hf; X = NMe2MX4 X2[PhNPN*]H2  41 2.2 Results and Discussion 2.2.1 Attempted Synthesis of [PhNPN*]TaCl3  As mentioned above, a simple way to access Ta coordination complexes of an anionic ligand set is through a salt metathesis reaction with a Ta halide.  The dioxane adduct of the lithium salt, [PhNPN*]Li2(dioxane) has already been synthesized and was used to investigate group 4 metal complexes.109,110 In addition, the potassium salt of [PhNPN*] can be prepared by reacting the protonated ligand, [PhNPN*]H2, with two equivalents of benzyl potassium.  [PhNPN*]K2(THF)0.5 (2.1) is more thermally robust than its lithium analogue, and is stable as a bright yellow powder at room temperature for weeks.  Compound 2.1 is somewhat soluble in toluene, and freely soluble in THF or pyridine; the 31P{1H} NMR spectrum of 2.1 in toluene-d8 (with a drop of pyridine-d5 for solubility) shows a broad singlet at δ -25.7.  The 1H and 13C{1H} spectra of 2.1 are similar to that of [PhNPN*]Li2(dioxane), with 4 inequivalent aryl methyl groups and resonances in the aromatic region consistent with a Cs symmetric complex, as well as the expected resonances for THF (in a ratio of 2 [PhNPN*]K2 : 1 THF, based on the comparison of 1H NMR resonance integrations).   Unfortunately, the reaction of [PhNPN*]Li2(dioxane) or 2.1 with TaCl5 in toluene led to a mixture of two products.  By both NMR spectroscopy and electron-ionization mass spectrometry (EI-MS), it appears that the putative [PhNPN*]TaCl3 is the major product (~75%); in particular, the 31P{1H} NMR resonance at δ 35 corresponds well to spectra of other Ta(V) diamidophosphine complexes.91,119  However, the reaction also yielded a minor product (~25%) with a significantly upfield 31P{1H} resonance (δ -42); numerous attempts to separate or unambiguously identify this second compound proved unsuccessful.  In light of these failures, it is unsurprising to note numerous examples in the literature where metathesis reactions with   42 TaCl5 have been shown to be problematic.120-122  Indeed, previous work in the Fryzuk group also illustrates this point.  In the case of the macrocyclic [P2N2] ligand set, attempts to synthesize the Ta trichloride from TaCl5 and a variety of ligand salts (Li, Mg, Zn) were fruitless; instead, success was only achieved when TaMe3Cl2 was employed to prepare the Ta trimethyl complex, [P2N2]TaMe3.123  2.2.2 Attempted Synthesis of [PhNPN*]TaR3 (R = Alkyl, Amido) Complexes Salt metathesis reactions with TaMe3Cl2 have proven to be a successful entry point into tantalum chemistry for both the [P2N2]119 and [PhNPNSi]90,91 ligand sets. Unfortunately, the reaction of [PhNPN*]Li2(dioxane) and TaMe3Cl2 ultimately led to a mixture of several new phosphorous-containing products.  According to 1H and 31P{1H} NMR spectroscopy, one of the minor products (~30%) is [PhNPN*]TaMe3, but given its low yield and thermal sensitivity, it proved exceedingly difficult to isolate this compound from the crude reaction mixture, and this route was ultimately deemed infeasible.  Later work by Dr. Dominik Nied, a post-doctoral fellow working in the Fryzuk group, demonstrated that the Ta trimethyl complex can be cleanly synthesized by using 2.1 instead of the ligand’s lithium salt.124  The [PhNPN*]TaMe3 system will be discussed in greater detail in Chapter 5. As mentioned above, protonolysis reactions are another potential route for preparing [PhNPN*]Ta complexes.  Unfortunately, the combination of Ta(NMe2)5 or Ta(Bn)5 and [PhNPN*]H2 led to no reaction, even in refluxing toluene over the course of several days.  It is probable that the steric shielding provided by the mesityl substituents on the amide arms of the [PhNPN*] ligand prevents interaction with these relatively bulky Ta reagents.      43 2.2.3 Synthesis of [PhNPN*]Ta Complexes Featuring an Activated Alkyne Unit Tantalum complexes of [PhNPN*] can be prepared via the salt metathesis reaction between 2.1 and Ta(alkyne)Cl3(DME) (1.16), as shown in Scheme 2.3.  As was discussed in Chapter 1, these simple alkyne compounds are synthesized through the addition of the desired alkyne to a DME/toluene slurry of TaCl5 and Zn metal.32   Scheme 2.3: The preparation of Ta(alkyne)Cl3(DME) complexes from TaCl5 (top), and their use in the synthesis of Ta alkyne chloride complexes of the [PhNPN*] ligand set (bottom).  The complex [PhNPN*]Ta(3-hexyne)Cl, 2.2, was synthesized as a dark yellow solid in good yield.  In C6D6, one observes a singlet at δ 32 in the 31P{1H} NMR spectrum.  The 1H NMR spectrum of 2.2 is consistent with a Cs symmetric complex; there are resonances due to 4 distinct aryl methyl groups, along with the expected aryl ligand resonances.  At room temperature, the Ta NP NRR Cl[PhNPN*]K2(THF)0.5THF(2.1) 2.2 (R = Et)2.3 (R = TMS)- 2 KClTa ClCl ClR ROODME/toluene1. Zn (xs)RC CR2.TaCl5 - ZnCl2 1.16Ta ClCl ClR ROO 1.16  44 ethyl arms of the 3-hexyne unit appear as two very broad singlets at δ 2.95 (methylene) and δ 1.03 (methyl), which integrate to the expected 4 and 6 protons, respectively.  The broadness of these resonances suggests that the position of the 3-hexyne unit is fluxional, likely ‘wagging’ in and out of the molecular plane of symmetry (i.e. there is some degree of rotation about the Ta–hexyne bond).  In the variable temperature 1H NMR experiment, the methyl and methylene protons of the 3-hexyne unit resolve into two pairs of triplets and quartets at 253 K, and coalesce into a single triplet and quartet at 358 K.  Accordingly, the ΔG‡rot was determined to be 68 kJ/mol; these variable temperature NMR experiments are addressed in greater detail in Appendix A. The 13C{1H} NMR spectrum for 2.2 features all the expected signals for the [PhNPN*] ligand, although the carbon resonances of the 3-hexyne unit are more complicated.  Resonances for the methyl and methylene carbons appear as very weak singlets at δ 14.1 and δ 29.2, respectively; no resonances attributable to the quaternary carbons are observable in a room temperature 13C{1H} spectrum, likely because of signal broadening (attributable to the fluxionality of the 3-hexyne unit) suppressing the already weak quaternary carbon resonances.  However, the 13C{1H} NMR spectrum collected at 243 K (in toluene-d8) contains two (albeit weak) downfield resonances at δ 182 and δ 200 attributable to these quaternary 3-hexyne carbons.  (At 243 K, the two weak resonances for the methyl and methylene carbons also resolve into two pairs of singlets: δ 13.7 and 15.7, and δ 26.7 and 30.6).  In addition, the quaternary 3-hexyne carbons can be detected indirectly using a 1H-13C HMBC NMR experiment, via their long-range coupling to the 3-hexyne methyl and methylene protons; in C6D6 at room temperature, this method correlates the quaternary carbons to a single resonance at δ 204.    45 An ORTEP representation of the solid-state molecular structure of 2.2 is shown in Figure 2.1.  The [PhNPN*] ligand coordinates facially to Ta, resulting in significantly distorted trigonal bipyramidal geometry about the metal centre; the apical positions are occupied by the Cl and P atoms, and the equatorial plane consists of N01, N02 and the centroid of the C41–C42 bond.  The P01–Ta01–N01 (74.41(6)°) and P01–Ta01–N02 (73.98(6)°) bond angles deviate from 90° due to structural demands of the ligand’s arene bridge.  In addition, the P01–Ta01–Cl01 bond angle (153.63(2)°) deviates significantly from 180° due to steric crowding from the 3-hexyne unit. The angle between the plane defined by P01, Ta01 and Cl01 and the plane defined by C41, C42 and Ta01 is only 2.8°; the alkyne unit essentially lies in the σv plane of the molecule. This leads to an alkyne ligand with two inequivalent halves in the solid-state, which agrees with what is observed in the low temperature 1H and 13C{1H} NMR spectra, although not with the more fluxional room temperature data. The sums of the bond angles around the amide nitrogen atoms (N01 and N02) are essentially 360°, indicating that both are sp2 hybridized and thus potential 4-electron donors; in this context, the Ta–N bond lengths are as expected.  In addition, the Ta–P and Ta–Cl bonds are unremarkable.  The bonds between the Ta centre and the 3-hexyne unit are slightly shorter than a typical Ta–C single bond90,119,125 (~2.20 Å), but similar to the distances observed in the starting trichloride complex; the Ta01–C41 bond (2.126(3) Å) is slightly longer than the Ta01–C42 bond (2.075(3) Å), likely due to steric crowding from the phosphine phenyl substituent. The C41–C42 bond is 1.295(4) Å, which is slightly shorter than the bond found in the starting trichloride (1.31(2) Å32); both cases indicate a bond order intermediate between two and three.     46  Figure 2.1: ORTEP drawing of the solid-state molecular structure of 2.2 (ellipsoids at 50% probability).  All hydrogen atoms and the mesityl group at N02 (except for Cipso) have been omitted for clarity.   Table 2.1: Selected bond lengths (Å) and angles (°) for 2.2 Parameter (Å) Parameter (°) Ta01–N01 2.060(2) N01–Ta01–N02 131.46(8) Ta01–N02 2.055(2) P01–Ta01–Cl01 153.63(2) Ta01–P01 2.573(6) N01–Ta01–P01 74.41(6) Ta01–Cl01 2.411(6) N02–Ta01–P01 73.98(6) Ta01–C41 2.126(3) N01–Ta01–Cl01 94.98(6) Ta01–C42 2.075(3) N02–Ta01–Cl01 97.34(6) C41–C42 1.295(4) C40–C41–C42 138.3(3)   C41–C42–C43 143.5(3) N02 N01 P01 Cl01 Ta01 C41 C40 C42 C43   47 As was discussed in Chapter 1, alkyne ligands are well known to be variable electron donors, mediated in part by the electrophilicity of the metal centre. Consequently, the interaction between the metal centre and the alkyne unit in 2.2 can be viewed as a dative bond between Ta(III) and a neutral (two electron donating) alkyne ligand.  Alternatively, the interaction can be viewed as the donation of four electrons via two formal covalent bonds between Ta(V) and an ‘alkenediyl’ dianion.  These two scenarios are depicted in Figure 2.2.    Figure 2.2: Two possible resonance structures for [PhNPN*]Ta(alkyne)Cl complexes.  Several pieces of evidence suggest that for 2.2 the Ta(V) formalism is a more accurate depiction.  In the 13C{1H} NMR spectrum, the extremely low-field chemical shift of the quaternary alkyne carbons (~δ 200) is indicative of a 4-electron donor coordinated to an electron deficient Ta(V) metal centre.23 Data from the solid-state structure also provides evidence for a Ta(V)-alkenediyl interaction. The length of the C–C multiple bond (1.295(4) Å, vide supra) is significantly longer than a typical triple bond (~1.2 Å) and is more analogous to the double bond that would be expected in an alkenediyl moiety.32  In addition, the C40–C41–C42 and C41–C42–C43 bond angles (~140o) deviate significantly from the 180o that would be expected of sp-hybridized carbons, which further detracts from the suitability of the Ta(III)-alkyne bonding TaRR Cl[PhNPN*]TaRR ClTa(III) + Ta(V) +[PhNPN*] 'alkenediyl' ligandalkyne ligand  48 model.  As was first addressed in Chapter 1, complex 2.2 and its various congeners discussed in this thesis will be referred to as metal-alkyne complexes. [PhNPN*]Ta(BTA)Cl, 2.3 (BTA = bis(trimethylsilyl)acetylene), was prepared via the reaction of 2.1 and Ta(BTA)Cl3(DME), and was isolated as a light orange powder in good yield.  In C6D6, 2.3 produces a singlet at δ 29 in the 31P{1H} NMR spectrum.  The 1H NMR spectrum of 2.3 is similar to 2.2 with 4 aryl methyl resonances; the aryl region of the spectrum is unremarkable and consistent with Cs symmetry.  Two distinct trimethylsilyl groups appear in the expected region (δ 0.16 and 0.08), suggesting that in solution the entire alkyne unit lies in the σv plane of symmetry.  The 13C{1H} spectrum is unremarkable and features all of the expected [PhNPN*] resonances; the quaternary carbons of the alkyne unit give rise to two singlets at δ 225 and 205, in contrast to the fluxional character of 2.2.   The ORTEP representation of the solid-state molecular structure of 2.3 is shown in Figure 2.3.  As with 2.2, the [PhNPN*] ligand coordinates facially to Ta, resulting in significantly distorted trigonal bipyramidal geometry about the metal centre; the apical positions are occupied by the Cl and P atoms, and the equatorial plane consists of N01, N02 and the centroid of the C39–C40 bond.  Compounds 2.2 and 2.3 have similar solid-state structures; in general, they exhibit small but unremarkable differences with respect to bond lengths and angles. These structural similarities include the coordinated alkyne; the Ta01–C39, Ta01–C40 and C39–C40 bond lengths and the C39–C40–Si01 and C40–C39–Si02 bond angles are close to the analogous parameters found in 2.2.  In addition, the plane defined by C39, C40 and Ta01 deviates from the P01–Ta01–Cl01 plane (the molecular plane of symmetry, σv) by only 5.2°.  This is consistent with the observation of distinct TMS groups in solution (as was the case with 2.2).    49  Figure 2.3: ORTEP drawing of the solid-state molecular structure of 2.3 (ellipsoids at 50% probability).  All hydrogen atoms and the mesityl group at N02 (except for Cipso) have been omitted for clarity.   Table 2.2: Selected bond lengths (Å) and angles (°) for 2.3 Parameter (Å) Parameter (°) Ta01–N01 2.058(2) N01–Ta01–N02 132.86(8) Ta01–N02 2.042(2) P01–Ta01–Cl01 145.59(2) Ta01–P01 2.619(6) N01–Ta01–P01 73.62(6) Ta01–Cl01 2.405(6) N02–Ta01–P01 73.99(6) Ta01–C39 2.168(2) N01–Ta01–Cl01 91.96(6) Ta01–C40 2.095(2) N02–Ta01–Cl01 95.91(6) C39–C40 1.326(3) C39–C40–Si01 142.16(19)   C40–C39–Si02 126.91(18) Cl01 Si01 Si02 C39 C40 N01 N02 P01 Ta01   50  A report by Cummins and coworkers41 described how Ta complexes similar to 2.2 and 2.3 could be converted to the corresponding diiodide (and the free alkyne) via the addition of one equivalent of I2 (scheme 2.4, i).    Scheme 2.4: i) The conversion of a Ta(alkyne) complex to the corresponding diiodide, (by Cummins and coworkers) and ii) the proposed conversion of [PhNPN*]Ta(alkyne)Cl complexes to their corresponding Ta trihalide, via the displacement of an alkyne ligand with I2.  It was hoped that a similar reaction between I2 and a [PhNPN*]Ta alkyne complex might offer a route to a [PhNPN*]Ta trihalide (scheme 2.3, ii), which could then be used as an entry point for dinitrogen reduction chemistry, similar to previous work done in the Fryzuk group with early metal NPN complexes.96,98,99,101,103,126,127  Unfortunately, treating either 2.2 or 2.3 with I2 under a variety of reaction conditions (-78 °C to 20 °C, one and two equivalents of I2) and solvents (toluene and diethyl ether) consistently yielded solid material of very low solubility, hampering attempts at definitive characterization or purification.  Samples submitted for analysis by EI–MS indicated the presence of [PhNPN*]TaI2Cl, as well as several related molecular TaRR Cl I2_ ClTaII[PhNPN*] [PhNPN*]TaEtEt I TaII2_ (NR2)3(NR2)3 EtC CEtRC CRi)ii)  51 fragments.  However, it was not possible to further separate or characterize the product (or products) from this reaction, and this route was abandoned.    2.2.4 Synthesis of [PhNPN*]Ta Monohydride Complexes  Both [PhNPN*]Ta(3-hexyne)H, 2.4, and [PhNPN*]Ta(BTA)H, 2.5, were prepared from their corresponding chloride complexes via a salt metathesis reaction with freshly prepared KBEt3H, as shown in Scheme 2.5.  Scheme 2.5: Synthesis of the [PhNPN*]Ta alkyne hydride complexes 2.4 and 2.5.  In C6D6, each complex produces a singlet (2.4 = δ 20, 2.5 = δ 16) in their respective 31P{1H} NMR spectra.  In general, the 13C{1H} NMR spectra of both 2.4 and 2.5 are unremarkable; all of the [PhNPN*] resonances are present in their expected regions and are indicative of Cs symmetric complexes.  In contrast to 2.2, the quaternary alkyne carbons for 2.4 are directly observable in the expected region (at δ 205 and 184); the quaternary alkyne carbons for 2.5 also appear as expected at δ 220 and 193. Ta NP NRR Cl2.2 (R = Et)2.3 (R = TMS) - KClKBEt3H 2.4 (R = Et)2.5 (R = TMS)Ta NP NRR H- BEt3  52 The 1H NMR spectra for the [PhNPN*] ligand protons for both 2.4 and 2.5 are similar to that of their corresponding chloride complex, as well as to each other; the appearance of 4 distinct aryl methyl peaks and the aromatic ligand protons are all indicative of Cs symmetry in solution.  As with 2.3, the trimethylsilyl groups in 2.5 appear as two singlets, again suggesting inequivalent TMS groups and thus that the alkyne unit lies in the σv plane of symmetry (a hypothesis supported by the inequivalent alkyne quaternary carbons mentioned above).  Whereas in the case of 2.2 the ethyl arms of the 3-hexyne unit displayed considerable fluxionality and consequently appeared as two very broad singlets at room temperature, in 2.4 these methyl and methylene protons appear as two pairs of well-resolved triplets and quartets, respectively. The appearance of these protons as two distinct pairs (in addition to the two inequivalent alkyne quaternary carbons seen in the 13C{1H} spectrum), implies inequivalent ethyl arms, and suggests that the 3-hexyne unit lies in the proposed σv plane of symmetry of the complex, similar to the examples discussed above.   A noteworthy feature of the 1H NMR spectrum for both 2.4 and 2.5 is the chemical shift of the hydride ligand: in both cases these resonances appear as doublets at δ ~21.  Viewed in the context of other early metal hydrides – particularly group 3128-130 and 4131-133 examples, which typically appear upfield of δ 8 – this data is surprising.  However, a survey of the literature shows that Ta hydride resonances appear over a wide range of chemical shifts.  In addition to a number of cases in the δ 0 – 12 range,90,91,134-138 there are several examples appearing upfield of δ 0,139 or as far downfield as δ 18 or more.41,52,140-145 The variability observed in these values owes much to the complicated interplay of physical properties that determine chemical shift values,146 and appears to have little bearing on chemical reactivity; as such, this NMR data for 2.4 and 2.5 appears to be little more than a spectroscopic curiosity.  The resonances for both hydride ligands   53 do exhibit strong coupling (2JHP = ~35 Hz) to the phosphorus-31 atom of [PhNPN*], suggesting that in solution these hydrides are trans to the phosphine.  This assertion is borne out by the solid-state structural data discussed below.  An ORTEP representation of the solid-state molecular structure of 2.4 is shown in Figure 2.4.  The structure of 2.4 resembles that of 2.2, with small differences in bond angles and lengths.  Again, the geometry at Ta is that of a significantly distorted trigonal bipyramid; the P atom of [PhNPN*] and the hydride ligand occupy the apical positions, with N01, N02 and the centroid of the C41–C42 bond constituting the equatorial plane.  In the solid-state the 3-hexyne unit lies in the σv plane of symmetry leading to two inequivalent ethyl arms, which is consistent with the solution-state NMR data.  The bond lengths and angles related to the coordinated 3-hexyne unit are similar to those found in 2.2, and reflect an analogous metal-ligand interaction.     54  Figure 2.4: ORTEP drawing of the solid-state molecular structure of 2.4 (ellipsoids at 50% probability).  All hydrogen atoms (except for H99, which was located from the difference map and refined isotropically), and the mesityl group at N01 (except for Cipso) have been omitted for clarity.  Table 2.3: Selected bond lengths (Å) and angles (°) for 2.4 Parameter (Å) Parameter (°) Ta01–N01 2.041(2) N01–Ta01–N02 130.27(10) Ta01–N02 2.037(2) P01–Ta01–H99 148.8(11) Ta01–P01 2.647(1) N01–Ta01–P01 74.84(7) Ta01–H99 1.82(3) N02–Ta01–P01 73.52(7) Ta01–C41 2.109(3) N01–Ta01–H99 94.60(11) Ta01–C42 2.084(3) N02–Ta01–H99 93.20(11) C41–C42 1.300(5) C40–C41–C42 138.8(3)   C41–C42–C43 141.3(3) C40 C41 C42 C43 Ta01 H99 N02 N01 P01   55  2.2.5 Synthesis of [PhNPN*]Ta Alkyl Complexes  [PhNPN*]Ta(3-hexyne)Bn, (2.6), and [PhNPN*]Ta(BTA)Bn, (2.7), were prepared via the salt metathesis reaction between the corresponding Ta chloride complex and benzyl potassium, as depicted in Scheme 2.6.  Scheme 2.6: Synthesis of the [PhNPN*]Ta alkyne benzyl complexes 2.6 and 2.7.  In C6D6, each complex generates a singlet (2.6 = δ 26.5, 2.7 = δ 26.2) in their respective 31P{1H} NMR spectra.  As with the Ta complexes discussed above, the 13C{1H} NMR spectra of the [PhNPN*] resonances for both 2.6 and 2.7 are indicative of Cs symmetric complexes.  Ta NP NTMSTMS Cl2.3Ta NP NEtEtBn Ta NP NTMSTMS BnTa NP NEtEt Cl2.2 2.72.6KBn- KClKBn- KCl  56 Additionally, the 1H NMR spectra for the [PhNPN*] ligand protons of both 2.6 and 2.7 are similar to that of their corresponding chloride complexes, as well as to each other; the aromatic ligand protons and the presence of 4 distinct aryl methyl peaks all reflect Cs symmetry in solution.  In 2.6, the benzyl methylene protons appear as a doublet (δ 2.75, JHP = 7.2 Hz) coupled to the phosphorus atom of [PhNPN*]; in contrast, the benzylic protons in 2.7 appear only as a broad singlet at δ 3.22.  By 1H-13C HSQC NMR spectroscopy, these benzylic protons correlate to phosphorus-coupled doublets at δ 75.5 (2.6, JCP = 4 Hz) and δ 89.8 (2.7, JCP = 16 Hz) in their respective 13C{1H} NMR spectra. The NMR data for the alkyne units in 2.6 and 2.7 are notably different from the complexes discussed previously.  While the quaternary alkyne carbons for 2.6 and 2.7 are not directly observable in the 13C{1H} spectra, they can be detected indirectly by a 1H–13C HMBC experiment (as was done with 2.2).  Using this approach, both of the 3-hexyne quaternary carbons give rise to one singlet at δ 241; coincidentally, the resonance for the quaternary alkyne carbons of 2.7 also appears at δ 241.  Additionally, in 2.6 there is only one peak for both of the methyl (δ 30) and methylene (δ 13) carbon atoms, and in 2.7, all six TMS methyl carbons give rise to one singlet at δ 2.2.  The 1H NMR data for both 2.6 and 2.7 is consistent with what is observed in the 13C{1H} NMR spectra.  There is only one set of methyl and methylene protons for the 3-hexyne unit in 2.6 (a quartet and a triplet that integrate to 4 and 6 protons, respectively), and the two trimethylsilyl groups in 2.7 produce one singlet (integrating to 18 protons). Overall this data suggests that in solution, both 2.6 and 2.7 have alkyne units with two equivalent halves, and are thus bound to the Ta centre perpendicular to the σv plane of symmetry in each molecule.    ORTEP representations of the solid-state molecular structures of 2.6 and 2.7 are shown in Figure 2.5 and Figure 2.6; the two benzyl complexes exhibit significant structural differences   57 with regards to the relative orientation of the ligands around the Ta centre.  In both cases, the geometry about the metal is distorted trigonal bipyramidal.  In 2.7 the equatorial plane is defined by N01, N02 and the centroid of the coordinated alkyne unit, with the apical positions occupied by the ligand phosphorus atom and the benzyl group; this ligand arrangement is similar to the structures of 2.2, 2.3 and 2.4.  In contrast, the position of the benzyl group and the alkyne unit are reversed in 2.6.  This orientation leads to a far less distorted geometry; the P1–Ta–C45 angle is almost 90°, and Ta01, P01 and the centroid of the C41–C42 bond lie in a nearly straight line (175.6°). The result is a nearly idealized trigonal bipyramid compared to 2.6 (or any of the previously discussed complexes).  In both cases, the angle between the metal centre and the carbons of the benzyl ligand (2.6: Ta01–C45–C46 = 114.8°; 2.7: Ta01–C39–C40 = 133.5°) is large enough so as to unambiguously designate these groups as η1-coordinated ligands.147  It appears that the difference in ligand orientation around the metal centre has little effect on the strength of the various Ta–donor atom interactions, as these bond lengths are essentially the same in 2.6 and 2.7.  However, this dramatic structural variation may explain the reactivity differences observed between these two complexes, which will be explored further in Chapters 3 and 5.     58  Figure 2.5: ORTEP drawing of the solid-state molecular structure of 2.6 (ellipsoids at 50% probability).  All hydrogen atoms and the mesityl group at N02 (except for Cipso) have been omitted for clarity.  The phenyl ring of the benzyl arm appears in the difference map as disordered over two discrete positions; although only one orientation is displayed, both orientations were located and modeled anisotropically. Table 2.4: Selected bond lengths (Å) and angles (°) for 2.6 Parameter (Å/°) Parameter (°) Ta01–N01 2.117(10) N01–Ta01–N02 118.9(4) Ta01–N02 2.043(9) P01–Ta01–C45 81.2(3) Ta01–P01 2.625(3) N01–Ta01–P01 74.9(2) Ta01–C45 2.238(10) N02–Ta01–P01 74.8(3) Ta01–C41 2.065(13) N01–Ta01–C45 104.9(4) Ta01–C42 2.096(11) N02–Ta01–C45 121.0(4) C41–C42 1.306(18) Ta01–C45–C46 114.8(10) C40–C41–C42 139.4(11) C41–C42–C43 132.4(13) C45 C46 C41 C40 C43 C42 P01 Ta01 N01 N02   59   Figure 2.6: ORTEP drawing of the solid-state molecular structure of 2.7 (ellipsoids at 50% probability).  All hydrogen atoms and the mesityl group at N01 (except for Cipso) have been omitted for clarity.     Table 2.5: Selected bond lengths (Å) and angles (°) for 2.7 Parameter (Å/°) Parameter (°) Ta01–N01 2.038(2) N01–Ta01–N02 122.39(9) Ta01–N02 2.106(2) P01–Ta01–C39 150.98(8) Ta01–P01 2.6393(7) N01–Ta01–P01 73.49(7) Ta01–C39 2.209(3) N02–Ta01–P01 73.51(7) Ta01–C100 2.098(3) N01–Ta01–C39 99.25(10) Ta01–C101 2.120(3) N02–Ta01–C39 88.17(10) C100–C101 1.328(4) Ta01–C39–C40 133.5(2) C100–C101–Si01 134.7(2) C101–C100–Si02 136.7(2) Si01 Si02 C101 C100 P01 Ta01 N02 N01 C39 C40   60 This solid-state structural data is in accordance with what is observed in solution via NMR spectroscopy, specifically with regards to the equivalency of both halves of the alkyne units. In 2.2 and 2.3 these units lie almost exactly in the σv plane of the molecule, leading to the inequivalent alkyne halves observed in the NMR spectra. In contrast, in 2.6 and 2.7 these alkyne units lie significantly out of the σv plane.  In complex 2.6, the angle between the plane defined by C41-Ta01-C42 and the σv plane (P01-Ta01-C45) is ~35°; in complex 2.7, the angle between the plane defined by C101-Ta01-C100 and the σv plane (P01-Ta01-C39) is ~30°.  While neither unit is oriented exactly perpendicular (i.e. 90°) to the σv plane – the orientation necessary for truly equivalent alkyne halves – it is probable that in solution the alkyne has some freedom of movement that allows it to ‘wag’ diagonally back and forth, and becomes trapped in one of these diagonal orientations upon crystallization.  The Ta ethyl complex, [PhNPN*]Ta(BTA)Et, (2.8) was prepared via two different routes (Scheme 2.7).  The first route involves the salt metathesis reaction between 2.3 and EtMgCl.  Complex 2.8 can also be synthesized via the insertion of ethylene into the Ta–H bond of the monohydride complex 2.5.    61  Scheme 2.7: Two routes for the preparation of [PhNPN*]Ta(BTA)Et (2.8).  In the 31P{1H} NMR spectrum, 2.8 displays a singlet at δ 27.7; in the 1H NMR spectrum, the resonances for the ethyl ligand appear as a well-resolved quartet (δ 1.73) and triplet (δ 1.25), with no apparent coupling to phosphorus-31.  The TMS methyl groups give rise to two very broad singlets, in stark contrast to the benzyl analogue (2.7) where these methyl groups appear as one sharp singlet.  It is possible that the reduced steric bulk of the ethyl moiety (compared to a benzyl arm) allows the coordinated alkyne greater freedom of movement, resulting in some fluxionality on the NMR timescale, and thus some signal broadening. Ta NP NTMSTMS Cl2.3Ta NP NTMSTMS H2.5 2.8EtMgCl, Et2O-35 oC; rt, 4 hethylene, toluenert, 1 h Ta NP NTMSTMS Et  62  An ORTEP representation of the solid-state molecular structure of 2.8 is shown in Figure 2.7. The overall structure is similar to that of the benzyl case (2.7).  The geometry about the metal is distorted trigonal bipyramidal; the equatorial plane is defined by N01, N02 and the centroid of the coordinated alkyne unit, and the ligand phosphorus atom and the ethyl group occupy the apical positions. The Ta-C100-C101 bond angle is 125.34°, indicative of an η1 bonding mode for the ethyl ligand. The orientation of the coordinated alkyne is rotated out of the σv plane, similar to complex 2.7: the angle between the C102-Ta01-C103 and P01-Ta01-C100 planes is ~31°.  Overall, it appears that in the complexes with less sterically bulky ligands, such as the Ta hydride complexes 2.4 or 2.5, the NMR and solid-state data suggest that the coordinated alkyne orients itself along the σv plane of the molecule; it follows that, in the context of relieving steric bulk, this is the most favourable orientation for the alkyne.  In the case of the Ta benzyl complexes 2.6 and 2.7, the alkyne appears to rotate significantly out of the σv plane of the molecule to reduce repulsive interactions with the bulkier benzyl ligand.  When compared in terms of relative steric bulk, the ethyl ligand is intermediate between the smaller hydride and larger benzyl moiety.  Thus in 2.8, while the slightly bulkier ethyl arm inhibits the coordinated alkyne from adopting its preferred orientation along the σv plane, it is not bulky enough to prevent some rotation along the Ta-alkyne axis in solution; this may explain the observed broadness of the signals for the BTA ligand in the 1H and 13C{1H} NMR spectra.  The broadness observed in the NMR resonances of the hexyne ligand in complex 2.2 can be rationalized in a similar fashion.    63  Figure 2.7: ORTEP drawing of the solid-state molecular structure of 2.8 (ellipsoids at 50% probability).  All hydrogen atoms and the mesityl group at N02 have been omitted for clarity.   Table 2.6: Selected bond lengths (Å) and angles (°) for 2.8. Parameter (Å/°) Parameter (°) Ta01–N01 2.046(5) N01–Ta01–N02 124.86(19) Ta01–N02 2.105(5) P01–Ta01–C100 153.75(16) Ta01–P01 2.6667(16) N01–Ta01–P01 73.40(13) Ta01–C100 2.212(6) N02–Ta01–P01 74.37(13) Ta01–C102 2.115(6) N01–Ta01–C100 100.9(2) Ta01–C103 2.077(6) N02–Ta01–C100 89.4(2) C102–C103 1.319(8) Ta01–C100–C101 125.2(4) C102–C103–Si01 136.3(5) C103–C102–Si02 139.4(5)  Si02 Si01 C102 C103 C101 C100 Ta01 N01 N02 P01   64 2.2.6 Thermal Decomposition of [PhNPN*]Ta Alkyl Complexes  The Ta benzyl (2.7) and ethyl (2.8) complexes are moderately heat-sensitive; the compounds are stable for weeks at room temperature in the solid-state, but when stirred in refluxing toluene for several days, both 2.7 and 2.8 formed the same decomposition product, 2.9.  The 31P{1H} NMR spectrum of 2.9 features a sharp singlet at δ 8.8.  The 1H NMR spectrum is indicative of a complex with C1 symmetry; aryl protons that were equivalent in the parent alkyl complexes give rise to two distinct resonances with similar chemical shifts, suggesting that the two amide arms of the [PhNPN*] ligand are inequivalent.  In addition, there are 7 signals in the aryl methyl region, each integrating to 3 protons; the 13C{1H} NMR data are consistent with these observations.  Finally, there is a pair of signals at δ 3.94 and δ 2.32 each integrating to 1 proton; COSY and HSQC NMR experiments confirm that these protons are coupled to each other, and are attached to the same carbon atom. Of note is the fact that only one of these diastereotopic methylene protons (at δ 3.94) exhibit coupling to the phosphorus-31 atom of the [PhNPN*] ligand.  Cognizant of previous work done by Dr. Erin MacLachlan on the decomposition of [PhNPN*]Zr dibenzyl complexes,110 this NMR data suggests that 2.9 features a metallated ortho-methyl group from one of the mesityl arms, generated by the loss of one of the methyl protons and the Ta alkyl ligand, as depicted in Scheme 2.8.   65  Scheme 2.8: Formation of the cyclometallated complex [PhNPNC*]Ta(BTA) (2.9), the proposed result of the thermal decomposition of the [PhNPN*]Ta(BTA) alkyl complexes 2.7 and 2.8.  2.3 Conclusions In this chapter, a route to a new alkali metal salt of a diamidophosphine ligand, [PhNPN*]K2(THF)0.5, is described. Attempts to prepare Ta(V) complexes such as [PhNPN*]TaCl3, [PhNPN*]TaBn3, and [PhNPN*]Ta(NMe2)3 through salt metathesis reactions with [PhNPN*]Li2・(dioxane) or [PhNPN*]K2(THF)0.5 and TaCl5, or protonolysis reactions with [PhNPN*]H2 and TaBn5 or Ta(NMe2)5 are also reported; these routes ultimately proved unsuccessful, and were abandoned.  Tantalum complexes of the [PhNPN*] ligand set can be prepared via the salt metathesis reaction between [PhNPN*]K2(THF)0.5 and Ta(alkyne)Cl3(DME), a series of Ta(V) species featuring a reduced alkyne unit.  Both [PhNPN*]Ta(3-hexyne)Cl and [PhNPN*]Ta(BTA)Cl are synthesized in good yields as dark yellow and orange powders, respectively.  Salt metathesis reactions with the appropriate reagent can convert each [PhNPN*]Ta(alkyne)Cl complex into a Ta NP NTMSTMS R Ta NP NTMSTMS110oC, 48 h- RH2.7 (R = Bn)2.8 (R = Et) 2.9  66 variety of organometallic complexes, [PhNPN*]Ta(alkyne)X (X = H, Bn, Et); in addition, [PhNPN*]Ta(BTA)Et can be synthesized from [PhNPN*]Ta(BTA)H and ethylene gas.   