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UBC Theses and Dissertations

Early transition metal complexes supported by amidophosphine and amidocarbene ligands Spencer, Liam Patrick 2006

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EARLY TRANSITION METAL COMPLEXES SUPPORTED BY AMIDOPHOSPHINE AND AMIDOCARBENE LIGANDS by LIAM PATRICK SPENCER B.Sc. (Hons.), The University of Victoria, 2000 M.Sc. The University of Windsor, 2002 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (CHEMISTRY) THE UNIVERSITY OF BRITISH COLUMBIA September 2006 © Liam Patrick Spencer,2006 ABSTRACT 1 0 The reactivity of the tantalum dinitrogen complex ([NPN]Ta)2(u-H)2(u-n :n -N2) (where [NPN] = [(PhNSiMe2CH2)2PPh]2") with several zirconium hydride reagents is explored. The addition of [Cp2Zr(Cl)H]x leads to the unanticipated reduction of the N-N bond without Zr-H addition. The coordinated N2 ligand is cleaved to form a triply bridging nitride and a phosphinimide functional group that bridges between Ta and Zr centres. A series of experiments to determine the mechanism of this reaction reveals that a "Cp2Zr" species promotes reduction of the N-N unit. This type of dinitrogen reduction is extended to include the insertion of a "Cp2Ti" fragment into the N-N bond. The synthesis of early transition metal complexes employing a tridentate diamido N-heterocyclic carbene (NHC) ligand set (denoted [NCN]) is also investigated. Aminolysis reactions with diamino-NHC precursors and M(NMe2)4 (M = Ti, Zr, Hf) provide bis(amido)-NHC-metal complexes that can be further converted to chloro and alkyl derivatives. Alkyl elimination reactions with the diamino-NHC ligands and Zr(CH2R)4 (R=Ph, SiMe3) yield dialkyl-NHC-zirconium complexes. The central position of the NHC donor in this tridentate architecture renders the carbene stable to dissociation from the metal centre in strongly coordinating solvents. The hafnium dialkyl complexes are thermally stable with the exception of the dialkyl complex, Mes[NCN]Hf(CH2CH3)2, (where Mes[NCN] = (2,4,6-Me3-C6H2NHCH2CH2)2N2C3H2) which undergoes (3-hydrogen transfer and subsequent C-H bond activation with an ortho-methyl substituent on the mesityl group. Activation of Mes[NCN]M(CH3)2 (M = Zr, Hf) with [Ph3C][B(C6F5)4] yields {Mes[NCN]MCH3}{B(C6F5)4}3 which is a moderately active ethylene polymerization catalyst. The hafnium dialkyl complexes also insert carbon monoxide, substituted isocyanides, and substituted cumulenes into a hafnium-sp3-carbon bond to yield expected insertion products. In some circumstances, further C-C bond coupling occurs to yield enediolate and eneamidolate metallacycles. Attempts to reduce Mes[NCN]ZrCl2 in the presence of dinitrogen lead to mixtures of products. In one case, an ether cleavage product is isolated, which is a result of C-0 bond activation of the solvent used in the reaction. ii Aminolysis and alkyl elimination reactions with the diamino-NHC ligand and tantalum(V) reagents provide complexes with an amide-amine donor configuration. Attempts to promote coordination of the remaining pendant amine donor have been unsuccessful. Metathesis reactions with the lithiated diamido-NHC ligand (Li2Ar[NCN]) and ClxTa(NR.2)5-x derivatives provide a successful method to coordinate both amide donors, yielding the desired Ar[NCN]TaClx(NR2)3-x complexes. Attempts to prepare trialkyl tantalum complexes by this methodology resulted in the formation of a metallaaziridine derivative. DFT calculations on model complexes suggest the lowest energy pathway involves a tantalum alkylidene intermediate, which undergoes amido C-H bond activation to form the metallaaziridine moiety. This mechanism was confirmed by examining the distribution of deuterium atoms in an experiment between Mes[NCN]Li2 and Cl2Ta(CD2Ph)3. The preparation of chiral [NCN] group 4 complexes is achieved by aminolysis and alkyl elimination reactions with a chiral diamino-NHC ligand and suitable group 4 reagents. The titanium and zirconium derivatives are investigated in the asymmetric intramolecular hydroamination of an aminoalkene in an attempt to promote selectivity in the N-heterocycle synthesized. While the titanium [NCN] complex shows no activity, the zirconium [NCN] complex is an efficient catalyst for the intramolecular formation of a substituted pyrollidine. Examination of the steroselectivity in the N-heterocyclic product formed reveals very low enantioselective excess. iii TABLE OF CONTENTS ABSTRACT ii TABLE OF CONTENTS iv LIST OF TABLES viiLIST OF FIGURES xii GLOSSARY OF TERMS xviiACKNOWLEDGEMENTS xxiiDEDICATION xxiv STATEMENT OF AUTHORSHIP xxChapter One Dinitrogen Chemistry and Ligand Design 1.1. Origins of Coordination and Organometallic Chemistry 1 1.2. Reactivity of Dinitrogen 3 1.3. Coordinated Dinitrogen Complexes 5 1.4. Dinitrogen Cleavage 7 1.5. Amidophosphine Ligands for N2 Activation 10 1.6. Introduction to Carbenes 14 1.7. Isolation of Stable Carbenes 1.7.1. Acyclic Carbenes 6 1.7.2. N-Heterocyclic Carbenes 19 1.8. Transition Metal Complexes With Carbene Ligands 1.8.1. Transition Metal Acyclic Carbene Complexes.. 21 1.8.2. Transition Metal NHC Complexes 23 1.9. Late Transition Metal NHC Complexes in Homogeneous Catalysis \ 24 1.10. Scope of This Thesis 25 1.11. References 7 Chapter Two Promoting Dinitrogen Cleavage and Functionalization with Zr(II) and Ti(II) reagents 2.1. Introduction 34 2.2. Attempted Hydrozirconation of 2.5 38 iv 2.3. Reduction and Functionalization of 2.5 with Cp2Ti(II) 43 2.4. Conclusions 44 2.5. Experimental 5 2.5.1. General Considerations 42.5.2. Materials and Reagents 5 2.5.3. Synthesis and Characterization of 2.10 and 2.13 46 2.6. References , 49 Chapter Three Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes 3.1. Introduction 51 3.2. Synthesis of Ar[NCN]H2 and Lithium Derivatives 54 3.3. Attempted Syntheses of an Ar[NCN] Ligand with an Aryl Backbone 59 3.4. Synthesis of Group 4 [NCN] Aamido, Chloride, and Alkyl Complexes 62 3.5. Conclusions 75 3.6. Experimental 7 3.6.1. General Considerations 73.6.2. Materials and Reagents 7 3.6.3. Synthesis and Characterization of Complexes 3.1 - 3.40 77 3.7. References 94 Chapter Four Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes 4.1. Introduction 97 4.2. Hf and Zr Cation Formation and Polymerization Studies 98 4.3. Formation of [NCN] Hafnium n2-Iminoacyls and an Eneamidolate Metallacycle 100 4.4. Formation of a Hafnium Vinyl-enolate and Enediolate Metallacycle 108 4.5. Formation of Amidate and Amidinate Metallacycles 115 4.6. Attempted Synthesis of Group 4 [NCN] Dinitrogen Complexes 120 4.7. Synthesis of Hydrazido(l-) Hafnium [NCN] Complexes 125 v 4.8. Conclusions 130 4.9. Experimental 2 4.9.1. General Considerations 134.9.2. Materials and Reagents 2 4.9.3. Synthesis and Characterization of Complexes 4.4 - 4.18, 4.24 -4.26 134.10. References 143 Chapter Five Synthesis and DFT Studies of Tantalum [NCN] Transition Metal Complexes 5.1. Introduction 148 5.2. Synthesis of Amine-Amide [NCNH] Tantalum Derivatives 150 5.3. Successful Synthesis of Ta[NCN] Amide Complexes 154 5.4. Isolation of Cyclometallated [NCCNJTa Dialkyl Derivatives 157 5.5. Mechanistic Insight Into the Formation of 5.7-5.9 160 5.6. Determination of Mechanism by DFT Calculations 161 5.7. Verification of the Mechanism Proposed by DFT Calculations 165 5.8. Conclusions 168 5.9. Experimental 170 5.9.1. General Considerations 175.9.2. Materials and Reagents 0 5.9.3. Synthesis and Characterization of Complexes 5.1 - 5.10 170 5.10. Reference s 177 Chapter Six Thesis Extensions: Chiral Group 4 [NCN] Complexes 6.1. Introduction 180 6.2. Synthesis of Group 4 [NCN] Complexes 184 6.3. Asymmetric Intramolecular Hydroamination Studies 191 6.4. Conclusions and Future Work 193 6.5. Experimental 197 6.5.1. General Considerations 19vi 6.5.2. Materials and Reagents 197 6.5.3. Synthesis and Characterization of Complexes 6.8 - 6.13, 6.14-6.15 196.7. References 203 Chapter Seven Thesis Summary and Future Work 206 Appendix A X-ray Crystal Structure Data A.l. General Considerations 208 A.2. References 210 A. 3. Tables of Crystallographic Data 211 Appendix B Evaluating the Formation of a Tantalum Metallaaziridine Complex by DFT Calculations B. l. Evaluation of a o-Bond Metathesis Mechanism 219 B.2. Investigation of an Alkylidene Intermediate Followed by C-H Bond Activation 221 B.3. Thermodynamic Considerations for the Formation of Metallated Ta [NCCN] Derivatives 224 B.4. General Considerations 232 B.5. References 233 vii LIST OF TABLES Chapter Two Promoting Dinitrogen Cleavage and Functionalization with Zr(II) and Ti(II) reagents Table Title Pase Table 2.1. Selected bond distances (A) and angles (deg) for ([NP(N)N]Ta(p-H)2(p-N)(Ta[NPN])(ZrCp2) (2.10) 41 Chapter Three Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Table Title Pase Table 3.1. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]H2-Cl (3.5) and Mes[NCN]H2, (3.8) 57 Table 3.2. Selected Bond Distances (A) and Bond Angles (°) for tol[NCN]Zr(NEt2)2, (3.17) 64 Table 3.3. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCNH]Ti(NMe2)3, (3.22) 66 Table 3.4. Selected Bond Distances (A) and Bond Angles (°) for tol[NCN]ZrCl2(py), (3.30) 68 Table 3.5. Selected Bond Distances (A) and Bond Angles (°) for tol[NCN]Zr(CH2SiMe3)2, (3.31) 71 Table 3.6. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]Hf Bu2, (3.38) 73 Chapter Four Synthesis of Group 4 Bis(amidq)-N-Heterocyclic Carbene Complexes Table Title Pase Table 4.1. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]Hf(ri2-XyNCCH3)(CH3), (4.7) 103 Table 4.2. Selected Bond Distances (A) and Bond Angles (°) for vm Mes[NCN]Hf(r)2-XyNCCH3)2, (4.8) 105 Table 4.3. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]Hf(OC(CH3)=C(CH3)NXy); (4.11) 108 Table 4.4. Selected Bond Distances (A) and Bond Angles (°) for (Mes[NCN]Hf)20-OC(iBu)=C(iBu)O)2, (4.16) 112 Table 4.5. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]Hf(Me)(n3-tBuNC(Me)0), (4.17) 117 Table 4.6. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]Hf(Me)(n3-iPrNC(Me)NiPr), (4.18) 119 Table 4.7. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]Zr(Cl)(OCH2CH2CH2CH3), (4.24) 124 Table 4.8. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]Hf(Me)(n2-NHNMe2), (4.25). 129 Chapter Five Synthesis and DFT Studies of Tantalum [NCN] Transition Metal Complexes Table Title Pase Table 5.1. Selected bond distances (A) and angles (°) for tol[NCNH]Ta(NMe2)4, 5.1 152 Table 5.2. Selected bond distances (A) and angles (°) for Mes[NCNH]Ta(CHPh)(CH2Ph)2 (5.2) 154 Table 5.3. Selected bond distances (A) and angles (°) for tol[NCN]Ta(NMe2)3 (5.3) '. 156 Table 5.4. Selected bond distances (A) and angles (°) for ^[NCCNJTa^Hz^u^ (5.7) 159 Table 5.5. Selected bond distances (A) and angles (°) for Mes[NCCN]Ta(Cl)(CH2tBu) (5.10) 167 ix Chapter Six Thesis Summary and Extensions: Chiral Group 4 [NCN] Complexes Table Title Pase Table 6.1. Selected Bond Distances (A) and Bond Angles (°) for (-)-(li?,2'5,47?)-2-(l,7,7-trimethylbicyclo[2.2.1]hept-2-ylamino)ethyl ammonium chloride, (6.8) 186 Appendix A Table Table A.l. Table A.2. Table A.3. Table A.4. Table A.5. Table A.6. Table A.7. X-ray Crystal Structure Data Title Pase Crystallographic and structure refinement for [NP(N)N]Ta(p-H)2(p-N)(Ta[NPN])(ZrCp2) (2.10), Mes(NCHN)H2-Cl (3.5), and Mes(NCN)H2 (3.8) 211 Crystallographic and structure refinement for t0'[NCN]Zr(NEt2)2 (3.17), Mes[NCNH]Ti(NMe2)3 (3.22), and tol[NCN]ZrCl2(py) (3.30).. 212 Crystallographic and structure refinement for tol[NCN]Zr(CH2SiMe3)2 (3.30), Mes[NCN]HfBu2 (3.38), and Mes[NCN]Hf(ri2-XyNCCH3XCH3) (4.7) 213 Crystallographic and structure refinement for Mes[NCN]Hf(ri2-XyNCCH3)2 (4.8), Mes[NCN]Hf(OC(CH3)=C(CH3)NXy) (4.11), and ^[NC^Hfl^-OCCB^^CB^O^ (4.16) 214 Crystallographic and structure refinement for Mes[NCN]Hf(Me)(r)3-tBuNC(Me)0) (4.17), Mes[NCN]Hf(Me)(n3-iPrNC(Me)NiPr) (4.18), and Mes[NCN]Zr(Cl)(OBu) (4.22) 215 Crystallographic and structure refinement for Mes[NCN]Hf(Me)(n2-NNMe2) (4.23), tol[NCNH]Ta(NMe2)4 (5.1), and Mes[NCNH]Ta(CHPh)(CH2Ph)2 (5.2) 216 Crystallographic and structure refinement for tol[NCN]Ta(NMe2)3 (5.3), Mes[NCCN]Ta(CH2tBu)2 (5.7), and ^[NCCNJTa^lXCH/Bu) (5.10) 217 Table A.8. Crystallographic and structure refinement for (-)-(17?,2'5',4i?)-2-(l,7,7-trimethylbicyclo[2.2.1]hept-2-ylamino)ethyl ammonium chloride (6.8) 218 Appendix B Evaluating the Formation of a Tantalum Metallaaziridine Complex by DFT Calculations Table Title . , Pa^e Table B.l. Gas-phase relative energies (kcal/mol) of the intermediates and transition states in a a-bond metathesis mechanism 221 Table B.2. Gas-phase relative energies (kcal/mol) of the intermediates and transition states in a mechanism involving a-H abstraction followed by alkylidene mediated C-H bond activation 223 Table B.3. NBO Occupancies of bonding and anti-bonding orbitals in the trimethyl complex A and the metallaaziridine complex D 227 Table B.4. Important second order perturbation theory analysis NBO donor-acceptor interactions AEy (kcal/mol) that contribute to shorter Ta-carbene and Ta-Me bonds in the metallaaziridine product D relative to the trimethyl complex A 228 xi LIST OF FIGURES Chapter One Dinitrogen Chemistry and Ligand Design Figure Title Page Figure 1.1. Interpretation of the coordination sphere of C0CI2 • 6 NH3 by (a) Blomstrand/Jorgensen and (b) Werner 2 Figure 1.2. Catalytic formation of N-containing compounds from N2 4 Figure 1.3. The first reported transition metal dinitrogen complex (X" = Br", T, BF4", PF6") 5 Figure 1.4. Examples of zirconocene-based dinitrogen complexes 6 Figure 1.5. Successful catalyst for the synthesis of NH3 from N2 10 Figure 1.6. Design of a diamido-N-heterocyclic carbene [NCN] ligand 14 Figure 1.7. Possible electronic configurations of carbenes 15 Figure 1.8. Attempted synthesis of an NHC by chloroform elimination 20 Figure 1.9. Reported methodology for the synthesis of NHCs 20 Figure 1.10. a) 71-Stabilization and b) inductive effects of NHCs 21 Figure 1.11. (a) An example and (b) schematic representation of donor-acceptor bonding in Fischer-carbene complexes 22 Figure 1.12. (a) An example and (b) schematic representation of covalent bonding in Schrock-alkylidene complexes 22 Figure 1.13. Metal-NHC complexes reported by Wanzlick and Ofele 23 Figure 1.14. Modification to a Ru catalyst with an NHC ligand 25 Figure 1.15. Examples of a) phosphine and b) pyridine NHC metal complexes for Heck catalysis 2Chapter Two Promoting Dinitrogen Cleavage and Functionalization with Zr(II) and Ti(II) reagents Figure Title Page Figure 2.1. ORTEP view of ([NP(N)N]Ta(^-H)2(p.-N)(Ta[NPN])(ZrCp2) (2.10) depicted with 50% ellipsoids; all hydrogen atoms, silyl methyl and phenyl ring carbon atoms except ipso carbons have xii been omitted for clarity 40 Chapter Three Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Figure Title Pase Figure 3.1. ORTEP view of Mes[NCHN]H2-Cl (3.5) depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity 56 Figure 3.2. ORTEP view of Mes[NCN]H2, (3.8) depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity 58 Figure 3.3. Imidazolium (3.12) and imidazolinium (3.13) candidates with an aryl backbone 59 Figure 3.4. ORTEP view of tol[NCN]Zr(NEt2)2 (3.17) depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity 64 Figure 3.5. ORTEP view of Mes[NCNH]Ti(NMe2)3 (3.22) depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity 65 Figure 3.6. ORTEP view of tol[NCN]ZrCl2(py) (3.30) depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity 68 Figure 3.7. ORTEP view of tol[NCN]Zr(CH2SiMe3)2 (3.31) depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity 71 Figure 3.8. ORTEP view of Mes[NCN]HfBu2 (3.39) depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity 73 Chapter Four Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Fisure Title Pase Figure 4.1. Examples of Group 4 olefin polymerization catalysts 98 xiii Figure 4.2. ORTEP view of Mes[NCN]Hf(Ti2-XyNCCH3)(CH3) (4.7) (THF omitted), depicted with 50% ellipsoids; all hydrogen atoms have been omitted for clarity 103 Figure 4.3. ORTEP view of Mes[NCN]Hf(n2-XyNCCH3)2 (4.8) (CH2C12 omitted), depicted with 50% ellipsoids; all hydrogen atoms and mesityl groups have been omitted for clarity 104 Figure 4.4. ORTEP view of Mes[NCN]Hf(OC(CH3)=C(CH3)NXy), (4.11) (1/2 Et20 omitted), depicted with 50% ellipsoids; all hydrogen atoms and mesityl groups have been omitted for clarity 108 Figure 4.5. ORTEP view of Mes[NCN]Hf)2(/u-OC(iBu)=C(iBu)0)2, (4.16) (4 C6H6 omitted), depicted with 50% ellipsoids; all hydrogen atoms and mesityl groups have been omitted for clarity 112 Figure 4.6. ORTEP view of Mes[NCN]Hf(Me)(n3-tBuNC(Me)0) (4.17) depicted with 50% ellipsoids; all hydrogen atoms have been omitted for clarity 117 Figure 4.7. ORTEP view of Mes[NCN]Hf(Me)(n3-iPrNC(Me)NiPr) (4.18) depicted with 50% ellipsoids; all hydrogen atoms have been omitted for clarity 119 Figure 4.8. ORTEP view of Mes[NCN]Zr(Cl)(OCH2CH2CH2CH3) (4.24) depicted with 50% ellipsoids; all hydrogen atoms have been omitted for clarity 124 Figure 4.9. Coordination modes of hydrazido ligands 126 Figure 4.10. Examples of hydrazido(l-) and hydrazido(2-) titanium complexes.. 127 Figure 4.11. ORTEP view of Mes[NCN]Hf(Me)(n2-NHNMe2) (4.25) depicted with 50% ellipsoids; with the exception of HI00, all hydrogen atoms have been omitted for clarity.' 128 Chapter Five Synthesis and DFT Studies of Tantalum [NCN] Transition Metal Complexes Figure Title Page Figure 5.1. ORTEP view of tol[NCNH]Ta(NMe2)4 (5.1) (CH3C6H5 omitted) xiv depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity 152 Figure 5.2. ORTEP view of Mes[NCNH]Ta(CHPh)(CH2Ph)2 (5.2), depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity with the exception of HI 01 154 Figure 5.3. ORTEP view of tol[NCN]Ta(NMe2)3 (5.3), depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity 156 Figure 5.4. ORTEP view of Mes[NCCN]Ta(CH2tBu)2 (5.7), depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity 159 Figure 5.5. Potential pathways for ligand P-H abstraction 161 Figure 5.6. Computational model of the trimethyl tantalum starting complex... 162 Figure 5.7. Relative energies of the intermediates and transition states in a potential a-bond metathesis mechanism 163 Figure 5.8. JIMP Pictures of the one-step a-bond metathesis pathway 163 Figure 5.9. Relative energies of the intermediates and transition states in a potential two-step a-H abstraction/alkylidene mediated C-H activation mechanism 164 Figure 5.10. JIMP Pictures of alkylidene mediated C-H activation of the ligand backbone 165 Figure 5.11. ORTEP view of Mes[NCCN]Ta(Cl)(CH2tBu) (5.10), depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity 167 Chapter Six Thesis Summary and Extensions: Chiral Group 4 [NCN] Complexes Figure Title Page Figure 6.1. 'H NMR spectrum of (l^,2'1S,4i?)-2-(l,7,7-trimethylbicyclo-[2.2.1]hept-2-ylamino)ethyl ammonium chloride (6.8) in CDCI3. (* denotes V2 equivalent of CH3C(0)CH3). 185 xv Figure 6.2. Figure 6.3. Figure 6.4. Figure 6.5. Figure 6.6. Figure 6.7. Figure 6.8. ORTEP view of (-)-(li?,2'5,4i2)-2-(l,7,7-trimethylbicyclo-[2.2.1]hept-2-ylamino)ethylammonium chloride (6.8) depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity 186 'H NMR spectrum of (17?,2'S,4i?)-2-(l,7,7-trimethylbicyclo-[2.2.1]hept-2-ylamino)ethyl chloride (6.9) in CDC13. (* denotes Et20 impurity) . 187 'H NMR spectrum of scam[NCHN]H2-Cl (6.10) in d4-CD3OD (* denotes THF impurity) 188 'H NMR spectrum of scam[NCN]Ti(NMe2)2 (6.12) in C6D6 190 Aminolysis of a Zr-N bond in scam[NCN]Zr(CH2Ph)2 193 'H NMR spectrum of scam[NCH]H-Cl (6.15) in d6-DMSO (* denotes contamination with H20 and § denotes a trace amount of Et20) 195 'H NMR spectrum of scam[NC]H"(6.16) in C6D6 , '.. ' 196 Appendix B Evaluating the Formation of a Tantalum Metallaaziridine Complex by DFT Calculations Figure Title Pase Figure B.l. Relative energies of the intermediates and transition states in a potential a-bond metathesis mechanism 220 Figure B.2. JIMP Pictures of the one-step a-bond metathesis pathway 221 Figure B.3. Relative energies of the intermediates and transition states in a potential two-step a-H abstraction/alkylidene mediated C-H activation mechanism 222 Figure B.4. JIMP Pictures of a-H abstraction by a methyl group to generate a [NCN]Ta(=CHR')R alkylidene intermediate 224 Figure B.5. JIMP view of A. Selected bond distances (A) and angles (deg): Tal-Cl 2.381, Tal-C2 2.252, Tal-C3 2.248, Tal-C4 2.238, Tal-Nl 2.053, Tal-N2 2.020, Cl-Tal-C2 139.6, Cl-Tal-C3 142.0, C2-Tal-C3 76.4 226 xvi Figure B.6. JIMP view of D. Selected bond distances (A) and angles (deg): Tal-Cl 2.248, Tal-C2 2.219, Tal-C3 2.173, Tal-C4 2.257, Tal-Nl 2.065, Tal-N2 1.982, Cl-Tal-C2 130.5, Cl-Tal-C3 122.7, C2-Tal-C3 106.1, C4-Tal-N2 38.2 226 Figure B.7. Gaussview representations of selected NBO bonding and antibonding orbitals in the trimethyl complex A 229 Figure B.8. Gaussview representations of selected NBO bonding and antibonding orbitals in the trimethyl complex D 230 Figure B.9. JIMP Pictures of A' and D' 231 xvii GLOSSARY OF TERMS , The following abbreviations, most of which are commonly found in the literature, are used in this thesis. A Angstrom a, b, c unit cell dimensions, lengths (A) a, P, Y unit cell dimensions, angles (°) Anal. analysis atm atmosphere Ar aryl Beq equivalent isotropic parameter BBI broad band inverse Bn benzyl, -CH2C6H5 br broad n-Bu «-butyl group, -CH2CH2CH2CH3 carbon-13 C\, C2, c2v Schoenflies symmetry designations cal calories Calcd calculated CCD charge coupled device cm centimetres Cp cyclopentadienyl, C5H5 Cp* pentamethylcyclopentadienyljCsMes cryst crystal Cy cyclohexyl, -C6H11 d doublet dd doublet of doublets dq doublet of quartets dt doublet of triplets deg (or °) degrees diox 1,4-dioxane xviii °c degrees Celsius DFT density functional theory DME 1,2-dimethoxyethane dt numbers of ^-electrons dn n-deuterated AE0 zero point corrected electronic energy AEe electronic energy EI-MS electron ionization/mass spectrometry Et ethyl group, -CH2CH3 ETM early transition metal AG0 free energy g grams gof goodness of fit GC-MS gas chromatography/mass spectrometry AH0 enthalpy 'H proton {'H} proton decoupled h hour HIPT hexaisopropylterphenyl, -(3,5-(2,4,6-'Pr HMD SO hexamethyldisiloxane HOMO highest occupied molecular orbital Hz Hertz, seconds"1 IR infrared N T JAB n-bond scalar coupling constant betwee K Kelvin kcal kilocalories kJ kiloJoules 6Li lithium-6 L neutral two-electron donor LTM late transition metal LUMO lowest unoccupied molecular orbital xix M central metal atom (or molar, when referring to concentration) M+ parent ion m meta m multiplet (NMR spectroscopy) mm millimetres mM millimolar Me methyl group Mes mesityl group, -2,4,6-Me3C6H2 mg milligram(s) MHz megaHertz MgADP adenosine diphosphate, magnesium salt MgATP adenosine triphosphate, magnesium salt mL millilitre mmol millimole MO molecular orbital mol mole MS mass spectrometry no. number 15N nitrogen-15 NHC N-heterocyclic carbene NMR nuclear magnetic resonance Np Neopentyl group, -CH2C(CH3)3 [NPN] diamidophosphine ligand, -(PhNSiMe2CH2)2PPh o ortho ORTEP Oakridge Thermal Ellipsoid Program p para p pentet Pi inorganic phosphate, P043" 31P phosphorus-31 {31P} phosphorus-31 decoupled Ph phenyl group, -C6H5 xx PhH benzene PhMe toluene R[PNP] amidodiphosphine ligand, -(R2PCH2SiMe2)2N [P2N2] diamidodiphosphine ligand, -PhP(CH2SiMe2NSiMe2CH2)2PPh ppm parts per million 'Pr isopropyl group, -CH(CH3)2 py pyridine q quartet R, R' hydrocarbon substituents R, Rw residual errors, (X-ray crystallography) R coefficient of determination for a linear regression re fins reflections (X-ray crystallography) rt room temperature s singlet sept septet syst system t triplet T temperature in Kelvin or °C THF tetrahydrofuran TMS trimethylsilyl group, -Si(CH3)3 U(eq) equivalent isotropic displacement parameter V unit cell volume VT variable temperature X halide substituent XHYDEX hydride location program Z asymmetric units per unit cell x-dn Complex x has n number of !H atoms replaced by 2H atoms x-157v2 Complex x has a 15N2 labeled dinitrogen moiety nn n-hapto [i bridging or absorption coefficient (X-ray crystallography) p density xxi pcaic calculated density A. wavelength 8 chemical shift in ppm a, 71, 8 notations for bonding symmetries xxii ACKNOWLEDGEMENTS I wish to acknowledge first and foremost, Dr. Michael D. Fryzuk. My studies at UBC under his supervision have been memorable and truly a learning experience. During this time, I have learned an immense amount of chemistry and have been challenged on a daily basis to thoroughly investigate not only chemistry that pertains to me, but research that exists outside my expertise. Despite his busy schedule, he has always found the time to listen to my ideas (and problems), and offered words of encouragement and guidance when needed. I would also like to thank those post-docs, graduate and undergraduate students who I have worked with in the Fryzuk in the past and present. I especially wish to thank: Erin MacLachlan for all her help and enthusiasm in the lab and always going for a coffee run whenever one was needed. Howard Jong for his assistance with crystallographic questions and his expertise in restaurants. Fiona Hess for all her assistance with editing and critical comments on this thesis. Kevin Noonan for his help with polarimetry and other thought provoking discussions, and also golf. Bryan Shaw for his golfing prowess and the introduction to a "turbo". The department of chemistry at UBC has also been an immense help in making my research here easy and enjoyable. I would especially like to thank Howie Jong and Dr. Brian Patrick for their expertise in X-ray crystallography. I would also like to thank Dr. Nick Burlinson and the NMR personnel for their assistance with any questions and concerns I had with running the NMR spectrometers. I am also grateful to Brian Ditchburn for his timely help in glassblowing the many broken J. Young tubes I brought to him. I also must acknowledge the personnel in the mechanical and electronics shops with the timely help they provided during the many crisis periods encountered with the glovebox. xxiii dedication This work is dedicated to the two important women in my life. To my mother, Carolyn Norris, you have always been there for me and I greatly appreciate all that you have given me (especially my sense of humour ("Illigitimus Non Carborundum")). If it wasn't for you, I hesitate to think where I would be. To my soon to be wife, Dr. Pauline Vykruta, your approach to life is truly inspirational. You have challenged me in many ways, and I look forward to spending the rest of my life with you, milacku. xxiv STATEMENT OF AUTHORSHIP Chapters two, three, four, five and six were conducted in collaboration with Professor Michael D. Fryzuk, the research supervisor for this thesis, who assisted with identification and design of the research presented. Chapter two features some initial research performed by a previous graduate student, Dr. Bruce MacKay, under the supervision of Professor Michael D. Fryzuk. Chapter three and appendix B presents DFT calculations that were performed by Dr. Chad L. Beddie at the Texas A&M University under the supervision of Professor Michael Hall. All experimental research, data analysis, and manuscript preparation were performed by the thesis author. xxv Chapter One: Dinitrogen Chemistry and Ligand Design Chapter One Dinitrogen Chemistry and Ligand Design 1.1. Origins of Coordination and Organometallic Chemistry Our modern view of coordination chemistry began at the end of the 19th century at a time when the understanding of valence bonding and geometry in transition metal complexes was in a state of confusion. Prior to this period, the configuration and composition of many inorganic compounds were written in accordance with a theory described by the Swedish chemist, Jons Jacob Berzelius.1 Berzelius ardently attempted to sort all chemical compounds using a paired system which he called "the two-component theory". The Swedish chemists, Christian Wilhelm Blomstrand and Sophus Mads Jorgensen,2'3 applied this theory to describe the bonding and geometry of hexaammine cobalt(II) complexes. Blomstrand and J0rgensen postulated that in a material with the composition C0CI2 • 6 NH3, the divalent Co2+ metal centre could only form two bonds to the ammonia molecules. As a result, the binding of the ammonia residues to the metal centre must occur in a linking fashion that would form chains (Figure 1.1). 1 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design CI 2® Co / \ 2CP N^—N-—N-H3 H3 H3 (a) CI (b) Figure 1.1. Interpretation of the coordination sphere of C0CI2 • 6 NH3 by (a) Blomstrand/Jorgensen and (b) Werner. Upon examination of Blomstrand and Jorgensen's research, Alfred Werner noted This evidence led Werner to postulate that "single atoms or ions act as central positions, where a certain number of other compounds, atoms, ions, or other molecules are ordered in simple geometrical patterns" (Figure l.l).6'7 Werner found that this hypothesis could successfully explain experimental discrepancies that were observed in the structures of the complexes proposed by Blomstrand and J0rgensen. Werner also introduced the term coordination number, which describes the number of atoms that are grouped around a central nucleus. For his seminal work in this area, Alfred Werner was awarded the Nobel Prize in Chemistry in 1913 "in recognition of his work on the linkage of atoms in molecules".3 Werner's contributions led to conception of the term ligand, which refers to o atoms or groups attached to a central atom in the formation of a coordination compound. The progress in coordination chemistry stimulated interest in the coordination of other ligands and one mature area of chemistry which has emerged is the study of organometallic complexes. Organometallic chemistry is defined as the study of 7 Q compounds containing a metal-carbon bond. ' Despite the prior isolation of organometallic compounds like Zn(CH2CH3)2 and Ni(CO)4,9 the origins of modern organometallic chemistry are believed to have begun with the discovery of ferrocene, reported separately by both Wilkinson and Fischer.10"12 The importance of this finding in the field of chemistry was embodied in the awarding of the Nobel Prize in Chemistry in 1973 to both scientists. The field of organometallic chemistry has grown since this discovery to include transition metal complexes with ligands such as CO, CN", and benzene.9 Bonding in these complexes is usually described by molecular orbital theory several anomalies that did not corroborate physical and chemical experimentation. 2 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design and the reactivity dictated by the effective atomic number (EAN) or "18-eIectron rule". ' Organometallic complexes are of interest from both an academic perspective in terms of their synthesis and bonding implications, and from an industrial perspective regarding their potential to mediate catalytic reactions. Indeed, two recent Nobel Prizes in Chemistry were awarded in 2001 to Knowles, Noyori, and Sharpless14 and in 2005 to Chauvin, Grubbs, and Schrock13 for research involving the use of organometallic catalysts in organic synthesis. The emergence of organometallic chemistry and the continued relevance of coordination chemistry has also been driven in part by its application and relevance in other areas of science. In particular, research in the biological field has relied on fundamental research in both organometallic and inorganic chemistry. Evidence of this comes from the ubiquitous role metal complexes play in living organisms. For example, iron plays a fundamental role in the daily function of human life forms.16 Other metals, such as copper, are also known to serve an integral part in enzymatic processes and vitamin synthesis.17 One important progression in the borderline field of inorganic chemistry and biology is the area of dinitrogen fixation, which is the topic of discussion in the next section. 1.2. Reactivity of Dinitrogen The utilization of molecular nitrogen, or dinitrogen, to form organic nitrogen-19 27 containing products is one of the substantial challenges in organometallic chemistry. Nitrogen appears in all biological systems and is vital for the synthesis of amino acids, 28 nucleotides, enzymes, and other biologically important compounds. One could envision the synthesis of these organic nitrogen-containing compounds from the catalytic reaction of dinitrogen with simple organic reagents (Figure 1.2). A formidable hurdle for the successful implementation of this catalytic cycle is the inertness of molecular nitrogen. Dinitrogen possesses a strong triple bond that must be completely cleaved and functionalized to form organic compounds. Furthermore, the robust dinitrogen molecule features a large dissociation energy (941 kJ mol"1), the absence of a dipole, and possesses a large HOMO-LUMO gap, which makes oxidation and reduction difficult. 3 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design N-2 + reagents Amines, N-heterocycles Figure 1.2. Catalytic formation of N-containing compounds from N2. Notwithstanding its poor reactivity, dinitrogen can be converted to ammonia by several well-known pathways. Several biological systems are known to activate and functionalize dinitrogen to form ammonia.29'30 For example, the enzyme nitrogenase is able to bind to N2 and other substrates, reducing these bound molecules using electrons provided by metal clusters. This facilitates the protonation of the reduced nitrogen atoms and produces H2 as a byproduct of the reaction. As a result of this protonation, the bound nitrogen atoms become "fixed" and are released as ammonia (Equation 1.1). It is important to note that the reduction of N2 in this process is energy intensive, requiring 16 equivalents of MgATP. While the mechanistic details of this transformation remain 31 unclear, the catalytic process has been scrutinized in detail. The most significant example in which dinitrogen is converted to ammonia is the Haber-Bosch process.32'33 Both N2 and H2 are reacted at high temperature (400-450°C) and pressure (270 atm) in the presence of a Fe or Ru catalyst to yield ammonia (Equation 1.2). In this process, H2 is both a reducing agent and substrate for the production of ammonia. The impact of this discovery to the field of chemistry was monumental as both Haber and Bosch each received the Nobel Prize in Chemistry in 1918 and 1931, respectively,34'35 for their discovery and refinement of the technique. N=N + 8 H+ + 8 e" + 16 MgATP *- 2 NH3.+H2 + 16 MgADP + 16 P, (1.1) Fe or Ru catalyst 2NH3(g) (1.2) N=N (g) + 3H '2(g) 100 to 300 atm 400 to 550°C 4 References begin on page 2 7. Chapter One: Dinitrogen Chemistry and Ligand Design One feature consistent with the known examples of dinitrogen fixation is the incidence of transition metals at the active site during catalysis. This fact has shifted research efforts towards the development of other transition-metal-assisted dinitrogen reduction systems with an aim to achieve a homogeneous version of the Haber-Bosch process. To this end, the coordination of dinitrogen to transition metals has been investigated and will be discussed further. 1.3. Coordinated Dinitrogen Complexes The necessity of a transition metal catalyst in the Haber-Bosch process suggests that the activation of N2 can be induced by a transition metal. This assumption has stimulated interest in the potential coordination of N2 to transition metals. In 1965, Allen and Senoff serendipitously discovered the first metal dinitrogen complex by the reaction of R.UCI3 with hydrazine hydrate.36,37 This result was revolutionary, as it became apparent that N2 could act as a ligand in a coordination complex. In these complexes, dinitrogen is typically "activated" when coordinated to the transition metal. This activation can decrease the bond order of the N=N bond to varying degrees. In general, late transition metal (LTM) complexes feature an N2 ligand that is weakly activated. An examination of the N-N bond lengths by X-ray crystallography in 1.1 and many other LTM dinitrogen complexes19 reveals distances that are close to free N2 (1.0975 A).38'39 2+ 2X" 1.1 Figure 1.3. The first reported transition metal dinitrogen complex (X" = Br", I", BF4", PF6")-H-sN HUN' N III N Ru-NH3 'NH, NH, 5 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design Early transition metals (ETMs) have also been extensively studied for the coordination and activation of dinitrogen. In general, stronger activation of the dinitrogen ligand is observed compared to LTM complexes. One series of ETM complexes that have been examined for their potential to activate molecular dinitrogen are substituted zirconium metallocenes. The reduction of (r^-CsMes^ZrCb with Na/Hg amalgam under nitrogen produces [(r^-CsMes^Zr (nl-N2)]2(|>V:T|1-N2) with three bound N2 ligands (Figure 1.4).40 A slight modification in the cyclopentadienyl ancillary ligand reveals a different bonding mode for N2 and higher degree of activation 41 The reduction of (n5-C5Me4H)2ZrCl2 with Na/Hg amalgam under N2 yields ((n5-C5Me4H)2Zr)2(u-r|2:ri2-N2) (1.3), a side-on bound dinitrogen complex with a N-N bond length of 1.377(3) A, which is significantly lengthened from free N2 (1.0976 A).38'39 1.2 1.3 Figure 1.4. Examples of zirconocene-based dinitrogen complexes. Although dinitrogen complexes of almost all the transition metals have been reported, including the actinides and lanthanides, the transformation of the dinitrogen ligand in these complexes has proven to be difficult. One goal of dinitrogen activation via transition metal complexes is the complete cleavage of the N=N bond. This objective is quite challenging since dinitrogen possesses characteristics that make N2 an unreactive ligand. Attempts to cleave the "activated" N=N bond with chemical reagents reveal the inert N2 ligand is susceptible to displacement by more reactive ligands such as H2,42"47 CO,44"49 and olefins.42'44'48'50'51 6 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design 1.4. Dinitrogen Cleavage The synthesis of ammonia from N2 by biological methods and the Haber-Bosch process requires the cleavage of the N=N bond. Several assays have focused on promoting the cleavage of molecular nitrogen with ETMs. Perhaps the most recognized example of dinitrogen cleavage involves molybdenum amide complexes reported by Cummins.52'53 A three-coordinate Mo[NR(Ar)]3 stabilized by bulky N-ter?-butyl anilide ligands (1.4) is able to bind to N2 to yield a dinuclear end-on bound dinitrogen complex at -35°C (Scheme 1.1). Warming solutions of this product to room temperature results in the cleavage of the N=N bond to form the terminal nitride complex N=Mo[NR(Ar)]3 (1.5). Density functional theory calculations on this transformation suggest that bond cleavage proceeds through a zig-zag dinuclear molybdenum dinitrogen transition state.54 This process is remarkable as the reduction of the N2 ligand occurs under mild conditions and requires no added reagents. In this example, the reducing power required for N-N bond scission originates from the two molybdenum metal centres from which six reducing equivalents reductively cleave the N=N triple bond. While ammonia synthesis from this nitride species has not been reported, a niobium analog of 1.5 has recently been used in a nitrogen atom transfer reaction with acid chlorides to generate new organic nitrile products.55 Unfortunately, the formation of these organic nitrogen containing compounds is still stoichiometric in nature and this process has yet to be developed into a catalytic system. 7 References begin on page 2 7. Chapter One: Dinitrogen Chemistry and Ligand Design Ar(R)N„ ,'Mo—N(R)Ar Ar(R)N Ar = 3,5-Me2C6H3 R = C(CD3)2CH3 1.4 N2 -35°C, N III I Ar(R)N" j ^-~N(R)Ar Ar(R)N Mo(N(R)Ar)3 Ar(R)N,,, ||j 2 Mo—N(R)Ar Ar(R)N*^ 1.5 Ar(R)N Ar(R)N' -N(R)Ar Mo' I N II N . ._..„, Mo Ar(R)N1' j ^~-N(R)Ar Ar(R)N Ar(R)N Ar(R)N" Mo' II N -N(R)Ar . ,„..„, Mo^ Ar(R)N' j N(R)Ar Ar(R)N Scheme 1.1. Vanadium complexes are also known to effect N=N bond cleavage. For example, the reduction of the diamidoamine vanadium complex (Me3Si(CH2CH2NSiMe3)2V)2(p-Cl)2 with potassium graphite (KCg) in the presence of N2 results in the coordination and cleavage of dinitrogen to form the dimeric vanadium bis(nitride) complex (Me3Si(CH2CH2NSiMe3)2V)2(p-N)2 (1.6) (Scheme 1.2).56 The vanadium-nitrogen bond from the cleaved dinitrogen unit can be further reduced with potassium graphite to generate 1.7. 