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Amidate complexes of the group 4 metals : sythesis, reactivity, and hydroamination catalysis Thomson, Robert Kenneth 2008

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AMIDATE COMPLEXES OF THE GROUP 4 METALS: SYNTHESIS, REACTIVITY, AND HYDROAMINATION CATALYSIS by ROBERT KENNETH THOMSON B.Sc. (Honours), University of British Columbia, 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 January 2008 © Robert Kenneth Thomson, 2008 ABSTRACT A series of bidentate amidate ligands with variable groups R' and R" abbreviated by [R"(NO)R'] and adamantyl substituted tetradentate amidate ligands abbreviated by Ad[02N2] were utilized as ancillaries for Ti, Zr, and Hf. Protonolysis routes into homoleptic amidate complexes, tris(amidate) mono(amido), bis(amidate) bis(amido), and bis(amidate) dibenzyl complexes are high yielding when performed with tetrakis(amido) and tetrabenzyl group 4 starting materials. Many of these complexes have been characterized in both the solid-state and in the solution phase, where in the latter case these complexes are fluxional and undergo exchange processes. Multiple geometric isomers are possible with the mixed N,0 chelate provided by the amidate ligands, and geometric isomerization of bis(amidate) bis(amido) complexes has been examined through X-ray crystallographic and density functional theory (DFT) calculations. Isomerization is dictated largely by the steric bulk present at the N of the amidate ligands, and is proposed to proceed through a 1(2-K1 -x2 ligand isomerization mechanism, which is supported by crystallographic evidence of K t -bound amidate ligands. The amidate ligand system binds to these metals in a largely electrostatic fashion, with poor orbital overlap, generating highly electrophilic metal centers. The bis(amidate) dibenzyl complexes of Zr and Hf are reactive towards insertion, abstraction, and protonolysis. Insertion of isocyanides into the Zr-C bonds of r(No ) tBu,12Zr(CH2Ph)2 results in the formation of i 2-iminoacyl complexes, which can either undergo thermally induced C=C coupling to generate an enediamido complex (aryl isocyanides), or rearrange to generate a bis(amidate) bis(vinylamido) complex (alkyl isocyanides). Benzyl abstraction to generate cationic Zr bis(amidate) benzyl complexes is also possible through reaction with [Ph3C][B(C6F5)4] or B(C6F5)3• Terminal imido complexes with novel pyramidal geometries are generated through protonolysis of bis(amidate) bis(amido) Ti and Zr complexes with primary aryl amines. DFT calculations demonstrate the existence of a ZraN triple bond for these complexes. Dimeric imido complexes have been characterized in the solid state, but are not maintained in solution. Cycloaddition reactions of the terminal Zr imido complexes with C=0 bonds result in the formation of proposed oxo complexes and organic ii metathesis products. Catalytic aminoalkene cyclohydroamination has also been realized with these complexes, generating N-heterocyclic products. A series of kinetic and labeling studies support an imido-cycloaddition mechanism for catalytic cyclohydroamination of primary aminoalkenes with neutral bis(amidate) Ti and Zr precatalysts. The intermediate Ti imido complex, K2- [Dipp(No)tsui _ Ki_ [DiPp(No) tBu i Ti=NCH2CPh2CH2CH=CH2(NHMe2), has been isolated and characterized in the solid-state and in solution. Amine stabilized imido complexes of this type are invoked as the resting state for the catalytic reaction, and solution phase data support a chair-like geometry, where the alkene is coordinated to the metal center. A diastereoselectivity study supports this proposed solution structure. Eyring and Arrhenius parameters, as well as isolation of a 7-coordinate model imido complex, support a seven-coordinate transition state for the rate-determining metallacycle protonolysis reaction. In contrast, secondary aminoalkene hydroamination catalysis with cationic Zr benzyl complexes is proposed to proceed through a o-bond insertion mechanism. Proton loss from cationic Zr amido complexes to generate imido species is proposed with primary aminoalkenes, and the resultant neutral imido complexes can catalyze the cyclization of these substrates by the aforementioned imido-cycloaddition mechanism. The ability of the amidate ligand system to promote both mechanisms is unique in the field of alkene hydroamination catalysis. iii TABLE OF CONTENTS Abstract^ ii Table of Contents^ iv List of Tables x List of Figures^ xiv List of Abbreviations xx Foreword^ xxv Acknowledgements^ xxvi CHAPTER ONE: AMIDATES AS NEW MODULAR LIGANDS FOR GROUP 4 METALS^ 1 ^1.1^Ligand Driven Reactivity of Early Transition Metals^ 1 ^ 1.2 Non-Cyclopentadienyl Ligand Systems for Early Transition Metals^ 4 1.3^Modular Ligand Systems 7 1.4 Four-Membered Chelate Systems^ 9 1.5^Amidate Ligands^ 12 1.5.1 Synthetic Methodology^ 12 1.5.2^Binding Motifs^ 13 1.5.3 Selected Applications of Amidate Ligands^ 16 1.5.3.1^Biological Models^ 16 1.5.3.2 Amidates as Structural Units^ 17 1.5.3.3^Amidates as Reactive Intermediates^ 18 1.5.3.3.1 Intermediates in Transamidation Reactivity^  18 1.5.3.3.2^Intermediates in Amidation of Aryl Chlorides 20 iv ^1.5.3.4^Amidates as Ancillary Ligands for Group 4 Metals^ 20 1.6^Scope of Thesis 21 1.7 References^ 24 CHAPTER Two: COORDINATION CHEMISTRY OF GROUP 4 AMIDATE COMPLEXES^ 33 2.1^Introduction^ 33 2.2 Homoleptic Amidate Complexes^ 36 2.2.1^Introduction^ 36 2.2.2 Results and Discussion^ 36 2.2.3^Summary^ 44 2.3 Amidate Amido Complexes^ 45 2.3.1^Introduction^ 45 2.3.2 Results and Discussion^ 46 2.3.2.1^Tris(amidate) Mono(amido) Complexes of Zr^ 46 2.3.2.2 Bis(amidate) Bis(amido) Complexes of Ti and Zr^ 49 2.3.2.3^Insertion Reactivity of Amido Ligands^60 2.3.3 Summary^ 62 2.4^Geometric Isomerization of Amido Complexes^ 62 2.4.1 Introduction^ 62 2.4.2^Results and Discussion^ 63 2.4.2.1 Structural Studies 63 2.4.2.2^Electronic Structure Analysis^ 72 2.4.3 Summary^ 77 2.5^Conclusions 79 2.6 Experimental^ 80 2.6.1^General Considerations^ 80 2.6.2 Starting Materials and Reagents^ 82 2.6.3^Synthesis^ 83 2.7 References 98 v CHAPTER 3: SYNTHESIS, STRUCTURE, AND REACTIVITY OF ZR AND HF AMIDATE BENZYL COMPLEXES^ 104 ^ 3.1^Introduction 104 3.2 Synthesis and Characterization of Dibenzyl Complexes^ 108 3.2.1^Introduction^ 108 3.2.2 Results and Discussion^  109 3.2.2.1^Bidentate Amidate Dibenzyl Complexes^ 109 3.2.2.2 Tethered Tetradentate Amidate Dibenzyl Complexes^ 117 3.2.3^Summary^ 120 3.3^Insertion Reactions of Isocyanides with [Dm (NO)tBu]2Zr(CH2Ph)2 (3.9)^ 121 3.3.1^Introduction^ 121 3.3.2 Results and Discussion^  121 3.3.3^Summary^ 132 3.4^Hydrolysis of Dibenzyl Complex [D (NO)P1 ]2Hf(CH2Ph)2(THF) (3.6)^  132 3.4.1^Introduction^ 132 3.4.2 Results and Discussion^  135 3.4.3^Summary^ 140 3.5^Abstraction Reactivity of Bis(amidate) Dibenzyl Complexes^ 140 3.5.1^Introduction^ 140 3.5.2 Results and Discussion^  141 3.5.3^Summary^ 143 3.6^Conclusions 144 3.7 Experimental^ 145 3.7.1^General Considerations^ 145 3.7.2 Starting Materials and Reagents^ 145 3.7.3^Synthesis^  146 3.8^References 155 vi CHAPTER 4: SYNTHESIS, STRUCTURE, AND REACTIVITY OF AMIDATE SUPPORTED IMIDO COMPLEXES^ 162 4.1^Introduction 162 4.2 Synthesis and Structure of Terminal Imido Complexes^ 171 ^ 4.2.1^Introduction^ 171 4.2.2 Results and Discussion^  173 4.2.2.1^Synthesis and Structure of Amido Anilido Complex 173 4.2.2.2^Synthesis and Structure of a Pentagonal Pyramidal Imido Complex^ 177 4.2.2.3^Electronic Structure of Pentagonal Pyramidal Imido Complex 4.25^ 182 4.2.2.4^Ligand Lability Investigations 185 4.2.3^Summary^ 186 4.3^Synthesis and Structure of a Seven-Coordinate Imido Complex^ 187 4.3.1^Introduction^ 187 4.3.2 Results and Discussion^  187 4.3.3^Summary^ 191 4.4^Structural Characterization of Dimeric Zr Imido Complexes ^ 192 4.4.1^Introduction^ 192 4.4.2 Results and Discussion^  193 4.4.3^Summary^ 199 4.5^Cycloaddition Reactivity with Terminal Imido Complexes^ 200 4.5.1^Introduction^ 200 4.5.2 Results and Discussion^ 201 4.5.2.1^Development of Spectroscopically Simpler Imido Complexes^ 201 4.5.2.2^Cycloaddition of C=0 Bonds 204 4.5.3^Summary^ 206 4.6^Catalytic Hydroamination of Aminoalkenes^ 207 4.6.1^Introduction^ 207 vii ^4.6.2^Results and Discussion^ 207 4.6.3 Summary^ 209 4.7^Conclusions 210 4.8 Experimental^ 211 4.8.1^General Considerations^ 211 4.8.2 Starting Materials and Reagents^211 4.8.3^Synthesis^ 211 4.9^References 222 CHAPTER 5: CATALYTIC HYDROAMINATION MECHANISTIC STUDIES^ 230 5.1 Introduction^ 230 5.1.1 General Introduction^ 230 5.1.2 Late Transition Metal Catalyst Mechanisms^ 234 5.1.3 Rare-Earth Metal Catalyst Mechanism^ 238 5.1.4 Group 4 Catalyst Mechanism^ 240 5.2 Alkyne Hydroamination^ 242 5.2.1 Introduction 242 5.2.2 Results and Discussion^ 244 5.2.3 Summary^ 246 5.3 Alkene Hydroamination 247 5.3.1 Introduction^ 247 5.3.1.1 Cationic Group 4 Catalysts^ 247 5.3.1.2 Neutral Group 4 Catalysts 250 5.3.2 Results and Discussion^ 252 5.3.2.1 Imido Precatalyst Kinetic Analysis^ 252 5.3.2.2 Bis(amido) Precatalyst Kinetic Analysis^ 258 5.3.2.3 Substrate Scope and Reaction Optimization  ^262 5.3.2.4 Protonolysis Transition State Studies^ 264 5.3.2.5 Stoichiometric Experiments^ 267 5.3.2.6 Diastereoselectivity^ 276 viii ^5.3.3^Summary of Neutral Catalyst Studies^277 5.4^Cationic Alkene Hydroamination Investigations 278 5.4.1^Introduction^ 278 5.4.2 Results and Discussion^ 279 5.4.3^Summary of Cationic Catalyst Systems^283 5.5^Conclusions^ 284 5.6 Experimental 285 5.6.1^General Considerations^ 285 5.6.2 Starting Materials and Reagents^ 286 5.6.3^Kinetic Investigations^ 286 5.6.4 Synthesis^ 288 5.7^References 291 CHAPTER 6: CONCLUDING REMARKS AND FUTURE DIRECTIONS^ 297 6.1^Summary and Conclusions^ 297 6.2 Future Work^ 299 6.2.1^Insertion Reactivity of Alkyl Complexes^ 299 6.2.2 Fluorinated Amidate Ligands for Enhanced Reactivity ^300 6.2.3^Phosphinidene Complexes^ 302 6.2.4 Further Catalytic Hydroamination Mechanistic Investigations^ 303 6.3^References^ 307 APPENDIX A: X-RAY CRYSTALLOGRAPHIC DATA^ 309 APPENDIX B: COMPUTATIONAL DETAILS^ 324 B1^Bis(amidate) bis(amido) DFT calculations^ 324 B2 Bis(amidate) imido DFT calculations 331 B3^References^ 332 ix LIST OF TABLES Table 2.1: Table 2.2: Table 2.3: Table 2.4: Table 2.5: Table 2.6: Table 2.7: Table 2.8: Table 2.9: Table 2.10: Table 2.11: Table 3.1: Table 3.2: Table 3.3: Table 3.4: Table 3.5: Table 3.6: Table 4.1: Selected Bond Distances (A) and Angles (°) for [DmP(NO)P1 ]4Hf, 2.8^ 38 Selected Bond Distances (A) and Angles (°) for [DmP(NO)`1314Zr, 2.9^40 Selected Bond Distances (A) and Angles (°) for Ad[02N2]2Zr, 2.10^ 43 Selected Bond Distances (A) and Angles (°) for [ DIPP(NO)P1 ]3ZrNHPh, 2.14^ 49 Selected Bond Distances (A) and Angles (°) for r u(NO)Ph]2Zr(NEt2)2, 2.15 51 Selected Bond Distances (A) and Angles (°) for r u(NO)P112Ti(NEt2)2, 2.16^ 52 Selected Bond Distances (A) and Angles (°) for [ DmP(N0r12Zr(NMe2)2, 2.18 55 Selected Bond Distances (A) and Angles (°) for K 2 - [DMA0)tBl_ Kl -[DNIP(NO)tBIZr(NMe2)2(Py), 2.19^ 58 Selected Bond Distances (A) and Angles (°) for [ Div1P(NO)Ph]2Ti(NEt2)2, 2.23^ 68 Selected Bond Distances (A) and Angles (°) for [ DIPP(NO)P1 ]2Ti(NEt2)2, 2.24 71 Theoretical and Experimental Amax Values for 2.16, 2.23, and 2.24^75 Selected Bond Distances (A) and Angles (°) for [DmP(NO)P1 ]2HACH2Ph)2(THF), 3.6^  111 [DmP(40) tB12Zr(CH2Ph)2,Selected Bond Distances (A) and Angles (°) for 3.9^ 115 Selected Bond Distances (A) and Angles (°) for Ad [02N2]Zr(CH2Ph)2(THF), 3.14^  119 Selected Bond Distances (A) and Angles (°) for [DmANOY1312Zr(r1 2-2,6-Me2C6H3N=CCH2Ph)2, 3.16^ 125 Selected Bond Distances (A) and Angles (°) for [DrviP(NO) ffil2Zr(r14-ArNC(CH2Ph)=C(CH2Ph)NAr), 3.17 (Ar = 2,6-Me2C6H3)^ 128 Selected Bond Distances (A) and Angles (°) for {[ DiviP(NO)Ph]2Hg[t-0)}4, 3.21^ 139 Selected Bond Distances (A) and Angles (°) forruipp(No)phi2zr-(NH_ 2,6-Me2C6H3)(NMe2), 4.17^ 176 x Table 4.2:^Selected Bond Distances (A) and Angles (°) for[Dipp(No) Ph,j2Zr=N(2,6-Me2C6H3)(TPPO), 4.25^ 181 Table 4.3:^Selected Bond Distances (A) and Angles (°) for [D^)M(Nos tBu,j2Zr=N(2,6-Me2C6H3)(TPP0)(Py), 4.27^ 190 Table 4.4:^Selected Bond Distances (A) and Angles (°) for[Dipp(No)pri2zr - tt_( N(2,6-Me2C6H3))2Zr[DIPP(NO)Ph ] 2, 4.31^ 195 Table 4.5: ^ ^Selected Bond Distances (A) and Angles (°) for [DmP(N0) rB12Zr(v,-N(2,6-Me2C6H3))2Zr[DmP(NO)fflu]\TH(2,6-Me2C6}13), 4.33^ 198 Table 4.6:^Unoptimized alkene hydroamination results with precatalyst 4.25^ 209 Table 5.1:^Effect of electronic modification of amidate ligands on alkyne cyclohydroamination efficiency^ 245 Table 5.2:^Comparison of bond distances for bis(amidate) bis(amido) complex 5.9 and fluorinated variant 5.10 245 Table 5.3:^Optimized hydroamination results with precatalytic complex 5.36^ 264 Table 5.4:^Selected Bond Distances (A) and Angles (°) for K2- [DIPP(No)rst]_KJ_ [DIPP(Ncy tBu, 1I) ]Ti=NCH2CPh2CH2CH=CH2(NHMe2), 5.49•NMe2^ 272 Table A2.1: Crystallographic Data and Refinement Details for [ Dmi)(NO)P1 ]4Hf, 2.8^ 309 Table A2.2: Crystallographic Data and Refinement Details for [ Div[P(NO)rBl4Zr, 