The Ta(V) alkyl complexes [PhNPN*]Ta(BTA)Et and [PhNPN*]Ta(BTA)Bn are thermally stable at room temperature for weeks in the solid-state, but both decompose in solution at high temperature to produce the C1 symmetric cyclometallated complex [PhNPNC*]Ta(BTA), through C–H activation of an aryl methyl group on the [PhNPN*] ligand and loss of the free alkane (ethane or toluene, respectively).  This transformation may be a means of relieving steric bulk at the tantalum centre.     67 Chapter  3: Synthesis and Reactivity of [PhNPN*]Ta Azide and Imide Complexes  3.1 Introduction Chapter 2 focused on the synthesis of a variety of [PhNPN*]Ta alkyne complexes, and outlined two different ways of considering the structure and bonding in transition metal complexes that feature coordinated alkyne ligands: the low valent tantalum(III)–alkyne and high valent tantalum(V)–‘alkenediyl’ and structural motifs (Scheme 3.1).   Scheme 3.1: Two possible resonance structures for [PhNPN*]Ta alkyne complexes.  While Chapters 4 and 5 will explore examples of chemical transformations of the coordinated alkyne unit, this chapter will deal with systems where the role of the alkyne is envisioned to be that of a ‘place-holder ligand,’ undergoing displacement and allowing for further reactivity at the Ta centre.  Accordingly, the systems discussed in this chapter are best understood by invoking the low valent–alkyne resonance form of the [PhNPN*]Ta alkyne complex (pictured on the right in Scheme 3.1); in this context, the synthesis and reactivity of Ta azide, nitride and imide complexes are investigated. TaRR X [PhNPN*]TaRR XTa(III) + Ta(V) +[PhNPN*] 'alkenediyl' ligandalkyne ligand  68  3.1.1 Transition Metal Azide and Nitride Complexes Transition metal nitrides are of great interest in field of dinitrogen (N2) activation and functionalization, as they are intermediates in the heterogeneous generation of ammonia from N2 and H2 (i.e., the Haber-Bosch process),148,149 and have also been implicated in several homogeneous catalytic systems,150-154 most notably in the work of Chatt155,156 and Schrock.157,158  There are a variety of examples in the literature of metal nitride formation via the cleavage of elemental N2, usually through a metal-mediated reduction process.103,153,159-165 An alternative method for generating metal nitrides is via the extrusion of N2 from a metal azide complex.  There are numerous examples of such a transformation, mainly involving the later transition metals: Group 6 (Mo166,167, W166,167, Cr168), Group 8 (Fe169-171, Ru172-174), Group 9 (Co175, Ir176) and actinide (U177) systems apply this method to synthesize nitride moieties. The azide complexes in which these transformations occur generally contain low valent, electron-rich metal centres, as the formation of a nitride from an azide is usually accompanied by the oxidation of the metal by two electrons (Equation 3.1).  In contrast, early transition metal (Group 3 – 5) complexes typically feature electron-poor, d0 metal centres that would generally be unable to undergo a similar metal-based oxidation.  However, as mentioned above, the Ta alkyne complexes of the type introduced in Chapter 2 are atypical, in that it is possible to envision a [PhNPN*]Ta alkyne azide complex as having a metal centre formally in the 3+ oxidation state (i.e. a d2 metal centre), which provides the possibility for a route to a Ta nitride that is analogous to a late metal system (Scheme 3.2).  Owing to the LnM -N2 (3.1)x x+2LnMNN3 hν / Δ  69 paucity of mononuclear terminal Ta nitride complexes in the literature, a dinuclear structure featuring two bridging nitrides is also a possibility; similar complexes have been reported by Sita and coworkers178 through the chemical reduction of a Ta-coordinated dinitrogen ligand.  Scheme 3.2: Possible synthetic route to a Ta(V) nitride, via extrusion of N2 from a Ta(III)-alkyne azide complex.  This approach is not without precedent, as a transformation of this kind is known for a low-valent Nb system; a 1985 report by Hubert-Pfalzgraf and coworkers detailed the synthesis of a formally Nb(III) azide (from Nb2Cl6(SMe2)3), which readily converted to an oligomeric Nb(V) nitride at ambient temperature (Scheme 3.3).179  To this end, we sought to prepare Ta alkyne azide complexes with the ultimate goal of converting the azide moiety into a nitride.   TaN [PhNPN*]Ta(V)-N2TaRR N3[PhNPN*] TaRR N3Ta(III)Ta(V) [PhNPN*] - alkyneTaN [PhNPN*]Ta N[PhNPN*] orhν / Δ  70  Scheme 3.3: Synthesis of a Nb(V) nitride via the extrusion of N2 from a Nb(III) azide complex (adapted from ref. 179).  3.2 Results and Discussion 3.2.1 Synthesis of Tantalum Azide Complexes, and the Attempted Synthesis of a Tantalum Nitride The reaction of sodium azide with the relevant Ta chloride complex (2.2 and 2.3, respectively), afforded [PhNPN*]Ta(3-hexyne)N3, 3.1, and [PhNPN*]Ta(BTA)N3, 3.2 (Scheme 3.4).  The products of these reactions were formed slowly, with complete conversion observed after 72 h at room temperature.  Surprisingly, heating the reaction mixtures did not lead to extrusion of N2 from the azide moiety, and was in fact the preferred method for the preparation of 3.1 and 3.2; the reaction times dropped to approximately 36 h when the mixtures were heated to 70 °C.    Nb2Cl6(SMe2)3 - 2 TMSCl2 TMSN3 Nb2(N3)2Cl4(SMe2)2 [NbNCl2(SMe2)]n- N2Nb(V)Nb(III)Nb(III)  71  Scheme 3.4: Synthesis of the [PhNPN*]Ta alkyne azide complexes 3.1 and 3.2.  In solution, the NMR spectra of 3.1 and 3.2 are generally unremarkable, and resemble their Ta chloride precursors.  The 31P{1H} NMR spectra feature sharp singlets (3.1: δ 29.6; 3.2: δ 28.3), and the 1H NMR spectra feature [PhNPN*] ligand resonances that are consistent with Cs symmetric complexes.  In the case of 3.2, all the expected resonances for the coordinated BTA moiety are readily located in the 1H and 13C{1H} NMR spectra, and indicate two inequivalent TMS groups (similar to the data seen with 2.3). In contrast, the methyl and methylene protons of the 3-hexyne moiety in 3.1 give rise to extremely broad singlets in the 1H NMR spectrum.  In the 13C{1H} NMR spectrum, the resonances for these carbon atoms are visible but very weak.  In addition, the quaternary 3-hexyne carbons are not observable in the 13C{1H} NMR spectrum.  This data is similar to that of complexes 2.2 and 2.6, where fluxional 3-hexyne units result in broad, low intensity resonances.  As was previously discussed (in Chapter 2), it is possible to indirectly observe the quaternary carbons in 2.2 and 2.6 via their correlation with the methyl and methylene protons of the 3-hexyne unit, using a 1H-13C HMBC NMR experiment. Unfortunately, Ta NP NRR Cl2.2 (R = Et)2.3 (R = TMS) - NaClNaN3 Ta NP NRR N3.1 (R = Et)3.2 (R = TMS)NN  72 this approach was unsuccessful in the case of 3.1, and as such resonances attributable to the quaternary alkyne carbon atoms have not been observed using NMR spectroscopy.    Small brown crystals of 3.1 were obtained from a toluene-pentane solution cooled to -35 °C, and were studied by single-crystal X-ray diffraction; a representation of the solid-state molecular structure of 3.1 is shown in Figure 3.1. The low quality of the data does not permit a meaningful discussion of the metrical parameters, but the overall connectivity in complex 3.1, including the presence of a 3-hexyne moiety and an intact azide group, is confirmed.     Figure 3.1: Solid-state molecular structure of 3.1.  All hydrogen atoms have been omitted for clarity. Only one of the three molecules present in the asymmetric unit is shown.  Poor quality data did not allow for the determination of metrical parameters.  The presence of a metal azide in 3.1 and 3.2 was also confirmed using solid-state FT-IR spectroscopy.  In each case, a strong band is observed in the expected region for terminal metal Ta01 Nα Nβ Nγ N01 N02 P01   73 azides168-171,175-177 (3.1: 2086 cm-1, 3.2: 2104 cm-1); as expected, this band is absent in the FT-IR spectra of the Ta chloride precursors (2.2 and 2.3).   With regards to nitride formation, complexes 3.1 and 3.2 proved to be frustratingly inert.  As mentioned above, these complexes were synthesized at elevated temperature (70 °C), and in this context it was not surprising that prolonged heating of 3.1 or 3.2 in refluxing toluene (110 °C for days) or the solid state (225 °C for 24 h) led to no further reaction (or decomposition).  In addition to thermolysis, a photolytic route for N2 extrusion was investigated.  Despite its cited use in the synthesis of a variety of metal nitrides, there appears to be no standardized approach to photolysis, aside from the predominance of UV light sources.168-170,175,177 Thus, solutions of 3.1 or 3.2 in THF were irradiated in a Rayonet Photochemical Reactor (3 x 8 W lamps, λmax = 300 and 350 nm) for 18 h; while UV-Vis spectra of the samples obtained before and after irradiation showed minor differences, there was no observable change in the 1H or 31P{1H} NMR spectra, and consequently this route was abandoned.   3.2.2 Reactions of Tantalum Alkyne Complexes with Organoazides: Synthesis of Tantalum Imides Organoazides have been shown to be valuable reagents for forming transition metal imide (M=NR) moieties.180-188  Their use in this transformation is believed to proceed via the formation of the metal terminal organoazido moiety (and concomitant oxidation of the metal centre), followed by extrusion of an equivalent of N2 to ultimately afford the metal imide.  Of particular interest to us was the work Bergman and coworkers have done investigating this type of reactivity with low-valent Ta complexes.189,190 They reported that the reaction of Cp2TaMe(PMe3) with aryl azides (ArN3) led to the formation of isolable Ta(V) terminal   74 arylazido complexes; gentle heating or irradiation with UV light led to the extrusion of N2 to give the Ta(V) imide (Equation 3.2).    A similar approach has been used by Chang and coworkers to synthesize a series of vanadium imides, oxidizing a low valent V(III) precursor with a variety of aryl azides.191  In light of the potential for a Ta(III) resonance form of the [PhNPN*]Ta alkyne complexes, it was hypothesized that it might be possible to synthesize [PhNPN*]Ta imide complexes via a route similar to that used by Bergman and Chang.   Treating a dark yellow toluene solution of 2.2 with one equivalent of para-isopropylphenyl (PIPP) azide at room temperature resulted in an opaque yellow suspension after stirring for several hours.  Upon cooling the crude reaction mixture complex 3.3 precipitated from solution, and upon filtration was isolated as a bright yellow powder.  Although 3.3 is only sparingly soluble in common organic solvents, satisfactory NMR spectra were acquired from a toluene-d8 solution.  The 31P{1H} NMR spectrum contains a sharp singlet at δ 30.9; the 1H NMR spectrum contains a septet and doublet (δ 2.56 and 1.02, 3JHH = 7 Hz) indicative of an isopropyl moiety (implying incorporation of the aryl ring of the organoazide into the complex), as well as [PhNPN*] ligand resonances indicative of Cs symmetry.  The 13C{1H} spectrum is consistent with these observations.  Of note is the absence in the NMR spectra of any resonances attributable to the 3-hexyne moiety, suggesting loss of the alkyne unit and potential synthesis of the Ta arylimido (or arylazido, as per Bergman’s work) complex. Cp2Ta ArN3 (3.2)-N2MePMe3 Cp2Ta MeN N NAr Cp2Ta MeNArTa(V)Ta(V)Ta(III) - PMe3 hν / Δ  75  Yellow crystals suitable for an X-ray diffraction study were obtained from a hexanes–THF solution cooled to -35 °C; an ORTEP representation of complex 3.3 is shown in Figure 3.2, and confirms that the coordinated alkyne unit in 2.2 was displaced to afford the Ta terminal arylimido complex.   Figure 3.2: ORTEP diagram of the solid-state molecular structure of 3.3 (ellipsoids at 50% probability). Half of the molecule is generated by the symmetry operation x, -y–½ , z.  All hydrogen atoms and the mesityl group at N01 (except for Cipso), as well as the disorder that affects the p-isopropylphenyl group and sections of the [PhNPN*] ligand have been omitted for clarity.   Cl01 Ta01 N01 N01’ N03 C21 P01   76 Table 3.1: Selected bond lengths (Å) and angles (°) for 3.3 Parameter (Å/°) Parameter (°) Ta01–N01 2.068(5) N01–Ta01–N01’ 127.1(3) Ta01–N03 1.794(8) N03–Ta01–N01 111.11(16) Ta01–P01 2.576(3) N03–Ta01–Cl01 105.0(3) Ta01–Cl01 2.363(2) P01–Ta01–N03 93.1(3) Ta01–N03–C21 176.4(6) P01–Ta01–Cl01 161.66(9)  The solid state molecular structure of 3.3 features slightly distorted trigonal bipyramidal geometry at Ta; the apical sites are occupied by the [PhNPN*] phosphorus atom and the chloro ligand, and the equatorial plane consists of the amido and imido nitrogen (N01 – N03) atoms.  Complex 3.3 also features a nearly linear Ta01–N03–C21 bond angle (176.4(6)o), which suggest that the imido-N atom is sp-hybridized, and allows for the possibility that the imide ligand acts as a 6-electron donor (and thus participates in a Ta–N triple bond). This hypothesis is supported by the relatively short Ta01–N03 distance of 1.794(8) Å, which is significantly shorter than a Ta –N double bond (~1.90 Å91,178,192), and is similar to distances found in other Ta imide complexes where the Ta–N interaction is best described as a triple bond (~1.76 Å193,194).  In the literature, these interactions are almost exclusively depicted with a Ta-N double bond,59,188-190,195-202 despite the fact that, as discussed above, the structural parameters often suggest that the interaction features some triple bond character; the Ta imide complexes discussed in this thesis will adhere to this convention, and are depicted with a Ta–N double bond. In addition, complex 2.3 reacted with PIPP azide to yield the same Ta imido chloride complex 3.3; the general reaction is depicted in Scheme 3.5.   77  Scheme 3.5: Synthesis of the [PhNPN*]Ta imido chloride complex 3.3.  This method for synthesizing an imido moiety is not limited to the Ta alkyne chloride complexes.  The Ta alkyne azide complexes 3.1 and 3.2 also reacted with PIPP azide to form the imido azide complex 3.4; the same product was formed via the reaction of 3.3 and NaN3 (Scheme 3.6).  The 1H and 13C{1H} NMR spectra are generally unremarkable, reflecting a Cs symmetric complex and closely resembling the data for 3.3.  The solid-state IR spectrum of 3.4 features a strong band at 2096 cm-1, which is in good agreement with the azide stretch in complexes 3.1, 3.2 and examples in the literature.  This data suggests that complex 3.4 features both an imide and an azide functional group (in addition to the two amide moieties present in the [PhNPN*] ancillary ligand).  Complex 3.4 shares the thermal stability of 3.1 and 3.2; both synthetic routes to 3.4 involved stirring the reaction mixtures at elevated temperature for extended periods of time.  In addition, heating toluene solutions of 3.4 at 110 °C for days led to no thermal decomposition or further reactivity, and suggests that the azide moiety is unreactive with regards to nitride formation. Ta NP NRR Cl2.2 (R = Et)2.3 (R = TMS) ArN3 Ta NP NCl3.3ArN- alkyne  78  Scheme 3.6: Two routes for the preparation of [PhNPN*]Ta(NAr)N3 (3.4).  The reactions of the Ta alkyne benzyl complexes (2.6 and 2.7) with PIPP azide were more complicated than the Ta alkyne chloride (2.2 and 2.3) and azide (3.1 and 3.2) systems, due to the fact that complex 2.6 exhibits a different ligand orientation (relative to the metal centre) than the other examples discussed above. As was addressed in Chapter 2, the alkyne unit in complex 2.6 is located in an apical coordination site in the solid state, essentially trans to the phosphorus atom of the [PhNPN*] ligand (and with the benzyl moiety located in an equatorial site). In contrast, the ligand orientation is reversed in complex 2.7 (and all of the other Ta alkyne 3.41 eq. ArN3Ta NP NRR NNN Ta NP NNNN ArNTa NP NCl3.3ArN 1 eq. NaN33.1 (R = Et)3.2 (R = TMS)  79 complexes discussed in this thesis); the solid-state structure shows the alkyne unit to be located in the equatorial plane, with the benzyl group in an apical site.  These structural differences are depicted in Figure 3.3.  Figure 3.3: The relative ligand orientations of various [PhNPN*]Ta alkyne complexes.  In the case of the Ta alkyne chloride complexes (2.2 and 2.3), the similar structure of the two congeners meant that reaction of either complex with PIPP azide led to the same product (vide supra, Scheme 3.5). This is not the case with the Ta alkyne benzyl complexes (2.6 and 2.7), and the structural differences led to markedly different products. Complex 2.7 – wherein the alkyne unit occupies an equatorial coordination site – reacted with PIPP azide to afford a product similar to complexes 3.3 and 3.4 (Scheme 3.7).  Treating a bright red toluene solution of 2.7 with one equivalent of PIPP azide at room temperature resulted in an orange suspension after several hours.  Upon workup, complex 3.5 was isolated as a bright orange powder.  As was the case with 3.3, complex 3.5 is only sparingly soluble in most organic solvents, though satisfactory NMR spectra were acquired from a toluene-d8 solution; unsurprisingly, the NMR data for 3.5 is similar to that of 3.3 and 3.4.  Overall, the 1H and Ta NP NEtEtBnTa NP NTMSTMS YTa NP NRR XComplexes 2.2 - 2.5, 2.7, 2.8, 3.1, 3.2(R = Et, TMS; X = Cl, H, N3; Y = Et, Bn):Alkyne unit in equitorial site Complex 2.6:Alkyne unit in apical site  80 13C{1H} NMR data are consistent with a Cs symmetric complex; the two benzylic methylene protons appear as a broad singlet at δ 2.06, and the methylene carbon of the benzyl moiety appears as a doublet at δ 77.3 (2JCP = 25 Hz), in the relevant spectra.  The 31P{1H} NMR spectrum features a sharp singlet at δ 26.5.   Scheme 3.7: Synthesis of the [PhNPN*]Ta imido benzyl complex, 3.5, from the [PhNPN*]Ta(BTA) benzyl complex, 2.7.  Orange crystals suitable for an X-ray crystallographic study were obtained by the slow evaporation of a concentrated toluene solution. An ORTEP representation of the solid-state structure of complex 3.5 is shown in Figure 3.4.  Ta NP NTMSTMS Bn Ta NP NBn3.5ArN1 eq. ArN3- alkyne2.7  81  Figure 3.4: ORTEP diagram of the solid-state molecular structure of 3.5 (ellipsoids at 50% probability).  All hydrogen atoms and the mesityl group at N01 (except for Cipso) have been omitted for clarity.    Table 3.2: Selected bond lengths (Å) and angles (°) for 3.5 Parameter (Å/°) Parameter (°) Ta01–N01 2.058(7) N01–Ta01–N02 128.3(3) Ta01–N02 2.083(8) N03–Ta01–N01 110.4(3) Ta01–P01 2.627(3) N03–Ta01–N02 111.1(3) Ta01–N03 1.796(8) N03–Ta01–C46 103.1(3) Ta01–C46 2.228(10) P01–Ta01–N03 95.8(3) Ta01–N03–C39 175.5(7) P01–Ta01–C46 161.1(3)  N03 N01 P01 Ta01 N02 C46 C47 C39   82 As was the case with 3.3, the solid-state structure of complex 3.5 features slightly distorted trigonal bipyramidal geometry at Ta; the apical sites are occupied by the [PhNPN*] phosphorus atom and the benzyl moiety, and the equatorial plane consists of the amido and imido nitrogen (N01 – N03) atoms. The nearly linear Ta01–N03–C39 bond angle (~176o), combined with the short Ta01–N03 distance (~1.79 Å), is consistent with a Ta-imide bond (cf. complex 3.3). The rest of the bond lengths and angles are unremarkable, and closely resemble those of the parent Ta alkyne benzyl complex (2.7). Additionally, complex 2.6 reacted cleanly with one equivalent of PIPP azide to yield a similar Ta imido benzyl complex as a dark orange-red powder (3.6, Scheme 3.8).  In contrast to 3.5, complex 3.6 is soluble in most organic solvents; in toluene-d8, the 31P{1H} NMR spectrum features a sharp singlet at δ 9.0.  The 1H NMR spectrum is indicative of Cs symmetry and is similar to that of 3.5, the sole major difference being the benzyl methylene protons, which give rise to a sharp doublet at δ 2.99 (3JHP = 7 Hz).  The 13C{1H} NMR spectrum is also quite similar to that of 3.5, save for the benzyl methylene carbon, which generates a doublet at δ 68.8 (2JCP = 7 Hz).  Scheme 3.8: Synthesis of the [PhNPN*]Ta imido benzyl complex, 3.6, from the [PhNPN*]Ta(3-hexyne) benzyl complex, 2.6. Ta NP NEtEtBn 3.62.6 1 eq. ArN3- alkyne [PhNPN*]Ta(NAr)Bn  83  The slight disparity between the solution-state NMR data for 3.5 and 3.6, as well as the differences in colour and solubility, are not unexpected considering the structural differences exhibited by complexes 2.6 and 2.7, the Ta alkyne precursors for these reactions.  In the case of 3.5, the crystallographically determined molecular structure shows that in the solid-state, the new imido functionality is located in the equatorial coordination site previously occupied by the alkyne unit in 2.7.  While it has not been possible to obtain a solid-state structure of 3.6 in order to definitively determine the relative orientation of the benzyl and imido moieties, analogies can be drawn based on trends in the NMR data of all four Ta benzyl complexes (Table 3.3), and the previously elucidated solid-state structures of complexes 2.6, 2.7 and 3.5.  Of particular interest is the coupling-constant data for the benzyl methylene carbon atoms: the two complexes whose solid-state structures feature benzyl groups in an apical coordination site (2.7 and 3.5) also exhibit strong 2JCP coupling.  This is likely due to the benzyl group being trans to the phosphorus atom of the [PhNPN*] ligand.  Table 3.3: Selected NMR data for the benzyl moiety in complexes 2.6, 2.7, 3.5 and 3.6 Complex 1H NMR data for [Ta]CH2Ph 13C{1H} data for [Ta]CH2Ph 2.6 δ 2.75, doublet, 3JHP = 7 Hz δ 75, doublet, 2JCP = 4 Hz 2.7 δ 3.22, singlet δ 89, doublet, 2JCP = 16 Hz 3.6 δ 3.02, doublet, 3JHP = 7 Hz δ 68, doublet, 2JCP = 7 Hz 3.5 δ 2.06, singlet δ 77, doublet, 2JCP = 25 Hz    84 In contrast, the 2JCP coupling is much weaker in complex 2.6, where the benzyl moiety has been shown to occupy an equatorial coordination site in the solid-state (and thus is cis to the phosphine ligand).  As complex 3.6 also exhibits weak 2JCP coupling it is presumed, by analogy, to have its ligands oriented in a fashion similar to that seen in complex 2.6: with the benzyl group in an equatorial site and the imide in an apical site.   Thus, both the solution-state and solid-state data seems to indicate that the orientation of the benzyl group about the Ta centre in the parent alkyne benzyl complex (2.6 or 2.7) is preserved in resulting imido benzyl product (3.5 or 3.6).  However, these relative ligand orientations are not immutable; toluene solutions of 3.5 cleanly convert to 3.6 over the course of several weeks at room temperature (or ~10 days at 105 °C).  The reactions discussed above are summarized in Scheme 3.9.   Mountford and coworkers recently published a computational study203 regarding the steric and electronic preferences of imido ligands in some related tungsten and tantalum diamido-pyridine complexes featuring trigonal bipyramidal geometry.  This study found that positioning the imido ligand in an apical coordination site allowed for optimal π-donation by both the imido and amido ligands.  In addition, the favourability of this orientation is influenced by the other ligands located in the equatorial plane (e.g. chloride (3.3) or benzyl (3.5 and 3.6) groups), which also affect the π-donor ability of the amide ligands.  In the case of complexes 3.5 and 3.6, additional experimental data (such as crystallographically determined Ta–Namide and Ta–Nimide bond lengths for 3.6) or computational modeling would be helpful in determining whether one isomer exhibits more favourable bonding Ta–imide and Ta–amide bonding interactions, and may help rationalize the structural rearrangement that is observed in this system.    85  Scheme 3.9: The synthesis of [PhNPN*]Ta imido benzyl complexes (3.5 and 3.6) from [PhNPN*]Ta alkyne benzyl complexes (2.6 and 2.7) and an organoazide.  A potential mechanism for the formation of 3.3–3.6 from their Ta alkyne precursors – modified from the mechanism for a similar reaction posited by Bergman189 – is shown in Scheme 3.10. Initially, the low-valent Ta complex is oxidized by an aryl azide to form a terminal arylazido Ta complex; complexes of this type, shown in square brackets in Scheme 3.10, are usually not isolable due to rapid loss of N2 (and were not identified in the course of this work).  Formation of the imido species proceeds via a four-centred transition state, similar to the well-Ta NP NTMSTMS BnTa NP NEtEtBn Ta NP NBn3.5Ta NP NBn NArN Ar3.6Δ2.72.6 1 eq. ArN3- alkyne1 eq. ArN3- alkyne  86 established Staudinger reaction.  The evolution of an equivalent of N2 and formation of a strong Ta–N multiple bond provides ample thermodynamic driving force for the reaction.     Scheme 3.10: Proposed mechanism for the formation of the Ta imide complexes 3.3 – 3.6 from their Ta alkyne precursors ([Ta] = [PhNPN*]TaX, X = Bn or Cl), adapted from Bergman and coworkers (ref 189).    The empirical observation that the newly formed arylimide moiety occupies the former coordination site of the alkyne ligand suggests that the formation of the Ta terminal arylazido intermediate (i.e. the first step in Scheme 3.10) is a bi-molecular process.  The uni-molecular alternative requires that after alkyne dissociation occurs, the position of the vacant coordination [Ta] RRTa(III) N NNAr[Ta] N Ar[Ta] Ta(V)ArN3- alkyne- N2N NN[Ta]ArN NNAr[Ta]  87 site relative to the other ligands is maintained prior to arylazide coordination (Scheme 3.11), which is improbable.     Scheme 3.11: Possible dissociative mechanisms for the reaction of an organoazide with the Ta alkyne complexes 2.2, 2.3, 2.7 (top) or 2.6 (bottom), where the relative position of the vacant coordination site is maintained.  Instead, if a uni-molecular mechanism were operative, the four coordinate intermediate that results from alkyne dissociation would most likely adopt tetrahedral geometry at Ta; in this scenario complexes 2.6 and 2.7 would generate the same four-coordinate intermediate (Scheme 3.12), which would not account for the different imide products (3.5 and 3.6) ultimately obtained.  Ta NP NXTa NP NBn EtEtRR Ta NP NXTa NP NBn Ta NP NXNNArN Ta NP NBn NNNArArN3- alkyne- alkyne ArN32.2, 2.3, 2.7(R = Et, TMS; X = Cl, Bn)2.6  88  Scheme 3.12: Dissociative mechanism for the reaction of an organoazide with complexes 2.6 or 2.7 that proceeds via a common 4-coordinate intermediate.  A more plausible mechanism involves coordination of an arylazide to the Ta(III) alkyne complex, generating a six-coordinate intermediate (Scheme 3.13).  Depending on the ligand arrangement in the initial Ta alkyne complex, an octahedral or trigonal prismatic structure can be envisioned that preserves the position of the benzyl or chloro group (labeled ‘X’) relative to the [PhNPN*] ligand set.  Subsequent displacement of alkyne would afford the Ta(V) arylazido complex (which in turn would go on to lose N2 and generate the Ta imide, as per Scheme 3.10). Ta NP NBnTa NP NBn EtEtTMSTMS Ta NP NBn ArN3- alkyne- alkyne [PhNPN*]Ta(NAr)Bn- N2 singleproduct2.72.6  89  Scheme 3.13: Proposed associative mechanisms for the reaction of an organoazide with the Ta alkyne complexes 2.2, 2.3, 2.7 (top) or 2.6 (bottom).   3.2.3 Reactions of Tantalum Benzyl Imides with Organoazides: Attempted Synthesis of Tetraazadiene and Triazenide Complexes  Along with being useful synthons for metal imides (Section 3.2.2), organoazides are also known to react with early transition metal complexes to generate a variety of nitrogen-rich metallacycles.204  One example of their use involves the insertion of an organoazide into a metal–carbon205-209 (or metal–hydride210) bond to generate a monoanionic triazenide moiety. In the case of the triazenido complex Cp*2Hf(H)(PhNNNH), the application of heat led to the loss of N2 and the formation of a metal amide;210 this process can be viewed as the insertion of a nitrene into an M–H bond (Scheme 3.14).   RR Ta NP NXNNArN Ta NP NBn NNArNTa NP NXTa NP NBn EtEtRR Ta NP NXNNArN Ta NP NBn NNNArEtEtArN3 - alkyne- alkyneArN32.2, 2.3, 2.7(R = Et, TMS; X = Cl, Bn)2.6  90  Scheme 3.14: Synthesis of a hafnium triazenide moiety, and the subsequent formation of a hafnium amide complex via thermal extrusion of N2 (adapted from ref. 210).  In addition, complexes that feature a metal imide have been known to react with organoazides to form tetraazadiene moieties: an N4 metallacycle generated via a [3+2] cycloaddition reaction209,211-215 (Scheme 3.15).  Scheme 3.15: Formation of a metal-tetraazadiene moiety, via the combination of an organoazide and a metal imide.  The Ta imide complexes discussed in the previous section – the Ta imido benzyl complexes 3.5 and 3.6 in particular, as they feature both Ta–carbon and Ta–imide bonds – provided a potential entry point for the synthesis of either of these nitrogen-rich metallacycles.  While treating the [PhNPN*]Ta imido chloride (3.3) or imido azide (3.4) complexes with an excess of PIPP azide led to no reaction (even when the mixture was heated for several days), Cp*2Hf(H)2 PhN3 NN NHPh [Hf] N HPh-N2[Hf] Δ[Hf] = Cp*2Hf(H)LnM NR NNNLnM NRArArN3  91 the Ta imido benzyl complexes (3.5 and 3.6) did exhibit further reactivity.  Complex 3.6 reacted with one equivalent of PIPP azide at 70 °C over the course of 2 days to ultimately afford a single new product, 3.7 (Scheme 3.16).    Scheme 3.16: The reaction of [PhNPN*]Ta(NAr)Bn with an organoazide, and the structures of three plausible products (I – III).  As outlined above, there are several plausible products that could result from the combination of 3.6 and an organoazide, depending on whether reactivity occurs at the imide or benzyl moiety.  Three possibilities are outlined in Scheme 3.16: reaction at the imide would result in a benzyl tetraazadiene complex (Structure I), whereas insertion of the organoazide into the Ta–benzyl bond would generate a Ta imido triazenide (Structure II), or the related Ta imido amide (Structure III, via thermal extrusion of N2 from Structure II). 1 eq. ArN3Ta NP NNNN NYXNNNN ArAr 3.6 3.7I II (X, Y = Ar, Bn)Ta NP NBn NAr Ta NP NNNYXTa NP NBn IIIAr Ar70oC, 48 h  92 Unfortunately, the 1H, 13C{1H} and 31P{1H} NMR data obtained for 3.7 (vide infra) is insufficient to distinguish between these possibilities. Unambiguous structural characterization was ultimately achieved via a single-crystal X-ray diffraction study, which revealed 3.7 to be a Ta imido triazenide complex, similar to structure II in Scheme 3.16.  The ORTEP representation of the solid-state structure of complex 3.7 is shown in Figure 3.5; both enantiomeric forms of 3.7 are present in the unit cell, though for clarity only one is depicted.     Figure 3.5: ORTEP diagram of the solid-state molecular structure of 3.7 (ellipsoids at 50% probability).  All hydrogen atoms and the mesityl group at N02 (except for Cipso) have been omitted for clarity.    Ta01 P01 N03 C39 N02 N01 N04 N06 N05   93 Table 3.4: Selected bond lengths (Å) and angles (°) for 3.7 Parameter (Å) Parameter (°) Ta01–N01 2.072(2) N01–Ta01–N02 102.06(9) Ta01–N02 2.075(2) N04–Ta01–N06 56.15(9) Ta01–P01 2.6384(11) C39–N03–Ta01 176.4(2) Ta01–N03 1.797(2) N03–Ta01–P01 177.84(8) Ta01–N04 2.235(2) N03–Ta01–N01 101.68(10) Ta01–N05 2.747(3) N03–Ta01–N02 106.06(10) Ta01–N06 2.216(3) N03–Ta01–N04 98.71(10) N04–N05 1.315(3) N03–Ta01–N06 102.67(10) N05–N06 1.304(3) N04–N05–N06 106.3(2) Ta01–N04–N05–N06 (dihedral) 4.42  In the solid-state, the geometry at Ta is that of a slightly distorted octahedron.  The axial coordination sites are occupied by the [PhNPN*] phosphorus atom and N03 from the imide functionality, and the equatorial coordination sites are occupied by 4 nitrogen atoms: the two amides from [PhNPN*] (N01 and N02), and two nitrogen atoms (N04 and N06) of the triazenide ligand.  The values for the Ta–imide (N03), Ta–amide (N01 and N02) and Ta–P01 bond lengths are similar to those seen in previous complexes and are generally unremarkable.  The N01–Ta01–N02 bond angle is much smaller (102.06(9)°) in 3.7 than in 3.3 or 3.5 (~130°), reflecting the more crowded coordination sphere around Ta.  The N03–Ta01–P01 (177.84(8)°) bond angle is essentially linear, and the angle between the N03–Ta01 axis and the four equatorial N atoms (N01, N02, N04, N06) are ~100°, which is in good agreement with the values expected (90°) for an octahedral coordination sphere at Ta.   94 The triazenide ligand is flat and essentially coplanar with the Ta centre (with a dihedral angle of 4.42°).  The equal (with experimental error) N04–N05 (1.315(3) Å) and N05–N06 (1.304(3) Å) bond lengths are intermediate between a single and double bond, and consistent with the monoanionic formal charge of the ligand being delocalized over the N04–N05–N06 fragment.  