8 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design The cleavage of molecular dinitrogen can also be achieved by the sterically WiPT WIPT encumbered molybdenum triamidoamine complex (N3N)MoCl (where (N3N) =[[(3,5-(2,4,6-iPr3C6H2)2C6H3)NCH2CH2]3N]3").57>58 The dinitrogen complex 1.8 (Figure 1.5) can be synthesized by the stepwise reduction and oxidation of HIPT(N3N)MoCl. While no discrete molybdenum nitride species has been isolated as a result of cleavage of the N2 ligand in 1.8, experiments with a compatible proton source and reducing agent reveal that 1.8 is capable of catalytically transforming dinitrogen into ammonia (four turnovers are reported before the catalyst loses function). This result and subsequent modeling studies of the intermediates formed during this reaction demonstrate that cleavage of N2 is possible under certain conditions. 9 References begin on page 2 7. Chapter One: Dinitrogen Chemistry and Ligand Design 'Pr 1.8 Figure 1.5. Successful catalyst for the synthesis of NH3 from N2 The activation and cleavage of molecular nitrogen by transition metal complexes can be largely influenced by the ligands encompassing a metal centre. It has been demonstrated that this chemistry can be regulated by slight modifications of the ligand architecture. One aspect of research in the Fryzuk group has been the design and synthesis of ancillary ligands effective for stabilizing transition metal dinitrogen complexes. For the most part, ancillary ligands involving mixed phosphine and amide donors have been investigated. /. 5. Amidophosphine Ligands for N2 Activation The combination of both phosphine and amido donors into a chelating ligand framework was anticipated to be suitable for the stabilization of different types of transition metals across the transition series in various oxidation states. Phosphine donors are well known to coordinate to LTM centres, while anionic amido donors have been used extensively in many ETM complexes. The first ligand that was investigated employing these donor groups was the [PNP] design, utilizing a centrally positioned 10 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design amido donor and two flanking phosphine groups.59 The preparation of this ligand is shown in Scheme 1.5, and involves the addition of three equivalents of LiP'P^ to commercially available HN(SiMe2CH2CI)2 to generate the lithiated [PNP] ligand. Both early and LTM complexes have been stabilized using this ligand and, in some cases, activated dinitrogen complexes (ie. 1.9) have been isolated.59"64 Despite these successful outcomes, one drawback of the [PNP] design in ETM chemistry is the potential for the phosphine groups to dissociate from the metal centre.62'65'66 Given this possibility, further modifications involving phosphine and amide donors have been investigated. 1.9 Scheme 1.5. A macrocyclic [P2N2] ligand was investigated that employs two neutral phosphine and two anionic amido donor groups. The preparation of this ligand follows a similar synthetic methodology to the [PNP] ligand. A diphosphinoamine precursor is synthesized by the addition of PhPHLi to HN(SiMe2CH2Cl)2. This reagent is deprotonated and reacted with a second equivalent of HN(SiMe2CH2Cl)2 to generate the lithiated [P2N2] ligand (Scheme 1.6).67 The arrangement of the phosphine atoms offers 11 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design the potential for the synthesis of two isomeric forms of the ligand; the syn isomer can be selectively synthesized by controlling the experimental conditions of the reaction. The [P2N2] macrocycle has been effectively used as a supporting ligand for the isolation of several ETM dinitrogen complexes (ie. 1.10).68"70 One drawback that has been found of certain [P2N2] complexes of tantalum has been their lack of reactivity. The additional donor in the ligand, as compared to [PNP], has resulted in several coordinatively and 71 electronically saturated [P2N2] systems that possess diminished reactivity. Scheme 1.6. Another variant of the amidophosphine ligand involves the [NPN] design that utilizes one phosphine donor and two anionic amido groups. This design reconciles the problems encountered with the reactivity of [P2N2] complexes and still maintains a dianionic bonding nature. The synthesis of the Li2[NPN] ligand involves the addition of four equivalents of o-BuLi to a mixture of one equivalent of PhPPb. and two equivalents 12 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design of PhNSiMe2CH2Cl (Scheme 1.7). Metathesis reactions with metal chloride reagents and the Li2[NPN] ligand were used successfully to synthesize [NPN]MC12 complexes. For example, the reaction between Li2[NPN] and ZrCL proceeds to give [NPN]ZrCl2. This complex can be reduced in the presence of nitrogen to form the dinitrogen complex Modifications to the [NPN] ligand design have generally focused on the replacement of the reactive N-Si bond.73'74 Not only is this bond sensitive to water and air, it can rupture during the reduction process forming products which result from deleterious modification of the [NPN] ligand.74"76 One aspect that remains relatively unexplored is the substitution of the phosphine group for another neutral donor. N-Heterocyclic carbenes (NHCs) have come to be regarded as phosphine equivalents and have been extensively studied as essential ligands in homogeneous catalysis and small 13 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design molecule activation processes.77 Given this parity with phosphines, the substitution of an NHC in place of the phosphine donor in the [NPN] architecture can be examined, which could generate a [NCN] ancillary ligand (Figure 1.6). The following sections of this chapter will describe the discovery of NHCs and their emergence as ligands in selected areas of organometallic catalysis. . [NPN] [NCN] Figure 1.6. Design of a diamido-N-heterocyclic carbene [NCN] ligand. 1.6. Introduction to Carbenes Our understanding of carbene chemistry has progressed in the past 20 years as a result of greater insight into the electronic structure and stability of carbenes.78'79 Carbenes are neutral species that contain a divalent carbon atom with only six electrons.80 Two nonbonding electrons can be found in two different frontier orbitals, commonly referred to as the a and p^ orbitals. The existence of these valence electrons in these orbitals presents the possibility of two unique electronic configurations (Figure 1.7). The singlet state involves the pairing of these electrons in the same a orbital. In the parallel triplet state, the two electrons can occupy unique a and p^ orbitals. 14 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design R o0R R singlet triplet Figure 1.7. Possible electronic configurations of carbenes. The nature of this electronic configuration directly influences the reactivity of the carbene molecule. Singlet carbenes possess a filled and a vacant orbital and thus exhibit an ambiphilic nature. As a result, singlet carbenes are very reactive species and difficult to isolate. Attempts to isolate singlet carbenes often result in rearrangement reactions such as 1,2-migration, dimerization, ' [l+2]-cycloadditions to C=C bonds,85'86 and insertion into C-H bonds.87 Conversely, triplet carbenes display a diradical nature owing to two singly occupied orbitals. As a result, these species have been reported to perform carbene dimerization,88"90 [l+2]-cycloadditions to C=C bonds,91 and insertion reactions89 that are similar to their singlet carbene counterparts but proceed by a different mechanism. The spin multiplicity of a carbene is dictated by the energetic difference between the a and p^ orbitals.92"99 A large o-pn energy separation favors a singlet ground configuration, whereas a small energy difference can induce the triplet spin state. The o-pn energy gap can be increased by the presence of a-electron withdrawing substituents adjacent to the divalent carbon atom. These groups inductively stabilize the a-nonbonding orbital by increasing its 5 character, while leaving the pn orbital unchanged. This gap can also be increased with adjacent 7i-donating atoms, which quenches the electron deficient nature of the carbene and results in a polarized four electron three-centered n system. Steric effects also influence the ground state multiplicity of a carbene.100"102 Bulky substituents on the carbene can dictate the multiplicity in the absence of electronic effects. For example, dimethylcarbene has a bent singlet ground state with a bond angle of 1110 at the central carbon atom.100,101 Conversely, carbenes with two bulky 15 References begin on page 2 7. Chapter One: Dinitrogen Chemistry and Ligand Design substituents, such as di(tert-butyl)carbene, exist in the triplet state and exhibit larger bond angles (ie. 143° for di(tert-butyl)carbene).102 1.7. Isolation of Stable Carbenes 1.7.1. Acyclic Carbenes The first discovery of a stable isolable carbene was reported by Bertrand in 1988 (Scheme 1.8).79'103'104 Phosphanyl(silyl) carbenes can be prepared by the thermolysis or photolysis of a diazo precursor (1.12). The carbene 1.13 is remarkably stable for weeks at room temperature and can be purified by distillation under vacuum at 75°C-80°C. At the time, the notion that this species was a carbene was met with skepticism as 1.13 does not exhibit reactivity typical of singlet carbenes. For example, 1.13 does not react with simple alkenes but can add to polar C=C bonds to give cyclopropane products (ie. 1.14)104'105 Nucleophilic reactions were also reported with 1.13 and isonitriles106"108 and phosphines109 to give 1.15 and 1.16, respectively. Despite this skepticism, the existence of a carbene was confirmed by both computational methods110 and an X-ray crystallographic study on an analog of 1.13."1 A P-C-Si bond angle of 152.6° was noted, in addition to short C-P and C-Si bond lengths, which suggests the ylide resonance form 1.13' contributes to the overall structure of the acyclic carbene. 16 References begin on page 2 7. Chapter One: Dinitrogen Chemistry and Ligand Design 'Pr2N > 'Pr2N 'SiMe3 1.13 R R 'Pr2Nx 'PrzN7 'SiMeg 1.14 'ProN 'Pr,N > N2 SiMe3 1.12 hv I A © P: [Pr2H XSiMe3 1.13" BuNC 'PrpN N'Bu / 'Pr2N SiMe3 1.15 Scheme 1.8. 'Pr2N 'Pr,N > P= SiMe3 1.13" PMe? 'Pr,N PMe, 'Pr2N ^SiMe3 1.16 Following these results, the isolation of other stable aminocarbenes were pursued. For example, an amino(aryl)carbene is accessible by deprotonation of the iminium salt 112 113 1.17 (Scheme 1.9). ' Although 1.17 could be isolated, its instability represents a common characteristic consistent of many acyclic carbenes. Compound 1.17 slowly decomposes over a period of 1 week to give 1.18, which is a product of an intramolecular cyclization reaction. 112 17 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design 1.18 Scheme 1.9. The combination of phosphanyl and amino groups on a divalent carbon centre has also been investigated, for example 1.19 in Scheme 1.10.114 This class of carbenes exhibits thermal sensitivity and decomposes at temperatures above -20°C. The stability of phosphanyl(amino) carbenes can be enhanced by the alkylation of 1.19 to afford the phosphonium salt 1.20. This phosphonium salt is indefinitely stable in the solid state at room temperature,115 but susceptible to displacement by nucleophiles like ter/-butoxide. This substitution of functional groups on the carbene allows for the formation of other amino heteroatom carbenes. 18 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design R a) X = O; R = *Bu b) X = S; R = Me Scheme 1.10. 1.7.2. N-Heterocyclic Carbenes In the early 1960's, Wanzlick proposed that the incorporation of a carbene unit in a cyclic structure between two amino substituents would enhance the stability and lead to the isolation of this reactive species.83'84'116 It was envisioned that the elimination of chloroform from 1.21 could yield the corresponding carbene product (Figure 1.8). Although the dimeric species 1.22 was the only product isolated from this reaction, recent findings suggest an equilibrium exists between the carbene and 1.22.117 While these results did not lead to the isolation of a stable carbene molecule, this idea did stimulate interest into the incorporation of a carbene unit in an N-heterocyclic manifold. 19 References begin on page 2 7. Chapter One: Dinitrogen Chemistry and Ligand Design Ph -N ^CCI3 Ph -N •N Ph -N Ph N-N N' Ph I Ph J Ph Ph 1.21 1.22 Figure 1.8. Attempted synthesis of an NHC by chloroform elimination. The first stable NHC was isolated by Arduengo in 1991.118 Deprotonation of bis(l-adamantyl)imidazolium chloride with NaH provides the first stable crystalline NHC compound, which melts at 240-241°C without decomposition. Since this discovery, other methods for the syntheses of NHCs have been reported, which includes the desulfurization of imidazole-2(3H)-thiones119 and methanol elimination by thermolysis of 5-methoxy-.l,3,4-triphenyl-4,5-dihydro-lH-l,2,4-triazoles (Figure 1.9). 120 CI R^N^-N^-R Base Figure 1.9. Reported methodology for the synthesis of NHCs. N N l-l OMe The stability of NHCs originates mainly from electronic factors, but steric factors also play a role. The presence of two adjacent nitrogen donors asserts a mesomeric effect 20 References begin on page 2 7. Chapter One: Dinitrogen Chemistry and Ligand Design on the divalent carbon carbene atom. The electron deficient nature of the carbene atom is reduced by the derealization of the two nitrogen lone pairs into a vacant p orbital on the carbon atom and results in a four electron three-centered % system. Furthermore, the carbene lone pair is stabilized inductively by the electronegative nitrogen atoms (Figure 1.10). As a result, a large o-pn energy separation is present, which allows NHCs to exist in the singlet ground state configuration. (a) (b) Figure 1.10. a) 7i-Stabilization and b) inductive effects of NHCs. 1.8. Transition Metal Complexes With Carbene Ligands 1.8.1. Transition Metal Acyclic Carbene Complexes 121 The first transition metal-carbene complex was discovered by Fischer in 1964. These complexes, historically referred to as Fischer-carbene complexes, feature a low-valent transition metal fragment and a carbene bearing at least one 71-donating substituent (Figure 1.11). As a result, the carbene atom has an electrophilic nature. The metal-carbon bond is best described as a donor-acceptor interaction resulting from the superposition of 122 carbene to metal o-donation and metal to carbene 7i-back donation. 21 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design OMe (OC)5W=C / \ Ph It RoC! Metal (a) (b) Figure 1.11. (a) An example and (b) schematic representation of donor-acceptor bonding in Fischer-carbene complexes. A second class of transition metal-carbene complexes was identified by Schrock in the 1970's.123 These Schrock-carbene or alkylidene complexes feature a high oxidation state metal and a carbene bearing two alkyl substituents (Figure 1.12). The carbene possesses a nucleophilic nature and forms a covalent bond with the metal. The metal-carbene bond can be regarded as an interaction between a carbene fragment in the 122 triplet state with two spin parallel electrons on the metal centre. (CH3)3 (tBuCH2)3Ta=C^ H (a) (b) Metal Figure 1.12. (a) An example and (b) schematic representation of covalent bonding in Schrock-alkylidene complexes. 22 References begin on page 2 7. Chapter One: Dinitrogen Chemistry and Ligand Design Transition metal complexes of the acyclic carbenes described in chapter 1.7.1 are also known. The coordination of aryl(phosphoranyl),124 amino(silyl),125 amino(alkyl),126 and aryl(amino)127 carbene ligands to rhodium were found to yield thermally stable metal complexes, which melt at temperatures above 150°C. In one case, a thermally sensitive aryl(phosphoranyl) carbene complex was observed to isomerize to an n'-phosphaalkene complex at -10°C.124 While the coordination of these carbenes has been investigated, their application in transition-metal-mediated catalysis has yet to be reported. 1.8.2. Transition Metal NHC Complexes In 1968, the first metal-NHC complexes were reported independently by Wanzlick and Ofele (Figure 1.13).128'129 Since this discovery, NHC complexes of almost all the transition metals have been reported. NHCs are strong a donors and very weak n-acceptors, despite having an empty p,t orbital. Unlike the Fischer- and Schrock-carbene complexes, NHC ligands have been reported to stabilize transition metals in both low and high oxidation states. Given their a-donating ability, NHCs have been compared to other strong a-donors such as phosphines. Probably the aspect that has propelled the use of NHCs to prominent levels has been their ability to replace phosphine ligands to generate catalytic precursors more robust and versatile than their phosphine 77 78 1 ^fl 1 "^7 counterparts. ' ' " These attractive features have stimulated the use of NHCs as essential ligands for LTM complexes in homogeneous catalysis and small molecule activation processes. 2+ 2 CI04" Figure 1.13. Metal-NHC complexes reported by Wanzlick and Ofele. 23 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design ETM complexes utilizing NHC ligands have generally been regarded as chemical curiosities, with reports focused on the routine coordination of an NHC ligand to an For example, the addition of TiCl4 to a 1,3-dialkyl substituted NHC gives the ETM 133-136 NHC titanium complex 1.23 (Equation 1.3).137 X-ray diffraction studies performed on this and other ETM NHC complexes show a pure a-donor character of the NHC-metal bond. Furthermore, the C-N bond distances in these metal complexes feature bond lengths that are intermediate between free carbenes and imidazolium salts, which suggests a 7c-delocalization in the NHC five membered ring. Me TiCL -N Me TiCL (1.3) 1.23 1.9. Late Transition Metal NHC Complexes in Homogeneous Catalysis NHC complexes of LTMs have found applications in many different catalytic processes, some of which have been traditionally carried out using phosphine-based systems. Due to their remarkable stability, NHC complexes have shown promise as catalysts in C-Si, C-C, C-N, and C-H bond activation processes.80,138"142 One example where NHCs have matched phosphine analogs in generating an active and robust catalyst involves the Ru-mediated ring closing metathesis of olefins.142 The replacement of one tertiary phosphine in complex 1.24 with an NHC ligand yields 1.25 (Figure 1.14). While both 1.24 and 1.25 exhibit similar catalytic activities for the ring closing metathesis of diethyldiallylmalonate, the NHC complex 1.25 shows remarkable stability in air and to prolonged heating. This observation is in marked contrast to 1.24, which decomposes after a short period of heating. 24 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design Cy3R PCy3 1.24 1.25 Figure 1.14. Modification to a Ru catalyst with an NHC ligand. NHC donors have also been effectively incorporated into a chelating ligand to yield catalysts that can be electronically and sterically attenuated. For example, pincer ligands containing NHC donors have been reported to be effective auxiliary ligands in several palladium catalyzed cross coupling reactions.143"145 Both bidentate and tridentate NHC ligands with phosphine and pyridine donors have been used with good success in the Heck reaction. Examples of these catalysts are given in Figure 1.15. (a) (b) Figure 1.15. Examples of a) phosphine and b) pyridine NHC metal complexes for Heck catalysis. 1.10. Scope of This Thesis This introduction has highlighted the origins of coordination and organometallic chemistry, and the potential for organometallic complexes and biological systems to 25 References begin on page 27. Chapter One: Dinitrogen Chemistry and Ligand Design promote dinitrogen activation and cleavage. This daunting task has been shown to be influenced by the ligands surrounding the metal centre. ETM complexes with amidophosphine ligands have been used extensively in the Fryzuk group for the activation and cleavage of N2. Recent work has focused on the effects that changing the electronic nature of the donating group in the ligand and the modification of the ligand backbone would have on dinitrogen activation. The focus of this thesis is to both explore the cleavage of an N2 ligand and to prepare a dinitrogen complex with a coordinated NHC ligand. In chapter 2 the reactivity of the tantalum dinitrogen complex ([NPN]Ta)2(u-H)2(|a-nl:n2-N2) with several zirconium hydride reagents is explored. Activation of the dinitrogen unit occurs and is followed by unique functionalization of one of the cleaved nitrogen atoms. The mechanism of this process is investigated and new methods to cleave a coordinated N2 ligand are suggested. The synthesis of group 4 metal complexes stabilized by the [NCN] ligand is the focus of chapter 3. Several [NCN] precursors are synthesized and used to prepare group 4-amido, -halide, and -alkyl complexes. The stability of the metal-NHC bond towards dissociation from the metal centre is probed, as is the thermal stability of several hafnium dialkyl compounds. Chapter 4 examines the reactivity and application of [NCN] complexes synthesized in chapter 3. The migratory insertion reactivity of the hafnium dialkyl derivatives with carbon monoxide, substituted isocyanides, and several cumulenes is investigated. A cationic zirconium-methyl complex is also evaluated for its potential to polymerize a-olefins. This chapter also explores the attempted synthesis of a group 4 dinitrogen complex in addition to other N-N containing complexes. Chapter 5 explores the synthesis of Ta(V) [NCN] complexes using both synthetic and theoretical methods. 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Chem. 2004, 43, 6822. (144) Peris, E.; Mata, J.; Loch, J. A.; Crabtree, R. H. Chem. Commun. 2001, 201. (145) Loch, J. A.; Albrecht, M.; Peris, E.; Mata, J.; Faller, J. W.; Crabtree, R. H. Organometallics 2002, 21, 700. 33 References begin on page 27. Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(ll) and Ti(II) reagents Chapter Two Promoting Dinitrogen Cleavage and Functionalization with Zr(II) and Ti(II) reagents 2.1. Introduction* The conversion of N2 to ammonia or to certain organic nitrogen-containing products requires the activation and cleavage of the N=N triple bond. As was highlighted in chapter 1, several early transition metal complexes are capable of performing these kinds of transformations. In some cases, further functionalization of the cleaved nitrogen atoms is possible, forming new element-nitrogen bonds. In each of these examples, an ancillary ligand plays an important role in stabilizing the various transition metal oxidation states necessary for N-N bond reduction. Transition metal complexes employing amidophosphine ancillary ligands have been examined extensively in the Fryzuk group for the cleavage and functionalization of coordinated dinitrogen. For example, a diamidodiphosphine ligand [P2N2] (where [P2N2] = (PhP(CH2SiMe2NSiMe2CH2)2PPh)) has been effectively used to stabilize a dinuclear dinitrogen complex of niobium (2.1).1 Thermolysis of 2.1 promotes N-N bond cleavage to generate a reactive molecular nitride, which immediately reacts with the [P2N2] ancillary ligand to yield 2.2 (Scheme 2.1). The formation of 2.2 provides a unique *A version of this chapter has been accepted for publication {Proc. Natl. Acad. Sci. USA). 34 Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(II) and Ti(II) reagents example of N-N bond cleavage followed by nitrogen atom insertion into the backbone of one of the [P2N2] macrocycles to form a bimetallic bridging nitride complex. The stabilization of early transition metal dinitrogen complexes has also been investigated in the Fryzuk group using a diamidophosphine [NPN] ancillary ligand (where [NPN] = [(PhNSiMe2CH2)2PPh]2'). For example, reduction of a titanium dichloride complex (2.3) employing a [NPN] ancillary ligand with 2.2 equivalents of KCs under 4 atmospheres of nitrogen yields a dinuclear titanium dinitrogen complex (Scheme 2 131 2.2). This species was only identified by H and P NMR spectroscopy and mass spectrometry. Thus far, the solid state structure of this dinitrogen complex has yet to be determined. This complex slowly rearranges in solution to form the bis(phosphinimide) titanium (III) dimer 2.4. The formation of 2.4 occurs as a result of complete N-N bond cleavage, followed by nitrogen atom functionalization by the [NPN] ancillary ligand to yield a new N-P bond. 35 References begin on page 49. Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(II) and Ti(ll) reagents Ph Ph Ph \ Me2Si'' KI ci ~~P. \ Ph 2.3 Ph. Ph VN Me2Sr MezSj'i ji(n-N2)Ti |SiMe2 V£i^SiMe2 \ Ph Ph Ph Ph \ SiMe, P\"H /N^ / X //SiMe2 Me2Siy ^Ti' Ti^N/ Me2Si, N -P I Ph V Ph Ph 23 2e" Ph. Ph Ph 29 VH \ _SiMe2 iMe2 \ Ph Ph Ph 2 e" Ph Ph- P==^X Phv'''N r/ ^\,SiMe2 / ^ //SiMe2 Me2Si/ *"Ti^ Ti^'N/ Me,sA N I Ph 2.4 Scheme 2.2. \ Ph Ph The diamidophosphine ligand [NPN] has also been used to stabilize a novel side-on end-on coordinated tantalum dinitrogen complex (2.5) (Scheme 2.3).3 The synthesis of this reduced dinitrogen complex is remarkable because it is formed upon exposure of a tetrahydride tantalum derivative to N2, a result that is not proliferated by the use of strong reducing reagents such as KCg. This dinitrogen complex has been shown to exhibit a wide range of reactivity patterns, which include reactions with electrophiles, adduct formation with Lewis acids,4 and displacement of the N2 moiety by terminal alkynes.5 36 References begin on page 49. Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(II) and Ti(II) reagents 4 atm N2 Et20 2.5 Scheme 2.3. The addition of simple main group hydride reagents (E-H) to the tantalum-dinitrogen complex (2.5) has also been studied. These hydride addition reactions feature several common outcomes and are summarized in Scheme 2.4. In every case, E-H addition occurs in a 1,2-addition manner across the exposed end of the coordinated dinitrogen moiety to form, in some cases, an isolable intermediate (2.6). This leads to elimination of H2, N-N bond cleavage, and subsequent rearrangement to yield 2.7, a tantalum nitride intermediate. The addition of a second equivalent of E-H reagent to this complex leads to different outcomes, which are dependent on the hydride source. For example, the addition of a B-H reagent leads to ancillary ligand degradation (2.8), limiting the overall, usefulness of these stoichiometric transformations.6,7 However, the addition of a Si-H reagent results in a clean conversion to the bis(silylimide) 2.9, which was observed for the addition of butylsilane.8 37 References begin on page 49. Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(II) and Ti(ll) reagents 2.9 2.8 Scheme 2.4. The extension of this E-H addition chemistry to include transition metal reagents was of particular interest. This would present a unique opportunity to functionalize a coordinated dinitrogen unit with a transition metal atom. In this chapter, the reaction of 2.5 with several zirconium-hydride reagents is examined. What results is an unanticipated reaction that involves N-N bond cleavage without zirconium-hydride addition, or H2 elimination from 2.5. 2.2. AttemptedHydrozirconation of 2.5 Schwartz's reagent or zirconocene chloro-hydride is known to add in a 1,2-addition fashion across the multiple bonds of alkenes, alkynes, ketones, aldehydes, and 38 References begin on page 49. Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(ll) and Ti(II) reagents nitriles.9 Given the success of main group hydride additions across the Ta-N bond of 2.5, a similar process was envisioned that would yield a terminal tantalum hydride moiety and a new Zr-N bond. Stirring a THF solution of sparingly soluble [Cp2Zr(Cl)H]x with 2.5 for 2 weeks generates an intense purple solution from which purple crystals of 2.10 are isolated in 35% yield (Equation 2.1). . The *H NMR spectrum reveals a Cs symmetric species in solution with one single Cp resonance at 5.38 ppm, four inequivalent silyl methyl resonances, and bridging hydrides at 11.4 ppm. Surprisingly, no terminal tantalum hydride resonances are observed indicating that hydrozirconation to form a species similar to 2.6 (Scheme 2.4) had not occurred. The 31P{'H} NMR spectrum features two singlets at 7.3 and 46.7 ppm with the downfield resonance split into a doublet ('jpN = 34.9 Hz) when 15N-labelled 3.5 is used. Similar coupling is also observed in the 15N{'H} spectrum with the resonance located at -185.1 ppm split into a doublet. A second 15N resonance is observed at 228.4 ppm. These two resonances are not mutually coupled, implying that N-N bond cleavage has occurred. D. Ph ou Ph Ph. \ Ph. / \ ' Ph FV'M M P^ , Ph-M ,H N ^ ^^Ta^ Na' I [Cp2ZrH(CI)] Me2srN^. >SiMe, n -n ( ^SiMe* THF Me2Si< Tf\ /T\ J , 2 (2A) \ t I -N VN^SiMe2 mr \V f XN N JJ N^„. ^P^N^ / f I N IN •<•/„. i^v^ / / \ Ph Ph CP: Ph 2.5 2.10 The solid state molecular structure of 2.10 was determined by an X-ray diffraction experiment. An ORTEP depiction is shown in Figure 2.1 with selected bond lengths and angles given in Table 2.1 and crystallographic details reported in appendix A. The addition of Schwartz's reagent to 2.5 appears to yield an N-N bond cleaved product in which a zirconocene fragment has inserted between the two nitrogen atoms. No bridging hydrides are observed in the solid state molecular structure; however, these ligands are successfully modeled using XHYDEX10 and their presence confirmed by *H NMR spectroscopy. A Ta-Ta bond interaction is observed with a Tal-Ta2 bond distance of 39 References begin on page 49. Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(ll) and Ti(II) reagents 2.7007(6) A, which is similar to other Ta(IV)-Ta(IV) dinuclear complexes.11 The phosphinimide P=N bond distance of 1.595(9) A is also typical of other reported group 5 phosphinimide complexes. ' A bridging zirconium nitride species is also observed. While the formation of this moiety is uncommon, the Zrl-N2 bond length (2.040(9) A) is shorter than other bridging zirconium nitrides which range from 2.21 to 2.35 A.14"16 The formation of the phosphinimide group in 2.10 is presumably a result of the intramolecular attack of one of the ancillary phosphine donors at an intermediate nitride species. Such an event has been reported earlier in the Fryzuk group in the formation of dinuclear titanium phosphinimide complexes and during the thermolysis of a preformed niobium dinitrogen complex. ' The intermolecular attack of phosphines on metal nitride species also has precedence and is analogous to the formation of 2.10.17-19 Figure 2.1. ORTEP view of [NP(N)N]Ta(Li-H)2(pJ-N)(TarNPN])(ZrCp2) (2.10) depicted with 50% ellipsoids; all hydrogen atoms, silyl methyl and phenyl ring carbon atoms except ipso carbons have been omitted for clarity. 40 References begin on page 49. Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(II) and Ti(ll) reagents Table 2.1. Selected bond distances (A) and angles (deg) for ((NP(N)N]Ta(u.-H)2(u.-N)(Ta[NPN])(ZrCp2) (2.10). Bond Lengths Bond Angles Tal-Ta2 2.7007(6) Nl-Tal-N2 87.2(3) N1--N2 2.925(9) Pl-Nl-Tal 121.5(5) Pl-Nl 1.595(9) Nl-Zrl-N2 88.6(3) Tal-Nl 2.102(9) Tal-N2-Ta2 82.9(3) Tal-N2 2.139(8) N3-Tal-N4 107.0(4) Ta2-N2 1.935(9) N5-Ta2-N6 105.6(4) Zrl-Nl 2.146(9) Zrl-N2 2.040(9) Tal-N3 2.054(9) Tal-N4 2.102(9) Ta2-N5 2.112(10) Ta2-N6 2.119(9) Ta2-P2 2.624(3) Tal-Zrl 3.0319(11) Surprisingly, the addition of [Cp2Zr(Cl)H]x to 2.5 does not result in hydrozirconation of the Ta-N bond. In this reaction, the fate of the hydride and chloride ligands derived from [Cp2Zr(Cl)H]x is not clear. Careful examination of the 'H NMR spectrum during the reaction reveals the presence of H2 (4.54 ppm in CeD6), although this was not quantified. To probe the origin of H2 formation, the addition of [Cp2ZrH2]2 to 2.5 was examined and found to yield 2.10 in excellent yield (90% by NMR spectroscopy) in addition to H2 liberation. The evolution of H2 was further examined with the reaction of the deuterated nitrogen complex ([NPN]Ta)2(|a-D)2(uJ-r|1:ri2-N2) (c/2-2.5) with [Cp2ZrH2]2 and found to yield free H2 and the absence of a bridging tantalum-hydride signal at 11.4 ppm in the 'H NMR spectrum. Given this information, a rational interpretation of the mechanism likely involves an initial reaction of [Cp2Zr(Cl)H]x with the parent dinitrogen complex 2.5 by chloride for hydride exchange, which generates [Cp2ZrH2]2 along with some unknown tantalum chloride species. This chloride for hydride exchange has been observed with the addition of several main group chloride reagents to 2.5 and would account for the low yield of 2.10 when the zirconocene chloro-hydride reagent was used.20 To examine this hypothesis, the addition of Cp2ZrCl2 to 2.5 was examined and found to yield the 41 References begin on page 49. Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(Il) and Ti(H) reagents phosphinimide product in 20% yield (by *H NMR spectroscopy). The low observed yield is consistent with a double chloride for hydride exchange that must occur to generate [Cp2ZrH2]2 from Cp2ZrCl2 and 2.5. The observation that free H2 is generated during the reaction of [Cp2ZrH2]2 with 2.5 suggests that the zirconocene hydride reagent was simply a source of a Cp2Zr(II) species. To probe this phenomenon, the reaction of a known Zr(II) precursor, [Cp2Zr(py)(Me3SiC=CSiMe3)], with 2.5 was investigated. The formation of 2.10 is nearly quantitative, which suggests that reductive elimination of H2 from [Cp2ZrH2]2 occurs to generate a Cp2Zr(II) species, which can then react with 2.5 to form 2.10. To the best of our knowledge, this reductive elimination of H2 from a Zr(IV) dihydride complex is quite rare.21'22 Directed by these experiments, a mechanism for the cleavage and functionalization of the dinitrogen unit in 2.5 can be postulated (Scheme 2.5). The addition of [Cp2Zr(Cl)H]x or [Cp2ZrH2]2 leads to generation of a Cp2Zr(II) species in solution, which can form a simple adduct (2.11) with the tantalum dinitrogen complex, 2.5. The two electrons from the zirconium centre can then cleave the N-N bond to form 2.12, in which the central atom of the metallocene has a formal M(IV) oxidation state. The nitride moiety in this complex can then undergo intramolecular attack with one of the phosphorus atoms on the [NPN] ancillary ligand to generate the final phosphinimide product 2.10. 42 References begin on page 49. Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(II) and Ti(II) reagents 2.10 2.12 Scheme 2.5. > 2.3. Reduction and Functionalization of 2.5 with Cp2Ti(II) The cleavage and functionalization of 2.5 with reduced group 4 transition metal derivatives represents a new procedure for the activation of dinitrogen. As an extension, the chemistry with a related titanocene(II) derivative [Cp2Ti(Me3SiC=CSiMe3)] was examined. The reaction with 2.5 proceeds smoothly in THF at room temperature to generate 2.13 in good yield, which was characterized by NMR spectroscopy and elemental analysis (Equation 2.2). Although no solid state molecular structure was determined, the spectroscopic data is quite similar to 2.10. A Cs symmetric species is observed in the *H NMR spectrum with equivalent silyl methyl resonances, one single Cp resonance at 5.20 ppm, and bridging hydrides at 12.4 ppm. The 31P{'H} NMR spectrum features two singlets at 8.6 and 46.4 ppm, with the latter resonance split into a doublet 43 References begin on page 49. Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(Il) and Ti(II) reagents ('JPN = 34 Hz) when 15N-2.5 is used. Two 15N resonances are observed in the 15N{'H} NMR spectrum at 227.5 and -185.5 ppm with the latter resonance split into a doublet with identical coupling to what was observed in the 31P NMR spectrum. (2.2) 2.4. Conclusions Investigation into the addition of Schwartz's reagent ([Cp2Zr(Cl)H]x) to the end-on side-on dinitrogen complex 2.5 led to the unanticipated reduction of the dinitrogen moiety without Zr-H addition. This reaction facilitates insertion of a Cp2Zr fragment into the N-N bond of the dinitrogen ligand. The origin of this zirconocene fragment was probed by a series of experiments, which traced its formation to the elimination of dihydrogen from [Cp2ZrH2]2 (formed in situ by hydride for chloride exchange). An independent reaction between 2.5 and [Cp2Zr(py)(Me3SiC=CSiMe3)] confirmed that a Cp2Zr(II) species induces the N-N bond cleavage. This Cp2Zr(II) species promotes a two-electron inner-sphere reduction of the N-N bond to generate a transient nitride species, which is susceptible to intramolecular attack by a coordinated phosphine atom of the [NPN] ancillary ligand to generate the observed phosphinimide in addition to a trimetallic nitride species. This process represents a new way to cleave and functionalize coordinated dinitrogen. 44 References begin on page 49. Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(II) and Ti(II) reagents 2.5. Experimental 2.5.1. General Considerations Unless otherwise stated, all manipulations were performed under an atmosphere of dry oxygen-free argon or nitrogen by means of standard Schlenk or glovebox techniques. Anhydrous hexanes and toluene were purchased from Aldrich, sparged with dinitrogen, and further dried by passage through a tower of silica followed by passage through a tower of Ridox (or Q-5) catalyst prior to use. Diethyl ether, pentane, and tetrahydrofuran were purchased anhydrous from Aldrich, sparged with nitrogen, and passed through an Innovative Technologies Pure-Solv 400 Solvent Purification System. Nitrogen gas was dried and deoxygenated by passage through a column containing activated molecular sieves and MnO. C6D6, CD3C6D5, and C5D5N were dried by refluxing with sodium/potassium alloy in a sealed vessel under partial pressure, then trap-to-trap distilled, and freeze-pump-thawed several times. Deuterated tetrahydrofuran was dried by refluxing with molten potassium metal in a sealed vessel under vacuum, then trap-to-trap distilled, and freeze-pump-thaw-degassed several times. Unless otherwise stated, 'H, 31P, 'HI3'?}, ""Pi'H}, 15N{'H}, 7Li{'H} NMR spectra were recorded on a Bruker AMX-400 instrument with a 5mm BBI probe operating at 400.0 MHz for *H. 3IP NMR spectra were referenced to either external or internal P(OMe)3 (8 141.0 ppm with respect to 85% H3PO4 at 8 0.0 ppm). Elemental analyses and mass spectrometry (EI/MS) were performed at the Department of Chemistry at the University of British Columbia or the Department of Chemistry at the University of Windsor. IR spectroscopy was performed on a Nicolet 4700 FT-IR spectrometer. 2.5.2. Materials and Reagents All materials were purchased from an appropriate supplier and purified by published methods prior to use. [({NPN}2Ta2)(p-H)2(p-r|1:n2-N2)],3 [({NPN}2Ta2)(p-D)2(p-n':ri2-N2)],3 [Cp2Zr(py)(Me3SiC=CSiMe3)],23 and [Cp2Ti(Me3SiC=CSiMe3)]24 were all prepared by literature procedures. 45 References begin on page 49. Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(II) and Ti(II) reagents 2.5.3. Synthesis and Characterization of Complexes 2.10 and 2.13 Synthesis of [N(^-P=N)N]Ta(^-H)2(^-N(ZrCp2))Ta[NPN] (2.10) a) using [Cp2Zr(Cl)H]x: To a mixture of Schwartz's reagent, [Cp2Zr(Cl)H]x, (70 mg, 0.265 mmol) and 2.5 (334 mg, 0.265 mmol, 1 equivalent) in a 50 mL Erlenmeyer flask equipped with as stir bar was added 10 mL toluene and 10 mL THF in a glove box. The mixture was capped and stirred for 1 week at 15°C, after which the red-brown color of 2.5 was converted to a dark purple. The crude purple solid 2.10 was recovered on a frit after evaporation of solvents and trituration with hexanes. X-ray quality crystals were obtained from a cooled solution of THF. Yield = 137 mg, 35%. These were also used for NMR spectroscopy and elemental analysis. 'H{31P} NMR (C6D6): 8 -0.32, -0.04, -0.02, 0.08 (s, 6H each, SiC//?), 1.17 (AMX, 2JHH = 11 Hz, 2JPH = 36Hz, 4H, ?CH2), 2.19 (AMX, 2JHH = 15Hz, 2JPH = 43 Hz, 4H, ?CH2), 5.38 (s, 10H, rj5-C5H5), 6.48, 6.55, 6.74, 6.95, 6.99, 7.62 (d, t, 20 H total, N-C6H5), 7.99, 8.26 (dd, 4H, o-?C6H5), 7.08, 7.12, 7.17, 7.68, (d, t, 6H total, PC6H5), 11.66 (d, 2JHp = 18 Hz, TaflTa). l3C{lB] NMR (C6D6): 8 0.2, 0.9, 1.0, 1.8 (SiCH3), 11.9, 20.4 (PCH2), 106.1 (tj 5-CjH5), 113.1, 113.9, 114.7, 117.8, 124.7, 125.2 (NC6U5), 123.2, 129.0, 135.4, 137.8 (P-C6U5), 153.2, 155.9 (0-PQH5). 31P{'H} NMR (C6D6): 8 7.3 ppm (s), 46.7 ppm (s). Anal. Calc'd for C58H74N6P2Si4Ta2Zr:.C 46.99; H 5.03; N 5.67. Found: C 47.12; H 5.21; N5.51. (b) using [Cp2ZrH2]2: To an intimate mixture of [Cp2ZrH2]2 (16 mg, 0.036 mmol) and 2.5 (92 mg, 0.072 mmol) in a J-Young tube was added 2 mL dg-THF in a glove box. A sealed capillary tube containing a P(OMe)3 standard solution was added and the mixture was sealed and 1 31 rotated on a mechanical stirrer for 2 weeks at room temperature. The H and P NMR confirmed the exclusive formation of 2.10 in addition to unreacted 2.5 and H2 formation (4.54 in rf8-THF) (76% yield by 31P NMR after 2 weeks). A similar experiment with D2-46 References begin on page 49. Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(II) and Ti(II) reagents 2.5 and [Cp2ZrH2]2 yielded 2.10 and no bridging hydride resonances in the lH NMR. c) using Cp2ZrCh: To an intimate mixture of Cp2ZrCl2 (24 mg, 0.081 mmol) and 2.5 (102 mg, 0.081 mmol) in a J-Young tube was added 2 mL C(,D(, in a glove box. A sealed capillary tube containing a P(OMe)3 standard solution was added and the mixture was sealed and rotated on a mechanical stirrer for 1 week at room temperature. The 'H and 31P NMR confirmed the formation of 2.10 (20% yield by 31P NMR after 2 weeks) in addition to H2 formation (4.47 ppm in CeDe). d) using Cp2Zr(py)(Me3SiOCSiMe3): To a stirred solution of 2.5 (350 mg, 0.28 mmol) in 20 mL toluene was added Cp2Zr(py)(Me3SiCsCSiMe3) (135 mg, 0.28 mmol) dissolved in 5 mL of toluene. The dark brown solution immediately darkened and the solution stirred for 8 hours. The solvent was removed under vacuum, leaving a purple residue, which was triturated with pentanes until a purple solid was retained. The resulting precipitate was recovered on a glass frit. Yield = 370 mg, 90%. Synthesis of /5/V2-[N(p-P=N)N]Ta(p-H)2(p-N(ZrCp2))Ta[NPN] (75/V2-2.10). 1 equivalent of Cp2Zr(py)(Me3SiOCSiMe3) was allowed to react with 1 equivalent of /57V2-1 in a manner similar to that outlined above for the synthesis of 2.10. 15N NMR (C6D6): 8 228.4 (d, JPN = 5 Hz), -185.1 (d, 'jPN = 35). Synthesis of [N(p-P=N)N]Ta(p-H)2(p-N(TiCp2))Ta[NPN] (2.13). To a stirred solution of 2.5 (300 mg, 0.24 mmol) in 20 mL toluene was added Cp2Ti(Me3SiOCSiMe3) (83 mg, 0.24 mmol) dissolved in 5 mL of toluene. The dark brown solution immediately darkened and the solution stirred for 8 hours. The solvent was removed under vacuum, leaving a purple residue, which was triturated with pentanes to yield a purple solid. Yield = 276 mg, 80%. 'H{31P} NMR (C6D6): 8 -0.45, -0.01, 0.07, 0.11 (s, 6H each, SiCHj), 1.50 (AMX, 2JHH = 10Hz, 2JPH = 36H.z, 4H, ?CH2), 2.05 (AMX, 2JHH = 15Hz, 2JPH = 42 Hz, 4H, ?CH2), 5.20 47 References begin on page 49. Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(II) and Ti(II) reagents (s, 8H, n5-C5H5), 6.62-7.24 (m, 26H total, -AiH), 9.32 (dd, 4H, o-?C6H5), 12.40 (d, 2JHp = 17 Hz, TaT/Ta). 13C{'H} NMR (C6D6): 8 0.2, 0.9, 1.1, 1.8 (SiCH3), 12.1, 20.6 (PCH2), 104.5 (n 5-C5H5), 112.5, 113.2, 113.5, 118.5, 123.6, 126.2 (NQH5J, 122.9, 129.6, 136.9, 139.4 (P-C6H5;, 155.6, 156.1 (0-PQH5). 31P{'H} NMR (C6D6): 8 8.6 ppm (s), 46.4 ppm (s). Anal. Calc'd for C58H74N6P2Si4Ta2Ti: C 48.40; H 5.18; N 5.84. Found: C 48.23; H 5.10; N5.49. Synthesis of /5/V2-[N(n-P=N)N]Ta(^H)2(ji-N(TiCp2))Ta[NPN] (/5/V2-2.13). 1 equivalent of Cp2Ti(Me3SiOCSiMe3) was allowed to react with 1 equivalent of l5N2-2.5 in a manner similar to that outlined above for the synthesis of 2.13. ^Nj'H} NMR (C6D6): 8 227.5 (d, JPN = 5 Hz), -185.5 (d, 'JPN = 34 Hz). 48 References begin on page 49. Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(II) and Ti(II) reagents 2.6. References (1) Fryzuk, M. D.; Kozak, C. M.; Bowdridge, M. R.; Patrick, B. O.; Rettig, S. J. J. Am. Chem. Soc. 2002,124, 8389. (2) Morello, L.; Yu, P.; Carmichael, C. D.; Patrick, B. O.; Fryzuk, M. D. J. Am. Chem. Soc. 2005, 127, 12796. (3) Fryzuk, M. D.; Johnson, S. A.; Patrick, B. O.; Albinati, A.; Mason, S. A.; Koetzle, T. F. J. Am. Chem. Soc. 2001,123, 3960. (4) Studt, F.; MacKay, B. A.; Johnson, S. A.; Patrick, B. O.; Fryzuk, M. D.; Tuczek, F. Chem. Eur. J. 2005, 11, 604. (5) Shaver, M. P.; Johnson, S. A.; Fryzuk, M. D. Can. J. Chem. 2005, 83, 652. (6) MacKay, B. A.; Johnson, S. A.; Patrick, B. O.; Fryzuk, M. D. Can. J. Chem. 2005, 83, 315. (7) Fryzuk, M. D.; MacKay, B. A.; Johnson, S. A.; Patrick, B. O. Angew. Chem. Int. Ed. 2002, 41, 3709. (8) Fryzuk, M. D.; MacKay, B. A.; Patrick, B. O. J. Am. Chem. Soc. 2003,125, 3234. (9) Bertelo, C. A.; Schwartz, J. J. Am. Chem. Soc. 1976, 98, 262. (10) Orpen, A. G. Dalton Trans. 1980, 2509. (11) Shaver, M. P.; Fryzuk, M. D. Organometallics 2005, 24, 1419. (12) Yue, N.; Hollink, E.; Guerin, F.; Stephan, D. W. Organometallics 2001, 20, 4424. (13) Courtenay, S.; Stephan, D. W. Organometallics 2001, 20, 1442. (14) Bai, G.; Mueller, P.; Roesky, H. W.; Uson, I. Organometallics 2000,19, 4675. (15) Banaszak Holl, M. M.; Wolczanski, P. T. J. Am. Chem. Soc. 1992,114, 3854. (16) Abarca, A.; Martin, A.; Mena, M.; Yelamos, C. Angew. Chem. Int. Ed. 2000, 39, 3460. (17) Bennett, B. K.; Saganic, E.; Lovell, S.; Kaminsky, W.; Samuel, A.; Mayer, J. M. Inorg. Chem. 2003, 42, 4127. (18) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2004,126, 6252. (19) Seymore, S. B.; Brown, S. N. Inorg. Chem. 2002, 41, 462. (20) Johnson, S. A., PhD Thesis, University of British Columbia, 2000. (21) Chirik, P. J.; Henling, L. M.; Bercaw, J. E. Organometallics 2001, 20, 534. 49 References begin on page 49. Chapter Two: Promoting Dinitrogen Cleavage and Functionalization with Zr(II) and Ti(II) reagents (22) Edelbach, B. L.; Rahman, A. K. F.; Lachicotte, R. J.; Jones, W. D. Organometallics 1999, 18, 3170. (23) Nitschke, J. R.; Zuercher, S.; Tilley, T. D. J. Am. Chem. Soc. 2000,122, 10345. (24) Varga, V.; Mach, K.; Schmid, G.; Thewalt, U.J. Organomet. Chem. 1994, 475, 127. 50 References begin on page 49. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Chapter Three Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes 3.1 Introduction * N-Heterocyclic carbenes have become popular ligands in coordination chemistry and in homogeneous catalysis. In particular, late transition metal complexes employing NHC ligands show strong metal carbene bonds and slow dissociation rates, properties that furnish robust and versatile catalysts.1"5 However, the use of these ligands in early transition metal chemistry has little precedent. Coordination of neutral donors, such as NHCs, to d° metal centres often results in complexes that display enhanced lability of the neutral ligand since back donation from the metal to ligand is not possible. In an effort to examine this tendency, NHCs with pendant anionic donors have been synthesized, in anticipation that this modification would anchor the ligand to the metal centre and enable the strength of the metal-carbene bond to be monitored. The first systematic study of the lability of ETM and lanthanide NHC complexes was performed utilizing an amido-functionalized NHC ligand.6 Aminolysis reactions involving a lithium bromide adduct of this bidentate ligand and M(N(SiMe3)2)3 (M=Y, *A portion of this chapter has been published (Spencer, L.; Winston, S.; Fryzuk, M.D. Organometallics 2004,25,3372). 51 Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes samarium and yttrium, the lability of the NHC donor was investigated and found to dissociate from the metal centre in the presence of Me2NCH2CH2NMe2 and Ph3P=0. M = Y, Sm, Nd, Ce Scheme 3.1 ETM alkoxide-functionalized NHC complexes have also been reported. ' Uranium(IV) complexes bearing this ancillary ligand display a unique example of a metal complex with a free unbound NHC donor (Scheme 3.2).7 This donor is capable of reacting with other Lewis acids to generate simple acid-base adducts. Further reactivity of the uranium-bound carbenes was observed with the addition of other metal fragments, such as Mo(CO)4, and functional groups like BH3. Although this may be useful for the introduction of molecules into the coordination sphere of the uranium(IV) metal, these results provide further proof of the hemilabile nature of NHCs when coordinated to electropositive metals. 52 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Scheme 3.2 Interest in NHC ligands with pendant anionic groups is not limited to bidentate examples. A tridentate carbene donor system has been reported employing two anionic phenoxide donors with a centrally disposed NHC unit (Scheme 3.3). Preliminary research with titanium derivatives has been reported with no mention of NHC dissociation. Although this aspect was not addressed, it seems likely that the central disposition of the NHC in this tridentate binding motif might anchor the NHC donor to an electropositive metal centre. 53 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes 'Bu 'BU Scheme 3.3 In light of these results, a tridentate donor system was designed that would incorporate an NHC flanked by two anionic amido arms. It was anticipated this pincer architecture would render the carbene stable to dissociation because of its central position between two anionic donors. This design creates a ligand which will be abbreviated by the formula ^[NCN], where Ar refers to an aryl substituent on the amido donor and [NCN] refers to the diamido-NHC ligand. The synthesis and coordination of this Ar[NCN] ligand to group 4 transition metals is the subject of discussion in this chapter. 3.2. Synthesis ofAr[NCN]H2 and Lithium Derivatives To construct a NHC with two pendant amine arms, the reduction of several bis(amide) imidazolium chloride precursors was examined. Addition of substituted 2-chloroacetamides to imidazole produces 3.1-3.3 in reasonable yields (Scheme 3.4). The 13C{'H} NMR spectrum of 3.1 features an amide resonance at 170.5 ppm, in addition to 54 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes appropriate imidazolium and aryl resonances. Reduction of the amide derivative 3.1 with borane-dimethylsulfide gives the desired bis(amino)-imidazolium chloride in reasonable yield. Both 'H and 13C{'H} NMR spectra are consistent with the expected product (3.4) as evidenced by the disappearance of the amide C resonance in the 1JC{'H} NMR spectrum. The !H NMR spectrum shows two multiplets for the newly formed ethylene spacers and an amino -NH resonance at 5.74 ppm. 2 ArNHCOCH2CI N^^NH W p-dioxane, NEt3, A -[HNEt3]CI 4.4 BH3-SMe2 THF Ar Ar Ar = 4-MeC6H4 3.4 Scheme 3.4. O' Ar CI0 NH HN / \ O Ar Ar = 4-MeC6H4 3.1 2,4,6-Me3C6H2 3.2 2,6-iPr2C6H3 3.3 Incorporation of more sterically demanding aryl amido groups required a different synthetic approach. Unfortunately, the reduction of bis(amide) imidazolium chlorides (3.2, 3.3) with borane was unsuccessful, leading only to decomposition of the starting materials. Introduction of mesityl (2,4,6-Me3CeH2) and 2,6-diisopropyl (2,6-'Pr2C6H3) groups was accomplished by melting the appropriately substituted imidazole and N-substituted 2-chloroethylamine. This provided the imidazolium chlorides 3.5 and 3.6 in near quantitative yield (Equation 3.2). The 'H and l3C{'H} NMR spectra of 3.5 and 3.6 are similar to 3.4 with the presence of symmetrical ethylene spacer and amino groups. 55 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes 150°C NvJS^N ArNHCH2CH2CI + ArHN N N (3.2) \=J Ar = 2,4,6-Me3C6H2 3.5 2,6-^20^3 3.6 X-ray quality crystals of 3.5 were grown from CH3CN and analyzed by X-ray crystallography. An ORTEP depiction of the solid state molecular structure of 3.5 is shown in Figure 3.1. Relevant bond lengths and angles are listed in Table 3.1, and crystallographic details are located in appendix A. The presence of a chloride counterion confirms the synthesis of an imidazolium moiety. In addition, a Cl-Nl bond length of 1.326(4) and N1-C1-N2 bond angle of 109.4(3)° is observed, typical of other reported imidazolium compounds.9"11 Figure 3.1. ORTEP view of Mes[NCHN]H2-Cl (3.5) depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity. 56 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Table 3.1. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]H2-Cl (3.5) and Mes[NCN]H2, (3.8). Bond Lengths (3.5) Bond Angles (3.5) Cl-Nl 1.326(4) N1-C1-N2 109.4(3) C1-N2 ' 1.325(4) Bond Lengths (3.8) Bond Angles (3.8) Cl-Nl 1.365(3) N1-C1-N3 102.2(2) Deprotonation of imidazolium halide precursors 3.4-3.6 with KN(SiMe3)2 proceeds cleanly to give NHCs 3.7-3.9 in near quantitative yield (Equation 3.3). Both !H and 13C{'H} NMR spectra of 3.7 and 3.8 are consistent with a symmetrical molecule. The presence of a carbene moiety is signified by the absence of the resonance attributed to the iminium proton (at C2), an upfield shift in the heterocycle protons in the C4;5 positions, and a weak 13C resonance at 211.4 ppm (3.7) and 215.0 (3.8) ppm for the carbene moiety. Deprotonation of 3.6 with one equivalent of KN(SiMe3)2 at -30°C resulted in the isolation of a highly soluble yellow oil. Although !H NMR spectroscopy of the oily product confirmed the presence of 3.9, other unidentifiable resonances were present. Given this observation, the highly soluble carbene 3.9 was generated in situ from 3.6 and used as a THF solution for further reactions. KN(SiMe3)2 > THF * - HN(SiMe3)2 - KCI NH -N-(3.3) HN Ar Ar Ar = 4-MeC6H4 3.7 2,4,6-Me3C6H2 3.8 2,6-iPr2C6H3 3.9 Colourless crystals of 3.8 were grown from a saturated hexane solution and were analyzed by X-ray diffraction. An ORTEP depiction of the solid state molecular structure of 3.8 is given in Figure 3.2. Relevant bond lengths and angles are listed in 57 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Table 3.1, and crystallographic details are located in appendix A. The N1-C1-N3 bond angle is 102.2(2)°, significantly smaller than the imidazolium chloride precursor 3.5, which is consistent with other reported systems.9"11 Figure 3.2. ORTEP view of Mes[NCN]H2, (3.8) depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity. Metathesis reactions with Li2[NPN] have successfully been used in the Fryzuk group in the synthesis of [NPN] metal complexes.12 Along this line, the synthesis of Li2[NCN] was investigated. The addition of two equivalents of fl-BuLi to 3.7 and 3.8 yielded Li2tol[NCN] (3.10) and Li2Mes[NCN] (3.11), respectively (Equation 3.4). Unfortunately, the low solubility of these complexes has prevented a solid state molecular structure determination; however, the 'H NMR spectra of both species in ds-pyridine shows broad resonances indicative of the desired products. For example, the !H NMR spectrum of 3.10 reveals two multiplets assigned to the ethylene spacers, one signal for aryl-methyl and imidazole environments, and two doublets for the para-substituted aryl ring. The ^Cj/H} NMR spectrum shows a weak resonance at 189.9 ppm, which is similar to other reported Li-NHC compounds.6 Additionally, a broad 7Li resonance is 58 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes observed at 2.86 ppm in the 7Li NMR spectrum. Reasonable structures are shown in Equation 3.4 and based on the analogy of the arrangement of lithium ions observed in the 12 dianionic diamidophosphine Li2[NPN]. Ar NH ,N + 2 n-BuLi PhMe, r\ -30°C \ ^\Li//,/#< -2BuH Ar Ar Ar = 4-MeC6H4 3.10 Ar = 2,4,6-Me3C6H2 3.11 (3.4) HN. \ Ar 3.3. Attempted Syntheses of an Ar[NCN] Ligand with an Aryl Backbone The substitution of the ethylene backbone for an aryl-derived spacer could impose a rigid six-membered metallacycle and potentially force a meridional geometry upon coordination of the ligand to a transition metal. With this in mind, the syntheses of appropriately substituted imidazolium (3.12) and imidazolinium (3.13) derivatives (Figure 3.3) were investigated. Figure 3.3. 'NH HN' A/ Ar 3.12 "NH HN' A/ Ar 3.13 Imidazolium (3.12) and imidazolinium (3.13) candidates with an aryl backbone. The synthesis of 3.12 could be envisioned from the ring closure of an appropriately substituted diimine with paraformaldehyde (Scheme 3.5). This type of 13 reaction has found success in the synthesis of other imidazolium complexes. 59 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Unfortunately, the synthesis of a substituted diimine precursor has proven to be problematic. The reaction between N-phenyl-orr/zo-phenylenediimine with glyoxal was examined and found to yield a bis(benzimidazole) derivative.14 Substitution of benzil or 2,3-butanedione for glyoxal was also investigated in an attempt to prevent the formation of benzimidazole derivatives. However, no reaction was observed between /Y-phenyl-or/Tzo-phenylenediimine and these glyoxal derivatives using experimental conditions that have been successful in the synthesis of diimines.13 \ / Ph Ph Ph Ph 3/|2 Scheme 3.5. The synthesis of N-substituted diamines was also examined with a goal to ring close these compounds to generate the imidazolinium complex 3.13. A precursor to 3.13, a substituted tetraamine (3.14), was synthesized in several steps from 2-nitrofluorobenzene and 1,2-ethylenediamine (Scheme 3.6).15 Palladium catalyzed coupling of the aniline 3.14 with two equivalents of mesityl bromide produced the desired mesityl-substituted tetraamine 3.15 in an overall yield of 86%. 60 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes + 1/2 HoN / \ DMA ^ NH2 K2CO3 100°C NH HN N02 02N H2NNH2-H20 .Graphite, EtOH NH HN NH HN / \ Mes Mes 3.15 Pd2(dba)3, rac-BlNAP 2 MesBr, Na04Bu .NH HN. Scheme 3.6. Ring closure of the tetraamine with triethylorthoformate and ammonium tetrafluoroborate was anticipated to yield the imidazolinium tetrafluoroborate derivative 3.13. !H and 13C{'H} NMR spectroscopy showed an asymmetric compound in solution with inequivalent ethylene spacer, aryl, and imidazole resonances, in addition to an imidazolinium resonance at 10.04 ppm in the *H NMR spectrum. Given this spectroscopic evidence, the formation of the asymmetric benzimidazolium tetrafluoroborate compound 3.16 is postulated (Scheme 3.7). 61 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Mes 3.16 Scheme 3.7. 3.4. Synthesis of Group 4 [NCN] Amido, Chloride, and Alkyl Complexes The coordination of the [NCN] ancillary ligand onto group 4 transition metals can be easily achieved by either amine or alkyl elimination reactions. For example, treatment of a THF solution Zr(NEt2)4 with 3.7 proceeded smoothly at room temperature to afford t0'[NCN]Zr(NEt2)2 (3.17) in high yield (Equation 3.5). Complex 3.17 was characterized by 'H and 13C{'H} NMR spectroscopy and shows equivalent NEt2 moieties along with equivalent amido side arms of the t0'[NCN] unit; on the basis of this data a C2v symmetry can be assigned to this five-coordinate complex. The 13C{'H} NMR spectrum shows a resonance at 188.8 ppm indicative of a metal-carbene moiety. The aminolysis reactions of 3.8 and 3.9 with Zr(NEt2)4 yielded no reaction at room temperature and when heated provided a mixture of intractable materials. The absence of a reaction may be attributed to the increase in steric bulk at the amido nitrogen position on the [NCN] architecture. Incorporation of the bulkier Mes[NCN] and D'PP[NCN] ancillary ligands on zirconium was achieved by aminolysis reactions with Zr(NMe2)4. These reactions occurred immediately at room temperature to provide the desired Ar[NCN]Zr(NMe2)2 products 3.18 and 3.19. 62 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes These aminolysis reactions have also been used with Ti(NMe2)4 and Hf(NMe2)4 to give 3.20 and 3.21, respectively. (3.5) R2N NR2 Ar M = Zr, Ar = 4-MeC6H4, R = Et M = Zr, Ar = 2,4,6-Me3C6H2, R = Me M = Zr, Ar = 2,6-iPr3C6H2, R = Me M = Ti, Ar = 4-MeC6H4, R = Me M = Hf, Ar = 2,4,6-Me3C6H2, R = Me 3.17 3.18 3.19 3.20 3.21 X-ray quality crystals of 3.17 were grown from Et20 and the solid state molecular structure was determined by X-ray crystallography. An ORTEP depiction of 3.17 is shown in Figure 3.4. Relevant bond lengths and angles are listed in Table 3.2, and crystallographic details are located in appendix A. The ligand assumes a quasi-planar orientation to produce a distorted trigonal pyramidal metal centre. One of the six-membered chelating rings of [NCN] is nearly planar, with a Nl-Cl-Zrl-N3 torsion angle of -0.8°. However, the other six-membered chelate ring is distorted from planarity as noted by the N2-Cl-Zrl-N4 torsion angle of 34.2°. The Zrl-Cl bond length of 2.421(6) A is slightly shorter than previously characterized lengths of Zr-based NHC compounds (2.432(3)-2.456(3) A),16,17 most likely a result of the ligand architecture that pulls the carbene donor closer to the metal. The Zr-N amido bond lengths average to 2.113(6) A and are comparable to other Zr-amide complexes.18"21 63 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes ellipsoids; all hydrogen atoms have been omitted for clarity. Table 3.2. Selected Bond Distances (A) and Bond Angles (°) for tol[NCN]Zr(NEt2)2, (3.17). Bond Lengths Bond Angles Zrl-Cl 2.421(6) N3-Zrl-N4 141.7(4) Zrl-N3 2.190(6) N3-Zrl-Cl 75.0(2) Zrl-N4 2.169(5) N4-Zrl-Cl 77.9(2) Zrl-N5 2.058(6) N5-Zrl-Cl 147.2(4) Zrl-N6 2.036(6) N6-Zrl-Cl 104.1(3) N5-Zrl-N6 108.6(3) Nl-Cl-Zrl-N3 -0.8 N2-Cl-Zrl-N4 34.2° The reaction of 3.8 with Ti(NMe2)4 does not produce the expected bis(amide) titanium complex. The *H and 13C{'H} NMR spectra showed a product with Cs symmetry (3.22) in solution with four multiplets observed for the ethylene spacers, two doublets for the imidazole groups, two distinct aryl signals, two aryl-methyl signals, and a broad resonance attributed to the NMe2 groups. This suggests that the [NCN] ligand is present in the amide-amine configuration shown in Equation 3.6. 64 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Ar / Hr^NMe2 H Ti / \ Me2N NMe2 Ar = 2,4,6-Me3C6H2 3.22 (3.6) The solid state molecular structure of 3.22 was determined by an X-ray diffraction experiment from crystals grown from an Et20 solution (Figure 3.5). Relevant bond lengths and angles are listed in Table 3.3, and crystallographic details are located in appendix A. The ligand coordinates in an amide-amine donor configuration on a distorted trigonal bipyramidal titanium metal centre. The Ti-N amide bond lengths are typical of other reported titanium amide complexes " as is the Til-CI NHC bond length.16-28 Although introduction of the [NCN] ligand is incomplete, the formation of a new Ti-C NHC bond was encouraging. Thus far, all attempts to promote the coordination of the other pendant amine donor by thermolysis have been unsuccessful. Figure 3.5. ORTEP view of Mes[NCNH]Ti(NMe2)3 (3.22) depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity. 65 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Table 3.3. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCNH]Ti(NMe2)3, (3.22). Bond Lengths Bond Angles Til-Cl 2.3382(18) N3-TH-C1 82.18(6) Til-N3 1.9983(14) N5-Til-Cl 88.37(6) Til-N5 1.9650(15) N7-TH-C1 177.91(6) N3-Til-N5 123.09(6) N3-TU-N7 97.37(6) Coordination of both amide donors to form Mes[NCN]Ti(NMe2)2 was achieved by a metathesis reaction between the dilithiated [NCN] derivative 3.11 and Cl2Ti(NMe2)2 (Equation 3.7). This reaction proceeded immediately in toluene to give a dark red product in 68% yield. In solution, the 'H NMR spectrum is consistent with a symmetrical species in solution with diagnostic 13C and 'H resonances expected for 3.23. Although elemental analysis studies agree with the formation of 3.23, attempts to grow crystals suitable for X-ray diffraction study have been unsuccessful. Ar = 2,4,6-Me3C6H2 3.23 Treatment of the group 4 bis(amide) metal complexes with excess MesSiCl in toluene gave the metal dichloride complexes in quantitative yield (3.24-3.29) (Equation 3.8). These complexes are extremely insoluble in many common organic solvents, which may indicate a dimeric structure in the solid state.29"32 Although no solution spectroscopic data could be obtained, the empirical formulae of 3.24-3.29 were confirmed by mass spectrometry and elemental analysis. The addition of pyridine to a suspension of 3.24 in toluene resulted in the isolation of the pyridine adduct 3.30. Complex 3.30 is soluble in many common organic solvents, which has allowed full 66 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes characterization by spectroscopic and X-ray diffraction studies. The NMR data for 3.30 clearly show the presence of resonances for coordinated pyridine along with ligand resonances. The Zr-carbene carbon in 3.30 appears as a weak singlet at 187.9 ppm in the 13C{'H} NMR spectrum. CT )-^CT ; ^ \. .". ./ - 2 Me3SiNR2 \. .T, / N M N J ^ N M N / / V \ / / V \ Ar R2N NR2 Ar Ar CI* CI Ar M = Zr, Ar = 4-MeC6H4 3.24 M = Zr, Ar = 2,4,6-Me3C6H2 3.25 M = Zr, Ar = 2,6-iPr3C6H2 3.26 M = Ti, Ar = 4-MeC6H4 3.27 M = Ti, Ar = 2,4,6-Me3C6H2 3.28 M = Hf, Ar = 2,4,6-Me3C6H2 3.29 Crystals of 3.30 were grown from a concentrated benzene solution and analyzed by single-crystal X-ray diffraction. An ORTEP depiction of 3.30 is given in Figure 3.6 with relevant bond lengths and angles listed in Table 3.4, and crystallographic details located in appendix A. In the solid state, the zirconium centre is coordinated by the tridentate tol[NCN] ligand in addition to pyridine. The two chlorides adopt a mutually cis position with the [NCN] ligand in a meridional orientation to generate a pseudo-octahedral arrangement around the central Zr atom. Both of the six-membered chelating rings of [NCN] are nearly planar, with N2-Cl-Zrl-N4 and Nl-Cl-Zrl-N3 torsion angles of 1.2° and 7.5°, respectively; however, each ring has the carbon a to the amido donor sitting above or below in the solid state. The Zr-C carbene bond length is similar to 3.17 but still shorter than previously characterized lengths of Zr-based NHC compounds.17 The Zr-N amido bond lengths average to 2.138(4) A and are comparable to other Zr-amide complexes. " ' The Zr-Cl bond distances are not unusual as is the Zr-N bond length of the pyridine donor (2.398(5) A). 67 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Figure 3.6. ORTEP view of tol[NCN]ZrCl2(py) (3.30) depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity. Table 3.4. Selected Bond Distances (A) and Bond Angles (°) for tol[NCN]ZrCl2(py), (3.30). Bond Lengths Bond Angles Zrl-Cl 2.391(5) N3-Zrl-N4 155.27(16) Zrl-N3 2.167(4) N5-Zrl-Cl 84.02(18) Zrl-N4 2.109(4) N4-Zrl-Cll 94.77(14) Cll-Zrl-Cll 170.38(14) N2-Cl-Zrl-N4 1.2 Nl-Cl-Zrl-N3 7.5 Given the ease of NHC dissociation in reported amide-NHC ETM complexes,6'7'34 the aspect of carbene lability in tol[NCN]ZrCl2(py) was investigated. This was accomplished in two ways: in the first, the 13C{*H} NMR spectrum of 3.24 in dy pyridine, a strongly coordinating solvent, was measured. This spectrum is compared to that obtained in non-coordinating ^-benzene. In particular the chemical shift of the N-heterocyclic carbene carbon resonance was monitored. In ^-pyridine this resonance is 68 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes observed at 181.0 ppm while in ^-benzene, it appeared at 187.9 ppm; the free carbene resonance observed for 3.7 is found at 211.4 ppm in ^-benzene. A second experiment involved the addition of 10 equivalents of Me2NCH2CH2NMe2 to a ^-benzene solution of 3.24; in this case no change in the 13C{'H} NMR resonance of the carbene carbon is observed. Both of these experiments are consistent with the carbene carbon atom of the NHC in the [NCN] ancillary ligand remaining bound to the Zr centre, confirming that the flanking amido donors do anchor the NHC to the Zr(IV) centre. The preparation of dialkyl zirconium complexes was performed by both alkylation and protonolysis methods. For example, the reaction of Zr(CH2SiMe3)4 with 3.7 produced the desired product 3.31 via SiMe4 elimination (Scheme 3.8). This complex was also synthesized from the reaction of dichloride 3.24 and two equivalents of LiCH2SiMe3; however, a better yield was obtained via the alkane elimination method at -30°C. 'H and 13C{'H} NMR spectroscopy showed the presence of the desired dialkyl groups in addition to expected ligand resonances. For example, the 'H NMR spectrum of 3.32 features equivalent zirconium-methyl groups with a single resonance at 0.62 ppm, in addition to equivalent amido side arms of the tol[NCN] unit. The 13C{'H} NMR spectrum shows a resonance at 39.7 ppm, which is characteristic of a zirconium-methyl moiety. 69 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Zr(CH2R)4 PhMe, -30°C - 2 CH3R R = SiMe-,, Ph 2 RMgCI or 2 LiCH?SiMe3/ Et20, -30°C 2 MgCI2/2 LiCI Scheme 3.8. Ar=tol; R=CH2SiMe3 3.31 Me 3.32 CH2Ph 3.33 Ar=Mes; R=Me 3.34 CH2Ph 3.35 The X-ray diffraction study of a single crystal of 3.31 revealed a distorted trigonal bipyramidal geometry around the zirconium metal centre. An ORTEP depiction of the solid state molecular structure of 3.31 is shown in Figure 3.7. Relevant bond lengths and angles are listed in Table 3.5, and crystallographic details are located in appendix A. The zirconium-carbene bond length (Zrl-Cl) is 2.415(3) A, which is slightly longer than that found in 3.31 but still shorter than the corresponding bond distances reported for NHC zirconium complexes of the type trans-TxCi^ (L = NHC). 17 70 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Figure 3.7. ORTEP view of lol[NCN]Zr(CH2SiMe3)2 (3.31) depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity. Table 3.5. Selected Bond Distances (A) and Bond Angles (°) for tolrNCN]Zr(CH2SiMe3)2, (3.31). Bond Lengths Bond Angles Zrl-Cl 2.415(3) N3-Zrl-N4 148.73(9) Zrl-N3 2.173(2) N3-Zrl-Cl 78.23(9) Zrl-N4 2.135(2) N4-Zrl-Cl 76.27(9) Zrl-C23 2.238(3) C24-Zrl-Cl 115.29(10) Zrl-C24 2.254(3) Alkylation of the hafnium dichloride 3.29 by Grignard reagents was the optimal method for the synthesis of dialkyl products 3.36-3.39 (Equation 3.9). In solution, the dialkyl products possess C2v symmetry with diagnostic 13C and 'H resonances in the NMR spectra. For example, the 'H NMR spectrum of 3.36 shows equivalent 'H resonances for hafnium-methyl groups at 0.12 ppm, in addition to appropriate ethylene spacer, aryl and imidazole resonances. 71 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes An ORTEP depiction of the solid state molecular structure of 3.38 is shown in Figure 3.8 and was determined from an X-ray diffraction experiment from crystals grown from Et20. Relevant bond lengths and angles are listed in Table 3.6, and crystallographic details are located in appendix A. The ligand assumes a puckered orientation with respect to a distorted trigonal bipyramidal metal centre. The mesitylamido donors are pseudo trans oriented with N4-Hfl-N3 being 151.49(8)°. The hafnium-carbene bond length (2.387(3) A) is similar to the previously reported zirconium [NCN] complexes as are Hf-N amido (avg. 1.361(3) A) and Hf-C 2.250(3) A) alkyl bond lengths. 72 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Figure 3.8. ORTEP view of Mes[NCN]HfBu2 (3.39) depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity. Table 3.6. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]Hf Bu2, (3.38). Bond Lengths Bond Angles Hfl-Cl 2.385(3) ' N3-HT1-N4 151.49(8) Hfl-N3 2.101(2) N3-Hfl-Cl 80.30(9) Hfl-N4 2.126(2) N4-Hfl-Cl 80.30(9) C30-Hfl-Cl 118.02(9) C26-Hfl-Cl 134.32(9) The hafnium dialkyls possess excellent thermal stability. For example, the H NMR spectrum of 3.39 remains unchanged after heating a CeD6 solution at 60°C for several weeks. One exception is the diethyl complex Mes[NCN]Hf(CH2CH3)2 (3.37), which decomposes at room temperature to give the metallated species 3.40 and ethane (identified by 'H NMR spectroscopy) (Equation 3.10). The 'H NMR spectrum of 3.40 shows a C\ symmetric species in solution with five inequivalent aryl-methyl resonances. There are two doublets at 1.00 and 2.51 ppm indicative of two diastereotopic protons on 73 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes the metallated -CH2 resonance, consistent with a previously described metallated mesityl group.35 Furthermore, two multiplets at -0.10 and 0.10 ppm can be assigned to the diastereotopic protons of the remaining -HfCi^ group, an observation previously made with a similarly metallated -HfCH2CH3 system.36 Further proof of ortho-methyl bond activation is observed with a downfield shifted Hf-C 13C resonance at 72.9 ppm, a chemical shift similar to that found for the benzylic carbons of the hafnium dibenzyl derivative 3.38. 25°C -H3CCH3 Mes pu pi_i^ Vw rw Mes 3.37 (3.10) Mes//H3CCH2 3.40 \ The decomposition of the deuterated diethyl complex Mes[NCN]Hf(CD2CD3)2 (d10-3.37) provides information on the mechanism of this metallation. Decomposition of dio-3.37 results in the liberation of tfVethane (CD3CD3: identified by GC-MS), which suggests that p-hydrogen transfer has occurred to give a reactive n -ethylene intermediate (Scheme 3.9). This intermediate is not observed in solution but readily undergoes C-H bond activation with a neighboring mesityl-methyl group to give the mono-protonated product, c^-3.40. The residual 'H resonance is observed in the 'H NMR spectrum as a broad singlet at 1.5 ppm integrating to one proton. Attempts to trap the n2-ethylene intermediate with PMe3 or pyridine were unsuccessful. 74 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Scheme 3.9. 3.5 Conclusions The synthesis of a potentially dianionic, tridentate NHC ligand system has been described. Incorporation of this ligand on group 4 transition metal was accomplished by protonolysis reactions with M(NMe2)4 and alkane elimination reactions with Zr(CHiR)4 (R = Ph, SiMe3) to generate bis(amide) and dialkyl derivatives, respectively. The bis(amide) complexes were converted to dichloro or other dialkyl complexes by straightforward procedures. Hafnium dialkyl complexes display excellent stability, with one exception: the hafnium diethyl complex 3.37 undergoes a facile P-hydrogen transfer reaction at room temperature to yield a metallated hafnium species. The addition of strong donors, such as pyridine and Me2NCH2CH2NMe2, to group 4 metal complexes containing the [NCN] ancillary ligand did not show any NHC dissociation. This confirms that the flanking amido donors anchor the NHC to the group 4 metal centre. 75 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes The following chapters will investigate the application of these group 4 complexes and extend the coordination chemistry of the [NCN] ligand to include high oxidation state metals such as tantalum. 76 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes 3.6. Experimental Section 3.6.1. General Considerations Unless otherwise stated, general procedures were performed as described in Section 2.5.1. 3.6.2. Materials and Reagents All chemicals were purchased from a chemical supplier and used as received. 4-MeC6H4NHC(0)CH2Cl,37 2,4,6-Me3C6H2NHC(0)CH2Cl,38 2,6-iPr2C6H3NHC(0)CH2Cl,39 (H2NC6H4NHCH2)2 (3.14),15 Zr(CH2SiMe3)440 and CD3CD2MgBr41 were all prepared by literature methods. 2,4,6-Me3C6H2NHCH2CH2Cl was prepared by a modification of literature procedure.42 3.6.3. Synthesis and Characterization of Complexes 3.1 - 3.40 Synthesis of tol[NCHN]coH2Cl (3.1), Mes[NCHN]coH2Cl (3.2), and Dipp[NCHN]coH2-Cl (3.3). The following procedure is representative of the synthesis of 3.1-3.3. To a 500 mL Schlenk flask was added imidazole (1.85 g, 27.1 mmol), 4-MeC6H4NHC(0)CH2Cl (10.0 g, 54.5 mmol), and NEt3 (8.35 mL, 60.0 mmol). The white slurry was dissolved in 300 mL />dioxane and heated to reflux overnight. After cooling to room temperature, the solvent was removed in vacuo and 100 mL CH2C12 added to yield a white slurry. This white suspension was filtered and washed with several portions of CH2C12 until the washings became clear. The colourless microcrystalline solid was dried in vacuo overnight and found to contain one equivalent of CH2C12 as determined by the !H NMR spectrum. Yield = 7.60 g, 58%. 3.1: *H NMR (</6-DMSO): 5 2.24 (s, 6H, -C//3), 5.32 (s, 4H, -CH2), 7.12 (d, J=8 Hz, 4H, -ArH), 7.52 (d, J=8 Hz, 4H, -ArH), 7.80 (s, 2H, -imidfl), 9.24 (s, 1H, -NC//N), 10.90 (s, 2H, -C(0)N/7). 77 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes nC{lW} NMR (</6-DMSO): 5 21.1 (-CH3), 55.4 (-CH2), 119.5 (-ArC), 125.3 (-imidQ, 129.8 (-ArC), 132.7 (-ArC), 134.8 (-ArC), 144.6 (-NCHN), 170.5 (-C(O)NH). Anal. Calcd for C22H25CI3N4O2: C, 54.61; H, 5.21; N, 11.58. Found: C, 54.45; H, 5.02; N, 11.22. 3.2: 'H NMR (</6-DMSO): 8 2.26 (s, 12H, -o-ArCH3), 2.36 (s, 6H, -p-ArCH3), 5.12 (s, 4H, -CH2), 7A3 (s, -4H, -Ar//), 7.65 (s, 2H, -imid//), 9.10 (s, 1H, -NC//N), 10.54 (s, 2H, -C(0)N//). I3C{'H} NMR (rf6-DMSO): 8 21.4 (-CH3), 22.3 (-CH3), 57.2 (-CH2), 121.2 (-ArQ, 123.8 (-imidC), 130.2 (-ArQ, 131.9 (-ArQ, 133.7 (-ArQ, 145.1 (-NCHN), 172.4 (-C(O)NH). Anal. Calcd. For C25H3iN402: C, 71.57; H, 7.45; N, 13.35. Found: C, 71.44; H, 7.10; N, 13.46. 3.3: 'H NMR (rf6-DMSO): 8 1.42 (d, J = 9 Hz, 24H, -CH(C//3)3), 3.78 (sept, J = 9 Hz, 4H, -C//(CH3)2), 5.24 (s, 4H, -C(0)C//2), 7.52-7.60 (m, 6H, -ArH), 7.83 (s, 2H, -imid//), 9.56 (s, 1H, -NC//N), 10.34 (s, 2H, -C(0)N//). 13C{'H} NMR (rf6-DMSO): 8 18.5 (-CH3), 39.5 (-CH), 55.8 (-NCH2), 120.8 (-imidQ, 124.8 (-ArQ, 131.3 (-ArQ, 133.4 (-ArQ (-ArQ, 139.5 (-ArQ, 143.0 (-NCHN), 175.2 (-C(O)NH). Anal. Calcd. For C3iH43ClN402: C, 69.06; H, 8.04; N, 10.39. Found: C, 69.30; H, 8.31; N, 10.28. Synthesis of tol[NCHN]H2Cl (3.4) A 250 mL Schlenk flask containing a magnetic stirbar was charged with 3.1-CH2CI2 (lO.lg, 20.8 mmol) and THF (100 mL) was added. BH3-SMe2 (5.0M in Et20, 18.3 mL) was added, and the white suspension was heated to reflux for 16 h with stirring. Vigorous gas evolution was noted as soon as heating commenced. The solvent was removed under reduced pressure leaving a white residue. Aqueous HCI (1M, 45.7 mL, 45.7 mmol) was added and the suspension refluxed for 1 h. After cooling to room temperature, solid NaOH (7.3 g, 0.18 mol) was added to the clear solution, which caused a white solid to precipitate. Addition of CH2CI2 (50 mL) caused further precipitation of a white solid. The solid was filtered, washed with CH2CI2 (3 x 20 mL), and dried in vacuo 78 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes overnight to give a white powder. The solid was recrystallized from MeOH giving colorless crystals. Yield = 3.70 g, 48%. 'H NMR (DMSO-rf6): 5 2.13 (s, 6H, -C//3), 3.40 (t, J = 5 Hz, 4H, -C//2NAT), 4.28 (t, J = 5 Hz, 4H, -C/fcNimid), 5.74 (br s, 2H, -Ni/), 6.50 (d, J = 8 Hz, 4H, -Ar//), 6.88 (d, J = 8 Hz, 4H, -Ar//), 7.78 (s, 2H, -imid//), 9.16 (s, 1H, -imid//). 13C{'H} NMR (DMSO-</6): 8 28.5 (-CH3), 50.5 (-CH2N), 52.1 (-CH2N), 115.3 (-ArQ, 125.2 (-ArQ, 126.1 (-ArQ, 130.0 (-imidQ, 138.6 (-NCHN), 140.5 (-ArQ. EI-MS: 370 [M+]. Anal. Calcd. for C2iH27ClN4: C, 68.00; H, 7.34; N, 15.10. Found: C, 67.82; H, 7.10; N, 15.13. Synthesis of 2,4,6-Me3C6H2NHC(0)CH2(imid) and 2,6-iPr3C6H2NHC(0)CH2(iniid) The following procedure is representative of the synthesis of 2,4,6-Me3C6H2NHC(0)CH2(imid) and 2,6-iPr3C6H2NHC(0)CH2(imid). NaH (3.8 g, 158.3 mmol) and imidazole (9.73 g, 142.9 mmol) were combined in DMF (75 mL) and stirred for 30 minutes at 50 °C to give a clear brown solution. 2,4,6-Me3C6H2NHC(0)CH2Cl (28.2 g, 142.9 mmol) was added portionwise over a period of 30 minutes to give an opaque solution. After stirring for 12 hours, water (20 mL) was added and all solvents were removed under reduced pressure. The resulting brown residue was acidified with 2M HC1 (250 mL) and washed with Et20 (3 x 150 mL). The aqueous solution was made basic with excess NaOH and extracted with CH2C12 (3 x 250 mL). The CH2C12 was removed to give a white crystalline material, which was washed with hexanes. Yield = 28.0 g, 86%. 2,4,6-Me3C6H2NHC(0)CH2(imid):  lH NMR (CDC13): 8 2.12 (s, 6H, -o-ArC//3), 2.25 (s, 3H, -/>ArC//3), 4.75 (s, 2H, -NC//2), 6.80 (s, 2H, -Ar//), 7.05 (s, 1H, -imid//), 7.10 (s, 1H, -imid//), 7.60 (s, 1H, -NC//N). 13C{'H} NMR (CDC13): 8 18.2 (-CH3), 20.5 (-CH3), 57.0 (-NCH2), 123.0 (-imidQ, 123.2 (-imidQ, 128.5 (-ArQ, 130.5 (-ArQ, 130.7 (-ArQ, 135.1 (-NCHN), 140.1 (-ArQ, 178.1 (-C(O)). Anal. Calc. for Ci4H17N30: C, 69.11; H, 7.04; N, 17.27. Found: C, 69.00; H, 7.01; N, 17.10. 79 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes 2,6-iPr3C6H2NHC(0)CH2(imid): 'H NMR (CDC13): 5 1.16 (d, J=7 Hz, -CH(C//3)2), 2.86 (sept, J=7 Hz, -C//(CH3)2), 4.87 (s, 2H, -C(0)C//2), 7.10 (s, 1H, -imid//), 7.15 (d, J=8 Hz, 2H, -Ar// ), 7.21 (s, 1H, -imid//), 7.29 (t, J=8 Hz, 1H, -Ar//), 7.64 (s, 1H, -imid//). 13C{'H} NMR (CDC13): 8 21.3 (-CH3), 36.4 (-CH), 58.7 (-NCH2), 124.1 (-imidQ, 124.6 (-imidQ, 127.2 (-ArQ, 130.9 (-ArQ, 131.8 (-ArQ, 137.1 (-NCHN), 142.3 (-ArQ, 174.7 (-C(O)). Anal. Calcd. for C,7H23N30 C, 71.55; H, 8.12; N, 14.72. Found: C, 71.23; H, 7.93; N, 14.55. Synthesis of 2,4,6-Me3C6H2NHCH2CH2(imid) and 2,6-iPr3C6H2HCH2CH2(imid) The following procedure is representative of the synthesis of 2,4,6-Me3C6H2NHCH2CH2(imid) and 2,6-iPr3C6H2HCH2CH2(imid). 2,4,6-Me3C6H2NHCOCH2(imidazole) (13.0 g, 56.8 mmol) and BH3-SMe2 (25.0 mL,125.0 mmol, 5.0 M in Et20) were combined in THF (500 ml) and refluxed overnight. The THF was removed under reduced pressure and 2 M HCI (63 mL, 2.2 equivalents) was added to the white residue. The solution was made basic with excess NaOH (8 equivalents) and extracted with CH2CI2 (3 x 250 mL). The CH2CI2 was removed, and the clear oil recrystallized with boiling hexanes. Yield = 9.8 g, 80%. 2,4,6-Me3C6H2NHCH2CH2(imid): ]H NMR (rf6-DMSO): 8 2.03 (s, 6H, -o-ArC//3), 2.15 (s, 3H, -/>ArC//3), 3.19 (t, J = 8 Hz, -NC//2), 3.68 (t, J = 8 Hz, 1H, -NH), 4.13 (t, J= 8Hz, -NC//2), 6.80 (s, 2H, -ArH), 6.85 (s, 1H, -imid//), 7.20 (s, 1H, -imid//), 7.64 (s, 1H, -NC//N). '^{'H} NMR (rf6-DMSO): 8 18.9 (-CH3), 22.1 (-CH3), 48.4 (-NCH2), 48.9 (-NCH2), 121.0 (-imidQ, 121.6 (-imidQ, 126.4 (-ArQ, 128.8 (-ArQ, 129.0 (-ArQ, 134.2 (-NCN), 142.1 (-ArQ. Anal. Calc. for Ci4H,9N3: C, 73.33; H, 8.35; N, 18.32. Found: C, 73.05; H, 8.09; N, 18.15. 