2.9 309 Table A2.3: Crystallographic Data and Refinement Details for Ad [02N2]2Zr, 2.10^ 310 Table A2.4: Selected Bond Distances (A) and Angles (°) for Ad [02N2]2Hf, 2.11^311 Table A2.5: Crystallographic Data and Refinement Details for Ad[02N2]2Hf, 2.11^ 311 Table A2.6: Crystallographic Data and Refinement Details for [ DIPP(NO)P113ZrNHPh, 2.14 312 Table A2.7: Crystallographic Data and Refinement Details for [rBu(NO)Ph]2Zr(NEt2)2, 2.15^ 312 Table A2.8: Crystallographic Data and Refinement Details for r u(NO)P112Ti(NEt2)2, 2.16 313 Table A2.9: Selected Bond Distances (A) and Angles (°) for ru(No)phvi(Nme2)2, 2.17^ 314 Table A2.10: Crystallographic Data and Refinement Details for [ fflu(NO)P1 ]2Ti(NMe2)2, 2.17 314 xi Table A2.11: Table A2.12: Table A2.13: Table A3.1: Table A3.2: Table A3.3: Table A3.4: Table A3.5: Table A3.6: Table A3.7: Table A3.8: Table A4.1: Table A4.2: Table A4.3: Table A4.4: Table A4.5: Crystallographic Data and Refinement Details for [Dm (NO)'1312Zr(NMe2)2,2.18^ 315 Crystallographic Data and Refinement Details for K2-[DmP(NO)'131- K1 -[DmP(NO)tBIZr(NMe2)2(Py),2.19 315 Crystallographic Data and Refinement Details for [DmP(NO)Ph]2Ti(NEt2)2, 2.23^ 316 Crystallographic Data and Refinement Details for [DmP(NO)P1 ]2Hf(CH2Ph)2(THF), 3.6^ 316 Crystallographic Data and Refinement Details for [DmP(NO)P1 ]2Zr(CH2Ph)2, 3.9 317 Crystallographic Data and Refinement Details for Ad[02N2]Zr(CH2Ph)2(THF), 3.14^ 317 Selected Bond Distances (A) and Angles (°) for Ad [02N2]Hf(CH2Ph)2(THF), 3.15 318 Crystallographic Data and Refinement Details for Ad [02N2]Hf(CH2Ph)2(THF), 3.15^ 319 Crystallographic Data and Refinement Details for [DmP(NO) t1312Zr(r12-2,6-Me2C6H3N=CCH2Ph)2, 3.16^319 Crystallographic Data and Refinement Details for [DmP(N0) tB12Zr014-ArNC(CH2Ph)=C(CH 2Ph)NAr), 3.17 (Ar = 2,6-Me2C6H3) Crystallographic Data and Refinement Details for Crystallographic Data and Refinement Details for Crystallographic Data and Refinement Details for ^ 320 {[DmA (T O)Ph]211f0A-0)}4, 3.21^ 320 [Dippo\r,ph, 2z,r kNTLT-4,/^e) ,v-ivievr,6.1-13Apt\ /NTiivLk e2 \ , 4 .1 71 ^iN L...321 CCrystallographic Data and Refinement Details for Table A5.1: xi i Table B2.1: Experimental and Calculated Bond Lengths (A) and Angles (°) for ^2.16^ 325 Table B2.2: Experimental and Calculated Bond Lengths (A) and Angles (°) for ^2.23 326 Table B2.3: Experimental and Calculated Bond Lengths (A) and Angles (°) for ^2.24^ 327 Table B2.4: Energetic Ordering of Geometric Isomers^ 328 LIST OF FIGURES Constrained geometry catalyst (CGC)^ 4 Selected early metal complexes with amido ancillary ligands^ 5 Selected aryloxy complexes and hydroamination regioselectivity^ 6 Living olefin polymerization phenoxyimine catalysts^ 7 Amidinate ligand and selected amidinate complexes 9 Guanidinate resonance structures and selected metal complexes^ 10 Carboxylate ligand framework and bimetallic paddlewheel complex ^ 11 Coordination modes of amidate ligands^  14 N-bound K'-amidate complexes 14 Ti complex with K2-0-bound and bidentate Ka -amidate ligands^ 15 Selected bridging bimetallic amidate complexes^ 16 Selected amidate complexes for bioinorganic studies 17 Amidate bridged Mo complex with Mo-Mo bonds (some para-anisyl units omitted for clarity)^ 18 Bis(amidate) bis(amido) Ti complex 21 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [DmP(NO)P1 ]4Hf, 2.8 (hydrogens and non-ipso phenyl carbons omitted for clarity)^ 38 View of [DivrP(NO)Ph]4Hf (2.8) down C2 axis of symmetry (left), and view of dodecahedral core structure (right)^ 39 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of D[ mANoyBu] 4L 2.9 (hydrogens omitted for clarity)^ 40 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of Ad[02N2]2Zr, 2.10 (hydrogens omitted for clarity)^ 42 ORTEP depiction (ellipsoids at 30% probability) of core structure of Ad [02Nd2Zr, 2.10 (hydrogens and adamantyl groups omitted for clarity)^ 42 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [DIPP(NO)Ph]3ZrNHPh, 2.14 (non-ipso phenyl carbons and hydrogens omitted)^ 48 Figure 1.1: Figure 1.2: Figure 1.3: Figure 1.4: Figure 1.5: Figure 1.6: Figure 1.7: Figure 1.8: Figure 1.9: Figure 1.10: Figure 1.11: Figure 1.12: Figure 1.13: Figure 1.14: Figure 2.1: Figure 2.2: Figure 2.3: Figure 2.4: Figure 2.5: Figure 2.6: xiv Figure 2.7: Figure 2.8: Figure 2.9: Figure 2.10: Figure 2.11: Figure 2.12: ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [tBu(NO)Ph] 2Zt(NEt2)2 , 2.15 (hydrogens omitted for clarity)^ 50 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [ tBu(NO)P1 ]2Ti(NEt2)2, 2.16 (hydrogens omitted for clarity) ^ 52 Tautomers of amidate ligand^ 53 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of[ DmP(NO) tB ]2Zr(NMe2 )2 , 2.18 (hydrogens omitted)^ 54 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of 1(2-[Dmp(No)ti3u]_KJ_[DmANor]zr(Nme2)2(py), 2.19 (hydrogens omitted for clarity)^ 57 Extended structure of 2.19 in the solid-state (A), and viewed down crystallographic C2 screw axis (B) (C = gray, H = green, Zr = purple, N = blue, 0 = red; hydrogen bonds = dashed red and blue lines)^ 59 Figure 2.13: Possible geometric isomers of bis(amidate) Ti complexes^ 63 Figure 2.14: Relative energetic ordering of geometric isomers of 2.16 (Ti = white, N = blue, 0 = red, C = gray)^ 65 Figure 2.15: ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [ DmiP(NO)Ph]2Ti(NEt2)2, 2.23 (hydrogens omitted for clarity)^ 67 Figure 2.16: Relative energetic ordering of geometric isomers of 2.23 (Ti = white, N = blue, 0 = red, C = gray)^ 69 Figure 2.17: ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [DIPP(NO)Ph)2Ti(NEt2)2 , 2.24 (hydrogens omitted for clarity)^ 71 Figure 2.18: Relative energetic ordering of geometric isomers of 2.24 (Ti = white, N = blue, 0 = red, C = gray)^ 72 Figure 2.19: Frontier bonding orbitals of ground state isomer of 2.16 (Ti = white, N = blue, 0 = red, C = gray) 73 Figure 2.20: Amido 6-bonding orbital of ground state isomer of 2.16 (Ti = white, N = blue, 0 = red, C = gray)^ 74 Figure 2.21: Frontier non-bonding orbital of ground state isomer of 2.16 (Ti = white, N = blue, 0 = red, C = gray) 74 Figure 2.22: Amidate bonding interactions of ground state isomer of 2.16 (Ti = white, N = blue, 0 = red, C = gray)^ 75 Figure 2.23: Experimental UV/vis absorption spectrum of 2.16^76 XV Figure 2.24: Figure 3.1: Figure 3.2: Figure 3.5: Figure 3.6: Figure 3.7: Figure 3.8: Figure 3.9: Figure 3.10: Figure 3.11: Figure 3.12: Figure 4.1: Figure 4.2: Figure 4.3: Figure 4.4: Calculated UV/vis absorption spectrum of 2.16^ 77 Hapticities of benzyl