While the Ta–N05 (2.747(3) Å) contact is too long to be considered as a formal bond, the similar values for the Ta–N04 (2.235(2) Å) and Ta–N06 (2.216(3) Å) bonds suggest that the bonding interaction between Ta and the triazenide is distributed essentially evenly between these two donor atoms.  This bond length data is in agreement the majority of crystallographically characterized metal-triazenide complexes,206,207,209,216 with the notable exception of the only known Ta example, reported by Heyduk and coworkers in 2011,208 which instead features Ta–N bond lengths that differ by nearly 0.2 Å (in addition to unequal N–N bond lengths).  Heyduk interprets this data as being the result of the predominance of one Ta –imine/amide resonance structure (III, Scheme 3.17) over a more delocalized metal–triazenide bonding description (II, Scheme 3.17).    Scheme 3.17: Three possible resonance structures that describe the bonding interaction between Ta and a triazenide ligand (adapted from ref. 208).  Explanations for the differences in bond lengths observed in metal-triazenide complexes have invoked either steric crowding at the metal centre, or electronic effects of the ligands trans TaRN N NR' TaRN N NR' IIII TaRN N NR' IIR = aryl, R' = alkyl  95 to the triazenide moiety.217,218 A third hypothesis, which follows from Heyduk’s resonance structure formalism, is that the N donor atom with an aryl substituent is better able to stabilize a formal negative charge than the alkyl-N donor, and hence the alkyl imine/ aryl amide resonance structure III is favoured over resonance structure I.  In general, the bond between Ta and an (anionic) amide would be expected to be shorter than a Ta – (neutral) imine interaction, which is consistent with the empirically observed bond lengths in the Heyduk system.  However, in the case of 3.7 the similar Ta–N04 and Ta–N06 bond lengths suggest that, despite the presence of both aryl and alkyl substituents, the sterically bulky – and symmetrical – [PhNPN*] ligand set enforces a Ta-triazenido interaction best described by the delocalized resonance structure II in Scheme 3.18.  The solution-state NMR spectra of 3.7 are consistent with the solid-state structure discussed above.  In toluene-d8, the 1H NMR spectrum reflects C1 molecular symmetry, with resonances for the 8 inequivalent methyl groups and related aryl protons of the [PhNPN*] ligand.  The two distinct aryl isopropyl groups give rise to two sets of septets and doublets (δ 2.63 and 1.09, δ 2.61 and 1.04, 3JHH = 7 Hz in both cases) in the appropriate regions.  The methylene protons of the benzyl group are diastereotopic, and give rise to a pair of AB coupled doublets (δ 5.16 and 4.92, 2JHH = 15 Hz).  The 13C{1H} spectrum is consistent with the 1H NMR data and generally unremarkable; the 31P{1H} spectrum contains a sharp singlet at δ 14.0. In contrast to 3.6, heating 3.5 in the presence of an equivalent of PIPP azide gave rise to a mixture of products (Scheme 3.18), of which 3.7 comprises approximately 60% (by 31P{1H}-NMR spectroscopy).  Since 3.5 also thermally rearranges to 3.6, it is difficult to ascertain whether the presence of 3.7 in the observed mixture of products is derived from the reaction of PIPP azide with the latter complex, or whether the product (or products) of the reaction between   96 PIPP azide and both 3.5 and 3.6 converges to a common product (3.7) upon heating. Given that the reaction times for the formation of 3.7 from either 3.5 or 3.6 are similar, and the rate of formation of 3.6 from 3.5 is extremely slow (vide supra), the second hypothesis seems more plausible.  However, more detailed competition experiments between PIPP azide and complexes 3.5 and 3.6 are needed to fully address this question.   Scheme 3.18: The reaction of 3.5 and an organoazide, which affords 3.7 (~60%) and several unidentified products.  3.3 Conclusions In this chapter, various reactions that sought to exploit the low valent Ta(III) resonance form of [PhNPN*]Ta alkyne complexes (introduced in Chapter 2) were explored.  Attempts were made to synthesize a Ta nitride, informed by examples of the synthesis of late transition metal nitrides via the extrusion of N2 from low-valent metal azide complexes.  The terminal azide complexes [PhNPN*]Ta(3-hexyne)N3, 3.1, and [PhNPN*]Ta(BTA)N3, 3.2 were prepared via the salt metathesis reaction of NaN3 and the relevant [PhNPN*]Ta alkyne chloride.  Despite attempts to thermally or photochemically induce N2 extrusion from the azide functionality, these complexes proved to be unreactive with regards to nitride formation.   1 eq. ArN33.5 3.770oC, 48 h Ta NP NNNN NYX ArTa NP NBnArN  + other 31P-containing products(X, Y = Ar, Bn)  97 A series of [PhNPN*]Ta imide complexes were synthesized via the reaction of PIPP (para-isopropylphenyl) azide with the relevant [PhNPN*]Ta alkyne complexes.  Mechanistically, this reaction is proposed to occur through the oxidation of the Ta(III) metal centre by the organic azide; rapid loss of N2 affords the final Ta(V) imide product.  The [PhNPN*]Ta arylimido chloride (3.3), arylimido azide (3.4) and arylimido benzyl (3.5 and 3.6) complexes were prepared in this fashion.  Through the analysis of the solid-state crystallographic and solution-state NMR data, it was posited that the newly formed imide moiety in the final product occupies the former position of the coordinated alkyne ligand in the starting material.  This observation suggests a bimolecular, or associative, mechanism for imide formation. The Ta arylimido benzyl complexes 3.5 and 3.6 react with an equivalent of aryl azide to form the Ta arylimido triazenide complex 3.7, via insertion of the azide moiety into the Ta-benzyl bond.  While the reaction with 3.6 proceeds cleanly, the reaction with complex 3.5 leads to a mixture of products, of which 3.7 comprises the major component; this latter reaction is possibly complicated by thermal rearrangement of 3.5 to 3.6. The synthesis of Ta arylimido compounds from Ta alkyne precursors reported in this chapter lends credence to the availability of a low-valent Ta(III) resonance form envisioned for the latter complexes, given the correct reagents and/or reaction conditions.  Indeed, as the compounds reported in Chapter 2 and the reactions discussed in Chapters 4 and 5 are more readily understood by invoking a Ta(V) metal centre, these results suggest that the ‘Ta(V)-alkenediyl / Ta(III)-alkyne’ formalisms are merely convenient simplifications of a more nuanced metal-ligand interaction.    98 Chapter  4: Reactions of Tantalum Alkyne–Hydride Complexes with Small Molecules 4.1 Introduction Some of the most important applications of organometallic chemistry have been in the field of homogeneous catalysis.  Crucial to the design and synthesis of effective organometallic catalysts is an understanding of the interactions – such as the formation or cleavage of chemical bonds – between small molecule substrates and metal complexes.  Towards this end, the activation and chemical functionalization of small molecules has been a fruitful, and challenging, field of study for organometallic chemists.  In recent years, the Fryzuk group has reported on reactivity studies of the dinuclear Ta(IV) tetrahydride ([NPNSi]Ta)2(µ-H)4  (1.71) with substrates such as CO,219 CO2220 and most notably, N2.89,90  One of the pivotal results from work with the resultant N2 complex ([NPNSi]Ta)2(η1:η2-µ-N2)(µ-H)2 (1.72) is the ability to form new chemical bonds between the metal-coordinated dinitrogen moiety, and substrates like boranes,105,106 alanes,104 silanes,107,108 and various cumulenes.221   Similarly, the formation of C–C bonds continues to be an area of great utility and interest for synthetic chemists. Aside from the seminal advances in the realm of Pd-catalyzed cross coupling, complexes of group 4 and 5 metals are well known to catalyze the coupling of amines and unsaturated hydrocarbons to afford new C–N and C–C bonds, via hydroamination and hydroaminoalkylation, respectively.222-235  In addition, there are a variety of examples of Ta and Nb complexes serving as hydride or alkyl transfer agents for ketones,85,86,236-239 imines,237-242 olefins and alkynes.36,37,63,64,243,244  These systems also provide further examples of metal-mediated C–C bond formation, as the intramolecular coupling of iminoacyl or alkyne ligands   99 have been known to result in the formation of more complex organic fragments, such as 1,3-diazabutadiene245-247 (DAD) and 1-aza-1,3-butadiene85-87 (AD) functionalities.  The work presented in this chapter focuses on the reactivity of the two monohydride tantalum complexes, [PhNPN*]Ta(3-hexyne)H (2.4) and [PhNPN*]Ta(BTA)H (2.5), with several small molecules.  The reactions of these monohydride complexes with 2,6-dimethylphenyl isocyanide and phenylacetylene, and the subsequent five-membered tantallacyclic products that result from C–C bond formation between the coordinated alkyne unit and the putative iminoacyl or phenylvinyl moieties are discussed.  Additionally, the reactions of carbon dioxide with multiple equivalents of a Ta monohydride were explored, which resulted in the synthesis of new mononuclear Ta alkyne-formate and dinuclear Ta alkyne-methylene diolate complexes.  4.2 Results and Discussion 4.2.1 Reactions of Complexes 2.4 & 2.5 with a Terminal Alkyne The addition of one equivalent of phenylacetylene to a benzene solution of [PhNPN*]Ta(3-hexyne)H (2.4) at room temperature results in an immediate brown to dark-red colour change (Scheme 4.1). After ca. 5 minutes the 31P{1H} NMR spectrum consists only of a new singlet at δ 26.4; the starting Ta hydride complex is rapidly converted to complex 4.1 via 1,2 – insertion of phenylacetylene into the Ta–H bond to generate a Ta alkyne-phenylvinyl complex.   100  Scheme 4.1: Synthesis of the [PhNPN*]Ta alkyne phenylvinyl complexes 4.1 and 4.3, via the reaction of 2.4 or 2.5 with phenylacetylene.  In C6D6, the 1H NMR spectrum of 4.1 features all the ligand resonances indicative of a Cs symmetric complex.  Two doublets at δ 8.62 and δ 6.08 correspond to the newly formed phenylvinylic protons; the large coupling constant shared by these protons (JHH = 18 Hz) is indicative of trans- rather than gem-substitution, hence the proposed stereochemistry of the vinyl moiety in Scheme 4.1.244  This assignment is buttressed by the results of a 1H-13C HSQC experiment, which indicates that the phenylvinylic protons are located on separate carbon atoms. Synthesis of the isotopologue of 4.1 with PhCCD (and the subsequent absence of the 1H NMR resonance at δ 8.62) allowed for the unambiguous identification of the phenylvinylic carbons: Cα = δ 204.3, Cβ = δ 142.1.  The methyl and methylene groups of the 3-hexyne unit appear as one quartet (δ 2.99) and one triplet (δ 0.99) integrating to 4 and 6 protons, respectively (rather than the two pairs of triplets and quartets seen in complex 2.4); as the static geometry depicted in Scheme 4.1 would result in diastereotopic methylene protons, the room temperature NMR data Ta NP NRR H Ta NP NCαCβPhHβ HαC C HPh RR2.4: R = Et2.5: R = TMS 4.1: R = Et4.3: R = TMS  101 implies fast exchange of the two halves of the alkyne ligand, via rotation about the Ta-alkyne bond. The rest of the 13C{1H} NMR spectrum is consistent with the 1H NMR data: the [PhNPN*] ligand resonances reflect overall Cs symmetry, and there is only one signal each for the methyl and methylene carbon atoms of the 3-hexyne ligand.   Complex 4.1 exhibited moderate thermal instability, and underwent an intramolecular rearrangement at room temperature over the course of approximately three days to yield the five-membered tantallacycle 4.2 (Scheme 4.2); this complex is discussed in detail in section 4.2.2. Complex 2.5 also reacts with phenylacetylene to generate 4.3, a Ta alkyne-phenylvinyl complex similar to 4.1 (Scheme 4.1). The 31P{1H} NMR spectrum of 4.3 in C6D6 features a singlet at δ 23.9. The 1H NMR spectrum contains two doublets at δ 8.92 and 5.83 (JHH = 18 Hz) that correspond well with the data for the phenylvinylic protons in 4.1.  The rest of the 1H NMR spectrum of 4.3 agrees with the proposed Cs symmetric structure, including a single resonance for the two TMS groups, which implies that the BTA moiety is either arranged perpendicular to the σv plane of symmetry, or undergoes fast exchange via rotation about the Ta-alkyne bond (as with 4.1). Further characterization of 4.3 was hampered by its thermal instability and rapid (~6 h at room temperature) structural rearrangement to complex 4.4 (Scheme 4.2); this latter compound is discussed in detail in section 4.2.2.  4.2.2 Structural Rearrangement of Alkyne-phenylvinyl Complexes 4.1 & 4.3    As mentioned in section 4.2.1, a benzene solution of 4.1 cleanly converted to a second complex, 4.2, over the course of approximately 3 days at room temperature (Scheme 4.2).     102  Scheme 4.2: Rearrangement of the [PhNPN*]Ta alkyne phenylvinyl complexes 4.1 and 4.3 to the tantallacyclic products 4.2 and 4.4.  The 1H and 13C{1H} NMR spectra of 4.2 are extremely complicated, and full structural elucidation was only possible after obtaining a solid-state molecular structure via a single crystal X-ray diffraction study; ORTEP representations for complex 4.2 are shown in Figures 4.1 and 4.2. Molecules representing both enantiomeric forms of 4.2 are present in the unit cell, although for clarity only one is depicted.    Ta R RHPhHTaCα CβPh HβHα RR[PhNPN*] (and isomer)4.1: R = Et4.3: R = TMS 4.2: R = Et4.4: R = TMSC6D6, RT4.1: 72 h4.3: 8 h [PhNPN*]  103  Figure 4.1: ORTEP depiction of the solid-state molecular structure of 4.2 (ellipsoids at 50% probability).  All hydrogen atoms (except H1 and H2) have been omitted for clarity; H1 and H2 were located from the difference map and refined isotropically.   Ta N02 N01 P01   104  Figure 4.2: Truncated ORTEP depiction of the core of 4.2. Table 4.1: Selected bond lengths (Å) and angles (°) for 4.2. Parameter (Å) Parameter (°) Ta–N01 2.052(3) N01–Ta–N02 109.33(10) Ta–N02 2.135(3) N01–Ta–P01 73.15(8) Ta–P01 2.5820(9) N02–Ta–P01 75.79(8) Ta–C1 2.034(3) C1–C2–C3 119.9(3) Ta–C2 2.377(3) C2–C3–C4 123.4(3) Ta–C3 2.282(4) C3–C4–Cipso 119.2(3) Ta–C4 2.225(4) C1–C2–C3–C4 (dihedral) 23.67 C1–C2 1.417(5)  C2–C3 1.426(5)  C3–C4 1.448(5)  Ta N02 N01 P01 Cipso C2 C3  C4  H1 C1 H2    105 Complex 4.2 features a 5-membered tantallacycle that arises from the coupling of one of the coordinated 3-hexyne carbons to the Cα of the phenylvinyl moiety in 4.1.  The exact nature of the bonding between the newly formed four-carbon chain and the metal centre is complicated; while the Ta–C1 (2.034(3) Å) and Ta–C4 (2.225(4) Å) distances are similar to typical tantalum-carbon double (1.95 – 2.05 Å119,122,125,248,249) and single (2.18 – 2.27 Å90,119,122,125,248,249) bond lengths, C2 and C3 are also located close enough to the metal centre to also be considered as involved in some form of bonding interaction.  Furthermore, there is evidence of bond delocalization around the four-carbon chain, with the C1–C2, C2–C3 and C3–C4 distances all equal (within experimental error). Other workers have synthesized group 5 metal complexes with similar structures, and therein the C4R5 fragment was described as an η3–butadienyl63,64,250 or η4–butadienyl36,37,251 moiety, depending on the length of the various metal-carbon bonds.  The solid-state structural data obtained for 4.2 is in good agreement with an η4-butadienyl metal complex, as all four Ta–C bonds are reasonably short (<2.4 Å); two resonance forms that describe this bonding motif are shown in Figure 4.3.     106  Figure 4.3: Resonance structures for the tantalum–η4-butadienyl moiety in 4.2 and 4.4.  The Ta(V) alkyl-alkylidene structure depicts a Ta(V)/ trianionic C4R5 pair that reflects the solid-state bond distance data most closely, particularly the Ta–C1 double and Ta–C4 single bonds.  This model is also supported by the 13C{1H} NMR data (vide infra) with the low-field resonance for C1 (δ 245) being typical for an alkylidene carbon. Alternatively, the Ta(III) butadienyl structure follows logically from the proposed mechanism for this transformation (Scheme 4.3): the (intramolecular) reductive elimination, from Ta(V), of the phenylvinyl group and the coordinated alkyne (formally alkenediyl) moiety resulting in the formation of a new C–C bond.  Accordingly, this structure invokes a formally monoanionic butadienyl moiety bound to tantalum through a single bond (at C1), and π-type interactions through the two double-bonds of the C4R5 chain. C3 C2C1TaC4 C3 C2C1TaC4 TaNN RRHPhTa(III) butadienylTa(V) alkyl-alkylidene HTaNN RRHPhH P P  107  Scheme 4.3: Possible mechanism for the formation of 4.2 and 4.4.   The C1 symmetric nature of complex 4.2 and its concomitant lack of a σv plane is reflected in the 1H and 13C{1H} NMR spectra, which contain twice the number of [PhNPN*] ligand resonances than are present in the spectra of 4.1.  The five-membered tantallacycle, formed from the 3-hexyne and phenylvinyl moieties, leads to a complicated 1H NMR spectrum between δ 0 – 5 (Figure 4.4); the assignments for these resonances are organized in Table 4.2, based on the results of a battery of NMR spectroscopic experiments (1H-1H COSY, 13C-APT, 1H-13C HSQC and 1H-13C HMBC).    Figure 4.4: Truncated (δ 0 – 5 region) 1H NMR spectrum (300 MHz, 298 K) of 4.2 in C6D6.  Some residual pentane (*) is also present. TaC3C4PhH H C2C1R R[PhNPN*] Ta(III) + 'butadienyl1-'reductive eliminationTa(V) +'alkenediyl2- / phenylvinyl1-' C3 C2C1TaC4 RRHHPh[PhNPN*]* * H1 H5 H5  H6 H6 H2   108   Table 4.2: 1H and 13C{1H} NMR spectral assignments for 4.2, and a schematic representation of the core of 4.2; this depiction is only meant to indicate connectivity, and does not accurately reflect the bonding in the tantallacycle. Proton(s)  Chemical Shift Coupling Observed Location (13C NMR) Me1 δ 0.48 H5a and H5b CMe1(δ 18.27) Me2 δ 1.34 H6a and H6b CMe2(δ 16.88) H5a/b δ 2.49, 2.89 H5b or H5a, and Me1 C5 (δ 30.36) H6a/b δ 2.86, 3.48 H6b or H6a, and Me2 C6 (δ 31.51) H2 (D2) δ 4.33 H1 C3 (δ 93.62) H1 δ 0.61 H2 and 31P C4 (δ 93.07)   Several points of interest arise from these NMR spectroscopic experiments.  In each case the two methylene protons at C5 (H5a and H5b) and C6 (H6a and H6b) are magnetically inequivalent; thus, the signals for the methyl protons at Me1 and Me2 are apparent triplets arising from coupling to two diastereotopic protons rather than ‘true’ triplets.  The methylene protons H5a/b and H6a/b appear as 4 multiplets that arise from coupling to a methyl group (Me1 or Me2) and a diastereotopic proton (e.g. H5a couples to Me1 and H5b); the signals for two such protons appear nearly coincident at δ ~2.9, although the HSQC experiment reveals that these C2C1TaC4C3 C5C6H2PhH1 Me2Me1H5aH5bH6bH6a  109 protons are located on separate carbon atoms (C5 and C6).  The lack of evidence for coupling between the protons at C5 and C6 is not surprising, as it would entail a 5JHH interaction, which would likely be very weak in magnitude.  In addition, coupling is not observed between H2 and H6a/b, as this would also involve a similarly small 4JHH coupling constant.  The signal for H2 (δ 4.33) appears as a doublet (JHH = 8 Hz) due to coupling with H1 (δ 0.61); H1 also exhibits weak coupling (JHP = 3 Hz) to the phosphorus-31 atom of [PhNPN*]. In the 13C{1H} spectrum, resonances for the methyl (CMe1 and CMe2) and methylene (C5 and C6) carbons appear as distinct singlets in their expected regions.  A 1H-13C HSQC experiment was employed to identify C3 (δ 93.62) and C4 (δ 93.07) through their correlation with H1 or H2.  Through the use of a 1H-13C HMBC experiment, it was possible to unambiguously assign C1 (δ 245.0) and C2 (δ 133.7) based on their correlation with the Me1 and C5, or Me2 and C6 protons, respectively; as mentioned above, the extremely downfield chemical shift for C1 lends credence to the Ta(V) alkyl-alkylidene structural motif. Despite its significant structural rearrangement, the 31P{1H} NMR spectrum of 4.2 features a singlet at δ 28.5, just slightly downfield of the resonance seen for 4.1.    Complex 4.3 undergoes a structural rearrangement analogous to 4.1, to yield a similar tantallacyclic complex, 4.4 (Scheme 4.2).  The NMR spectra of 4.4 are similar to 4.2, although not as complicated.  In C6D6, the 31P{1H} NMR spectrum of 4.4 features a singlet at δ 31.9, slightly downfield of 4.3 (δ 23.9).  As with 4.2, the 1H NMR spectrum of 4.4 features resonances for 8 inequivalent aryl methyl groups, as well as all the aryl ligand resonances expected for a C1 symmetric complex.  Selected 1H and 13C{1H} NMR data for the tantallacycle moiety are collected in Table 4.3, and are in good agreement with the data for the similar complex 4.2; this   110 information was derived from several NMR experiments (1H-1H COSY, 13C-APT, 1H-13C HSQC and 1H-13C HMBC).    Table 4.3: Selected 1H and 13C NMR spectra assignments for 4.4, and a schematic representation of the core of 4.4; this depiction is only meant to indicate connectivity, and does not accurately reflect the bonding in the tantallacycle. Proton(s)  Chemical Shift Coupling Observed Location (13C NMR) TMS2 δ 0.15 none CTMS2 (δ 0.36) C2 (δ 122.8) TMS1 δ 0.39 none CTMS1 (δ 4.35) C1 (δ 251.9) H1 δ 1.51 H2 C3 (δ 98.3) H2 δ 4.95 H1 and 31P C4 (δ 104.8)   4.2.3 Kinetic Study of the Rearrangement of Complex 4.1 Although the initial reaction between 2.4 and phenylacetylene occurs too rapidly to monitor by NMR spectroscopy, the rearrangement of 4.1 to 4.2 proceeds slowly enough to permit a study of its kinetic parameters.  The 1H NMR spectra of four samples of a 45 mM solution of 4.1 (freshly prepared from equimolar amounts of 2.4 and phenylacetylene) in C6D6 C2C1TaC4C3 TMS1TMS2H2PhH1  111 were acquired periodically in a NMR spectrometer heated to 318 K, 328 K, 333 K (900 s or 1200 s periods) or 303 K (8-12 h periods) until at least 90% (> 3 t½) of the starting material had been consumed.  Based on the linearity of a ln[4.1] versus time plot, the rearrangement was determined to be first order in 4.1; details regarding this determination, as well as a discussion of NMR spectrum processing, integral choice, error analysis and a sample plot of ln[4.1] versus time (at 318 K) used in the determination of kobs can be found in Appendix B.   The Eyring plot of ln(kobs/T) versus 1/T is shown in Figure 4.5; from the line of best fit for this plot the activation parameters ΔH‡ = 92.7 ± 1.2 kJ/mol and ΔS‡ = -36.47 ± 0.82 J/(mol)(K) were extracted.  This negative ΔS‡ value is indicative of an ordered transition state, while the observed first-order kinetics are consistent with an intramolecular rearrangement.   Figure 4.5: Eyring plot of ln(kobs/T) versus 1/T for the rearrangement of 4.1 to 4.2. y = -11154x + 19.374 R² = 0.99969 -18 -17 -16 -15 -14 -13 -12 0.0029 0.003 0.0031 0.0032 0.0033 0.0034 ln(kobs/T) 1/T (K-1)   112 The five-membered tantallacyclic products (4.2 and 4.4) discussed in the previous sections (4.2.2 and 4.2.3) arose via the formation of a new C–C bond between a metal-coordinated alkyne and a vinyl ligand, with the latter functional group generated by the insertion of phenylacetylene into the Ta–H bond of complexes 2.4 or 2.5.  The next section extends this chemistry to an isocyanide substrate, with the intention of exploring the synthesis of nitrogen-containing tantallacycles (similar to 4.2 and 4.4) via the potential coupling of iminoformyl moieties with a metal-coordinated alkyne.   4.2.4 Reactions of Complexes 2.4 & 2.5 with an Aryl Isocyanide The addition of one equivalent of 2,6-dimethylphenyl isocyanide to a toluene solution of 2.4 or 2.5 led to the immediate (< 5 mins.) consumption of the starting hydride complex and the quantitative formation of a new species, 4.5 or 4.6, indicated by new singlet resonances at δ 16.1 and δ 28.8 in their respective 31P{1H} NMR spectra.  Upon workup, the 1H NMR spectrum of 4.5 or 4.6 in C6D6 features aryl [PhNPN*] ligand resonances suggestive of a C1 symmetric complex, in addition to a new singlet (4.5 = δ 4.85; 4.6 = δ 5.24) for a proton that a 1H-13C HSQC experiment indicates to be carbon-bound (4.5 = δ 93; 4.6 = δ 107).  As well, there are ten distinct singlets attributable to aryl methyl groups (eight from the [PhNPN*] ligand, and two from the isocyanide moiety) in the expected regions of the 1H NMR spectra for both compounds (δ 2.3 − 1.6).  An obvious structure consistent with this data is that of an iminoformyl complex generated from insertion of the isocyanide into the Ta-H bond (square brackets, Scheme 4.4).    113  Scheme 4.4: Probable synthetic route for the tantallacyclic complexes 4.5 and 4.6, via the reaction of 2.4 or 2.5 with an aryl isocyanide.  Single crystals of 4.6 were obtained from a concentrated hexanes solution cooled to -35 °C, and the ORTEP representations (Figures 4.6 and 4.7) show a more complicated structure: the putative iminoformyl has coupled with the coordinated alkyne to generate an 1-aza-1,3-butadiene (AD) moiety.  Molecules representing both enantiomeric forms of 4.6 are present in the unit cell, although for clarity only one is depicted.  Ta NP NRR H N CAr Ta NP NCN HArRRTa NP NNRR H Ar2.4: R = Et2.5: R = TMS 4.5: R = Et4.6: R = TMS  114  Figure 4.6: ORTEP depiction of the solid-state molecular structure of 4.6 (ellipsoids at 40% probability).  All hydrogen atoms (except H3) have been omitted for clarity; H3 was located from the difference map and refined isotropically.    Ta N02 N01 P01   115  Figure 4.7: Truncated ORTEP depiction of the core of 4.6.  Table 4.4: Selected bond lengths (Å) and angles (°) for 4.6 Parameter (Å) Parameter (°) Ta–N01 2.077(2) N01–Ta –N02 124.46(9) Ta–N02 2.103(3) P01–Ta –N03 168.97(9) Ta–P01 2.595(1) Ta–N03–C3 87.2(2) Ta–N03  2.022(2) Ta–C1–C2 89.3(2) Ta–C1 1.976(3) N03–Ta–C1 89.59(10) Ta–C2 2.445(3) N03–C3–C2 121.9(3) Ta–C3 2.418(3) H3–C3–C2 118.4(20) C1–C2 1.463(4) C1–C2–C3 117.2(2) C2–C3 1.401(4) N03–C3–C2–C1 (dihedral) 1.4 C3–N03 1.412(4)  Ta N02 N01 P01 N03 Cipso Si02 Si01 C1 C2  C3 H3    116 There are several important structural features to be gleaned from this data.  Most notable is the near planarity (dihedral angle ≅1o) of the N03–C3–C2–C1 fragment, a defining feature of the AD (and the related 1,4-diazabutadiene, or DAD) structural motif. In addition, AD (X = NR, Y = CR) and DAD (X, Y = NR) moieties are known to coordinate to metal centres in both the ‘prone’ and ‘supine’ orientations,252-254 as depicted in Figure 4.8; the solid-state structure of 4.6 is indicative of the latter orientation.  Figure 4.8: Possible coordination modes of 1-aza-1,3-butadiene (X = NR, Y = CR) and 1,4-diazabutadiene (X = Y = NR) moieties to a metal centre (‘M’).  Similar to the case of 4.3, the bonding in the metal–AD fragment of 4.6 is best understood as a combination of the two resonance structures shown in Figure 4.9. The solid-state structural data for 4.6 agrees well with the Ta(V) alkylidene-amide form; the short C2–C3 (1.401(4) Å) and long C1–C2 (1.463(4) Å) distances are suggestive of double and single bonds, respectively, and the Ta–N03 (2.022(2) Å) distance is similar to the Ta-amido bond lengths of the [PhNPN*] ligand (cf., Ta–N01: 2.077(2), Ta–N02: 2.103(3) Å).  In addition, the short Ta–C1 (1.976(3) Å) distance is consistent with a metal-alkylidene type interaction; the 13C{1H} NMR data also supports this assertion, with the low-field resonance for C1 (δ 242, vide infra) being typical for an alkylidene carbon.  MYX MYX'prone'                            'supine'  117  Figure 4.9: Resonance structures for the tantalum–AD moiety in 4.5 & 4.6.  The 13C{1H} NMR spectra of 4.5 and 4.6 are typical of C1 symmetric complexes featuring a 1-aza-1,3-butadiene moiety.  The 13C{1H} NMR spectrum of 4.5 features two pairs of singlets (δ 29.0 and 28.9, δ 17.9 and 15.1) corresponding to the inequivalent methylene and methyl carbons of the former 3-hexyne moiety.  In the case of 4.6, there are two distinct singlets at δ 5.22 and δ 1.14 that correspond to inequivalent trimethylsilyl groups.  Resonances for the 1-aza-1,3-butadiene fragment’s carbon atoms C1 (4.5: δ 232, 4.6: δ 242),  C2 (4.5: δ 110 , 4.6: δ 111) and C3 (4.5: δ 94 , 4.6: δ 107) are in excellent agreement with other similar complexes.87   The synthesis of a 1-aza-1,3-butadiene moiety via the reaction of an organic isocyanide and an early transition metal complex is not without precedent.  There are numerous examples in the literature of the related η2-imine177,237-241 (Figure 4.10 i) or η2-iminoacyl85-87,236-239,245-247 TaNRR H ArTa N NNRR H Ar Ta(III) alkenyl-imineTa(V) alkylidene-amide Ta NArC3C2C1Ta NArC3C2C1 NP NP  118 (Figure 4.10 ii) functionalities being generated by isocyanide insertion into Ta–C bonds.  In addition, there are several examples of iminoacyl units coupling with coordinated alkynes,85-87 and indeed other iminoacyls,245-247,255-259 to form 1-aza-1,3-butadiene (AD) or 1,4-diazabutadiene (DAD) ligands, respectively (Figure 4.10 iii). However, examples of stable, well-characterized iminoformyl complexes generated from isocyanide insertion into early metal-hydride bonds are rare (Figure 4.10 ii).244  Instead, most reports feature products that are either incompletely characterized (usually due to rapid decomposition),260-262 feature a µ-η2:η2 RN=CH unit bridging between two metal centres,263-265 or a strongly coordinated phosphine adduct at the iminoformyl C atom.266,267 In this context, it appears that the highly electrophilic iminoformyl moiety strongly favours further reactivity, which frustrates isolation and characterization of a mononuclear, adduct-free example; consequently the synthesis of 4.5 and 4.6 via the rapid coupling of an (presumed) iminoformyl functionality and a coordinated alkyne is unsurprising.   119  Figure 4.10: Possible synthetic routes to η2–imine, η2–iminoacyl/iminoformyl, and 1,3-diazabutadiene/ 1-aza-1,3-butadiene moieties.  LnM XY LnM CN XRYLnM CN XRLnM CN XRZ CR'' ZLnM N CCR XR''N CR + η2 - imineX, Y = alkylη2 - iminoacylX = alkylη2 - iminoformylX = H1,3-diazabutadieneZ = N1-aza-1,3-butadieneZ = CR'R' (iii)(ii)(i)Y  120 4.2.5 Reactions of Complex 2.4 with Carbon Dioxide A significant body of work has focused on the use of carbon dioxide as a potential synthon for higher value C1-molecules such as methanol and formic acid.268-271  The work of Floriani272,273 and others274 have highlighted the role that homogeneous metal-based hydrides like Schwartz’s reagent, Cp2Zr(H)Cl, can play in the reduction of CO2; a key first step in these systems is the formation of metalloformate complexes through the insertion of CO2 into a metal-hydride bond (Scheme 4.5).  Subsequent reduction with a second equivalent of metal hydride ultimately produces formaldehyde and µ–oxo–bis(chlorozirconocene); it was long hypothesized that this transformation proceeded via the generation of a bridged bimetallic methylene diolate species (square brackets, Scheme 4.5).  Scheme 4.5: The reduction of CO2 to formaldehyde by an early metal hydride complex (e.g. Schwartz’s reagent, [Zr] = Cp2ZrCl).  Adapted from ref. 274.  While some spectroscopic evidence existed,275,276 the isolation and full structural characterization of a methylene diolate complex derived directly from CO2 remained elusive for decades.  It was not until 2004, when a report by Hou and workers277 detailing the reaction of CO2 with a tetrametallic yttrium tetrahydrido cluster provided the first crystallographically O OCH2[Zr] [Zr][Zr] HCO2 [Zr] O CO H [Zr] HO[Zr] [Zr]CO HH +  121 characterized example of such a complex.  This work was followed by a report from Cummins and coworkers in 201041 who synthesized and fully characterized a family of bimetallic methylene diolate complexes generated via the reduction of CO2 with a variety of sterically bulky Ta, Nb and Ti monohydrides (one example is shown in Scheme 4.6).   Scheme 4.6: Synthesis of a dinuclear Ta methylene diolate complex by Cummins and coworkers (adapted from ref. 41).  A third example of a bimetallic methylene diolate complex comes from previous work in the Fryzuk group.220  Treating the dinuclear Ta(IV) tetrahydride ([NPNSi]Ta)2(µ-H)4 (1.71) with an equivalent of CO2 results in the formation of ([NPNSi]Ta)2(µ-OCH2O)4(µ-H)2 (Scheme 4.7).  Scheme 4.7: Synthesis of the dinuclear Ta methylene diolate complex ([NPNSi]Ta)2(µ-OCH2O)4(µ-H)2  (adapted from ref. 220)  TaH NR2NR2NR Ta OR2N R2NRN CH2 O Ta NR2R2N NRCO2 (1 atm.)2 C6H6TaNN PhMe2SiMe2SiPhPh P HH PhTa PNNPh SiMe2SiMe2PhO OH2CTa TaHHHH [NPNSi][NPNSi] CO2 (1 eq.)toluene  122 Mindful of these examples, we sought to investigate the reduction of CO2 with Ta monohydride complexes such as 2.4 with the goal of preparing similar methylene diolate moieties.  In addition, the presence of an activated alkyne unit offered the potential of generating new metallacyclic structures through reactivity with the reduced CO2 unit (similar to the chemistry discussed previously in this chapter).   