2,6-iPr3C6H2HCH2CH2(imid): 'H NMR (rf6-DMSO): 8 1.10 (d, J=7 Hz, -CH(C//3)2), 2.95 (sept, J = 7 Hz, -C//(CH3)2), 3.03 (t, J = 8 Hz, -NArC//2), 3.98 (t, J = 8 Hz, -80 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes NimidC//2), 7.02 (s, 1H, -imid//), 7.11 (d, J=8 Hz, 2H, -Ar//), 7.24 (s, 1H, -imid//), 7.35 (t, J=8 Hz, 1H, -Ar//), 7.80 (s, 1H, -imid//). l3C{ lH} NMR (</6-DMSO): 8 22.5 (-CH3), 29.6 (-CH), 50.7 (-NCH2), 51.3 (-NCH2), 118.7 (-imidQ, 120.2 (-imidQ, 121.3 (-ArQ, 128.3 (-NCN), 129.4 (-ArQ, 134.8 (-ArQ, 138.4 (-ArQ. Anal. Calcd. for C17H25N3 C, 75.23; H, 9.28; N, 15.48. Found: C, 74.97; H, 9.45; N, 15.33. Synthesis of 2,6-iPr2C6H3NHCH2CH2Cl 2,6-iPr2C6H3NHC(0)CH2Cl (10 g, 35.1 mmol) and BH3-SMe2 (15.4 mL, 77.1 mmol, 5.0 M in Et20) were dissolved in THF (250 ml) in a 500 mL Schlenk flask and refluxed overnight. The THF was removed under reduced pressure and 2 M HC1 (38.6 mL, 2.2 equivalents) was added to the white residue. The solution was made basic with excess NaOH (14.04 g, 8 equivalents) and extracted with CH2C12 (3 x 50 mL). The CH2C12 was removed and the clear oil was extracted with pentane. The solvent was removed to yield a colorless oil that was used without further purification. Yield = 8.19 g, 86 %. *H NMR (CDCI3): 8 1.23 (d, J = 7 Hz, 12H, -CH(C//3)2), 3.19 (t, J = 8 Hz, 2H, -NC//2), 3.18 (sept, J = 7 Hz, 2H, -C//(CH3)2), 3.73 (t, J = 8 Hz, 2H, -C//2C1), 7.08 (m, 3H, -Ar//). 13C{'H} NMR (CDCI3): 20.3 (-CH3), 30.6 (-CH), 45.8 (-CH2C1), 51.3 (-NCH2), 125.7 (-ArQ, 128.9 (-ArQ, 130.1 (-ArQ, 137.9 (-ArQ. Synthesis of Mes[NCHN]cl (3.5) and Dipp[NCHN]cl (3.6) The following procedure is representative of the synthesis of 3.5 and 3.6. 2,4,6-Me3C6H2NHCH2CH2Cl (6.0 g, 30.4 mmol) and 2,4,6-Me3C6H2NHCH2CH2(imid) (6.5 g, 30.4 mmol) were combined and stirred at 150 °C for 2 hours. The resulting white solid was washed with THF (50 mL) and filtered to give a white crystalline powder. Yield = 11.29 g, 87%. Recrystallization with boiling acetonitrile gave long colorless crystals. 81 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes 3.5: 'H NMR (rf6-DMSO): 5 2.01 (s, 12H, -o-ArC//3), 2.12 (s, 6H, -p-ArCH3), 3.15 (q, J = 8 Hz, 4H, -NArC//2), 3.92 (t, J= 8 Hz, 1H, -N//), 4.40 (t, J= 8 Hz, 4H, -NimidC//2), 6.81 (s, 4H, -ArH), 7.88 (s, 2H, -imid//), 9.35 (s, 1H, -NC//N). 13C{'H} NMR (rf6-DMSO): 5 17.8 (-CH3), 20.2 (-CH3), 47.2 (-NCH2), 49.2 (-NCH2), 122.6 (-imidQ, 129.0 (-ArQ, 130.2 (-ArQ, 130.7 (-ArQ, 137.2 (-NCHN), 142.4 (-ArQ. Anal. Calc. for C25H35C1N4: C, 70.32; H, 8.26; N, 13.12. Found: C, 70.15; H, 8.13; N, 13.02. 3.6: *H NMR (</6-DMSO): 8 1.19 (d, J = 7 Hz, -CH(Cr73)2), 2.92 (sept, J = 7 Hz, -Ctf(CH3)2), 3.44 (q, J = 8 Hz, 4H, -NArC//2), 3.89 (t, J = 8 Hz, 1H, -N//), 4.90 (t, J = 8 Hz, 4H, -NimidC//2), 7.01 (m, 6H, -ArH), 7.76 (s, 2H, -imid//), 9.16 (s, 1H, -NC//N). ,3C{'H} NMR (rf6-DMSO): 8 20.2 (CH3), 33.5 (-CH), 45.9 (-NCH2), 50.3 (-NCH2), 121.5 (-imidQ, 127.6(-ArQ, 129.7 (-ArQ, 131.6 (-ArQ, 139.5 (-NCHN), 143.1 (-ArQ. Anal. Calcd. for C31H47C1N4 C, 72.84; H, 9.27; N, 10.96. Found: C, 72.78; H, 9.55; N, 10.74. Synthesis of ,0|[NCN]H2 (3.7) and Mes[NCN]H2 (3.8) The following procedure is representative of the synthesis of 3.7 and 3.8. The following procedure was used in the synthesis of 3.9; however, the product was not isolated. A THF solution (10 mL) of KN(SiMe3)2 (664 mg, 3.3 mmol) was slowly added dropwise to 3.5 (1.24 g, 3.3 mmol) dissolved in 40 mL THF, creating a slightly yellow suspension. The suspension was stirred for LA hr and the solvent removed in vacuo. The pale yellow residue was extracted with toluene (20 mL) and the solution filtered through celite. Removal of the solvent yielded a white solid which was washed several times with hexane and dried in vacuo. Yield = 1.10 g, 100%. 3.7: 'H NMR (C6D6): 8 2.13 (s, 6H, -CH3), 3.40 (t, J = Hz, 4H, -C//2N), 4.28 (t, J = Hz, 4H, -C//2N), 5.74 (br s, 2H, -NH), 6.25 (s, 2H, -imid//), 6.53 (d, J = Hz, 4H, -ArH), 6.85 (d, J = Hz, 4H, -ArH). "CCU} NMR (C6D6): 8 22.5 (-CH3), 46.1 (-CH2N), 50.9 (-CH2N), 114.5 (-ArQ, 118.9 (-imidQ, 120.6 (-ArQ, 130.7 (-ArQ, 149.8 (-ArQ, 211.4 (-NCN). Anal. Calcd. for C2,H26N4: C, 75.41; H, 7.84; N, 16.75. Found: C, 75.20; H, 7.49; N, 16.88. 82 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes 3.8: lU NMR (C6D6): 8 2.35 (s, 6H, -/>ArC//3), 2AO (s, 12H, -o-ArC//3), 3.22 (q, J = 8Hz, 4H, -NC//2), 3.76 (t, J = 8Hz, 4H, -NCH2), 4.15 (t, J = 8Hz, 1H, -NH), 6.30 (s, 2H, -imid//), 6.82 (s, 4H, -ArH). 13C{'H} NMR (C6D6): 8 20.0 (-o-CH3), 24.5 (-/>CH3), 48.5 (-NCH2), 49.1 (-NCH2), 121.7 (-imidQ, 126.9 (-ArQ, 127.5 (-ArQ, 130.1 (-ArQ, 145.0 (-ArQ, 215.0 (-NCN). Anal. Calc. for C25H34N4: C, 76.88; H, 8.77; N, 14.35. Found: C, 76.54; H, 8.41; N, 14.28 Synthesis of Li2,ol[NCN] (3.10) and Li2Mes[NCN] (3.11), The following procedure is representative of the synthesis of 3.10 and 3.11. A toluene solution of 3.7 (530 mg, 1.6 mmol) was cooled to -30°G and 2.0 mL of 1.6M n-BuLi (3.2 mmol) was added slowly dropwise. The solution immediately darkened after the first equivalent was added and a white precipitate formed after addition of the second equivalent of base was complete. The white suspension was allowed to warm slowly to room temperature where it is stirred overnight. Filtration yielded a white powder which was washed with several portions of toluene. Yield = 549 mg, 99% yield. 3.10: 'H NMR (C5D5N): 8 2.29 (s, 6H, -Ar.CH3), 3.62(m, 4H, -NC//2), 4.13 (m, 4H, -NC//2), 6.69 (s, 4H, -Ar//), 7.00 (s, 2H, -imid//). ,3C{'H} NMR (C5D5N): 8 21.4 (-CH3), 45.6 (-NCH2), 47.2 (-NCH2), 117.9 (-ArQ, 120.5 (-ArQ, 122.9 (-imidQ, 126.9 (-ArQ, 148.6 (-ArQ, 189.9 (-NCN). 7Li NMR (C5H5N): 8 2.86. Anal. Calcd. for C2iH24Li2N4: C, 72.83; H, 6.98; N, 16.18; Found: C, 72.79; H, 6.78; N, 16.01. 3.11: !H NMR (C5D5N): 8 2.35 (s, 6H, -/>ArCH3), 2.46 (s, 6H, -o-ArCH3), 4.08 (m, 4H, -NC//2), 4.18 (m, 4H, -NC//2), 6.81 (s, 4H, -Ar//), 6.91 (s, 2H, -imid//). UC{XU} NMR (C5D5N): 8 20.4 (-CH3), 21.3 (-CH3), 46.8 (-NCH2), 47.8 (-NCH2), 119.5 (-ArQ, 119.9 (-ArQ, 123.6 (-imidQ, 127.9 (-ArQ, 149.5 (-ArQ, 190.2 (-NCN). 7Li NMR (C5D5N): 8 2.88. Anal. Calcd. for C25H32Li2N4: C, 74.61; H, 8.01; N, 13.92; Found: C, 74.55; H, 7.95; N, 13.86. 83 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes Synthesis of (2,4,6-Me3C6H2NHC6H4NHCH2)2 (3.15) A 100 mL Schlenk was charged with Pd2(dba)3 (151 mg, 0.16 mmol), rac-BINAP (103 mg, 0.16 mmol), NaOlBu (1.11 g, 11.6 mmol), 3.14 (1.0 g, 4.1 mmol), mesityl bromide (1.81 g, 9.0 mmol) and 50 mL of toluene. The mixture was slowly warmed to 100°C and stirred overnight. The dark brown suspension was filtered through Celite and the solvent removed to yield a dark red residue. Pentane (10 mL) was added to yield a brown powder that was further purified by dissolving the solid in toluene and filtering through a silica plug. Yield = 1.69 g, 86%. 'H NMR (CDC13): 5 2.03 (s, 12H, -o-ArC//3), 2.27 (s, 12H, -p-ArCH3), 3.56 (s, 4H, -NC//2), 4.60 (br s, 2H, -N//), 6.22 (d, J - 8 Hz, 2H, -Ar//), 6.62 (t, J = 8 Hz, 2H, -Ar//), 6.80-6.86 (m, 4H, -Ar//). 13C{'H} NMR (CDCI3): 5 18.9 (-CH3), 20.1 (-CH3), 52.2 (-NCH2), 116.5 (-ArQ, 117.9 (-ArQ, 118.5 (-ArQ, 120.3 (-ArQ, 125.6 (-ArQ, 127.9 (-ArQ, 130.4 (-ArQ, 135.2 (-ArQ. Satisfactory elemental analysis was not obtained. Synthesis of Asymmetrical Imidazolinium tetrafluoroborate (3.16) A 25 mL Schlenk flask was charged with 3.15 (980 mg, 2.0 mmol), NH4BF4 (215 mg, 2.0 mmol), and HC(OEt)3 (0.31 mL, 2.0 mmol) and slowly heated to 120°C. The slurry was stirred at this temperature for 1 hr, then cooled to room temperature. THF was added to give a white suspension, which was filtered and washed with several portions of THF. The white solid was dried in vacuo overnight. Yield = 968 mg, 84 %. *H NMR (rf6-DMSO): 5 1.87 (s, 6H, -p-AxCHf), 1.89 (s, 6H, -p-AvCH3), 2.21 (s, 3H, -o-ArC//3), 2.37 (s, 6H, -o-ArC//3), 3.81 (m, 2H, -NC//2), 4.89 (m, 2H, -NC//2), 5.04 (m, 1H, -NH), 5.71 (m, 1H, -N//), 5.82 (m, 1H, -Ar//), 6.40 (m, 1H, -Ar//), 6.55 (m, 1H, -Ar//), 6.61 (m, 1H, -Ar//), 6.87 (s, 2H, -Ar//), 7.20 (m, 2H, -Ar//), 7.39 (m, 1H, -Ar//), 7.66 (m, 1H, -Ar//), 7.76 (m, 1H, -Ar//), 8.26 (m, 1H, -Ar//), 10.04 (s, 1H, -NC//N). '^{'H} NMR (rfe-DMSO): 5 19.6 (-CH3), 20.4 (-CH3), 20.9 (-CH3), 21.8 (-CH3), 51.3 (-NCH2), 55.0 (-NCH2), 113.5 (-ArQ, 115.8 (-ArQ, 117.9 (-ArQ, 118.3 (-ArQ, 118.9 (-ArQ, 119.5 (-ArQ, 120.4 (-ArQ, 120.9 (-ArQ, 123.3 (-ArQ, 124.7 (-ArQ, 125.2 (-84 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes ArC), 126.3 (-ArQ, 127.5 (-ArQ, 128.5 (-ArQ, 129.3 (-ArQ, 131.6 (-ArQ, 132.7 (-ArQ, 133.6 (-ArQ, 135.2 (-ArQ, 137.2 (-ArQ, 143.4 (-NCHN). Satisfactory elemental analysis was not obtained. Synthesis of Ar(NCN)M(NMe2)2 [(M=Zr, Ar=tol, R=Et (3.17); M=Zr, Ar=Mes, R=Me (3.18); M=Zr, Ar=Dipp, R=Me (3.19); M=Ti, Ar=tol, R=Me (3.20); M=Hf, Ar=Mes, R=Me (3.21)] The following procedure is representative of the synthesis of 3.17-3.21. In the case of 3.19, the carbene 3.9 was generated in situ and used as a THF solution. A cooled (-30°C) THF (10 mL) solution of Hf(NMe2)4 (415 mg, 1.17 mmol) was slowly added dropwise to 3.8 dissolved in THF (20 mL). The mixture was warmed to room temperature gradually and stirred overnight. After the solvent was removed in vacuo and toluene (15 mL) added, the solution was filtered and the solvent removed to give a pale orange solid, that was recrystallized with Et20/hexanes at -30°C to give a white powder Yield = 662 mg, 85%. 3.17: 'H NMR (C6D6): 5 1.10 (t, J=8 Hz, 12H, -NCH2C//3), 2.35 (s, 6H, -CH3), 3.40 (m, 4H, -C//2N), 3.44 (q, J = 8 Hz, 8H, -NC//2CH3), 3.92 (m, 4H, -C//2N), 5.80 (s, 2H, -imid//), 7.10 (d, J=8 Hz, 4H, -ArH), 7.15 (d, J = 8Hz, 4H, -ArH). 13C{lH} NMR (C6D6): 5 17.5 (-CH2CH3), 23.0 (-CH3), 50.7 (-CH2N), 51.2 (-CH2N), 53.4 (-ZrNCH2), 119.9 (-ArQ, 120.6 (-imidQ, 127.2 (-ArQ, 130.1 (-ArQ, 157.6 (-ArQ, 188.8 (-NCN). Anal. Calcd. for C29H44N6Zr: C, 61.33; H, 7.81; N, 14.80. Found: C, 61.11; H, 7.74; N, 14.56. 3.18: !H NMR (C6D6): 5 2.21 (s, 6H, -/>ArC//3), 2.40 (s, 12H, -o-ArC//3), 2.67 (s, 12H, -NMe2), 3.34 (m, 4H, -NCH2), 3.61 (m, 4H, -NC//2), 6.02 (s, 2H, -imid//), 6.96 (s, 4H, -ArH). 13C{'H} NMR (C6D6): 5 19.6 (-o-CH3), 23.4 (-p-CR3), 45.6 (-NCH3), 48.8 (-NCH2), 49.2 (-NCH2), 122.4 (-imidQ, 130.5 (-ArQ, 132.3 (-ArQ, 132.9 (-ArQ, 140.2 (-ArQ, 190.9 (-ZrCcarbene). Anal. Calc. for C29H44N6Zr: C, 61.33; H, 7.81; N, 14.80. Found: C, 61.20; H, 7.58; N, 14.45. 85 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes 3.19: 'H NMR (C6D6): 5 1.20 (d, J = 8 Hz, 12H, -CH(C//3)2), 1.35 (d, J = 8 Hz, 12H, -CH(C//3)2), 2.43 (s, 12H, -N(CH3)2), 3.24 (sept, J = 8 Hz, 4H, -C//(CH3)2), 3.86 (m, 4H, -NC//2), 4.13 (m, 4H, -NC//2), 5.98 (s, 2H, -imid//), 6.98-7.03 (m, 6H, - Ar//). '^{'H} NMR (C6D6): 8 21.4 (CH3), 31.6 (-CH), 46.7 (-NCH3), 49.6 (-NCH2), 51.5 (-NCH2), 119.9 (-imidQ, 121.2 (-ArQ, 125.8 (-ArQ, 130.4 (-ArQ, 140.2 (-ArQ, 192.1 (-NCN). Anal. Calc. for C35H56N6Zr: C, 64.47; H, 8.66; N, 12.89. Found: C, 64.37; H, 8.83; N, 12.75. 3.20: 'H NMR (C6D6): 5 2.38 (s, 6H, -C//3), 2.65 (s, 12H, -N(C//3)2), 3.45 (m, 4H, -C//2N), 4.08 (m, 4H, -C//2N), 5.78 (s, 2H, -imid//), 7.11 (d, J=8 Hz, 4H, -Ar//), 7.20 (d, J = 8Hz, 4H, -Ar//). '^{'H} NMR (C6D6): 8 24.3 (-CH3), 49.6 (-CH2N), 51.7 (-NCH3), 52.4 (-CH2N), 118.4 (-ArQ, 120.3 (-imidQ, 127.2 (-ArQ, 132.3 (-ArQ, 155.7 (-ArQ, 187.2 (-NCN). Anal. Calc. for C25H36N6Ti: C, 64.10; H, 7.75; N, 17.94. Found: C, 63.98; H, 7.88;N, 17.84. 3.21: *H NMR (C6D6): 8 2.19 (s, 6H, -/>ArC//3), 2.39 (s, 12H, -o-ArC//3), 2.70 (s, 12H, -NMe2), 3.36 (m, 4H, -NC//2), 3.55 (m, 4H, -NC//2), 5.95 (s, 2H, -imid//), 6.91 (s, 4H, -ArH). l3C{ lH} NMR (C6D6): 8 19.7 (-o-CH3), 24.3 (-/?-CH3), 44.5 (-NCH3), 49.0 (-NCH2), 50.1 (-NCH2), 122.0 (-imidQ, 129.8 (-ArQ, 132.8 (-ArQ, 133.3 (-ArQ, 142.3 (-ArQ, 195.7 (-HfCcarbene). Anal. Calc. for C29H44HfN6: C, 53.16; H, 6.77; N, 12.83. Found: C, 52.89; H, 6.44; N, 12.68. Synthesis of Mes[NCNH]Ti(NMe2)3 (3.22) A cooled (-30°C) THF (10 mL) solution of Ti(NMe2)4 (200 mg, 0.89 mmol) was slowly added dropwise to 3.8 (348 mg, 0.89 mmol) dissolved in THF (20 mL). The mixture was warmed to room temperature gradually and stirred overnight. After the solvent was removed in vacuo and toluene (15 mL) added, the solution was filtered and the solvent removed to give a dark red solid, that was recrystallized with Et20/hexanes at -30°C to give a red powder. Yield = 662 mg, 85%. 86 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes 'H NMR (C6D6): 5 2.13 (s, 6H, -o-ArC7/3), 2.18 (s, 3H, -/>ArC//3), 2.22 (s, 3H, -p-AvCHj), 2.38 (s, 6H, -o-ArC7/3), 2.6 (t, J = 8 Hz, 1H, -NH), 3.0 (q, J = 8 Hz, 2H, -C//2NH), 3.1-3.2 (br s, 18H, -NC//3), 3.40 (m, 2H, -NC7/2), 3.50 (m, 2H, -NC//2), 3.77 (t, J = 8 Hz, -NC7/2), 6.04 (d, J = 2 Hz, 1H, -imid//), 6.42 (d, J = 2 Hz, 1H, -imid//), 6.78 (s, 2H, -Ar//), 7.01 (s, 2H, -Ar//). ,3C{'H} NMR (C6D6): 8 19.5 (-CH3), 20.2 (-CH3), 22.6 (-CH3), 22.9 (-CH3), 45.8 (-NCH2), 47.8 (-NCH2), 49.8 (-NCH2), 51.3 (-NCH2), 117.8 (-ArQ, 118.5 (-ArQ, 119.6 (-imidQ, 120.6 (-imidQ, 128.9 (-ArQ, 129.8 (-ArQ, 130.5 (-ArQ, 132.6 (-ArQ, 148.7 (-ArQ, 149.9 (-ArQ, 192.6 (-NCN). Satisfactory elemental analysis was not obtained. Synthesis of Mes[NCN]Ti(NMe2)2 (3.23) To a cooled (-30°C) solution of Cl2Ti(NMe2)2 (103 mg, 5.0 mmol) in THF (10 mL) was slowly added a THF solution of 3.11 (200 mg, 5.0 mmol). The solution was left to stand at -30°C for 15 minutes and then slowly warmed to room temperature. The red solution was stirred overnight at which time the solvent was removed to dryness, toluene added, and the solution filtered through Celite. The volume was reduced to several mL's and hexane added to yield a dark red powder. Yield = 1.78 g, 68%. 'H NMR (C6D6): 8 2.22 (s, 6H, -p-ArCH3), 2.40 (s, 6H, -o-ArC//3), 2.78 (br s, 12H, -N(C773)2), 3.35 (m, 4H, -C772), 3.60 (m, 4H, -CH2), 6.05 (s, 2H, -imid//), 6.95 (s, 4H, -Ar//). ,3C{'H} NMR (C6D6): 8 20.5 (-CH3), 22.4 (-CH3), 50.6 (-NCH3), 52.4 (-NCH2), 54.1 (-NCH2), 117.9 (-imidQ, 120.4 (-ArQ, 128.0(-ArQ, 131.7 (-ArQ, 143.7 (-ArQ, 193.7 (-NCN). Anal. Calc. for C29H44N6Ti: C, 66.40; H, 8.45; N, 16.02. Found: C, 66.52; H, 8.80; N, 15.89. Synthesis of Ar(NCN)MCl2 [(M=Zr, Ar=tol (3.24), M=Zr, Ar=Mes (3.25), M=Zr, Ar=Dipp (3.26), M=Ti, Ar=tol (3.27), M=Ti, Ar=Mes (3.28), M=Hf, Ar=Mes (3.29)] The following procedure is representative of the synthesis of 3.24-3.29. Chlorotrimethylsilane (1.03 mL, 8.14 mmol) was added dropwise to a toluene (20 mL) 87 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes solution of 4 (533 mg, 0.81 mmol) with vigorous stirring. The white suspension was stirred overnight and collected by filtration to give a white powder. Yield = 495 mg, 95%. 3.24: EI-MS: 494 [M+] Anal. Calcd. for C2iH24Cl2N4Zr: C, 51.00; H, 4.89; N, 11.33. Found: C, 50.65; H, 4.53; N, 11.28. 3.25: EI-MS: 562 [M+] Anal. Calc. for C25H32Cl2N4Zr: C, 54.53; H, 5.86; N, 10.17. Found: C, 54.36; H, 5.44; N, 10.01. 3.26: EI-MS: 634 [M+] Anal. Calcd. for C3iH44Cl2N4Zr: C, 58.65; H, 6.99; N, 8.83. Found: C, 58.95; H, 7.21; N, 8.95. 3.27: EI-MS: 451 [M+] Anal. Calcd. for C2iH24Cl2N4Ti: C, 55.90; H, 5.36; N, 12.42. Found: C, 55.78; H, 5.52; N, 12.23. 3.28: EI-MS: 507 [M+] Anal. Calcd. for C25H32Cl2N4Ti: C, 59.19; H, 6.36; N, 11.04. Found: C 59.26; H, 6.45; N, 10.89. 3.29: EI-MS: 638 [M+] Anal. Calc. for C25H32Cl2HfN4: C, 47.07; H, 5.06; N, 8.78. Found: C, 46.85; H, 4.86; N, 8.53. Synthesis of ,oI[NCN]ZrCl2(py) (3.30) The pyridine adduct was formed by suspending a portion of the solid 3.24 in toluene and adding an excess of pyridine. The solution was filtered, and the solvent was reduced in volume (~ 2 mL). Hexane was then added to precipitate a red-orange solid. Further recrystallization from a saturated solution of benzene afforded red-orange crystals of3.30-l/2C6H6. 'H NMR (C6D6): 5 2.18 (s, 6H, -CH3), 3.60 (m, 4H, -C//2N), 4.15 (m, 4H, -C//2N), 5.92 (s, 2H, -imid//), 6.42 (br s, 2H, -py//), 6.74 (br s, 1H, -py//), 6.90 (d, J = Hz, 4H, -Ar//), 7.49 (d, J = Hz, 4H, -ArH), 8.58 (br s, 2H, -py//). 88 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes ,3C{'H} NMR (C6D6): 5 23.0 (-CH3), 51.4 (-CH2N), 52.9 (-CH2N), 119.3 (-ArQ, 121.3 (-imidQ, 124.1 (-ArQ, 126.9 (-ArQ, 131.4 (-ArQ, 136.3 (-ArQ, 150.4 (-ArQ, 156.1 (-ArQ, 187.9 (-NCN). Anal. Calcd. for C29H32Cl2N5Zr: C, 56.85; H, 5.26; N, 11.43. Found: C, 56.80; H, 5.33; N, 11.40. Synthesis of Ar[NCN]ZrR2 [(Ar = tol, R = CH2SiMe3 (3.31), Me (3.32), CH2Ph (3.33); Ar = Mes, R = Me (3.34), CH2Ph (3.35)] Method A (SiMe4 elimination): To a cooled (-30°C) THF solution (10 mL) of 3.7 (90 mg, 0.27 mmol) was added dropwise a cooled (-30°C) THF solution (5 mL) of Zr(CH2SiMe3)4 (121 mg, 0.27 mmol). The solution was kept at the reduced temperature for Vi hour then gradually warmed to room temperature for 15 min. The solvent was removed and the yellow residue was extracted with hexane/toluene (5:1) (3 x 10 mL). The extracts were cooled to -30°C over a period of 1 week giving a large portion of pale yellow crystal. Yield =110 mg, 67%. Method B (Alkylation): A suspension of 3.24 (103 mg, 0.21 mmol) in Et20 was cooled to -30°C and an Et20 solution of LiCH2SiMe3 (39 mg, 0.42 mmol) was slowly added dropwise. The solution was allowed to gradually warm to room temperature where stirring was continued for Vz hour. The solvent was removed and the yellow residue extracted with hexane/toluene (5:1) (3 x 10 mL). The extracts were cooled to -30°C over a period of 1 week to give the same pale yellow crystalline material described above. Yield = 50 mg, 40%. 3.31: (Method A) !H NMR (C6D6): 8 0.11 (s, 18H, -CH2SiMe3), 0.95 (s, 4H, -C//2SiMe3), 2.23 (s, 6H, -C//3), 3.40 (m, 4H, -C//2N), 3.95 (m, 4H, -C//2N), 5.79 (s, 2H, -imid//), 7.18 (d, J = 8 Hz, 4H, -Ar//), 7.30 (d, J = 8 Hz, 4H, -Ar//). nC{lW) NMR (C6D6): 6 1.4 (-SiQ, 23.5 (-CH3), 50.9 (-CH2N), 54.6 (-CH2N), 69.0 4 (-ZrCH2), 121.0 (-ArQ, 121.4 (-imidQ, 129.4 (-ArQ, 132.7 (-ArQ, 155.2 (-ArQ, 186.8 (-NCN). Anal. Calcd. For C29H46N4Si2Zr: C, 58.24; H, 7.75; N, 9.37. Found: C, 58.05; H, 7.66; N, 9.20. 89 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes 3.32: (Method B) !H NMR (C6D6): 5 0.62 (s, 6H, -ZrC//3), 2.35 (s, 6H, -C//3), 4.01 (m, 4H, -C//2N), 4.33 (m, 4H, -C//2N), 5.86 (s, 2H, -imid//), 6.93 (d, J = 8 Hz, 4H, -Ar//), 7.05 (d, J = 8 Hz, 4H, -Ar//). 13C{'H} NMR (C6D6): 6 19.9 (-CH3), 39.7 (-ZrCH3), 50.2 (-CH2N), 51.4 (-CH2N), 118.5 (-ArQ, 120.4 (-imidQ, 127.8 (-ArQ, 130.2 (-ArQ, 148.6 (-ArQ, 193.5 (-NCN). Anal. Calc. for C23H30N4Zr C, 60.88; H, 6.66; N, 12.35. Found: C, 60.53; H, 6.78; N, 12.14. 3.33: (Method A) 'H NMR (C6D6): 5 1.85 (s, 4H,-ZrC//2), 2.35 (s, 12H, -p-AvCHf), 3.14 (m, 4H, -NC//2), 3.47 (m, 4H, -NC//2), 5.89 (s, 2H, -imid//), 6.79 (d, J = 8Hz, 4H, -o-CH2P/7), 6.90 (t, J = 8Hz, 2H, -p-CU2Ph), 7.01 (s, 4H, -Ar//), 7.26 (t, J = 8Hz, 4H, -m-CH2Ph). 13C{'H} NMR (C6D6): 5 23.4 (-/>CH3), 48.3 (-NCH2), 54.6 (-NCH2), 70.3 (-ZrCH2), 119.8 (-imidQ, 123.1 (-ArQ, 129.6 (-ArQ, 130.8 (-ArQ, 137.6 (-ArQ, 145.8 (-ArQ, 150.1 (-ArQ, 195.3 (-ZrCcarbene)- Some aromatic resonances obscured by C(,De solvent. Anal. Calc. for C33H38N4Zr C, 68.11; H, 6.58; N, 9.63. Found: C, 68.22; H, 6.67; N, 9.52. 3.34: (Method B) lU NMR (C6D6): 8 0.33 (s, 6H, -ZrC//3), 2.20 (s, 6H, -/?-ArC//3), 2.43 (s, 12H, -o-ArC//3), 3.37 (m, 4H, -NC//2), 3.45 (m, 4H, -NC//2), 5.88 (s, 2H, -imid//), 6.98 (s, 4H, -Ar//). ,3C{'H} NMR (C6D6): 5 18.6 (-o-CH3), 20.0 (-/>CH3), 48.4 (-ZrCH3), 51.7 (-NCH2), 52.3 (-NCH2), 118.5 (-imidQ, 129.9 (-ArQ, 133.3 (-ArQ, 135.0 (-ArQ, 148.9 (-ArQ, 189.8 (-ZrCcal-bene)-Anal. Calc. for C27H38N4Zr: C, 63.61; H, 7.51; N, 10.99. Found: C, 63.33; H, 7.32; N, 10.75. 3.35: (Method B) 'H NMR (C6D6): 5 1.92 (s, 4H,-ZrC//2), 2.24 (s, 6H, -p-AxCHf), 2.31 (s, 12H, -o-ArC//3), 3.03 (m, 4H, -NC//2), 3.56 (m, 4H, -NC//2), 5.77 (s, 2H, -imid//), 6.84 (d, J=8Hz, 4H, -o-CH2Ph), 6.92 (t, J=8Hz, 2H, -p-CH2Ph), 6.98 (s, 4H, -Ar//), 7.17 (t, J=8Hz, 4H, -m-CU2Ph). 13C{'H} NMR (C6D6): 8 20.2 (-0-CH3), 24.3 (-p-CHf), 49.3 (-NCH2), 52.7 (-NCH2), 65.6 (-ZrCH2), 121.6 (-imidQ, 122.4 (-ArQ, 130.3 (-ArQ, 131.5 (-ArQ, 136.6 (-ArQ, 90 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes 148.4 (-ArQ, 156.3 (-ArQ, 190.1 (-ZrCCarbene)- Some aromatic resonances obscured by C6D6 solvent. Anal. Calc. for C39H46N4Zr: C, 70.75; H, 7.00; N, 8.46. Found: C, 70.43; H, 6.92; N, 8.35. Synthesis of Mes(NCN)Hf(R)2 [R=Me (3.36), CH2CH3 (3.37), CD2CD3 (</io-3.37), CH2Ph (3.38), CH2CH(CH3)2 (3.39)] The following procedure is representative of the synthesis of 3.36-3.39. 3.29 (400 mg, 0.63 mmol) was dissolved in 5 mL THF and cooled to -30 °C. MeMgBr (0.42 mL, 1.3 mmol, 3M in Et20) was added dropwise and the slightly yellow solution was stirred in the dark for 30 minutes. The THF was removed under reduced pressure and to the resulting residue was added a few drops 1,4-dioxane and Et20 (5 mL). The resulting suspension was filtered through Celite and volatiles were removed to give a white solid, 3.36, which was recrystallized with Et20. Yield = 293 mg, 78 %. 3.36: *H NMR (C6D6): 8 0.12 (s, 6H, -HfC/fc), 2.17 (s, 6H, -p-ArCH3), 2.49 (s, 12H, -o-ArC//3), 3.40 (m, 4H, -NC/fc), 3.49 (m, 4H, -NC/fc), 5.90 (s, 2H, -imid//), 6.99 (s, 4H, -Ar//). 13C{'H} NMR (C6D6): 8 19.6 (-CH3), 20.7 (-CH3), 52.2 (-NCH2), 54.1 (-HfCH3), 54.2 (-NCH2), 119.5 (-imidQ, 129.6 (-ArQ, 132.3 (-ArQ, 135.3 (-ArQ, 152.1 (-ArQ, 196.1 (-HfCcarbene)-Anal. Calc. for C27H38HfN4: C, 54.31; H, 6.41; N, 9.38. Found: C, 54.22; H, 6.26; N, 9.16. 3.37: *H NMR (C6D6): 8 0.62 (q, J = 8Hz, 4H, -HfC/fc), 1.49 (t, J = 8Hz, 6H, -CH2C//3), 2.24 (s, 6H, -p-ArCH3), 2.46 (s, 12H, -o-ArC//3), 3.32 (m, 2H, -NC//2), 3.55 (m, 4H, -NC//2), 5.90 (s, 2H, -imid//), 6.70 (s, 4H, -ArH). l3C{lll} NMR (C6D6): 8 12.8 (-HfCH2CH3), 19.7 (-CH3), 20.9 (-CH3), 51.4 (-NCH2), 52.5 (-NCH2), 66.1 (-HfCH2CH3), 119.9 (-imidQ, 129.6 (-ArQ, 131.8 (-ArQ, 134.9 (-ArQ, 152.9 (-ArQ, 196.4 (-HfCcarbene). Anal. Calc. for C29H42HfN4: C, 55.72; H, 6.77; N, 8.96. Found: C, 55.24; H, 6.59; N, 8.87. 91 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes </io-3.37: 'H and 13C NMR spectra identical to 11 with the absence of -HfCH2CH3 resonances. 3.38: 'H NMR (C6D6): 5 1.85 (s, 4H,-HfC//2), 2.26 (s, 6H, -p-ArCH3), 2.36 (s, 12H, -o-ArC//3), 3.04 (m, 4H, -NC//2), 3.46 (m, 4H, -NC//2), 5.73 (s, 2H, -imid//), 6.82 (d, J = 8Hz, 4H, -o-CH2Ph), 6.90 (t, J = 8Hz, 2H, -p-CU2Ph), 7.02 (s, 4H, -AiH), 7.18 (t, J = 8Hz, 4H, -m-CU2Ph). I3C{'H} NMR (C6D6): 5 21.2 (-CH3), 22.1 (-CH3), 50.0 (-NCH2), 51.4 (-NCH2), 73.5 (-HfCH2), 121.3 (-ArQ, 122.9 (-imidQ, 129.6 (-ArQ, 130.9 (-ArQ, 135.5 (-ArQ, 147.9 (-ArQ, 155.6 (-ArQ, 196.5 (-HfCcarbene)- Some aromatic resonances obscured by C6D6 resonances. Anal. Calc. for C39H46HfN4: C, 62.51; H, 6.19; N, 7.48. Found: C, 62.42; H, 6.14; N, 7.42. 3.39: 'H NMR (C6D6): 5 0.68 (d, J = 8Hz, 4H, -HfC//2), 1.02 (d, J = 8Hz, 12H, -CH(C//3)2), 2.24 (s, 6H, -p-ArCH3), 2.35 (n, J = 8Hz, 2H, -CH2C//(CH3)2), 2.46 (s, 12H, -O-ATCHJ), 3.23 (m, 2H, -NC//2), 3.67 (m, 4H, -NC//2), 5.86 (s, 2H, -imid//), 7.00 (s, 4H, -ArH). l3C{lH} NMR (C6D6): 8 13.7 (-CH2CH(CH3)2), 19.4 (-o-CH3), 20.5 (-p-CU3), 32.3 (-CH2CH(CH3)2), 50.5 (-NCH2), 52.1 (-NCH2), 68.4 (-HfCH2), 118.4 (-imidQ, 128.4 (-ArQ, 132.1 (-ArQ, 133.9 (-ArQ, 148.4 (-ArQ, 194.9 (-HfCCarbene). Anal. Calc. for C33H5oHfN4: C, 58.18; H, 7.40; N, 8.22. Found: C, 57.91; H, 7.05; N, 8.01. Decomposition of Mes(NCN)Hf(CH2CH3)2 to form 3.40 and rf4-3.40. 3.37 (110 mg, 0.18 mmol) was dissolved in Et20 (20 mL) and the pale yellow solution was stirred for 5 days. The Et20 was removed under reduced pressure and hexanes added to precipitate a dark orange powder. Yield = 99 mg, 95 %. 'H NMR (C6D6): 5 -0.10 (dq, J = 5, 8Hz, 1H, -HfC//2CH3), 0.010 (dq, J = 5, 8Hz, 1H, -HfC//2CH3), 1.00 (d, J = 12Hz, 1H, -HfC//2Ar), 1.01 (t, J = 8Hz, 3H, -HfCH2C/Z3), 2.23 (s, 3H, -CH3), 2.25 (s, 3H, -CH3), 2.37 (s, 3H, -CH3), 2.47 (s, 3H, -CH3), 2.51 (d, J = 12Hz, 1H, -HfC//2Ar), 2.58 (s, 3H, -C//3), 3.19 (m, 3H, -NC//2), 3.22 (m, 3H, -NC//2), 3.27-3.31 (m, 3H, -NC//2), 3.47 (m, 3H, -NC//2), 3.62 (m, 3H, -NC//2), 3.93 (m, 3H, -92 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes NC//2), 4.16 (m, 3H, -NC//2), 5.89 (d, J=2Hz, 1H, -imid//), 5.92 (d, J=2Hz, 1H, -imid/O, 6.77 (s, 1H, -Ar//), 6.97 (s, 1H, -Ar//), 7.04 (s, 2H, -Ar//). 13C{'H} NMR (C6D6): 8 9.7 (-HfCH2CH3), 19.1 (-ArCH3), 19.3 (-ArCH3), 21.1 (-ArCH3), 21.3 (-ArCH3), 52.1 (-NCH2), 53.8 (-NCH2), 55.2 (-NCH2), 55.3 (-NCH2), 58.2 (-HfCH2CH3), 72.9 (-HfCH2Ar), 1.19.5 (-imidQ, 129.3 (-ArQ, 129.9 (-ArQ, 135.0 (-ArQ, 138.5 (-ArQ, 138.6 (-ArQ, 142.9 (-ArQ, 145.6 (-ArQ, 197.3 (-HfCCarbene). Some aromatic resonances obscured by C6D6 solvent. Anal. Calc. for C27H36HfN4: C, 54.49; H, 6.10; N, 9.41. Found: C, 54.13; H, 5.86; N, 9.12. rf4-3.40: 'H and 13C NMR spectra identical to 3.40 with the absence of -HfCH2CH3 resonances. 93 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes 3.7. References (1) Regitz, M. Angew. Chem., Int. Ed. Engl. 1996, 35, 725. (2) Herrmann, W. A.; Kocher, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 2162. (3) Dullius, J. E. L.; Suarez, P. A. Z.; Einloft, S.; de Souza, R. F.; Dupont, J.; Fischer, J.; De Cian, A. Organometallics 1998, 17, 815. (4) Arduengo, A. J., Ill Acc. Chem. Res. 1999, 32, 913. (5) Arduengo, A. J., Ill; Krafczyk, R. Chem. Z. 1998, 32, 6. (6) Arnold, P. L.; Mungur, S. A.; Blake, A. J.; Wilson, C. Angew. Chem. Int. Ed. 2003, 42, 5981. (7) Arnold, P. L.; Blake, A. J.; Wilson, C. Chem. Eur. J. 2005, 11, 6095. (8) Aihara, H.; Matsuo, T.; Kawaguchi, H. Chem. Commun. 2003, 2204. (9) Arduengo, A. J., Ill; Dias, H. V. R.; Calabrese, J. C; Davidson, F. J. Am. Chem. Soc. 1992, 114, 9724. (10) Arduengo, A. J., Ill; Dias, H. V. R.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1992, 114, 5530. (11) Arduengo, A. J., Ill; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991,113, 361. (12) Fryzuk, M. D.; Johnson, S. A.; Patrick, B. O.; Albinati, A.; Mason, S. A.; Koetzle, T. F. J. Am. Chem. Soc. 2001,123, 3960. (13) Arduengo, A. J., Ill; Krafczyk, R.; Schmutzler, R.; Craig, H. A.; Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron 1999, 55, 14523. (14) Malek, A.; Fresco, J. M. Can. J. Chem. 1973, 51, 1981. (15) Chandra, S.; Kumar, R. Transition Met: Chem. 2004, 29, 269. (16) Niehues, M.; Erker, G.; Kehr, G.; Schwab, P.; Froehlich, R.; Blacque, O.; Berke, H. Organometallics 2002, 21, 2905. (17) Niehues, M.; Kehr, G.; Erker, G.; Wibbeling, B.; Frohlich, R.; Blacque, O.; Berke, H. J. Organomet. Chem. 2002, 663, 192. (18) Zhang, X.; Zhu, Q.; Guzei, I. A.; Jordan, R. F. J. Am. Chem. Soc. 2000,122, 8093. (19) Skinner, M. E. G.; Li, Y.; Mountford, P. Inorg. Chem. 2002, 41, 1110. 94 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes (20) Schrock, R. R.; Seidel, S. W.; Schrodi, Y.; Davis, W. M. Organometallics 1999, 75, 428. (21) Scott, M. J.; Lippard, S. J. Inorg. Chim. Acta 1997, 263, 287. (22) Petersen, J. R.; Hoover, J. M.; Kassel, W. S.; Rheingold, A. L.; Johnson, A. R. Inorg. Chim. Acta 2005, 358, 687. (23) Straus, D. A.; Kamigaito, M.; Cole, A. P.; Waymouth, R. M. Inorg. Chim. Acta 2003,349,65. (24) Lee, Y.-J.; Lee, J.-D.; Ko, J.; Kim, S.-H.; Kang, S. O. Chem. Commun. 2003, 1364. (25) Novak, A.; Blake, A. J.; Wilson, C; Love, J. B. Chem. Commun. 2002, 2796. (26) Harris, S. A.; Ciszewski, J. T.; Odom, A. L. Inorg. Chem. 2001, 40, 1987. (27) Boisson, C; Berthet, J. C; Ephritikhine, M.; Lance, M.; Nierlich, M. J. Organomet. Chem. 1997, 531, 115. (28) Mungur, S. A.; Blake, A. J.; Wilson, C; McMaster, J.; Arnold, P. L. Organometallics, ACS ASAP. (29) Arndt, S.; Okuda, J. Chem. Rev. 2002,102, 1953. (30) Fagan, P. J.; Manriquez, J. M.; Marks, T. J.; Day, C. S.; Vollmer, S. H.; Day, V. W. Organometallics 1982,1, 170. (31) Okuda, J. Dalton Trans. 2003, 2367. (32) Evans, W. J.; Drummond, D. K.; Grate, J. W.; Zhang, H.; Atwood, J. L. J. Am. Chem. Soc. 1987, 109, 3928. (33) Scott, M. J.; Lippard, S. J. Organometallics 1997,16, 5857. (34) Arnold, P. L.; Liddle, S. T. Chem. Commun. 2005, 5638. (35) Warren, T. H.; Schrock, R. R.; Davis, W. M. Organometallics 1996, 15, 562. (36) Alt, H. G.; Denner, C. E.; Milius, W. Inorg. Chim. Acta 2004, 357, 1682. (37) Matulenko, M. A.; Hakeem, A. A.; Kolasa, T.; Nakane, M.; Terranova, M. A.; Uchic, M. E.; Miller, L. N.; Chang, R.; Donnelly-Roberts, D. L.; Namovic, M. T.; Moreland, R. B.; Brioni, J. D.; Stewart, A. O. Bioorg. Med. Chem. 2004,12, 3471. (38) Gowda, B. T.; Svoboda, I.; Fuess, H. Z. Naturforsck, A: Phys. Sci. 2000, 55, 779. 95 References begin on page 94. Chapter Three: Synthesis of Group 4 Bis(amido)-N-Heterocyclic Carbene Complexes (39) Wilde, R. G.; Billheimer, J. T.; Germain, S. J.; Hausner, E. A.; Meunier, P. C.; Munzer, D. A.; Stoltenborg, J. K.; Gillies, P. J.; Burcham, D. L.; et al. Bioorg. Med. Chem. 1996, 4, 1493. (40) McAlexander, L. H.; Li, L.; Yang, Y.; Pollitte, J. L.; Xue, Z. Inorg. Chem. 1998, 37, 1423. (41) Whitesides, G. M.; Hackett, M.; Brainard, R. L.; Lavalleye, J. P. P. M.; Sowinski, A. F.; Izumi, A. N.; Moore, S. S.; Brown, D. W.; Staudt, E. M. Organometallics 1985,4,1819. (42) Bird, R.; Knipe, A. C.; Stirling, C. J. M. J. Chem. Soc, Perkin Trans. 2 1973, 1215. 96 References begin on page 94. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes Chapter Four Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes 4.1. Introduction* To date, research with NHC donors has focused on the application of late transition metal NHC complexes in areas such as homogeneous catalysis. In general, the chemistry of electropositive NHC complexes, which include groups 4, 5, and the s- and f-block metals, has been limited to studying the binding of the NHC donor and the coordination properties of the metal complexes. Surprisingly, little is known about the application and reactivity of these complexes in homogeneous catalysis and small molecule activation.1 In chapter 3, we demonstrated that by flanking a centrally disposed NHC with two pendant amido donors in a tridentate motif, the carbene donor has been forced to bind to group 4 transition metals by virtue of its position in the chelate array. Given this stability, we were interested in the reactivity and application of these complexes in fundamental processes such as homogeneous catalysis and small molecule activation. *A portion of this chapter has been published (Spencer, L.; Fryzuk, M.D. J. Organomet. Chem. 2005, 690, 5788). 97 Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes In this chapter, the applications of group 4 transition metal [NCN] complexes in olefin polymerization and migratory insertion processes will be presented. In addition, the attempted syntheses of coordinated dinitrogen complexes will be outlined. As the topics discussed are unique from each other, separate introductions are included in each section for each reactivity pattern investigated. 4.2. Hf and Zr Cation Formation and Polymerization Studies Group 4 transition metal complexes with substituted amide ligands have been 2 9 extensively examined as olefin polymerization catalysts. " Perhaps the most recognized examples of highly active amide-based olefin polymerization complexes are the constrained geometry catalysts (CGC's) (4.1, M = Ti, Zr, Figure 4.1).10 This catalyst and derivatives thereof are active for both ethylene and 1-hexene polymerization. Group 4 complexes stabilized by NHC ligands (4.2-4.3) have also been investigated for the polymerization of a-olefins (Figure 4.1)." These simple N-alkyl substituted NHC supported complexes exhibit moderate ethylene polymerization activity at room temperature. The activity dramatically decreases at elevated temperatures, which is potentially a result of NHC dissociation from the metal centre. Another example of an NHC-based polymerization catalyst is the bis(aryloxido)NHC stabilized titanium complex described in chapter 3.1, which exhibits high ethylene polymerization activity upon MMAO activation.12 4.1 4.2 4.3 Figure 4.1. Examples of Group 4 olefin polymerization catalysts. 98 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes In light of the success with the CGC design and spurred by the recent accomplishments of NHC supported complexes, the synthesis of activated zirconium and hafnium [NCN] complexes was investigated. The methyl cations, {Mes[NCN]M(CH3)}{B(C6F5)4}, (M = Zr, 4.4; M = Hf, 4.5) were generated in situ from dimethyl precursors 3.33 and 3.35 at -10°C with [Ph3C][B(C6F5)4] (Scheme 4.1). Due to the thermal sensitivity of the species, the products were identified by *H NMR spectroscopy in solution and not isolated. !H NMR spectroscopy of both species shows anticipated ligand resonances for Cs symmetric products in solution with M-Cr73 resonances at 0.50 ppm for 4.4 and 0.26 ppm for 4.5. Treatment of 3.35 with triflic acid generates the hafnium methyl-triflate complex, Mes[NCN]Hf(OTf)(CH3) (4.6), which is isolable at room temperature (Scheme 4.1). The *H NMR spectrum of 4.6 shows similar ligand resonances to 4.4 and 4.5, indicative of a Cs symmetric species in solution with a Hf-C#3 resonance at 0.19 ppm. These findings 13 are similar to those of previously described hafnium triflate complexes. M = Zr 4.4 Hf 4.5 Scheme 4.1. 99 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes Cationic complexes 4.4 and 4.5 were evaluated as 1-hexene polymerization catalysts. The addition of -500 equivalents of 1-hexene to a chlorobenzene solution of 4.4 resulted in the isolation of a viscous oily polymer. 13C{'H} NMR spectroscopy of the substance reveals the formation of atactic poly-1-hexene in 4% yield.14 The low yield observed is quite surprising in light of the many active amido-based non-metallocene group 4 catalysts.2"9 It was recently found that 1-hexene polymerization with group 4 metal bearing diamido Mes[N(NMe)N] ancillary ligands underwent ortho-methyl C-H bond activation during polymerization.15 Investigation of the decomposition products revealed an ortho-methyl C-H bond activated compound similar to 3.39, in addition to other unidentifiable materials. The exposure of 4.4 to ethylene in toluene resulted in a different outcome. A large amount of polyethylene was recovered, which shows that 4.4 is a moderately active catalyst for the polymerization of ethylene (125 g mmol"1 h"1 atm"1). It is important to note that immediately after exposure to ethylene, a noticeable exothermic event was observed. Halting the polymerization experiment at increasingly longer times resulted in a decreased activity. This suggests that the active species, albeit catalytically active, is short-lived in solution. 4.3. Formation of [NCN] Hafnium tj -Iminoacyls and an Eneamidolate„ metallacycle The migratory insertion of carbon monoxide into metal alkyl and hydride bonds represents a fundamental reaction type in organometallic chemistry.16 Interest in this area stems from the importance of carbonylation reactions where the migratory insertion of CO is a key step in the catalytic cycle.17 CO insertion into transition metal alkyls normally generates r\ or r)2-acyl derivatives, with the latter binding mode typically occurring in electron-deficient early d-block, actinide and lanthanide metal centres. Analogous reactivity has been observed in the migratory insertion of isocyanides, 2 16 isoelectronic equivalents of CO, generating n -iminoacyl groups. 100 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes Migratory insertion reactions involving transition metal NHC complexes have also been investigated and, in some cases, the destructive modification of the NHC donor has been observed.18'19 For example, the facile insertion of an NHC group into a Pd-Me bond was recently reported (Equation 4.1).19 DFT calculations on model complexes suggests all three functionalities of the tridentate ligand coordinate to the palladium centre prior to methyl migration from the metal centre to the NHC moiety. Ar Ar With respect to early transition metal (ETM) complexes and migratory insertion reactions, the participation of the metal-NHC bond during these reactions has yet to be addressed. Along this line, the migratory insertion of substituted isocyanides into the halfnium-alkyl bonds of 3.35 was investigated. The hafnium dimethyl derivative reacts immediately in solution with one equivalent of xylyl isocyanide (XyNC) to give the mono-insertion product 4.7 (Scheme 4.2). The *H NMR spectrum is consistent with a Cs symmetric structure (equivalent N-mesityl groups and backbone linkers); the remaining hafnium-methyl resonance is found at 0.17 ppm. The r|2 coordination of the iminoacyl group is confirmed by 13C{'H} NMR (N=C, 259 ppm) and IR (uc=N 1575 cm"1) spectroscopy and is a typical outcome for this kind of reaction.16 In solution, only one of the two possible orientations of the r|2-iminoacyl unit is observed. NOE measurements show a through space enhancement of the or/7zo-methyls of the N-xylyl group upon irradiation of the remaining Hf-Me resonance, which supports the isomer having the N-xylyl group pointing towards the Hf-Me. 101 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes N Hf N Mes H3CN CH3 Mes 3.35 Me Me Mes=—0"Me Xy= —y) Me Me XyNC THF r\ ^^^^ ^^^^ Mes N-—.Hf—N /H3C*/V \ Me N-^Cv Mes CH3 Me 4.7 THF RNC -Nv. ,.N-'^^^^ ^^^^ ^N—:.Hf^-N^ Mes ^NN^.i M^CV MES R • I CH3 H3C Xy R = Xy 4.8 R = jPr 4.9 Scheme 4.2. The solid-state molecular structure of 4.7 determined by an X-ray diffraction experiment also verified the nMminoacyl coordination mode. An ORTEP depiction of 4.7 is shown in Figure 4.2 with relevant bond lengths and angles listed in Table 4.1 and crystallographic details given in appendix A. The imino carbon atom is directly bound to the hafnium centre with the HT1-C36 bond length at 2.251(2) A, which is similar to related iminoacyl-zirconium complexes (2.23-2.25 A).16 The bond angles of the triangle defined by Hf-C26-N5 are typical of other structurally characterized group 4 n2-iminoacyl complexes as is the imino N5-C26 bond length.16 Surprisingly, the ancillary [NCN] ligand is distorted towards facial coordination with the N4-HA-N3 angle being 133.65(7)°. 102 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes depicted with 50% ellipsoids; all hydrogen atoms have been omitted for clarity. Table 4.1. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]Hf(r|2-XyNCCH3)(CH3), (4.7). Bond Lengths Bond Angles Hfl-Cl 2.387(2) N3-HT1-N4 133.65(7) Hfl-N3 2.1352(18) N5-C26-HT1 73.05(11) Hfl-N4 2.1117(18) N4-Hfl-Cl 77.25(7) Hfl-C26 2.251(2) N3-HH-C1 77.72(7) Hfl-N5 2.2461(16) N5-C26 Addition of a second equivalent of xylyl isocyanide to 4.7 resulted in an immediate color change from pale yellow to dark purple, with the exclusive formation of the bis(n2-iminoacyl) product 4.8 (Scheme 4.2). The presence of an r|2-iminoacyl group is noted by a UC=N band at 1568 cm"1. The room temperature *H NMR spectrum of 4.8 in CD2CI2 is consistent with a Cs symmetric species (four inequivalent ethylene spacer 103 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes resonances); however, cooling the solution to -40°C is necessary to fully resolve all resonances. At this lower temperature, two unique environments are observed for each r)2-iminoacyl group consistent with the solid-state molecular structure of 4.8. An ORTEP depiction of 4.8 is shown in Figure 4.3. Bond lengths and angles for 4.8 are given in Table 4.2 and crystallographic details are given in appendix A. The characteristic iminoacyl 13C resonance (~ 260 ppm) was not observed in the 13C NMR spectrum. This is likely due to fast exchange under the normal acquisition conditions. depicted with 50% ellipsoids; all hydrogen atoms and mesityl groups have been omitted for clarity. 104 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes Table 4.2. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]Hf(r)2-XyNCCH3)2, (4.8). Bond Lengths Bond Angles Hfl-Cl 2.377(10) N3-Hfl-N4 122.6(3) HT1-N3 2.154(7) C26-Hfl-N5 34.0(3) HT1-N4 2.146(8) N4-HT1-C1 78.2(3) HT1-C26 2.249(9) N3-Hfl-Cl 76.2(3) Hfl-N5 2.250(8) N6-Hfl-C36 32.3(3) Hfl-N6 2.305(7) Hfl-C36 2.306(10) In the solid state, there are two independent molecules in the unit cell with subtle variations in the n2-iminoacyl bond lengths (only one of the molecules is used for structural analysis described below). Although formally seven-coordinate, 4.8 is best described as distorted trigonal bipyramidal about hafnium, with each of the n -iminoacyl groups occupying a single coordination site, one axial and one equatorial. The Mes[NCN] ligand is puckered towards a facial orientation with N4-HA-N3 being 122.6(3)°, presumably to accommodate the additional steric constraints of two xylyl units. The two iminoacyl groups are oriented perpendicular to each other, which is further evidence of the considerable steric interactions in the solid state. In general, thermolysis of groups 4 and 5 bis(r) -iminoacyl) complexes results in the formation of enediamido metallacycles.20'21 No reaction was observed by !H NMR spectroscopy when 4.8 was heated in toluene (110°C for >8h). This is somewhat surprising given that this transformation is reported to be facilitated by lowering the 7i*c=N orbital via the presence of electron-withdrawing substituents or by having a relatively electron-rich metal centre.16 Because NHCs are considered strongly a-donating,22'23 this should have facilitated this C-C bond coupling process. To probe the effects of sterics on this process, isopropylisocyanide ('PrNC) was added to 4.7 to generate the mixed bis(iminoacyl) species 4.9 (Scheme 4.2). Thermolysis of this material did not result in the formation of an enediamido metallacycle even after extended reaction times at 110°C. Finally, the bis(isopropyl) iminoacyl was prepared by the addition of two equivalents of isopropyl isocyanide to the dimethyl complex 3.35 to generate 4.10 in excellent yield (Equation 4.2). This compound also turned out to be 105 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes stable to thermolysis as evidenced by no change in the 'H NMR spectrum after one day at 110°C. Formation of an enediamido metallacycle is known to be affected by the steric bulk located on the nitrogen atom.24'25 Moreover, for enediamide formation to occur, both n -iminoacyls must rotate into the preferred coplanar configuration, which would appear to be difficult in complexes 4.8-4.10 due to the increased steric bulk around the metal centre.20'21'26"28 The insertion of carbon monoxide into the remaining metal-alkyl bond in 4.7 was investigated with a view that this smaller molecule would insert and facilitate C=C bond formation. Indeed, a facile reaction is observed when the hafnium methyl-iminoacyl complex 4.7 is exposed to one atmosphere of CO. Interestingly, the product was identified as the eneamidolate complex 4.11 (Equation 4.3). The likely first step is insertion of CO into the remaining Hf-CH.3 bond; however, monitoring this process by NMR spectroscopy did not provide any evidence for the presence of a mixed acyl-iminoacyl compound implying that the C=C bond formation process is quite facile. To our knowledge, only one other example of eneamidolate synthesis has been reported; however, in contrast to our work, forcing conditions (200-1000 psi of CO) were required.20 106 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes (4.3) The solid-state molecular structure of 4.11 was determined by an X-ray diffraction experiment and an ORTEP depiction is shown in Figure 4.4. Relevant bond lengths and angles are listed in Table 4.3, and crystallographic details are located in appendix A. The C=C bond length (1.340(9) A) compares well with previously described metallocyclopentene metallocycles.20'29 The eneamidolate ring is distorted from a planar coordination, an observation prominent in most enediolate, eneamidolate and 2 * * enediamides systems. This bending has been attributed to a n ,7i bonding interaction between the electron deficient metal centre and the olefinic portion of the metallacyclic 29 backbone, a phenomenon observed in a similar Zr-butadiene system. A fold angle of 24.0° was found for 4.11, significantly less than the previously reported fold angles (-50°) of other compounds.20,30 As a result, the Hfl»«C26 and Hfl»»C27 distances (2.728(7) and 2.660(6) A, respectively) are significantly longer than a previously reported eneamidolate complex (2.549(8) and 2.581(8) A).21 Once again, the Mes[NCN] ancillary ligand is distorted towards facial coordination as evidenced by the N4-HT1-N3 bond angle of 124.5(2)°. The molecule possesses C\ symmetry in the solid state as a result of a weak n2,7i interaction from the olefin; however, Cs symmetry is observed in solution at room temperature due to ring flipping of this eneamidolate ring on the NMR time scale. The AG* of 54.1 kJ mol"1 for the ring flipping process (Equation 4.4) is estimated from the coalescence temperature (268 K), and is very similar to the value estimated for a Hf-enediamide complex (59.4 kJ mol"1).20 Broad resonances for the 0-C(CH3)= and O-C(CH3)= carbon nuclei are observed in the 13C{'H} NMR spectrum at room temperature at 137.0 ppm and 19.4 ppm, respectively. 107 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes Figure 4.4. ORTEP view of Mes[NCN]Hf(OC(CH3)=C(CH3)NXy), (4.11) (1/2 Et20 omitted), depicted with 50% ellipsoids; all hydrogen atoms and mesityl groups have been omitted for clarity. Table 4.3. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]Hf(OC(CH3)=C(CH3)NXy), (4.11). Bond Lengths Bond Angles Hfl-Cl 2.348(6) 01-Hfl-N5 83.14(19) Hfl-N3 2.077(5) N3-Hfl-N4 124.5(2) Hfl-N4 2.082(5) N3-Hfl-Cl 78.1(2) Hfl-N5 2.061(5) N4-Hfl-Cl 81.2(2) Hfl-C26 2.728(7) Hfl-C27 2.660(6) Hfl-Ol 2.032(4) C6-C27 1.340(9) Hfl—C26 2.728(7) Hfl»»C27 2.660(6) 4.4. Formation of a Hafnium Vinyl-enolate and Enediolate Metallacycle Due to its ease of preparation and its enhanced thermal stability relative to zirconium dialkyl complexes, the hafnium dimethyl derivative 3.35 was chosen for 108 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes reactivity studies with CO. Exposure of 3.35 to one atmosphere of carbon monoxide for one day results in the formation of Mes[NCN]Hf(CH3)(0-CH=CH2) (4.13), a hafnium vinyl-enolate (Scheme 4.3). Diagnostic resonances appear in the !H NMR spectrum at 3.47 ppm, 3.54 ppm, and 5.75 ppm, with appropriate geminal and vicinal coupling constants. The downfield resonance at 5.75 ppm, assigned to the a-0-CH= proton, splits further when 13CO is substituted, giving typical 'ji3C.iH coupling of 145 Hz.31 The Mes[NCN] ligand resonances are diagnostic of a Cs symmetric compound and also consistent with the Hf-CH3 resonance located at 0.37 ppm. The 13C{'H} NMR spectrum reveals a Hf-NHC carbene signal at 196.2 ppm, as well as vinyl 13C resonances at 120.2 ppm and 139.0 ppm. From the solution NMR data, a definitive orientation of the ligand remains unknown, and for this reason, a mer geometry is shown, despite the fact that the mono insertion adduct of XyNC (4.7) has a facial coordination of the ancillary ligand. H 4.13 Scheme 4.3. Monitoring the reaction of CO with the dimethyl complex showed the formation of the ri2-acyl intermediate Mes[NCN]Hf(n2-COCH3)(CH3) 4.12, as evidenced by a singlet at 1.62 ppm for the acetyl methyl protons. This resonance splits into a doublet with the use of 13CO ('ji3CjH = 7 Hz), confirming that simple insertion has occurred. The l3C{'H} NMR spectrum features a resonance at 339.6 ppm, and is typical for the acyl carbonyl carbon of reported Hf(r]2-acyl) complexes.16 IR spectroscopy was also useful in 109 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes the determination of the Hf(r)2-acyl) moiety with a t)c=o stretch observed at 1540 cm"1, characteristic of similar compounds. The rearrangement of the hafnium methyl-acetyl complex 4.12 to the methyl-vinyl enolate derivative 4.13 was unexpected. The high reactivity of group 4 methyl-acetyl complexes has been ascribed to the oxy-carbene resonance form of the r\ -acyl moiety.31'32 This species can undergo intramolecular coupling with the adjacent methyl group,33 which is followed by hydrogen abstraction by the metal to generate a hydridomethylvinyl-enolate complex (Scheme 4.4). Cp'2 .Zr. O CH, Cp'2 /Zr\ :c CH, Cp'2 HoCil o Cp H2C=C CH, Scheme 4.4. In the case of the hafnium methyl-acetyl complex 4.12, the formation of the vinyl enolate suggests that the oxy-carbene resonance form preferentially undergoes a hydrogen-atom shift from the methyl substituent of the carbene carbon to generate the observed vinyl enolate (Scheme 4.5). Presumably, the r)2-acetyl unit of 4.12 is oriented in such a way as to disfavor C-C coupling with the Hf-CH3 unit, and instead, hydrogen transfer from the methyl occurs. Whether or not this is a result of a geometric constraint caused by the different ancillary ligands or an electronic effect is unknown. Similar hydrogen and silyl group migrations have been reported for Cp3ThR and Cp2*ThR(Cl) derivatives upon reaction with CO.34 110 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes 4.13 Scheme 4.5. Continued exposure of 4.13 to CO results in the formation of an insoluble, white powder. ES-MS shows a molecular ion peak at 652 m/z indicative of a second CO insertion; however, the insolubility of this product has hampered further characterization efforts. It has been shown that the nature of carbonylation products is dependent on substituents on the metal.35 With this in mind, the carbonylation of the hafnium diisobutyl complex, 3.38, was investigated. Exposure of 3.38 to one atmosphere of CO for an extended period (5 days) results in the precipitation of colorless crystals in reasonable yield. Analysis of these crystals by solid-state X-ray diffraction indicated that this material is the dihafnium bis(enediolate) complex, (Mes[NCN]Hf)2(«-OC(iBu)=C(iBu)0)2, 4.16 (Equation 4.6). The ORTEP diagram is shown in Figure 4.5, bond lengths and angles are shown in Table 4.4, and crystallographic details are located References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes in appendix A. Examination of the C-C bond length suggests double bond character (avg. 1.342(15) A) and is similar to other early transition metal enediolate complexes.36 The [NCN] ligand distorts to a facial geometry having an N4-HT1-N3 bond angle of 124.4(3)°. Hf-C alkyl and Hf-N amido bond lengths are similar to previously discussed complexes. The 'lT NMR spectrum is consistent with a Cs symmetric species in solution with two distinct wo-butyl resonances. In addition, 4.16 exhibits a weak 13C resonance at 140.0 1 13 ppm characteristic of an olefinic C=C bond ( Ji3Cj3C = 20 Hz with CO). omitted), depicted with 50% ellipsoids; all hydrogen atoms and mesityl groups have been omitted for clarity. Table 4.4. Selected Bond Distances (A) and Bond Angles (°) for (Mes[NCN]Hf)2Gu-OCCBu^COBuP):, (4.16). Bond Lengths Bond Angles Hfl-Cl 2.387(9) Ol-Hfl-02 105.4(2) Hfl-N3 2.109(7) N3-HA-N4 124.4(3) Hfl-N4 2.097(7) N3-HA-C1 77.8(3) Hfl-Ol 1.937(5) , N4-HA-C1 77.1(3) Hfl-02 1.912(6) C51-01-Hfl 169.4(6) C51-C61 1.341(12) C52-01-Hfl 163.1(6) 112 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes The mechanism of the formation of the dinuclear bis(enediolate) was examined by monitoring the reaction of 3.38 with CO as a function of time using NMR spectroscopy. The first intermediate formed is the hafnium isobutyl-acyl species, 4.14, clearly distinguished by a 13C{!H} NMR singlet at 338.4 ppm, attributed to a Hf(0=C'Bu) resonance. This finding is analogous to other early transition metal n -acyl complexes (Scheme 4.6).16 The 'H NMR spectrum reveals a Cs symmetric species in solution with two distinct /so-butyl resonances. An upfield resonance at 1.68 ppm, assigned to the methylene protons a to the acyl carbonyl of one z'so-butyl unit, splits into a doublet of doublets when 13CO was used (2Ji3C_iH = 4.7 Hz). Again, the coordination mode of the Mes[NCN] ligand in 4.14, mer vs fac, is not assignable with the NMR data available. N Hf-CO ^ CeH6 co -N. Mes V \ / igu' 'Bu" 'Bu Mes Mes 3.38 N -Hf Nv n^Cs, Mes ° 'Bu 4.14 -Hf N Mes . ? y 'Bu \ 'Bu Mes 'Bu /'Bu C=C / \ Mes [NCN]Hf / \ \ / O O \ / c=c 'Bu 'Bu HfTNCN] Mes 4.16 4.15 Scheme 4.6. Continued exposure of 4.14 to CO results in the formation of a new product having C2v symmetry (one set of wo-butyl resonances) in solution and distinct from the 113 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes final dihafnium macrocycle 4.16 (Scheme 4.6). Only one /so-butyl resonance is present in the 'H NMR spectrum at room temperature, suggestive of a CO insertion into the remaining Hf-CH2 alkyl bond of 4.14. The 13C{'H} NMR spectrum of this species displays a weak resonance at 140.6 ppm which suggests that a new C=C bond is formed. This spectroscopic evidence is consistent with the formation of the mononuclear hafnium-enediolate, 4.15 (Scheme 4.6). The synthesis of this enediolate likely proceeds through a bis(n -acyl) species, the presence of which was not observed by H NMR studies. In solution, the nuclearity of the enediolate 4.15 is assumed to be mononuclear; however, there are early transition metal enediolates reported in both monomeric and dimeric forms, depending on the steric bulk of the alkyl group.35 For example, when the alkyl group is bulky (R=CH2C(CH3)3, CH2Si(CH3)3) monomeric species have been observed, and with smaller groups (R=H, CH3, CH2Ph), higher nuclearity species have been reported. Indeed, there are several reports of monomeric enediolates dimerizing to form dinuclear complexes (Scheme 4.7).37'38 In one example, such complexes show dynamic behavior interconverting oxygen atoms through a low activation-energy process by a ten-membered metallacycle intermediate, similar in structure to the final product 4.16.37 A reasonable proposal for the formation of 4.16 is shown in Scheme 4.7, and involves dimerization of 4.15 via dative oxygen-hafnium interactions that are converted to covalent bonds. This dimerization is followed by ring-opening to generate the 10-membered dihafnium macrocycle. The driving force for the formation of the dinuclear complex 4.16 may be the better oxygen-to-hafnium 7i-donation, an overlap that is more difficult in the mononuclear complex 4.15 with the five-membered enediolate ring.39"41 Examination of the solid state structure of 4.16 reveals an average Hf-O-C bond angle of 163.5(3)° and reflects such a 7i-donation. The reason for the differences in reactivity between alkyl substituents may be attributed to competitive dimerization and insertion pathways. Such considerations have been attributed to the nucleophilic character of the alkoxy-carbene moiety and have been well-documented in actinide and tantalum metallocene systems.34'42 114 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes 2 Mes[NCN]Hf / \ Bu 'Bu Mesi [NCN]Hf, / 'Bu 'Bu \ / C=C Bu \ \ / O O \ / Hf[NCN] Mes c=c 'Bu 'Bu /'Bu \ / C = C / \ Mes[NCN]Hf .0 V Hf[NCN]Mes O xo \ / c^c 'Bu 'Bu 'Bu .'Bu / \ Mes[NCN]Hf \ x Hf[NCN]Mes 0 O \ / c=c 'Bu^ ^'Bu Scheme 4.7. 4.5. Formation of Amidate and Amidinate Metallacycles The transition-metal-assisted formation of new C-C bonds from insertion reactions with other simple organic molecules was also investigated. The insertion of cumulenes, such as isocyanates and carbodiimides, into zirconium-alkyl bonds has been shown to yield amidate43 and amidinate43"46 ligands, respectively. With this in mind, the reaction of terr-butyl isocyanate with the hafnium-dimethyl complex 3.35 was investigated (Scheme 4.8). This reaction proceeds immediately at room temperature to yield 4.17 as an off-white powder in 84% yield. The 'H and '^{'H} NMR spectra are consistent with the symmetrical bidentate coordination of an amidate ligand. Most notably, a 13C resonance at 180.4 ppm is indicative of an -NC(Me)0 moiety, which is similar to other reported metal amidate compounds.43 Interestingly, the addition of 115 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes excess isocyanate did not produce a bis(amidate) product, which could result from increased steric interactions at the metal centre. r\ N, ,N N Hf N Mes |_|3Q c\-\3 ^es 3.35 'PrN=C=N'Pr PhMe f=\ N Hfr N Mes^'Pr'"NN \^CHs ^ ?>\ H3C 'Pr 4.18 Mes 'BUNCO PhMe H3C, .N-Mes «Bu-V/° Hf Nv H3C 4.17 Mes Scheme 4.8. A single crystal X-ray diffraction experiment confirmed the identity of the product as 4.17. An ORTEP depiction of 4.17 is shown in Figure 4.6 with relevant bond lengths and angles listed in Table 4.5 and crystallographic details located in appendix A. The geometry about the hafnium metal centre is a distorted trigonal bipyramid with the amidate group occupying one of the coordination sites. The metallacycle core as defined by N5-Hfl-01-C26 is essentially planar (torsion angle of N-Hf-O-C = 8.9°). The N5-C26 and 01-C26 bond lengths (1.299(3) A and 2.1351(17) A, respectively) are similar to previously reported group 4 amidate compounds as is the amidate bite angle defined by 01-Hfl-N5 bond angle (58.02(8)°) 43 . 116 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes Figure 4.6. ORTEP view of Mes[NCN]Hf(Me)(ri3-tBuNC(Me)0) (4.17) depicted with 50% ellipsoids; all hydrogen atoms have been omitted for clarity. Table 4.5. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]Hf(Me)(n3-tBuNC(Me)0), (4.17). Bond Lengths Bond Angles Hfl-Cl 2.385(3) N3-HA-N4 145.47(8) Hfl-N3 2.116(2) N3-Hfl-Cl 78.59(9) Hfl-N4 2.124(2) N4-HH-C1 78.35(8) Hfl-N5 2.374(2) HA-N5-C26 87.02(8) Hfl-Ol 2.1351(17) HA-01-C26 97.65(8) Hfl-C26 2.646(3) 01-C26-N5 115.04(8) Hfl-C32 2.270(3) Ol-Hfl-NS 58.02(8) C26-N5 1.299(3) C26-N5-Hfl-01 8.9 C26-01 1.305(3) Carbodiimides have also been shown to insert into titanium- and zirconium-alkyl bonds to form amidinate complexes.43 Indeed, the addition of one equivalent of 'PrN=C=N'Pr to a toluene solution of 3.35 results in the formation of 4.18 as the exclusive product (Scheme 4.8). The *H NMR spectrum reveals that the desired 117 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes amidinate was produced with no evidence of multiple insertions of the carbodiimide molecule. In solution, a single hafnium-methyl resonance was observed at -0.06 ppm in addition to a methyl resonance at 1.74 ppm. Two inequivalent isopropyl moieties are also observed, implying the orientation of the amidinate ligand shown in Scheme 4.8. The 13C{'H} NMR spectrum is also informative with a 13C resonance at 179.4 ppm, indicative of a -NC(Me)N group. The formation of 4.18 was confirmed by an X-ray diffraction experiment performed on crystals grown from a concentrated Et20 solution. An ORTEP depiction of 4.18 is shown in Figure 4.7 with bond lengths and angles given in Table 4.6 and crystallographic details located in appendix A. Although formally six-coordinate, the geometry at the hafnium centre is best described as a distorted trigonal bipyramid, with the amidinate moiety occupying one coordination site. The N3-Hfl-N4 and N3-Hfl-Cl bond angles in 4.18 are similar to the amidate complex 4.17. The amidinate bite angle defined by the N5-Hfl-N6 bond angle is 58.45(7)°, similar to other reported metal-amidinate complexes.44'47'48 The metal carbene bond length is 2.418(3) A, typical of other [NCN] Hf complexes. The RC(NR')2Hf core forms a nearly planar metallacycle as defined by the torsion angle formed by C27-N5-Hfl-N6. The C-N bond distances are approximately equal and are intermediate between C=N double bond distances in carbodiimides (1.16-1.22 A)49 and C(sp2)-N single bond distances (1.47 A).50 118 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes Figure 4.7. ORTEP view of Mes[NCN]Hf(Me)(ri3-iPrNC(Me)NiPr) (4.18) depicted with 50% ellipsoids; all hydrogen atoms have been omitted for clarity. Table 4.6. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]Hf(Me)(n3-iPrNC(Me)NiPr), (4.18). Bond Lengths Bond Angles Hfl-Cl 2.418(3) N3-Hfl-N4 147.38(7) Hfl-N3 2.130(2) N3-Hfl-Cl 78.91(6) Hfl-N4 2.120(2) N4-HT1-C1 77.88(7) Hfl-N5 2.235(2) Hfl-N5-C27 92.80(7) HT1-N6 2.292(2) HT1-N6-C27 96.06(8) Hfl-C26 2.282(2) N6-C27-N5 112.05(8) Hfl-C(7 2.714(2) N5-HA-N6 58.45(7) C27-N5 1.345(2) C7-N5-HT1-N6 4.9 C27-N6 1.321(2) 119 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes 4.6. Attempted Synthesis of Group 4 [NCN] Dinitrogen Complexes Research in the Fryzuk group has centered on the use of [PNP], [P2N2] and [NPN] ligands for the preparation of early transition metal dinitrogen complexes. In particular, the reduction of metal chlorides with strong reducing agents in the presence of dinitrogen 5157 generally affords dinitrogen complexes with different N2 binding modes. " For example, the reduction of [P2N2]ZrCb with KCg in the presence of dinitrogen yields the side-on dinitrogen complex ([P2N2]Zr)(//-ri2:ri2-N2) in good yields (Scheme 4.9).54 Although this reduction method is successful for many amidophosphine stabilized ETM complexes, reduction of the cyclohexyl-substituted analog Cy[P2N2]ZrCl2 fails to yield a dinitrogen complex, instead producing a paramagnetic Zr(III) complex.58 Evidently, a slight modification in the electronic nature of the ligand can influence the products obtained from the reduction process. Paramagnetic Zr(lll) product Scheme 4.9. The first molecular dinitrogen complex stabilized by an NHC ligand was recently reported.59 - The reduction of the iron complex 4.19 with an excess of Na/Hg amalgam in THF under dinitrogen gives the bis-dinitrogen complex 4.20 (Equation 4.4). Like many 120 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes LTM dinitrogen complexes, the nitrogen ligands are weakly activated by the iron centre and display N-N bond lengths (avg. 1.114 A) that are similar to free N2 (1.0975 A). N—Ar N—Ar Ar = 2,6-'Pr2C6H3 4.19 Na/Hg t THF, N2 -2 NaBr N—Ar N—Ar (4.4) 4.20 The reduction of metal-NHC complexes does not necessarily result in the formation of dinitrogen complexes.60'61 Treatment of the samarium derivative of 4.21 with 2.2 equivalents of KCg in the presence of DME results in the isolation of 4.22, a bridging ether cleavage product (Scheme 4.10). The formation of 4.22 was postulated to proceed via a reduced Sm(II) intermediate, which could activate the DME solvent. The reduction of an yttrium derivative of 4.21 reveals that the NHC is also capable of participating in this reduction chemistry. The olefinic carbon backbone of the five-membered NHC ring undergoes reduction during the reaction of KCgHio with the yttrium derivative 4.21, which facilitates deprotonation of the NHC ring to generate 4.23. These results suggest that both the metal centre and the NHC unit are capable of engaging in reduction chemistry. 121 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes 4.23 Scheme 4.10. The addition of 2.2 equivalents of potassium graphite to 3.25 in a solution of THF under 4 atmospheres of nitrogen results in the immediate formation of a yellow solution. A small amount of a yellow crystalline solid was obtained upon removal of the solvent, addition of Et20, and filtration through Celite (Equation 4.5). Surprisingly, the *H NMR spectrum is consistent with a Cs symmetric species in solution with the appropriate ligand resonances and unexpected alkyl resonances between 1.0 and 1.5 ppm. Additionally, a triplet is observed at 3.5 ppm indicative of a -OC//2 moiety. The 13C{'H} NMR spectrum confirms a metal-carbene interaction in addition to a downfield -OCH2 13C resonance. The presence of these functionalities in the NMR spectra suggests that THF is incorporated in the final product. 122 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes r\ r\ cn N Zr N 2.2 KC: (4.5) THF 4 atm N2 - KCI Mes' Mes Mes 3.25 4.24 Single crystals of 4.24 were grown from a concentrated Et20 solution and the molecular solid state structure was determined by X-ray crystallography. An ORTEP depiction is shown in Figure 4.8 with bond lengths and angles given in Table 4.7 and crystallographic details located in appendix A. Remarkably, the solid state structure shows the presence of an rc-butoxide group, in addition to the typical arrangement of the [NCN] ligand. The ligand assumes a meridional orientation with respect to a distorted trigonal bipyramidal zirconium metal centre. The Zrl-Ol bond distance of 1.912(3) A is similar to other reported Zr-alkoxide complexes,62'63 as are the Zr-N amide and Zr-C NHC bond distances. 123 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes Figure 4.8. ORTEP view of Mes[NCN]Zr(Cl)(OCH2CH2CH2CH3) (4.24) depicted with 50% ellipsoids; all hydrogen atoms have been omitted for clarity. Table 4.7. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]Zr(Cl)(OCH2CH2CH2CH3), (4.24). Bond Lengths Bond Angles Zrl-Cl 2.379(3) N3-Zrl-N4 126.35(11) Zrl-N3 2.101(3) N3-Zrl-Cl 78.70(11) Zrl-N4 2.103(3) N4-Zrl-Cl 78.84(10) Zrl-Ol 1.912(2) Ol-Zrl-Cl 167.44(12) Zrl-Cll 2.4649(14) Cll-Zrl-Cl 87.85(8) This phenomenon of reductive ring-opening of THF has precedent in several reduced early transition metal complexes.64"66 For example, a titanium(III) hydride dimer [Cp2TiH]2 was reported to add THF to form solvated monomers that add the 0-Ca bond of THF across the Ti-H bond in a concerted step.64 In light of these findings and the C-0 bond activation reported in the NHC-derived complex 4.24, it is plausible that reduction of 3.25 generates a monochloro Zr(III) intermediate, which could promote scission of the 124 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes C-0 bond in THF. It is important to note that no conclusive evidence of this intermediate was observed as attempts to trap or observe a Zr(III) intermediate were not performed. Given that the reduction of 3.25 in the presence of THF yielded a species that reacts with the solvent, the reduction in toluene was examined. Unfortunately, no reaction between 3.25 and KCg was observed, potentially a result of the insolubility of 3.25 in toluene. In an attempt to increase the solubility of the metal dichlorides in toluene, the diisopropylphenyl-substituted 3.26 was used in the same reduction conditions, however, no reaction was observed. Other reducing reagents such as Mg, Na/Hg amalgam, and Na have been investigated in similar conditions described above, however, no tractable materials were recovered. 4.7. Synthesis of Hydrazido(l-) Hafnium [NCN] Complexes The coordination of hydrazine and substituted hydrazines has generated considerable interest, in particular for modeling transition metal intermediates in the biological fixation of dinitrogen.67"69 In light of our inability to isolate a dinitrogen complex using the [NCN] ancillary ligand, the coordination of substituted hydrazines was investigated. It was hoped that such studies would form N-N bonded complexes that resemble intermediates in the metal-mediated reduction of dinitrogen to ammonia. Hydrazido ligands have been reported to coordinate to a metal centre in a variety of binding modes. This coordination is classified according to the charge carried by the hydrazido donor.70,71 Widespread examples of this ligand include hydrazido(l-) (NRNR2), hydrazido(2-) (NNR2) (where R signifies an organic group or H), and hydrazido(4-) complexes. Examples of the known coordination modes are given in Figure 4.9. 125 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes R FT N' M n1-hydrazido(l-) R R N N \/ M r\ -hydrazido(l-) R. .R N' N M r) -hydrazido(2-) R N .N. R M M p2: r|',r|1-hydrazido(2-) R .N N' "M M' [i2: ri2,r|1-hydrazido(2-) M=N N=M (j,2: r]1,r|1-hydrazido(4-) Figure 4.9. Coordination modes of hydrazido ligands. Despite many advances in the area of hydrazido transition metal chemistry, there are relatively few reports of early transition metal complexes incorporating this ligand. For example, mononuclear titanium cyclopentadienyl and porphyrin-supported complexes have been reported with n -hydrazido(l-) ligands. 72-74 A slight modification of the supporting ligand can result in a dinuclear complex, as has been observed with a N,N-di(pyrrolyl-a-methyl)-A^-methylamine (dpma) titanium complexes (Figure 4.10).71 The synthesis of hydrazido(2-) ligands also has precedent in ETM chemistry with reports of both terminal mononuclear complexes71 and binuclear bridging hydrazido(2-) derivatives 75 126 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes Me Me Me Me N\ N\ T/NH l/(mer-dpma)Ti^ (mer-dpmaJTi^ ^(/ac-dpma) N N Me Me Me Me NMe2 . . ,ac-dpma)< Ph"NXTi>-N;Ph 'Bu Figure 4.10. Examples of hydrazido(l-) and hydrazido(2-) titanium complexes. The addition of 1 equivalent of 1,1-Me2NNH2 to 3.35 at -30°C resulted in the formation of 4.25, which was identified by NMR spectroscopy (Equation 4.6). The *H NMR spectrum reveals the presence of one amino -NH moiety as a singlet at 4.13 ppm and a -NMe2 resonance at 2.0 ppm. Based on this data, the exact coordination mode of the hydrazido(l-) moiety cannot be determined, although given the coordination mode of the amidate ligands in 4.17 and 4.18, an n -orientation of the hydrazido(l-) group might be expected. This r\2 coordination mode has precedent in hydrazido (1-) early transition metal complexes (Figure 4.10).71 ,N N Hf N Me2NNH2 PhMe -CH4 Mes' /V. \ H3C CH3 3.35 Mes Mes' N :Hf / H3cf (4.6) -N -NH 'Mi 4.25 N Me' 'Me Mes 127 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes The expected ^-coordination mode is observed in the solid state structure of 4.25, which was determined by an X-ray diffraction experiment on crystals grown from a saturated solution of Et20. An ORTEP depiction of 4.25 is shown in Figure 4.11. Relevant bond lengths and angles are listed in Table 4.8, and crystallographic details are located in appendix A. The molecular dimensions of the hydrazido(l-) complex are similar to reported Ti hydrazido(l-) complexes.71'75 The Hf-N bond length for the anionic nitrogen containing the hydrogen is shorter than the nitrogen containing the two methyl substituents. The N3-N4 bond length is 1.437(5) A, reminiscent of the bond length found in hydrazine itself {ca. 1.47 A),76 and consequently can be considered a single N-N bond. Figure 4.11. ORTEP view of Mes[NCN]Hf(Me)(n2-NHNMe2) (4.25) depicted with 50% ellipsoids; with the exception of HI00, all hydrogen atoms have been omitted for clarity. 128 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes Table 4.8. Selected Bond Distances (A) and Bond Angles (°) for Mes[NCN]Hf(Me)(n2-NHNMe2), (4.25). Bond Lengths Bond Angles Hfl-Cl Hfl-N3 Hfl-N5 Hfl-N6 Hfl-CT4 N5-N6 2.423(4) 2.123(3) 2.042(4) 2.355(3) 2.232(4) 1.437(5) N3-HA-N4 N3-Hfl-Cl HA-N5-N6 HH-N6-N5 N5-HA-N6 r 153.11(14) 78.06(7) 83.3(2) 59.5(2) 37.2(2) The addition of a second equivalent of 1,1 -Me2NNH2 to 4.25 at -30°C produced 4.26, a bis(hydrazido)(l-) complex (Equation 4.7). The 'H NMR spectrum reveals equivalent -N(C//3)2, aryl, and imidazole resonances, suggestive of a species with C2V symmetry in solution. Although the solid state molecular structure of 4.26 remains unknown, two possible structural isomers could exist in solution which are consistent with the symmetry observed in the *H NMR spectrum. The two possibilities are: 1) the -NMe2 moieties of the n -NHNMe2 ligands are trans to each other (structure 4.26a, Equation 4.7); or 2) the -NMe2 moieties of the n2-NHNMe2 ligands are cis to each other (structure 4.26b, Equation 4.7). Unfortunately, NOE measurements did not shed insight into the configuration of the hydrazido(l-) groups, but based on possible steric interactions between -NMe2 groups, and the reported structures of other bis(hydrazido)(l-) complexes,71 the structure is most likely 4.26a. 129 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes (4.7) N N Me2 Me2 4.26b 4.8. Conclusions In chapter 3, it was demonstrated that having the NHC situated between two anionic amide donors prevented the dissociation of a carbene moiety from an early transition metal. What was not known was how stable early transition metal NHC interactions would be during processes such as olefin polymerization and migratory insertion. Driven by the precedent for activated NHC-derived group 4 catalysts to polymerize a-olefins, the potential of several [NCN] group 4 metal alkyl compounds was investigated. Activation of the zirconium-dimethyl derivative with [Ph3C][B(C6Fs)4] in the presence of ethylene yielded a moderately active polymerization catalyst. In the presence of 1-hexene, the activated zirconium- and hafnium-dimethyl catalysts yielded a marginal amount of polymer. Investigation into the migratory insertion of isocyanides and CO into the hafnium-sp3-carbon bond of several hafnium-alkyl derivatives revealed the NHC moiety remains coordinated to the metal centre and does not participate in a manner that would alter the NHC donor. Multiple insertions of substrates was 130 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes accomplished; in some examples further C-C bond coupling was observed to generate new eneamidolate and enediolate metallacycles. Attempts to synthesize zirconium [NCN] dinitrogen complexes by previously successful reduction methods were unsuccessful. In one case, an ether cleavage product was recovered (4.22), which is a result of solvent C-0 bond activation. In the next chapter, the coordination of the [NCN] ligand to tantalum will be examined in an attempt to promote the reduction of dinitrogen. 131 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes 4.9. Experimental 4.9.1. General Considerations Unless otherwise stated, general procedures were performed as described in Section 2.5.1. 4.9.2. Materials and Reagents All materials were purchased from an appropriate supplier and purified by published methods prior to use. KCg was synthesized by the method described in the literature.77 4.9.3. Synthesis and Characterization of Complexes 4.4 - 4.18, 4.24 - 4.26 In situ generation of [Mes[NCN]M(CH3)][B(C6F5)4] (M=Zr (4.4), Hf (4.5)). The following procedure is representative of the synthesis of 4.4 and 4.5. To a cooled solution of 3.33 (35 mg, 0.069 mmol) in CD2C12 (0.5 mL) was added a cooled CD2C12 (0.5 mL) solution of [Ph3C][B(C6F5)4] (64 mg, 0.069 mmol). The pale orange solution was immediately transferred to an NMR tube and then frozen in liquid N2. The NMR spectrum was taken immediately after warming the solution to -10°C. 4.4: 'H NMR (CD2C12): 8 0.50 (s, 3H, -ZrC//3), 2.10 (br s, 12H, o-ArC//3), 2.15 (s, 6H, />ArC//3), 3.45 (m, 2H, -NC//2), 3.94 (m, 2H, -NC//2), 4.14 (m, 2H, -NCH2), 4.29 (m, 2H, -NCH2), 6.85 (br s, 4H, -ArH), 7.04 (s, 2H, -imid//). 4.5: 'H NMR (CD2C12): 8 0.26 (s, 3H, -HfC//3), 2.15 (br s, 12H, o-ArC//3), 2.20 (s, 6H, />ArC//3), 3.70 (m, 2H, -NC//2), 4.05 (m, 2H, -NC//2), 4.28 (m, 2H, -NCH2), 4.60 (m, 2H, -NCH2), 6.99 (br s, 4H, -ArH), 7.15 (s, 2H, -imid//). Synthesis of Mes[NCN]Hf(OTf)(CH3) (4.6) To a cooled ethereal solution (5 mL) of 3.35 (200 mg, 0.33 mmol) was added an ethereal solution (2 mL) of CF3S03H (48 mg, 0.32 mmol). The solution was gradually allowed to warm to room temperature and stirred overnight. The solvent was removed 132 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes and the residue recrystallized from Et20/hexane at -30°C to give a colorless solid. Yield = 205 mg, 85%. 'H NMR (CD2C12): 5 0.19 (s, 3H, -UfCH3), 2.19 (br s, 12H, o-ArC//3), 2.21 (s, 6H, p-ArC//3), 3.34 (m, 2H, -NC//2), 4.00 (m, 2H, -NCH2), 4.13 (m, 2H, -NC//2), 4.55 (m, 2H, -NCH2), 6.87 (br s, 4H, -ArH), 7.09 (s, 2H, -imidtf). Anal. Calcd. for C27H35F3HfN403S: C, 44.35; H, 4.83; N, 7.66. Found: C, 43.85; H, 4.61; N, 7.09. Synthesis of Mes[NCN]Hf(ii2-XyNCCH3)(CH3) (4.7) To a cooled toluene (5 mL) solution of 3.35 (200 mg, 0.33 mmol) was added a cooled toluene (2 mL) solution of xylyl isocyanide (44 mg, 0.33 mmol). The pale yellow solution was allowed to gradually warm to room temperature and stirred overnight. The solvent was removed and suspended in cold (-30°C) hexanes. The solid was filtered and washed with cold hexanes to give a white powder. The solid was further recrystallized with Et20 at -30°C. Yield = 183 mg, 74%. 'H NMR (C6D6): 5 0.17 (s, 3H, -HfC//3), 1.61 (s, 6H, -xylylC//3), 1.63 (s, 3H, -CCH3), 2.21 (s, 6H, -p-ArMes-CH3), 2.39 (s, 6H, -o-ArMes-CH3), 2.48 (s, 6H, -o-ArMes CH3), 3.08 (m, 2H, -NC/fc), 3.18 (m, 2H, -NCH2), 3.81 (m, 2H, -NCH2), 4.07 (m, 2H, -NCH2), 6.06 (s, 2H, -imid/f), 6.82-6.92 (m, 7H, -ArH). 13C{'H} NMR (C6D6): 8 18.2 (-ArCH3), 19.7 (-ArCH3), 19.8 (-ArCH3), 20.5 (-ArCH3), 22.4 (-ArCH3), 34.2 (-HfCH3), 52.2 (-NCH2), 56.9 (-NCH2), 118.7 (-imidQ, 124.3 (-ArQ, 128.8 (-ArQ, 129.2 (-ArQ, 129.6 (-ArQ, 131.1 (-ArQ, 134.4 (-ArQ, 134.6 (-ArQ, 147.2 (-ArQ, 154.8 (-ArQ, 197.5 (-HfCcarbene), 259.0 (-HfCimin0acyi). IR (nujol): u(C=N) 1575 cm"1. Anal. Calc. for C36H47N5Hf: C, 59.37; H, 6.50; N, 9.62. Found: C, 59.23; H, 6.33; N, 9.46. Synthesis of Mes[NCN]Hf(ti2-RNCCH3)2 (R = Xy (4.8); R = jPr (4.10)) The following procedure is representative of the synthesis of 4.8 and 4.10. To a toluene (2 mL) solution of 4.7 (200 mg, 0.33 mmol) was added a toluene (2 mL) solution 133 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes of xylyl isocyanide (92 mg, 0.70 mmol). The pale yellow solution gradually turned dark purple (or orange in the case of 4.8) and was stirred overnight, whereupon the solvent was removed quickly and Et20 (2 mL) added to precipitate a white solid. Cooling of the solution, followed by filtration yielded colorless microcrystals. Yield = 260 mg, 92%. 4.8: *H NMR (CD2C12): 8 1.45 (br s, 18H, -CCH3 and -ArXyC7/3), 1.85 (br s, 12H, -o-ArMesCf73), 2.14 (br s, 6H, -/?-ArMesC//3), 4.2 (br s, 8H, -NC//2), 6.60 (br s, 4H, -ArMes#), 6.80-6.85 (br s, 6H, -ArXy//). 6.90 (s, 2H, -imid//). 'H NMR (CD2C12, 233 K): 8 1.21 (s, 3H, -CC//3), 1.27 (s, 6H, -ArXyC//3), 1.57 (s, 6H, -ArXyC//3), 1.78 (s, 6H, -o-ArMesC//3), I. 85 (s, 6H, -o-ArMesC//3), 2.10 (s, 6H, -p-ArMesCH3), 2.24 (s, 3H, -CCH3), 2.80 (m, 2H, -NC//2), 3.98 (m, 4H, -NC//2), 4.27 (m, 1H, -NC//2), 6.56 (s, 2H, -ArMesH), 6.64 (s, 2H, -ArMes#), 6.81 (m, 1H, -ArXy//), 6.85 (m, 2H, -ArXy//), 6.90 (s, 2H, -imid/f). i3C{lU} NMR (C6D6): 8 18.5 (-ArCH3), 20.4 (-ArCH3), 20.8 (-ArCH3), 23.6 (-ArCH3), 52.7 (-NCH2), 57.9 (-NCH2), 118.5 (-imidQ, 124.7 (-ArQ, 125.6 (-ArQ, 129.1 (-ArQ, 129.9 (-ArQ, 130.3 (-ArQ, 135.1 (-ArQ, 156.5 (-ArQ, 196.5 (-HfCcarbene). IR(nujol): u(C=N) 1568 cm"1. Anal. Calc. for C45H56HfN6: C, 62.89; H, 6.57; N, 9.78. Found: 4.9: 'H NMR (C6D6): 8 0.88 (d, J=7Hz, 12H, -CH(C//3)2), 1.32 (s, 6H, n2-iPrNCC//3), 2.25 (s, 12H, -o-ArMes-C//3), 2.31 (s, 6H, -p-AvMES-CH3), 3.28 (m, 2H, -NATC//2), 3.47 (sept, J=7Hz, 2H, -C//(CH3)2), 3.55 (m, 2H, -NC//2), 3.67 (m, 2H, -NC//2), 4.01 (m, 2H, -NC//2), 6.07 (s, 2H, -imid//), 6.93-7.07 (m, 13H, -Ar//). 13C{'H} NMR (C6D6): 8 20.4 (-ArCH3), 20.9 (-ArCH3), 21.1 (-CCH3) 23.2, 48.8 (-NCH), 53.3 (-NCH2), 56.4 (-NCH2), 119.4 (-imidQ, 128.7 (-ArQ, 129.8 (-ArQ, 135.3 (-ArQ, 157.6 (-ArQ, 194.5 (-HfCcarbene) 266.2 (-HfCirnn0acyi). IR(nujol): u(C=N) 1562 cm"1. Anal. Calc. for C^F^HfNe: C, 57.17; H, 7.13; N, 11.43. Found: C, 56.89; H, 7.00; N, II. 26. Synthesis of Mes[NCN]Hf(n2-XyNCCH3)(ii2-iPrNCCH3) (4.9). To a toluene (2 mL) solution of 4.7 (100 mg, 0.14 mmol) was added a toluene (2 mL) solution of 'PrNC (10 mg, mmol). No observable color change was noted. The 134 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes solution was stirred for 1 hour, whereupon the solvent was removed quickly and Et20 (2 mL) added to precipitate a white solid, which was washed with hexanes and dried in vacuo. Yield = 96 mg, 86%. LH NMR (C6D6): 5 0.97 (d, J=7Hz, 6H, -CH(C//3)2), 1.23 (s, 3H, r^-'PrNCC/^), 1.91 (s, 6H, -xylylC//3), 2.26 (s, 6H, -p-ArMes-CH3), 2.38 (s, 6H, -o-ArMes-CH3), 2.39 (s, 6H, -o-ArMes-C//3), 2.47 (s, 3H, -n2-XyNCC//3), 3.10 (m, 2H, -NC//2), 3.43 (m, 2H, -NC//2), 3.56 (sept, J=7Hz, 1H, -C//(CH3)2), 3.69 (m, 2H, -NC//2), 3.88 (m, 2H, -NC//2), 6.01 (s, 2H, -imid//), 6.89 (s, 2H, -ArMes//), 6.95 (s, 2H, -ArMes//), 6.97-7.04 (m, 3H, -Arxyly,/7). ,3C{'H} NMR (C6D6): 8 18.5 (-CCH3), 18.7 (-CCH3), 20.3 (-CH(CH3)2), 20.7 (-ArCH3), 20.9 (-ArCH3), 26.0 (-ArCH3), 50.3 (-NCH), 52.6 (-NCH2), 56.5 (-NCH2), 118.7 (-imidQ, 124.3 (-ArQ, 128.9 (-ArQ, 129.4 (-ArQ, 130.2 (-ArQ, 134.7 (-ArQ, 135.8 (-ArQ, 148.9 (-ArQ, 157.4 (-ArQ, 195.1 (-HfCcarbene), 262.4 (-HfCiminoacyl), 264.1 (-HfCjminoacyl)-IR (nujol): u(C=N) 1558, 1570 cm"1. Anal. Calc. for C^F^HfNe: C, 60.25; H, 6.83; N, 10.54. Found: C, 60.01; H, 6.82; N, 10.36. Synthesis of Mes[NCN]Hf(OC(CH3)=C(CH3)NXy) (4.11). A toluene (10 mL) solution of 4.7 (102 mg, 0.14 mmol) was freeze-pumped-thawed with 1 atm CO several times and left to stand for 1 day. The solvent was removed in vacuo and the yellow powder was washed with hexane (5 mL). The yellow solid was recrystallized from Et20 at -30°C to give crystals suitable for X-ray diffraction. Yield = 88 mg, 83%. 'H NMR (C6D6, 298K): 8 1.37 (s, 3H, -NCC//3), 1.59 (s, 3H, -OCC//3), 2.21 (s, 6H, -C//3), 2.37 (s, 6H, -C//3), 3.01 (m, 4H, -NC//2), 3.51 (m, 2H, -NjC//2), 3.77 (m, 2H, -NC//2), 5.81 (s, 2H, -imid//), 6.90-6.96 (m, 5H, -ArH), 7.07-7.09 (m, 2H, -ArH). 'H NMR (CD3C6D5, 223 K): 8 1.31 (s, 3H, -NCC//3), 1.60 (s, 3H, -OCC//3), 1.98 (s, 6H, -ArC//3), 2.24 (s, 6H, -ArC//3), 2.38 (s, 6H, -ArC//3), 2.69 (s, 6H, -ArC//3), 2.94 (m, 4H, -NC//2), 3.47 (m, 2H, -NC//2), 3.73 (m, 2H, -NC//2), 5.74 (s, 2H, -imid//), 6.81 (s, 2H, -ArMesH), 6.99 (m, 3H, -ArxyH), 7.08 (s, 2H, -ArMesH). 135 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes i3C{lH} NMR (C6D6, 298K): 5 15.2 (-NCCH3), 17.4 (-ArCH3), 19.0 (-OCCH3), 19.4 (-ArCH3), 20.9 (-ArCH3), 52.9 (-NCH2), 57.3 (-NCH2), 116.2 (-CN), 118.7 (-imidQ, 123.1 (-ArQ, 129.6 (-ArQ, 131.5 (-ArQ, 133.0 (-ArQ, 136.0 (-ArQ, 137.0 (-CO), 147.9 (-ArQ, 149.6 (-ArQ, 196.7 (-HfCcarbene).. Anal. Calc. for C^yHfNsO: C, 58.76; H, 6.26; N, 9.26. Found: C, 58.29; H, 6.05; N, 9.33. Synthesis of Mes(NCN)Hf(n.2-C(0)CH3)(CH3) (4.12), 13C-4.12 The product was identified in situ by NMR and IR spectroscopy. A CeD6 solution (1 mL) of 3.35 was freeze-pumped-thawed three times with CO, and the solution left at 1 atmosphere. The solution was left to stand for 6 hours and the solvent removed in vacuo to yield a pale yellow solid. The product was contaminated with -5% 4.13. 