ligands^ 109 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [ DmP(NO)Ph]2Hf(CH2Ph)2(THF), 3.6 (hydrogens omitted)^  111 Stacked plot 1 H NMR spectra showing THE exchange on [DmP(NO)P1 ]2Hf(CH2Ph)2(THF), 3.6^ 113 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [ DmP(NO) tBI2Zr(CH2Ph)2, 3.9 (hydrogens omitted for clarity)^  114 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of Ad[02N2]Zr(CH2Ph)2(THF), 3.9 (hydrogens omitted)^ 119 Common geometric isomers of bis(r1 2-iminoacyl) complexes^ 123 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [ DmP(NO)tI2Zr(r12-2,6-Me2C6H3N=CCH2Ph)2, 3.16 (hydrogens omitted for clarity)^ 124 ORTEP depiction (ellipsoids at 30% probability) of simplified structure of 3.16 viewed down the C2 axis of symmetry (amidate substituents and hydrogens omitted for clarity)^ 125 ORTEP depictions (ellipsoids at 30% probability) of [D1'ANOP12Zr(r1 4-ArNC(CH2Ph)=C(CH2Ph)NAr), 3.17 (Ar = 2,6-Me2C6H3) (A — hydrogens omitted, B — hydrogens and amidate substituents omitted)^  127 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of {[DmP(NO)Ph]211f(-1-0)}4 , 3.21 (hydrogens omitted)^ 137 ORTEP depiction (ellipsoids at 30% probability) of core solid-state molecular structure of {[ DmP(NO)Ph]2111(-1-0)}4 , 3.21 (hydrogens and amidate substituents omitted)^  139 Proposed structure of 3.24 in solution^  143 Grubbs and Schrock olefin metathesis catalysts^ 164 New Ti imido polymerization catalysts^  165 The first group 4 imido complexes 167 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [DIPP(NO) P1 ]2Zr(NH-2,6-Me2C6H3)(NMe2), 4.17 (non-N-H hydrogens omitted)^ 175 Figure 3.3: Figure 3.4: xvi Figure 4.5: Figure 4.6: Figure 4.7: Figure 4.8: Figure 4.9: Figure 4.10: Figure 4.11: Figure 4.12: Figure 4.13: Figure 4.14: Figure 4.15: Figure 4.16: Figure 4.17: Figure 5.1: Figure 5.2: Figure 5.3: Figure 5.4: Figure 5.5: Figure 5.6: Five-coordinate bis(guanidinate) imido complex and guanidinate resonance forms^ 177 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [DIPP(NO)P1 ]2Zr—N(2,6-Me2C6H3)(TPPO), 4.25 (hydrogens omitted for clarity)^ 180 ORTEP depiction (ellipsoids at 30% probability) of bottom view of solid-state molecular structure of [ DIP (NO)P1 ]2Zr=N(2,6-Me2C6H3) (TPPO), 4.25 (hydrogens omitted for clarity) 181 HOMO of [DIPP(NO)Ph]2Zr=N(2,6-Me2C6H3)(TPPO), 4.25^ 183 HOMO-1 of [DIPP(NO)P1 ]2Zr=N(2,6-Me2C6H3)(TPPO), 4.25^ 183 HOMO-59 of [DIPP(NO)P1 ]2Zr=1\1(2,6-Me2C6H3)(TPPO), 4.25^ 184 Space-filling models of 4.25 (imido ligand = purple balls, amidate ligands = green balls, TPPO ligand = gray balls)^ 185 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [ DmANO)thuJ2Zr=N(2,6-Me2C6H3)(TPP0)(Py), 4.27 (non-ipso phenyl carbons and hydrogens omitted for clarity)^ 189 Space-filling models of 4.27 with pyridine removed (imido ligand = purple balls, amidate ligands = green balls, TPPO ligand = gray balls)^ 191 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [DIPP(NO)Ph]2ZO-N(2,6-Me2C6H3))2Zr[DIPP(NO)P1 ]2, 4.31 (hydrogens omitted)^ 194 ORTEP depiction (ellipsoids at 30% probability) of core molecular structure of [DIPP(NO)P ]2Zr(li-N(2,6-Me2C6H3))2Zr[ DIPP(NO)Ph]2, 4.31 (bond distances in A)^ 195 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular NH(2,6-Me2C6H3), 4.33 (non-N-H hydrogens omitted)^ 197 Metallacycles relevant to catalytic hydroamination 200 Structures of alkaloid natural products^ 231 Biologically active N-heterocycles synthesized via hydroamination^ 239 Selected group 4 alkyne hydroamination catalysts^ 243 Cationic precatalysts for alkene hydroamination 248 Selected neutral Ti and Zr alkene hydroamination precatalysts^ 251 Amidate Ti and Zr aminoalkene cyclohydroamination precatalysts^ 252 structure of [DNIP(NO) °]2Zr( NO 6 M C 14 1)2___.76-3„r[DIvrp(Norl_ xvii Time-resolved 300 MHz I li NMR spectra showing consumption of substrate (olefinic signals (*) at 8 4.96 and 5.44)^253 Natural log of [5.32] vs. time plot showing first-order substrate dependence^ 254 Ln [5.32'] vs. time plot for 4 different catalyst loadings showing first- order substrate dependence over a wide range of catalyst loadings^ 256 Plot of kobs vs. [catalyst] (5.31) showing first-order catalyst dependence^ 257 Kinetic profile comparison of Zr amido 5.29 vs. imido 5.31 for cyclization of aminoalkene 5.32 ([5.32] vs. time)^ 259 Kinetic profile comparison of Zr imido 5.31 and amido 5.29 + TPPO for cyclization of aminoalkene 5.32 ([5.32] vs. time)^260 Kinetic isotope effect experiment showing small primary kinetic isotope effect for cyclization of aminoalkene 5.32 (Ln [5.32'] vs. time)^262 ORTEP depiction of solid-state molecular structure of 7-coordinate imido (A) and proposed protonolysis transition state (B)^265 Eyring plot for cyclization of substrate 5.32 between 90 and 117 °C using amido precatalyst 5.36^ 266 Arrhenius plot for cyclization of substrate 5.32 between 90 and 117 °C using amido precatalyst 5.36 267 400 MHz I ff NMR spectrum of Zr mixed amido complex 5.47^ 268 400 MHz 1 H NMR spectrum of Ti imido complex 5.49^ 270 ORTEP depiction of solid-state molecular structure (ellipoids at 30% probability) of K-2-[DIPP(NO) tB1-1(1 - NCH2CPh2CH2CH=CH2(NHMe2), 5.49•HNMe2 (non-N-H hydrogens omitted for clarity)^ 272 ORTEP depiction of core structure of 5.49•1-1NMe2 (hydrogen atoms and amidate/dimethylamine substituents removed for clarity)^ 273 Ln [substrate] vs. time for Ti imido 5.49 and Ti bis(amido) 5.48 showing similar rates and first-order substrate consumption^ 275 Rationalization of diastereoselectivity of cyclization^ 276 [5.34] vs. time plot for cationic Zr precatalyst 5.51 showing zero- order substrate dependence and product inhibition^ 281 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [DIPP(No)Ph-p-CF3]21z_fxrik 4,, \ , U4 .4.Avouvtczn (hydrogens omitted)^ 301 Figure 5.7: Figure 5.8: Figure 5.9: Figure 5.10 Figure 5.11: Figure 5.12: Figure 5.13: Figure 5.14: Figure 5.15: Figure 5.16: Figure 5.17: Figure 5.18: Figure 5.19: Figure 5.20: Figure 5.21: Figure 5.22: Figure 5.23: Figure 6.1: [Dipp(No) tsuiTi= xviii Figure A 1 :^ORTEP depiction (ellipsoids at 30% probability) of core solid-state molecular structure of Ad[02N2]2Hf, 2.11(hydrogens and adamantyl groups omitted for clarity)^ 310 Figure A2:^ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of r u(NO)P112Ti(NMe2)2, 2.17 (hydrogens omitted for clarity)^ 313 Figure A3:^ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of Ad[02N2]Hf(CH2Ph)2(THF), 3.15 (hydrogens omitted for clarity)^ 318 Figure B 1:^Experimental UV/vis spectrum of 2.23^ 329 Figure B2:^Experimental UV/vis spectrum of 2.24 329 Figure B3:^Calculated UV/vis spectrum of 2.23^ 330 Figure B4:^Calculated UV/vis spectrum of 2.24 330 xix LIST OF ABBREVIATIONS 2D^ 2-Dimensional A Angstrom Ad^ Adamantyl Anal. Analysis atm^ Atmosphere Ar Aryl BBI^ Broadband inverse Bn Benzyl br^ Broad °C Degrees Celsius CGC^ Constrained geometry catalyst Calcd. Calculated cat.