At room temperature, exposure of complex 2.4 to 1 atm of CO2 results in a complicated mixture of products.  However, when stoichiometry and reaction conditions were carefully regulated, 2.4 reacted cleanly with 1 equivalent of CO2 to give the formate complex 4.7 (Scheme 4.8).    Scheme 4.8: Synthesis of the [PhNPN*]Ta hexyne formate complex 4.7.  The 1H and 13C NMR data for 4.7 reflects a solution-state structure with Cs symmetry: along with the expected aryl resonances, there are signals for 4 distinct aryl methyl groups (each integrating to 6 protons). The diagnostic hydride resonance (δ 21) from the 1H NMR spectrum of 2.4 is absent, and a new singlet appears at δ 8.27 that corresponds to the formate proton.  A 1H-13C HSQC experiment correlates this proton to a carbon atom at δ 167, typical of early metal formates.41,278,279 These assignments are supported by NMR data from the isotopologue of 4.7 Ta NP NEtEt H2.4 -35oC to rttoluene 4.7Ta NP NEtEt H(O)CO1 eq. CO2  123 (synthesized from 2.4 and 13CO2), whereupon the signal at δ 8.27 appears as a doublet strongly coupled (JHC = 209 Hz) to a 13C atom (I = ½).  The 3-hexyne unit in 4.7 appears to be fluxional at room temperature; the methyl and methylene protons appear as two broad singlets, whereas the carbon signals for these groups are unobservable in the 13C{1H} spectrum; the 1H-13C HMBC experiment locates these quaternary carbons at 204 ppm.  This closely matches the NMR data for complex 2.2, which featured a 3-hexyne unit that is fluxional in the solution state at room temperature, and lies in the σv plane of molecular symmetry in the solid state. Formate ligands can bind to metal centres in several ways; for early transition metals, the CO2H– moiety usually adopts either the η1-O or η2-O, O coordination mode, as depicted in Figure 4.11.278-280    Figure 4.11: Two possible formate binding modes for 4.7 – monodentate (η1-O-O(O)CH, left) and bidentate (η2-O, O-O(O)CH, right).  Ta NP NEtEt O C HO Ta NP NEtEt O CH O  124 Attempts at growing single crystals of 4.7 suitable for an X-ray diffraction study have thus far been unsuccessful.  In the absence of a crystallographically derived solid-state structure, it is difficult to definitively assign which coordination mode the formate ligand adopts in 4.7.  In some metal-formate complexes, vibrational IR spectroscopy has been used as a means for distinguishing between the two binding modes, based on the separation between the νsym(CO2–) and νasym(CO2–) bands (referred to in the literature as Δ).281  Empirically, smaller values (<100 cm-1) of Δ correspond to bidentate coordination, and larger values (~200 cm-1) to monodentate coordination.278-280  The FT-IR spectrum of 4.7 features a strong band at 1670 cm-1, similar to the νasym(CO2–) stretch observed in other early metal-formate complexes.  However, the region of the spectrum below 1650 cm-1 is crowded, which makes identification of the νasym(CO2–) band exceedingly difficult; consequently, an assignment of the formate binding mode in 4.7 could not be made on the basis of the FT-IR data.  Nevertheless, a useful comparison can be made between the tantalum-formate complex 4.7 and tantalum-amidate complexes, which feature a related monoanionic ligand with both an oxygen and nitrogen donor atom (Figure 4.12).  With rare exception, crystallographically characterized tantalum-amidate complexes feature ligands that adopt the bidentate coordination mode in the solid-state.282-284  By analogy, it seems more likely that the formate ligand in 4.7 would also adopt the bidentate coordination mode.  Figure 4.12: Bidentate ligand coordination in generalized Ta formate (left) and Ta amidate (right) complexes [Ta]O CH O [Ta]O CR NR'  125  When complex 2.4 is instead treated with ½ equivalent of CO2, the result is the methylene diolate complex 4.8.  Complex 4.8 may also be generated via the stoichiometric reaction of the Ta hydride, 2.4, and the aforementioned formate derivative, 4.7 (Scheme 4.9).  Scheme 4.9: Two routes for the synthesis of the dinuclear Ta methylene diolate complex 4.8.  Compared to complex 4.7, the 1H NMR spectrum for 4.8 features twice the expected number of 3-hexyne and [PhNPN*] ligand resonances.  The resonance attributed to the formate proton in 4.7 is absent, replaced by a new singlet at δ 5.47; a 1H-13C HSQC experiment correlates this new resonance to a 13C NMR signal at δ 102, which the 13C-APT experiment suggests is a secondary (–CH2–) carbon.  Additionally, the NMR data for the 13C-isotopologue of 4.8 (synthesized from 2.4 and 13CO2) shows a doublet at δ 5.47 in 1H NMR spectrum (JHC = 163 Hz).  This is in good agreement with the NMR data for the bridging –OCH2O– moiety in 2.4 1 eq. 4.7rt, 1 htoluene-35oC to rttoluene1 eq. CO2 {[PhNPN*]Ta(hexyne)}2CO2H24.82 eq.  [PhNPN*]Ta(hexyne)H2.41 eq.  [PhNPN*]Ta(hexyne)H  126 ([NPNSi]Ta)2(µ-OCH2O)4(µ-H)2,220 as well as the Ta and Nb methylene diolate complexes studied by Cummins and coworkers.41 Thus, if this singlet at δ 5.47 is integrated as 2 protons, the relative integrations for the rest of the signals in the 1H NMR spectrum reflect a dinuclear Ta complex with two inequivalent (and Cs symmetric) [PhNPN*] and 3-hexyne ligands.  This proposed formulation is further supported by the 31P{1H} NMR spectrum, which features two sharp singlets at δ 28.6 and δ 20.5, indicative of two inequivalent [PhNPN*] units. Attempts at growing single crystals suitable for an X-ray diffraction study have thus far been unsuccessful, but a plausible solution-state structure for 4.8 that is consistent with the NMR data is shown in Figure 4.13.  As the oxygen atoms of the methylene diolate moiety can bind to Ta in two distinct positions relative to the phosphorous atom of the [PhNPN*] ligand (i.e. cis to P in the left half of the molecule, trans to P in the right half), it is possible to envision a structure which would result in a complex with magnetically inequivalent halves, while maintaining overall Cs symmetry.   Figure 4.13: Proposed structure of the dinuclear Ta methylene diolate complex 4.8.  TaN PNEt EtTaNPN Et EtO OH2C  127 Somewhat disappointingly, none of the structural data available for 4.7 and 4.8 indicates that the insertion of CO2 into the Ta–H bond is followed by any further reactivity with the coordinated alkyne ligand.  This suggests that the formation of strong Ta–O bonds in 4.7 and 4.8 may result in complexes that are less prone to further intramolecular reactivity.   4.3 Conclusions The work described in this chapter explored the reactivity of the Ta alkyne monohydride complexes [PhNPN*]Ta(3-hexyne)H (2.4) and [PhNPN*]Ta(BTA)H (2.5) with phenylacetylene, 2,6-dimethylphenyl isocyanide and carbon dioxide.  These reactants each feature different kinds of C-X (X = C, N, O) multiple bonds, and their varied reactivity has allowed us to probe the insertion behavior of these Ta–H moieties. In addition, these studies have allowed us to gain insight into the reactivity patterns of the coordinated alkyne unit with the Ta – X bonds that result from these insertion processes. Both 2.4 and 2.5 react rapidly with one equivalent of phenylacetylene to generate the Ta alkyne phenylvinyl complexes 4.1 and 4.3, respectively, which were characterized via solution-state NMR spectroscopy; at ambient temperature, complexes 4.1 and 4.3 undergo an intramolecular structural rearrangement to cleanly generate complexes 4.2 and 4.4.  These rearrangement products feature a 5-membered tantallacycle that arises from the coupling of the coordinated alkyne unit and the phenylvinyl moiety of the parent compound.  Complexes 4.2 and 4.4 give rise to complicated 1H and 13C{1H} NMR spectra, and full structural characterization was only achieved with the aid of a crystallographically derived solid-state structure of the 3-hexyne analogue, 4.2.  The bonding in these newly formed tantallacycles is similar to an η4-butadienyl metal complex, and is best described as a combination of two resonance forms: a   128 Ta(V) alkyl-alkylidene and a Ta(III) butadiene.  The latter resonance form follows from the proposed mechanism for the formation of 4.2 and 4.4, which invokes reductive elimination from a formally Ta(V) metal centre.  A kinetic study of the conversion of 4.1 to 4.2 suggests that the transformation proceeds via a highly ordered transition state (ΔS‡ = -36.47 ± 0.82 J/(mol)(K)).  In addition, the conversion is first-order with respect to 4.1, which is consistent with an intramolecular process. Complexes 2.4 and 2.5 undergo a similar reaction with one equivalent of 2,6-dimethylphenyl isocyanide to produce complexes 4.5 and 4.6. These compounds feature a 5-membered tantallacycle, composed of a 1-aza-1,3-butadiene unit generated from the coupling of the coordinated alkyne and a proposed (though not observed) iminoformyl moiety, itself derived from isocyanide insertion into a Ta–H bond.  The structure of 4.6, confirmed by an X-ray diffraction study, is similar to that of 4.2, in that it can be described as a combination of two resonance forms: a Ta(V) alkylidene-amide and a Ta(III) alkenyl-imine. The reactions of carbon dioxide with one and two equivalents of the Ta monohydride 2.4 were explored, which resulted in the synthesis of the Ta hexyne-formate (4.7) and dinuclear Ta bis(3-hexyne)-methylene diolate complexes (4.8), respectively.  In contrast to the phenylacetylene and aryl isocyanide systems, the coordinated 3-hexyne ligand does not appear to participate in further reactivity with the formate or methylene diolate moieties, likely a reflection of the stability of the newly formed Ta–O bonds.  These observations contrast with the insertion processes where the formation of a Ta–C bond (4.1–4.6) results in complexes that do undergo further transformations; in these cases, it is probable that the intermediacy of an insertion product with a weaker Ta–X bond is essential to the subsequent reactivity.   129 Chapter  5: The Hydrogenolysis Chemistry of Tantalum Alkyne Complexes, and the Synthesis of a Dinuclear Tantalum Tetrahydride  5.1 Introduction The hydrogenolysis of early transition metal-carbon σ-bonds is a well-established method for installing hydride ligands.132,285-291 Transformations of this type are one example of a broader class of reactions known as ‘σ-bond metathesis.’ Scheme 5.1 depicts an example of such a process involving the hydrogenolysis of a metal-alkyl bond.  In addition to H2, a wide variety of substrates including alkanes, alkenes, alkynes, silanes, phosphines and others are known to interact with metal–H or metal–C bonds in a similar fashion to generate new metal–E (E = C (sp, sp2, sp3), Si, P, H, etc.) functionalities.291 The general mechanism for such a reaction is simple: in the case of Scheme 5.1, the metal alkyl fragment forms a four-centred transition state with H2, which leads to the formation of the metal hydride and an equivalent of free alkane.    Scheme 5.1: The hydrogenolysis of a metal–alkyl bond to generate a metal hydride functionality.  As an example of σ-bond metathesis, this reaction proceeds via a concerted 4-centred transition state.  LnM R LnM RH H LnM H-δ+ δ−δ− δ+H2 R H  130 There are several important points that follow from this mechanism.  Perhaps the most salient is the fact that the formal oxidation state of the metal remains unchanged throughout the reaction.  This is essential for complexes of high valent, early transition metals, as the oxidative addition of a substrate (such as H2) is generally not a viable pathway for these electron deficient (usually d0) metal centres.   In addition, there is compelling experimental evidence which suggests that a key factor in determining the rate of reactivity of a particular substrate for σ-bond metathesis is the degree of s-character in the orbitals that constitute the 4-membered transition state.  Seminal investigations by Bercaw and coworkers288 revealed that the rates of reaction of a series of scandium alkyl complexes with various hydrocarbons (Scheme 5.2) followed the trend: sp > sp2 > sp3.    Scheme 5.2: The relative rates of σ-bond metathesis of scandium alkyl complexes ([Sc] = Cp*2Sc) with various hydrocarbons: R’ = sp carbon > R’ = sp2 carbon > R’ = sp3 carbon.  These results were interpreted as being due to substrates with carbon atoms with more s-character allowing for better orbital overlap, and consequently a more stabilized (and lower energy) transition state, relative to carbon atoms with less s-character, and thus poorer orbital overlap.291  In this context, the potential hydrogenolysis chemistry of the [PhNPN*]Ta alkyne complexes introduced in Chapter 2 is of interest.  These complexes feature metal–C (formally [Sc] R' R H+[Sc] R R' H+  131 sp2 hybridized) bonds between the Ta centre and the ‘alkyne’ (formally alkenediyl) ligand that could potentially be transformed into hydride moieties (Scheme 5.3).  Scheme 5.3: The possible synthesis of a [PhNPN*] Ta hydride complex via the hydrogenolysis of the corresponding Ta alkyne complex.  Previous work in the Fryzuk group has demonstrated that the hydrogenolysis of amidophosphine tantalum alkyl complexes can be an effective synthetic methodology for the formation new tantalum hydrides.  As Scheme 5.4 illustrates, the dinuclear tantalum tetrahydrides ([P2N2]Ta)2(µ-H)4 (5.2)138 and ([NPNSi]Ta)2(µ-H)4 (1.71)90 were formed via the hydrogenolysis of [P2N2]TaMe3 (5.1) or [NPNSi]TaMe3 (1.71), respectively.  H2Ta NP NRR X Ta NP NHH X?  132  Scheme 5.4: Previous examples from the Fryzuk group of the synthesis of the dinuclear Ta tetrahydride complexes via the hydrogenolysis of the corresponding Ta alkyl complexes. Adapted from refs. 90 and 138.  Remarkably, complex 1.71 reacts with N2 at ambient pressure to form the dinitrogen complex ([NPNSi]Ta)2(η1:η2-µ-N2)(µ-H)2 (1.72), as shown in Scheme 5.5.90 As discussed in Chapter 1, the formation of a dinitrogen complex under mild conditions and without the use of a harsh external reducing agent is attractive, particularly with regards to the potential functionalization of the coordinated N2 unit.  The Ta(IV)–Ta(IV) bond, along with the loss of an TaNN P PhPhSiSiSiSi PMe2Me2Me2Me2 MeMeMe TaNN P PhPhSiSiSiSi PMe2Me2Me2Me2 HH Ta NNPPhPh SiSiSiSiPHH Me2Me2Me2Me2TaNN PhMe2SiMe2SiPhPh MeMeP Me PhTa TaNN PNNPh PhMe2SiMe2Si SiMe2SiMe2PhPh PhHHP HHH2- 6 CH42 1.70 1.71H2- 6 CH42 5.1 5.2  133 equivalent of H2, is hypothesized to provide 1.71 with a ‘built-in’ source of electrons for the reduction of N2.90    Scheme 5.5: Synthesis of the Ta dinitrogen complex 1.72 from the dinuclear tetrahydride 1.71.   However, subsequent investigations with a variety of electrophilic substrates (e.g. boranes,105,106 alanes104 and silanes107,108) revealed that 1.72 lacked the necessary chemical stability to facilitate further reactivity at the coordinated N2 unit, as it was prone to engage in various ligand-based side-reactions that ultimately led to the decomposition of the [NPNSi] moiety (Section 1.2).  Nevertheless, dinuclear tantalum tetrahydrides similar to 1.71 remain an attractive synthetic target for the activation and functionalization of dinitrogen. Consequently, the work presented in this chapter is focused on the synthesis of a dinuclear Ta tetrahydride featuring the more chemically robust [PhNPN*] ligand set.   Towards this end, the hydrogenolysis chemistry of a variety of [PhNPN*]Ta alkyne complexes (2.4 – 2.8) was explored. Concurrent with this work, Dr. Dominik Nied, a post-doctoral fellow working in the Fryzuk group, studied the hydrogenolysis chemistry of the Ta trimethyl complex [PhNPN*]TaMe3 (5.3).  These two separate lines of investigation revealed that PhTa TaNN PNNPh PhMe2SiMe2Si SiMe2SiMe2PhPh PhHHP HH Ta TaHHNNN2- H2 [NPNSi][NPNSi] 1.721.71  134 the hydrogenolysis of the Ta alkyne complexes (2.4 – 2.8), and the Ta trimethyl complex (5.3) both result in the formation of the Ta tetrahydride complex ([PhNPN*]Ta)2(µ-H)4  (5.4). Therefore, a discussion of the synthesis and structure (Section 5.2.1) and hydrogenolysis chemistry (Section 5.2.2) of [PhNPN*]TaMe3 (5.3) is also included in this chapter; all of the experimental work involving complex 5.3 reported in Sections 5.2.1 and 5.2.2 was conducted by Dr. Nied.  However, all other experimental work reported in this chapter was completed by the author of this dissertation (K. Parker).  In addition, the analysis and discussion of all results in all sections of this chapter was written by the author (K. Parker).  5.2 Results and Discussion 5.2.1 The Synthesis and Structure of [PhNPN*]TaMe3 As was mentioned in Chapter 2, attempts at preparing [PhNPN*]TaMe3 from TaMe3Cl2 and [PhNPN*]Li2(dioxane) were unsuccessful, and instead yielded an intractable mixture of the desired Ta trimethyl complex and other unidentified phosphorus-containing compounds (Section 2.2.2).  However, subsequent investigations by Dr. Dominik Nied focused on the reactivity of various Ta trialkyl diamidophosphine complexes; in the course of this work, [PhNPN*]TaMe3 (5.3) was prepared from TaMe3Cl2 and the potassium salt [PhNPN*]K2(THF)0.5 (2.1, first discussed in Chapter 2), as shown in Scheme 5.6.    135  Scheme 5.6: Synthesis of the [PhNPN*]Ta trimethyl complex 5.3. A C6D6 solution of 5.3 generates a 1H NMR spectrum that contains [PhNPN*] ligand resonances indicative of a Cs symmetric complex.  The three Ta methyl groups give rise to a single broad resonance at δ 1.33, indicative of fluxional behavior at room temperature; the exact nature of the fluxional process that exchanges the three methyl groups was not investigated further.  However, a possible explanation might involve the dissociation of the phosphine donor of the [PhNPN*] ligand, which would generate a five-coordinate intermediate that could undergo a ligand rearrangement process (such as Berry pseudorotation). The 13C{1H} NMR data are also consistent with Cs symmetry; similar to what was observed in the 1H NMR data, the three carbons of the Ta methyl moieties generate a single resonance at δ 73.  The 31P{1H} NMR spectrum features a sharp singlet at δ 42, which is typical of the other [PhNPN*]Ta complexes discussed in this thesis.      Single crystals of 5.3 suitable for an X-ray diffraction study were obtained by Dr. Dominik Nied from a saturated toluene solution cooled to -35 °C.  Figure 5.1 shows an ORTEP representation of the solid-state molecular structure of [PhNPN*]TaMe3.  In contrast to the Cs symmetry that is observed in the solution-state NMR spectra, complex 5.3 adopts a structure with C1 molecular symmetry in the solid-state.  The [PhNPN*] ligand coordinates facially to the Ta NP NMe MeMe 5.3TaMe3Cl22.1      + Et2O, -78oC- 2 KCl  136 Ta centre, and the geometry at the metal is distorted trigonal prismatic, with one of the trigonal faces consisting of C39, N02 and P01, and the second trigonal face made up of C40, C41 and N01.  Figure 5.1: ORTEP diagram of the solid-state molecular structure of 5.3 (ellipsoids at 50% probability).  All hydrogen atoms and the mesityl group at N01 (except for Cipso) have been omitted for clarity.  Table 5.1: Selected bond lengths (Å) and angles (°) for 5.3 Parameter (Å) Parameter (°) Ta01–N01 2.055(4) N01–Ta01–N02 121.59(10) Ta01–N02 2.137(4) P01–Ta01–C39 74.82(10) Ta01–P01 2.6034(14) C39–Ta01–N02 88.17(12) Ta01–C39 2.221(3) P01–Ta01–N02 73.49(8) Ta01–C40 2.210(3) C40–Ta01–C41 79.12(14) Ta01–C41 2.192(3) C41–Ta01–N01 99.45(12)   N01–Ta01–C40 99.54(12)  C40 C39 C41 P01 N02 N01 Ta01   137  The bond lengths between the [PhNPN*] ligand and the metal centre (Ta01–N01, Ta01–N02 and Ta–P01) are similar to those found in complexes that have been discussed previously, and are generally unremarkable.  In addition, the three Ta–Me bond lengths (Ta01–C39, Ta01–C40 and Ta–C41) are essentially equal (within experimental error), and agree well with the Ta–Me bond lengths found in [P2N2]TaMe3 (5.1)119 and [NPNSi]TaMe3 (1.70).30    5.2.2 The Synthesis and Structure of ([PhNPN*]Ta)2(µ-H)4  (5.4) Cognizant of the precedent set with [P2N2]TaMe3 and [NPNSi]TaMe3 (vide supra, Scheme 5.4), the hydrogenolysis chemistry of [PhNPN*]TaMe3 was of great interest. Thus, it was gratifying, though not unexpected, to note that complex 5.3 reacts with H2 (4 atm.) over the course of approximately 24 h to generate the dinuclear Ta tetrahydride complex 5.4 as a dark red-brown powder (Scheme 5.7).  (This experiment was performed by Dr. Dominik Nied.)   Scheme 5.7: Synthesis of the dinuclear [PhNPN*]Ta tetrahydride complex 5.4 from complex 5.3; performed by Dr. Dominik Nied.  Ta NP NMe MeMe 5.3 C6D6, 24 h- 6 CH4H2, 4 atm. ([PhNPN*]Ta)2(µ-3H)45.42  138 In addition, the hydrogenolysis of the Ta alkyne alkyl complexes (2.6 – 2.8) also afforded the Ta tetrahydride 5.4 (Scheme 5.8).  (These investigations were conducted by K. Parker.)  The synthesis of ([PhNPN*]Ta)2(µ-H)4  (5.4) from the Ta alkyne complexes was significantly slower than with 5.3; with [PhNPN*]Ta(3-hexyne)Bn (2.6), full conversion required approximately 14 days, while the BTA versions (2.7 and 2.8) were slightly faster, requiring approximately 10 days (under 4 atm. H2).  Subsequent investigations revealed that the hydrogenolysis of the Ta alkyne alkyl complexes initially led to the formation of isolable alkene hydride intermediates (5.5 and 5.6, Scheme 5.8); these latter complexes will be addressed in detail in Section 5.2.3.  Scheme 5.8: Synthesis of complex 5.4 from the Ta alkyne alkyl complexes 2.6 – 2.8, via the intermediacy of complexes 5.6 and 5.5; performed by K. Parker.  {[PhNPN*]Ta}2(µ-3H)45.4- 1,2-bis(TMS)ethaneH2[PhNPN*]Ta(BTA)Bn(2.7)or[PhNPN*]Ta(BTA)Et(2.8)[PhNPN*]Ta(3-hexyne)Bn (2.6) - HBn (2.7) or HEt (2.8) - n-hexaneH2Ta NP NTMSH H5.5HTMSTa NP NnBuH H5.6HHH2H2- HBn  139 Small brown crystals of 5.4 suitable for an X-ray diffraction study were obtained (by Dr. Dominik Nied via the hydrogenolysis of 5.3).  Figure 5.2 shows an ORTEP representation of the solid-state molecular structure of 5.4.  The complex adopts a C2 symmetric structure, with the two [PhNPN*] ligands rotated nearly perpendicular (~86°) relative to one another to reduce interactions between the sterically bulky mesityl amide arms.   Figure 5.2: ORTEP diagram of the solid-state molecular structure of 5.4 (ellipsoids at 30% probability).  All hydrogen atoms and the mesityl groups at N01 and N02 (except for Cipso) have been omitted for clarity.  Half of the molecule is generated by the symmetry operation:  2 - x, y, ½ - z.  The bridging hydride moieties could not be located in the difference map.   Ta01 Ta01’ N01' N01 N02’ N02 P01 P01’   140 Table 5.2: Selected bond lengths (Å) and angles (°) for 5.4 Parameter (Å) Parameter (°) Ta01–N01 2.081(9) N01–Ta01–N02 117.0(3) Ta01–N02 2.108(10) N01–Ta01–P01 76.7(2) Ta01–P01 2.574(3) N02–Ta01–P01 75.3(3) Ta01–Ta01’ 2.6244(12) P01–Ta01–Ta01’–P01’ (dihedral) 86.39  Unfortunately, the data obtained from the X-ray crystallographic study was of insufficient quality to allow for the unambiguous identification of the bridging hydrogen atoms.  However, the assignment of 5.4 as a dinuclear Ta(IV)–Ta(IV) complex bridged by four hydrides is justified, based on solution-state NMR data (vide infra) and by comparison to similar crystallographically characterized compounds. The two Ta atoms in 5.4 are separated by 2.6244(12) Å, which is slightly longer than the Ta–Ta distances found in similar dinuclear tetrahydride-bridged M(IV) complexes of Ta292,293 and Nb294 (2.51 – 2.57 Å) reported by other workers, but in good agreement with complexes 5.2 (2.6165(5) Å138) and 1.71 (2.565(1) Å295); it is probable that the slight Ta–Ta bond elongation observed in 5.4 is due to steric interactions between the bulky mesityl amide groups of the two [PhNPN*] ligands. The NMR spectroscopic data suggests that 5.4 also adopts a diamagnetic, dinuclear structure in solution, with two [PhNPN*]Ta(IV) units bridged by four hydrides; in C6D6, the 31P{1H} NMR spectrum contains only one singlet at δ 32.3.   The room temperature (293 K, C6D6) 1H NMR spectrum of 5.4 features broad resonances that suggest that the two [PhNPN*]Ta units may rotate relative to one another along the axis of the Ta–Ta bond. However, in toluene-d8 at 263 K, the 1H NMR spectrum features sharp resonances that are indicative of a C2 symmetric   141 structure similar to what is depicted in Figure 5.2, where the two [PhNPN*]Ta moieties are rotated 90° relative to one another.  While the two [PhNPN*]Ta units that constitute 5.4 are equivalent to each other due to the C2 rotational axis of symmetry, the two halves of each individual [PhNPN*]Ta unit are inequivalent, resulting in resonances for 8 distinct aryl methyl groups, and the concomitant aromatic protons of the ligand backbone.  In contrast, the 1H NMR spectrum at 363 K is indicative of a more symmetric structure, where the two [PhNPN*] ligands are either eclipsed, or oriented anti with respect to one another (i.e. C2v or C2h, as shown in Scheme 5.9).  The resulting spectrum features resonances for only 4 aryl methyl groups and the associated aromatic protons of the [PhNPN*] backbone.     142  Scheme 5.9: Relative orientations of the [PhNPN*] ligands in complex 5.4, as reflected by the low (263 K) and high (363 K) temperature 1H NMR spectra.  The only non-[PhNPN*] ligand resonance found in the 1H NMR spectrum of 5.4 appears at δ 14.7, and integrates to 2 hydrides per [PhNPN*]Ta unit. This hydride resonance appears as a well-resolved triplet coupled (3JHP = 3 Hz) to two equivalent phosphorus-31 nuclei (from two [PhNPN*] ligands) over the temperature range studied (263 K to 363 K).   TaP NN TaP NNTa PNNTaP NN([PhNPN*]Ta)2(µ-H)4(5.4)C2vC2363 K263 Kor TaPN NTaP NN C2h  143 As mentioned in Section 5.1, an obvious motivation for these investigations into the hydrogenolysis chemistry of [PhNPN*]Ta complexes was the possibility of generating a dinuclear Ta(IV) tetrahydride complex analogous to ([NPNSi]Ta)2(µ-H)4 (1.71), which could subsequently activate molecular nitrogen.  Unfortunately, the synthesis of the analogue of ([NPNSi]Ta)2(η1:η2-µ-N2)(µ-H)2 (1.72), proved not to be possible.  Tetrahydride 5.4 shows no signs of reactivity with N2 under a variety of conditions (1 or 4 atm N2, temperatures up to 100 °C).  In this regard, complex 5.4 is similar to ([P2N2]Ta)2(µ-H)4 (5.2), which also does not react with N2.138  There does not appear to be a simple or obvious explanation as to why complex 1.71, or similar dinuclear Nb(IV) tetrahydride complexes reported by Kawaguchi and coworkers,163,294 react spontaneously with N2, and why 5.2, 5.4, or other Group 5 M(IV) tetrahydrides292,293 reported in the literature do not.  Indeed, subsequent work in the Fryzuk group has demonstrated how seemingly small changes to the steric and electronic properties of the [NPNSi] ligand had a dramatic, and unpredictable, impact on reactivity.  As shown in Scheme 5.10, variants of the Ta trimethyl complex 1.70 where the amide arms of the [NPNSi] ligand featured R groups other than phenyl (C6H5) or p-tolyl (4-MeC6H4) either do not react with H2, or lead to an intractable mixture of products.296  However, changing the phosphine substituent of [NPNSi] from a phenyl to a cyclohexyl group still allowed for the synthesis of a dinuclear Ta tetrahydride (Cy-1.71) similar to 1.71, which also formed the related Ta dinitrogen dihydride complex (Cy-1.72) upon exposure to N2 (albeit more slowly than 1.72).297  Investigations into the reactivity of Cy-1.72 with various electrophiles are ongoing.298   144  Scheme 5.10: Examples of the reactivity of Ta complexes featuring variations of the [NPNSi] ligand set.  In this context, the inertness of complex 5.4 towards N2 further demonstrates that, of the possible modifications that can be made to the [NPNSi] motif, only a small subset of these amidophosphine ligands will facilitate the synthesis of complexes similar to the Ta dinitrogen dihydride complex 1.72.  PhTa TaNN PNNCy CyMe2SiMe2Si SiMe2SiMe2PhPh PhHHP HH Ta TaHHNNN2- H2 [CyNPNSi][CyNPNSi]TaNN PhMe2SiMe2SiRR MeMeP Me RTa TaNN PNNPh PhMe2SiMe2Si SiMe2SiMe2RR RHHP HHH2- 6 CH4X2 R = tBu, 2,4,6-Me3C6H2, 2-MeSC6H4, 3-CF3C6H4, 2-MeC6H4Cy-1.71 Cy-1.72  145 5.2.3 The Synthesis of Tantalum Alkene Hydrides from Tantalum Alkyne Alkyl Complexes  As discussed in Section 5.2.2, the hydrogenolysis of the [PhNPN*]Ta alkyne alkyl complexes 2.6 – 2.8 ultimately affords the Ta tetrahydride, 5.4 (Scheme 5.8).  Although the complete conversion to 5.4 requires between 10 and 14 days (with 4 atm H2), complexes 2.6 – 2.8 do react relatively rapidly with H2 to form Ta alkene hydride intermediates. For example, exposing a benzene solution of [PhNPN*]Ta(BTA)Bn (2.7) to H2 (1 atm) over the course of approximately 36 h resulted in the quantitative formation of complex 5.5.  Bright red crystals of 5.5 suitable for an X-ray diffraction study were obtained from a concentrated toluene/pentane solution cooled to -35 °C; an ORTEP representation of solid-state molecular structure of 5.5 is shown in Figure 5.3.  Figure 5.3: ORTEP diagram of the solid-state molecular structure of 5.5 (ellipsoids at 50% probability).  All hydrogen atoms (except for H39, H43 and H100) and the mesityl group at N02 (except for Cipso) have been omitted for clarity; H39, H43 and H100 were located from the difference map and refined isotropically.   Si02 Si01 H39 H43 C39 C43 P01 N02 N01 H100 Ta01   146  Table 5.3: Selected bond lengths (Å) and angles (°) for 5.5 Parameter (Å) Parameter (°) Ta01–N01 2.050(2) N01–Ta01–N02 124.29(10) Ta01–N02 2.023(2) P01–Ta01–H99 151.17(95) Ta01–P01 2.685(1) N01–Ta01–P01 74.75(7) Ta01–H100 1.72(3) N02–Ta01–P01 72.26(7) Ta01–C39 2.188(3) Si01–C43–H43 113(2) Ta01–C43 2.154(3) Si02–C39–H39 109.80(16) C39–C43 1.498(4) Si01–C43–C39–Si02 (dihedral) 111.54  Complex 5.5 resulted from hydrogenolysis of both the alkyne and alkyl moieties of 2.7 (Scheme 5.11), and thus can be viewed as a Ta alkene hydride complex; hydrogenolysis of the ethyl analogue, 2.8, also afforded complex 5.3, albeit at a slightly slower rate (~3 days).  Scheme 5.11: Synthesis of the [PhNPN*]Ta alkene hydride complex 5.5.  Just as the structures of the Ta complexes introduced in Chapter 2 (2.2 – 2.9) were discussed as being intermediate between the Ta(V)–‘alkenediyl’ and Ta(III)–alkyne resonance forms, it is Ta NP NTMSTMS R2.7 (R = Bn)2.8 (R = Et) Ta NP NTMSH H5.5HTMSC6H6H2, 1 atm.- HR  147 also possible to view 5.5 as a combination of the Ta(V)–‘alkanediyl’ and Ta(III)–alkene forms (Scheme 5.12).   Scheme 5.12: Two possible resonance structures for [PhNPN*]Ta alkene hydride complexes.  The solid-state structural details of 5.5 suggest that the Ta(V) formalism is the better description.  The C39–C43 distance is quite long (1.498(4) Å), placing it on the upper limit of the range of C–C bond lengths found in most other structurally characterized Ta and Nb olefin complexes (1.43 – 1.49 Å299-304).  In addition, the Ta01–C39 (2.188(3) Å) and Ta01–C43 (2.154(3) Å) bonds are short compared to the same literature examples cited above (2.18 – 2.30 Å299-304).  Consequently, it is more appropriate to refer to the (Me3Si)CH–CH(SiMe3) moiety as a formally dianionic ‘alkanediyl’ ligand, bound to a Ta(V) centre by two Ta–C single bonds.  However, just as the complexes introduced in Chapter 2 (2.2 – 2.9) are referred to as Ta alkyne complexes for the sake of simplicity, by analogy 5.5 will be referred to as a Ta alkene hydride complex.  The solution-state NMR spectroscopic data for 5.5 are in good agreement with solid-state structure shown above.  A C6D6 solution of 5.5 generates a 1H NMR spectrum that is indicative of C1 molecular symmetry, with resonances for the eight inequivalent aryl methyl groups and related aryl protons of the [PhNPN*] ligand backbone.  The hydride ligand gives rise to a doublet TaH [PhNPN*]RH HRTa(V) +'alkanediyl' ligand TaHTa(III) +[PhNPN*]alkene ligandRH HR  148 at δ 24.3, strongly coupled to phosphorus-31 (JHP = 44 Hz); both the extremely downfield chemical shift, and the large JHP coupling constant are similar to the Ta alkyne hydride complexes 2.4 (δ 21.6, JHP = 35 Hz), and 2.5 (δ 20.6, JHP = 35 Hz).  The ‘alkenic’ protons of the (Me3Si)CH–CH(SiMe3) fragment give rise to a pair of AB coupled doublets (δ -1.06 and -1.11, JHH = 22 Hz); this large coupling constant is consistent with the trans-stereochemistry observed in the solid-state structure.  