'H NMR (C6D6): 5 0.56 (s, 3H, -HfC//3), 1.62 (s, 3H, -HfC(0)C7/3), 2.11 (s, 6H, -p-ArC//3), 2.34 (s, 6H, -o-ArC//3), 2.44 (s, 6H, -o-ArC//3), 2.92 (m, 2H, -CH2), 3.20 (m, 2H, -CH2), 3.65 (m, 2H, -CH2), 3.96 (m, 2H, -CH2), 6.01 (s, 2H, -imid//), 6.76 (s, 2H, -AIH), 6.86 (s, 2H, -ArH). l3C{lH} NMR (C6D6): 8 19.7 (-ArCH3), 19.8 (-ArCH3), 20.8 (-ArCH3), 31.1 (-HfC(0)CH3), 34.7 (-HfCH2), 53.7 (-NCH2), 55.9 (-NCH2), 119.1 (-imidQ, 128.8 (-ArQ, 129.5 (-ArQ, 130.0 (-ArQ, 132.1 (-ArQ, 134.9 (-ArQ, 135.5 (-ArQ, 151.5 (-ArQ, 153.0 (-ArQ, 195.6 (-HfCcarbene), 339.6 (-HfCacyl). IR(nujol): u(C=0) 1540 cm"1. 13C-4.12: 'H and 13C NMR spectra identical to 19 except 1.62 (d, J=7Hz, 3H, -HfC(0)C//3). Synthesis of Mes(NCN)Hf(OCH=CH2)(CH3) (4.13) and 13C-4.13 A benzene solution (10 mL) of 3.35 (250 mg, 0.42 mmol) was freeze-pumped-thawed three times with CO, and the solution left to stir under 1 atmosphere for three days. The solvent was removed residue recrystallized with Et20/hexanes at -30°C. Yield = 162mg, 62%. 136 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes 'H NMR (C6D6): 8 0.37 (s, 3H, -HfC//3), 2.21 (s, 6H, -p-ArCH2), 2.34 (s, 6H, -o-ArC//3), 2.62 (s, 6H, -o-ArC//3), 2.92 (m, 2H, -NC//2), 3.15 (m, 2H, -NC//2), 3.47 (d, J = MHz, -CH), 3.54 (d, J = 6 Hz, -CH), 3.74 (m, 2H, -NCH2), 3.96 (m, 2H, -NC//2), 5.75 (dd, J = 6, 14 Hz, 1H, -CH), 5.94 (s, 2H, -imid//), 6.95 (s, 2H, -Ar//), 7.01 (s, 2H, -Ar//). '^{'H} NMR (C6D6): 8 19.5 (-o-CH3), 20.9 (-/>CH3), 38.4' (-HfCH3), 52.1 (-NCH2), 55.1 (-NCH2), 117.4 (-imidC), 120.2 (-HfOCH=CH2), 129.4 (-ArQ, 130.4 (-ArQ, 133.7 (-ArQ, 139.0 (-HfOCH=CH2), 145.3 (-ArQ, 196.2 (-HfCcarbene). Anal. Calcd. for C28H38HfN40: C, 53.80; H, 6.13; N, 8.96. Found: C, 53.53; H, 5.89; N, 8.61. 13C-4.13: 'H NMR resonances are identical except 8 5.75 (ddd, J= 6,14,145 Hz, 2H, -Hf013C/Z=CH2). In situ generation of ^[NCNJHf^-COCBuNBu) (4.14) and 13C-4.14. A C6D6 (1 mL) solution of 3.38 was freeze-pumped-thawed three times with CO. The reaction was followed by *H NMR spectroscopy and upon complete conversion to the monoacyl complex, a 13C NMR spectroscopy experiment was performed. 'H NMR (C6D6): 8 0.66 (d, J = 7Hz, 6H, -CH(C//3)2), 1.06 (d, J = 7Hz, 2H, -HfC//2), 1.40 (d, J = 7Hz, 6H, -C(0)CH(C//3)2), 1.68 (d, J = 7Hz, 2H, -HfC(0)C//2), 1.80 (sept, J = 7Hz, 1H, -C//(CH3)2), 2.18 (-ArC//3), 2.34(-ArC//3), 2.38 (-ArC//3), 2.75 (sept, J = 7Hz, 1H, -C(0)C//(CH3)2), 3.07 (m, 2H, -NC//), 3.28 (m, 2H, -NC//), 3.51 (m, 2H, -NC//), 4.00 (m, 2H, -NC//), 6.01 (s, 2H, -imid//), 6.78 (s, 2H, -ArH), 6.85 (s, 2H, -ArH). l3C{ lH} NMR (C6D6): 8 19.8 (-ArCH3), 20.0 (-ArCH3), 21.2 (-ArCH3), 32.3 (-HfC(0)CH2), 36.4 (-HfCH2), 54.2 (-NCH2), 55.6 (-NCH2), 118.4 (-imidQ, 128.4 (-ArQ, 128.6 (-ArQ, 129.2 (-ArQ, 130.4 (-ArQ, 131.2 (-ArQ, 132.4 (-ArQ, 149.8 (-ArQ, 153.2 (-ArQ, 196.1 (-HfCcarbene), 338.4 (-HfCacy,). 13C-21: *H NMR resonances are identical except 8 1.68 (dd, J=4,7Hz, 2H, -Hf13C(0)C//2). 137 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes In situ generation of Mes[NCN]Hf(OC(iBu)=C(iBu)0) (4.15) and 13C-4.15. The same procedure was followed as described in the synthesis of 4.14; however the reaction was further monitored by [H NMR spectroscopy after conversion to the monoacyl complex. Within 1 day, the presence of 4.15 was noted. 'H NMR (C6D6): 8 1.06 (d, J = 8Hz, 12H,-CH(C//3)2), 1-99 (n, J = 8Hz, 2H, -CH2C//(CH3)2), 2.02 (d, J = 8Hz, 4H, -OCC//2), 2.22 (s, 6H, -p-AxCH3), 2.40 (s, 12H, -o-ArC//3), 3.41(m, 4H, -NC//2), 3.61 (m, 4H, -NC//2), 5.89 (s, 2H, -imid//), 6.92 (s, 4H, -ArH). l3C{lH} NMR (C6D6): 8 15.5 (-CH(CH3)2), 19.0 (-o-CH3), 21.2 (-/>CH3), 33.8 (-CH(CH3)2), 38.3 (-CCH2), 49.4 (-NCH2), 52.1 (-NjCH2), 120.4 (-imidQ, 129.2 (-ArQ, 132.1 (-ArQ, 134.3 (-ArQ, 140.6 (-C=Q, 153.5 (-ArQ, 198.5 (-HfCcarbene). 13C-4.15: 13C NMR resonances are identical except 8 140.6 (d, J=30Hz, -C=Q. Synthesis of [Mes[NCN]Hf(OC(iBu)=C(iBu)0)]2 (4.16) and 13C-4.16. A benzene solution (20 mL) of 3.38 (115 mg, 0.17 mmol) was freeze-pumped-thawed three times with CO, and the solution left to stand under 1 atmosphere for five days. Colorless crystalline material began to precipitate after three days. The solution was filtered and the crystalline material was washed with pentane. Yield - 82 mg, 66%. 'H NMR (C6D6): 8 0.79 (d, J = 8Hz, 6H,-CH(C//3)2), 0.87 (d, J = 8Hz, 6H,-CH(C//3)2), 1.09 (m, 2H, -CC//2CH), 1.51 (m, 2H, -CC//2CH), 2.21 (s, 6H, -ArC//3), 2.33 (s, 6H, -ArC//3), 2.74 (s, 6H, -ArC//3), 3.40 (m, 4H, -NC//2), 3.55 (m, 2H, -NC//2), 4.16 (m, 2H, -NC//2), 6.01 (s, 2H, -imid//), 6.96 (s, 2H, -ArH), 7.05 (s, 2H, -ArH) (multiplet from -CH2C//(CH3)2 obscured by aryl resonances). '^{'H} NMR (C6D6): 8 14.6 (-CH(CH3)2), 14.8 (-CH(CH3)2), 19.6 (-ArCH3), 19.7 (-ArCH3), 20.4 (-ArCH3), 34.3 (-CH(CH3)2), 34.6 (-CH(CH3)2), 39.4 (-CCH2), 39.6 (-CCH2), 51.3 (-NCH2), 54.4 (-NCH2), 119.8 (-imidQ, 130.4 (-ArQ, 131.6 (-ArQ, 131.8 (-ArQ, 133.4 (-ArQ, 134.1 (-ArQ, 140.0 (-C=Q, 149.4 (-ArQ, 153.2 (-ArQ, 195.4 (-HfCcarbene)-,3C-4.16: 13C NMR resonances are identical except 8 140.0 (d, J=30Hz, -C=Q. 138 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes Anal. Calc. for C7oH10oHf2N804: C, 57.02; H, 6.84; N, 7.60. Found: 56.72; H, 6.59; N, 7.43. Synthesis of Mes[NCN]Hf(Me)(ri3-N(tBu)C(Me)0) (4.17), Mes[NCN]Hf(Me)(r]3-N(iPr)C(Me)N(iPr)) (4.18) The following procedure is representative of the synthesis of 4.17 and 4.18. In a 50 mL Erlenemeyer flask, 3.35 (100 mg, 0.17 mmol) was dissolved in 10 mL toluene and cooled to -30°C. At this temperature, a toluene (3 mL) solution of TSuNCO (17 mg, 0.17 mmol) was added dropwise and the colorless solution was slowly warmed to room temperature. Upon warming, the solution turned a pale yellow color and was stirred overnight. The solution was filtered through Celite and solvent was reduced in volume. Addition of hexane and cooling to -30°C resulted in the formation of colorless crystals which were recovered by filtration. Yield = 99 mg, 84%. 4.17: 'H NMR (C6D6): 5 0.26 (s, 3H, -HfC//3), 1-05 (s, 9H, -C(C//3)3), 1.30 (s, 3H, -CC//3), 2.21 (s, 6H, -/>ArC//3), 2.35 (s, 6H, -o-ArCHf), 2.52 (s, 6H, -o-ArC//3), 3.42 (m, 2H, -NC//2), 3.45 (m, 2H, -NC//2), 3.59 (m, 2H, -NC//2), 3.64 (m, 2H, -NC//2), 6.07 (s, 2H, -imid//), 6.91 (s, 4H, -Ar//) I3C{!H} NMR (C6D6): 5 17.7 (-CH3), 20.3 (-CH3), 21.7 (-CH3), 23.2 (-CH3), 40.1 (-C(CH3)3), 48.3 (-NCH2), 52.9 (-NCH2), H8.6 (-ArQ, 119.0 (-imidQ, 128.2 (-ArQ, 131.4 (-ArQ, 148.9 (-ArQ, 180.4 (-NCO), 193.8 (-NCN). Anal Calcd. for C^yHfNsO: C, 55.20; H, 6.80; N, 10.06. Found: C, 55.35; H, 6.64; N, 10.30. 4.18: *H NMR (C6D6): 5 -0.06 (s, 3H, -HfC//3), 0.88 (d, J = 9 Hz, 6H, -CH(C//3)2), 0.90 (d, J = 9 Hz, 6H, -CH(C//3)2), 1.74 (s, 3H, -CC//3), 2.26 (s, 6H, -/>ArC//3), 2.46 (s, 6H, -o-AxCHf), 2.49 (s, 6H, -o-ArC//3), 3.32 (m, 2H, -NC//2), 3.41 (m, 2H, -NC//2), 3.50 (sept, J = 9 Hz, 1H, -C//(CH3)2), 3.68, (m, 2H, -NC//2), 3.78 (sept, J = 9 Hz, 1H, -C//(CH3)2), 4.34 (m, 2H, -NC//2), 6.06 (s, 2H, -imid//), 6.97 (s, 2H, -Ar//), 7.01 (s, 2H, -Ar//). 139 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes 13C{'H} NMR (C6D6): 8 19.6 (-CH3), 20.4 (-CH3), 21.9 (-CH3), 30.2 (-CH3), 30.4 (-CH3), 38.6 (-CH), 38.9 (-CH), 49.7 (-NCH2), 54.8 (-NCH2), 117.9 (-ArQ, 120.4 (-miidQ, 127.9 (-ArQ, 132.6 (-ArQ, 150.2 (-ArQ, 179.5 (-NCN), 193.8 (-NCN). Anal Calcd. for C34H52HfN6: C, 56.46; H, 7.25; N, 11.62. Found: C, 56.21; H, 7.35; N, 11.42. Synthesis of Mes[NCN]ZrCl(OCH2CH2CH2CH3) (4.24) To a 250 mL thick-walled bomb charged with 3.25 (500 mg, 0.91 mmol) and KC8 (270 mg, 2.0 mmol) was vacuum transferred 40 mL THF at -196°C. The frozen solution was freeze-pump-thawed three times with N2 and sealed at -196°C. The brown slurry was slowly warmed to room temperature to yield a dark black suspension. After stirring for three days, the pressure in the bomb was reduced to one atmosphere and filtered through Celite. The solvent was removed and the yellow residue was extracted with Et20. The solvent was concentrated and left to stand at -30°C upon which time yellow block crystals formed. Yield = 187 mg, 35%. 'H NMR (C6D6): 8 0.9-1.1 (m, 7H, -C//2C//2C//3), 2.06 (s, 12H, -o-ArC//3), 2.24 (s, 6H, -p-AxCH3), 3.42 (m, 2H, -NC//H), 3.50 (t, J = 8 Hz, 2H, -OCH2\ 3.78 (m, 2H, -NC//H), 3.89 (m, 2H, -NC//H), 4.03 (m, 2H, -NC//H), 5.82 (s, 2H, -imid//), 6.89 (s, 2H, -Ar//), 6.94 (s, 2H, -Ar//). '^{'H} NMR (C6D6): 8 16.9 (-CH3), 20.2 (-CH3), 20.9 (-CH3), 21.2 (-CH3), 22.4 (-CH3), 48.9 (-NCH2), 51.3 (-NCH2), 60.4 (-OCH2), 118.5 (-ArQ, 120.3 (-imidQ, 128.6(-ArQ, 130.5 (-ArQ, 149.2 (-ArQ, 192.5 '(-NCN). Anal Calcd. for C29H4iClN4OZr: C, 59.20; H, 7.02; N, 9.52. Found: C, 59.35; H, 7.33; N,9.67. Synthesis of Mes[NCN]Hf(Me)(NHNMe2) (4.25), Mes[NCN]Hf(NHNMe2)2 (4.26) The following procedure is representative of the synthesis of 4.25 and 4.26. In a 50 mL Erlenmeyer flask, 3.35 (200 mg, 0.33 mmol) was dissolved in 10 mL of toluene and cooled to -30°C. A toluene solution of Me2NNH2 (20 mg, 0.33 mmol) was carefully added dropwise at this temperature and the colorless solution was slowly warmed.to room 140 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes temperature. After stirring overnight, the solution was filtered through Celite, and the solvent removed in vacuo. E12O was added and the solution was carefully concentrated and cooled to -30°C to yield large block colorless crystals. Yield =131 mg, 62%. 4.25: 'H NMR (C6D6): 5 0.35 (s, 3H, -HfC//3), 2.00 (s, 6H, -N(C#3)2), 2.32 (s, 6H, -o-AxCHf), 2.44 (s, 6H, -o-ArC//3), 2.63 (s, 6H, -p-ArCH2), 3.44 (m, 2H, -NC//2), 3.62-3.70 (m, 6H, -NCH2), 4.13 (s, 1H, -NH), 6.08 (s, 2H, -imid//), 7.05 (s, 2H, -ArH), 7.09 (s, 2H, -Ar//). 13C{'H} NMR (C6D6): 5 22.4 (-CH3), 23.1 (-CH3), 35.2 (-NCH3), 49.6 (-NCH2), 50.2 (-HfCH3), 53.1 (-NCH2), 118.6 (-ArQ, 119.2 (-imidQ, 128.4 (-ArQ, 130.1 (-ArQ, 143.2 (-ArQ, 191.4 (-NCN). Anal Calcd. for C28H42HfN6: C, 52.45; H, 6.60; N, 13.11. Found: C, 52.58; H, 6.51; N, 13.29. 4.26: 'H NMR (C6D6): 2.12 (s, 12H, -N(C//3)2, 2.31 (s, 6H, -p-ArCH3), 2.50 (s, 12H, -o-ArC//3), 3.60 (m, 4H, -NC//2), 3.78 (m, 4H, -NC//2), 6.04 (s, 2H, -imid//), 7.02 (s, 4H, -ArH). ,3C{lH} NMR (C6D6): 8 20.3 (-CH3), 21.5 (-CH3), 39.5 (-NCH3), 48.6 (-NCH2), 52.3 (-NCH2), 53.8 (-HfCH3), 118.0 (-ArQ, 118.5 (-imidQ, 127.9 (-ArQ, 133.2 (-ArQ, 145.9 (-ArQ, 192.5 (-NCN). Anal Calcd. for C29H46HfN8: C, 50.83; H, 6.77; N, 16.35. Found: C, 50.77; H, 6.93; N, 16.18. 4.8.4. Polymerization protocols a) Ethylene polymerization: A 100 mL Schlenk flask was charged with 50 mL toluene inside a glove box and attached to a vacuum line. The toluene was degassed with ethylene for 30 minutes at room temperature, whereby a toluene (5mL) solution of [Ph3C][B(CeF5)4] was syringed into the flask. The orange solution was stirred for 5 minutes under ethylene. A toluene (5 mL) solution of the catalyst was quickly added and the opaque solution stirred for 15 minutes. The experiment was stopped by venting the ethylene and quenching the reaction with 10% methanolic HC1. The resulting 141 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes polyethylene was filtered and washed with 10% methanolic HCI and methanol and air-dried overnight. b) 1-hexene polymerization: 4.4 and 4.5 were generated in chlorobenzene and 1-hexene was added immediately (-500 equivalents) by syringe. The solution was stirred for 1 hour and quenched with 10% methanolic HCI. The solvents were removed, the residue dissolved in pentane, and filtered through silica gel. The solvents were removed in vacuo to yield a minor amount of a viscous gel. 142 References begin on page 143. Chapter Four: Reactivity and Applications of Group 4 [NCN] Transition Metal Complexes 4.10. References (1) Arnold, P. L.; Mungur, S. A.; Blake, A. J.; Wilson, C. Angew. Chem. Int. Ed. 2003, 42, 5981. (2) Scollard, J. D.; McConville, D. 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Chapter Five: Synthesis and DFT Studies of Tantalum [NCN] Transition Metal Complexes Chapter Five Synthesis and DFT Studies of Tantalum [NCN] Transition Metal Complexes 5.7. Introduction* Given the absence of NHC dissociation in group 4 [NCN] complexes discussed in chapter 4, we were interested in the synthesis of other early transition metal [NCN] complexes. As was previously discussed in chapter 2, a tantalum [NPN] complex has been isolated displaying a unique side-on end-on coordination mode for dinitrogen. The synthesis of this complex is quite remarkable as this occurs in the absence of strong reducing agents such as KCg or Na/Hg amalgam. In chapter 4, the synthesis of group 4 [NCN] dinitrogen complexes was attempted; however, no dinitrogen containing products were recovered. While the reasons for this are unclear, the potential for other [NCN] transition metal complexes to promote dinitrogen activation is of interest. Tantalum [NCN] complexes are of particular appeal, in light of the successful synthesis of the tantalum [NPN] dinitrogen complex 2.5. *A version of this chapter has been accepted for publication (J. Am. Chem. Soc). 148 Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes To the best of our knowledge, only one previous report describes the chemistry of tantalum-NHC complexes.1 The addition of a simple alkyl-substituted NHC to TaCi4(py)2 results in the displacement of both pyridine ligands to give TaCl4(NHC)2 (Scheme 5.1). Although these results appear promising, no solid state molecular structures, reactivity, or applications of the complexes were mentioned. + TaCI4(py)2 2py Scheme 5.1. Pincer {NCN}-based ligands that have an anionic aryl group flanked by two amino donors have also been a subject of investigation in organotantalum chemistry (Scheme 5.2).2"8 In particular, a monoanionic {NCN} ligand has been found to be an excellent spectator ligand that stabilizes and controls tantalum centered reactions. For example, the central metal-aryl unit of the {NCN} ligand was found to be chemically inert during a variety of alkylidene-based transformations reactions that resulted in the formation of reactive tantalum alkene adducts. 149 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes Scheme 5.2. This chapter details the synthesis of tantalum-amide, -halide, and -alkyl compounds bearing an [NCN] ancillary ligand. In addition, our unsuccessful attempts to prepare a coordinated dinitrogen stabilized by this ligand are also included. In the case of tantalum alkyl compounds, an unexpected C-H activation process occurs, which was investigated by DFT calculations and NMR experiments using deuterium labeled compounds. 5.2. Synthesis of Amine-Amide fNCNH] Tantalum Derivatives In light of the success using aminolysis reactions described in chapter 3, a similar approach was examined as a convenient entry into tantalum [NCN] derivatives. Disappointingly, there was no reaction between Mes[NCN]H2 (3.8) and Ta(NMe2)5, even at elevated temperatures. Decreasing the steric bulk on the amide donors did promote a reaction. The addition of tol[NCN]H2 (3.7) to Ta(NMe2)s in toluene yielded a product 150 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes that displayed a set of !H and 13C resonances in the NMR spectra indicative of Cs symmetric species (5.1). This is in contrast to the expected C2v symmetry anticipated for tol[NCN]Ta(NMe2)3. NMR spectroscopy shows four multiplets for the ethylene spacers, two doublets for the imidazole groups, two distinct sets of doublets typical of para-substituted aryl rings, two aryl-methyl signals, and a broad resonance attributable to the -NMe2 groups. Furthermore, a triplet at 3.32 ppm is observed which can be ascribed to an amino -NH group. The 13C{'H} NMR spectrum features a weak downfield resonance at 198.5 ppm, typical of a metal-carbene carbon atom. From these results, it appears coordination of the [NCN] ligand to tantalum is incomplete, one amine donor is still present, giving the molecule an overall Cs symmetry in solution (Equation 5.1). ^NH HN' / \ tol tol tol = Ta(NMe?)5, PhMe, -HNMe2 (5.1) -Me Orange crystals of 5.1 were grown from a saturated solution of toluene and were studied by X-ray crystallography. An ORTEP depiction of the solid state molecular structure 5.1 is shown in Figure 5.1, with selected bond angles and lengths given in Table 5.1 and crystallographic details located in appendix A. The ligand assumes an amide-amine donor configuration with respect to a distorted octahedral metal centre. The Ta-alkyl carbene bond length is 2.407(4) A and represents, to the best of our knowledge, the first crystallographically characterized Ta-C NHC bond. The Ta-N amido bond lengths are similar to other reported compounds.9"13 Although introduction of the [NCN] ligand is incomplete, we were encouraged by the formation of a new Ta-C NHC bond. Thus far, all attempts to promote the coordination of the other pendant amine donor have been unsuccessful. 151 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes Figure 5.1. ORTEP view of tol[NCNH]Ta(NMe2)4 (5.1) (CH3C6H5 omitted) depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity. Table 5.1. Selected bond distances (A) and angles (°) for tol[NCNH]Ta(NMe2)4, 5.1. Bond Lengths Bond Angles Tal-Cl 2.407(4) Tal-N3 2.181(3) Tal-N8 2.050(3) N8-Tal-N3 90.58(12) N6-Tal-N3 178.48(11) N3-Tal-Cl 80.14(11) In chapter 3, alkyl elimination reactions between 3.7 and Zr(CH2R)4 (R = SiMe3, Ph) provided the desired five-coordinate dialkyl tol[NCN]Zr(CH2R)2 complexes. A similar approach was examined with Mes[NCN]H2 (3.8) and Ta(CH2Ph)5. The reaction proceeds immediately in toluene to give dark brown 5.2, which displays a 'H NMR spectrum with inequivalent imidazole and aryl resonances (Equation 5.2). A benzylidene resonance is observed at 4.81 ppm and correlates with a C resonance located at 236.3 13 1 ppm in the C{ H} NMR spectrum. Presumably, alkylidene formation proceeds through a tetraalkyl Mes(NCNH)Ta(CH2Ph)4 intermediate, which is not observed in solution. This species undergoes a-hydrogen abstraction to generate the benzylidene product, a phenomenon that has been observed in other tantalum alkyl complexes.14 Interestingly, 152 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes there is no change in the 13C carbene resonance (or that in 5.1) in the presence of pyridine over a long period of time, a surprising result despite the carbene dissociation reported in similar bidentate amido-carbene early transition metal complexes.15 Ta(CH2Ph)5 PhMe V Js^"2"' (5.2) 'NH HN' -2 PhMe / \ Mes Mes An ORTEP depiction of the solid state molecular structure for 5.2 is shown in Figure 5.2 as determined by an X-ray diffraction experiment. Relevant bond lengths and angles are listed in Table 5.2, and crystallographic details are located in Appendix A. A benzylidene moiety is clearly observed in addition to an amide-amine ligand configuration on a distorted square pyramidal metal centre. The alkylidene moiety is clearly defined by a large Ta-C-C bond angle of 164.5(4)° and a short Ta-C alkyl bond (1.940(4) A), with the latter being significantly shorter than that of the other two Ta-C alkyl bonds (-2.26 A). The Ta=C bond compares well with previously described Ta-alkylidene complexes.16 The Ta-C carbene and Ta-N amido bond lengths are similar to 5.1. Attempts to promote coordination of the pendant amine arm by thermolysis have proven futile leading only to decomposition. 153 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes Figure 5.2. ORTEP view of Mes[NCNH]Ta(CHPh)(CH2Ph)2 (5.2), depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity with the exception of HI 01. Table 5.2. Selected bond distances (A) and angles (°) for Mes[NCNH]Ta(CHPh)(CH2Ph)2 (5.2). Bond Lengths Bond Angles Tal-C28 1.940(3) N3-Tal-Cl 80.90(9) Tal-N3 2.011(2) C41-C28-Tal 164.5(2) Tal-C26 2.243(3) C29-C26-Tal 126.44(18) Tal-C27 2.259(3) Tal-Cl 2.290(3) 5.3 Successful Synthesis of TafNCNJ Amide Complexes Given the difficulty of coordinating both amide donors to a Ta(V) centre by aminolysis and alkane elimination reactions, an alternative method was sought as a means to generate the desired tridentate coordination mode for this ligand system. Metathesis reactions with Li2[NPN] have successfully been used by many groups 154 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes including our own in the synthesis of tantalum [NPN] metal complexes.17 The metathesis reaction of 3.10 and Cl2Ta(NMe2)3 proceeds at -30°C to produce tol[NCN]Ta(NMe2)3 (5.3) in good yield. The *H NMR spectrum features two multiplets for the ethylene spacers and one set of resonances for the imidazole, aryl, and /?ara-methyl aryl groups. There are two sets of resonances for the N-methyl protons of the NMe2 groups in a ratio of 2:1 at 3.22 and 3.68 ppm. A weak 13C resonance in the 13C{'H} NMR spectrum is also observed at 186.7 ppm, indicative of a metal-carbene carbon atom (Equation 5.3). ,„»^L|'".„, . tol tol 3.10 CI2Ta(NMe2)3> -30°C, THF -2 LiCI -N N-(5.3) t0''Me2N/Le2 5.3 tol Crystals suitable for an X-ray diffraction experiment were grown from toluene and an ORTEP depiction of the solid state molecular structure of 5.3 is shown in Figure 5.3. Relevant bond lengths and angles are listed in Table 5.3, and crystallographic details are located in appendix A. Clearly the ligand exists in a dianionic state with both pendant amide donors coordinating to a single tantalum centre. The ligand adopts a meridional orientation with respect to a distorted octahedral metal centre as evidenced by the cis oriented amido donors (N2-Tal-N3 = 89.25(4)° and N2-Tal-N4 = 98.46(4)°). The Ta-C carbene and Ta-N amido bond lengths are similar to previously discussed complexes. 155 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes Figure 5.3. ORTEP view of tol[NCN]Ta(NMe2)3 (5.3), depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity. Table 5.3. Selected bond distances (A) and angles (°) for tol[NCN]Ta(NMe2)3 (5.3). Bond Lengths Bond Angles Tal-N3 2.055(3) Tal-N4 2.078(4) Tal-N2 2.132(3) Tal-Cl 2.365(6) N3-Tal-N2 89.25(13) N3-Tal-Cl 82.93(10) N2-Tal-Cl 81.54(9) N4-Tal-Cl 180.000(2) Modification of the tantalum starting material serves as a useful entry into mixed amide-chloride metal complexes. The reaction of 3.10 and Cl3Ta(NMe2)2(THF) yields the expected product tol[NCN]TaCl(NMe2)2 (5.4) (Scheme 5.3). The !H NMR spectrum reveals a Cs symmetric species in solution with four sets of multiplets for the ethylene spacers and most noticeably three singlets integrating to six protons each. This evidence suggests a cis arrangement of NMe2 groups on the tantalum centre. Although no crystals of the product could be recovered, NMR evidence is quite compelling given that trans deposited NMe2 groups would yield a species with C2v symmetry in solution. A similar reaction with 3.10 and TaCl4(NEt2)(Et20) also yields the expected tol[NCN]TaCl2(NEt2) product. The 'H NMR spectrum reveals the presence of two structural isomers in 156 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes solution. The minor species possesses Cs symmetry, a result that would be expected with cis deposited chloride groups on the tantalum centre (5.5). The major species possesses C2v symmetry, indicative of trans oriented chloride groups on the metal centre (5.6). 5.5 5.6 Scheme 5.3. The reactivity of these amide-substituted TafNCN] complexes was investigated, in particular the synthesis of [NCNJTaCh. and alkyl derivatives. These complexes could be used in reductive processes to activate N2. Unfortunately, attempts to convert the amide groups of 5.3-5.6 to chloride ligands with TMSC1, BCI3, or NEt3-HCl yields a mixture of intractable materials. Similar results were found when alkylation of 5.3-5.6 was attempted with Al2Me6, MeLi, MeMgBr, or (lBuCH2)2Zn. The reduction of 5.3-5.6 with strong reducing reagents under N2 was also investigated; however, no dinitrogen containing products were identified. 5.4. Isolation of Cyclometallated [NCCNJTa Dialkyl Derivatives One of the precursors for the synthesis of the [NPN] tantalum dinitrogen complex 2.5 is an [NPN]-substituted tantalum trialkyl derivative. Given the success of coordinating the [NCN] ligand to tantalum by metathesis reactions with mixed alkyl-157 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes chloro precursors of Ta(V), C^TaR^ (R = Ctb'Bu, Me, CH2PI1), was examined. The reaction of 3.11 with Cb-Taf^CHVBu^ is representative of these metathesis reactions and proceeds immediately at -30°C in THF to give a brown solution. The 'H NMR spectrum of the product reveals a complicated set of resonances indicative of a low-symmetry species. The ethylene arms of the ligand backbone are observed as sets of complex, coupled resonances. The asymmetry of the product is further exemplified by inequivalent imidazole-hydrogen resonances at 5.82 and 5,88 ppm. Two different tantalum alkyl 'H resonances are also observed, which is unexpected for the anticipated ^[NCNJTaCCHz'BuJs product. The 13C{'H} NMR spectrum shows two distinct Ta-C alkyl resonances around 70 ppm, and one weak resonance at 85 ppm suggestive of another Ta-C moiety (Equation 5.4). (5.4) R = CH2lBu 5.7 CH3 5.8 CH2Ph 5.9 The solid state molecular structure of 5.7 was determined from an X-ray diffraction experiment and an ORTEP depiction is shown in Figure 5.4. Selected bond angles and lengths are given in Table 5.4 and crystallographic details are located in appendix A. Surprisingly, one of the six-membered metallacycles has undergone cyclometallation to form new five- and three-membered rings. In lieu of the amide C-H bond activation, only two neopentyl groups are observed rather than the expected trialkyl substitution. The C-H bond activated ligand (denoted [NCCN]) adopts a distorted facial orientation about a distorted trigonal bipyramidal metal centre. The bond angles defined by Tal-C7-N4 are typical of another structurally characterized metallaaziridine tantalum 158 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes complex.18 All Ta-C alkyl bonds are similar in length (-2.24 A), and are representative of other reported Ta-C alkyl bond lengths.16'17'19 Figure 5.4. ORTEP view of Mes[NCCN]Ta(CH2tBu)2 (5.7), depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity. Table 5.4. Selected bond distances (A) and angles (°) for Mes[NCCN]Ta(CH2tBu)2 (5.7). Bond Lengths Bond Angles Tal-N4 1.9877(15) N4-Tal-N3 153.91(6) Tal-N3 2.0738(17) N4-Tal-Cl 89.46(6) Tal-C31 2.1833(17) N3-Tal-Cl 79.59(6) Tal-Cl 2.2247(17) N4-Tal-C7 39.37(6) Tal-C7 2.2257(19) N4-C7-Tal 61.34(9) Tal-C26 2.255(2) C7-N4-Tal 79.29(10) 159 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes 5.5. Mechanistic Insight Into the Formation of 5.7-5.9 Intramolecular C-H bond activation of ligands in Ta complexes is relatively common,20'21 particularly with amido-based systems.18'19,22"25 Among these reports, the formation of metallaaziridine rings by exocyclic18'24'25 C-H bond activation is known; the only example of endocyclic19 C-H bond activation involves a tripodal tris(aryloxy)amine system that activates next the amine donor. In most examples, metallaaziridine ring formation appears to follow a a-bond metathesis mechanism involving direct elimination of an alkane,24 although in some cases the mechanism was not fully investigated.19 Two plausible mechanisms were explored for the formation of 5.7-5.9. Assuming that metathesis reactions between Li2Ar[NCN] and Cl2TaR3 provide the trialkyl Ar[NCN]TaR3 derivatives, the ligand backbone P-H abstraction process may occur by either a one-step a-bond metathesis pathway (Path 1, Figure 5.5) or a two-step pathway that involves a-H abstraction to produce a [NCN]Ta(=CHR')R alkylidene intermediate (Path 2, Figure 5.5). This alkylidene intermediate could then mediate C-H bond activation of the ligand backbone. Given the literature precedence for both mechanisms, density functional calculations and isotopic labeling experiments were used to elucidate a mechanism for the formation of 5.7-5.9. 160 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes r=\ Path 1 i) a-bond metathesis r=\ Ar = 2,4,6-Me3C6H2 R = benzyl, neopentyl Ar xNLi LiN 7\r TaCI2R3 - 2 LiCI Ar c R' Path 2 ii) a-H abstraction followed by iii) alkylidene mediated C-H activation Figure 5.5. Potential pathways for ligand P-H abstraction. 5.6. Determination of Mechanism by DFT Calculations DFT calculations were performed using the model complex shown in Figure 5.6 and the B3LYP/BS1 level of theory. All the DFT calculations were performed by Dr. Chad Beddie at the Texas A&M University under the supervision of Professor Michael Hall. The proposed mechanisms were investigated by calculating the structures and energies of intermediates and transition state structures along both pathways (Figure 5.5). The results of these calculations are discussed in detail in appendix B. 161 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes r=\ r=\ N N H H Experimental Ligand Mes[NCN] Computational Ligand H[NCN] Figure 5.6. Computational model of the trimethyl tantalum starting complex. The computational results suggest the lowest energy pathway involves a tantalum alkylidene intermediate, which can then mediate C-H bond activation with a neighboring backbone linker C-H group to form a new metallaaziridine. The energies of the intermediates and transition states for both path 1 and 2 are shown in Figures 5.7 and 5.9, respectively. The structures of the energy minimized transition states are given in Figures 5.8 (Path 1) and 5.10 (Path 2). 162 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes TS-A-B (49.6) These isomers vary in the conformation of the 6 member rings a Gas-phase relative free energies at the B3LYP/BS1 level of theory based on the energy of separated 1 set to 0.0 kcal/mol are provided in parentheses in kcal/mol. Electronic energies, corrected zero-point energies, enthalpies, and free energies are provided in Table 1. Figure 5.8. JIMP Pictures of the one-step a-bond metathesis pathway. 163 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes TS-A-E (36.4) A F-J D These isomers vary in the conformation of the 6 member rings a Gas-phase relative free energies at the B3LYP/BS1 level of theory based on the energy of separated 1 set to 0.0 kcal/mol are provided in parentheses in kcal/mol. Electronic energies, corrected zero-point energies, enthalpies, and free energies are provided in Tabiei. Figure 5.9. Relative energies of the intermediates and transition states in a potential two-step a-H abstraction/alkylidene mediated C-H activation mechanism. 164 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes J TS-J-D D Figure 5.10. JIMP26 Pictures of alkylidene mediated C-H activation of the ligand backbone. 5.7. Verification of the Mechanism proposed by DFT Calculations Based on the computational support for the intermediacy of a tantalum alkylidene in the endocyclic C-H activation process, the synthesis of an [NCN] tantalum alkylidene complex was attempted. Beginning with Cl3Ta=CHtBu(THF)2, the reaction with 3.11 proceeds to give 5.10 in good yield (Scheme 5.4). Both the ]H and l3C{'H} NMR spectra display resonances similar to the previous cyclometallated complexes 5.7-5.9. Examination of the !H NMR spectrum during the reaction reveals the presence of 2,2-dimethylpropane formation and no observable intermediate. 165 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes 5.10 Scheme 5.4. An X-ray diffraction experiment was performed on crystals grown from Et20 with an ORTEP depiction of 5.10 given in Figure 5.11. Relevant bond lengths and angles are listed in Table 5.5, and crystallographic details are located in appendix A. The solid state molecular structure reveals an endocyclic C-H bond activated complex with similar structural characteristics to 5.7. Clearly, there is no neopentylidene present as the Ta-C alkyl bonds from both complexes are of similar length. The Tal-C5 alkyl bond length of 2.216(4) A is similar to length to 5.7, as are the bond angles defined by the N3-C5-Tal metallaaziridine ring. 166 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes Figure 5.11. ORTEP view of Mes[NCCN]Ta(Cl)(CH2tBu) (5.10), depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity. Table 5.5. Selected bond distances (A) and angles (°) for Mes[NCCN]Ta(Cl)(CH2tBu) (5.10). Bond Lengths Bond Angles Tal-N3 1.967(4) N3-Tal-N4 115.35(15) Tal-N4 2.010(3) N3-Tal-C5 39.64(16) Tal-C5 2.216(4) N3-Tal-Cl 84.96(15) Tal-C26 2.224(4) N4-Tal-Cl 77.84(15) Tal-Cl 2.241(4) C5-N3-Tal 79.6(2) Tal-Cl 1 2.3996(11) N3-C5-Tal 60.8(2) Although these experimental results cannot confirm the presence of an alkylidene intermediate in the decomposition of [NCN]Ta(alkyl)3 complexes, they infer that an alkylidene species can undergo rapid amido C-H bond activation. Further evidence for the mechanism postulated by DFT calculations was observed in NMR experiments using deuterium labeled complexes. Utilizing Cl2Ta(CD2Ph)3 as a starting material, we can examine the proton distribution in the final C-H bond activated product was examined 167 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes (Scheme 5.5). If the mechanism proceeds as suggested by calculations, the tantalum alkylidene intermediate would abstract a proton from the ethylene spacer to yield a TaC//(D)Ph moiety (Path 1, Scheme 5.5). If a concerted a-bond metathesis was the operant mechanism, this reaction would yield two -CD2Ph groups and no benzylic proton resonances would be observed (^4-5.9, Path 2, Scheme 5.5). The reaction of 3.11 with Cl2Ta(CD2Ph)3 yields <i3-5.9. The 'H NMR spectrum clearly shows a 1:1:1 triplet at 2.70 ppm, evidence for the -TaC//(D)Ph moiety in d3-5.9. This result suggests DFT calculations are correct and implies that the decomposition of [NCN]Ta(alkyl)3 complexes proceeds through an alkylidene intermediate which rapidly undergoes C-H bond activation with an amido donor. Scheme 5.5. 5.8. Conclusions In this chapter, the potential of the [NCN] ligand architecture to stabilize tantalum amide, halide, and alkyl complexes has been presented. Aminolysis and alkyl 168 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes elimination reactions, which were previously successful in the synthesis of group 4 [NCN] complexes, provided a route to incorporate one flanking amido arm. Thus far, attempts to promote coordination of the second pendant amine arm have been unsuccessful. Coordination of both pendant amide donors was achieved by metathesis reactions between Lia^fNCN] and substituted tantalum chlorides. In the case of trialkyl tantalum derivatives, amide C-H bond activation occurs to generate a new cyclometallated metallaaziridine. DFT calculations on model complexes suggested that the mechanism for this phenomenon proceeds through a tantalum alkylidene intermediate, which can then mediate C-H bond activation with a neighboring amido group to form a new metallaaziridine. In light of these findings, we examined the synthesis of tantalum alkylidene complexes stabilized by an [NCN] ancillary ligand and found that similar amide C-H bond activation occurs. While this result cannot confirm the mechanism postulated by DFT calculations, deuterium labeling experiments have shown that a tantalum alkylidene intermediate is involved during this process. 169 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes 5.9. Experimental Section 5.9.1. General Considerations Unless otherwise stated, general procedures were performed as described in Section 2.5.1. DFT calculations were performed at the Department of Chemistry at the University of Texas A&M by Dr. Chad Beddie. 5.9.2. Materials and Reagents Ta(NMe2)5 were purchased from Strem Chemicals and used as received. All other 27 chemicals were purchased from Aldrich and used as received. Ta(CH2Ph)5, Cl2Ta(NMe2)3,13 Cl3Ta(NMe2)2(THF),28 Cl4Ta(NEt2)(Et20),28 Cl2TaMe3,29 Cl2Ta(CH2Ph)3,3() Cl2Ta(CD2Ph)3,30 Cl2Ta(CH2tBu)3,31 Cl3Ta=CHtBu(THF)232 were all synthesized by literature methods. 5.9.3. Synthesis and Characterization of Complexes 5.1 - 5.10 Synthesis of tol[NCNH]Ta(NMe2)4 (5.1) To a stirred toluene solution (5 mL) of Ta(NMe2)5 (216 mg, 0.54 mmol) in a 50 mL Erlenmeyer flask was added a toluene solution (5 mL) of 3.7 (181 mg, 0.54 mmol). The orange solution was stirred overnight and filtered thru Celite. The solvent was removed until the volume was ~2 mL, and then cooled to -30°C to yield orange blocks. Yield - 246 mg, 66%. 'H NMR (C6D6): 2.22 (s, 3H, -ArC//3), 2.44 (s, 3H, -ArC//3), 3.06 (br s, 12H, -NC//3), 3.16 (m, 2H, -NC//2), 3.27 (m, 2H, -NC//2), 3.32 (br m, 1H, -N//), 3.52 (br s, 6H, -NC//3), 3.75 (br s, 6H, -NC//2), 4.03 (m, 2H, -NC//2), 4.19 (m, 2H, -NC//2), 5.98 (br s, 1H, -imid//), 6.29 (br s, 1H, -imid//), 6.38 (d, J=7 Hz, 2H, -Ar//), 6.91 (d, J=7 Hz, 2H, -Ar//), 7.01 (d, J=7 Hz, 2H, -Ar//), 7.24 (d, J=7 Hz, 2H, -Ar//). 13C{'H} NMR (C6D6): 18.1 (-CH3), 20.2 (-CH3), 45.5 (br, -NCH3) 46.9 (-NCH2), 47.0 (-NCH2), 47.5 (-NCH2), 48.9 (-NCH2), 118.6 (-ArQ, 119.3 (-ArQ, 120.5 (-imidQ, 122.6 170 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes (-imidQ, 124.8 (-ArQ, 124.9 (-ArQ, 128.7 (-ArQ, 129.2 (-ArQ, 140.5 (-ArQ, 141.6 (-ArQ, 198.5 (-NCN). Anal. Calcd. for C29H49N8Ta: C, 50.43; H, 7.15; N, 16.22; Found: C, 50.11; H, 7.00; N, 15.95. Synthesis of Mes[NCNH]Ta=CHPh(CH2Ph)2 (5.2) To a stirred toluene solution (5 mL) of Ta(CH2Ph)5 (328 mg, 0.51 mmol) was added a toluene solution (5 mL) of 3.8 (200 mg, 0.51 mmol) in a 50 mL Erlenmeyer flask. The dark brown solution was stirred overnight and filtered thru Celite. The solvent was removed to yield a brown residue, which was triturated with hexanes (10 mL) to yield a brown powder. Yield = 327 mg, 76%. X-ray quality crystals were obtained from a cooled (-30°C) toluene solution. !H NMR (C6D6): 2.02 (s, 3H, -/?-ArCH3), 2.08 (s, 6H, -o-ArCH3), 2.20 (s, 6H, -o-ArCH3), 2.31 (s, 3H, -p-ArCR3), 2.42 (m, 4H,-TaC//2), 2.43-3.10 (m, 7H, -Ni/ and -NC//2J and 3.58 (dt, J = 6Hz, 1H, -NC//), 4.40 (dt, J = 6Hz, 1H, -NC//), 4.81 (s, 1H, -C//Ph), 5.81 (d, J=2 Hz, 1H, -imid//), 6.23 (d, J=2 Hz, 1H, -imid//), 6.60-6.85 (m, H, -ArH), 7.42 (d, J=8 Hz, 4H, -ArH). l3C{ lH} NMR (C6D6): 17.9 (-CH3), 18.2 (-CH3), 20.7 (-CH3), 21.0 (-CH3), 48.7 (-NCH2), 48.8 (-NCH2), 51.6 (-NCH2), 54.5 (-NCH2), 69.9 (-TaCH2), 85.0 (-TaCH2), 119.6 (-ArQ, 120.3 (-ArQ, 121.2 (-imidQ, 123.7 (-imidQ, 125.6 (-ArQ, 129.3 (-ArQ, 129.8 (-ArQ, 129.9 (-ArQ, 130.6 (-ArQ, 131.0 (-ArQ, 132.0 (-ArQ, 136.1 (-ArQ, 137.6 (-ArQ, 137.8 (-ArQ, 138.2 (-ArQ, 142.7 (-ArQ, 147.8 (-ArQ, 148.7 (-ArQ, 155.3 (-ArQ, 205.0 (-NCN), 236.3 (-TaCH). Anal. Calcd. for C46H53N4Ta: C, 65.55; H, 6.34; N, 6.65; Found: C, 65.26; H, 6.22; N, 6.59. Synthesis of tol[NCN]Ta(NMe2)3 (5.3), tol[NCN]Ta(NMe2)2Cl (5.4), tol[NCN]Ta(NEt2)Cl2 (5.5-5.6) . The following procedure is representative of the synthesis of 5.3-5.6. To a cooled (-30°C) THF solution (5 mL) of Cl2Ta(NMe2)3 (288 mg, 0.75 mmol) was added a chilled (-30°C) THF solution (5 mL) of 3.10 (408 mg, 0.75 mmol) in a 50 mL Erlenmeyer flask. 171 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes The solution slowly darkened to brown upon warming to room temperature. The solution was stirred overnight whereupon the solvent was removed and toluene added. The solution was filtered thru Celite and the solvent removed to yield an orange/brown residue. Trituration of the residue with hexane yielded an orange solid which was recrystallized from toluene. Yield = 383 mg, 79%. 5.3: 'H NMR (C6D6): 2.45 (s, 6H, -ArCH3), 3.22 (s, 12H, -NC//3), 3.53 (m, 4H, -NC//2), 3.68 (s, 6H, -NCr73), 4.15 (m, 4H, -NC//2), 6.00 (s, 2H, -imid//), 7.11 (d, J = 8Hz, 4H, -ArH), 7.27 (d, J = 8Hz, 4H, -ArH). l3C{lH} NMR (C6D6): 18.7 (-CH3), 45.4 (-NCH3), 47.0 (-NCH3), 48.0 (-NCH2), 50.7 (-NCH2), 117.3 (-ArQ, 118.3 (-ArQ, 124.4 (-imidQ, 126.6 (-ArQ, 152.6 (-ArQ, 186.7 (-NCN). Anal. Calcd. for C27H42N7Ta: C, 50.23; H, 6.56; N, 15.19; Found: C, 50.10; H, 6.35; N, 15.05. 5.4: 'H NMR (C6D6): 2.26 (s, 6H, -ArC//3), 3.26 (s, 6H, -NC//3), 3.36 (m, 2H, -NC//H), 3.60 (m, 2H, -NC//H), 3.80 (s, 6H, -NC//3), 3.88 (m, 2H, -NC//H), 4.35 (m, 2H, -NC//H), 5.96 (s, 2H, -imid//), 7.10 (d, J = 8 Hz, 4H, -ArH), 7.36 7.10 (d, J = 8 Hz, 4H, -ArH). 13C{'H} NMR (C6D6): 19.5 (-CH3), 45.6 (-NCH3), 46.5 (-NCH3), 47.5 (-NCH2), 48.9 (-NCH2), 116.8 (-ArQ, 119.1 (-ArQ, 123.9 (-imidQ, 127.5 (-ArQ, 149.9 (-ArQ, 187.1 (-NCN). Anal. Calcd. for C25H36ClN6Ta: C, 47.14; H, 5.70; N, 13.19; Found: C, 47.09; H, 5.52; N, 12.95. Minor isomer (5.5): *H NMR (C6D6): 0.60 (t, J = 9 Hz, 6H, -N(CH2C//3)2), 2.28 (s, 6H, -ArC//3), 3.25 (m, 2H, -NC//H), 3.52 (m, 2H, -NC//H), 3.56 (q, J = 9 Hz, 4H, -N(C//2CH3)2), 3.98 (m, 2H, -NC//H), 4.70 (m, 2H, -NC//H), 6.10 (s, 2H, -imid//), 7.17 (d, J = 8 Hz, 4H, -ArH), 7.61 (d, J = 8 Hz, 4H, -ArH). l3C{lU} NMR (C6D6): 15.0 (-NCH2CH3), 22.5 (-CH3), 45.4 (-NCH2CH3), 48.0 (-NCH2), 50.7 (-NCH2), 119.5 (-ArQ, 120.5 (-ArQ, 125.6 (-imidQ, 131.5 (-ArQ, 151.6 (-ArQ, 189.5 (-NCN). 172 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes Major isomer (5.6): 'H NMR (C6D6): 0.81 (t, J = 9 Hz, 6H, -N(CH2C//3)2), 2.21 (s, 6H, -ArCH3), 3.30 (m, 4H, -NC//2), 3.74 (q, J = 9 Hz, 4H, -N(C//2CH3)2), 4.52 (m, 4H, -NC//2), 5.78 (s, 2H, -imid//), 7.02 (d, J = 8 Hz, 4H, -Ar//), 7.49 (d, J = 8 Hz, 4H, -ArH). 