^ Catalytic CCD Charge-coupled device cm^ Centimeter -1cm Wavenumber CI^ Chemical impact COSY Correlation spectroscopy Cp^ Cyclopentadienyl Cp* Pentamethylcyclopentadienyl Cy^ Cyclohexyl d Doublet dn^Number of d-electrons DCC Dicyclohexylcarbodiimide DD^ Dodecahedron d.e. Diastereomeric excess DFT^ Density functional theory DIPP 2,6-Diisopropylphenyl DME^ Dimethoxyethane DMP 2,6-Dimethylphenyl E Element E Energy e-^Electron Ea^Activation energy EI Electron impact Eq.^ Equation Equiv Equivalent Et^ Ethyl e.u. Entropy units eV^ Electon volts Fig. Figure FT^ Fourier-transform g Gas g^ Gram G Gibbs free energy AG1^Activation energy GC Gas chromatography GPC^ Gel permeation chromotography h Hours AH1^Enthalpy of activation HMQC Heteronuclear multiquantum correlation HOMO^Highest occupied molecular orbital HSAB Hard-soft acid-base Hz^ Hertz, s-I `Pr Isopropyl IR^ Infrared J Coupling constant k^ Rate constant K Equilibrium constant xxi K^ Kelvin kcal Kilocalorie KIE^ Kinetic isotope effect kJ Kilojoule L^ Ligand LMCT Ligand-to-metal charge transfer LUMO^Lowest unoccupied molecular orbital m Multiplet m^ Meta M Metal M^ Molar MALDI Matrix assisted laser desorption ionization MAO^ Methylalumoxane max Maximum Me^ Methyl Mg Milligram MHz^ Megahertz min Minute mL^ Millilitre [xL Microlitre mm^ Millimeter MOCVD Metal-organic chemical vapour deposition mol^ Mole mmol Millimole MS^ Mass spectrometry m/z Mass-to-charge ratio NA^ Natural abundance 1 3u n-butyl n^ Number (integer) NOE Nuclear overhauser effect NOESY^Nuclear overhauser effect spectroscopy NMR^ Nuclear magnetic resonance nm Nanometer o^ Ortho obs Observed ORTEP^Oakridge Thermal Ellipsoid Program OTf Triflate, trifluoromethylsulfonate p^ Para Ph Phenyl Py^ Pyridine R Organic group RCM^ Ring closing metathesis ROMP Ring-opening metathesis polymerization r.t.^ Room temperature [R"(No)R1 R"-N(CO)R' amidate anion (R" = variable group on N, R' = variable group on carbonyl) [R(02N2)]^R(CO)NCH2CMe2CH2N(CO)R bis(amidate) dianion (R = variable group on carbonyl) s^ Second s Singlet AS^ Entropy of activation SAP Square antiprismatic geometry sept^ Septet t Triplet 43u^ Tert-butyl tacn Triazacyclononane TC^ Coalescense temperature THE Tetrahydrofuran TM^ Trademark TMS Trimethylsilane TMS^ Trimethylsilyl TON Turnover number TPPO^ Triphenylphosphine oxide UBC^ University of British Columbia TTP p-Tolyl-NC(CH3)CHC(CH3)N-p-toly1 UV^ Ultraviolet Vis. Visible w^ Wide xs Excess Copyright Degree 8^ Delta, chemical shift A Delta, change in A^ Delta, elevated temperature Lambda, wavelength Kappa, denticity Mu, bridging rl^ Eta, hapticity FOREWORD The uniting theme within this dissertation is the development and utility of the amidate ligand set for group 4 metals. This thesis covers the topics of coordination chemistry and organometallic chemistry of group 4 amidate complexes, as well as imido chemistry and hydroamination catalysis. This is a manuscript based thesis and each chapter is meant to be a stand-alone document. As such, there are some instances of repetition between chapters, and certain compounds are utilized in more than one chapter. Compound numbering is consistent within each chapter; however, in cases where a compound is used in multiple chapters, the compound number from its first appearance may also be given in subsequent chapters for reference purposes. The work presented in this dissertation represents a new project within the Schafer research group. A literature review of each area of investigation is provided at the beginning of each chapter, and is meant to be a resource for future researchers in the Schafer group. Subsections within each chapter are divided into introduction, results and discussion, and summary sections, to clearly separate background material from experimental results. Each experimental chapter ends with a detailed conclusions section, and as such, Chapter 6 provides only an overall summary and conclusion, without many of the details present in Chapters 2-5. X-ray crystallographic data and refinement details for all crystallographically characterized compounds are presented in Appendix A. Appendix B contains details regarding the theoretical (DFT) calculations performed during the course of this work. xxv ACKNOWLEDGEMENTS I would like to first thank my research supervisor, Dr. Laurel Schafer, for her incredible support, encouragement, and guidance during the course of this research. The exploration of new ideas was strongly encouraged by Laurel, and this cultivated an atmosphere of independent thought and creativity within the research group. Working as a member of the Schafer research group has been an extremely rewarding experience. I have enjoyed my time with each and every member to come through the group. In particular, I would like to thank Ali, Louisa, Jason, Mark, Rashidat, Dave and Courtney for many engaging discussions about chemistry, life, and everything in between. I would also like to thank Jason Bexrud and Dr. Federico Zahariev for our fruitful scientific collaborations. I am grateful to Dr. Mike Fryzuk and Dr. Michael Shaver for their encouragement and support during my graduate work, and for getting me interested in chemical research in the first place. I would also like to thank Dr. Jennifer Love, Dr. Brian James, and Dr. Pierre Kennepohl for many fruitful discussions and assistance in technical areas. The support staff in the UBC chemistry department has been instrumental in moving the research presented within this thesis forward. I would especially like to thank Ken Love and Brian Snapkauskas for their expertise and assistance with glovebox related issues, and Brian Ditchburn for always quickly repairing my J-Young NMR tubes and Schlenk lines. NMR spectroscopy was utilized extensively in this thesis, and I must thank Dr. Nick Burlinson, Zorana Danilovic, and Dr. Maria Ezhova for their expertise and assistance with some of the more complicated experiments presented in this work. X-ray crystallography was instrumental to the success of this project, and I would like to thank Dr. Brian Patrick for his numerous crystallography lessons, and for teaching me how to collect, process, and solve crystallographic