A 1H–13C HSQC experiment correlates these protons with carbon atoms at δ 57.9 and 75.0, respectively; these 13C NMR resonances are slightly more downfield than the data for most Ta and Nb-olefin complexes, but still within the empirically observed range (~ δ 30 – 80).53,299-302,304-308 The trans-stereochemistry of the bis(trimethylsilyl)ethylene ligand in complex 5.5 was surprising. Typically, the hydrogenolysis of alkynes by an organometallic complex, such as Wilkinson’s catalyst (Scheme 5.13),309 proceeds via migratory insertion of a metal hydride, and then reductive elimination of the resulting vinyl moiety and the remaining hydride ligand; such a mechanism necessarily leads to a cis-alkene.309-312 The trans-alkene observed in 5.5 suggests that a different mechanism may be operative for the hydrogenolysis of the alkyne ligand in 2.7.    149  Scheme 5.13: Simplified mechanism for the hydrogenolysis of alkynes by Wilkinson’s catalyst (P = PPh3): i) oxidative addition of H2; ii) alkyne coordination; iii) migratory insertion; iv) reductive elimination; v) dissociation of cis-alkene.  Although examples of the generation of trans-alkene moieties via the insertion of an alkyne into a metal-hydride bond are known (vide supra, 1.47 and 1.48 in Section 1.1.3), they are rare, and the mechanism for their formation is not well understood.63,66-68  In an effort to gain some insights into the mechanism for the conversion of 2.7 to 5.5, the progress of the reaction was monitored at several points by 1H and 31P{1H} NMR spectroscopy.  These data suggest that the reaction proceeds via the formation of several Ta hydride-containing intermediates, evinced by the appearance of new resonances in the downfield region (δ >15) of the 1H NMR spectrum; H2 RhH ClHRhP ClP PPRhHH PP Cl RR RRRhH ClPPRR HRR HH RhP ClP i) ii)iii)iv) v)HRR H RhP3Cl- P  150 attempts at isolating these intermediates proved unsuccessful, and consequently the structure and composition of these proposed Ta hydride complexes are presently unknown.  Notably absent from this NMR data is any evidence for the formation of [PhNPN*]Ta(BTA)H (2.5).  Although subsequent investigations revealed that 2.5 does indeed react with H2 to generate 5.5 (vide infra, Section 5.2.5), the rate of this reaction (several days, 4 atm H2) was significantly slower than the route starting from 2.7 (36 h, 1 atm H2).  This observation rules out the possibility that complex 2.5 is a short-lived (and unobserved) intermediate in the latter reaction (Scheme 5.14).  Scheme 5.14: Synthesis of the Ta alkene hydride complex 5.5 from complex 2.7, which does not proceed via the intermediacy of the Ta alkyne hydride complex 2.5.  A possible mechanism for the synthesis of 5.5 from 2.7 that is consistent with these observations is shown in Scheme 5.15.  This postulate requires that hydrogenolysis first occurs at Ta NP NTMSTMS Bn2.7 Ta NP NTMSTMS H2.5Ta NP NTMSH H5.5HTMSX 4 atm H2> 10 days1 atm H2H236 h  151 one of the Ta–Calkyne bonds, rather than at the Ta–Cbenzyl bond; assuming that this step proceeds via a σ-bond metathesis reaction, this is not an unreasonable assertion.  The Ta–Cbenzyl bond features a carbon atom that is sp3 hybridized, as compared to the Ta–Calkyne bond, where the carbon atom is (formally) sp2 hybridized.  As was discussed in Section 5.1, the greater degree of s-orbital character in the Ta–C alkyne bond means that it would better stabilize the attendant 4-membered transition state (Scheme 5.1), and thus react more readily than the Ta–Cbenzyl bond. Accordingly, the first step results in the conversion of the coordinated alkyne ligand into a vinyl moiety (Scheme 5.15); an alkylidene resonance structure can also be envisioned for this latter complex. The α–insertion of the alkylidene moiety into the Ta–H bond (and not the Ta–R bond, which would be significantly slower313) results in a zwitterionic species (square brackets, Scheme 5.15) that undergoes a ring-closing step to afford a Ta alkene alkyl complex; free rotation about the C–C bond (labeled in red) enables the alkene moiety to adopt the more sterically favourable trans-geometry. The remaining alkyl ligand of this Ta alkene complex would then undergo σ-bond metathesis with H2 to generate complex 5.5.    152  Scheme 5.15: Possible mechanism for the formation of 5.5 ([Ta] = [PhNPN*]Ta); the trans-geometry of the alkene ligand is achieved via free rotation around the bond labeled in red.    In addition to the [PhNPN*]Ta(BTA) alkyl complexes, the 3-hexyne analogue [PhNPN*]Ta(3-hexyne)Bn (2.6) reacts with H2 to afford 5.6, a Ta alkene hydride complex that is analogous to 5.5 (Scheme 5.16), but with the added complexity of isomerization of the 3-hexene ligand to the 1-hexene isomer. [Ta]RR R'[Ta]HHR R[Ta]H R'HR R[Ta]R'HR H R R' [Ta]H R'HR Rα-insertionH22.7 (R' = Bn)2.8 (R' = Et)5.5H2 [Ta]HHR R H- HR'  153  Scheme 5.16: Synthesis of the Ta 1-hexene hydride complex 5.6 from complex 2.6, which may proceed via the intermediacy of a Ta 3-hexene hydride complex.  The conversion of complex 2.6 to 5.6 occurs over approximately 6 h (1 atm H2), significantly faster than the analogous reaction with complexes 2.7 or 2.8 (36 – 72 h, 1 atm H2).  However, it is important to note that the alkyl and alkyne ligands of complex 2.6 are coordinated to the Ta centre in a different arrangement than in complexes 2.7 and 2.8 (Section 2.2.5).  While the mechanistic implications that this alternate ligand arrangement might have are unclear, it may explain the more rapid rate of formation of the Ta alkene hydride product from complex 2.6 than from the latter examples.   Regardless of these differences in ligand arrangement, the formation of 5.6 from 2.6 is believed to proceed via a mechanism similar to the one depicted in Scheme 5.15; the progression Ta NP NEtEtBn 2.6 Ta NP NnBuH H5.6HHC6H6, 6 hH2, 1 atm.- HBn3-hexene intermediate ?Ta NP NEtH HHEt  154 of the reaction was monitored periodically by 1H and 31P{1H} NMR spectroscopy, and, similar to the formation of 5.5, the presence of several new resonances downfield of δ 15 indicate that the reaction proceeds via the intermediacy of several unidentified Ta hydride-containing complexes. While this NMR spectroscopic data does not show any evidence for the formation of 2.4, this is not surprising.  The putative result from the hydrogenolysis of the benzyl ligand in 2.6 would be a structural isomer of 2.4, with the position of the alkyne and hydride ligands interchanged (Scheme 5.17). As the NMR spectroscopic details of this isomer of 2.4 are not presently known, its role as a potential intermediate in the formation of 5.6 is unclear.   Scheme 5.17: The putative result of the hydrogenolysis of the benzyl group in complex 2.6: an isomer of complex 2.4 with the position of the alkyne and hydride ligands interchanged.  However, the intermediacy of complex 2.4 in the hydrogenolysis of 2.6 may be ruled out: in addition to there being no NMR spectroscopic evidence for its formation, exposure of 2.4 to H2 (1 atm) for a period of 24 h leads to no observable reaction.  As with the case of 2.5, the hydrogenolysis of complex 2.4 does ultimately lead to the formation of complex 5.6, over a Ta NP NEtEtBn 2.6 Ta NP NEtEtHisomer of2.4H2, - HBn?  155 period of several days (under 4 atm H2); this transformation will be discussed in more detail in Section 5.2.5.  Although the solid-state molecular structure of 5.6 has not yet been determined, solution-state NMR spectroscopy provides compelling evidence for a 1-hexene hydride complex (shown in Scheme 5.16).  In C6D6, the 1H NMR spectrum of 5.6 features [PhNPN*] ligand resonances indicative of a C1 symmetric complex, as well as an extremely downfield doublet resonance characteristic of a hydride ligand strongly coupled to a phosphorus-31 atom (δ 23.7, JHP = 36 Hz).  Based on the similarly large JHP coupling constants, this last piece of data suggests that the relative orientation of the ligands around Ta is the same as is seen in 5.5 (as well as 2.4 and 2.5), with the hydride ligand in an apical site opposite the phosphorus-31 atom of [PhNPN*]. The 1H and 13C NMR data for 5.6 also suggests that the 3-hexyne ligand in complex 2.6 is ultimately converted to a 1-hexene, rather than a 3-hexene (square brackets, Scheme 5.16), moiety.  This result has precedent: the isomerization of alkene ligands is a well-known process for early transition metal hydride complexes.  A classic example is the hydrozirconation reaction between an internal alkene and Schwartz’s reagent, Cp2Zr(H)Cl.  Insertion of the alkene into the Zr–H bond ultimately affords the terminal Zr alkyl moiety;314-316 this isomerization process is hypothesized to occur via a ‘chain walking’ mechanism317 consisting of successive olefin insertion and β-hydride elimination cycles, with the terminal isomer being preferred as a means of reducing steric interactions at the metal centre.62   Thus, presuming that the hydrogenolysis of 2.6 initially results in the formation of the [PhNPN*]Ta(3-hexene)H intermediate (square brackets, Scheme 5.16), the alkene ligand would likely undergo rearrangement to generate the terminal isomer (5.6, Scheme 5.16). Scheme 5.18 depicts one such iteration, in which the putative 3-hexene is converted to the 2-hexene ligand; a   156 second iteration can be envisioned that would convert the 2-hexene ligand to the 1-hexene isomer consistent with the NMR spectroscopic data for complex 5.6.  Drawing on the analogy of Schwartz’s reagent, this process is envisioned as proceeding via a series of olefin insertion and β-hydride elimination steps involving the Ta(III)-alkene resonance form of 5.6 (cf. Scheme 5.12).  Scheme 5.18: One iteration of the proposed ‘chain walking’ mechanism for the formation of the 1-hexene ligand in complex 5.6 from the putative 3-hexene isomer.  The 1H and 13C NMR resonances attributable to the 1-hexene ligand of 5.6 are tabulated in Table 5.4; in addition, Figure 5.4 shows a truncated (δ 0 – 3) 1H NMR spectrum with the 1-hexene resonances labeled.  These spectral assignments were made based on the results of the 1H-1H COSY, 1H-13C HSQC, 1H-13C HMBC and 13C-APT NMR experiments.  The protons of Ta NP NCCEtH HHCH2Me Ta NP NCCEt H HHCH2Me Ta NP NCCCH HHMeEt HHolefininsertion3-hexene isomer 2-hexene isomerolefininsertionβ-H elim. Ta NP NCCCH HHHnPr HH1-hexene isomer(5.6)β-H elim.Ta NP NCCCH HHHnPr HH  157 the methyl group (C6H3) generate a well-resolved triplet resonance at δ 0.77 (JHH = 7 Hz), and the methylene protons (C5H2, C4H2, C3H2) give rise to three broad multiplets in the region between ~ δ 1.2 and 1.8; the resonance for the C2H proton is nearly coincident with that of the two C3H2 protons.  The two C1H2 protons give rise to a multiplet at δ 2.07 (obscured by a singlet attributed to an aryl methyl group of [PhNPN*]) and a multiplet at δ 0.43 with well-resolved coupling to phosphorus-31 (JHP = 4 Hz), H2 (JHH = 17 Hz) and the other proton at C1 (JHH = 13 Hz).  The results of several deuterium-labeling experiments with 5.6 support a mechanism that involves successive hexene isomerization steps, similar to the olefin insertion/ β-hydride elimination process depicted in Scheme 5.18.  Treating 2.6 with D2 gas resulted in the deuterated analogue of [PhNPN*]Ta(1-hexene)H, 5.6–dn, where the hydride ligand and protons of the 1-hexene moiety are replaced by deuterium atoms.  Although the mass spectroscopic data (low resolution electron-ionization) for 5.6–dn indicates a significant level of deuterium incorporation, evidence for the fully deuterated isotopologue 5.6–d13 was not found.  Nevertheless, the 1H NMR spectrum of 5.6–dn (Figure 5.5, ii) lacks all of the resonances attributed to the protons of the 1-hexene ligand in 5.6 (Figure 5.5, iii) along with the Ta hydride resonance at δ 23.7.  However, the resonances for these deuterons are readily located at the expected chemical shifts in the 2H NMR spectrum of 5.6–dn (Figure 5.5, i), with the exception of C1Da/b, whose 2H NMR resonances are either very weak in intensity or too broad to be observed.  In addition, in the 13C{1H} NMR spectrum of 5.6–dn, the resonances attributed to the carbons of the 1-hexene chain are not visible.  This is a common phenomenon for carbon atoms attached to deuterons, due to both a decrease in signal intensity as a result of longer spin relaxation times (T1), as well as signal broadening due to 13C-2H J-coupling.146    158   Figure 5.4: Truncated (δ 0 – 3 region) 1H NMR spectrum (400 MHz, 298 K) of 5.6 in C6D6.  Some residual pentane (*) and polydimethylsiloxane grease (#) is also present. Assignments for C atoms are labeled in red.  Table 5.4: 1H and 13C NMR spectral assignments for the 1-hexene ligand of complex 5.6.  Proton(s)  Chemical Shift Location (13C NMR) C1Ha/b δ 0.43, 2.07 C1 (δ 43.9) C2H δ 1.17 C2 (δ 74.7) C3H2 δ 1.21 C3 (δ 23.4) C4H2 δ 1.84 C4 (δ 36.8) C5H2 δ 1.52 C5 (δ 37.5) C6H3 δ 0.77 C6 (δ 14.5)    C HC CH2 H2C CH2 CH31 2 3 4 5 6[Ta]HH# * * C1Ha/b C6H3 C2H C3H2 C5H2 C4H2 C1Ha/b   159   Figure 5.5: i) 2H NMR spectrum (61 MHz, 293 K, C6H6) of 5.6–dn. ii) 1H NMR spectrum (400 MHz, 293 K, C6D6) of 5.6–dn. iii) 1H NMR spectrum (400 MHz, 293 K, C6D6) of 5.6. Some residual pentane (*) and polydimethylsiloxane grease (#) is also present.  Complexes 5.6 and 5.6–dn both readily undergo H/D exchange with D2 or H2 gas, respectively, to generate the corresponding isotopologue (Scheme 5.19); the synthesis of 5.4 from 5.6 is exceedingly slow under these conditions, and thus the formation of 5.4 (or 5.4–d4) is not observed alongside these H/D exchange processes.  However, experimental work performed by Dr. Dominik Nied and the author confirms that 5.4 and 5.4–d4 also undergo H/D exchange C DC CD2 D2C CD2 CD31 2 3 4 5 6[Ta]DDiii) ii) i)     * * * *  # [Ta]–D C6D3 C2D C3D2 C5D2 C4D2   160 with D2 (D. Nied) or H2 (K. Parker), respectively, to generate the corresponding isotopologue, as is also shown in Scheme 5.19.   Scheme 5.19: H/D exchange reactions between 5.6 & 5.6–dn, and 5.4 & 5.4–d4.  In addition, the Ta alkyne hydride complex [PhNPN*]Ta(3-hexyne)H (2.4) undergoes fast H/D exchange with D2 (1 atm, ~16 h) to generate the monodeuteride complex, [PhNPN*]Ta(3-hexyne)D (Scheme 5.20), as evidenced by the disappearance of the Ta hydride 1H NMR resonance at δ 21; this exchange process is slow, but significantly faster than the conversion of 2.4 to 5.6 (as will be discussed in Section 5.2.5), and thus no deuteration of the alkyne unit is observed.  Scheme 5.20: Formation of the Ta alkyne monodeuteride complex 2.4–d1. [PhNPN*]Ta(1-hexene)H [PhNPN*]Ta(dn-1-hexene)DD2H2 5.6-dn5.6([PhNPN*]Ta)2(µ-H)4 ([PhNPN*]Ta)2(µ-D)45.4-d45.4 ~24 h(1 atm)7 days(4 atm)H2 slow   (1 atm) slow   (1 atm)D2D2H2[PhNPN*]Ta(3-hexyne)H [PhNPN*]Ta(3-hexyne)DD2 2.4-d12.4 ~16 h(1 atm)  161 Scheme 5.21 depicts a proposed mechanism for the incorporation of deuterium into the hexene ligand of 5.6, based on the data presented above; similar to Scheme 5.18, the Ta(III)-alkene resonance form of 5.6 is invoked for the sake of simplicity and consistent terminology.  In the presence of excess D2 gas, the hydride ligand of 5.6 would exchange for a deuteride via a σ-bond metathesis-type interaction. This deuterium atom could then be incorporated into the hexene chain via an olefin insertion/ β-hydride elimination cycle to afford a new hydride ligand; Scheme 5.21 depicts two possible olefin insertion products (A-I or B-I), which lead to two possible β-hydride elimination results (A-II or B-II).  While the steric bulk of the [PhNPN*] ligand ensures that 1-hexene is ultimately the favoured regioisomer, work done by Bercaw and coworkers on the reactivity of alkenes with Nb(III) metallocene hydride complexes300,302 suggests that the kinetic barrier between the possible olefin insertion/ β-hydride elimination pathways (e.g. A and B in Scheme 5.21) could be low enough such that in solution a number of different regioisomers (e.g. A-I and B-I) exist in equilibrium with one another. Thus, as the hydride ligand in A-II or B-II can also exchange with D2 to generate a deuteride, subsequent olefin insertion/ β-hydride elimination cycles can be envisioned that will eventually lead to all the hydrogen atoms along the hexene chain being replaced by deuterium.      162  Scheme 5.21: One iteration of the proposed ‘chain walking’ mechanism for H/D exchange in the 1-hexene ligand in complex 5.6.  Ta NP NCCH2CH DHH Ta NP NCH2C Ta NP NCCnPr HHHolefinins.β-H elim.olefinins.β-D elim. nPr nPrDH2CDH2CD2- HD Ta NP NCH2CH2C DHnPr olefinins.β-D elim.Ta NP NCCH2CH HDHnPrHβ-H elim.olefinins.5.6Ta NP NCCH2CH HHHnPr B-IA-IB-IIA-II  163 5.2.4 The Synthesis of ([PhNPN*]Ta)2(µ-H)4 from Tantalum Alkene Hydrides (and Tantalum Alkyne Benzyl Complexes) As mentioned briefly in Section 5.2.2 (Scheme 5.8), the hydrogenolysis of either the Ta alkyne benzyl complexes (2.6 or 2.7), or the alkene hydride complexes (5.5 and 5.6), ultimately afforded the dinuclear tetrahydride ([PhNPN*]Ta)2(µ-H)4 (5.4).  Both of these synthetic routes are slow, requiring either 10 (from 2.7) or 14 (from 2.6) days (at 4 atm H2); as the formation of 5.5 or 5.6 from 2.7 or 2.6 is comparably fast (between 6 and 36 h at 1 atm H2), the time required to form 5.4 via either of these two synthetic routes is essentially the same (Scheme 5.22).  In the course of the formation of 5.4, the alkyne or alkene ligand is converted to the fully hydrogenated alkane: n-hexane (2.6 and 5.6) or 1,2-bis(TMS)ethane (2.7 and 5.5), based on 1H and 13C{1H} NMR data.    Scheme 5.22: Formation of the tetrahydride complex 5.4 from the Ta alkyne benzyl complexes 2.6 and 2.7.    [PhNPN*]Ta(alkyne)Bn (2.6 or 2.7)[PhNPN*]Ta(alkene)H (5.5 or 5.6) ([PhNPN*]Ta}2(µ-3H)45.410 - 14 d4 atm H210 - 14 d4 atm H26 - 36 h1 atm H2 + n-hexane(2.6, 5.6)or1,2-bis(TMS)ethane(2.7, 5.5)  164 Unfortunately, NMR spectroscopic data provides no information regarding any further intermediates that may be formed during the synthesis of 5.4 from 5.5 or 5.6. Once the formation of the Ta alkene hydride complexes are complete (or, if the reaction is started from this point) the only new resonances that are observed in the 1H or 31P{1H} NMR spectra correspond to 5.4 (and the alkane byproduct).   Despite the fact that the specific details are unknown, a plausible mechanism for the formation of 5.4 is shown in Scheme 5.23.  It is likely that, in the presence of H2, the Ta–C(sp3) bonds of 5.5 or 5.6 would undergo σ-bond metathesis to generate hydride ligands; the Ta trihydride complex (5.7) is one possible (although unobserved) outcome for the hydrogenolysis of 5.5 or 5.6. Complex 5.4 could form via the dimerization of two such [PhNPN*]TaH3 units, followed by the reductive elimination of an equivalent of H2 (which would provide the two electrons necessary for the requisite Ta(IV)–Ta(IV) bond).  A route similar to the one shown in Scheme 5.23 could be easily adapted to the formation of 5.4 from [PhNPN*]TaMe3 (5.3).  Scheme 5.23: Possible route for the formation of the tetrahydride complex 5.4 from the Ta alkene hydride complexes 5.5 and 5.6. Ta NP NRH HHR - alkane0.5  ([PhNPN*]Ta)2(µ-H)42 H2 Ta NP NHH Hdimerization- H25.45.5 or5.6 5.7  165 Of course, Scheme 5.23 reflects only one of many possible routes to ([PhNPN*]Ta)2(µ-H)4 (5.4).  As with the case of ([P2N2]Ta)2(µ-H)4  (5.2) and ([NPNSi]Ta)2(µ-H)4 (1.71), dinuclear Ta tetrahydrides both formed via the hydrogenolysis of their mononuclear Ta alkyl precursors, a more definitive mechanism for the formation of 5.4 remains elusive.  5.2.5 The Hydrogenolysis Chemistry of [PhNPN*]Ta(alkyne)H Complexes The Ta alkyne hydrides 2.4 and 2.5 also react with H2 to generate the corresponding Ta alkene hydride complexes (5.6 and 5.5, respectively), albeit slowly; as 5.5 and 5.6 go on to form 5.4, complexes 2.4 and 2.5 also provided a route for the synthesis of ([PhNPN*]Ta)2(µ-H)4 (5.4) (Scheme 5.24).  Scheme 5.24: Formation of the tetrahydride complex 5.4 from the Ta alkyne hydride complexes 2.4 and 2.5.  In contrast to the relatively rapid synthesis of 5.5 or 5.6 from the Ta alkyne benzyl complexes (2.6 and 2.7) discussed in Section 5.2.3, exposure of 2.4 or 2.5 to H2 (1 atm) for a period of 36 h resulted in no observable reaction.  However, in the case of 2.4, an increase in [PhNPN*]Ta(alkene)H (5.5 or 5.6)[PhNPN*]Ta(alkyne)H (2.4 or 2.5)3 days4 atm H2(~30%) 10 d4 atm H2(~60 %) ([PhNPN*]Ta}2(µ-3H)45.410 - 14 d4 atm H2 + n-hexane(2.4, 5.6)or1,2-bis(TMS)ethane(2.5, 5.5)  166 both H2 pressure (4 atm) and reaction time (3 days) led to a mixture of unreacted starting material (~65%) and the Ta alkene hydride 5.6 (~30%), along with trace amounts of 5.4 (<5%).  After 10 days (4 atm H2), the mixture of products was mainly composed of 5.4 (~60%), with small amounts of 5.6 (~25%) and 2.4 (~15%) also present.  Similar observations were made in the case of 2.5. Qualitatively, this evidence suggests that the formation of the Ta alkene hydride complex (5.6 or 5.5) from the Ta alkyne hydride complex (2.4 or 2.5), and the subsequent conversion of the Ta alkene hydride to the tetrahydride complex 5.4 are processes with similar, and very slow, rates of reaction. However, a detailed kinetic study of the transformation of 2.4 or 2.5 to 5.4 has not yet been conducted, and thus these conclusions are necessarily preliminary.	  Unfortunately, periodically monitoring the conversion of 2.4 or 2.5 to 5.6 or 5.5 by 1H and 31P{1H} NMR spectroscopy provided no further information regarding potential intermediates for these reactions; the only resonances present in the NMR spectra correspond to either the alkyne hydride starting material or the alkene hydride product (along with 5.4, which arises from the concurrent reaction of the latter complex with H2).  It is possible that the conversion of the Ta alkyne hydrides 2.4 and 2.5 to the Ta alkene hydrides 5.5 and 5.6 proceeds via a σ-bond metathesis mechanism similar to the one discussed for the Ta alkyne alkyl complexes (2.6 – 2.8) in Section 5.2.3, beginning with the hydrogenolysis of a Ta–Calkyne bond (Scheme 5.25).  However, in order for this to be the case, the mechanism must also account for the observation that the rates of formation of 5.5 or 5.6 from 2.6 – 2.8 is significantly faster than the rate of formation from 2.4 or 2.5 (as summarized in Table 5.5).       167 Complex Alkyne R’ p(H2) Time to 5.5/5.6 2.4 3-hexyne H 4 atm > 10 days 2.5 BTA H 4 atm  > 10 days 2.6 3-hexyne Bn 1 atm ~6 h 2.7 BTA Bn 1 atm  ~36 h 2.8 BTA Et 1 atm ~72 h  Table 5.5: Summary of the experimental conditions for the formation of the [PhNPN*]Ta alkene hydrides 5.5 or 5.6, via the hydrogenolysis of the alkyne alkyl and hydride complexes 2.4 – 2.8.  All reactions were conducted in 200 mL thick-walled Kontes-sealed glass reactors charged with 1 or 4 atm of H2 and 32 mM (0.11 mmol in 3.5 mL C6D6) solutions of the relevant Ta complex.         Scheme 5.25: Possible mechanism for the formation of the Ta alkene hydride complexes (5.5 or 5.6) from the Ta alkyne alkyl (2.6 – 2.8) and alkyne hydride (2.4 and 2.5) complexes, which proceeds via a common Ta vinyl hydride intermediate ([Ta] = [PhNPN*]Ta).  Adapted from Scheme 5.15; the trans-geometry of the alkene ligand is achieved via free rotation around the bond labeled in red.   168  Ta NP NRR R'2.4 (R = Et, R' = H)2.5(R = TMS, R' = H) Ta NP NRR R'2.7 (R = TMS, R' = Bn)2.8 (R = TMS, R' = Et)[Ta]HHR R[Ta]H R'HR R[Ta]R'HR H R R' [Ta]H R'HR Rα-insertionH2 [Ta]HHR R H- HR'H2'Ta vinyl hydride'Ta NP NRRR' 2.6 (R = Et, R' = Bn)or or5.5 or 5.6  169 As shown in Scheme 5.25, it is necessary that the initial hydrogenolysis step (i.e. the formation of the ‘Ta vinyl hydride’ intermediate) is significantly slower in the cases of 2.4 and 2.5 (Scheme 5.25, R’ = H) as compared to 2.6 – 2.8 (Scheme 5.25, R’ = alkyl).  Support for this hypothesis can be found by taking into account the factors that determine the rate of reactivity of a particular substrate for σ-bond metathesis.  As discussed in Section 5.1, a greater degree of s-character in the orbitals that constitute the 4-membered transition state correlates with a faster rate of reaction;291 in Section 5.2.3, this was cited as a reason for the increased reactivity of the Ta-Calkyne bond over the Ta-Cbenzyl bond in complexes 2.6 – 2.8.  However, the acknowledged288,291 rapidity of σ-bond metathesis between metal hydrides and H2 also follows from this point (Scheme 5.26); the low kinetic barrier for this transformation is believed to be due to the high degree of overlap between the s-orbitals of three hydrogen atoms in the 4-centred transition state.  Scheme 5.26: The σ-bond metathesis reaction between H2 and the Ta–H moiety of complex 2.4 or 2.5 ([Ta] = [PhNPN*]Ta(alkyne)).   Thus, in the case of complexes 2.4 and 2.5, it is likely that the Ta–H bond, rather than the Ta–Calkyne bond, is most reactive with regards to σ-bond metathesis with H2.  As the analogous σ-bond metathesis H/D exchange reaction between the Ta–H bond of 2.4 and D2 occurs readily over a period of ~16 h (vide supra, Scheme 5.20), it is reasonable to conclude that ‘H-atom [Ta] H [Ta] HH H [Ta] H-δ+ δ−δ− δ+H2 H2  170 exchange’ between the Ta–H bond of 2.4 and H2 (Scheme 5.26) is similarly facile.  Thus, it is possible that the σ-bond metathesis reaction between H2 and the Ta–H bond of 2.4 or 2.5 is simply far more kinetically favourable than the reaction at the Ta–Calkyne bond, and that interactions between H2 and 2.4 or 2.5 proceed along the former pathway far more readily than the latter.  However, as illustrated in Scheme 5.26, σ-bond metathesis between a metal hydride and H2 results in no net structural change, and so the predominance of this reaction pathway would lead to the observation that no overall reaction between 2.4 or 2.5 and H2 is occurring. Of course, as these reactions are conducted in the presence of a large excess of H2, the kinetic barrier for the hydrogenolysis of the Ta-Calkyne bond in 2.4 and 2.5 would need to be tremendously high in order to adequately explain the much slower rate of reaction at this site.  Thus, while this hypothesis does provide a possible rationalization for the experimental observations, it falls short of a completely satisfying explanation as to why the relative rates of reaction are so drastically different.  Clearly, further experimental and computational studies are required in order to better understand the mechanism for, and relative rates of, the hydrogenolysis of these Ta alkyne complexes.  5.3 Conclusions The work in this chapter investigated the hydrogenolysis chemistry of the Ta alkyne alkyl and alkyne hydride complexes [PhNPN*]Ta(3-hexyne)R (R = H, 2.4; R = Bn, 2.6) and [PhNPN*]Ta(BTA)R (R = H, 2.5; R = Bn, 2.7; R = Et, 2.8).  It was discovered that hydrogenolysis of these Ta alkyne complexes eventually resulted in the formation of the dinuclear Ta tetrahydride ([PhNPN*]Ta)2(µ-H)4 (5.4), after a period of 10 – 14 days (under 4 atm H2).  The synthesis of the trimethyl complex [PhNPN*]TaMe3 (5.3) was also investigated; as   171 expected, the hydrogenolysis of 5.3 (4 atm H2, 24 h) also resulted in the formation of 5.4.  Unfortunately, ([PhNPN*]Ta)2(µ-H)4 (5.4) does not react with N2 to form a Ta dinitrogen compound analogous to ([NPNSi]Ta)2(η1:η2-µ-N2)(µ-H)2 (1.72).  The reason for this inertness with respect to N2 is presently unknown. Low pressure hydrogenolysis of either the Ta alkyne alkyls (2.6 – 2.8) or Ta alkyne hydrides (2.4 – 2.5) results in the intermediacy of the Ta alkene hydride complexes [PhNPN*]Ta(trans-BTE)H (5.5) or [PhNPN*]Ta(1-hexene)H (5.6).  The alkene ligand of complex 5.6 is of interest, since the result of hydrogenolysis of a 3-hexyne ligand would be expected to be 3-hexene, rather than the observed 1-hexene.  However, as complex 5.6 also features a hydride ligand, the isomerization of the hexene unit is hypothesized to occur via successive olefin insertion/ β-hydride elimination steps.  The terminal hexene isomer is favoured as a result of the steric bulk of the [PhNPN*] ligand set. As synthetic precursors to 5.4, the hydrogenolysis of each of the various Ta alkyne alkyl and alkyne hydride complexes lead to a common intermediate, the Ta alkene hydride complex (5.5 or 5.6, depending on the alkyne in the starting reagent).  In the case of the Ta alkyne alkyl complexes (2.6 – 2.8), the Ta alkene hydride intermediate forms relatively rapidly; thus, the time required to form 5.4 from 2.6 – 2.8, or 5.5 and 5.6 are nearly equal.  In contrast, the formation of 5.5 or 5.6 from the Ta alkyne hydrides (2.4 and 2.5) is extremely slow.  An explanation for this slow rate of reaction is not immediately obvious, but may have to do with the predominance of rapid σ-bond metathesis exchange between the Ta–H moiety and H2.    172 Chapter  6: Thesis Synopsis and Future Directions 6.1 Thesis Synopsis The overall goal of the work presented in this thesis was to explore the synthesis and reactivity of tantalum complexes supported by the diamidophosphine ligand [PhNPN*].  The initial inspiration for this project came from previous work from the Fryzuk group involving tantalum complexes of the [NPNSi] ligand set, particularly the strongly reducing tetrahydride ([NPNSi]Ta)2(µ-H)4  (1.71).  However, as noted in Chapters 1 and 5, reactivity studies with [NPNSi]Ta complexes, particularly the dinitrogen complex ([NPNSi]Ta)2(η1:η2-µ-N2)(µ-H)2 (1.72), have been hampered by undesirable side-reactions with the ligand, which often resulted in complex degradation or decomposition.  In an effort to address these issues, former doctoral student Dr. Erin MacLachlan investigated the synthesis of the new, ortho-phenylene bridged diamidophosphine ligand, [PhNPN*], which was expected to be more chemically robust than its silyl-amine bridged precursor, while at the same time mimicking the steric and electronic properties of the [NPNSi] ligand set.  While Dr. MacLachlan thoroughly investigated the synthesis and reactivity of a number of [PhNPN*] complexes featuring zirconium and hafnium, tantalum complexes of this new amidophosphine ligand had yet to be explored.  Chapter 2 described the initial, unsuccessful attempts made at synthesizing [PhNPN*]Ta complexes using [PhNPN*]Li2(dioxane) or [PhNPN*]K2(THF)0.5 (2.1) and TaCl5, TaBn5 or Ta(NMe2)5.  Instead, combining 2.1 with Ta(alkyne)Cl3(DME), Ta(V) reagents featuring a reduced alkyne unit, afforded [PhNPN*]Ta(3-hexyne)Cl (2.2) and [PhNPN*]Ta(BTA)Cl (2.3).  Subsequent functionalization of the Ta–Cl moiety in 2.2 or 2.3 allowed for the synthesis of a number of Ta hydride and alkyl complexes: [PhNPN*]Ta(alkyne)X (X = H, Bn, Et).  An examination of the solid-state molecular structures and NMR spectroscopic data for these   173 [PhNPN*]Ta(alkyne)X complexes indicated that the bond between the Ta and the alkyne ligand is best described as a interaction between a Ta(V) metal centre and an ‘alkenediyl’ dianion. However, the Ta(III)–alkyne bonding motif was also found to be an apt description for several examples discussed in later chapters. For example, Chapter 3 focuses on a series of [PhNPN*]Ta imide complexes (3.3 – 3.6) prepared via the reaction of PIPP (para-isopropylphenyl) azide with various [PhNPN*]Ta alkyne complexes.  This reaction is proposed to occur through the oxidation of a low-valent Ta(III) metal centre by the organic azide; extrusion of an equivalent of N2, along with expulsion of the alkyne ligand, would generate the Ta(V) imide product.  Mechanistically, the reaction between the Ta alkyne complex and the organic azide is thought to be an associative process, based on the observation that the newly formed imide moiety in the final product occupies the former position of the coordinated alkyne ligand in the starting material.  The synthesis and structural characterization of a novel Ta triazenide imide complex (3.7), prepared via the insertion of an equivalent of PIPP azide into a Ta–C bond of the Ta benzyl imide complex (3.6), is also reported. The possible synthesis of a [PhNPN*]Ta(V) nitride complex via the extrusion of N2 from a Ta(III) terminal azide was also explored in Chapter 3.  Although this is a common means of synthesizing nitride moieties in late-metal complexes, the Ta alkyne azide complexes [PhNPN*]Ta(3-hexyne)N3 (3.1) and [PhNPN*]Ta(BTA)N3 (3.2) proved to be stable to N2 loss, despite attempts to thermally or photochemically induce nitride formation.   The work described in Chapter Four explored the reactivity of the Ta alkyne monohydride complexes [PhNPN*]Ta(3-hexyne)H (2.4) and [PhNPN*]Ta(BTA)H (2.5) with 2,6-dimethylphenyl isocyanide, phenylacetylene and carbon dioxide.  In contrast to the reactivity   174 described in Chapter 3, where the addition of an aryl azide to a Ta alkyne complex resulted in the displacement of free alkyne, the addition of an isocyanide or alkyne reagent to either 2.4 or 2.5 resulted in the formation of 5-membered tantallacyclic products via the coupling of the alkyne ligand with a newly formed phenylvinyl (4.2 and 4.4) or iminoacyl moiety (4.5 and 4.6).  In the phenylacetylene case, conversion of Ta alkyne phenylvinyl intermediate to the tantallacyclic product occurs slowly enough to allow for characterization by solution-state NMR spectroscopy; a kinetic study of this process was also conducted, and indicated that the formation of the tantallacycle proceeds through a highly ordered transition state.  