13C{'H} NMR (C6D6): 15.2 (-NCH2CH3), 23.4 (-CH3), 45.9 (-NCH2CH3), 48.6 (-NCH2), 51.6 (-NCH2), 117.6 (-ArQ, 120.9 (-ArQ, 126.8 (-imidQ, 132.6 (-ArQ, 153.9 (-ArQ, 185.6 (-NCN). Anal. Calcd. for C25H34Cl2N5Ta: C, 45.74; H, 5.22; N, 10.67; Found: C, 45.45; H, 5.01; N, 10.52. Ratio of minor isomer 5.5: major isomer 5.6 =1:1.3. Synthesis of ^[NCCNJTafCH^Buk (5.7) Mes[NCCN]TaMe2 (5.8) Mes[NCCN]Ta(CH2Ph)2 (5.9) Mes[NCCN]Ta(CD2Ph)2 (d3-5.9) The following procedure is representative of the synthesis of 5.7-5.9. To a cooled (-30°C) THF solution (5 mL) of Cl2Ta(CH2'Bu)3 (360 mg, 0.77 mmol) in a 50 mL Erlenmeyer flask was added a chilled (-30°C) THF solution (5 mL) of 3.11 (270 mg, 0.77 mmol). The dark brown solution was slowly warmed to room temperature. The solution was stirred overnight whereupon the solvent was removed and toluene added. The solution was filtered thru Celite and the solvent removed to yield a dark orange solid. Yield = 356 mg, 68%. Yellow crystals were obtained from recrystallization in hexane. 5.7: *H NMR (C6D6): 0.80 (d, J = 8 Hz, 1H, -TaC/ffl), 0.92 (s, 9H, -C(C//3)3), 1.10 (d, J = 8 Hz, 1H, -TaC//H), 1.18 (d, J = 8 Hz, 1H, -TaC//H), 1.29 (d, J = 8 Hz, 1H, -TaCMf), 1.30 (s, 9H, -C(C/73)3), 2.18 (s, 3H, -ArC//3), 2.25 (s, 3H, -ArC//3), 2.28 (s, 3H, -ArC//3), 2.67 (m, 1H, -NC//H), 2.86(s, 3H, -ArC//3), 3.60 (m, 1H, -NC//H), 3.67 (m, 1H, -NC//H), 3.84 (m, 2H, -NC//H), 4.34 (m, 1H, -NC//H), 4.48 (m, 1H, -NC//H), 5.82 (d, J = 2Hz, 1H, -imid//), 5.88 (d, J = 2Hz, 1H, -imid//) 6.80 (br s, 2H, -ArH), 7.05 (s, 1H, -ArH), 7.14 (s, 1H, -ArH). l3C{lH} NMR (C6D6): 18.8 (-CH3), 19.2 (-CH3), 20.5 (-CH3), 21.9 (-CH3), 22.9 (-CH3), 35.1 (-C(CH3)), 36.0 (-C(CH3)), 51.2 (-NCH2), 55.9 (-NCH2), 56.3 (-NCH2), 68.1 (-TaQ, 72.1 (-TaQ, 85.6 (-TaQ, 117.5 (-ArQ, 119.5 (-ArQ, 120.5 (-imidQ, 121.6 (-imidQ, 129.6 (-ArQ, 130.9 (-ArQ, 132.1 (-ArQ, 133.9 (-ArQ, 148.6 (-ArQ, 149.9 (-ArQ, 197.5 (-NCN). 173 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes Anal. Calcd. for C33H47N4Ta: C, 59.14; H,' 7.52; N, 7.88; Found: C, 58.95; H, 7.26; N, 7.59. 5.8: 'H NMR (C6D6): 0.46 (s, 3H, -TaCZZ3), 0.62 (s, 3H, -TaCZZ3), 1.89 (s, 3H, -ArCZZ3), 2.21 (s, 3H, -AvCH3), 2.27 (s, 3H, -ArCZZ3), 2.70 (s, 3H, -ArCZZ3), 2.72 (m, 1H, -NCZZH), 3.31 (m, 1H, -NC/ZH), 3.49 (m, 1H, -NCZZH), 3.71 (m, 1H, -NCZZH), 3.86 (m, 1H, -NCZZH), 4.19 (m, 1H, -NCZZH), 4.37 (m, 1H, -NCZZH), 5.91 (d, J=2 Hz, 1H, -imid//), 5.94 (d, J=2 Hz, 1H, -imid//), 6.84 (br s, 2H, -ArH), 7.02 (s, 1H, -ArH), 7.09 (s, 1H, -ArH). A satisfactory 13C NMR spectrum and elemental analysis could not be obtained due to the sensitivity of the product. 5.9: 'H NMR (C6D6): 1.67 (d, J = 7 Hz, -TaC/ZH), 2.27 (s, 3H, -ArC//3), 2.29 (s, 3H, -ArC//3), 2.32 (s, 3H, -ArC//3), 2.45 (d, J = 7 Hz, -TaC//H), 2.61 (d, J = 7 Hz, -TaCZZH), 2.76 (s, 3H, -ArC//3), 2.82 (d, J = 7 Hz, -TaCZZH), 3.28 (m, 1H, -TaC//H), 3.45 (m, 1H, -TaC//H), 3.55 (m, 1H, -TaC//H), 3.88 (m, 1H, -TaCZZH), 4.12 (m, 1H, -TaC//H), 4.55 (m, 1H, -TaCZZH), 5.90 (d, J=2 Hz, 1H, -imid//), 5.92 (d, J=2 Hz, 1H, -imid//), 6.64-6.71 (m, 3H, -ArH), 6.81-6.93 (m, 6H, -ArH), 6.99-7'.17 (m, 7H, -ArH). 13C{JH} NMR (C6D6): 17.6 (-CH3), 18.9 (-CH3), 19.6 (-CH3), 20.5 (-CH3), 50.1 (-NCH2), 54.6 (-NCH2), 55.4 (-NCH2), 79.2 (-TaQ, 81.6 (-TaQ, 90.1 (-TaQ, 119.2 (-ArC), 120.2 (-imidQ, 121.9 (-ArQ, 122.3 (-imidQ, 124.6 (-ArQ, 124.9 (-ArQ, 129.6 (-ArQ, 130.6 (-ArQ, 131.5 (-ArQ, 135.6 (-ArQ, 145.6 (-ArQ, 148.6 (-ArQ, 149.6 (-ArQ, 151.2 (-ArQ, 196.2 (-NCN). Some aryl resonances obscured by solvent signals. Anal. Calcd. for C43H4iN4Ta: C, 62.39; H, 6.04; N, 7.46; Found: C, 61.98; H, 5.89; N, 7.19. ^3-5.9: 'H NMR (C6D6): NMR is identical to 5.9 with the absence of the peaks at ppm and ppm and the presence 8 2.70 (t, J=l 1 Hz, 1H, -TaC/ZD). Synthesis of Mes[NCCN]Ta(CH2tBu)Cl (5.10) To a cooled (-30°C) THF solution (5 mL) of Cl3Ta=CH(tBu)(THF)2 (311 mg, 0.62 mmol) in a 50 mL Erlenmeyer flask was added a chilled (-30°C) THF solution (5 mL) of 3.11 (249 mg, 0.62 mmol). The dark brown solution was slowly warmed to room temperature. The solution was stirred overnight whereupon the solvent was removed and 174 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes toluene added. The solution was filtered thru Celite and the solvent removed to yield a dark orange solid. Yield = 344 mg, 84%. Yellow crystals were obtained from recrystallization in hexane. 'H NMR (C6D6): 1.26 (s, 9H, -C(CH3)3), 1.93 (d, J = 12 Hz, 1H, -TaC//H), 2.10 (s, 3H, -ArC//3), 2.12 (s, 3H, -ArC//3), 2.21 (d, J = 12 Hz, 1H, -TaC//H), 2.24 (s, 3H, -ArC//3), 2.73 (s, 3H, -ArC//3), 2.91 (m, 1H, -NC//H), 3.13 (m, 1H, -NC//H), 3.60 (m, 1H, -NC7/H), 3.83 (m, 2H, -NC//H), 3.96 (m, 1H, -NC//H ), 4.42 (m, 1H, -NC//H), 5.82 (br s, 1H, -imid//), 5.85 (br s, 1H, -imid//), 6.60 (s, 1H, -ArH), 6.87 (s, 2H, -ArH), 7.01 (s, 1H, -ArH). ,3C{'H} NMR (C6D6): 19.1 (-CH3), 20.1 (-CH3), 21.1 (-CH3), 21.9 (-CH3), 36.1 (-C(CH3)), 52.0 (-NCH2), 54.6 (-NCH2), 57.2 (-NCH2), 75.2 (-TaQ, 83.3 (-TaQ, 116.9 (-ArQ, 121.5 (-imidQ, 122.8 (-imidQ, 132.0 (-ArQ, 134.2 (-ArQ, 136.2 (-ArQ, 138.6 (-ArQ, 150.6 (-ArQ, 150.8 (-ArQ, 196.3 (-NCN). Anal. Calcd. for C29H39ClN4Ta: C, 53.37; H, 6.27; CI, 5.25; N, 8.30; Found: 53.05; H, 6.02; N, 8.22. 5.9.4. Theoretical Calculations 33 All calculations were performed using the Gaussian 03 suite of programs. Optimized gas-phase geometries were obtained using the Becke3 exchange functional,34 in combination the Lee, Yang, and Parr correlation functional,35 i.e. the B3LYP method, as implemented in Gaussian 03. The basis set (BS1) used for geometry optimizations and energy calculations was implemented as follows: for tantalum, the valence double-^ LANL2DZ36"38 basis set was supplemented with a set of 6p functions for transition metals developed by Couty and Hall,39 while for all hydrogen, carbon, and nitrogen atoms, the 6-31G(d',p') basis sets40"45 were used. All structures were calculated in singlet spin states using the restricted B3LYP method. Calculating the harmonic vibrational frequencies and noting the number of imaginary frequencies confirmed the nature of all intermediates (NImag = 0) and transition state structures (NImag = 1). All gas-phase relative free energies are reported in kcal mol"1, with the energy of H[NCN]TaMe3 (H[NCN] = (HNCH2CH2)2N2C3H2) set to 0.0 kcal mol"1. Relative electronic energies, zero-point 175 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes corrected energies, and enthalpies are provided in the supplemental information. For the computational investigation, H[NCN] was used in place of the experimental ligand (p-Me-C6H4NCH2CH2)2N2C3H2 (Tol[NCN]) in order to reduce the computational demands, while still providing two amide donors and one ./V-heterocyclic carbene donor to the tantalum centre. 176 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes 5.10. References (1) Herrmann, W. A. O., Karl; Elison, Martina; Kuehn, Fritz E.; Roesky, Peter W. J. Organomet. Chem. 1994, 480, C7. (2) Rietveld, M. H. P.; Klumpers, E. G.; Jastrzebski, J. T. B. H.; Grove, D. M.; Veldman, N.; Spek, A. L.; van Koten, G. Organometallics 1997,16, 4260. (3) Rietveld, M. H. P.; Lohner, P.; Nijkamp, M. G.; Grove, D. 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T.; Yap, G. P. A.; Richeson, D. S. Inorg. Chem. 1999, 38, 998. (12) Guzei, I. A.; Yap, G. P. A.; Winter, C. H. Inorg. Chem. 1997, 36, 1738. (13) Chisholm, M. H.; Huffman, J. C.; Tan, L.-S. Inorg. Chem. 1981, 20, 1859. (14) Schrock, R. R. Chem. Rev. 2002,102, 145. (15) Arnold, P. L.; Mungur, S. A.; Blake, A. J.; Wilson, C. Angew. Chem. Int. Ed. 2003,42,5981. (16) Messerle, L. W.; Jennische, P.; Schrock, R. R.; Stucky, G. J. Am. Chem. Soc. 1980,102, 6744. 177 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes (17) Fryzuk, M. D.; Johnson, S. A.; Patrick, B. O.; Albinati, A.; Mason, S. A.; Koetzle, T. F. J. Am. Chem. Soc. 2001,123, 3960. (18) Bazinet, P.; Yap, G. P. A.; Richeson, D. S. Organometallics 2001, 20, 4129. (19) Groysman, S.; Goldberg, I.; Kol, M.; Genizi, E.; Goldschmidt, Z. Organometallics 2004, 23, 1880. (20) Chamberlain, L. R.; Kerschner, J. L.; Rothwell, A. P.; Rothwell, I. P.; Huffman, J. C. J. Am. Chem. 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Organomet. Chem. 1976,122, 209. (31) Li, L.; Diminnie, J. B.; Liu, X.; Pollitte, J. L.; Xue, Z. Organometallics 1996, 75, 3520. (32) Boncella, J. M.; Cajigal, M. L.; Abboud, K. A. Organometallics 1996, 75, 1905. (33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A. J.; Vreven, T.; Kudin, K. N.; Burant, J. C; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; 178 References begin on page 177. Chapter Five: Synthesis and DFT Studies on Tantalum [NCN] Transition Metal Complexes Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C; Pople, J. A. Gaussian 03, Revision B.4; Gaussian, Inc.: Pittsburgh, PA, 2003. (34) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (35) Lee, C; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter 1988, 37, 785. (36) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (37) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (38) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (39) Couty, M.; Hall, M. B. J. Comput. Chem. 1996, 17, 1359. (40) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724. (41) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (42) Hariharan, P. C; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (43) Petersson, G. A.; Al-Laham, M. A. J. Chem. Phys. 1991, 94, 6081. (44) Petersson, G. A.; Bennett, A.; Tensfeldt, T. G.; Al-Laham, M. A.; Shirley, W. A.; Mantzaris, J. J. Chem. Phys. 1988, 89, 2193. (45) Foresman, J. B.; Frisch, A. E. Exploring Chemistry with Electronic Structure Methods, 2nd Ed. (Gaussian, Inc, Pittsburgh, PA), p. 110. The 6-31G(d',p') basis set has the d polarization functions for C, N, O, and F taken from the 6-31 lG(d) basis set, instead of the original arbitrarily assigned value of 0.8 used in the 6-31G(d) basis set. 179 References begin on page 177. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes Chapter Six Thesis Extensions: Chiral Group 4 [NCN] Complexes 6.1. Introduction Several of the preceding chapters have explored the synthesis of a diamido-N-heterocyclic carbene ligand set and the coordination to group 4 and 5 transition metals. A logical extension of this research is to introduce chirality into the [NCN] architecture to produce chiral transition metal complexes that could facilitate enantioselective catalysis. The development of efficient and practical chiral ligands and derived catalysts is believed to be "one of the most critical research objectives in modern organic synthesis".1 Because of the incredible success enjoyed by the application of chiral phosphine ligands in asymmetric catalysis,2'3 interest in introducing chirality into NHCs, so-called phosphine analogs, has dramatically increased. To date, several distinct classes of chiral NHC ligands have emerged, which can be characterized by the position of the chiral unit in relation to the NHC donor. Furthermore, each of these classes can be broadly subdivided into (i) monodentate and (ii) bidentate NHC ligands, the former being further divided into substitution of chiral groups on either the carbon backbone or the nitrogen atoms of the NHC ring, and the latter into chirality on either the spacer group or substituent on the donor atom. 180 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes In the case of monodentate ligands, chiral information can be provided by the incorporation of chiral units at different positions of the five-membered heterocyclic ring.4"14 For example, introduction of chiral substituents on the nitrogen atoms as shown in the use of chiral imidazolinium precursors 6.1 and 6.2 was examined in the palladium-catalyzed asymmetric oxindole reaction (Scheme 6.1).13 Moderate enantioselectivities were obtained using 6.1 and 6.2 as ligand precursors in the cyclization of a-napthyl-a-methyl amides. Substitution on the carbon backbone of the five-membered NHC ring has also been exploited.15"22 Chiral ruthenium complexes 6.3 and 6.4 were examined in the desymmetrization of triolefins and found to give the ring-closed metathesis products with high enantioselectivities (Scheme 6.2).19 In this example, steric repulsion between the chiral backbone phenyl groups and the ortho-aryl substituents stabilizes a mutual anti configuration that permits efficient transmission of chiral information at the active site of the catalyst. 181 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes Scheme 6.2. Chiral multidentate NHC ligands have also been exploited in asymmetric catalysis. In contrast to monodentate ligands, this class has the benefit that chiral substituents can be introduced at unique positions on the chelating ligand. For example, the introduction of chiral spacer units between the NHC and a pendant donor atom has been reported.23"27 A ruthenium complex employing an anionic bidentate ligand with a chiral 1,1-binaphthyl spacer (6.5) has been investigated in the asymmetric ring opening cross-metathesis (AROM-CM) of olefins (Scheme 6.3).26 Examination of this catalyst in the AROM-CM of tricyclic norbornenes with a terminal monoalkene revealed excellent stereoselective control on the product formed. 182 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes 6.5 Scheme 6.3. Transition metal complexes incorporating chiral oxazoline-NHC ligands have also emerged as promising asymmetric catalyst precursors.28"33 The design of this ligand was inspired by chiral bidentate phosphine-oxazoline complexes, which are highly selective in the enantioselective hydrogenation of alkenes.34 In both the phosphine and NHC examples, the chiral information is provided by a stereodirecting substituent located adjacent to the N-donor on the oxazoline heterocyclic ring. This substituent is positioned in close proximity to the metal centre and can control the space available for substrate coordination. For example, the rhodium complex 6.6 has been shown to be a highly selective catalyst for the asymmetric hydrosilylation of ketones (Scheme 6.4).30 O 1.0% 6.6 OH 6.6 Scheme 6.4. 183 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes The use of chiral NHC derivatives in asymmetric catalysis continues to expand. However, research in this area has been focused on late transition-metal-mediated catalysis. In this chapter, the synthesis of chiral group 4 transition metal [NCN] complexes will be investigated. Several complexes will be examined for their potential to promote asymmetric hydroamination catalysis. 6.2. Synthesis of Group 4 [NCN] Complexes The construction of a chiral [NCN] ligand introduces the possibility to incorporate chiral substituents at discrete positions in the [NCN] framework. Chiral groups could be substituted at: 1) the amide-N-donor; 2) along the ethylene spacer; or 3) at the 4- and 5-positions on the NHC ring. The latter two possibilities introduce stereocentres remote from the metal centre, options that may not effectively transfer stereochemical information during • the catalytic process. Given this possibility, a ligand design incorporating chiral amine groups was investigated, which would introduce chiral information in close proximity to the metal centre. In chapter 3, aryl amido substituted imidazolium precursors were synthesized utilizing a substituted chloroethylamine precursor. Given this result, a similar approach was investigated for the preparation of a chiral [NCN] ligand. A well-known procedure for the synthesis of these derivatives is the reaction of a substituted aminoethanol derivative with S0C12.35 (li?,2'1S,47?)-2-(l,7,7-Trimethylbicyclo[2.2.1]-hept-2-ylideneamino)ethanol 6.7 offers the desired chiral amine component and has been previously prepared.36 Treatment of optically pure 6.7 with SOCl2 in CHCI3 yields 6.8 in near quantitative yield as an air-stable white solid (Equation 6.1). The 'H NMR spectrum of 6.8 is shown in Figure 6.1 and features three distinct methyl resonances, several complicated cyclohexyl methylene resonances, and methylene resonances attributable to a -NC//2CrY2Cl moiety. Two broad diastereotopic -N// resonances are also observed at 8.31 and 9.45 ppm. 184 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes (ppm) Figure 6.1. *H NMR spectrum of (li?,2'5,4i?)-2-(l,7,7-trimethylbicyclo[2.2.1]hept-2-ylamino)ethyl ammonium chloride (6.8) in CDCI3. (* denotes V2 equivalent of CH3C(0)CH3). X-ray quality crystals of 6.8 were grown from a saturated solution of methanol and the molecular solid state structure was determined by an X-ray diffraction study. Selected bond lengths and angles are given in Table 6.1 and crystallographic details are presented in Appendix A. The orientation of the stereocentre on the cyclohexyl ring 185 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes shows there has been no change in configuration as a result of the reaction of 6.7 with SOCb. Furthermore, the presence of a chloride counterion infers that an ammonium moiety is present. Unfortunately, the data collected from X-ray diffraction is quite poor and the results presented here are used to establish the connectivity and orientation of the atoms in the molecule. Figure 6.2. ORTEP view of (-)-(li?,2'5,4i?)-2-(l,7,7-trimethylbicyclo[2.2.1]hept-2-ylamino)ethyl ammonium chloride (6.8) depicted with 50% thermal ellipsoids; all hydrogen atoms have been omitted for clarity. Table 6.1. Selected Bond Distances (A) and Bond Angles (°) for (-)-(li?,2'5,4i?)-2-(1,7,7-trimethylbicyclo[2.2.1 ]hept-2-ylamino)ethyl ammonium chloride, (6.8). Bond Lengths Bond Angles N1-C2 1.453(13) C11-C1-C2 107.6(10) N1-C3 1.571(13) C3-N1-C2 110.1(10) Cll-Cl 1.771(19) Conversion of the ammonium salt 6.8 to the corresponding amine was achieved by the reaction of 6.8 with an excess of K2CO3 (Equation 6.2). This reaction provides the free amine 6.9 in quantitative yield as a colorless oil. The *H NMR spectrum of 6.9, shown in Figure 6.3, reveals the absence of the ammonium resonances at 8.31 and 9.45 186 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes ppm that were observed in 6.8, in addition to ethylene spacer units at 2.88 and 3.63 ppm. The -NH resonance was not located in the *H NMR spectrum. K2C03> Et?0 (6.2) 6.8 6.9 6.9 -CH2C\ -NHCtf, 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 25 2.0 1.5 1.0 OS (ppm) Figure 6.3. 'H NMR spectrum of (li?,2'5,4/?)-2-(l,7,7-trimethylbicyclo[2.2.1]hept-2-ylamino)ethyl chloride (6.9) in CDCI3. (* denotes Et20 impurity). The first approach to the synthesis of a chiral [NCN] imidazolium species was the reaction of two equivalents of 6.9 with imidazole in the presence of NEt3 (Equation 6.3). This reaction produced a CH2CI2 insoluble solid in low yield that was identified as 6.10 by *H and l3C{'H} NMR spectroscopy. The formation of 6.10 was confirmed by the 187 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes presence of an iminium resonance at 9.05 ppm, along with anticipated ethylene spacer, imidazole and cyclohexylamine resonances. The resonances in the 'H NMR spectrum are broad, possibly a result of a fluxional process in the molecule or fast exchange with ^4-CD3OD (Figure 6.4). (ppm) Figure 6.4. 'H NMR spectrum of scam[NCHN]H2-Cl (6.10) in d4-CD3OD (* denotes THF impurity). Intrigued by the low yield of 6.10 in this reaction, the CH2CI2 soluble fraction was examined to ascertain the formation of other products. The *H NMR spectrum of the crude product suggested the synthesis of the neutral chiral 2-aminoethyl imidazole compound 6.11 (Equation 6.3), with imidazole resonances at 7.00, 7.08, and 7.69 ppm. Although the synthesis or purification of this compound was not optimized, the crude CH2Ci2-soluble fraction could be used as a precursor for the synthesis of 6.10. Treatment of the crude chiral 2-aminoethyl imidazole 6.11 with the chiral 2-chloroethylamine 6.9 at 160°C provided the imidazolium chloride 6.10 in excellent yields. 188 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes 6.11 Deprotonation of 6.10 with one equivalent of KN(SiMe3)2 at -30°C resulted in the isolation of a highly soluble yellow oil. The disappearance of the iminium resonance at 9.05 ppm in the *H NMR spectrum supports the presence of an NHC, however, other unidentifiable resonances were present. Over an extended period of time (ca. 2 days), the resonances attributed to the NHC disappear, which suggests decomposition of the NHC ligand. As a result, the synthesis of the NHC ligand was performed in situ and used immediately in further reactions. Given the success with aminolysis reactions in the synthesis of group 4 [NCN] complexes described in chapter 3, a similar approach was investigated for the synthesis of chiral group 4 [NCN] complexes. The reaction of 6.10 with Ti(NMe2)4 in the presence of one equivalent of KN(SiMe3)2 at -30°C yielded 6.12 as a highly soluble orange solid (Equation 6.4). The 'H NMR spectrum of 6.12 is shown in Figure 6.5 and shows 13 1 equivalent ethylene spacer, imidazole, and N{CH3)2 groups. The C{ H} NMR spectrum features a weak resonance at 190.5 ppm indicative of a Ti-Ccarbene bond. Single crystals suitable for an X-ray structure determination have yet to be obtained due to the high solubility of 6.12 in organic solvents. 189 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes Figure 6.5. *H NMR spectrum of scam[NCN]Ti(NMe2)2 (6.12) in C6D6. Alkyl elimination reactions were also successful for the synthesis of chiral group 4 [NCN] complexes. The addition of a solution of 6.10 to Zr(CH2Ph)4 in the presence of one equivalent of KN(SiMe3)2 at -30°C yielded 6.12 as a highly soluble yellow solid (Equation 6.5). The 'H NMR spectrum of 6.12 shows a C2 symmetric species in solution with equivalent ethylene spacer and imidazole resonances. The ZrC//2 groups are observed as a diastereomeric set of doublets and are obscured by the resonances of an 13 1 ethylene spacer group. A ZrCH2 resonance is also observed at 77.2 ppm in the IJC{'H} NMR spectrum. Unfortunately, crystals of 6.13 suitable for X-ray diffraction have yet to be obtained. 190 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes KN(SiMe3)2 Zr(CH2Ph)4 THF -30°C - KCI f=\ N h 1 11 ss 1 Me HN Me Me -HN(SiMe3)2 Me (6.5) 6.10 6.13 6.3. Asymmetric Intramolecular Hydroamination Studies The catalytic formation of organic nitrogen containing molecules is of great academic and industrial interest. One method that has shown success is the hydroamination reaction, in which an amine is added to an olefin in either an intermolecular or intramolecular fashion.37,38 Transition metal complexes have been extensively examined as catalysts for this process with examples reported for many transition metals.39,40 One common goal for the design of these catalysts has been to gain control over the diastereoselectivity and regiochemistry of the hydroamination product. The enantioselective addition of amines to olefins is a logical extension of this chemistry and remains a challenging task. With respect to early transition metals and lanthanides, several examples that display moderate to high enantioselectivities are known. Chiral yttrium binaphtholate complexes and cationic zirconium aminophenolate complexes are known to accomplish this with high enantioselectivity values. 41,42 Chiral lanthanide complexes with salicylaldimine43 and binaphthyl diamine44 ligands have also been reported to catalyze intramolecular hydroamination with moderate enantiomeric excess values. Preliminary experiments in our laboratory with the titanium and zirconium species, 6.12 and 6.13, focused on the intramolecular hydroamination of 2,2-diphenyl-4-pentenylamine (Equation 6.6). The catalytic reaction was initially performed at 110°C in dg-toluene with a 10 mol % precatalyst loading. 'H NMR spectroscopy was used to monitor the disappearance of the olefinic signals in the amine substrate and the presence of diagnostic signals for the pyrrolidine product. A heated (is-toluene solution of 191 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes diphenyl-4-pentenylamine and the titanium precursor 6.12 was monitored by 'H NMR spectroscopy and revealed no conversion to the corresponding heterocycle after 24 hours. H 6.14 [cat]= 6.12 0% yield 0% ee 6.13 >99% yield 4% ee It has been shown that the activities of hydroamination catalysts increase with the increasing ionic radius of the metal centre.39 With this in mind, the zirconium precursor 6.13 was examined under the same conditions used for 6.12 (110°C, dg-toluene). The reaction was followed by 'H NMR spectroscopy, which revealed complete conversion to 2-methyl-4,4-diphenylpyrrolidine 6.14 after heating for 1 hour (Equation 6.6). The enantiomeric excess of each of the two pyrrolidine products was determined by derivatizing the heterocycle 6.14 with (-)-Mosher's acid chloride.42 'H NMR spectroscopic analysis of the diastereomers formed showed that a nominal enantiomeric excess (4%) was achieved during the catalytic process. While catalytic formation of the N-heterocycle 6.14 is encouraging, there are several possible explanations for the low enantioselectivity obtained with the zirconium precatalyst 6.13. This low value may be a result of the ineffective transmission of chiral information by the camphor chiral unit. Alternatively, the presence of the dialkylamido donors in the chiral [NCN] ligand set presents the potential for aminolysis of the Zr-N bond by the amine precursor. Although it is anticipated that the hydroamination reaction with the aminoalkene and 6.14 would proceed to eliminate toluene (Path 1, Figure 6.6), the presence of an excess amount of aminoalkene at high temperatures could react to form a metal derivative with a new M-N amido bond and an amido-amino substituted 192 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes scam[NCNH] ligand array (Path 2, Figure 6.6). As a result of this protonolysis, the transfer of chiral information during the catalytic process may be disrupted. Figure 6.6. Aminolysis of a Zr-N bond in scam[NCN]Zr(CH2Ph)2. 6.4. Conclusions and Future Work In this chapter, the synthesis of a chiral camphor-based [NCN] ligand set was investigated. Coordination of this ligand to titanium and zirconium was accomplished by aminolysis and alkyl elimination reactions, respectively. The complexes 6.12 and 6.13 were investigated in the asymmetric intramolecular hydroamination of an aminoalkene in an attempt to promote selectivity in the N-heterocycle synthesized. While the titanium complex.6.12 showed no activity, the zirconium complex 6.13 was an efficient catalyst 193 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes for the intramolecular formation of a substituted pyrollidine. Examination of the steroselectivity in the N-heterocyclic product revealed nominal enantioselective excess. Chiral group 4 metallocene complexes have attracted a great deal of attention as precursors for stereoregular a-olefin polymerization. It is well established that the tacticities of synthesized polyolefins are highly dependent on the structure of the precatalyst. When activated, achiral complexes such as Cp2ZrCl2 produce atactic polymers, whereas C2-symmetric and Cs-symmetric chiral complexes produce isotactic and syndiotactic polymers, respectively.45 Given the potential C2 symmetry of 6.13, the ability of activated 6.13 to form isotactic polypropylene would be of interest. This chapter has demonstrated that the chiral dianionic, tridentate [NCN] ligand is well-suited for the stabilization of titanium and zirconium complexes. An analogous bidentate chiral amido-NHC ligand would be of great interest, in particular for late transition-metal-mediated asymmetric catalysis. The synthesis of a chiral amino-imidazolium chloride precursor has been investigated. Thermolysis of 1-mesitylimidazole and 6.9 at 160°C for 1 hour produces the desired imidazolium compound 6.15 in near quantitative yield (Equation 6.7). The *H NMR spectrum (Figure 6.7) shows an iminium resonance at 9.48 ppm in addition to signals for the cyclohexylamine, aryl, and ethylene spacer groups. 194 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes (6.7) 6.16 (ppm) Figure 6.7. *H NMR spectrum of scam[NCH]H-Cl (6.15) in d6-DMSO (* denotes contamination with H2O and § denotes a trace amount of Et20). 195 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes Deprotonation of 6.14 with one equivalent of KN(SiMe3)2 proceeds immediately at room temperature to give the chiral NHC 6.16 in near quantitative yield (Equation 6.7). The *H NMR spectrum shown in Figure 6.8 features a loss of the iminium resonance at 9.50 ppm, which is diagnostic for NHC formation. Unfortunately, the 13C resonance expected for the divalent carbon atom of the NHC ligand was not observed in the 13C{'H} NMR spectrum. -aryl// -imid// -ary!C//3 1 -NC//2 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.U 4.5 4.0 3.5 3.0 2.5 2.0 1.5 l.C (ppm) Figure 6.8. 'H NMR spectrum of scam[NC]H (6.16) in C6D6. The coordination of the chiral bidentate NHC 6.16 remains unexplored and could yield a number of potentially chiral metal complexes. Enantioselective catalysis involving these ligands, in particular with late transition metals, offers a vast potential, which will be explored in the near future. 196 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes 6.5. Experimental Section 6.5.1. General Considerations Unless otherwise stated, general procedures were performed as described in Section 2.5.1. 6.5.2. Materials and Reagents All chemicals were purchased from a chemical supplier and used as received. (-)-(l^,2',S,4i?)-2-(l,7,7-trimethylbicyclo[2.2.1]-hept-2-ylideneamino)ethanol (6.7) was prepared by a literature method. 6.5.3. Synthesis and Characterization of Complexes 6.8 - 6.13, 6.14 - 6.15 Synthesis of (^-(lif^'.S'^^^^l^J-trimethylbicyclo^^.ll-hept^-ylideneamino)-ethyl ammonium chloride (6.8) To a stirred solution of 6.7 (3.25 g, 16.5 mmol) in 50 mL CHC13 in a 100 mL Schlenk flask was added SOCl2 (5.89 g, 49.5 mmol) dropwise. The orange solution was carefully heated at 80°C for 4 hours, cooled to room temperature, and the solvent removed in vacuo to yield a brown powder. This solid was triturated with acetone to yield a white crystalline solid that was recovered by filtration and recrystallized from MeOH. Yield = 3.87 g, 93%. 'H NMR (CDC13): 5 0.88 (s, 3H, -C//3), 1.09 (s, 3H, -CH3), 1.15 (m, 2H, -camphorC//2), 1.25 (s, 3H, -CH3), 1.63-1.97 (m, 4H, -camphorC//2), 2.22 (m, 1H, - camphorC/7), 3.13 (m, 1H, - camphorC//), 3.42 (m, 2H, -NHC//2), 4.12 (m, 2H, -C//2C1), 8.31 (br s, 1H, -N//H), 9.45 (br s, 1H, -N//H). '^{'H} NMR (CDCI3): 6 12.8 (-CH3), 20.0 (-CH3), 20.8 (-CH3). 26.7 (-CH2), 34.9 (-CH2), 36.7 (-CH2), 38.8 (-CH), 44.8 (-CH2C1),.47.5.(-CHCH3), 49.1 (-C(CH3)2), 49.5 (-CH2N), 67.2 (-CHN). [<x]D= -67.0° (c 0.01; MeOH). 197 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes Anal Calcd. for C12H23C12N: C, 57.14; H, 9.19; N, 5.55. Found: C, 57.52; H, 9.06; N, 5.32. Synthesis of (li?,2'5,4^)-2-(l,7,7-trimethylbicyclo[2.2.1]-hept-2-ylideneamino)-ethyl chloride (6.9) A 250 mL Schlenk flask was charged with 6.8 (3.85 g, 14.2 mmol), 100 mL distilled H20, and 100 mL Et20. An aqueous solution (10 mL) of K2C03 (2.16 g, 15.6 mmol) was slowly added and the biphasic solution stirred for 1 hour. The Et20 layer was removed and the aqueous layer washed with 2 portions of Et20 (50 mL). The ethereal solutions were combined, dried with MgSO-4, and filtered. Upon removal of the solid in vacuo a colourless oil was recovered. Yield = 3.06 g, 100%. 'H NMR (CDC13): S 0.81 (s, 3H, -CH3), 0.92 (s, 3H, -C//3), 1.03 (s, 3H, -CH3), 1.17 (m, 2H, -camphorC//2), 1.48-1.71 (m, 5H, -camphorC//2 and -camphorC//), 2.54 (m, 1H, -camphorC//), 2.91 (m, 2H, -NHC//2), 3.63 (m, 2H, -CH2C\). "Cf/H} NMR (CDCI3): 8 12.3 (-CH3), 20.6 (-CH3), 20.7 (-CH3), 27.4 (-CH2), 37.0 (-CH2), 38.7 (-CH2), 44.9 (-CH2C1), 45.3 (-CH), 46.9 (-CHCH3), 48.7 (-C(CH3)2), 49.8 (-CH2N), 66.2 (-CHN). Synthesis of (-)-scam[NCHN]H2-Cl (6.10) (scam = (l/?,2'S,4#)-2-(l,7,7-trimethylbicyclo-[2.2.1]-hept-2-ylideneamino) A 500 mL Schlenk flask was charged with 6.9 (1.12 g, 5.2 mmol), imidazole (177 mg, 2.6 mmol), NEt3 (0.37 mL, 5.3 mmol), and 250 mL of p-dioxane. The slurry was heated to 120°C and stirred overnight. Upon cooling to room temperature, the solvent was removed in vacuo and 50 mL CH2C12 added. Filtration of this solution yielded a white solid, which was washed several times with CH2C12. The solid was dried in vacuo overnight as it is mildly hygroscopic. Yield = 252 mg, 21%). JH NMR (CD3OD): 8 0.83 (s, 6H, -CH3), 0.90 (s, 6H, -CH3), 0.93 (s, 6H, -CH3), 1.11 (m, 4H, -camphorC/^), 1.56-1.72 (m, 10H, -camphorC//2 and -camphorC//), 2.71 (br m, 2H, - camphorC//), 3.06 (br m, 4H, -NC//2), 4.35 (br m, 4H, -NC//2), 7.68 (s, 2H, -imid//), 9.05 (br s, 1H, -NC//N). 198 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes ,3C{'H} NMR (CD3OD): 5 14.7 (-CH3), 21.2 (-CH3), 21.9 (-CH3), 29.1 (-CH2), 38.3 (-CH2), 38.9 (-CH2), 46.2 (-CH), 47.7 (-CH), 48.3 (-CH2N), 49.0 (-CHCH3), 51.3 (-CH2N), 67.9 (-C(CH3)2), 126.4 (-imidC), 138.6 (-NCHN). [a]D = -83.5° (c. 0.01; MeOH). Anal Calcd. for C27H46CIN4: C, 70.17; H, 10.03; N, 12.12. Found: C, 70.02; H, 9.68; N, 12.35. Synthesis of (lif,2'5',4i?)-2-(l,7,7-trimethylbicyclo[2.2.1]-hept-2-ylideneamino)ethyl imidazole (6.11) The CH2C12 filtrate that was obtained in the synthesis of 6.10 was recovered and the solvent removed in vacuo to yield a dark oil. *H analysis of this residue revealed the formation of 6.11 in about -85% purity. lH NMR (CDCI3): 5 0.73 (s, 3H, -CH3), 0.86 (s, 3H, -C//3), 0.94 (s, 3H, -CH3), 1.05 (m, 2H, -camphorC//2), 1.48-1.74 (m, 5H, -camphorC//2 and -camphorC//), 2.52 (m, 1H, -camphorC//), 2.95 (m, 1H, - camphorC//), 2.54 (m, 2H, -NcamC//2), 4.14 (m, 2H, -Nimid C/72), 7.00 (s, 1H, -imid//), 7.08 (s, 1H, -imid//), 7.69 (s, 1H, -NC//N). Alternative synthesis of (-)-scam[NCHN]H2-Cl (6.10) A 25 mL Schlenk flask was charged with 6.9 (262 mg, 1.2 mmol) and crude 6.11 (300 mg, 1.2 mmol) and slowly heated to 160°C for 1 hour. The reaction mixture was cooled to room temperature and THF added to give a white solid (6.10) which was collected by filtration and washed with several portions of THF. Yield = 505 mg, 91%. Synthesis of scam[NCN]Ti(NMe2)2 (6.12) A suspension of 6.10 (296 mg, 0.64 mmol) in THF (5 mL) was cooled to -30°C in a 50 mL Erlenmeyer flask and a THF (5 mL) solution of KN(SiMe3)2 (127 mg, 0.64 mmol) was slowly added dropwise. The solution was left to stand at -30°C for 15 minutes without stirring. A THF (5 mL) solution of Ti(NMe2)4 (143 mg, 0.64 mmol) was slowly added dropwise and the entire mixture slowly warmed to room temperature and stirred overnight. The solvent was removed and the yellow residue extracted with hexane 199 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes (2 xlO mL). The solvent was removed in vacuo and HMDSO added to yield a bright orange powder. Yield = 143 mg, 40%. 'H NMR (C6D6): 8 0.82 (s, 6H, -CH3), 0.92 (s, 6H, -CH3), 1.03 (m, 4H, -camphorC//2), 1.18 (s, 6H, -CH3), 1.41-1.70 (m, 10H, -camphorC// and camphorC//2), 2.45 (m, 2H, -camphorC//), 2.70-2.91 (m, 4H, -NC//2), 3.32 (s, 12H, -N(C//3)2, 3.85 (m, 4H, -NC//2), 6.52 (s, 2H, -imid//) l3C{ lH) NMR (C6D6): 8 13.2 (-CH3), 20.6 (-CH3), 21.7 (-CH3), 28.5 (-CH2), 36.9 (-CH2), 37.3 (-CH2), 45.1 (-CH), 45.9 (-NCH3), 47.8 (-CH), 47.9 (-CHCH3), 48.2 (-NCH2), 51.0 (-NCH2), 66.8 (-C(CH3)2), 120.4 (-imidQ, 190.5 (-TiCcarbene). Satisfactory elemental analysis has yet to be obtained. Synthesis of scam[NCN]Zr(CH2Ph)2 (6.13) A suspension of 6.10 (286 mg, 0.62 mmol) in THF (5 mL) was cooled to -30°C in a 50 mL Erlenmeyer flask and a THF (5 mL) solution of KN(SiMe3)2 (136 mg, 0.62 mmol) was slowly added dropwise. The solution was left to stand at -30°C for 15 minutes without stirring. A THF (5 mL) solution of Zr(CH2Ph)4 (282 mg, 0.62 mmol) was slowly added dropwise and the entire mixture slowly warmed to room temperature and stirred overnight. The solvent was removed and the yellow residue extracted with hexane (2 xlO mL). The solvent was removed in vacuo and HMDSO added to yield a bright yellow powder. Yield = 295 mg, 68%. 'H NMR (C6D6): 8 0.86 (s, 6H, -C//3), 0.96 (s, 6H, -C//3), 1.11 (m, 4H, -camphorC//2), 1.16 (s, 6H, -C//3), 1.50-1.71 (m, 8H, -camphorC// and camphorC//2), 2.20 (m, 2H, -camphorC//), 2.26-2.51 (m, 10H, - camphorC//, ZrC//2, and NC//2), 3.11 (m, 4H, -NC//2), 6.90 (s, 2H, -imid//), 6.95-7.01 (m, 6H, -Ar//), 7.19 (t, J = 8 Hz, 4H, -Ar//) 13C{ lH} NMR (C6D6): 8 15.2 (-CH3), 20.0 (-CH3), 20.9 (-CH3), 27.7 (-CH2), 37.4 (-CH2), 38.2 (-CH2), 44.9 (-CH), 45.1 (-CH), 46.3 (-CHCH3), 49.3 (-NCH2), 51.8 (-NCH2), 66.2 (-C(CH3)2), 77.3 (-ZrCH2), 118.5 (-ArQ, 119.7 (-imidQ, 128.6 (-ArQ, 130.1 (-ArQ, 148.2 (-ArQ, 193.0 (-ZrCcarbene). Anal Calcd. for C4iH58N4Zr: C, 70.53; H, 8.37; N, 8.03. Found: C, 70.22; H, 8.56; N, 8.16. 200 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes Synthesis of (-)-scam[NCH]H-Cl (6.15) A 25 mL Schlenk flask was charged with 6.9 (1.05 g, 4.9 mmol) and 1-mesitylimidazole (0.906 mg, 4.9 mmol) then slowly heated to 160°C for 1 hour. The reaction mixture was cooled to room temperature and THF added to give a white solid (6.15) which was collected by filtration and washed with several portions of THF. Yield = 1.87 g, 95%. *H NMR (rf6-DMSO): 8'0.72 (s, 6H, -CH3), 0.75 (s, 6H, -CH3), 0.82 (s, 6H, -CH3), 0.99 (m, 4H, -camphorC//), 1.30-1.68 (m, 4H, -camphorC// and camphorCrY2), 2.00 (s, 3H, -o-ArCH3), 2.32 (s, 3H, -/>ArCH3), 2.48 (m, 1H, - camphorC//), 2.87 (m, 2H, -NC//2), 4.35 (m, 2H, -NC//2), 7.13 (s, 2H, -ArH), 7.90 (br s, 1H, -imid//), 8.07 (br s, 1H, -imid//), 9.50. nC{lH} NMR (<4-DMSO): 8 13.0 (-CH3), 18.6 (-CH3), 22.6 (-CH3), 28.2 (-CH2), 35.6 (-CH2), 38.3 (-CH2), 45.1 (-CH), 47.0 (-CH), 48.1 (-CHCH3), 48.9 (-NCH2), 51.9 (-NCH2), 67.1 (-C(CH3)2), 118.0 (-imidQ, 119.1 (-imidQ, 127.8 (-ArQ, 130.2 (-ArQ, 131.2 (-ArQ, 139.4 (-ArQ, 142.1 (-NCHN). [<x]D = -41.3° (c 0.01; MeOH). Satisfactory elemental analysis has yet to be obtained. Synthesis of scam[NC]H (6.16) A THF solution (5 mL) of KN(SiMe3)2 (390 mg, 2.0 mmol) was slowly added dropwise to 6.15 (786 mg, 2.0 mmol) dissolved in 5 mL THF, creating a slightly yellow suspension. The suspension was stirred for Vi hr and the solvent removed in vacuo. The pale yellow residue was extracted with toluene (20 mL) and the solution filtered through celite. Removal of the solvent yielded a white solid which was washed several times with hexane and dried in vacuo. Yield = 702 mg, 96%. 'H NMR (C6D6): 8 0.89 (s, 6H, -CH3), 1.00 (s, 6H, -CH3), 1.12 (m, 2H, -camphorC//2), 1.29 (s, 6H, -CH3), 1.49.-1.92 (m, 4H, -camphorC//and camphorC//2), 2.18 (s, 6H, -o-ArCH3), 2.21 (s, 3H, -/>ArCH3), 2.60 (m, 1H, - camphorC//), 2.91 (m, 2H, -NC//2), 4.09 (m, 2H, -NC//2), 6.46 (br s, 1H, -imid//), 6.72 (br s, 1H, -imid//), 6.85 (s, 2H, -ArH). 13C{'H} NMR (C6D6): 8 12.2 (-CH3), 17.9 (-CH3), 20.7 (-CH3), 27.6 (-CH2), 36.9 (-CH2), 38.8 (-CH2), 45.5 (-CH), 46.7 (-CH), 48.4 (-CHCH3), 49.6 (-NCH2), 51.0 (-NCH2), 201 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes 66.7 (-C(CH3)2), 119.3 (-imidQ, 120.0 (-imidQ, 128.8 (-ArQ, 135.1 (-ArQ, 136.9 (-ArQ, 138.9 (-ArQ. Satisfactory elemental analysis has yet to be obtained. Procedure for Intramolecular Hydroamination A J. Young's tube was charged with a ferrocene internal standard sealed in a glass capillary tube, catalyst (0.025 mmol), and the amino alkene (0.5 mmol) and dissolved in c/5-toluene (~ 1 ml). The NMR tube was heated to 110°C and the progress of the reaction was monitored by *H NMR spectroscopy at regular intervals. The NMR yields were determined by comparing the integration of the internal standard with a well-resolved signal for the heterocyclic product. The 'H NMR spectrum of 2-methyl-4,4-diphenylpyrrolidine recovered during 6.13-mediated hydroamination is identical to reported values.47 Enantioselective Excess Determination42 The NMR solution was transferred to a 50 mL Erlenmeyer flask and the tube rinsed with several portions of CHCI3. To this solution was added (-)-Mosher chloride and ~ 1 mL triethylamine. The solution was stirred for 15 minutes and solvent removed in vacuo. The residue was dissolved in CDCI3 and the enantiomeric excess was determined by 'H NMR spectroscopy. 202 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes 6.7. References (1) Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H.; Editors Comprehensive Asymmetric Catalysis I-III, Volume J; Springer: Berlin, Germany, 1999. (2) Cesar, V.; Bellemin-Laponnaz, S.; Gade, L. H. Chem. Soc. Rev. 2004, 33, 619. (3) Roland, S.; Mangeney, P. Top. Organomet. Chem. 2005,15, 191. (4) Herrmann, W. A.; Goossen, L. J.; Koecher, C; Artus, G. R. J. Angew. Chem., Int. Ed. Engl. 1997, 35, 2805. (5) Enders, D.; Gielen, H. J. Organomet. Chem. 2001, 617-618, 70. (6) Enders, D.; Gielen, H.; Breuer, K. Tetrahedron: Asymmetry 1997, 8, 3571. (7) Enders, D.; Breuer, K.; Runsink, J.; Teles, J. H. Helv. Chim. Acta 1996, 79, 1899. (8) Enders, D.; Breuer, K.; Teles, J. H. Helv. Chim. Acta 1996, 79, 1217. (9) Knight, R. L.; Leeper, F. J. J. Chem. Soc, Perkin Trans. 