data. Finally, I would like to thank my parents for supporting me in all senses of the word during all of my academic endeavors. Without their endless love and support, this thesis could never have been written. I must also thank Shiva Shoai, she has been a bastion of strength when I have needed it most, and has always pushed me to succeed and be a better person. I dedicate this work to them. xxvi Chapter 1: Amidates as New Modular Ligands for Group 4 Metals CHAPTER 1 Amidates as New Modular Ligands for Group 4 Metals 1.1 Ligand Driven Reactivity of Early Transition Metals There are a number of unique properties of transition metals that give them the capacity to facilitate novel transformations otherwise not possible by standard organic synthesis. Transition metals, having d-orbitals available for bonding, are capable of adopting expanded coordination numbers of 4 through 9 and higher." In addition, many metal complexes are known to have multiple stable oxidation states, which can facilitate interactions with a variety of different reagents. 1^References begin on page 24 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals Metals facilitate reactivity by bringing reagents together in a such a way that they have the appropriate energy and orientation to allow for their combination. While reaction geometry is important to allow for successful chemical transformations, in many cases the thermodynamic requirements of the reaction are a larger obstacle to overcome. In particular, reactions that require the cleavage of very strong bonds typically require metals. One of the best examples of this is in the cleavage and functionalization of dinitrogen. The N2 molecule has an extremely strong N=N triple bond. The ability to utilize N2 from the atmosphere as a source of nitrogen in the formation of new N-E bonds (E = C, H, 0, etc.) represents one of the biggest challenges for synthetic chemistry. 9 One of the most important industrially synthesized molecules is ammonia, which is generated on a massive scale every year.' ° ' 11 The Haber-Bosch process has been developed to produce ammonia from nitrogen and hydrogen as shown in Eq. 1.1. 9-11 This process utilizes heterogeneous metal catalysts, typically containing iron or ruthenium. 12 ' 13 The gases are passed at high temperatures and pressures over an activated catalyst surface, where they are physisorbed and chemisorbed onto the surface. The adsorbed N and H atoms form new N-H bonds on the surface of the metal catalyst, and NH3 is subsequently eliminated as a gas. N2 (g) + 3 H2 (g) Metal Catalyst 2 NH 3 (g)^(1.1) 100-300 atm 400-550 °C Heterogeneous catalysis is prevalent in other industrial processes such as ethylene polymerization, where stereocontrol is not an issue. In fact, the vast majority of polyethylene is produced with highly effective heterogeneous Ziegler-Natta catalysts, which are mixtures of titanium chloro complexes and aluminum alkyls. 14-18 The development of homogeneous catalysts has enabled polymerization of a-olefins, which can generate polymers of a particular tacticity. 19-23 For example, the symmetry of cationic Ti and Zr complexes influences the tacticity of the polymer produced, and therefore the physical properties of the material generated. 23 With these homogeneous polymerization catalysts, the ligands play two important roles. First, the ligands solubilize the metal centers so that the catalyst and reactants are 2^References begin on page 24 H2 - N2 1.1 1.3 H2 22 °C H2 85 °C 1.51.2^ 1.4 N ..........cZi.--N -=--N— N N• Chapter 1: Amidates as New Modular Ligands for Group 4 Metals in the same phase. Second, the ancillary ligands determine the symmetry of the overall complex, which dictates the nature of the products formed. In the case of propylene polymerization, this involves the tacticity of the polypropylene generated.22' 24-30 While the factors determining reactivity at metal centers are complex, in general, the ancillary ligands control steric access to the metal through the presence of groups that can be varied in size. Additionally, the relative electronic properties of the metal center can be modulated by the ancillary ligand. An excellent example of how small changes to the ancillary ligand result in large changes in observed reactivity is shown in Scheme 1.1. The complex (T1 5-05Me5)2ZrC12 has long been known to generate the end-on bound bridging dinitrogen complex [(Il i -C5Me5)2Zr(Ti 1 -N2)10-T1 1 :1 1 -N2) (1.1) upon reduction with Na/Hg amalgam in the presence of N2. 31 ' 32 Chirik and coworkers have recently shown that a very similar complex (11 5 -05Me4H)2ZrC12, when reduced under the same conditions, generates the side-on bound bridging dinitrogen complex [(1 5-05Me4E1)2Zr]2([1,41 2 :11 2-N2) (1.2). 33 This difference in binding geometry of the N2 is critical to the resulting reactivity displayed by these complexes. Scheme 1.1 + NH3 While the end-on bound complex 1.1 reacts with H2 and simply eliminates N2, generating the dihydrido complex 1.3,31 ' 32 the side-on bound complex 1.2 adds H2 to the 3^References begin on page 24 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals bound N2 ligand, generating bridging hydrazido ligands, as shown in 1.4. 33 This complex can react further with H2 to generate NH3 and the dihydrido complex 1.5. 33 By changing the ancillary ligand by one methyl group a dramatic change in reactivity is observed, 33-36 allowing for the functionalization of dinitrogen, an extremely challenging, and rarely observed reaction. 37-44 While cyclopentadienyl (Cp) derived ligands have been heavily exploited in organometallic chemistry, it is relatively difficult to modify the substitution of these ligands in comparison to other systems. 1.2 Non-Cyclopentadienyl Ligand Systems for Early Transition Metals While metallocenes offer rich stoichiometric and catalytic chemistry, the difficulties and limitations associated with modifying these ancillary ligands resulted in an explosion of interest in the study of non-metallocene complexes, mainly for the development of new polymerization catalysts. 45-49 In particular, early transition metal complexes were heavily studied, and several important classes of ligands were found to act as excellent stabilizing groups for these metals. Due to the hard, highly electrophilic nature of early transition metals, the most successful ligand designs for these metals incorporate hard donors, such as N and 0. One important success in this area was the development of the constrained geometry catalysts (CGC) 1.6, shown in Fig. 1.1, which incorporate a Cp ligand, and a hard amido ligand that are tethered together. 