In contrast, treating the Ta alkyne hydride complexes with different amounts of carbon dioxide resulted in the formation of a formate (4.7) or a methylene diolate moiety (4.8) which does not undergo coupling with the Ta–coordinated alkyne ligand, possibly due to the formation of strong Ta–O bonds that are resistant to further reactivity.  The structure of the tantallacyclic products formed from phenylacetylene or isocyanide insertion were confirmed by X-ray diffraction studies, and can be described as a combination of two resonance forms: a Ta(V) alkylidene-amide and a Ta(III) alkenyl-imine (in the case of isocyanide insertion), or a Ta(V) alkyl-alkylidene and a Ta(III) butadienyl complex (for the phenylacetylene examples). The work presented in Chapter 5 addresses the hydrogenolysis chemistry of both the [PhNPN*]Ta trimethyl complex (5.3), and the Ta alkyne alkyl and monohydrides (2.4 – 2.8).  Exposure of these compounds to high pressures (4 atm) of H2 resulted in the formation of the dinuclear Ta(IV) tetrahydride, ([PhNPN*]Ta)2(µ-H)4 (5.4).  In case of the Ta alkyne complexes this transformation is markedly slower, and occurs over the course of 10 – 14 days, in contrast to complex 5.3, which forms the dinuclear Ta tetrahydride product in approximately 24 h.  Low pressure (1 atm) hydrogenolysis of complexes 2.4 – 2.8 resulted in the formation of a common   175 Ta alkene hydride intermediate, depending on the identity of the alkyne in the starting material: [PhNPN*]Ta(trans-BTE)H (5.5) or [PhNPN*]Ta(1-hexene)H (5.6).  In the case of the Ta alkyne alkyl complexes (2.6 – 2.8), NMR spectroscopic evidence suggests that hydrogenolysis occurs at the alkyne ligand before the alkyl ligand.  Unexpectedly, while the formation of 5.5 or 5.6 from the Ta alkyne alkyl complexes is relatively rapid, the formation of Ta alkene hydride intermediates from the Ta alkyne hydrides (2.4 and 2.5) is extremely slow.  An explanation for this slow rate of reaction is not immediately obvious, but may have to do with the predominance of rapid σ-bond metathesis exchange between the Ta–H moiety and H2.  The structures of the Ta alkene hydride complexes, as determined by solution-state NMR spectroscopy (both 5.5 and 5.6), and X-ray crystallography (5.5 only), were somewhat surprising.  The alkene ligand of complex 5.6 is of interest, since the result of hydrogenolysis of a 3-hexyne ligand would be expected to be 3-hexene, rather than the observed 1-hexene.  However, as complex 5.6 also features a hydride ligand, the isomerization of the hexene unit is hypothesized to occur via successive olefin insertion/ β-hydride elimination steps, which favour the terminal hexene isomer as a result of the steric bulk of the [PhNPN*] ligand.  Similarly, the trans-geometry of the bis(trimethylsilyl)ethene moiety in complex 5.5 was unexpected, as metal-mediated alkyne hydrogenolysis typically affords cis-alkene products.  It is unclear whether a hydrogenolysis mechanism that favours trans-alkenes is operative instead, or if the trans-geometry is a result of the rearrangement of a cis-alkene ligand akin to complex 5.6.  Finally, no reaction occurs upon exposure of ([PhNPN*]Ta)2(µ-H)4 (5.4) to high pressures of N2 gas.  The reason for this inertness is presently unknown, and further investigation into the reactivity of complex 5.4 with other small molecule substrates, such as CO, CO2 or alkynes, may help elucidate this point.   176 In conclusion, [PhNPN*] diamidophosphine ligand set was found to be capable of supporting a variety of tantalum organometallic complexes.  The use of the novel Ta(alkyne) reagents resulted in [PhNPN*]Ta complexes where the bonding and reactivity at the metal centre is best understood as a combination of both the high-valent Ta(V) and low-valent Ta(III) structural formalisms.  An analogue of ([NPNSi]Ta)2(µ-H)4 ( 1.71) featuring the new ortho-phenylene bridged diamidophosphine ligand [PhNPN*] was successfully synthesized, although the inertness of ([PhNPN*]Ta)2(µ-H)4 (5.4) with respect to N2 means that a comparison of the stability of ([NPNSi]Ta)2(η1:η2-µ-N2)(µ-H)2 ( 1.72) to a similar [PhNPN*]Ta dinitrogen complex could not be made.  6.2 Future Directions 6.2.1 Synthesis and Potential Reactivity of a Cationic Tantalum Imide  The synthesis of a family of [PhNPN*]Ta imide complexes (3.3 – 3.6), and the reaction of the Ta benzyl imide complex 3.6 with PIPP azide, was presented in Chapter 3 (Scheme 6.1).    177  Scheme 6.1: The various Ta imide complexes reported in Chapter 3, and the synthesis of a Ta triazenide imide complex via azide insertion into a Ta–C bond.  Complex 3.7 arose from the insertion of the azide reagent into the Ta–Cbenzyl bond; in this example, the imide moiety functioned as a spectator ligand.  However, there are also many examples in the literature of Ta imide complexes where reactivity involves the Ta=NR bond.  Recently, there have been several reports where Ta imide complexes have been employed as nitrene transfer reagents,196,208 or catalysts for the C=N bond metathesis of carbodiimides197 or imines.318  Royo and coworkers have investigated the reactivity of Ta imides with a variety of substrates, including CO2 and benzaldehyde, which afford the corresponding isocyanate and imine via the exchange of a Ta=NR moiety for a Ta oxide.319   3.6 Ta NP NR3.3: R = Cl3.4: R = N33.5 R = BnArN Ta NP NBn NAr3.61 eq. ArN3 3.770oC, 48 h Ta NP NNNN NYX Ar(X, Y = Ar, Bn)  178 While the examples cited above involve the transfer of the imide ligand to an organic substrate from a neutral Ta complex, Bercaw and coworkers320 have studied the reactivity of a tantallocene imide cation (6.1) that instead adds a variety of substrates across the Ta=NR bond.  As shown in Scheme 6.2, treating 6.1 with H2 or a terminal alkyne, such as phenylacetylene, leads to formation of the Ta amide hydride (6.2) or amide acetylide (6.3).  In these examples, the electrophilic metal centre of the cation results in a highly polarized Ta–N bond, which is hypothesized to be a key factor in the heterolytic cleavage of the H–H or H–C bond of the substrate.    Scheme 6.2: Examples of the reactivity of cationic Ta imides (6.1) with H2 and phenylacetylene to generate the amide hydride (6.2) and amide acetylide (6.3) products, by Bercaw and coworkers. Ta NCMe3THFX = [B(C6F5)4]- PhC CH Ta NHCMe3C CPhH2 Ta NHCMe3H6.1 6.26.3X XX  179  In addition, Bergman, Arnold and coworkers227 have described the use of neutral and cationic Ta alkyl imide complexes (6.4 and 6.5, respectively) as hydroamination catalysts for the coupling of alkynes and aryl amines (Scheme 6.3).  Although the role of the imide moiety in the mechanism for this transformation is still under investigation,195,321,322 the study did demonstrate that the cationic version (6.5) was a more active catalyst, likely due to a combination of a reduction in steric bulk and a more electrophilic metal centre.    Scheme 6.3: An example of the hydroamination of alkynes and aryl amines using neutral and cationic Ta imides, by Bergman and coworkers.  Cognizant of the work discussed above, the synthesis of a cationic [PhNPN*]Ta imide complex was of interest.  To this end, the reactions of 3.5 and 3.6 with the alkyl abstracting reagents B(C6F5)3 and [CPh3][B(C6F5)4] were investigated. Unexpectedly, treating 3.5 with one equivalent of B(C6F5)3 did not lead to benzyl abstraction, and instead ultimately afforded its isomer, 3.6 (Scheme 6.4); treating 3.6 with + H2NPhPhC CPh Ph PhNPh H HPh PhNHPhH +[Ta][Ta] = TaNCMe3RRR R = Bn, nPn6.4 6.5TaNCMe3RR [B(C6F5)4]  180 B(C6F5)3 resulted in no reaction.  The presence of a Lewis acid greatly accelerated this isomeric rearrangement, which reached completion over the course of 3 days at room temperature, in contrast to the 10 days at 105 °C otherwise required (Section 3.2.2).     Scheme 6.4: Lewis acid-mediated rearrangement of the Ta benzyl imide complex 3.5 to its structural isomer 3.6.  In 2012, Gomez and coworkers reported a similar example of a borane-mediated structural rearrangement between the two enantiomeric forms of a series of Nb and Ta imide complexes.323  They proposed that this rearrangement occurred via the abstraction of a methyl group by B(C6F5)3, followed by rapid alkylation of the resulting cationic species by the [B(C6F5)3Me]– anion; a similar process may be operative in the case of 3.5 (Scheme 6.5). Nevertheless, the 1H and 31P{1H} NMR spectra of the reaction mixture only contain resonances corresponding to the reactant (3.5) and product (3.6), and the 11B{1H} and 19F{1H} NMR spectra only feature resonances for free B(C6F5)3, which suggests that any intermediate formed during this transformation may be exceedingly short-lived, or present in concentrations too low to be observed.  Ta NP NBn3.5ArN Ta NP NBn NAr3.6BArF3ArF = C6F5  181  Scheme 6.5: Possible mechanism for the structural rearrangement of the Ta benzyl imide complex 3.5 to its structural isomer 3.6.  (Adapted from ref. 323.)  In contrast, the addition of one equivalent of [CPh3][B(C6F5)4] to a toluene solution of either 3.5 or 3.6 led to the formation of an insoluble red oil.  If the reaction was instead carried out in the presence of several equivalents of THF, the solvent-stabilized salt [[PhNPN*]Ta(NR)(THF)x][B(C6F5)4] (6.6) formed quantitatively, along with an equivalent of BnCPh3 (Scheme 6.6).   Ta NP NBn3.5ArN Ta NP NBn NAr3.6Ta NP NArN- [BnBArF3]-BArF3 [BnBArF3]-- BArF3BArF3  182  Scheme 6.6: Synthesis of the Ta imide cation 6.6 via benzyl group abstraction by [CPh3][B(C6F5)4].  In toluene-d8 the NMR spectra of 6.6 are indicative of Cs symmetry.  In the 1H NMR spectrum, in addition to the expected [PhNPN*] ligand aryl proton resonances there are 4 singlets attributable to aryl methyl groups.  The two resonances for the ortho-Me protons of the mesityl amide arms appear as broadened singlets, indicating that rotation about the N-Cipso bond is perhaps less hindered than in 3.5 or 3.6 (due to reduced steric bulk around the metal centre); the signals for these ortho-Me groups are also broadened in the 13C{1H} NMR spectrum.  The proton resonances for all three THF molecules appear as two broad singlets (δ 3.75 and 1.41) shifted slightly from the anticipated values for free THF (cf. δ 3.54 and 1.43), which suggests rapid coordination and de-coordination on the NMR timescale.  The 13C{1H} NMR data is consistent with these assignments, including broad THF resonances that are slightly shifted (δ 65.3 and 26.1, compared to δ 67.8 and 25.7) from literature values.  The 11B{1H} and 19F{1H} NMR spectroscopic data are consistent with the usual values for a weakly coordinated B(C6F5)4 anion.195,320 3.5 or 3.6 1 eq. [CPh3][B(C6F5)4] 6.6- BnCPh3 [B(C6F5)4]3 eq. THF Ta NP NNAr (THF)x  183 The in vacuo removal of volatiles from 6.6 afforded a red oily residue that was insoluble in common aromatic or aliphatic solvents, presumably due to loss of THF.  However, the addition of several equivalents of THF to a toluene suspension of the red oil regenerated 6.6 with no observable decomposition. Due to the challenges of unambiguously characterizing 6.6, these results must be viewed as preliminary.  Further investigations into the reactivity of both the neutral (3.3 – 3.6) and cationic (6.6) Ta imide complexes are recommended.    6.2.2 Expansion to [PhNPN*] Niobium Complexes  The work described in Chapter 5 focused on synthesizing the Ta tetrahydride ([PhNPN*]Ta)2(µ-H)4 (5.4), in large part due to the remarkable reactivity of ([NPNSi]Ta)2(µ-H)4 (1.71).  Recent examples in the literature suggest that niobium complexes of the [PhNPN*] ligand set may also be of interest.  Kawaguchi and coworkers163,294 have reported the synthesis of a dinuclear Nb(IV) tetrahydride complex (6.7) supported by a triaryloxide ligand set.  Similar to the Ta tetrahydride 1.71, complex 6.7 is a strong reducing agent that reacts with N2 at ambient temperature and pressure to form the corresponding Nb dinitrogen (formally dinitride) complex (6.8), as shown in Scheme 6.7.    184  Scheme 6.7: Generation of the Nb dinitrogen complex 6.8 via reduction of N2 by the dinuclear Nb(IV) tetrahydride 6.7, as reported by Kawaguchi and coworkers.  Unfortunately, preliminary attempts to extend the coordination chemistry of the [PhNPN*] ligand set to Nb using reagents similar to Ta(alkyne)Cl3(DME) have been unsuccessful.  Treating [PhNPN*]Li2(dioxane) or [PhNPN*]K2(THF)0.5 (2.1) with Nb(hexyne)Cl3(DME) (1.15) produces an intractable mixture of products; experimenting with versions of the Nb(alkyne) reagent that feature different alkyne ligands (i.e. BTA, diphenylacetylene, etc.) may prove more successful.   Furthermore, despite the problems encountered with the use of TaCl5 (Section 2.2.1), NbCl5 or NbBr5 are obvious candidates of the synthesis of [PhNPN*] complexes, due both to the easy accessibility of these reagents, and the potential versatility of the putative [PhNPN*]NbX3 trihalide products via functionalization of the Nb–X bonds (e.g. salt metathesis reactions with R Nb ClClOO ClORRR RRH KBEt3HNb NN[OOO] Nb[OOO]- KCl N2- 2 H2 6.76.8 2-2-R = tBu Nb NbHHHH [OOO][OOO]  185 KBEt3H or alkylating reagents).  In addition to the Nb pentahalides, previous workers in the Fryzuk group have used NbCl2Me3 to synthesize the Nb analogue of [NPNSi]TaMe3. Although the hydrogenolysis of [NPNSi]NbMe3 resulted in a mixture of decomposition products,113 the use of this reagent with the more robust [PhNPN*] ligand set might prove more successful.  Aside from the Nb(V) options, there are also several easily accessible low-valent Nb starting materials, such as NbCl4(THF)2, Nb2Cl6(THT)3 (1.9) or NbCl3(DME) (1.14) that could yield interesting results.  These reagents are particularly attractive, since a Nb(III) or Nb(IV) metal centre can potentially serve as a reductant for small molecule substrates without the need for an external reducing agent, as is the case with complexes 1.71 or 6.7.  For example, previous workers in the Fryzuk group have investigated the coordination chemistry of NbCl3(DME) (1.14) with several amidophosphine ligand sets.  The lithium salts of both [NPNSi]90,112 and the [P2N2] macrocycle324 react with 1.14 to form the corresponding Nb(III) chloride complexes, 6.9 and 6.10 (Scheme 6.8).     186  Scheme 6.8: Synthesis of low-valent Nb amidophosphine complexes from NbCl3(DME).  In the case of 6.9, the reducing ability of the Nb(III) centre is demonstrated by the facile generation of the dinitrogen complex 6.11 (Scheme 6.9) upon exposure to high pressure (4 atm) N2 gas.  In the case of the [P2N2] system, although the Nb chloride complex (6.10) is inert the Nb methyl derivative 6.12 combines readily with N2 to afford a Nb dinitrogen complex (6.13) similar to 6.11.126  [P2N2]Li2(dioxane) - 2 LiCl+ NbCl3(DME)[NPNSi]Li2(dioxane)+ NbCl3(DME) - 2 LiCl NbNN PhMe2SiMe2SiPhPh ClPNbNN P R RSiSiSiSi PMe2Me2Me2Me2 Cl6.96.10R = Cy, Ph  187  Scheme 6.9: The generation of Nb dinitrogen complexes via the reaction of low-valent Nb amidophosphine complexes with N2.  The hydrogenolysis chemistry of low-valent Nb complexes is also of interest.  The addition of H2 to the [P2N2]Nb alkyl complex 6.14 (derived from 6.10) generates a hydride species capable of the catalytic hydrogenation of arenes, as shown in Scheme 6.10.325  Although it was not isolated, the intermediacy of a Nb hydride was inferred by the isolation of the Nb(η5-arene) complex (6.15), presumably formed via the addition of a hydride to benzene or toluene.  Complex 6.15 can also be formed via the reduction of 6.10 with KC8, in the presence of H2 and the appropriate arene substrate.   Nb NbCl ClNN [NPNSi][NPNSi]0.5N2 (4 atm.)[NPNSi]NbCl6.9 6.116.12[P2N2]NbCl [P2N2]NbMeMeMgCl- MgCl26.10 Nb NbMe MeNN [P2N2][P2N2]0.5 6.13 N2  188  Scheme 6.10: The hydrogenolysis chemistry of some low-valent Nb complexes supported by the [P2N2] ligand set.  The synthetic methodologies presented above featuring NbCl3(DME) (1.14) could easily be extended to the [PhNPN*] ligand set, and potentially result in new Nb hydride or dinitrogen complexes, as shown in Scheme 6.11. NbNN P PhPhSiSiSiSi P[P2N2]NbCl [P2N2]NbCH2SiMe3H2C6H5RKC8, H2C6H5R R = H, Me6.10 R 6.14, H2 RMe2Me2Me2Me2 RH 6.14LiCH2SiMe3- LiClR = H, Me6.15  189  Scheme 6.11: Potential synthesis and reactivity of a low-valent [PhNPN*]Nb complex, synthesized from NbCl3(DME).  6.2.3 Investigating the Reactivity of ([PhNPN*]Ta)2(µ-H)4 (5.4) As has been noted above, the Ta(IV) tetrahydride complex synthesized in Chapter 5, ([PhNPN*]Ta)2(µ-H)4 (5.4), although structurally similar to ([NPNSi]Ta)2(µ-H)4 (1.71), is unlike the latter complex in that it does not reduce N2 gas and form a dinitrogen complex analogous to ([NPNSi]Ta)2(η1:η2-µ-N2)(µ-H)2 ( 1.72).  However, previous work in the Fryzuk group has demonstrated that complex 1.71 engages in a number of other transformations with a variety of small molecule substrates that are also of interest, as shown in Scheme 6.12.  For example, primary and secondary phosphines,326 hydrazines,115 terminal alkynes116 and carbon dioxide220 all undergo reduction by complex 1.71 to afford an array of organometallic products that feature [PhNPN*]K2(THF)0.52.1 [PhNPN*]Nb[PhNPN*]Nb Nb[PhNPN*]Nb NbCl ClNN [PhNPN*][PhNPN*] HHHHClNbCl3(DME) H2, KC8N2, KC8N2 [PhNPN*]Nb Nb[PhNPN*]NNMR, - MCl[PhNPN*]Nb R H2R = alkyl  190 reduced substrates as ligands.  In the case of carbon disulfide327 and carbon monoxide,219 complex 1.71 is able to convert the C1 substrate to methane (along with the degradation of the [NPNSi] ligand in the case of CO).  Revisiting these studies with complex 5.4 might provide valuable insight into the effect that the more rigid, ortho-phenylene bridged [PhNPN*] has on the reactivity of the Ta(IV) tetrahydride moiety.   191  Scheme 6.12: Reactions of the Ta tetrahydride 1.71 with a variety of small molecule substrates. Ta TaHHHH [NPNSi][NPNSi] - H2Ta TaHR2PHH [NPNSi][NPNSi] Ta TaRPHH [NPNSi][NPNSi]H2PR- 2 H2HPR2Ta TaHHNNHH [NPNSi][NPNSi] H2N-NH2- 2 H2 Ta TaHHC C [NPNSi][NPNSi] RHRC CH- H2Ta TaHHO O [NPNSi][NPNSi] CH2Ta TaSS [NPNSi][NPNSi] CS2- CH4Ta TaHHHH [NPNSi][NPNSi] CO PhTa Ta PNNPh SiMe2SiMe2OH[NPNSi]- CH4CO2  192 6.3 Final Conclusions Comparing the reactivity, or lack thereof, of the dinuclear Ta(IV) tetrahydride complexes 1.71 and 5.4 with N2 illustrates how difficult it can be to anticipate the effect that changes in ligand structure will have on the reactivity of the resulting complexes.  A recent, and perhaps more dramatic, demonstration of this fact can be found in comparing the reactivity of two structurally similar zirconocene dichloride complexes (η5-C5Me5)2ZrCl2 (6.13)328,329 and (η5-C5Me4H)2ZrCl2 (6.14).330,331 As shown in Scheme 6.13, while the reduction of either 6.13 or 6.14 in the presence of N2 results in the formation of a dinuclear zirconocene dinitrogen complex (6.15 or 6.16, respectively), the seemingly minor alteration of the substituents of the cyclopentadienyl ligand from pentamethyl (C5Me5) to tetramethyl (C5Me4H) leads to products whose structure and reactivity are markedly different.  Complex 6.15 features three dinitrogen units bound in an end-on fashion to the two zirconium centres as both terminal and bridging ligands; exposure of 6.15 to H2 results in the dissociation of the weakly bound N2 units and formation of a zirconocene dihydride complex.  In contrast, complex 6.16 features a side-on bound N2 unit bridging the two zirconium atoms; instead of displacing the N2 ligand, treating 6.16 with excess H2 leads to the formation of a diazenido moiety, and ultimately an equivalent of ammonia (NH3).   The differences in the reactivity of complexes 6.13 – 6.16 are truly surprising, and demonstrate that the effect of even (seemingly) small changes to ancillary ligands is difficult to predict.  Similarly, it was hard to anticipate how modifications made to the silyl-bridged [NPNSi] ligand might affect the reactivity of potential analogues of 1.71.  Indeed, while the synthesis of the ortho-phenylene bridged [PhNPN*] analogue was undertaken with laudable intentions – namely, to increase the stability of [NPNSi] by replacing the reactive N-Si bond in the ligand   193 backbone – the fact that the resulting Ta tetrahydride complex 5.4 does not react with N2 underlines the occasionally capricious nature of this chemistry, and reflects the challenges that organometallic chemists face when trying to tailor the steric and electronic characteristics of ligands to achieve specific synthetic goals.  Scheme 6.13: The divergent reactivity displayed by two structurally similar zirconocene dinitrogen complexes with H2.  Zr ClCl [Zr] NN NN [Zr]NN[Zr] HHZr ClCl [Zr] NN [Zr][Zr] HNNH [Zr]HH[Zr] HH+ NH3H2- N2Na/Hg,N2Na/Hg,N2 H2H2, Δ6.136.14 6.156.16  194 Chapter  7: Experimental Details 7.1 General Procedures 7.1.1 Laboratory Equipment and Procedures Unless otherwise noted, all experiments were conducted by means of standard Schlenk line techniques or in a glovebox (Innovative Technology) equipped with a freezer (-35 °C), under an atmosphere of dry oxygen-free dinitrogen, using oven-dried (200 °C) glassware cooled under dynamic vacuum.  7.1.2 Solvents Anhydrous hexanes, toluene, diethyl ether and tetrahydrofuran were purchased from Aldrich, sparged with dinitrogen and dried further by passage through towers containing activated alumina and molecular sieves.  Pentane was refluxed over sodium benzophenone ketal, distilled under positive Ar pressure and degassed via several freeze-pump-thaw cycles.  THF-d8 and C6D6 were stirred over sodium benzophenone ketal, vacuum transferred and freeze-pump-thaw degassed; toluene-d8 and pyridine-d5 were stirred over activated molecular sieves and freeze-pump-thaw degassed.  Gaseous reagents (H2, D2, CO2, 13CO2) were dried by passage through a trap containing activated molecular sieves prior to use.  7.1.3 Starting Materials and Reagents  B(C6F5)3, [CPh3][B(C6F5)4], 2,6-dimethylphenyl isocyanide and EtMgCl (2.0 M in Et2O) were purchased from commercial sources and used as received. KBEt3H (1.0 M in THF) was purchased from Aldrich, evaporated to dryness and used as a solid; KBEt3H was also prepared by stirring a THF solution of BEt3 over KH, then filtering and evaporating the filtrate to dryness.   195 Phenylacetylene was purchased from Aldrich, distilled, degassed and stored over molecular sieves; phenylacetylene-d1 was prepared by treating dry phenylacetylene with 1 equivalent of nBuLi and quenching with an excess of DCl (35 % w/w in D2O).  Hydrogen and carbon dioxide gases were obtained from Praxair; D2 (HD 0.4%) and 13CO2 (99%, <1% 18O) were obtained from Cambridge Isotope Laboratories.  Benzyl potassium,332 TaMe3Cl2,333 PIPP azide,334,335 Ta(hexyne)Cl3(DME),32 Ta(BTA)Cl3(DME),31 [PhNPN*]Li2(dioxane) and [PhNPN*]H2109 were prepared according to literature methods.  7.1.4 Instrumentation and Methods of Analysis NMR spectra were recorded on a Bruker AV-400 MHz or AV-300 MHz spectrometer.  Except where noted, all spectra were recorded at room temperature.  1H NMR spectra were referenced to residual proton signals in C6D6 (δ 7.16), toluene-d8 (δ 2.09) or THF-d8 (δ 1.73). 31P{1H} NMR spectra were referenced to an external sample of P(OMe)3 (δ 141.0 with respect to 85% H3PO4 at δ 0.0). 13C{1H} NMR spectra were referenced to the solvent resonances of C6D6 (δ 128.06), toluene-d8 (δ 20.9) or THF-d8 (δ 25.31). 11B{1H} NMR chemical shifts were referenced to an external sample of BF3OEt2 (50% in CHCl3) at δ 0.0. 19F{1H} NMR chemical shifts were referenced to an external sample of neat CFCl3 at δ 0.0.   FT-IR spectra were recorded on a Perkin Elmer Frontier Spectrometer with Attenuated total reflectance.  Samples (powders or crystalline solids) were coated in nujol to prevent oxidation, and nujol backgrounds were subtracted from the FT-IR spectra. Elemental analyses (EA) were performed using a FISONS Elemental Analyzer 1108 by Mr. David Wong or Mr. Derek Smith at the Department of Chemistry, University of British Columbia.   196 Electron ionization – mass spectrometry (EI-MS) analyses were performed using a Kratos MS-50 spectrometer (70 eV source) by Mr. Marshall Lapawa at the Department of Chemistry, University of British Columbia. Single crystal X-ray diffraction data was collected on a Bruker (X8 or DUO) Apex II diffractometer equipped with a graphite (X8) or TRIUMPH (DUO) monochromator, using Mo Kα radiation (λ = 0.71073 Å), and integrated using the Bruker SAINT software package.336  Suitable single crystals were selected, coated in Fomblin oil and mounted on a glass fiber.  All absorption corrections were performed using the multi-scan technique (SADABS).337  Structures were solved by direct methods and refined using the SHELX-97338 and WinGX (version 1.80.05)339 software packages.  All non-hydrogen atoms were refined anisotropically unless otherwise noted.  All hydrogen atoms were placed in calculated positions and assigned to an isotropic displacement parameter, unless otherwise noted; hydrogen atoms so specified were located in the difference map and were refined isotropically.  All X-ray crystallographic data was collected by the author (KP) or Dr. Nathan Halcovitch, with assistance from Dr. Brian Patrick.  All structures (except 5.3 and 5.4) were solved and refined by the author (KP), with assistance from Dr. Brian Patrick; the structures of 5.3 and 5.4 were solved and refined by Dr. Nathan Halcovitch. Tables containing crystallographic unit cell and refinement details are located in Appendix C.       197 7.2 Synthesis of Compounds 7.2.1 Complexes Pertaining to Chapter 2 [PhNPN*]K2(THF)0.5 (2.1) At room temperature, 30 mL of THF was added to [PhNPN*]H2 (1.00 g, 1.80 mmol) to give a clear, pale yellow solution. Solid benzyl potassium (468 mg, 3.60 mmol) was added to the solution and the mixture was left to stir for 30 mins. The resulting bright yellow solution was evaporated to dryness, and then triturated with 30 mL of hexanes to generate a bright yellow solid.  This solid was collected on a sintered-glass frit and was washed with hexanes (3 x 30 mL) and dried in vacuo to yield 1.10 g (1.74 mmol, 97%).  Samples for NMR spectroscopy were prepared in toluene-d8, with a drop for pyridine-d5 for additional solubility.   1H NMR (toluene-d8, 300 MHz): δ = 8.06 (bs, 2H), 7.2 – 6.9 (overlapping resonances, 9H plus residual toluene-d8), 6.66 (d, JHH = 8 Hz, 2H), 6.07 (dd, JHP = 6 Hz, JHH = 6 Hz, 2H) (ArH), 3.55 (THF, 2H), 2.36 (s, 6H), 2.12 (bs, 12H), 2.04 (s, 6H) (ArCH3), 1.48 (THF, 2H). 31P{1H} NMR (toluene-d8, 120 MHz): δ = -25.7 (s). 13C{1H} NMR (toluene-d8, 75 MHz): δ =159.3 (d, JCP = 21 Hz), 152.79, 142.65 (d, JCP = 12 Hz), 136.11, 134.8 (d, JCP = 17 Hz) 131.92, 131.37, 130.52, 130.38, 129.56, 128.28, 126.20, 127.05, 117.68, 115.58, 111.50 (ArC), 67.7, 25.82 (THF), 21.05, 20.59, 20.12, 19.86 (ArCH3). Elemental analysis of 2.1 was hampered by its pronounced air sensitivity. Despite several attempts, results that were significantly low in carbon were found.  The data for one representative attempt is reported: Anal. Calcd. for C80H86K4N4O2P2: C, 71.82; H, 6.48; N, 4.19; Found: C, 63.68; H, 6.47; N, 5.40. [PhNPN*]Ta(3-hexyne)Cl (2.2)  A 200 mL Kontes-seal glass reactor was charged with a magnetic stir bar, 2.1 (3.57 g, 5.65 mmol), Ta(3-hexyne)Cl3(DME) (2.60 g, 5.66 mmol) and 60 mL THF.  The resulting dark   198 orange-brown solution was stirred at 54 °C for 36 h, during which the formation of a light-coloured precipitate was observed.  This suspension was filtered through a pad of silica on a sintered glass frit, and the filtrate was evaporated to dryness in vacuo to afford a dark orange powder.  This powder was triturated with 30 mL of pentane and cooled to -35 °C, whereupon a yellow precipitate formed.  This dark yellow solid was collected on a frit and washed with cold pentane (2 x 10 mL) to afford 3.47 g (4.08 mmol, 72%).  1H NMR (C6D6, 300 MHz, 298 K): δ = 7.62 (bd, JHP = 7.8 Hz, 2H), 7.52 (m, 2H), 7.05 (m, 3H), 6.93 (s, 2H), 6.84 (bd, JHH = 8.7 Hz, 2H), 6.70 (s, 2H), 6.09 (dd, JHP = 5 Hz, JHH = 8.7 Hz, 2H) (ArH), 2.95 (bs, 4H, hexyne CH2), 2.58 (s, 6H), 2.07 (s, 6H), 2.01 (s, 6H), 1.76 (s, 6H) (ArCH3), 1.03 (bs, 6H, hexyne CH3). 1H NMR (toluene-d8, 400 MHz, 298 K): δ = 7.57 (bd, JHP = 7.8 Hz, 2H), 7.44 (m, 2H), 7.1 – 6.9 (overlapping signals, approx. 3 aromatic protons and residual toluene-d8 protons), 6.87 (s, 2H), 6.77 (bd, JHH = 8.7 Hz, 2H), 6.63 (s, 2H), 5.98 (dd, JHP = 5 Hz, JHH = 8.7 Hz, 2H) (ArH), 2.90 (bs, 4H, hexyne CH2), 2.51 (s, 6H), 2.05 (s, 6H), 1.99 (s, 6H), 1.68 (s, 6H) (ArCH3), 0.98 (bs, 6H, hexyne CH3). 1H NMR (toluene-d8, 400 MHz, 243 K): δ = 7.60 (bd, JHP = 7.8 Hz, 2H), 7.47 (m, 2H), 7.1 – 6.9 (overlapping signals, approx. 3 aromatic protons and residual toluene-d8 protons), 6.87 (s, 2H), 6.75 (bd, JHH = 8.7 Hz, 2H), 6.64 (s, 2H), 6.05 (dd, JHP = 5 Hz, JHH = 8.7 Hz, 2H) (ArH), 3.49 (q, 2H, JHH = 7 Hz, hexyne CH2), 2.59 (s, 6H, ArCH3), 2.33 (q, 2H, JHH = 7 Hz, hexyne CH2), 2.05 (s, 6H), 1.99 (s, 6H), 1.73 (s, 6H) (ArCH3), 1.27 (t, 3H, JHH = 7 Hz), 0.79 (t, 3H, JHH = 7 Hz) (hexyne CH3). 1H NMR (toluene-d8, 400 MHz, 343 K): δ = 7.57 (bd, JHP = 7.8 Hz, 2H), 7.44 (m, 2H), 7.1 – 6.9 (overlapping signals, approx. 3 aromatic protons and residual toluene-d8 protons), 6.87 (s, 2H), 6.77 (bd, JHH = 8.7 Hz, 2H), 6.63 (s, 2H), 5.98 (dd, JHP = 5 Hz, JHH = 8.7 Hz, 2H) (ArH), 2.90 (bs, 4H, hexyne CH2), 2.51 (s, 6H), 2.05 (s, 6H), 1.99 (s, 6H), 1.68 (s, 6H) (ArCH3), 0.98 (bs, 6H, hexyne CH3).   199 31P{1H} NMR (C6D6, 120 MHz): δ = 32.4 (s). 13C{1H} NMR (C6D6, 75 MHz): d = 163.10 (d, JCP = 23 Hz), 139.25, 138.48, 136.9 (d, JCP = 32 Hz), 136.1 (d, JCP = 10 Hz), 135.27, 134.19, 132.81 (d, JCP = 9 Hz), 131.15, 130.42, 129.9, 129.6, 129.2, 125.7, 121.8 (d, JCP = 32 Hz), 115.5 (d, JCP = 8 Hz) (ArC), 29.2 (hexyne CH2), 21.12, 20.28, 20.19, 18.66 (ArCH3), 14.1 (hexyne CH3). 13C{1H} NMR (toluene-d8, 100 MHz, 243 K): δ = 200.6, 182.1 (d, JCP = 2 Hz) (hexyne EtC≡CEt), 163.10 (d, JCP = 32 Hz), 139.05, 137.4, 135.8 (d, JCP = 10 Hz), 135.27, 134.6 (d, JCP = 32 Hz), 134.19, 132.81 (d, JCP = 9 Hz), 131.15, 130.42, 129.9, 129.6, 129.2, 125.6, 121.8 (d, JCP = 32 Hz), 115.3 (d, JCP = 8 Hz) (ArC), 30.6, 26.7 (hexyne CH2), 21.05, 20.17, 20.165, 18.6 (ArCH3), 15.7, 13.7 (hexyne CH3). Anal. Calcd. for C44H49Cl1N2P1Ta1: C, 61.94; H, 5.79; N, 3.28; Found: C, 61.89; H, 5.87; N, 3.10.  [PhNPN*]Ta(BTA)Cl (2.3)  A 200 mL round-bottom Schlenk flask was charged with a magnetic stir bar, 2.1 (3.79 g, 6.00 mmol), Ta(BTA)Cl3(DME) (2.92 g, 5.99 mmol) and 60 mL THF.  The resulting dark brown solution was stirred at room temperature for 18 h, during which the formation of a light-coloured precipitate was observed.  This suspension was filtered through a pad of silica on a sintered glass frit, and the filtrate was evaporated to dryness in vacuo to afford a dark brown powder.  This powder was triturated with 30 mL of pentane and cooled to -35 °C, whereupon an orange precipitate formed.  This bright orange solid was collected on a frit and washed with cold pentane (2 x 10 mL) to afford 3.78 g (4.01 mmol, 68%).  1H NMR (C6D6, 300 MHz): δ = 7.62 (bd, JHP = 8 Hz, 2H), 7.55 (m, 2H), 7.05 (m, 3H), 6.93 (s, 2H), 6.80 (bd, JHH = 8 Hz, 2H), 6.69 (s, 2H), 6.05 (dd, JHP = 5 Hz, JHH = 9 Hz, 2H) (ArH), 2.48 (s, 6H), 2.12 (s, 6H), 1.98 (s, 6H), 1.73 (s, 6H) (ArCH3), 0.16, (s, 9H) 0.08 (s, 9H) (Si(CH3)3). 31P{1H} NMR (C6D6, 120 MHz): δ = 29.7 (s). 13C{1H} NMR (C6D6, 75 MHz): δ = 225.3, 205.4   200 (d, JCP = 9 Hz) (TMSC≡CTMS), 161.9 (d, JCP = 30 Hz), 139.5 (d, JCP = 4 Hz), 138.8, 137.3 (d, JCP = 39 Hz), 136.4, 135.2, 134.9 (d, JCP = 2 Hz), 134.2 (d, JCP = 5 Hz), 134.1 (d, JCP = 4 Hz), 131.3, 130.4 (d, JCP = 2 Hz), 130.0 (d, JCP = 5 Hz), 129.7, 128.8 (d, JCP = 10 Hz), 122.6 (d, JCP = 41 Hz), 115.2 (d, JCP = 10 Hz) (ArC), 21.0, 20.5, 20.3, 20.1 (ArCH3), 2.8, 2.0 (SiCH3). Anal. Calcd. for C46H57ClN2PSi2Ta: C, 58.68; H, 6.10; N, 2.98; Found: C, 58.87; H, 6.37; N, 3.30. [PhNPN*]Ta(3-hexyne)H (2.4)  At room temperature, solid KHBEt3 (164 mg, 1.22 mmol) was added at once to a stirring toluene solution (20 mL) of 2.2 (1.03 g, 1.20 mmol).  The resulting dark brown solution was stirred for 3 h, during which the formation of a light-coloured precipitate was observed.  This suspension was filtered through a pad of silica on a sintered glass frit, and the filtrate was evaporated to dryness in vacuo to afford a dark brown residue.  This residue was triturated with 20 mL of pentane and cooled to -35 °C, whereupon a tawny-brown precipitate formed.  This tawny solid was collected on a frit and washed with cold pentane (2 x 5 mL) to afford 589 mg (0.72 mmol, 61%).  