1 1998, 1891. (10) Kerr, M. S.; Rovis, T. Synlett 2003, 1934. (11) Kerr, M. S.; Read de Alaniz, J.; Rovis, T. J. Am. Chem. Soc. 2002, 124, 10298. (12) Culkin, D. A.; Hartwig, J. F. Acc Chem. Res. 2003, 36, 234. (13) Lee, S.; Hartwig, J. F. J. Org. Chem. 2001, 66, 3402. (14) Seo, H.; Kim, B. Y.; Lee, J. H.; Park, H.-J.; Son, S. U.; Chung, Y. K. Organometallics 2003, 22, 4783. (15) Fraser, P. K.; Woodward, S. Tetrahedron Lett. 2001, 42, 21Al. (16) Alexakis, A.; Benhaim, C; Rosset, S.; Humam, M. J. Am. Chem. Soc. 2002,124, 5262. (17) Feringa, B. L. Acc Chem. Res. 2000, 33, 346. (18) Masamune, S.; Choy, W.; Petersen, J. S.; Sita, L. R. Angew. Chem., Int. Ed. 1985, 97, 1. (19) Seiders, T. J.; Ward, D. W.; Grubbs, R. H. Org. Lett. 2001, 3, 3225. (20) Jensen, D. R.; Sigman, M. S. Org. Lett. 2003, 5, 63. (21) Bappert, E.; Helmchen, G. Synlett 2004, 1789. (22) Chianese, A. R.; Li, X.; Janzen, M. C; Faller, J. W.; Crabtree, R. H. Organometallics 2003, 22, 1663. 203 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes (23) Clyne, D. S.; Jin, J.; Genest, E.; Gallucci, J. C.; RajanBabu, T. V. Org. Lett. 2000, 2, 1125. (24) Albrecht, M.; Crabtree, R. H.; Mata, J.; Peris, E. Chem. Commun. 2002, 32. (25) Van Veldhuizen, J. J.; Gillingham, D. G.; Garber, S. B.; Kataoka, O.; Hoveyda, A. H. J. Am. Chem. Soc. 2003, 125, 12502. (26) Van Veldhuizen, J. J.; Garber, S. B.; Kingsbury, J. S.; Hoveyda, A. H. J. Am. Chem. Soc. 2002, 124, 4954. (27) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2000.122, 8168. (28) Perry, M. C; Cui, X.; Powell, M. T.; Hou, D.-R.; Reibenspies, J. H.; Burgess, K. J. Am. Chem. Soc. 2003, 125, 113. (29) Powell, M. T.; Hou, D.-R.; Perry, M. C; Cui, X.; Burgess, K. J. Am. Chem. Soc. 2001.123, 8878. (30) Gade, L. H.; Cesar, V.; Bellemin-Laponnaz, S. Angew. Chem. Int. Ed. 2004, 43, 1014. (31) Glorius, F.; Altenhoff, G.; Goddard, R.; Lehmann, C. Chem. Commun. 2002, 2704. (32) Bolm, C; Focken, T.; Raabe, G. Tetrahedron: Asymmetry 2003, 14, 1733. (33) Enders, D.; Kallfass, U. Angew. Chem. Int. Ed. 2002, 41, 1743. (34) Pfaltz, A.; Blankenstein, J.; Hilgraf, R.; Hormann, E.; Mclntyre, S.; Menges, F.; Schonleber, M.; Smidt, S. P.; Wustenberg, B.; Zimmermann, N. Adv. Synth. Catal. 2003, 345, 33. (35) Foster, P.; Chien, J. C. W.; Rausch, M. D. J. Organomet. Chem. 1997, 545-546, • 35. (36) Squire, M. D.; Burwell, A.; Ferrence, G. M.; Hitchcock, S. R. Tetrahedron: Asymmetry 2002, 13, 1849. (37) Pohlki, F.; Doye, S. Chem. Soc. Rev. 2003, 32, 104. (38) Mueller, T. E.; Beller, M. Chem. Rev. 1998, 98, 675. (39) Roesky, P. W.; Mueller, T. E. Angew. Chem. Int. Ed. 2003, 42, 2708. (40) Hultzsch, K. C. Adv. Synth. Catal. 2005, 347, 367. (41) Gribkov, D. V.; Hultzsch, K. C. Chem. Commun. 2004, 730. 204 References begin on page 203. Chapter Six: Synthesis and Applications of Chiral Group 4 [NCN] Complexes (42) Knight, P. D.; Munslow, I.; O'Shaughnessy, P. N.; Scott, P. Chem. Commun. 2004, 894. (43) O'Shaughnessy, P. N.; Knight, P. D.; Morton, C; Gillespie, K. M.; Scott, P. Chem. Commun. 2003, 1770. (44) Collin, J.; Daran, J.-C; Schulz, E.; Trifonov, A. Chem. Commun. 2003, 3048. (45) Nakayama, Y.; Shiono, T. Molecules 2005,10, 620. (46) Danopoulos, A. A.; Wright, J. A.; Motherwell, W. B. Chem. Commun. 2005, 784. (47) Bexrud, J. A.; Beard, J. D.; Leitch, D. C; Schafer, L. L. Org. Lett. 2005, 7, 1959. 205 References begin on page 203. Chapter Seven Thesis Summary and Future Work This thesis has investigated the chemistry of two different ligand sets on early transition metals. In chapter 2, the reactivity of a tantalum [NPN] dinitrogen complex with several transition metal hydrides was investigated. The addition of Schwartz's reagent, ([Cp2Zr(Cl)H]x), led to the unanticipated reduction of the N-N unit without Zr-H addition. Examination of the product revealed the formation of a phosphinimide derivative with insertion of a "Cp2Zr" fragment into the N-N bond. A series of experiments determined that the origin of the "Cp2Zr" was from the reductive elimination of H2 from [Cp2ZrH2]2- An independent reaction with a "Cp2Zr" source verified that this species induced N-N bond cleavage. This type of dinitrogen reduction was extended to include the insertion of a "Cp2Ti" fragment into the N-N bond and represents a new approach in dinitrogen chemistry to cleave an N2 ligand. Early transition metal chemistry with a unique diamido-N-heterocyclic carbene ligand set was pursued in chapter 3. Ligands with an ethylene spacer between the N-heterocyclic ring and the amido donor were successfully synthesized. Aminolysis and alkyl elimination reactions with group 4 transition metal precursors afforded a successful way to synthesize group 4 [NCN] complexes. The central position of the NHC in this tridentate architecture renders the carbene stable to dissociation from the metal centre in strongly coordinating solvents. 206 The application and reactivity of group 4 [NCN] complexes was the focus of chapter 4. Fundamental processes such as olefin polymerization, migratory insertion, and dinitrogen activation were examined and revealed the NHC moiety remains coordinated to the metal centre and does not participate in a manner that would alter the NHC donor. Activation of a zirconium-dimethyl derivative with [Ph3C][B(CeF5)4] in the presence of ethylene yielded a moderately active polymerization catalyst. Migratory insertion of simple organic molecules, such as isocyanides, carbon monoxide and cumulenes, into the hafnium-sp -carbon bond of several hafnium-alkyl derivatives yielded the expected insertion products. In some examples, further C-C bond coupling was observed to generate new eneamidolate and enediolate metallacycles. Despite many attempts, no early transition metal dinitrogen complexes were recovered utilizing the [NCN] ligand set. In chapter 5, the coordination of the [NCN] ligand set to tantalum(V) was examined in anticipation that a dinitrogen complex could be synthesized. While tantalum-amide and -halide complexes were isolated with an intact [NCN] architecture, reduction of these complexes in the presence of dinitrogen resulted in a complicated mixture of products. Attempts to synthesize tantalum alkyl complexes resulted in the isolation of compounds where the ligand has undergone C-H bond activation at one of the backbone positions. Density functional calculations and isotopic labeling studies examined the mechanism of this C-H bond activation process and suggested the intermediacy of an alkylidene species. The research described in this thesis has provided insight into the use of two different ligand sets in different areas of early transition metal chemistry. Future research with [NPN] chemistry will be aimed at the addition of other reduced transition metal and main group complexes to 2.5, in an attempt to extend the functionalization of coordinated nitrogen atoms with other elements. In light of the inability to obtain an ETM dinitrogen complex stabilized by a [NCN] ancillary ligand, other transition metals may assist in accomplishing this elusive goal. Further research utilizing the chiral tridentate [NCN] and bidentate [NC] ligands may also yield a highly active and selective ETM or LTM catalyst. In conclusion, the research presented herein has provided solid groundwork for future research in both [NPN] and [NCN] chemistry. 207 Appendix A: X-ray Crystal Structure Data Appendix A X-ray Crystal Structure Data A.l. General Considerations In all cases, suitable crystals were selected and mounted on a glass fibre using Paratone-N oil and frozen to -100°C. Measurements for structures 2.10, 3.5, 3.17, 3.22, 3.24C5H5N, 3.30, 4.22, 5.1 and 5.2 were made on a Rigaku/ADSC CCD area detector with graphite monochromated Mo-Ka radiation by either Dr. Brian O. Patrick or Dr. Christopher Carmichael. Data was processed using the d*TREK' module, part of the CrystalClear software package, version 1.3.6 SPO,2 and corrected for Lorentz and polarization effects and absorption. Neutral atom scattering factors for all non-hydrogen atoms were taken from Cromer and Waber.3 Anomalous dispersion effects were included in Fca|C.4 Measurements for structure 6.8 were made on a Bruker X8 area detector with graphite monochromated Mo-Ka radiation by Howard Jong. Data was determined to be a two component twin using the Twinsolve module of the CrystalClear software package, version 1.3.6 SPO.2 Measurements for structures 3.8, 3.38, 4.7, 4.8, 4.11, 4.16, 4.17, 4.18, 4.23, 5.3, 5.7, and 5.10 were made on a Bruker X8 area detector with graphite monochromated Mo-Ka radiation by either Brian O. Patrick or Howard Jong. Data was 208 References located on page 210. Appendix A: X-ray Crystal Structure Data processed and integrated using the Bruker SAINT software package5 and corrected for absorption effects using the multi-scan technique (SADABS).6 Neutral atom scattering factors for all non-hydrogen atoms were taken from Cromer and Waber.3 Anomalous dispersion effects were included in Fcaic;4 the values for A/" and Af"' were those of Creagh and McAuley.7 The values for the mass attenuation coefficients are those of Creagh and Hubbell.8 All structures were solved by direct methods using the program SIR97.9 All non-hydrogen atoms were refined anisotropically by least square procedures on F2 using SHELXL-97.10 Hydrogen atoms were included but not refined; their positional parameters were calculated with fixed C-H bond distances of 0.99 A for sp2 C, 0.98 A for sp3 C, and 0.95 A for aromatic sp C, with U\so set to 1.2 times the Ueq of the attached sp 2 3 or sp C and 1.5 times the Ueq values of the attached sp C atom. Methyl hydrogen torsion angles were determined by electron density. Structure solution and refinements were conducted using the WinGX software package, version 1.64.05." Structural illustrations were created using ORTEP-III for Windows.12 209 References located on page 210. Appendix A: X-ray Crystal Structure Data A.2. References 1) Pflugrath, J. W. Acta Cryst. 1999, D55, 1718. 2) CrystalClear: An Integrated Program for the Collection and Processing of Area-Detector Data, Rigaku Corporation, 2002. 3) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystallography; Kynoch Press, 1974; Vol. IV. 4) Ibers, J. A.; Hamilton, W. C. Acta Cryst. 1964, 77, 781. 5) SAINT Software User Guide, Version 7.03A, Bruker Analytical X-ray Systems, Inc., Madison, WI, 1997. 6) Sheldrick, G. M. SADABS, Version 2.05, Bruker Analytical X-ray Systems, Inc., Madison, WI, 2003. 7) Creagh, D. C; McAuley, W. J. In Internation Tables for Crystallography; Wilson, A. J. C, Ed.; Kluwer Academic Publishers; Boston, 1992; Vol. C, pp 219-222. 8) Creagh, D. C; Hubbell, J. H. In Internation Tables for Crystallography; Wilson, A. J. C, Ed.; Kluwer Academic Publishers; Boston, 1992; Vol. C, pp 200-206. 9) Altomare, A.; Burla, M. C; Cammali, G.; Cascarano, M.; Giacovazzo, C; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, A. J. Appl. Crystallogr. 1999, 32, 115. 10) Sheldrick, G. M. SHELXL-97: Programs for Crystal Structure Analysis (Release 97-2), University of Gottingen, Gottingen, Germany, 1998. 11) Farrugia, L. J. J. Appl. Cystallogr. 1999, 32, 837. 12) Farrugia, L. J. J. Appl. Cystallogr. 1997, 30, 565. 210 References located on page 210. Appendix A: X-ray Crystal Structure Data A.3. Tables of Crystallographic Data Table A.l. Crystallographic and structure refinement for [N(LA-P=N)N]Ta(|a-H)2((x-N(ZrCp2))-Ta[NPN] (2.10), (3.8). Mes (NCHN)H2-C1 (3.5), and Mes(NCN)H2 [NP(N)N]Ta(u-H)2(u-N)(Ta[NPN]) (ZrCp2) (2.10) Mes(NCHN)H2-Cl (3.5) Mes(NCN)H2 (3.8) Formula C58H74N6P2Si4 C25H35C1N4 C25H34N4 Ta2Zr Formula 1482.65 427.02 390.56 weight Colour, Habit Purple, plate Colourless, plate Colourless, prism Crystal size, 0.4x0.2x0.1 0.25x0.15x0.05 0.40 x 0.40 x 0.20 mm Crystal system Orthorhombic Monoclinic Orthorhombic Space group Pccn P2fc P22,2, a, A 13.4310(10) 7.5827(13) 8.8133(2)A b,A 38.984(3) 25.577(4) 9.2024(2) c,A 28.4871(19) 12.453(2) 14.0787(3) a, deg 90 90 90 P, deg 90 91.802(10) 90 y, deg 90 90 90 v,A? 14915.6(18) 2414.0(7) 1141.83(4) z 8 • 4 2 Pcalc, g Cm"3 1.320 1.175 1.136 Fooo 5888 920 424 p, (MoKa), 3.205 0.177 0.068 cm"1 transmission 0.7238-1.0000 0.7870-1.0000 0.8403-1.0000 factors 20max, deg 55.76 54.98 55.72 total no. of 93218 19828 13829 reflns no. of unique 17760 9450 4303 reflns Rint 0.563 0.0820 0.0306 no. of 666 289 139 variables R. (F2,I> 0.0766 0.0510 0.0370 2a(I)) Rw(F2,all 0.0889 0.1056 0.0463 data) Gof 1.221 0.979 0.969 2,2x1/2 Ri = H|Fo| - 1FC11 / S|FQ|; Rw = (Sw( F02| - FC 2\Y / Sw F02 2) 211 References located on page 210. Appendix A: X-ray Crystal Structure Data Table A.2. Crystallographic and structure refinement for [NCN]Zr(NEt2)2 (3.17), Mes[NCNH]Ti(NMe2)3 (3.22), and tol[NCN]ZrCl2(py) (3.30). tol[NCN]Zr(NEt2)2 Mes[NCNH]Ti(NMe2)3 tol[NCN]ZrCl2(py) (3.17) (3.22) (3.30) Formula C2gH44N6Zr C3,H51N7Ti C29H29Cl2N5Zr Formula 567.92 569.69 609.69 weight Colour, Habit Colourless, platelet Red, prism Red, prism Crystal size, 0.40x0.20x 0.05 0.40 x 0.40 x 0.20 0.05 x 0.04x0.03 mm Crystal system Monoclinic Monoclinic Monoclinic Space group P2Jn P2\lc P2xln a, A 9.8762(14) 13.1845(12) 8.2213(8) b,A 12.3743(19) 18.7121(17) 31.764(3) c,A 24.325(4) 13.2384(11) 10.8884(11) a, deg 90 90 90 P, deg 93.812(9) 98.130(4) 91.757(3) y, deg 90 90 90 v,A? 2963(37) 3233.2(5) 2842.0(5) z 4 4 4 pcalc, g Cm"3 1.273 1.170 1.425 Fooo 1200 1232 1248 p, (MoKa), 0.398 0.295 0.602 cm"1 transmission 0.8765-1.0000 0.8516-1.0000 0.8419-1.0000 factors 20max, deg 55.00 55.12 55.76 total no. of 24721 26981 25486 reflns no. of unique 6345 9771 6283 reflns Rint 0.0871 0.0826 0.0793 no. of 151 368 336 variables R. (F2,I> 0.0743 0.0384 0.0751 2c(I)) Rw (F2, all 0.1168 0.0584 0.1036 data) Gof 1.034 0.958 1.088 Rt = SlJFoi - [Fell / £|F0|; Rw = (£w(|F02[ - jFc2 f I £wlF0f )l/2 212 References located on page 210. Appendix A: X-ray Crystal Structure Data Table A.3. Crystallographic and structure refinement for tol[NCN]Zr(CH2SiMe3)2 (3.30), (4.7). Mesr [NCN]HfBu2 (3.38), and Mes^CN]Hf(nz-XyNCCH3)(CH3) '[NCN]Zr Mes [NCN]HfBu2 (3.38) [NCN]Hf(n (CH2SiMe3)2 (3.30) XyNCCH3)(CH3) (4.7) Formula C29H46N4Si2Zr C33H5oHfN4-l/2C4H80 C36H47HfN5-C4H80 Formula 598.10 726.33 800.39 weight Colour, Habit Yellow, block colourless, plate colourless, plate Crystal size, 0.25 x 0.25 x 0.2 0.15x0.15x0.15 0.40x0.30x 0.10 mm Crystal Monoclinic Monoclinic Monoclinic system Space group PlxIc C2/c C2/c a, A 13.105(3) 19.9312(3) 32.883(5) b,A 12.676(3) 13.7149(3) 11.614(5) c,A 19.448(4) 28.0013(6) 23.793(5) a, deg 90 90 90 P, deg 99.398(2) 113.0590(10) 112.217(5) y, deg 90 90 90 v,A? 3198.6(6) 7042.7(2) 8412(4) z 4 4 4 pcalc, g Cm"3 1.242 1.355 1.378 Fooo 1264 2952 3600 p., (MoKa), 0.442 2.991 2.521 cm"1 transmission 0.7550-1.0000 0.8448-1.0000 0.7118-1.0000 factors 20max, deg 53.24 56.30 55.24 total no. of 27267 97131 115842 reflns no. of unique 7173 8473 9958 reflns Rint 0.0374 0.0396 0.0388 no. of 333 . 377 479 variables R. (F2,I> 0.0301 0.0217 0.0285 2a(I)) Rw(F2,all 0.0629 0.0576 0.0454 data) Gof 0.821 1.066 1.021 Rx = £ 1FQ| - Fc||/I|F0|;Rw = (£w(jF02|- FC2 )l I £wF02|2)'/2 213 References located on page 210. Appendix A: X-ray Crystal Structure Data Table A.4. Crystallographic and structure refinement for Mes[NCN]Hf(Ti2-XyNCCH3)2 (4.8), Mes[NCN]Hf(OC(CH3)=C(CH3)NXy) (4.11), and Mes[NCN]Hf)2C«-OC(iBu)=C(iBu)0)2 (4.16). Mes[NCN]Hf(r|2- Mes[NCN]Hf(OC(CH3) (Mes[NCN]Hf)2(p-XyNCCH3)2 (4.8) =C(CH3)NXy) (4.11) OC(iBu)=C(iBu)0)2 (4.16) Formula C9oHii2Hf2N]2- C37H47HfN50-C2H50 C7oH10oHf2N804-4C6H6 CH2C12 Formula 1803.82 801.35 1786.99 weight Colour, Habit Colourless, irregular Yellow, irregular Colourless, tablet Crystal size, 0.20x 0.07x 0.04 0.10x0.05x0.03 0.15 x 0.07x0.04 mm Crystal system Triclinic Monoclinic Triclinic Space group P-l Pljc P-\ a, A 15.6616(8) 10.9435(6) 12.377(5) b,A 16.3972(7) 20.0210(11) 18.530(5) c,A 21.5514(10) 17.3179(8) 21.408(5) a, deg 70.757(2) 90 107.656(5) P, deg 73.887(2) 92.451(2) 103.930(5) Y, deg 68.167(2) 90 100.104(5) v,A? 4774.1(4) 3790.9(3) 4374(2) z 2 4 2 Pcalc, g Cm"3 1.255 1.404 1.357 Fooo 1844 1636 1840 p., (MoKa), 2.275 2.790 2.425 cm transmission 0.8093-1.0000 0.7300-1.0000 0.6694-1.0000 factors 20max, deg 48.10 41.20 46.92 total no. of 108780 25826 44117 reflns no. of unique 19273 6703 11428 reflns Rint 0.0649 0.1030 0.1051 no. of 988 420 993 variables R. (F2,I> 0.0656 0.0479 0.0473 2a(I)) RW(F2, all 0.0963 0.1058 0.1106 data) Gof 1.281 0.999 0.982 R, = E||F0|- FC||/I|F0| ;Rw = (£w(|F02|- Fc2|)2/Sw|F0ty." 214 References located on page 210. Appendix A: X-ray Crystal Structure Data Table A.5. Crystallographic and 'BuNC(Me)O) (4.17), Mes[NCN]Zr(Cl)(OBu) (4.22) structure refinement for Mes[NCN]Hf(Me)(n3-Mes[NCN]Hf(Me)(n3-iPrNC(Me)NiPr) (4.18), and Mes[NCN]Hf(Me)(ii3- Mes [NCN] Hf(Me)(nJ- Mes[NCN]Zr(Cl)(OBu) lBuNC(Me)0) (4.17) iPrNC(Me)N'Pr) (4.22) (4.18) Formula C32H47HfN50 C34H52HfN6 C29H41CI N4OZr Formula 696.24 723.31 588.33 weight Colour, Habit Colourless, prism Colourless, plate Yellow, irregular Crystal size, 0.3 x 0.2x0.15 0.35 x 0.25 x0.1 0.4x0.4x0.35 mm Crystal system Orthorhombic Triclinic Triclinic Space group Pbca P-\ P-\ a, A 10.1327(2) 10.4245(2) 10.367(5) b,A 21.6299(4) 19.3938(4) 10.894(5) c,A 28.8509(6) 19.9828(4) 13.550(5) a, deg 90 113.8580(10) 80.965(5) P, deg 90 104.8770(10) 81.615(5) y, deg 90 96.0180(10) 80.095(5) v,A? 6323.2(2) 3469.92(12) 1477.7(11) z 8 4 2 pcalc, g Cm"3 . 1.463 1.385 1.322 Fooo 2832 1480 616 p, (MoKa), 3.331 3.037 0.490 cm transmission 0.7831-1.0000 0.6466-1.0000 0.7971-1.0000 factors 26max, deg 55.68 55.88 55.74 total no. of 58871 94064 13213 reflns no. of unique 7501 16394 6026 reflns Rint 0.0448 0.0378 0.0331 no. of 363 763 332 variables Ri (F2,I> 0.0227 0.0237 0.0488 2a(I)) Rw (F2, all 0.0379 0.0407 0.0561 data) Gof 1.028 1.028 1.090 Rt = S|]FQ1 - ]Fel| / £[F0 ; Rw = (Sw( F02 - 1FC2|)2 / £w|F02 lfl 215 References located on page 210. Appendix A: X-ray Crystal Structure Data Table A.6. Crystallographic and structure refinement for Mes[NCN]Hf(Me)(r|2-NNMe2) (4.23), tol[NCNH]Ta(NMe2)4 (5.1), and Mes[NCNH]Ta(CHPh) (CH2Ph)2 (5.2). Mes[NCN]Hf(Me)(n2- tol[NCNH]Ta(NMe2)4 Mes [NCNH] Ta(CHPh) NNMe2) (4.23) (5.1) (CH2Ph)2 (5.2) Formula C28H42HfN6 C36H57N8Ta C46H50N4Ta, Formula 641.17 782.85 839.85 weight Colour, Habit Colourless, prism Yellow, fiber Orange, prism Crystal size, 0.25x0.2x0.05 0.4x0.25x0.15 0.2x0.1 x0.05 mm Crystal system Monoclinic Monoclinic Monoclinic Space group P2xlm PlxIc PlxIc a, A 8.1361(4) 12.669(5) 11.0925(10) b,A 22.1536(10) 11.681(5) 27.769(3) c, A 8.5276(4) 25.201(5) 13.3194(14) a, deg 90 90 90 P, deg 113.3560(10) 87.021(5) 96.253(4) y, deg 90 90 90 v,A? 1411.10(12) 3724(2) 4078.3(7) z 2 4 4 pcalc, g Cm"3 1.509 1.396 1.368 Fooo 648 1608 1708 p, (MoKa), 3.723 2.986 2.730 cm transmission 0.7458-1.0000 0.6390-1.0000 0.8017-1.0000 factors 20max, deg 55.82 55.00 55.66 total no. of 19552 13436 105524 reflns no. of unique 3452 7557 9567 reflns Rint 0.0400 0.0245 0.0619 no. of 174 421 474 variables Ri (F2,I> 0.0257 0.0294 0.0255 2a(I)) RW(F2, all 0.0312 0.0438 0.0463 data) Gof 1.087 1.044 1.011 R, = S||F0|-|FC||/I|F0|; Rw = (2w(|F02 - |FC2 )2/EwF02|2)iyi 216 References located on page 210. Appendix A: X-ray Crystal Structure Data Table A.7. Crystallographic and structure refinement for [NCN]Ta(NMe2)3 (5.3), Mes[NCCN]Ta(CH2tBu)2 (5.7), and Mes[NCCN]Ta(Cl)(CH2tBu) (5.10). tol[NCN]Ta(NMe2)3 Mes[NCCN]Ta(CH2tBu)2 Mes[NCCN]Ta(Cl) (5.3) (5.7) (CH2lBu) (5.10) Formula C3,H42N7Ta C35H53N4Ta C30H42C1N4 Formula 693.67 710.76 675.08 weight Colour, Habit Orange, irregular Yellow, irregular Yellow, irregular Crystal size, 0.25x0.18x0.12 0.35x0.2x0.1 0.12 x 0.05 x 0.05 mm Crystal Monoclinic Monoclinic Triclinic system Space group C2/c P2\ln P-l a, A 15.668(5) 9.67930(10) 9.3783(8) b,A 11.495(5) 22.4522(3) 11.5711(10) c, A 19.926(5) 16.0990(2) 14.6775(11) a, deg 90 90 109.568(3) P, deg 102.002(5) 104.6490(10) 90.206(3) y, deg 90 90 94.935(3) v,A? 3510(2) 3384.93(7) 1494.3(2) z 4 4 2 Pcalc, g Cm"3 1.313 1.395 1.500 Fooo 1400 680 1456 p, (MoKa), 3.159 3.275 3.791 cm"1 transmission 0.8542-1.000 0.7666-1.0000 0.8698-1.0000 factors 20max, deg 55.46 55.92 55.84 total no. of 39215 31089 49582 reflns no. of unique 4086 7008 8005 reflns Rim 0.0315 0.0678 0.0316 no. of 182 334 , ' 373 variables R. (F2,I> 0.0285 0.0357 0.0181 2a(I)) Rw(F2,all 0.0307 0.0518 0.0271 data) Gof 1.122 1.056 1.072 R, = I |F0| - |Fc|| / S|F0|; Rw = (Sw( F02 - Fc2 )2 / Ew|F02|2)1/2 217 References located on page 210. Appendix A: X-ray Crystal Structure Data TableA.8. Crystallographic and structure refinement for (-)-(li?,2'S',4i?)-2-(l,7,7-trimethylbicyclo[2.2. l]hept-2-ylamino)ethyl ammonium chloride (6.8). (-)-(lR,2'S,4R)-2-(l,7,7-trimethylbicyclo[2.2.1]hept-2-ylamino)ethyl ammonium chloride (6.8) Formula C12H24C12N Formula 253.22 weight Colour, Habit Colourless, prism Crystal size, 0.40x0.20x0.10 mm Crystal system Triclinic Space group Pl a, A 10.661(2) b,A 10.659(2) c,A 14.141(3) a, deg 108.308(7) P, deg 114.205(7) y, deg 92.053(8) v,A? 1366.0(8) z 4 Pcalc, g Cm"3 1.231 Fooo 548 p., (MoKa), 0.448 cm"1 transmission 0.5679-1.0000 factors 20max, deg 51.50 total no. of 34817 reflns no. of unique 18462 reflns Rint 0.0802 no. of 559 variables Ri (F2,I> 0.1523 2a(I)) Rw (F2, all 0.1873 data) Gof 1.659 R, = 2 |F 0| - |FC|| / I|F0|; Rw = (2w( F02| - |FC2 )2 / Ew F02 2)1/2 218 References located on page 210. Appendix B: Evaluating the Formation of Tantalum Metallaaziridine Complex by Density Functional Calculations Appendix B Evaluating the Formation of a Tantalum Metallaaziridine Complex by Density Functional Calculations B.l. Evaluation of a a-Bond Metathesis Mechanism In the calculated a-bond metathesis pathway (Figure B.l), the lowest energy trimethyl complex (A, 0.0 kcal/mol, Table B.l) is converted to a methane adduct of the ligand activated metallaziridine dimethyl complex, (B, -7.9 kcal/mol), via the a-bond metathesis transition state (TS-A-B, 49.6 kcal/mol). The calculated structures of A, TS-A-B, and B are provided in Figure B.2. In the transition state TS-A-B, the breaking Ta-C bond distance is 2.96 A, while the forming and breaking C-H bond distances are 1.36 and 1.47 A, respectively. The Ta-H distance in TS-A-B is 2.79 A, which indicates that there is very little interaction between the migrating hydrogen and the tantalum centre in this transition state. Separating methane from the methane adduct complex B generates the metallaaziridine complex C and free methane (-10.5 kcal/mol). The metallaaziridine complex C is able to change conformations to the most stable metallaaziridine complex D via the low barrier isomerization transition state (TS-C-D, -10.4 kcal/mol). Overall the reaction is exergonic, with the free energy of the metallaziridine D and CH4 products being 18.6 kcal/mol lower in energy that the starting trimethyl complex A. However, the 219 References begins on page 233. Appendix B: Evaluating the Formation of Tantalum Metallaaziridine Complex by Density Functional Calculations high energy point on this pathway, the transition state TS-A-B, is 49.6 kcal/mol higher in energy than the starting trimethyl complex A, which is too large of an energy difference between the starting complex and the transition state for this transformation to readily occur without heating the solution, which indicates that this pathway may not be operating under the experimental conditions employed in this study. TS-A-B (49.6) These isomers vary in the conformation of the 6 member rings a Gas-phase relative free energies at the B3LYP/BS1 level of theory based on the energy of separated 1 set to 0.0 kcal/mol are provided in parentheses in kcal/mol. Electronic energies, corrected zero-point energies, enthalpies, and free energies are provided in Table B.l. Figure B.l. Relative energies of the intermediates and transition states in a potential a-bond metathesis mechanism. 220 References begins on page 233. Appendix B: Evaluating the Formation of Tantalum Metallaaziridine Complex by Density Functional Calculations Table B.l. Gas-phase relative energies (kcal/mol) of the intermediates and transition states in a a-bond metathesis mechanism. Structure AEeW AE0(b) AHo(c; AG0(d) A 0.00 0.0 0.0 0.0 TS-A-B 55.33 49.7 49.9 49.6 B 1.99 -1.3 0.6 -7.9 C + CH4 2.36 -1.1 -0.2 -10.5 TS-C-D + CH4 2.89 -1.0 -0.4 -10.4 D + CH4 -5.85 -9.3 -8.4 -18.6 (a) based on the gas-phase relative electronic energy of A set to 0.00 kcal/mol (b) based on the gas-phase relative zero point corrected energy of A set to 0.0 kcal/mol (c) based on the gas-phase relative enthalpy of A set to 0.0 kcal/mol (d) based on the gas-phase relative free energy of A set to 0.0 kcal/mol TS-A-B B Figure B.2. JIMP1 Pictures of the one-step a-bond metathesis pathway. B.2. Investigation of an Alkylidene Intermediate Followed by C-H Bond Activation In the calculated two-step pathway for metallaaziridine formation (Figure B.3), in which a-H abstraction is followed by alkylidene mediated C-H activation, the starting trimethyl complex (A, 0.0 kcal/mol, Table B.2) is initially converted to the methane-adduct methylidene-methyl complex (E, -10.3 kcal/mol) via the a-H abstraction transition state (TS-A-E, 36.4 kcal/mol) (Figure B.4). Separating methane from the adduct-complex yields the methylidene-methyl complex and free methane (F + CH4, -14.8 kcal/mol). The a-H abstraction sequence (A -» TS-A-E -» E -» F + CH4) is exergonic by -14.8 kcal/mol, thus indicating that this step is an energetically favorable process. The a-H abstraction transition state (TS-A-E) is 36.4 kcal/mol higher in energy 221 References begins on page 233. Appendix B: Evaluating the Formation of Tantalum Metallaaziridine Complex by Density Functional Calculations than the starting trimethyl complex A, which shows a-H abstraction is energetically favored over a-bond metathesis for the trimethyl complex A by 13.5 kcal/mol. TS-A-E (36.4) A F-J D These isomers vary in the conformation of the 6 member rings 3 Gas-phase relative free energies at the B3LYP/BS1 level of theory based on the energy of separated 1 set to 0.0 kcal/mol are provided in parentheses in kcal/mol. Electronic energies, corrected zero-point energies, enthalpies, and free energies are provided in Table 1. Figure B.3. Relative energies of the intermediates and transition states in a potential two-step a-H abstraction/alkylidene mediated C-H activation mechanism. 222 References begins on page 233. Appendix B: Evaluating the Formation of Tantalum Metallaaziridine Complex by Density Functional Calculations Table B.2. Gas-phase relative energies (kcal/mol) of the intermediates and transition states in a mechanism involving a-H abstraction followed by alkylidene mediated C-H bond activation. Structure AEeW AEc/b) AG°W A 0.00 0.0 0.0 0.0 TS-A-E + CH4 38.30 36.0 35.6 36.4 E + CH4 -1.90 -4.7 -2.8 -10.3 F + CH4 -1.56 -4.8 -3.6 -14.8 TS-F-G + CH4 8.09 4.8 5.4 -4.3 G + CH4 -0.01 -3.0 -2.1 -12.5 TS-G-H + CH4 9.69 6.3 7.0 -3.1 H + CH4 5.67 2.7 3.7 -7.3 TS-H-I + CH4 6.35 2.9 3.6 -6.5 I + CH4 6.20 3.0 4.0 -6.7 TS-I-J + CH4 7.65 3.7 4.6 -5.7 J + CH4 7.63 3.8 5.2 -7.0 TS-J-D + CH4 31.43 25.8 26.2 17.3 D + CH4 -5.85 -9.3 -8.4 -18.6 (a) based on the gas-phase relative electronic energy of A set to 0.00 kcal/mol (b) based on the gas-phase relative zero point corrected energy of A set to 0.0 kcal/mol (c) based on the gas-phase relative enthalpy of A set to 0.0 kcal/mol (d) based on the gas-phase relative free energy of A set to 0.0 kcal/mol A series of low energy rearrangements (F -> TS-F-G -> G -> TS-G-H -> H -> TS-H-I -> I -> TS-I-J J) positions the CH2CH2 carbene-amido linker in the proper position for alkylidene mediated C-H activation of the ligand backbone. In the ligand C-H bond activation process, the methylidene unit of the rearranged methylidene-methyl complex (J, -7.0 kcal/mol, Figures B.3 and B.4) abstracts a proton from the ligand backbone via the alkylidene mediated C-H activation transition state (TS-J-D, 17.3 kcal/mol) to yield the ligand activated metallaziridine product D (Figures B.3 and B.4). Overall, the final products D and CH4 are lower in energy than all of the methylidene-methyl complexes (F-J), thus indicating that the metallaaziridine complex and CH4 are the favored products for methane elimination from the trimethyl complex A in agreement with the experimental observations. In addition, the energy difference between the lowest energy methylidene-methyl complex (F) and the alkylidene mediated C-H activation transition state (TS-H-D) is 32.1 kcal/mol, which indicates that the alkylidene mediated C-H bond activation occurs more readily than the initial a-H abstraction step. It should also be noted that the barrier for the reaction to proceed in the reverse direction 223 References begins on page 233. Appendix B: Evaluating the Formation of Tantalum Metallaaziridine Complex by Density Functional Calculations from lowest energy methylidene-methyl complex (F), for example the energy difference between F and CH4 and the a-H abstraction transition state (TS-A-E), is 51.2 kcal/mol, which implies that endocyclic C-H activation of the ligand backbone is highly favored over C-H activation of alkanes in solution. • A TS-A-F E J TS-J-D D Figure B.4. JIMP1 Pictures of a-H abstraction by a methyl group to generate a [NCN]Ta(=CHR')R alkylidene intermediate. B.3. Thermodynamic Considerations for the Formation of Metallated Ta [NCCNJ Derivatives In addition to studying the mechanism of the ligand activation, part of the motivation for the DFT investigation was to address the question, why are the ligand activated metallaaziridine products formed instead of the trialkyl derivatives? For our model complexes, the metallaaziridine product D and separated CH4 are favored relative to the trimethyl complex A both in terms of both enthalpy and entropy (Table B.l). Although we expected that formation of two molecules (D + CH4) instead of one molecule (A) would be entropically favored, it was not obvious why the metallaaziridine 224 References begins on page 233. Appendix B: Evaluating the Formation of Tantalum Metallaaziridine Complex by Density Functional Calculations product D and CH4 were enthalpically favored over the trimethyl complex A, especially because D contains a three-membered ring, which is a potential source of ring strain. An examination of the bond lengths and angles in the trimethyl complex A (Figure B.5) and the metallaaziridine product D (Figure B.6) revealed several features that contribute to. D and CH4 being enthalpically favored relative to A: (1) The Ta-carbene bond is much shorter in D (2.248 A) than in A (2.381 A), indicative of a much stronger bond; (2) The Ta-CH3 bonds in D (2.173 and 2.219 A) are shorter than the Ta-CH3 bonds in A (2.238, 2.248, and 2.252 A); and (3) The Ta-N bonds in D (1.982 and 2.065 A) are, on average, shorter than the Ta-N bonds in A (2.020 and 2.053 A). Furthermore, the ring strain due to the metallaaziridine fragment in complex D is reduced relative to the strain in organic three-membered rings because D can be viewed as a six-coordinate cf Ta complex in which Ta-L a-bonding occurs through ligand orbital interactions with Ta sd5 hybrids. In six coordinate, cf early transition metal complexes such as WrLj, formation of localized, electron pair bonds draws from all s and d orbitals to form six sd5 hybrids, which have energy minimas at angles of 63° and 1170.2'3 The calculated C4-Tal-N2 angle in the metallaaziridine ring of D is 38.2°, which represents less than a 25° distortion from the smaller angle energy minima. 225 References begins on page 233. Appendix B: Evaluating the Formation of Tantalum Metallaaziridine Complex by Density Functional Calculations Figure B.5. JIMP1 view of A. Selected bond distances (A) and angles (deg): Tal-Cl 2.381, Tal-C2 2.252, Tal-C3 2.248, Tal-C4 2.238, Tal-Nl 2.053, Tal-N2 2.020, Cl-Tal-C2 139.6, Cl-Tal-C3 142.0, C2-Tal-C3 76.4. Figure B.6. JIMP1 view of D. Selected bond distances (A) and angles (deg): Tal-Cl 2.248, Tal-C2 2.219, Tal-C3 2.173, Tal-C4 2.257, Tal-Nl 2.065, Tal-N2 1.982, Cl-Tal-C2 130.5, Cl-Tal-C3 122.7, C2-Tal-C3 106.1, C4-Tal-N2 38.2. 226 References begins on page 233. Appendix B: Evaluating the Formation of Tantalum Metallaaziridine Complex by Density Functional Calculations In addition, formation of the metallaaziridine ring in D facilitates the close proximity of two Ta substituents, which provides more space around the Ta centre for the remaining substituents to adopt more favorable bonding positions. Natural Bond Orbital4 (NBO) analyses of the trimethyl complex A, and the metallaaziridine product D, indicate that shortening of the Ta-carbene and Ta-Me bonds in D relative to A results from changes in the occupancy of the a-bonding and a*-anti-bonding orbitals. The occupancies of the Ta-carbene and Ta-Me3 a-bonding orbitals in D are higher than the occupancies of the corresponding a-bonding orbitals in A, while the occupancies of the Ta-carbene and Ta-Me3 a*-anti-bonding orbitals in D are lower than the occupancies of the corresponding a-bonding orbitals in A (Table B.3). Table B.3. NBO Occupancies of bonding and anti-bonding orbitals in the trimethyl complex A and the metallaaziridine complex D. A D Tal-Cl a 1.853 1.933 Tal-C2a 1.833 1.946 Tal-C3 a 1.795 1.892 Tal-Cl a* 0.168 0.091 Tal-C2a* 0.126 0.094 Tal-C3 a* 0.129 0.103 Examination of the second order perturbation theory analysis of the NBO orbitals of A reveals the origin of the differences in the orbital occupancies between A and D (Table B.4). Strong donation from the a-bonding orbitals to the trans Ta-C a* orbitals (Table B.4, Figure B.7) is shown by the large interaction terms for A. It should be noted that in D, the corresponding donation from the Ta-C a-bonding orbitals the Ta-C a* orbitals is significantly reduced (Table B.4, Figure B.8) because the more favorable Cl-Ta-Me bond angles of D can avoid the mixing of bonding and anti-bonding orbitals observed in A. Thus, it appears that formation of the metallaaziridine ring is a key 227 References begins on page 233. Appendix B: Evaluating the Formation of Tantalum Metallaaziridine Complex by Density Functional Calculations component that allows the remaining methyl and carbene substituents in D to adopt preferable bonding positions. Table B.4. Important second order perturbation theory analysis NBO donor -acceptor interactions AEy (kcal/mol) that contribute to shorter Ta-carbene and Ta-Me bonds in the metallaaziridine product D relative to the trimethyl complex A. NBO Donor orbital NBO Acceptor orbital AEy (kcal/mol) A D Tal-Cl a Tal-C2 o* 26.13 1.02 Tal-Cl a Tal-C3 o* 31.50 2.64 Tal-C2 a Tal-Cl a* 38.32 1.81 Tal-C3 a Tal-Cl a* 54.62 3.02 228 References begins on page 233. Appendix B: Evaluating the Formation of Tantalum Metallaaziridine Complex by Density Functional Calculations Tal-Cl a Tal-C2a Tal-C3 a Figure B.7. Gaussview representations of selected NBO bonding and antibonding orbitals in the trimethyl complex A. 229 References begins on page 233. Appendix B: Evaluating the Formation of Tantalum Metallaaziridine Complex by Density Functional Calculations Tal-Cl a Tal-C2o Tal-C3 a Figure B.8. Gaussview representations of selected NBO bonding and antibonding orbitals in the trimethyl complex D. The other factor that causes the Ta-carbene distance to be shorter in the metallaaziridine complex D than in the trimethyl complex A is the formation of the five-membered ring. To gauge how much of the shortening of the Ta-carbene bond in D relative to A is a result of the five-membered ring pulling the carbene toward the Ta-centre, calculations were conducted on model complexes in which atoms linking the 230 References begins on page 233. Appendix B: Evaluating the Formation of Tantalum Metallaaziridine Complex by Density Functional Calculations carbene unit to the amido donors were removed. In the first calculation, both CH2CH2 linkages in A were removed and replaced with H atoms to generate (H4N2C3)Ta(NH2)2(Me)3 ,A' (Figure B.9). Optimization of A' resulted in a Ta-carbene bond length of 2.378 A, which is only 0.003 A shorter than for A, and indicates that the two six-membered rings do not significantly influence the Ta-carbene distance. In the second calculation, the CH2CH2 linkage between the carbene and amido units, and the CH2 linkage between the carbene and the metallaaziridine ring were removed to generate (H4N2C3)Ta(NH2)(NHCH2)(Me)2, D' (Figure B.9). The optimized Ta-carbene distance in D' is 2.275 A, which is 0.027 A longer than is observed in D. Thus, the calculations suggest that approximately 20% of the Ta-carbene bond shortening observed in D can be attributed to the five-membered ring pulling the carbene toward the Ta-centre, while the remaining shortening can be attributed to electronic effects brought upon by the formation of the metallaaziridine ring (vide ante). A' D' Figure B.9. JIMP Pictures of A' and D'. 231 References begins on page 233. Appendix B: Evaluating the Formation of Tantalum Metallaaziridine Complex by Density Functional Calculations B.4. General Considerations All calculations were performed using the Gaussian 03 suite of programs.5 Optimized gas-phase geometries were obtained using the Becke3 exchange functional,6 in combination the Lee, Yang, and Parr correlation functional,7 i.e. the B3LYP method, as implemented in Gaussian 03. The basis set (BS1) used for geometry optimizations and energy calculations was implemented as follows: for tantalum, the valence double-^ LANL2DZ8"10 basis set was supplemented with a set of 6p functions for transition metals developed by Couty and Hall," while for all hydrogen, carbon, and nitrogen atoms, the 6-12 17 31G(d',p') basis sets " were used. All structures were calculated in singlet spin states using the restricted B3LYP method. Calculating the harmonic vibrational frequencies and noting the number of imaginary frequencies confirmed the nature of all intermediates (NImag = 0) and transition state structures (NImag = 1). All gas-phase relative free energies are reported in kcal mol"1, with the energy of H[NCN]TaMe3 (H[NCN] = (HNCH2CH2)2N2C3H2) set to 0.0 kcal mol"1. Relative electronic energies, zero-point corrected energies, and enthalpies are provided in the supplemental information. For the computational investigation, H[NCN] was used in place of the experimental ligand (p-Me-C6H4NCH2CH2)2N2C3H2 (Tol[NCN]) in order to reduce the computational demands, while still providing two amide donors and one 7V-heterocyclic carbene donor to the tantalum centre. Natural Bond Orbitals (NBO) calculations were conducted with Gaussian NBO Version 3.1.4'18-21 232 References begins on page 233. Appendix B: Evaluating the Formation of Tantalum Metallaaziridine Complex by Density Functional Calculations B.5. References (1) Manson, J.; Webster, C. E.; Hall, M. B. In JIMP Development Version 0. l.vlU (built for Windows PC and Redhat Linux 7.3) Department of Chemistry, Texas A&M University, College Station, TX 77842, 2006. (2) Landis, C. R.; Cleveland, T.; Firman, T. K. J. Am. Chem. Soc. 1995, 77 7, 1859. (3) Bayse, C. A.; Hall, M. B. J. Am. Chem. Soc. 1999, 727, 1348. (4) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899. (5) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A. J.; Vreven, T.; Kudin, K. N.; Burant, J. C; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C; Pople, J. A. Gaussian 03, Revision B.4; Gaussian, Inc.: Pittsburgh, PA, 2003. (6) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (7) Lee, C; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter 1988, 37, 785. (8) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (9) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (10) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (11) Couty, M.; Hall, M. B. J. Comput. Chem. 1996, 7 7, 1359. (12) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724. 233 References begins on page 233. Appendix B: Evaluating the Formation of Tantalum Metallaaziridine Complex by Density Functional Calculations (13) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (14) Hariharan, P. C; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (15) Petersson, G. A.; Al-Laham, M. A. J. Chem. Phys. 1991, 94, 6081. (16) Petersson, G. A.; Bennett, A.; Tensfeldt, T. G.; Al-Laham, M. A.; Shirley, W. A.; Mantzaris, J. J. Chem. Phys. 1988, 89, 2193. (17) Foresman, J. B.; Frisch, JE. Exploring Chemistry with Electronic Structure Methods, 2nd Ed. (Gaussian, Inc, Pittsburgh, PA), p. 110. The 6-31G(d',p) basis set has the d polarization functions for C, N, O, and F taken from the 6-31 IG(d) basis set, instead of the original arbitrarily assigned value of 0.8 used in the 6-31G(d) basis set. (18) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211. (19) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735. (20) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1985, 83, 1736. (21) Carpenter, J. E.; Weinhold, F. Theochem 1988, 46, 41. 234 References begins on page 233. 

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