50-53 This combination effectively stabilizes the highly electrophilic metal center, and ties the ligand back, opening a "reactivity pocket" in which polymerization catalysis can occur. 50 Me2Si /Ti ^Ph 1^Ph 1.6 Figure 1.1 Constrained geometry catalyst (CGC) 4^References begin on page 24 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals The success of the CGC ligand spurred the development of a wide variety of amido ligands for early transition metals. 45 ' 54 Many permutations of the amido ligand have been developed, including monodentate, bidentate, tridentate, and tetradentate motifs. 45 ' 54-56 While it is impossible to introduce this entire area, a few notable examples are shown in Fig. 1.2.  Na0 (^N\ . s me Ti .., ....„ N /^Me 1.7 ^ 1.8 ^ 1.9 Figure 1.2 Selected early metal complexes with amido ancillary ligands In 1996, McConville and coworkers discovered the first catalyst capable of living polymerization of a-olefins. 57' 58 Complex 1.7, once activated with B(C6F5)3, was capable of polymerizing 1-hexene, 1-octene, and 1-decene in a living manner, with polydispersities of nearly 1.00. This new diamide ligand essentially eliminated chain- termination processes, and opened new avenues to access block copolymers with extended aliphatic side groups. The mixed aryl-alkyl amido ligand developed by Cummins and coworkers has allowed access to a plethora of metal-element multiple bonds, including oxo (M=0),59 nitride (mmN),43, 60 and phosphide (M=P) (1.8) complexes, 61 which are accessed through reduction chemistry. The steric protection afforded by these monodentate amido ligands, and their electron rich nature, allows for the stabilization of low-valent, low-coordinate trigonal planar metal species, which are extremely reactive toward normally inert molecules like N2 and P4. 62, 63 A third important ligand system which has been developed is the bidentate 13-diketiminate, or NacNac, ligand architecture. This ligand system has been widely exploited by numerous research groups for a wide variety of applications. 64 ' 65 In particular, Mindiola and coworkers have 5^References begin on page 24 NR" C6H 13 < CH 3 Markovnikov when R = tBu, R' = Me NR" C6H13 H13 anti-Markovnikov when R = i Pr, R' = H C6H 13^H + R"—NH2 1 + Zr` N- / C 1.10 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals had great success stabilizing reactive alkylidene (M=CHR) (1.9) ,66, 67 imido (m=NR)968-70 and phosphinidenem( =Hz . 66) complexes with NacNac ligands on group 4 and 5 metals. 71 Oxygen-based ligands have also proven to be successful ancillary groups for early transition metals. In particular, aryloxy ligands have been used to great success by Rothwell and coworkers forgroup 4 organometallic chemist 72-77 especially in the synthesis and reactivity of 11 2-iminoacyl complexes like 1.10 (Fig 1.3). 78-81 Rothwell also first reported that the aryloxy ligand system is capable of facilitating the catalytic hydroamination of alkynes. 82 Recently, Beller and coworkers have elaborated on this work demonstrating that the substitution at the 2- and 6- positions of the aryl rings in 1.11 dictates the regioselectivity of the hydroamination reaction. 83 ' 84 When R = tert-butyl, the predominant product formed from the hydroamination of octyne with aryl amines and aliphatic amines is the Markovikov product. 83 However, when R = isopropyl, the regioselectivity reverses to favor the anti-Markovnikov product. 84 Figure 1.3 Selected aryloxy complexes and hydroamination regioselectivity Since both nitrogen and oxygen donors proved to be highly effective for the formation of stable, but reactive early transition metal complexes, the combination of these donors would logically generate similarly stable complexes capable of promoting novel reactivity. One particular mixed N,0 chelating ligand that has been extremely successful is the phenoxyimine ligand. The most prevalent use of this ligand type has 6^References begin on page 24 tBu C6F5 N ,,,,, , N tBu Ti /Cr CIC6F5 tBu C6F5 N ,,,,, T . .......... N /Cr SCI C6F5 0 tBu tBu Chapter 1: Amidates as New Modular Ligands for Group 4 Metals been in the generation of exceptionally reactive olefin polymerization catalysts. 85-87 Coates and coworkers have shown 1.12 (Fig. 1.4) to be a living catalyst for the polymerization of propylene. 88' 89 A closely related complex 1.13, was also shown to produce polyethylene in a living manner by Fujita and coworkers at Mitsui.90 ' 1.12^ 1.13 Figure 1.4 Living olefin polymerization phenoxyimine catalysts While these complexes demonstrate how different ligand systems can promote varied reactivity, and small differences within a class of ligands can lead to divergent reactivity, systematic investigation of structure-activity relationships with metal complexes can only be effectively accomplished with modular ligand systems. 1.3 Modular Ligand Systems Modular ligands are generated by combining simple modifiable building blocks, where variations in the final ligand are brought about by simply changing the nature of the building blocks in the synthesis. A generic scheme for such a synthesis is illustrated in Eq. 1.2, where Ax is one reagent in the synthesis, with side groups Xi through Xn, where these substituted groups are either present in the commercially available starting material, or are installed by simple synthetic routes. Likewise, B Y is a second reagent with similarly variable side groups Y 1 through Yn . The number of components involved in the synthesis of the proligand can vary, but ideally all reagents should be combined in a single-pot reaction, resulting in an easily isolated and purified proligand C XY . This type 7^References begin on page 24 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals of synthesis readily lends itself to combinatorial methods, where libraries of systematically varied proligands can be generated rapidly for use in the synthesis of catalyst candidates for various reactions. 92-94 Ax + BY  CXY^(1.2) The simplest possible modular ligands are alkoxide or aryloxide ligands, where the monoanionic ligands are derived from alcohols (ROH), of which numerous examples are known and commercially available. Similarly, monoanionic amido ligands are very simple modular ligands, derived from primary (NH 2R) or secondary (NHRR') amines, where the amines are typically either commercially available, or are easily synthesized in a few steps. The phenoxyimine ligands shown in Fig. 1.4 are an excellent example of a highly variable modular ligand architecture. Synthesis of the phenolimine proligand is accomplished in a single step, through Schiff base condensation of substituted salicylaldehydes and substituted anilines, as shown in Eq. 1.3. 