1H NMR (C6D6, 400 MHz): δ = 21.6 (d, JHP = 34.8 Hz, 1H, TaH), 7.71 (d, JHP = 7 Hz, 2H), 7.62 (dd, JHP = 9 Hz, JHH = 6 Hz, 2H), 7.12 (m, 3H), 6.93 (s, 2H), 6.86 (d, JHH = 8 Hz, 2H), 6.67 (s, 2H), 6.00 (dd, JHP = 6 Hz, JHH = 8 Hz, 2H) (ArH), 3.10 (q, JHH = 7 Hz, 2H), 2.78 (q, JHH = 7 Hz, 2H) (hexyne CH2), 2.68 (s, 6H), 2.10 (s, 6H), 2.01 (s, 6H), 1.79 (s, 6H) (ArCH3) 1.16 (t, JHH = 7 Hz, 3H), 0.77 (t, JHH = 7 Hz, 3H) (hexyne CH3). 31P{1H} NMR (C6D6, 160 MHz): δ = 20.1 (s). 13C{1H} NMR (C6D6, 75 MHz): δ = 205. 3 (d, JCP = 4 Hz), 184.2 (d, JCP = 11 Hz) (EtC≡CEt), 162.80 (d, JCP = 34 Hz), 141.8, 137.0, 136.3 (JCP = 35 Hz) 135.2, 134.8, 133.4 (d, JCP = 6 Hz), 132.2 (d, JCP = 12 Hz) 129.9 (d, JCP = 3 Hz), 129.69, 129.67, 129.65, 129.60, 129.2 (d, JCP = 9 Hz), 121.81 (d, JCP = 37 Hz), 115.8 (d, JCP = 11 Hz) (ArC), 30.54 (d, JCP = 4.9 Hz), 30.40 (JCP = 2.8 Hz) (hexyne CH2), 21.43, 20.81, 20.34, 18.72 (ArCH3), 15.75, 14.21 (hexyne CH3). EI-MS   201 (m/z): 817 (100, [M – H]+), 735 (40, [Ta{PhNPN*}]+), 541 (20, [{PhNPN*} – Me]+). Anal. Calcd. for C44H50N2PTa: C, 64.54; H, 6.15; N, 3.42;. Found: C, 62.71; H, 5.83; N, 3.98  [PhNPN*]Ta(BTA)H (2.5)  At room temperature, solid KHBEt3 (149 mg, 1.10 mmol) was added at once to a stirring toluene solution (20 mL) of 2.3 ( 1.04 g, 1.1 mmol).  The resulting dark red-brown solution was stirred for 3 h, during which the formation of a light-coloured precipitate was observed.  This suspension was evaporated to dryness in vacuo to afford a dark red residue.  This residue was dissolved in a mixture of HMDSO (20 mL) and pentane (5 mL), cooled to -35 °C, and then filtered through a pad of silica on a sintered glass frit.  The resulting filtrate was evaporated to dryness in vacuo to afford 581 mg (0.64 mmol, 58 %).  1H NMR (C6D6, 300 MHz): δ = 20.6 (d, JHP = 35 Hz, 1H, TaH), 7.72 (d, JHP = 7 Hz, 2H), 7.62 (dd, JHP = 8 Hz, JHH = 7 Hz, 2H), 7.02 (m, 3H), 6.91 (s, 2H), 6.82 (bd, JHH = 8 Hz, 2H), 6.61 (s, 2H), 5.92 (dd, JHP = 5 Hz, JHH = 8 Hz, 2H) (ArH), 2.67 (s, 6H), 2.12 (s, 6H), 1.98 (s, 6H), 1.64 (s, 6H) (ArCH3), 0.13, (s, 9H), -0.1 (s, 9H) (Si(CH3)3).31P{1H} NMR (C6D6, 120 MHz): δ = 16.3 (s).13C{1H} NMR (C6D6, 100 MHz): δ = 220.3, 193.2 (d, JCP = 5 Hz) (TMSC≡CTMS), 161.9 (d, JCP = 32 Hz), 141.8, 137.6, 136.5, 135.7, 134.9 (d, JCP = 2 Hz), 134.7 (d, JCP = 3 Hz), 133.3 (d, JCP = 12 Hz), 133.2, 131.8, 130.0 (d, JCP = 5 Hz), 129.9 (d, JCP = 5 Hz), 129.6 (d, JCP = 9 Hz), 128.8 (d, JCP = 9 Hz), 123.2 (d, JCP = 35 Hz), 115.3 (d, JCP = 10 Hz) (ArC), 21.1, 20.9, 20.4, 19.5 (ArCH3), 2.3, 0.4 (SiCH3). Anal. Calcd. for C46H58N2PSi2Ta: C, 60.91; H, 6.45; N, 3.09. Found: C, 57.10; H, 7.46; N, 2.83  [PhNPN*]Ta(3-hexyne)CH2Ph (2.6) A stirring slurry of benzyl potassium (180 mg, 1.38 mmol) in toluene (10 mL) was cooled to -35 °C.  A solution of 2.2 (1.19 g, 1.39 mmol) in toluene (10 mL) was added via canula over   202 approximately 10 minutes.  The resulting dark brown solution was warmed to room temperature and stirred for 48 h, during which the formation of a light-coloured precipitate was observed.  This suspension was filtered through a pad of silica on a sintered glass frit, and the filtrate was evaporated to dryness in vacuo to afford a dark brown residue.  This residue was triturated with 20 mL of pentane and cooled to -35 °C, whereupon a yellow precipitate formed.  A yellow solid was collected on a frit and washed with cold pentane (2 x 10 mL) to afford 860 mg (0.95 mmol, 68%).  1H NMR (C6D6, 400 MHz): δ = 7.54 (ddd, JHP = 11 Hz, JHH = 2, 8 Hz, 2H), 7.46 (dd, JHP = 8 Hz, JHH = 2 Hz, 2H), 7.05 (m, 5H), 6.93 (d, JHH = 8 Hz, 2H), 6.86 (d, JHH = 8 Hz, 4H), 6.79 (t, JHH = 8 Hz, 3H), 6.07 (dd, JHP = 5.6 Hz, JHH = 8.4 Hz, 2H) (ArH), 2.75 (d, JHP = 7.2 Hz, 2H, CH2Ph), 2.22 (q, JHH = 7.2 Hz, 4H, hexyne CH2), 2.28 (s, 6H), 2.21 (s, 6H), 2.16 (s, 6H), 1.93 (s, 6H) (ArCH3) 0.65 (t, JHH = 7.2 Hz, 6H, hexyne CH3). 31P{1H} NMR (C6D6, 160 MHz): δ = 26.5 (s). 13C{1H} NMR (C6D6, 100 MHz): δ = 163.6 (d, JCP = 33 Hz), 151.4 (d, JCP = 7 Hz), 151.3 (d, JCP = 4 Hz), 135.4 (d, JCP = 2 Hz), 134.8 (d, JCP = 2 Hz), 134.1, 134.0 (d, JCP = 21 Hz) 133.2 (d, JCP = 11 Hz), 133.0, 132.6, 130.2 (d, JCP = 2 Hz), 129.9 (d, JCP = 4 Hz), 129.5 (d, JCP = 5 Hz), 129.2 (d, JCP = 9 Hz), 128.4, 128.1, 127.6, 122.5, 117.8 (d, JCP = 38 Hz), 115.7 (d, JCP = 12 Hz) (ArC), 75.5 (d, JCP = 4 Hz, CH2Ph), 29.9 (hexyne CH2), 20.8, 20.1, 19.7, 19.5 (ArCH3), 13.2 (hexyne CH3). Anal. Calcd. for C51H56N2PTa: C, 67.39; H, 6.21; N, 3.08; Found: C, 59.67; H, 5.67; N, 3.67 [PhNPN*]Ta(BTA)CH2Ph (2.7) A slurry of benzyl potassium (200 mg, 1.53 mmol) in toluene (10 mL) was cooled to -35 °C and stirred vigorously.  To this, a solution of 2.3 (1.44 g, 1.53 mmol) in toluene (10 mL) was added via canula over approximately 10 minutes.  The resulting dark red solution was warmed to room   203 temperature and stirred for 48 h, during which the formation of a light-coloured precipitate was observed.  This suspension was filtered through a pad of silica on a sintered glass frit, and the filtrate was evaporated to dryness in vacuo to afford a dark red residue.  This residue was triturated with 20 mL of pentane and cooled to -35 °C, whereupon a red precipitate formed.  A maroon solid was collected on a frit and washed with cold pentane (2 x 10 mL) to afford 920 mg (0.92 mmol, 60%).  1H NMR (C6D6, 300 MHz): δ = 7.62 (m, 4H), 7.05 (m, 3H), 6.90 (t, JHH = 7 Hz, 3H), 6.81 (bs, 1H), 6.79 (bs, 1H), 6.67 (s, 2H), 6.55 (d, JHH = 7 Hz, 2H), 6.49 (s, 2H), 5.97 (dd, JHP = 5.4 Hz, JHH = 8.4 Hz, 2H) (ArH), 3.22, (s, 2H, CH2Ph) 2.33 (s, 6H), 2.01 (s, 6H), 1.97 (s, 6H), 1.80 (s, 6H) (ArCH3), 0.1 (s, 18H) (Si(CH3)3). 31P{1H} NMR (C6D6, 160 MHz): δ = 26.2 (s). 13C{1H} NMR (C6D6, 100 MHz):  δ = 163.6 (d, JCP = 32 Hz), 148.5, 142.1 (d, JCP = 4 Hz), 136.8, 136.4 (d, JCP = 32 Hz), 135.1, 135.0 (d, JCP = 38 Hz), 134.3, 134.2, 133.7, 130.4 (d, JCP = 34 Hz), 129.9, 129.1 (d, JCP = 5 Hz), 128.6 (d, JCP = 9 Hz), 128.4, 128.1, 126.5, 122.7, 120.6 (d, JCP = 38 Hz), 115.6 (d, JCP = 11 Hz), 89.8 (d, JCP = 16 Hz, CH2Ph), 20.9, 20.4, 20.2, 19.3 (ArCH3), 2.2 (SiCH3). Anal. Calcd. for C53H64N2PSi2Ta: C, 63.84; H, 6.47; N, 2.81; Found: C, 65.81; H, 6.56; N, 2.63  [PhNPN*]Ta(BTA)CH2CH3 (2.8)  Method A: A stirring solution of 2.3 (500 mg, 0.53 mmol) in 20 mL of Et2O was cooled to -35 °C.  To this dark red solution, 0.27 mL of EtMgCl in Et2O (2.0 M, 0.54 mmol) was added by syringe over approximately 5 minutes.  The reaction mixture was allowed to warm up to room temperature and left to stir for 4 h, during which time the reaction mixture darkened in colour and became slightly cloudly.  After 4 h, the reaction mixture was evaporated to dryness in vacuo to yield a red-coloured foam, which was then dissolved in 20 mL of hexanes and filtered through   204 a pad of silica gel on a sintered-glass frit.  This filtrate was evaporated to dryness in vacuo to yield 360 mg (0.38 mol, 73 %) of red powder. Method B: A 50 mL Kontes-seal glass reactor was charged with a magnetic stir bar, 2.4 (100 mg, 0.11 mmol) and 10 mL toluene.  This dark red solution was degassed via three freeze-pump-thaw cycles, and then 1 atm of ethylene was introduced at room temperature.  The solution was left to stir for 1 h, and was then evaporated to dryness in vacuo to yield 95 mg (0.10 mmol, 93 %) of red powder.  1H NMR (C6D6, 400 MHz): δ = 7.59 (m, 4H), 7.05 (m, 4H), 6.93 (s, 2H), 6.81 (d, JHH = 8 Hz, 1H), 6.80 (s, 2H), 5.99 (dd, JHP = 5 Hz, JHH = 8 Hz, 2H) (ArH), 2.33 (s, 6H), 2.17 (s, 6H), 1.97 (s, 6H), 1.82 (s, 6H) (ArCH3), 1.73 (q, JHH = 6 Hz, 2H, TaCH2CH3), 1.25 (t, 3H, JHH = 6 Hz, TaCH2CH3), 0.17 (bs, 9H), 0.08 (bs, 9H) (SiCH3). 31P{1H} NMR (C6D6, 160 MHz): δ = 27.7 (s). 13C{1H} NMR (C6D6, 100 MHz):  δ = 164.0 (d, JCP = 44 Hz), 142.4 (d, JCP = 7 Hz), 137.3, 136.6 (d, JCP = 44 Hz), 135.3 (d, JCP = 29 Hz), 134.9, 134.4, 134.1 (d, JCP = 14 Hz), 130.9, 130.4, 129.7 (d, JCP = 5 Hz), 128.8 (d, JCP = 7 Hz), 128.6, 128.4, 120.3 (d, JCP = 48 Hz), 115.2 (d, JCP = 15 Hz), 82.2 (d, JCP = 22 Hz, TaCH2CH3), 21.0, 20.2, 20.0, 19.1 (ArCH3), 18.6 (TaCH2CH3), 2.38, 1.49 (SiCH3). Anal. Calcd. for C48H62N2PSi2Ta: C, 61.65; H, 6.68; N, 3.00; Found: C, 60.50; H, 7.26, N, 2.63  [PhNPNC*]Ta(BTA) (2.9) A 50 mL Kontes-seal glass reactor was charged with a magnetic stir bar, 2.7 or 2.8 (0.10 mmol) and 10 mL toluene.  This dark red solution was heated at 110 °C for 60 h; over this period, the solution became slightly lighter in colour.  The solution was evaporated to dryness in vacuo to afford a dark red residue.  This residue was triturated with 20 mL of pentane and cooled to -35   205 °C, whereupon a red precipitate formed.  A bright red solid was collected on a frit and washed with cold pentane (2 x 10 mL) to afford 81 mg (0.89 mmol, 90%).  1H NMR (C6D6, 400 MHz): δ = 7.75 (d, JHP = 7 Hz, 1H), 7.68 (d, JHP = 7 Hz, 1H), 7.58 (m, 2H), 7.11 (dt, JHH = 7 Hz, JHP = 3 Hz, 3H), 7.04 (d, JHH = 7 Hz, 1H), 7.02 (s, 1H), 6.93 (s, 1H), 6.87 (dd, JHH = 8 Hz, JHH = 2 Hz, 1H), 6.83 (dd, JHH = 8 Hz, JHH = 2 Hz, 1H), 6.70 (s, 1H), 6.65 (s, 1H), 6.43 (dd, JHP = 5.2 Hz, JHH = 8.4 Hz, 1H), 5.95 (dd, JHH = 8.4 Hz, JHP = 5.2 Hz, 1H) (ArH), 3.94 (dd, JHH = 11 Hz, JHP = 2 Hz, 1H), 2.32 (d, JHH = 11 Hz, 1H) (TaCHaHbXylyl) 2.24 (s, 3H), 2.18 (s, 3H), 2.12 (s, 3H), 2.06 (s, 3H), 2.00 (s, 6H), 1.69 (s, 3H) (ArCH3), 0.09 (s, 9H), 0.06 (s, 9H) (Si(CH3)3). 31P{1H} NMR (C6D6, 160 MHz): δ = 8.8 (s). 13C{1H} NMR (C6D6, 100 MHz): δ = 231.4, 220.3 (d, JCP = 6 Hz) (TMSC≡CTMS) 160.4 (d, JCP = 32 Hz), 157.9 (d, JCP = 28 Hz), 141.7, 139.3, 137.5 (d, JCP = 4 Hz), 137.3, 137.2, 135.7, 135.6 (d, JCP = 4 Hz), 135.55, 135.5 (d, JCP = 2 Hz), 135.2 (d, JCP = 7 Hz), 134.4 (d, JCP = 3 Hz), 133.3, 133.2, 132.4 (d, JCP = 5 Hz), 130.6, 130.3 (d, JCP = 5 Hz), 130.2, 129.5 (d, JCP = 2 Hz), 129.4, 128.9, 128.8, 128.5 (d, JCP = 6 Hz), 127.6, 126.3, 123.0 (d, JCP = 36 Hz), 119.4 (d, JCP = 38 Hz), 115.4 (d, JCP = 9 Hz), 115.1 (d, JCP = 10 Hz) (ArC) 62.6 (d, JCP = 16 Hz, TaCH2Xylyl), 21.5, 21.1, 20.5, 20.4, 20.1, 19.7, 18.5 (ArCH3), 1.41 (SiMe3). Anal. Calcd. for C46H56N2PSi2Ta: C, 61.05; H, 6.24; N, 3.10; Found: C, 61.42; H, 6.37; N, 3.06.  7.2.2 Complexes Pertaining to Chapter 3  [PhNPN*]Ta(3-hexyne)N3 (3.1) A 100 mL Kontes-seal glass reactor was charged with a magnetic stir bar, 2.2 (1.25 g, 1.46 mmol), sodium azide (95 mg, 1.46 mmol) and 25 mL THF.  This dark orange-brown solution was stirred at 70 °C for 48 h, during which the formation of a light-coloured precipitate was   206 observed.  This suspension was filtered through a pad of silica on a sintered glass frit, and the filtrate was evaporated to dryness in vacuo to afford a dark brown residue.  This residue was triturated with 20 mL of pentane and cooled to -35 °C, whereupon a yellow precipitate formed.  This solid was collected on a frit and washed with cold pentane (2 x 10 mL) to afford 879 mg (1.02 mmol, 70%).  1H NMR (C6D6, 400 MHz): δ = 7.60 (bd, JHH = 8 Hz, 2H), 7.52 (m, 2H), 7.06 (m, 5H), 6.84 (bd, JHH = 8 Hz, 2H), 6.72 (bs, 2H), 6.00 (dd, JHH = 8 Hz, JHP = 6 Hz, 2H), 3.20 (bs, 4H, hexyne CH-2), 2.50 (s, 6H), 2.11 (s, 6H), 2.00 (s, 6H), 1.74 (s, 6H) (ArCH3) 1.01 (bs, 6H, hexyne CH3). 31P NMR (C6D6, 120 MHz): δ = 29.6 (s). 13C{1H} NMR (C6D6, 100 MHz): δ = 162.9 (d, JCP = 31 Hz), 139.26, 136.2 (d, JCP = 26 Hz), 136.7 (d, JCP = 5 Hz), 136.3, 136.2 (d, JCP = 26 Hz), 135.21, 134.29, 132.5 (d, JCP = 11 Hz), 130.55, 130.19, 129.77, 129.5 (d, JCP = 5 Hz), 129.1 (d, JCP = 10 Hz), 121.5 (d, JCP = 32 Hz), 115.3 (d, JCP = 11 Hz) (ArC), 31.9 (hexyne CH2), 20.96, 20.16, 18.86, 18.34 (ArCH3), 14.3 (hexyne CH3). EI-MS (m/z): 860 (8, [M+]), 831 (5, [M – N2]+), 749 (100, [[PhNPN*]TaN]+), 734 (20, [[PhNPN*]Ta]+).  FT-IR (Nujol): ν(TaN3) 2086 cm-1.  Anal. Calcd. for C44H49N5P1Ta1: C, 61.46; H, 5.74; N, 8.15; Found: C, 61.30; H, 5.59; N, 7.81. [PhNPN*]Ta(BTA)N3  (3.2) A 100 mL Kontes-seal glass reactor was charged with a magnetic stir bar, 2.3 (1.25 g, 1.33 mmol), sodium azide (88 mg, 1.35 mmol) and 25 mL THF.  This dark red solution was stirred at 70 °C for 36 h, during which the formation of a light-coloured precipitate was observed.  This suspension was filtered through a pad of silica on a sintered glass frit, and the filtrate was evaporated to dryness in vacuo to afford a dark brown residue.  This residue was triturated with 20 mL of pentane and cooled to -35 °C, whereupon a brown precipitate formed.  This tan-brown   207 solid was collected on a frit and washed with cold pentane (2 x 10 mL) to afford 890 mg (0.94 mmol, 71%).  1H NMR (C6D6, 300 MHz): δ = 7.59 (d, JHH = 8 Hz, 2H), 7.55 (m, 2H), 7.07 (m, 5H), 6.80 (bd, JHH = 8 Hz, 2H), 6.70 (bs, 2H), 5.95 (dd, JHH = 8 Hz, JHP = 6 Hz, 2H), 2.46 (s, 6H), 2.14 (s, 6H), 1.98 (s, 6H), 1.66 (s, 6H) (ArCH3) 0.07 (s, 9H), 0.04 (s, 9H) (Si(CH3)3). 31P NMR (C6D6, 120 MHz): δ = 28.3 (s). 13C{1H} NMR (C6D6, 75 MHz): δ = 219.7, 204.3 (JCP = 8 Hz) (TMSC≡CTMS), 161.6 (d, JCP = 30 Hz), 138.8, 137.6 (JCP = 5 Hz), 137.0 (d, JCP = 40 Hz), 136.4, 135.6, 134.9, 134.2 (d, JCP = 10 Hz), 131.0, 130.3, 130.0, 129.8 (d, JCP = 6 Hz), 129.6, 128.7 (d, JCP = 10 Hz), 122.5 (d, JCP = 40 Hz), 115.1 (d, JCP = 11 Hz) (ArC), 21.0, 20.2, 19.5, 18.6 (ArCH3) 2.64, 0.68 (Si(CH3)3). FT-IR (Nujol): ν(TaN3) 2104 cm-1. Anal. Calcd. for C46H57N5P1Si2Ta1: C, 58.28; H, 6.06; N, 7.39; Found: C, 53.89; H, 5.62; N, 7.76. [PhNPN*]Ta(=N-p-C6H4iPr)Cl (3.3) At room temperature, p-isopropylphenyl azide (70 mg, 0.43 mmol in 2 mL of toluene) was added to a toluene solution (10 mL) of 2.2 or 2.3 (0.41 mmol), and the mixture is left to stir at room temperature for 24 h.  The resulting bright yellow slurry was evaporated to dryness in vacuo, triterated with 10 mL of pentane and filtered through a sintered glass frit. The bright yellow solid thus collected was dried in vacuo to yield 334 mg (0.37 mmol, 93%).  1H NMR (C6D6, 400 MHz): δ = 7.51 (ddd, JHH = 13 Hz, JHH = 8 Hz, JHP = 2 Hz, 2H), 7.43 (dd, JHH 9 = Hz, JHP = 2 Hz, 2H), 7.06 (m, 2H), 6.97 (bs, 2H), 6.88 (m, 3H), 6.79 (d, 2H, JHH =8 Hz ), 6.71 (bs, 2H), 6.30 (d, JHH = 8 Hz, 2H), 6.24 (d, JHH = 8 Hz, 2H) (ArH), 2.56 (sept, JHH = 7 Hz, 1H, (CH)Me2), 2.54 (s, 6H), 2.43 (s, 6H), 2.07 (s, 6H), 1.99 (s, 6H) (ArCH3), 1.02 (d, JHH = 7 Hz, 6H, (CH)Me2). 31P{1H} NMR (C6D6, 160 MHz): δ = 30.9 (s). 13C{1H} NMR (C6D6, 100 MHz): δ = 165.0 (d, JCP = 30 Hz), 157.1, 141.9, 139.9 (d, JCP = 6 Hz), 139.0, 137.4, 136.8, 136.1,   208 134.9 (d, JCP = 50 Hz), 134.3, 132.0, 131.0, 130.7, 130.05 (d, JCP = 7 Hz), 129.5 (d, JCP = 10 Hz), 128.3, 126.0, 123.7, 117.6 (d, JCP = 50 Hz), 116.4 (d, JCP = 11 Hz) (ArC), 33.8 (CHMe2), 24.29 (CHMe2), 21.15, 20.25, 19.41, 19.14 (ArCH3). EI-MS (m/z): 903 (75, [M]+), 769 (100, [2.2 - hexyne]+). Anal. Calcd. for C47H50Cl1N3P1Ta1: C, 62.42; H, 5.57; N, 4.65; Found: C, 61.69; H, 5.82; N, 5.39 [PhNPN*]Ta(=N-p-C6H4iPr)N3 (3.4) Method A: A 100 mL Kontes-seal glass reactor was charged with a magnetic stir bar, 3.1 or 3.2 (0.17 mmol), p-isopropylphenyl azide (30 mg, 0.19 mmol) and 20 mL of toluene.  This mixture was stirred at 100 °C for 18 h, after which the resulting orange solution was evaporated to dryness in vacuo to yield a dark orange/red residue.  This residue was triturated with ~15 mL of pentane to afford a light orange powder, which was collected on a sintered-glass frit and washed with cold pentane (2 x 5 mL) to afford 131 mg (0.14 mmol, 82%).  Method B: A 100 mL Kontes-seal glass reactor was charged with a magnetic stir bar, 3.4 (150 mg, 0.16 mmol), sodium azide (11 mg, 0.16 mmol) and 20 mL of THF.  This mixture was stirred at 80 °C for 18 h, after which the resulting orange solution was filtered through a pad of silica on a sintered glass frit.  The filtrate was evaporated to dryness in vacuo to afford a dark orange residue.  This residue was triturated with 20 mL of pentane and cooled to -35 °C, whereupon an orange precipitate formed.  This solid was collected on a frit and washed with cold pentane (2 x 5 mL) to afford 113 mg (0.12 mmol, 75%).  1H NMR (C6D6, 300 MHz): δ = 7.63 (ddd, JHH = 13 Hz, JHH = 8 Hz, JHP = 2 Hz, 2H), 7.43 (dd, JHH 9 = Hz, JHP = 2 Hz, 2H), 7.09 (m, 3H), 7.07 (bs, 2H), 6.88 (d, 2H, JHH =8 Hz), 6.78 (d, 2H, JHH =8 Hz), 6.74 (bs, 2H), 6.27 (d, JHH = 8 Hz, 2H), 6.15 (dd, JHH = 8 Hz, JHP = 6 Hz, 2H) (ArH), 2.59 (sept, JHH = 7 Hz, 1H, (CH)Me2), 2.47 (s, 6H), 2.43 (s, 6H), 2.10 (s, 6H), 1.99 (s,   209 6H) (ArCH3), 1.02 (d, JHH = 7 Hz, 6H, (CH)Me2). 31P{1H} NMR (C6D6, 120 MHz): δ = 27.3 (s). 13C{1H} NMR (C6D6, 100 MHz): δ = 164.6 (d, JCP = 30 Hz), 157.3, 141.6, 138.7 (d, JCP = 6 Hz), 138.9, 137.6, 136.7, 136.1 (d, JCP = 2 Hz), 134.7 (d, JCP = 50 Hz), 134.5, 132.0 (d, JCP = 10 Hz), 131.0, 130.7 (d, JCP = 7 Hz), 130.5, 129.8 (d, JCP = 8 Hz), 129.5 (d, JCP = 14 Hz), 126.0, 124.0, 116.4 (d, JCP = 50 Hz), 116.1 (d, JCP = 11 Hz) (ArC), 33.8 (CHMe2), 24.32 (CHMe2), 21.11, 20.26, 18.96, 18.55 (ArCH3). FT-IR (Nujol): ν(TaN3) 2096 cm-1.  Anal. Calcd. for C47H50N6P1Ta1: C, 61.97; H, 5.53; N, 9.23; Found: C, 59.34; H, 5.88; N, 7.91. [PhNPN*]Ta(=N-p-C6H4iPr)CH2Ph (3.5)  At room temperature, p-isopropylphenyl azide (44 mg, 0.27 mmol in 2 mL toluene) was added to a toluene solution (10 mL) of 2.7 (298 mg, 0.30 mmol) and the mixture was left to stir at room temperature for 6 h.  The resulting bright orange slurry was evaporated to dryness in vacuo, triterated with 10 mL of pentane and filtered through a sintered glass frit.  A bright orange solid is collected on the frit and dried in vacuo to yield 242 mg (0.25 mmol, 93% based on PIPP azide).  1H NMR (toluene-d8, 300 MHz): δ = 7.47 (m, 4H), 7.1 – 6.7 (overlapping signals, approx. 14 aromatic protons and residual toluene-d8 protons), 6.07 (dd, JHP = 6 Hz, JHH = 8 Hz, 2H), 5.92 (d, JHH = 8 Hz, 2H), 5.58 (d, JHH = 8 Hz, 2H), (ArH), 2.56 (sept, JHH = 7 Hz, (CH)Me2 ,1H), 2.35 (s, 6H), 2.31 (s, 6H), 2.19 (s, 6H) (ArCH3), 2.06 (s, 2H, TaCH2Ph), 1.99 (s, 6H) (ArCH3), 1.00 (d, 6H, JHH = 7 Hz, (CH)Me2). 31P{1H} NMR (toluene-d8, 120 MHz): δ = 26.5 (s). 13C{1H} NMR (toluene-d8, 75 MHz): δ = 165.7 (d, JCP = 33 Hz), 157.5, 153.2 (d, JCP = 2 Hz), 141.0, 139.0 (d, JCP = 7 Hz), 138.8, 138.4, 135.85 (d, JCP = 42 Hz), 135.8, 134.6, 131.7 (d, JCP = 11 Hz), 131.2 (d, JCP = 13 Hz), 130.0 (d, JCP = 2 Hz), 129.65, 129.60, 129.5, 127.6, 127.2, 125.6, 123.5 (d, JCP = 2 Hz), 121.0, 117.1 (d, JCP = 45 Hz), 116.3 (d, JCP = 11 Hz) (ArC), 77.6 (d, JCP = 25 Hz, CH2Ph),   210 33.8 (CHMe2), 24.3 (CHMe2) 21.1, 20.1, 19.0, 18.9 (ArCH3). EI-MS (m/z): 959 (3, [M+]), 868 (100, [M – CH2Ph]+). Anal. Calcd. for C54H57N3P1Ta1: C, 67.56; H, 5.99; N, 4.38; Found: C, 68.47; H, 6.62; N, 5.23. [PhNPN*]Ta(=N-p-C6H4iPr)CH2Ph (3.6)  At room temperature, p-isopropylphenyl azide (44 mg, 0.27 mmol in 2 mL toluene) was added to a toluene solution (10 mL) of 2.6 (272 mg, 0.30 mmol) and the mixture was left to stir at room temperature for 18 h.  The resulting bright red solution was evaporated to dryness in vacuo, triterated with 5 mL of HMDSO and filtered through a sintered glass frit.  A dark orange-red solid was collected on the frit and dried in vacuo to yield 220 mg (0.23 mmol, 85% based on PIPP azide).  1H NMR (toluene-d8, 300 MHz): δ = 7.60 (m, 2H), 7.45 (dd, JHP = 2 Hz, JHH = 8 Hz, 2H), 7.2 – 6.7 (overlapping signals, approx. 16 aromatic protons and residual toluene-d8 protons), 6.08 (dd, JHP = 6 Hz, JHH = 8 Hz, 2H), 5.63 (d, JHH = 8 Hz, 2H), (ArH), 2.99 (d, JHP = 7 Hz, TaCH2Ph), 2.65 (sept, JHH = 7 Hz, (CH)Me2, 1H), 2.25 (s, 6H), 2.20 (s, 6H) 2.09 (s, 6H), 1.94 (s, 6H) (ArCH3), 1.07 (d, 6H, JHH = 7 Hz, (CH)Me2). 31P{1H} NMR (toluene-d8, 120 MHz): δ = 9.00 (s). 13C{1H} NMR (toluene-d8, 75 MHz): δ = 161.6 (d, JCP = 31 Hz), 154.1 (d, JCP = 13 Hz), 148.8, 146.4 (d, JCP = 2 Hz), 142.7, 135.4 (d, JCP = 2 Hz), 134.9, 134.2 (d, JCP = 4 Hz), 133.8, 132.7 (d, JCP = 12 Hz), 132.3 (d, JCP = 37 Hz), 130.1, 129.8, 129.4, 129.3, 128.5, 128.1, 126.9, 125.0, 122.8, 118.3 (d, JCP = 34 Hz), 116.1 (d, JCP = 11 Hz) (ArC), 68.8 (d, JCP = 7 Hz, TaCH2Ph), 33.9 (CHMe2), 24.5 (CHMe2) 20.6, 19.7, 19.1, 18.0 (ArCH3). Anal. Calcd. for C54H57N3P1Ta1: C, 67.56; H, 5.99; N, 4.38; Found: C, 63.96; H, 7.18; N, 6.27.     211 [PhNPN*]Ta(=N-p-C6H4iPr)(η2-BnNNN-p-C6H4iPr) (3.7)  At room temperature, p-isopropylphenyl azide (49 mg, 0.30 mmol in 2 mL toluene) was added to a toluene solution (10 mL) of 3.6 (288 mg, 0.30 mmol) and the mixture was heated at 70 °C for 48 h.  The resulting bright red solution was evaporated to dryness in vacuo, triterated with 10 mL of pentane and filtered through a sintered glass frit.  A dark red solid was collected on the frit and dried in vacuo to yield 299 mg (0.26 mmol, 88%). 1H NMR (toluene-d8, 300 MHz): δ = 7.42 (ddd, JHH = 8 Hz, JHH = 1 Hz, JHP = 7 Hz, 2H), 7.27 (ddd, JHH = 10 Hz, JHH = 2 Hz, JHP = 9 Hz, 2H), 7.2 – 6.7 (overlapping signals, approx. 16 aromatic protons and residual toluene-d8 protons), 6.48 (d, JHH = 8 Hz, 2H), 6.10 (dd, JHH = 8 Hz, JHP = 6 Hz, 1H), 6.07 (dd, JHH = 8 Hz, JHP = 6 Hz, 1H), (ArH), 5.15 (d, JHH = 7 Hz, benzyl CHaHbPh), 4.92 (d, JHH = 7 Hz, benzyl CHaHbPh), 2.63 (sept, JHH = 7 Hz, (CH)Me2, 1H), 2.61 (sept, JHH = 7 Hz, (CH)Me2, 1H), 2.29 (s, 3H), 2.27 (s, 3H), 2.20 (s, 3H), 2.18 (s, 3H), 1.92 (s, 6H), 1.67 (s, 3H) (ArCH3), 1.09 (d, JHH = 7 Hz, (CH)Me2, 6H), 1.04 (d, JHH = 7 Hz, (CH)Me2, 6H). 31P{1H} NMR (toluene-d8, 120 MHz): δ = 14.0 (s). 13C{1H} NMR (toluene-d8, 100 MHz): δ = 164.3 (d, JCP = 39 Hz), 160.0 (d, JCP = 38 Hz), 154.6 (d, JCP = 20 Hz), 148.7 (d, JCP = 2 Hz), 148.2 (d, JCP = 2 Hz), 147.5, 145.4, 142.7, 139.7, 136.3, 135.7, 135.4, 134.7 (d, JCP = 2 Hz), 134.3, 134.1, 133.9, 133.7, 132.9 (d, JCP = 15 Hz), 132.5, 130.28, 130.25, 129.9, 129.88, 129.85, 129.8, 129.7, 129.52, 129.45, 129.3, 129.2, 128.9, 128.45, 128.4, 127.9, 126.8, 126.4, 125.3, 121.5, 120.5 (d, JCP = 49 Hz), 117.8 (d, JCP = 14 Hz), 116.5 (d, JCP = 14 Hz) (ArC), 58.8 (s, NCH2Ph), 34.0, 33.8 (CHMe2), 24.4, 24.35, 24.3, 24.1 (CHMe2) 20.95, 20.9, 20.8, 20.4, 20.2, 20.1, 19.8, 18.8 (ArCH3). EI-MS (m/z): 1121 (5, [M]+), 1093 (50, [M – N2]+), 1002 (50, [M – N2 – CH2Ph]+), 868 (100, [3.5 – CH2Ph]+). Anal. Calcd. for C63H68N6P1Ta1: C, 67.49; H, 6.11; N, 7.50; Found: C, 67.23; H, 6.29; N, 7.77.   212 7.2.3 Complexes Pertaining to Chapter 4  [PhNPN*]Ta(EtC≡CEt)(CH=CHPh) (4.1) Phenylacetylene (5 uL, 4.1 mg, 43 umol) was added to a dark brown C6D6 solution (~0.5 mL) of 2.4 (33 mg, 42 umol), which leads to an immediate bright red colour change.  By NMR, the reaction is quantitative and complete within 5 mins; due to slow thermal decomposition (to 4.2) this complex was characterized by NMR spectroscopy only.  1H NMR (C6D6, 400 MHz): δ = 8.69 (dd, JHH = 18 Hz, JHP = 3 Hz, 1H, TaCH=CHPh), 7.65 (d, JHH = 8 Hz, 2H), 7.52 (m, 2H), 7.21 (s, 2H), 7.10 (m, 5H), 6.85 (m, 5H), 6.74 (s, 2H) (ArH), 6.21 (d, JHH = 18 Hz, 1H, TaCH=CHPh), 6.03 (dd, JHH = 8 Hz, JHP = 7 Hz, 2H, ArH), 2.99 (q, JHH = 7.5 Hz, 4H, hexyne CH2), 2.34 (s, 6H), 2.13 (s, 6H), 2.04 (s, 6H), 1.78 (s, 6H) (ArCH3), 0.99 (t, JHH = 7.5 Hz, 6H, hexyne CH3). 31P{1H} NMR (C6D6, 160 MHz): δ = 26.4 (s). 13C{1H} NMR (C6D6, 75 MHz): δ = 204.3 (d, JCP = 18 Hz, TaCH=CHPh), 164.1 (d, JCP = 25 Hz), 143.3, 142.0 (TaCH=CHPh), 139.3, 137.9 (d, JCP = 5 Hz), 137.1 (d, JCP = 28 Hz), 135.1, 134.7, 132.8 (d, JCP = 9 Hz), 131.1, 130.1, 129.8, 129.5 (d, JCP = 4 Hz), 129.1 (d, JCP = 7 Hz), 128.5, 126.1, 125.6, 121.1 (d, JCP = 28 Hz), 115.7 (d, JCP = 8 Hz) (ArC), 29.2 (bs, hexyne CH2), 21.1, 20.3, 19.6, 18.7 (ArCH3), 14.6 (bs, hexyne CH3). [PhNPN*]Ta(EtC≡CEt)(CD=CHPh) (4.1-d1) A sample of 4.1-d1 was prepared using PhC≡CD and 2.4, in a manner identical to that for 4.1.  1H NMR (C6D6, 400 MHz): δ = 7.65 (d, JHH = 8 Hz, 2H), 7.52 (m, 2H), 7.21 (s, 2H), 7.10 (m, 5H), 6.85 (m, 5H), 6.74 (s, 2H) (ArH), 6.20 (bs, 1H, TaCD=CHPh), 6.03 (dd, JHH = 8 Hz, JHP = 7 Hz, 2H, ArH), 2.99 (q, JHH = 7.5 Hz, 4H, hexyne CH2), 2.34 (s, 6H), 2.13 (s, 6H), 2.04 (s, 6H), 1.78 (s, 6H) (ArCH3), 0.99 (t, JHH = 7.5 Hz, 6H, hexyne CH3,). 31P{1H} NMR (C6D6, 160 MHz): δ = 26.7 (s).    213 [PhNPN*]TaC(Et)C(Et)C(H)C(H)Ph (4.2). A 50 mL Kontes-seal glass reactor was charged with a magnetic stir bar, 2.4 (300 mg, 0.37 mmol) phenylacetylene (40 uL, 37 mg, 0.37 mmol) and 20 mL of toluene.  The resulting solution was stirred at 54 °C for 6 h, during which a brown to red-brown colour change was observed.  After heating, the volatiles were removed in vacuo; the resulting red-brown residue was triturated with ~20 mL of cold pentane and filtered to yield solid 4.2. (231 mg, 68%).  1H NMR (C6D6, 400 MHz): δ = 7.83 (m, 2H), 7.60 (d, JHP = 9 Hz, 1H), 7.45 (d, JHP = 9 Hz, 1H), 7.10 (m, 3H), 6.96 (m, 4H), 6.81 (d, JHH = 8 Hz, 1H), 6.79 (s, 1H), 6.72 (d, JHH = 8 Hz, 1H), 6.69 (s, 1H), 6.57 (s, 1H), 6.21 (dd, JHH = 8 Hz, JHP = 5 Hz, 1H), 5.70 (dd, JHH = 8 Hz, JHP = 5 Hz, 1H), 5.59 (d, JHH = 6.5 Hz, 2H) (ArH), 4.33 (d, JHH = 8 Hz, 1H, ‘H2’), 3.48 (m, 1H), 2.89 (m, 2H), 2.49 (m, 1H) (‘H5a/b, H6a/b’) 2.31, 2.15, 2.09, 2.02, 1.94, 1.90, 1.86, 1.62, (s, 3H) (ArCH3), 1.34 (t, JHH = 7.5 Hz, 3H, ‘Me2’), 0.56 (dd, JHH = 8 Hz, JHP = 3 Hz, 1H, ‘H1’), 0.45 (t, JHH = 7.5 Hz, 3H, ‘Me1’). 31P{1H} NMR (C6D6, 120 MHz): δ = 28.5 (s). 13C{1H} NMR (C6D6, 75 MHz): δ = 245.3 (d, JCP = 11 Hz, C1), 168.4 (d, JCP = 32 Hz), 160.4 (d, JCP = 28 Hz), 151.2 (d, JCP = 5 Hz), 147.6, 141.7 (d, JCP = 31 Hz), 137.9, 135.9, 135.6, 134.6, 134.5, 134.4, 133.9, 133.7 (C2), 133.6, 133.5 (d, JCP = 14 Hz), 133.3, 132.5 (d, JCP = 14 Hz), 131.3 (d, JCP = 25 Hz), 130.9 (d, JCP = 6 Hz), 130.1 (d, JCP = 4 Hz), 129.9 (d, JCP = 3 Hz) 129.6, 129.4, 129.2, 129.1, 129.0, 126.1, 126.0, 125.1 (d, JCP = 42 Hz), 123.2, 117.6 (d, JCP = 34 Hz), 116.7 (d, JCP = 10 Hz), 116.1 (d, JCP = 10 Hz) (ArC), 93.6 (C3), 93.1 (d, JCP = 15 Hz, C4), 31.5 (d, JCP = 6 Hz, C6), 30.4 (C5), 21.2, 20.91, 20.89, 20.6, 20.1, 19.7, 19.1, 18.4 (ArMe), 18.3 (Me1), 16.9 (Me2). Anal. Calcd. for C52H56N2PTa: C, 67.82; H, 6.13; N, 3.04. Found: C, 60.90; H, 6.22; N, 2.64.     214 [PhNPN*]TaC(Et)C(Et)C(D)C(H)Ph (4.2-d1)   A sample of 4.2-d1 was prepared using PhC≡CD and 2.4 in a manner similar to that for 4.2; the reaction was scaled down by a factor of 10 and performed in a sealed J-Young NMR tube.  1H NMR (C6D6, 400 MHz): δ = 7.83 (m, 2H), 7.60 (d, JHP = 9 Hz, 1H), 7.45 (d, JHP = 9 Hz, 1H), 7.10 (m, 3H), 6.96 (m, 4H), 6.81 (d, JHH = 8 Hz, 1H), 6.79 (s, 1H), 6.72 (d, JHH = 8 Hz, 1H), 6.69 (s, 1H), 6.57 (s, 1H), 6.21 (dd, JHH = 8 Hz, JHP = 5 Hz, 1H), 5.70 (dd, JHH = 8 Hz, JHP = 5 Hz, 1H), 5.59 (d, JHH = 6.5 Hz, 2H) (ArH), 3.48 (m, 1H), 2.89 (m, 2H), 2.495 (m, 1H) (‘H5a/b, H6a/b’) 2.31, 2.15, 2.09, 2.02, 1.94, 1.90, 1.86, 1.62, (s, 3H) (ArCH3), 1.34 (t, JHH = 7.5 Hz, 3H, ‘Me2’), 0.55 (d, JHP = 3 Hz, 1H, ‘H1’), 0.45 (t, JHH = 7.5 Hz, 3H, ‘Me1’). 31P{1H} NMR (C6D6, 120 MHz): δ = 28.5 (s) [PhNPN*]Ta(BTA)(CH=CHPh) (4.3) Phenylacetylene (5 uL, 4.1 mg, 43 umol) was added to a red C6D6 solution (~0.5 mL) of 2.5 (38 mg, 42 umol), which leads to no observable colour change.  By NMR, the reaction is quantitative and complete within 5 mins; due to rapid thermal decomposition (to 4.4), this complex was characterized by NMR spectroscopy only.  1H NMR (C6D6, 300 MHz): δ = 8.92 (dd, JHH = 18 Hz, JHP = 3 Hz, 1H, TaCH=CHPh), 7.65 (d, JHH = 8 Hz, 2H), 7.5-6.7 (overlapping signals, 16H plus residual C6D6 protons) (ArH), 6.03 (dd, JHH = 8 Hz, JHP = 7 Hz, 2H, ArH), 5.83 (d, JHH = 18 Hz, 1H, TaCH=CHPh), 2.32 (s, 6H), 2.09 (s, 6H), 1.97 (s, 6H), 1.78 (s, 6H) (ArCH3), 0.10 (s, 18H, Si(CH3)3). 31P{1H} NMR (C6D6, 160 MHz): δ = 23.9 (s). [PhNPN*]TaC(TMS)C(TMS)C(H)C(H)Ph (4.4) A 50 mL Kontes-seal glass reactor was charged with a magnetic stir bar, 2.5 (332 mg, 0.37 mmol), phenylacetylene (40 uL, 37 mg, 0.37 mmol) and 20 mL of toluene.  The resulting   215 solution was stirred at 54 °C for 6 h, during which the deep red mixture became slightly darker in colour.  After heating, the volatiles were removed in vacuo; the resulting red-brown residue was triturated with ~20 mL of cold pentane and filtered through a sintered glass frit to yield solid 4.4. (284 mg, 0.28 mmol, 76 %). 1H NMR (C6D6, 300 MHz): δ = 8.30 (m, 2H), 7.60 (d, JHP = 9 Hz, 1H), 7.45 (dd, JHH = 7 Hz, JHP = 2 Hz, 1H), 7.33 (dd, JHH = 7 Hz, JHP = 2 Hz, 1H), 7.23 (dt, JHH = 9 Hz, JHP = 2 Hz, 2H), 7.15 – 6.95 (overlapping multiplets, 5H), 6.76 (t, JHH = 9 Hz, JHP = 2 Hz, 2H), 6.62 (s, 1H), 6.51 (s, 1H), 6.21 (dd, JHH = 8 Hz, JHP = 5 Hz, 1H), 5.70 (dd, JHH = 8 Hz, JHP = 5 Hz, 1H), 6.14 (ddd, JHH = 8.5 Hz, JHH = 5 Hz, JHP = 2 Hz, 2H) (ArH), 4.94 (dd, JHH = 9 Hz, JHP = 2 Hz, 1H, ‘H2’), 2.23 (s, 3H), 2.09 (s, 3H), 2.03 (s, 3H), 1.98 (s, 3H), 1.92 (s, 3H), 1.89 (s, 3H), 1.70 (s, 3H), 1.67 (s, 3H), (ArCH3), 1.53 (d, JHH = 9 Hz, 1H, ‘H1’), 0.38 (s, 9H), 0.15 (s, 9H) (Si(CH3)2). 31P{1H} NMR (C6D6, 160 MHz): δ = 31.9 (s). 13C{1H} (C6D6, 75 MHz): δ = 251.4 (C1), 162.0 (d, JCP = 22 Hz), 156.7 (d, JCP = 18 Hz), 150.9 (d, JCP = 7 Hz), 147.1, 144.9, 135.0 (d, JCP = 9 Hz), 134.9 (d, JCP = 9 Hz), 134.6 (d, JCP = 11 Hz), 134.4 (d, JCP = 20 Hz), 134.3, 133.75, 133.7, 133.0, 132.7, 132.65, 132.3, 131.4, 131.35, 129.8 (d, JCP = 13 Hz), 129.2 (d, JCP = 8 Hz), 129.0, 128.8, 128.65, 128.6, 128.3, 124.6, 122.8 (C2), 119.5 (d, JCP = 5 Hz), 119.1 (d, JCP = 34 Hz), 116.5 (d, JCP = 8 Hz), 115.5 (d, JCP = 27 Hz) (ArC), 104.8 (C4), 98.3 (C3), 21.8, 20.9, 20.7, 20.6, 20.5, 20.4, 20.2, 18.5 (ArCH3), 4.35, 0.36 (Si(CH3)3). Anal. Calcd. for C54H64N2PSi2Ta: C, 64.27; H, 6.39; N, 2.78. Found: C, 64.07; H, 6.77; N, 3.12.  [PhNPN*]TaC(Et)C(Et)C(H)N(xylyl) (4.5) To a mixture of 2.4 (94 mg, 0.11 mmol) and 2,6-dimethylphenyl isocyanide (15 mg, 0.11 mmol) was added 10 mL of toluene.  This solution was stirred for 15 min., after which the volatiles   216 were removed in vacuo.  The resulting dark brown residue was triturated with ~10 mL of cold pentane and filtered to yield solid 4.5 (94 mg, 90%).  1H NMR (C6D6, 300 MHz): δ = 7.38 (dd, JHH = 8 Hz, JHP = 7 Hz, 1H), 7.2–6.6 (15H plus residual C6D6 protons), 6.04 (dd, JHH = 8 Hz, JHP = 5 Hz, 1H), 5.91 (dd, JHH = 8 Hz, JHP = 5 Hz, 1H) (ArH), 4.88 (s, 1H, CHNR), 3.38 (dt, 2JHH = 7 Hz, 3JHH = 7 Hz, 1H, H5a/b), 2.61 (s, 3H), 2.36 (s, 3H) (ArCH3), 2.29 (m, 1H, H5a/b), 2.27 (s, 3H), 2.22 (s, 3H), 2.20 (s, 6H), 1.97 (s, 3H), 1.94 (s, 6H), 1.87 (s, 3H), 1.86 (s, 3H) (ArCH3), 1.60 (m, 1H, ‘H6a/b’), 1.09 (t, JHH = 7 Hz, ‘Me2’), 0.77 (m, 1H, ‘H6a/b’) 0.716 (t, JHH = 7 Hz, ‘Me1’). 31P{1H} NMR (C6D6, 120 MHz): δ = 16.1 (s). 13C{1H} (C6D6, 75 MHz): δ = 231.6 (d, JCP = 30 Hz, C1), 163.7 (d, JCP = 30 Hz), 163.2 (d, JCP = 25 Hz), 151.8, 147.1 (d, JCP = 7 Hz), 146.8 (d, JCP = 3 Hz), 137.2, 136.4, 135.6, 135.4, 134.6, 134.4, 134.2, 133.9, 133.7, 133.6, 133.3, 133.2, 133.0, 132.2, 131.8, 130.2 (d, JCP = 15 Hz), 129.8 (d, JCP = 5 Hz), 129.6, 129.3, 129.2, 128.7, 128.6, 128.4, 127.0 (d, JCP = 5 Hz), 125.2, 120.5 (d, JCP = 35 Hz), 116.8 (d, JCP = 10 Hz), 116.1 (d, JCP = 10 Hz), 113.8 (d, JCP = 36 Hz) (ArC), 111.3 (C2), 93.9 (C3), 29.0 (C5), 21.5, 21.1, 20.8, 20.52 (ArCH3), 20.50 (C6), 20.15, 20.12, 20.10, 20.08, 19.7, 19.0, (ArCH3), 17.7 (Me1), 15.2 (Me2). Anal. Calcd. for C53H59N3PTa: C, 67.01; H, 6.26; N, 4.42. Found: C, 67.20; H, 6.24; N, 4.09.  [PhNPN*]TaC(TMS)C(TMS)C(H)N(xylyl) (4.6) To a mixture of 2.5 (104 mg, 0.11 mmol) and 2,6-dimethylphenyl isocyanide (15 mg, 0.11 mmol) was added 10 mL of toluene.  This solution was stirred for 15 min., after which the volatiles were removed in vacuo.  The resulting dark red residue was triturated with ~10 mL of cold pentane and filtered to yield solid 4.6 (102 mg, 86%).  1H NMR (C6D6, 300 MHz): δ = 8.22 (dd, JHH = 8 Hz, JHP = 7 Hz, 2H), 7.44 (d, JHP = 7 Hz, 1H), 6.90 (d, JHH = 7 Hz, 1H), 6.75 (m, 4H), 6.63 (d, JHH = 7 Hz, 1H), 6.55 (bs, 2H), 6.31 (dd, JHH = 8   217 Hz, JHP = 5 Hz, 1H), 6.18 (bs, 1H), 6.03 (dd, JHH = 8 Hz, JHP = 5 Hz, 1H) (ArH), 5.24 (s, 1H, ‘H3’), 2.20 (s, 6H), 2.09 (s, 3H), 2.06 (s, 3H), 2.04 (s, 3H), 2.02 (s, 3H), 1.94 (s, 6H), 1.64 (s, 3H), 1.59 (s, 3H) (ArCH3), 0.49 (s, 9H), 0.11 (s, 9H) (SiCH3). 31P{1H} NMR (C6D6, 120 MHz): δ = 28.8 (s). 13C{1H} (C6D6, 75 MHz): δ = 242.6 (C1), 162.0 (d, JCP = 30 Hz), 158.1 (d, JCP = 25 Hz), 148.7 (d, JCP = 7 Hz), 147.5, 146.8 (d, JCP = 7 Hz), 136.4, 135.9, 135.8, 134.8, 134.7, 134.6, 134.55, 134.3, 134.2, 133.9, 133.2, 133.1, 132.3 (d, JCP = 5 Hz), 131.0 (d, JCP = 3 Hz), 130.8, 130.4, 130.3, 130.2, 130.0 (d, JCP = 36 Hz), 129.3, 129.2, 129.0, 128.95, 128.9, 128.6, 128.5, 123. 5, 120.4 (d, JCP = 45 Hz), 120.2 (d, JCP = 7 Hz), 116.7 (d, JCP = 10 Hz), 111.72 (d, JCP = 2 Hz, C2), 111.6 (d, JCP = 36 Hz) (ArC), 107.6 (C3), 22.3, 22.1 (d, JCP = 2 Hz), 21.7, 21.2, 21.2, 20.9, 20.6, 20.5, 20.4, 19.2 (ArCH3), 5.22, 1.14 (Si(CH3)3). Anal. Calcd. for C55H67N3PSi2Ta: C, 63.63; H, 6.50; N, 4.05. Found: C, 65.01; H, 7.09; N, 4.26  [PhNPN*]Ta(hexyne)CO2H (4.7) A 50 mL Kontes-seal glass reactor was charged with a magnetic stir bar, 2.4 (400 mg, 0.44 mmol) and 10 mL toluene to yield a clear yellow solution.  The reactor was placed in a liquid nitrogen bath (-197 °C) and the headspace was removed in vacuo, after which CO2 (10 mL, 0.45 mmol) was sublimed in from a calibrated glass bulb. The reaction vessel was sealed under static vacuum and allowed to warm to room temperature while stirring for 1h; no obvious colour change was observed during this time.  The resulting solution was evaporated to dryness in vacuo to yield a dark yellow residue.  This residue was triturated with 20 mL of pentane and cooled to -35 °C, whereupon a tawny-yellow precipitate formed.  This precipitate was collected on a frit and washed with cold pentane (2 x 5 mL) to afford 275 mg (0.32 mmol, 73%) of a tawny solid.    218 1H NMR (C6D6, 400 MHz): δ = 8.27 (s, 1H, CO2H), 7.61 (d, JHH = 8 Hz, 2H), 7.53 (m, 2H), 7.03 (m, 5H), 6.85 (d, JHH = 8 Hz, 2H), 6.70 (s, 2H), 6.07 (dd, JHH = 8 Hz, JHP = 8 Hz, 2H) (ArH), 2.71 (bs, 4H, hexyne CH2), 2.51 (s, 6H), 2.10 (s, 6H), 2.02 (s, 6H), 1.79 (s, 6H) (ArCH3), 0.83 (bs, 6H, hexyne CH3). 31P{1H} NMR (C6D6, 160 MHz): δ = 27.6 (s). 13C{1H} NMR (C6D6, 75 MHz): δ = 167.7 (CO2H), 162.6 (d, JCP = 41 Hz), 139.6, 136.8, 136.5 (d, JCP = 10 Hz), 136.4 (d, JCP = 41 Hz), 135.3, 134.4, 132.8 (d, JCP = 11 Hz), 130.9, 130.3 (d, JCP = 2 Hz), 130.0 (d, JCP = 6 Hz), 129.7, 129.3, 129.1, 121.1 (d, JCP = 42 Hz), 115.6 (d, JCP = 10 Hz) (ArC), 29.4 (bs, hexyne CH2), 21.1, 20.3, 19.3, 18.9 (ArCH3), 14.2 (bs, hexyne CH3). EI-MS (m/z): 862 (1, [M]+), 833 (45, [M – COH]+), 817 (30, [M – CO2H]+). FT-IR (Nujol): ν(CO2–) 1670 cm-1.  Anal. Calcd. for C45H50N2O2P1Ta1: C, 62.64; H, 5.84; N, 3.25; Found: C, 59.44; H, 5.58, N, 3.02. [PhNPN*]Ta(hexyne)13CO2H (13C-4.7) A sample of 13C-4.7 was prepared in a manner identical to that of 4.7, using 13C-enriched carbon dioxide.  1H NMR (C6D6, 400 MHz), selected peaks only: δ = 8.27 (d, JCH = 209 Hz, 1H, CO2H). 13C NMR (gated-decoupling, C6D6, 101 MHz), selected peaks only: δ = 167.7 (d, JCH = 209 Hz, CO2H). {[PhNPN*]Ta(hexyne)}2(µ-OCH2O) (4.8) Method A: Complex 4.2 can be prepared in a fashion analogous to 4.1, from 2.4 (176 mg, 0.2 mmol) and CO2 (2.2 mL, 0.1 mmol). Method B: Compound 2.4 (93 mg, 0.12 mmol) was added as a solid to a stirring solution of 4.1 (100 mg, 0.12 mmol) in 10 mL of toluene.  The solution was stirred for 1 h, and then evaporated to dryness in vacuo to yield a dark brown residue.  This residue was triturated with 10 mL of pentane and cooled to -35 °C, whereupon a brown precipitate formed.  A brown solid was   219 collected on a frit and washed with cold pentane (2 x 5 mL) to afford 140 mg (8.3 x10-5 mol, 69%) of a pale brown solid.  1H NMR (C6D6, 300 MHz): δ = 8.21 (dd, JHH = 7 Hz, JHP = 7 Hz, 2H), 7.45 (m, 4H), 7.42 (d, 2H), 7.03 (s, 6H), 6.89 (bs, 6H), 6.85 (s, 4H), 6.72 (s, 2H), 6.20 (dd, JHH = 8 Hz, JHP = 8 Hz, 2H), 6.01 (dd, JHH = 8 Hz, JHP = 8 Hz, 2H) (ArH), 5.86 (s, 2H, O2CH2), 2.79 (q, JHH = 7 Hz, 4H, hexyne CH2) 2.46 (s, 6H), 2.28 (bs, 12H), 2.24 (s, 6H), 2.15 (s, 6H) (ArCH3), 2.02 (q, JHH = 8 Hz, 4H, hexyne CH2), 2.00 (s, 6H), 1.99 (s, 6H), 1.82 (s, 6H) (ArCH3), 0.89 (t, JHH = 8 Hz, 6H), 0.86 (t, JHH = 7 Hz, 6H) (hexyne CH3). 31P{1H} NMR (C6D6, 120 MHz): δ = 28.6 (s), 20.5 (s). 13C{1H} NMR (C6D6, 75 MHz): δ = 229.0 (d, JCP = 11 Hz), 200.6 (d, JCP = 12 Hz), (EtC≡CEt), 163.8 (d, JCP = 31 Hz), 163.0 (d, JCP = 31 Hz), 152.7, 152.6, 139.7, 138.3 (d, JCP = 41 Hz), 138.2, 138.1, 136.7, 135.15 (d, JCP = 34 Hz), 135.1, 134.5, 134.5 (d, JCP = 15 Hz), 134.1, 133.8, 133.4, 132.85 (d, JCP = 11 Hz), 130.7, 130.6, 130.4, 130.1, 129.8, 129.6, 129.3, 129.0, 128.9, 128.8, 128.5, 120.1 (d, JCP = 42 Hz), 117.5 (d, JCP = 42 Hz), 115.8 (d, JCP = 10 Hz), 115.2 (d, JCP = 10 Hz) (ArC), 102.9 (O2CH2), 29.7, 28.5 (hexyne CH2), 21.2, 21.0, 20.8, 20.35, 20.25, 20.2, 19.0, 18.6 (ArCH3), 14.7, 14.3 (hexyne CH3). Anal. Calcd. for C89H100N4O2P2Ta2: C, 63.57; H, 5.99; N, 3.33; Found: C, 60.20; H, 5.88, N, 3.25. {[PhNPN*]Ta(hexyne)}2(µ-O13CH2O) (13C-4.8) A sample of 13C-4.8 was prepared in a manner identical to that of {[PhNPN*]Ta(hexyne)}2(µ-OCH2O), using 13C-enriched carbon dioxide.  1H NMR (C6D6, 400 MHz), selected peaks only: δ = 5.86 (d, JCH = 162 Hz, 2H, O2CH2). 13C NMR (gated-decoupling, C6D6, 101 MHz), selected peaks only: δ = 102.9 (t, JHC = 162 Hz, O2CH2).    220 7.2.4 Complexes Pertaining to Chapter 5 [PhNPN*]TaMe3 (5.3)  Performed by Dr. Dominik Nied: A solution of TaMe3Cl2 (680 mg, 2.29 mmol) in Et2O (30 mL) was added dropwise via canula to a -78 °C solution of 2.1 (1.53 g, 2.29 mmol) in 200 mL Et2O over the course of approximately 10 minutes.  The resulting dark brown solution was stirred for 30 mins at -78 °C before it was allowed to warm to 0°C and stirred for an additional 30 mins, during which the formation of a light-coloured precipitate was observed.  The reaction mixture was then evaporated to dryness in vacuo to afford a dark brown residue, which was extracted with 50 mL of toluene and filtered through a pad of silica on a sintered glass frit.  The filtrate was again evaporated to dryness, and the resulting brown residue was triturated with 20 mL of pentane and cooled in a glovebox freezer (-35 °C), whereupon a yellow-orange precipitate formed.  This precipitate was collected on a frit and washed with a minimum amount of cold pentane and dried in vacuo to afford 1.55 g (1.99 mmol, 87%) of a yellow-orange powder.  1H NMR (C6D6, 400 MHz): δ = 7.60 (dd, JHP = 11 Hz, JHH = 8 Hz, 2H), 7.32 (d, JHH = 8 Hz, 2H), 7.06 (m, 3H), 6.86 (s, 4H), 6.80 (d, JHH = 8 Hz, 2H), 6.06 (dd, JHP = 6 Hz, JHH = 8 Hz, 2H) (ArH), 2.34 (s, 6H), 2.14 (s, 6H), 2.12 (s, 6H), 1.95 (s, 6H) (ArCH3), 1.33 (bs, 9H, Ta(CH3)3). 31P{1H} NMR (C6D6, 160 MHz): δ = 41.9 (s). 13C{1H} NMR (C6D6, 100 MHz): δ = 165 (d, JCP = 32 Hz), 144 (d, JCP = 5 Hz), 138, 137.1, 135.9, 135 (d, JCP = 2 Hz), 134.2 (d, JCP = 2 Hz), 133.3 (d, JCP = 9.9 Hz), 132.2 (d, JCP = 39.3 Hz), 130.8, 130.7, 130.4, 129.7 (d, JCP = 5.3 Hz), 129.0 (d, JCP = 9.4 Hz), 120.9 (d, JCP = 41.1 Hz), 117.26 (d, JCP = 11.2 Hz) (ArC), 72.9 (TaCH3), 21.2, 20.5, 20.31, 19.3 (ArCH3). Anal. Calcd. for C41H48N2PTa C 63.07, H 6.20, N 3.59, found C 62.82, H 6.34, N 3.50.    221 ([PhNPN*]Ta)2(µ-H)4 (5.4) Method A (from 5.3, performed by Dr. Dominik Nied): A thick-walled 200 mL glass vessel equipped with a Teflon valve was charged with a yellow solution of 5.3 (500 mg, 0.64 mmol) in toluene (20 ml), and the headspace gas was removed via one freeze-pump-thaw cycle. The vessel was then cooled in liquid nitrogen, filled with H2 gas and sealed under atmospheric pressure. The reaction mixture was allowed to warm to room temperature, while the pressure inside the vessel slowly rose to approximately four atmospheres. After stirring at room temperature for one week, a dark brick red solution of ([PhNPN*]Ta)2(µ-H)4 was obtained. The excess H2 gas was carefully vented from the reaction vessel, and the reaction mixture was evaporated to dryness in vacuo to yield a dark red-brown residue.  This residue was then extracted with Et2O (20 mL) and cooled in a glovebox freezer (-35 °C), whereupon a dark coloured solid precipitated out of solution.  The precipitate was collected on a sintered glass frit, washed with a minimum amount of cold pentane and dried in vacuo to afford 320 mg (0.21 mmol, 69%) of a dark red-brown powder. Method B (from 2.6 or 2.7): A thick-walled 200 mL glass vessel equipped with a Teflon valve was charged with a benzene solution (4 mL) of 2.6 (100 mg, 0.11 mmol) or 2.7 (110 mg, 0.11 mmol), and the headspace gas was removed via one freeze-pump-thaw cycle.  The vessel was then cooled in liquid nitrogen, filled with H2 gas and sealed under atmospheric pressure. The reaction mixture was allowed to warm to room temperature, while the pressure inside the vessel slowly rose to approximately four atmospheres. After stirring at room temperature for 14 days (2.6) or 10 days (2.7), a dark brick red solution of ([PhNPN*]Ta)2(µ-H)4 was obtained. The excess H2 gas was carefully vented from the reaction vessel, and the reaction mixture was evaporated to dryness in vacuo to yield a dark red-brown residue.  This residue was then extracted with Et2O (10 mL) and cooled in a glovebox freezer (-35 °C), whereupon a dark   222 coloured solid precipitated out of solution.  The precipitate was collected on a sintered glass frit, washed with a minimum amount of cold pentane and dried in vacuo to afford 55 mg (0.037 mmol, 34%, from 2.6) or 64 mg (0.043 mmol, 39%, from 2.7).  Low yields were obtained due to the appreciable solubility of 5.4 in pentane.  1H NMR (C6D6, 300 MHz, 298 K): δ = 14.70 (t, JHP = 3.2 Hz, 4H, Ta(µ-H)4Ta), 7.57 (m, 4H), 7.20 (bs, 4H), 7.13 (m, 6H), 6.99 (s, 4H), 6.93 (s, 2H), 6.60 (bd, JHH = 8.7 Hz, 4H), 6.33 (s, 2H), 5.95 (m, 4H) (ArH), 2.45 (bs, 6H), 2.22 (bs, 6H), 2.17 (bs, 6H), 2.10 (bs, 6H), 1.87 (bs, 12H), 1.82 (bs, 12H) (ArCH3). 31P{1H} NMR (C6D6, 120 MHz): δ = 32.3 (s). 13C{1H} NMR (C6D6, 75 MHz): δ = 162.6, 161.0, 152.9, 151.4, 136.5 (d, JCP = 41 Hz), 135.8, 134.4, 134.1, 133.4 (d, JCP = 11 Hz), 133.3, 132.9, 132.7, 130.8, 130.2, 129.5, 129.1, 128.4, 123.7 (d, JCP = 41 Hz), 116.9, 115.8 (ArC), 21.3, 21.25, 20.7, 20.2, 19.9, 19.5 (ArCH3). 1H NMR (toluene-d8, 400 MHz, 298 K): δ =14.67 (t, JHP = 3.2 Hz, 4H, TaHTa), 7.56 (m, 4H), 7.15 (s, 10H), 6.97 (bs, 4H), 6.88 (bs, 2H), 6.53 (bd, JHH = 8.8 Hz, 4H), 6.26 (bs, 2H), 5.89, (bs, 2H) 5.79 (bs, 2H) (ArH), 2.45 (bs, 6H), 2.19 (bs, 6H), 2.02 (bs, 6H), 1.88 (bs, 12H), 1.82 (bs, 12H) 1.72 (bs, 6H) (ArCH3). 1H NMR (toluene-d8, 400 MHz, 253 K): δ =14.54 (t, JHP = 3.2 Hz, 4H, TaHTa), 7.56 (m, 4H), 7.2 – 6.9 (overlapping signals, approx. 16 aromatic protons and residual toluene-d8 protons), 6.53 (bd, JHH = 8.8 Hz, 4H), 6.28 (bs, 2H), 5.97, (dd, JHP = 6 Hz, JHH = 8 Hz, 2H), 5.86 (dd, JHP = 6 Hz, JHH = 8 Hz, 2H) (ArH), 2.47 (s, 6H), 2.23, (s, 6H), 2.18 (s, 6H), 2.06 (s, 6H), 1.88 (s, 6H), 1.85 (s, 6H), 1.80 (s, 6H) 1.77 (s, 6H) (ArCH3). 1H NMR (toluene-d8, 400 MHz, 363 K): δ = 14.81 (t, JHP = 4 Hz, 4H, TaHTa), 7.56 (m, 4H), 7.20 (bs, 6H), 7.12 (s, 2H), 6.95 (s, 4H), 6.86 (s, 4H), 6.53 (d, JHH = 8 Hz, 6H), 5.74 (dd, JHP = 6 Hz, JHH = 8 Hz, 4H) (ArH), 2.28 (s, 12H), 1.90 (bs, 12H), 1.87 (s, 12H), 1.81 (bs, 12H) (ArCH3). Anal. Calcd. for C76H82N4P2Ta2: C, 61.87; H, 5.60; N, 3.80; Found: C, 59.61; H, 6.03; N, 3.46.   223 [PhNPN*]Ta(trans-1,2-bis-trimethylsilylethene)H (5.5) A thick-walled 200 mL glass vessel equipped with a Teflon valve was charged with a bright red benzene solution (4 mL) of 2.7 (110 mg, 0.11 mmol) and the headspace gas was removed via one freeze-pump-thaw cycle.  The vessel was back-filled with H2 gas and sealed under atmospheric pressure at room temperature. After stirring at room temperature for 36 hours, a dark brown-red solution of 5.5 was obtained; the reaction mixture was evaporated to dryness in vacuo to yield a dark red-brown residue.  This residue was then extracted with Et2O (10 mL) and cooled in a glovebox freezer (-35 °C), whereupon a dark red coloured solid precipitated out of solution.  The precipitate was collected on a sintered glass frit, washed with a minimum amount of cold pentane and dried in vacuo to afford 67 mg (0.074 mmol, 68%).  1H NMR (C6D6, 400 MHz): δ = 24.3 (d, JHP = 44 Hz, 1H, TaH), 7.68 (s, 1H), 7.58 (d, JHH = 7 Hz, 1H), 7.52 (s, 1H), 7.11 (m, 4H), 6.84 (s, 1H), 6.77 (d, JHH = 10 Hz, 2H), 6.76 (s, 1H), 6.70 (d, JHH = 10 Hz, 2H), 6.20 (dd, JHP = 5 Hz, JHH = 8 Hz, 1H), 5.78 (dd, JHH = 8 Hz, JHP = 5 Hz, 1H) (ArH), 2.80 (s, 3H), 2.24 (s, 3H) 2.12 (s, 3H), 2.10 (s, 3H), 2.07 (s, 3H), 2.04 (s, 3H), 1.90 (s, 3H), 1.82 (s, 3H) (ArCH3), 0.04 (s, 9H), -0.12 (s, 9H) (Si(CH3)3), -1.06 (d, JHH = 22 Hz, 1H), -1.11 (d, JHH = 22 Hz) (TMS(H)C=C(H)TMS). 31P{1H} NMR (C6D6, 160 MHz): δ = 16.0 (s). 13C{1H} NMR (C6D6, 100 MHz): δ = 160.4 (d, JCP = 32 Hz), 158.7 (d, JCP = 28 Hz), 142.9, 139.0 (d, JCP = 4 Hz), 138.2 (d, JCP = 4 Hz), 137.8, 136.3, 135.9, 134.8, 134.65, 134.5, 134.4, 133.95, 133.9, 133.8, 131.0 (d, JCP = 5 Hz), 130.6, 130.55, 130.2 (d, JCP = 5 Hz), 129.9, 129.8 (d, JCP = 2 Hz), 129.6, 128.8, 128.7, 124.6, 124.5, 123.5 (d, JCP = 36 Hz), 122.0 (d, JCP = 38 Hz), 116.0 (d, JCP = 9 Hz), 115.4 (d, JCP = 10 Hz) (ArC) 75.0, 57.9 (TMS(H)C=C(H)TMS), 22.4, 21.1, 20.8, 20.3, 20.2, 19.9, 19.7, 19.1 (ArCH3), 2.87, 0.58 (SiMe3). Anal. Calcd. for C46H60N2Si2P1Ta1: C, 60.78; H, 6.65; N, 3.08; Found: C, 61.06; H, 6.60; N, 3.31.   224 [PhNPN*]Ta(1-hexene)H (5.6) A thick-walled 200 mL glass vessel equipped with a Teflon valve was charged with a dark yellow benzene solution (4 mL) of 2.6 (100 mg, 0.11 mmol) and the headspace gas was removed via one freeze-pump-thaw cycle.  The vessel was back-filled with H2 gas and sealed under atmospheric pressure at room temperature. After stirring at room temperature for 6 hours, a dark brown solution of 5.6 was obtained; the reaction mixture was evaporated to dryness in vacuo to yield a dark brown residue.  This residue was then extracted with Et2O (10 mL) and cooled in a glovebox freezer (-35 °C), whereupon a dark brown solid precipitated out of solution.  The precipitate was collected on a sintered glass frit, washed with a minimum amount of cold pentane and dried in vacuo to afford 40 mg (0.049 mmol, 44%). Low yields were obtained due to the appreciable solubility of 5.6 in pentane.  1H NMR (C6D6, 400 MHz): δ = 23.7 (d, JHP = 36 Hz, 1H, TaH), 7.77 (dd, JHH = 20 Hz, JHP = 12 Hz, 2H), 7.63 (dd, JHH = 11 Hz, JHP = 7 Hz, 2H), 7.11 (m, 2H), 6.91 (s, 1H), 6.86 (s, 2H), 6.83 (dd, JHH = 11 Hz , JHP = 8, 2H), 6.59 (s, 2H), 6.00 (dd, JHH = 12 Hz, JHP = 8 Hz, 1H), 5.83 (dd, JHH = 12 Hz, JHP = 8 Hz, 1H) (ArH), 2.87 (s, 3H), 2.64 (s, 3H), 2.08 (s, 3H) (ArCH3), 2.07 (m, 1H, H1a/b), 2.03 (s, 3H), 2.01 (s, 6H) (ArCH3), 1.84 (m, 1H, H4), 1.72 (s, 3H), 1.60 (s, 3H) (ArCH3), 1.52 (m, 1H, H5), 1.21 (m, 1H, H3), 1.17 (m, 1H, H2), 0.77 (t, JHH = 7 Hz, 1H, H6), 0.43 (ddd, JHH = 17 Hz, 13 Hz, JHP = 4 Hz, 1H, H1b). 31P{1H} NMR (C6D6, 160 MHz): δ = 30.0 (s). 13C{1H} NMR (C6D6, 100 MHz): δ = 162.1 (d, JCP = 34 Hz), 161.4 (d, JCP = 34 Hz), 141.4, 140.9, 136.9, 136.8, 136.75 (d, JCP = 25 Hz), 135.5, 135.0, 134.9 (d, JCP = 2 Hz), 134.8 (d, JCP = 2 Hz), 134.5, 134.48, 134.45, 134.4, 134.3, 132.1 (d, JCP = 12 Hz), 131.4 (d, JCP = 6 Hz), 130.5 (d, JCP = 4 Hz), 130.4, 130.2, 129.9, 129.65, 129.6, 129.1 (d, JCP = 9 Hz), 125.5 (d, JCP = 36 Hz), 124.5 (d, JCP = 36 Hz), 116.3 (d, JCP = 11 Hz), 115.5 (d, JCP = 10 Hz) (ArC) 74.73 (C2), 43.93   225 (C1), 37.50, 36.84, 23.35 (C3, C4, C5), 21.65, 21.21, 21.12, 20.90, 20.42, 20.38, 18.64, 17.69 (ArCH3), 14.47 (C6). Anal. Calcd. for C46H60N2Si2P1Ta1: C, 64.38; H, 6.39; N, 3.41; Found: C, 62.30; H, 5.91; N, 5.04. EI-MS (m/z): 816 (15, [M – 4 H]+), 734 (80, [Ta{PhNPN*}]+), 556 (30, [PhNPN*]+), 541 (100, [{PhNPN*} – Me]+).  [PhNPN*]Ta(1-hexene–dn)D (5.6–dn) A sample of 5.6–dn was prepared using 2.6 and D2, in a manner identical to that for 5.6. 1H NMR (C6D6, 400 MHz): δ = 7.77 (dd, JHH = 20 Hz, JHP = 12 Hz, 2H), 7.63 (dd, JHH = 11 Hz, JHP = 7 Hz, 2H), 7.11 (m, 2H), 6.91 (s, 1H), 6.86 (s, 2H), 6.83 (dd, JHH = 11 Hz , JHP = 8, 2H), 6.59 (s, 2H), 6.00 (dd, JHH = 12 Hz, JHP = 8 Hz, 1H), 5.83 (dd, JHH = 12 Hz, JHP = 8 Hz, 1H) (ArH), 2.87 (s, 3H), 2.64 (s, 3H), 2.08 (s, 3H), 2.03 (s, 3H), 2.01 (s, 6H), 1.72 (s, 3H), 1.60 (s, 3H) (ArCH3). 31P{1H} NMR (C6D6, 160 MHz): δ = 30.0 (bs). 13C{1H} NMR (C6D6, 100 MHz): δ = 162.1 (d, JCP = 34 Hz), 161.4 (d, JCP = 34 Hz), 141.4, 140.9, 136.9, 136.8, 136.75 (d, JCP = 25 Hz), 135.5, 135.0, 134.9 (d, JCP = 2 Hz), 134.8 (d, JCP = 2 Hz), 134.5, 134.48, 134.45, 134.4, 134.3, 132.1 (d, JCP = 12 Hz), 131.4 (d, JCP = 6 Hz), 130.5 (d, JCP = 4 Hz), 130.4, 130.2, 129.9, 129.65, 129.6, 129.1 (d, JCP = 9 Hz), 125.5 (d, JCP = 36 Hz), 124.5 (d, JCP = 36 Hz), 116.3 (d, JCP = 11 Hz), 115.5 (d, JCP = 10 Hz) (ArC), 21.65, 21.21, 21.12, 20.90, 20.42, 20.38, 18.64, 17.69 (ArCH3). EI-MS (m/z): 825 ((5, [5.6–d5]+), 816 (5, [5.6 – 4 H]+), 738 (80), 556 (30, [PhNPN*]+), 541 (100, [{PhNPN*} – Me]+).   7.2.5 Complexes Pertaining to Chapter 6 [[PhNPN*]Ta(NR)(THF)x][B(C6F5)4] (6.6) At room temperature, [CPh3][B(C6F5)4] (50 mg, 0.054 mmol) was added to a stirring toluene solution (5 mL) of 3.5 or 3.6 (54 mg, 0.054 mmol), and the mixture was left to stir at room   226 temperature for 30 mins.  The resulting oily suspension was evaporated to dryness in vacuo to yield a bright red glassy residue.  The reaction vessel was charged with pentane (10 mL), vigorously shaken and then allowed to stand for 10 minutes, after which the supernatant solution was decanted away by Pasteur pipette; this procedure was repeated twice to remove residual BnCPh3. The remaining red residue was dissolved in a mixture of toluene-d8 (2 mL) and THF (~12 mg), for analysis by NMR spectroscopy. 1H NMR (toluene-d8, 400 MHz): δ = 7.53 (2H, m), 7.2 – 6.8 (overlapping signals, approx. 7 aromatic protons and residual toluene-d8 protons), 6.73 (2H, d, JHH =8 Hz), 6.67 (bs, 2H), 6.63 (2H, bd, JHH =8 Hz), 6.02 (d, JHH = 8 Hz, 2H), 5.91 (dd, JHH = 8 Hz, JHP = 6 Hz, 2H) (ArH), 3.75 (bs, THF), 2.66 (sept, JHH = 7 Hz, 1H, (CH)Me2), 2.19 (s, 6H), 2.16 (bs, 6H), 1.95 (s, 6H), 1.73 (bs, 6H) (ArCH3), 1.44 (bs, THF), 1.05 (d, JHH = 7 Hz, 6H, (CH)Me2). 31P{1H} NMR (toluene-d8, 120 MHz): δ = 12.1 (s).  13C{1H} NMR (toluene-d8, 100 MHz): δ = 161.97 (d, JCP = 30 Hz), 156.95, 151.6 (d, JCP = 6 Hz), 150.33 (m, C6F5), 147.89 (m, C6F5), 147.87, 147.14, 146.53, 144.42, 138.94, 138.28 (m, C6F5), 135.82 (m, C6F5), 135.62, 135.30, 131.54, 131.35 (d, JCP = 10 Hz), 130.19, 129.85 (d, JCP = 7 Hz), 128.62 (d, JCP = 8 Hz), 127.81, 127.10 (d, JCP = 14 Hz), 126.14, 125.52, 117.8 (d, JCP = 11 Hz), 117.2 (ArC), 65.4 (bs, THF), 33.8 (CHMe2), 26.1 (bs, THF), 24.15 (CHMe2), 20.61, 20.22, 19.44, 18.89 (ArCH3). 11B{1H} NMR (toluene-d8, 128 MHz): δ = -16.1 (s). 19F{1H} NMR (toluene-d8, 282 MHz): δ = -132.0 (m), -163.1 (t, JFF = 21 Hz), -166.9 (bt, JFF = 18 Hz).     227 Bibliography (1) Rosenthal, U.; Burlakov, V. 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Basic One- and Two-Dimensional NMR Spectroscopy; Third Revised Edition ed.; Wiley-VCH: Weinheim, 1998.     248 Appendices Appendix A  : Variable-temperature NMR Study of [PhNPN*]Ta(3-hexyne)Cl (2.2)  In toluene-d8 at room temperature (298 K), the methyl and methylene 1H NMR resonances for the hexyne unit in 2.2 appear as a two broad singlets (in the case of the methylene protons, so broad as to be essentially unobservable) at δ 0.98 and δ 2.90, respectively.  The exchange between the two sets of methyl and methylene protons (via rotation about the Ta–hexyne bond) of 2.2 is depicted in Scheme A.1.   Scheme A.1: Rotation about the Ta – alkyne bond leads to exchange between the two ethyl arms (blue and red) at elevated temperature.  Coalescence of these signals is observed at 358 K; at 253 K, the methyl and methylene resonances are separated by 460 ± 5 Hz and 210 ± 5 Hz, respectively.  Figures A.1 and A.2 show stacked plots of the 1H NMR spectra of 2.2 in d8-toluene, acquired every 10 K from 358 K to 298 K and 283 K to 223 K, respectively.  Ta NP NCCH2CCH2 ClH3CH3C (A2.1)Ta NP NCCH2CCH2 ClH3CH3C  249  Figure A.1: 1H NMR spectra of 2.2 in toluene-d8, from 358 K to 298 K.  At 358 K (Tc), the methyl (^) and methylene resonances (#) of the hexyne unit (and residual pentane(*)) are labeled.  Figure A.2: 1H NMR spectra of 2.2 in toluene-d8, from 298 K to 223 K.  At 253 K, the methyl (^) and methylene resonances (#) of the hexyne unit (and residual pentane(*)) are labeled. # ^ * * 358 K 348 K 338 K 328 K          318 K     308 K           298 K    * * 298 K 283 K 273 K 253K 243 K          233 K     223 K          # # ^ ^   250  At its coalescence temperature the rate constant for an exchange reaction can be expressed as:  kc = πΔν/(√2) = 2.22Δν where kc is the rate constant at Tc, and Δν is the separation between the two peaks at a temperature well below Tc.340  For this calculation, Tc = 358 K, and the Δν values were taken from the 253 K experiment: 460 ± 5 Hz (methyl) and 210 ± 5 Hz (methylene).  Thus, for the reaction depicted in Scheme A.1, kc = 1021.2 s-1 (methyl) and 466.2 s-1 (methylene).  The Eyring equation can be used to calculate ΔG‡rot,  ΔG‡rot = –RTcln(kch/kBTc) where R is the ideal gas constant (8.314 JᐧK-1mol-1), Tc is the coalescence temperature, kc is the rate constant, h is Planck’s constant (6.626 x 10-34 Jᐧs) and kB is the Boltzmann constant (1.380 x 10-23 JᐧK-1).   From this analysis, ΔG‡rot = 67.6 ± 1.2 kJ/mol (methyl) and 69.9 ± 1.3 kJ/mol (methylene), resulting in an average value of ΔG‡rot = 68.7 ± 1.2 kJ/mol.  The major source of error in this experiment is related to the determination of the coalescence temperature; as spectra were acquired in 10 K intervals, a value of 358 ± 5 K was used for Tc.           251 Appendix B  : Kinetic Study of the Thermal Rearrangement of 4.1 to 4.2 At room temperature, complex 4.1, [PhNPN*]Ta(EtC≡CEt)(CH=CHPh), rearranges to form complex 4.2, [PhNPN*]TaC(Et)C(Et)C(H)C(H)Ph, over the course of approximately 3 days. In order to study the kinetic parameters of this rearrangement, the 1H NMR spectra of four samples of a 45 mM solution of 4.1 (freshly prepared from equimolar amounts of 2.4 and phenylacetylene) in C6D6 were acquired periodically in a NMR spectrometer heated to 318 K, 328 K, 333 K (900 s or 1200 s periods) or 303.8 K (8-12 h periods).  The decrease in the concentration of 4.1 over time (-d[4.1]/dt) was measured until at least 90% (> 3 t½) of the starting material had been consumed.  As depicted in the equation below, [4.1] can be expressed as the integral measured for Ta(CH=CHPh) in 4.1 (δ 8.69), divided by the sum of that integral and the integral for ‘H2’ in 4.2 (δ 4.33).   –d[4.1]/dt = –kobs[4.1] = –kobs [int Ta(CH=CHPh){4.1}] / ([int Ta(CH=CHPh){4.1}] + [int H2{4.2}])  In this way, the fraction of 4.1 remaining in solution (relative to 1.0) is used, rather than an absolute molar concentration.  The resonances used in this determination were chosen due to their lack of overlap with other signals, and were normalized to an internal standard (ferrocene) of unchanging concentration. Using the data from the 318 K run as an example, the rearrangement was determined to be first order in 4.1 based on the linearity of the ln[4.1] versus time plot (Figure B.1); a reaction order of two or zero was discounted based on the poor linear fit of the (1/[4.1]) versus time (Figure B.2) or [4.1] versus time (Figure B.3) plots, respectively.    252  Figure B.1: Plot of ln[4.1] versus time at 318 K; linearity suggests first-order kinetics.  Figure B.2: Plot of 1/[4.1] versus time at 318 K. R²	  =	  0.99579	  -­‐3	  -­‐2.5	  -­‐2	  -­‐1.5	  -­‐1	  -­‐0.5	  0	  0	   10000	   20000	   30000	   40000	   50000	   60000	  ln([4.1])	  ⬬e	  (s)	  0	  2	  4	  6	  8	  10	  12	  14	  16	  0	   10000	   20000	   30000	   40000	   50000	   60000	  1/[4.1]	  ⬬e	  (s)	    253  Figure B.3: Plot of [4.1] versus time at 318 K.  Thus, the rate constants (kobs) at 4 temperatures (303.8, 318, 328 and 333 K) were determined from the slopes of the lines of best fit of relevant ln[4.1] versus time plots: k303.8 = 8.81 x 10-6 ± 2 x 10-7 s-1 k318 = 5.01 x 10-5 ± 6 x 10-7 s-1 k328 = 1.44 x 10-4 ± 2 x 10-6 s-1 k333 = 2.43 x 10-4 ± 4 x 10-6 s-1 It is possible to use this data to extract several kinetic parameters for the rearrangement of 4.1 to 4.2, in the context of Transition State Theory.  The Eyring equation relates the enthalpy of activation (ΔH‡) and entropy of activation (ΔS‡) to the temperature (T) and observed rate constant (kobs) of a reaction: kobs = (kBT)/h exp(ΔS‡/R) exp(–ΔH‡/RT)  0	  0.1	  0.2	  0.3	  0.4	  0.5	  0.6	  0.7	  0.8	  0.9	  1	  0	   10000	   20000	   30000	   40000	   50000	   60000	  [4/1]	  ⬬e	  (s)	    254 This can be rearranged to give a new expression that resembles the equation of a straight line,  y = mx + b:    ln (kobs/T) = (–ΔH‡/R)T-1 + [ln(kB/h) + (ΔS‡/R)] A plot of ln(kobs/T) versus 1/T was constructed and is shown in Figure B.4; the slope and y-intercept of the line of best fit from this graph were used to determine: ΔH‡ = 92.73 ± 1.2 kJ/mol         ΔS‡ = -36.47 ± 0.82 J/(mol)(K)  Figure B.4: Eyring plot of ln(kobs/T) versus 1/T for the rearrangement of 4.1 to 4.2.  For these experiments, the error in temperature is estimated to be ± 1 K (except for 303.8 K, where it is estimated to be ± 1.5 K).  Errors in the integration of NMR spectra resonances is estimated to be ± 5%.  The errors in the slope of the ln[4.1] versus time plots (used to determine kobs) and the slope and y-intercept of the Eyring plot (used to determine ΔH‡ and ΔS‡) were determined using the linear regression statistics package in Microsoft Excel for Mac 2011.  y = -11154x + 19.374 R² = 0.99969 -18 -17 -16 -15 -14 -13 -12 0.0029 0.003 0.0031 0.0032 0.0033 0.0034 ln(kobs/T) 1/T (K-1)   255 Appendix C  : X-ray Crystallographic Structure and Refinement Data C1: Crystallographic structure and refinement data Compound 2.2 2.3 Dataset ID mf822 mf878 Empirical Formula C44H49Cl1N2P1Ta1 C46H57Cl1N2P1Si2Ta1 Formula Weight 853.25 941.49 λ/Å 0.71073 0.71073 Temperature 100(2) K 100(2) K Crystal System Triclinic Monoclinic Space group P-1 P2(1)/n a/Å 12.2733(5) 12.2140(5) b/Å 13.1929(5) 20.6230(10) c/Å 18.2941(8) 18.3430(8) α/° 85.154(2) 90 β/° 75.726(2) 107.955(2) ɣ/° 62.928(2) 90 V/Å3 2554.89(18) 4395.4(3) Z 4 4 ρ/ g cm-1 1.211 1.423 µ/mm-1 2.265 2.686 F(000) 948 1920 Absorption Correction Multi-scan Multi-scan Crystal size/mm3 0.36 x 0.25 x 0.20 0.30 x 0.15 x 0.15 θmin - θmax /° 1.73 to 25.23 1.53 to 32.73 Reflections collected 32636 84639 Independent reflections (Rint) 9163 (0.0318) 14436 (0.0456) Completeness to θmax 99.70% 89.00% Max. and min. transmission 0.6357 and 0.5113 0.6683 and 0.4865 Data/ restraints/ parameters 9163/0/496 14436/0/478 Goodness-of-fit on F2 1.042 1.049 R [I > 2σ(I)] (R1, wR2) (0.0218, 0.0569) (0.0278, 0.0673) R (all data) (R1, wR2) (0.0240, 0.5777) (0.0373, 0.0728) Largest diff. peak and hole (e/Å3) 1.070 and -0.713 2.042 and -1.081 R1 = Σ||Fo|-|Fc|| / Σ|Fo|; wR2 = [ Σ(w(Fo2 - Fc2)2) / Σ w(Fo2)2]½     256 C2: Crystallographic structure and refinement data Compound 2.4 2.6 Dataset ID mo_mf919 mf868 Chemical Formula C44H50N2P1Ta1 C51H56N2P1Ta1 Formula Weight 818.78 908.9 λ/Å 0.71073 0.71073 Temperature 100(2) K 100(2) K Crystal System Triclinic Triclinic Space group P-1 P-1 a/Å 9.038(5) 11.424(5) b/Å 11.789(5) 12.606(5) c/Å 17.915(5) 18.071(5) α/° 79.960(5) 82.590(5) β/° 87.070(5) 89.160(5) ɣ/° 84.776(5) 77.086(5) V/Å3 1870.6(14) 2515.1(16) Z 2 2 ρ/ g cm-1 1.454 1.2 µ/mm-1 3.014 2.248 F(000) 832 928 Absorption Correction Multi-scan Multi-scan Crystal size/mm3 0.42 x 0.20 x 0.10 0.40 x 0.35 x 0.25  θmin - θmax /° 1.16 to 25.01 1.67 to 24.95 Reflections collected 23892 31085 Independent reflections (Rint) 6567 (0.0280) 8690 (0.0332) Completeness to θmax 99.4 98.6 Max. and min. transmission 0.7398 and 0.5447 0.5701 and 0.3756  Data/ restraints/ parameters 6567/0/447 8690/54/521 Goodness-of-fit on F2 1.098 1.092 R [I > 2σ(I)] (R1, wR2) (0.0222, 0.0582) (0.0767, 0.2295) R (all data) (R1, wR2) (0.0246, 0.0597) (0.088, 0.2432) Largest diff. peak and hole (e/Å3) 1.499 and -0.597 5.868 and -1.993 R1 = Σ||Fo|-|Fc|| / Σ|Fo|; wR2 = [ Σ(w(Fo2 - Fc2)2) / Σ w(Fo2)2]½      257 C3: Crystallographic structure and refinement data Compound 2.7 2.8 Dataset ID mf896 mo_mf931 Chemical Formula C53H64N2P1Si2Ta1(Et2O) C48H62N2P1Si2Ta1(Et2O) Formula Weight 1071.28 1009.22 λ/Å 0.71073 0.71073 Temperature 100(2) K 100(2) K Crystal System Monoclinic Triclinic Space group P2(1)/n P-1 a/Å 12.1271(2) 11.595(5) b/Å 20.5083(5) 14.517(5) c/Å 21.0150(5) 17.823(5) α/° 90.0060(10) 98.849(5) β/° 92.1840(10) 92.363(5) ɣ/° 89.9970(10) 109.344(5) V/Å3 5222.8(2) 2783.3(17) Z 4 2 ρ/ g cm-1 1.362 1.204 µ/mm-1 2.221 2.08 F(000) 2216 1044 Absorption Correction Multi-scan Multi-scan Crystal size/mm3 0.35 x 0.20 x 0.15  0.38 x 0.30 x 0.15 θmin - θmax /° 1.91 to 27.15 1.51 to 27.69 Reflections collected 45376 45908 Independent reflections (Rint) 11584 (0.0396) 12842 (0.0198) Completeness to θmax 99.9 98.6 Max. and min. transmission 0.7166 and 0.6092  0.7320 and 0.6725 Data/ restraints/ parameters 11584/0/572 12842/0/532 Goodness-of-fit on F2 0.998 2.428 R [I > 2σ(I)] (R1, wR2) (0.0270, 0.0540) (0.0488, 0.1358) R (all data) (R1, wR2) (0.0432, 0.0601) (0.0559, 0.1398) Largest diff. peak and hole (e/Å3) 0.635 and -0.584 7.390 and -1.979 R1 = Σ||Fo|-|Fc|| / Σ|Fo|; wR2 = [ Σ(w(Fo2 - Fc2)2) / Σ w(Fo2)2]½      258 C4: Crystallographic structure and refinement data Compound 3.3 3.5 Dataset ID mo_mf960 mo_mf945 Chemical Formula C45H43Cl1N3P1Ta1 C54H57N3P1Ta1 Formula Weight 873.19 959.95 λ/Å 0.71073 0.71073 Temperature 100(2) K 100(2) K Crystal System Orthorhombic Orthorhombic Space group P nma P212121 a/Å 20.881(3) 13.320(5) b/Å 16.562(3) 17.789(5) c/Å 11.709(2) 21.868(5) α/° 90 90 β/° 90 90 ɣ/° 90 90 V/Å3 4049.1(12) 5182(3) Z 4 4 ρ/ g cm-1 1.432 1.231 µ/mm-1 2.854 2.187 F(000) 1756 1960 Absorption Correction Multi-scan Multi-scan Crystal size/mm3 0.18 x 0.15 x 0.11 0.16 x 0.14 x 0.10 θmin - θmax /° 1.95 to 26.10 1.48 to 25.12 Reflections collected 27443 35831 Independent reflections (Rint) 4154 (0.0376) 9214 (0.0347) Completeness to θmax 99.4 99.5 Max. and min. transmission 0.7306 and 0.5528 0.8036 and 0.5187 Data/ restraints/ parameters 4154/619/308 9214/0/542 Goodness-of-fit on F2 1.239 1.156 R [I > 2σ(I)] (R1, wR2) (0.0519, 0.1077) (0.0505, 0.1511) R (all data) (R1, wR2) (0.0691, 0.1198) (0.0592, 0.1621) Largest diff. peak and hole (e/Å3) 3.751 and -2.046 1.964 and -1.292 R1 = Σ||Fo|-|Fc|| / Σ|Fo|; wR2 = [ Σ(w(Fo2 - Fc2)2) / Σ w(Fo2)2]½      259 C5: Crystallographic structure and refinement data Compound 3.7 4.2 Dataset ID mf992 mf904 Chemical Formula C63H68N6P1Ta1 C52H56N2P1Ta1 Formula Weight 1121.15 920.91 λ/Å 0.71073 0.71073 Temperature 100(2) K 100(2) K Crystal System Triclinic Monoclinic Space group P-1 P2(1)/c a/Å 11.280(3) 11.1759(4) b/Å 13.715(4) 33.5707(11) c/Å 19.464(6) 12.7008(4) α/° 96.726(15) 90 β/° 96.357(16) 99.1010(10) ɣ/° 110.692(14) 90 V/Å3 2759.6(14) 4705.1(3) Z 2 4 ρ/ g cm-1 1.349 1.3 µ/mm-1 2.065 2.404 F(000) 1152 1880 Absorption Correction Multi-scan Multi-scan Crystal size/mm3  0.24 x 0.20 x 0.10 0.35 x 0.18 x 0.10 θmin - θmax /° 1.61 to 27.91 1.73 to 25.07 Reflections collected 43154 33385 Independent reflections (Rint) 12848 (0.0398) 8328 (0.0393) Completeness to θmax 99.5 99.7 Max. and min. transmission  0.8120 and 0.7157 0.7863 and 0.6935 Data/ restraints/ parameters 12848/0/640 8328/0/503 Goodness-of-fit on F2 1.044 1.016 R [I > 2σ(I)] (R1, wR2) (0.0308, 0.0695) (0.0281, 0.0587) R (all data) (R1, wR2) (0.0387, 0.0730) (0.0419, 0.0617) Largest diff. peak and hole (e/Å3) 1.392 and -0.856 0.862 and -0.441 R1 = Σ||Fo|-|Fc|| / Σ|Fo|; wR2 = [ Σ(w(Fo2 - Fc2)2) / Σ w(Fo2)2]½      260 C6: Crystallographic structure and refinement data Compound 4.6 5.3 Dataset ID mo_mf978 mf890 Chemical Formula C55H67N3P1Si2Ta1 C41H48N2P1Ta1 Formula Weight 1038.22 780.73 λ/Å 0.71073 0.71073 Temperature 100(2) K 100(2) K Crystal System Triclinic Monoclinic Space group P-1 P2(1)/n a/Å 19.722(5) 11.467(5) b/Å 21.120(5) 12.400(5) c/Å 22.644(5) 25.080(5) α/° 113.594(5) 90 β/° 97.971(5) 99.420(5) ɣ/° 112.323(5) 90 V/Å3 7511(3) 3518(2) Z 6 4 ρ/ g cm-1 1.377 1.474 µ/mm-1 2.314 3.201 F(000) 3204 1584 Absorption Correction Multi-scan Multi-scan Crystal size/mm3 0.25 x 0.10 x 0.09 0.30 x 0.15 x 0.08  θmin - θmax /° 1.19 to 28.09 1.84 to 25.10 Reflections collected 137218 21985 Independent reflections (Rint) 36296 (0.0305) 6227 (0.0208) Completeness to θmax 99.2 99.3 Max. and min. transmission 0.8119 and 0.6502 0.7741 and 0.6259 Data/ restraints/ parameters 36296/0/1687 6227/0/406 Goodness-of-fit on F2 1.015 1.053 R [I > 2σ(I)] (R1, wR2) (0.0263, 0.0637) (0.0238, 0.0566) R (all data) (R1, wR2) (0.0323, 0.0669) (0.0270, 0.0576) Largest diff. peak and hole (e/Å3) 0.861 and -1.040 0.817 and -0.725 R1 = Σ||Fo|-|Fc|| / Σ|Fo|; wR2 = [ Σ(w(Fo2 - Fc2)2) / Σ w(Fo2)2]½      261 C7: Crystallographic structure and refinement data Compound 5.4 5.5 Dataset ID mf895 mo_mf906 Chemical Formula C76H78N4P2Ta2 C46H60N2P1Si2Ta1 Formula Weight 1471.26 909.06 λ/Å 0.71073 0.71073 Temperature 100(2) K 100(2) K Crystal System Monoclinic Triclinic Space group C 2/c P-1 a/Å 16.309(5) 11.263(5) b/Å 32.774(5) 12.398(5) c/Å 14.072(5) 18.608(5) α/° 90 83.546(5) β/° 90.320(5) 89.917(5) ɣ/° 90 78.366(5) V/Å3 7522(4) 2528.3(17) Z 4 2 ρ/ g cm-1 1.299 1.194 µ/mm-1 2.99 2.281 F(000) 2952 932 Absorption Correction Multi-scan Multi-scan Crystal size/mm3  0.27 x 0.25 x 0.15 0.35 x 0.30 x 0.10 θmin - θmax /° 1.91 to 22.74 1.85 to 25.37 Reflections collected 19515 33054 Independent reflections (Rint) 5046 (0.0526) 9148 (0.0340) Completeness to θmax 99.4 98.6 Max. and min. transmission 0.6385 and 0.5384 0.7960 and 0.6620 Data/ restraints/ parameters 5046/36/333 9148/0/481 Goodness-of-fit on F2 1.475 1.062 R [I > 2σ(I)] (R1, wR2) (0.0604, 0.1692) (0.0244, 0.0625) R (all data) (R1, wR2) (0.0981, 0.1834) (0.0281, 0.0638) Largest diff. peak and hole (e/Å3) 1.667 and -0.891 0.720 and -0.902 R1 = Σ||Fo|-|Fc|| / Σ|Fo|; wR2 = [ Σ(w(Fo2 - Fc2)2) / Σ w(Fo2)2]½    

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