95 Combinatorial methods have been applied to this ligand system to generate heteroligated bis(phenoxyimine) catalysts for olefin polymerization, which exhibit high activity, and excellent tacticity selectivity. 96 (1.3) R3 Most bidentate ligand systems generate 5- or 6-membered metallacycles upon coordination to the metal center, where the phenoxyimine ligand set is a prime example. Several modular nitrogen-based bidentate ligand systems have been exploited, which form 4-membered metallacycles when coordinated to metal centers, and these show novel reactivity and interesting coordination chemistry. 8^References begin on page 24 Br^ Br I / N--( f■ I^,,,,N NY N 1.15 ^ Me3Si^SiMe3 I^1 N Me3Si Tli^/SiMe3 N^N --, 1 Me3Si^SiMe Chapter 1: Amidates as New Modular Ligands for Group 4 Metals 1.4 Four-Membered Chelate Systems Amidinate ligands are monoanionic bidentate donors, which are derived from organic amidines. While amidinate ligands have been widely utilized for many metals, 97 ' 98 they have been most heavily exploited for groups 4 and 5 metal chemistry. 97" 1 " These modular ligands can be easily varied at the R and R' positions (Fig. 1.5), and the resulting ligand offers substantial steric protection, while also satisfying the metal center's electronic requirements by donating four electrons in a a-fashion. While an additional lone pair of electrons is available for xr-donation to the metal center, these electrons are generally thought to delocalize into the backbone of the amidinate ligand, and are essentially uninvolved in bonding to the metal center. 97' 104 1.14 Figure 1.5 Amidinate ligand and selected amidinate complexes While amidinate complexes have been applied to many types of reactivity, including olefin polymerization, 105-1°7 one of the most interesting areas of study has been in the activation of N2. Bis(amidinate) dichloro complexes were shown by Arnold and coworkers to generate end-on bound 1,t-dinitrogen complexes upon reduction with Na/Hg amalgam (1.14, Fig. 1.5).108-110 Likewise, Sita and coworkers have utilized mixed cyclopentadienyl amidinate complexes for a wide range of reactivity, but have recently found that these species generate dinitrogen complexes, which exhibit side-on N2 binding (1.15), and undergo reaction with H2, alkyl halides, and silanes. 41 ' 42 9^References begin on page 24 R' N. ^1,2,R' 1 1=t N^N^ N^N^e e e ,R' /s\ N N e A B C Chapter 1: Amidates as New Modular Ligands for Group 4 Metals Another well studied ligand motif that generates 4-membered metallacycles when bound to metals are the guanidinates, or deprotonated guanidines. 111 These ligands are closely related to amidinates, where guanidinates bear an additional amine group in the backbone of the ligand, as illustrated in Fig. 1.6. As with the amidinates, these ligands can be modified sterically and electronically at both of the side groups (R), as well as the backbone amine group (R'). The Arnold, 112, 113 Bergman, 112, 113 and Richeson 114-119 groups have invested considerable effort in generating novel group 4 complexes supported by guanidinate ancillary ligands. Dinitrogen complexes supported by guanidinate ligands have been isolated (1.16), and are quite stable, due in large part to the electron rich nature of the guanidinate ligands. 112 The source of this stability is also featured in Fig 1.6, where resonance structure B illustrates how the amine group in the ligand backbone can effectively provide additional electron donation to the metal center. 112 ^'Pr^'Pr 'Pr HN^ I^I \ pri -N1 ''Pr N NI Pry^I^Pr 'PrHN^ / NH'Pr ^ I^I 'Pr^'Pr 1.16 Nrr =N 'Pr HN^'Pr M = Ti, Zr 1.17 Figure 1.6 Guanidinate resonance structures and selected metal complexes 10^References begin on page 24 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals Novel 5-coordinate imido complexes of group 4 metals have also been characterized with guanidinate ancillary ligands (1 .17) . 115, 116 These low-coordinate complexes are stabilized by the aforementioned electron rich nature of the guanidinate ligands, as well as the steric protection afforded by the substituents on the two nitrogens coordinated to the metal center. These imido complexes have been isolated for both Ti and Zr, 115, 116 and the Ti complexes show modest activity towards alkynes in catalytic hydroamination. 116 It is also interesting to note that the imido complexes (1.17) can be accessed by a novel and rare rearrangement reaction with arylisocyanides, 115' 120 suggesting that the guanidinate ligands are able to promote unique modes of reactivity. The mechanism for this interesting reaction will be revisited in Chapter 3.  R 1.18 Figure 1.7 Carboxylate ligand framework and bimetallic paddlewheel complex While the carboxylate ligands shown in Fig. 1.7 are inherently less variable than the amidinate and guanidinate ligands, they have nonetheless proven successful as structurally rigid bridging ligands for bimetallic transition metal complexes. 121-123 These ligands have been instrumental in the quest to understand metal-metal bonding (1.18 in Fig. 1.7), and bond orders greater than 3. 124-128 Extension of carboxylate based paddlewheel complexes as core units for supramolecular chemistry has also been very fruitful, with supramolecular arrays of various shapes and sizes being successfully isolated. 123 The interesting magnetic properties of these materials may prove important in the development of new nanodevices for future applications. 123 While carboxylate ligands are excellent bridging ligands, their application to monomeric metal complexes has not been widely explored. 11^References begin on page 24 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals Despite the fact that the structurally related amidinate, guanidinate, and carboxylate ligands have been widely investigated, their mixed N,O chelating congeners, the amidates, have not been extensively studied. Considering the success seen with the mixed N,O chelating phenoxyimine ligands, described in sections 1.2 and 1.3, it was hypothesized that amidate ligands, possessing a similar N,O chelate, would offer unique reactivity. Additionally, the 4-membered chelates generated upon coordination of amidinate and guanidinate ligands to metal centers result in novel structures and reactivity. Thus, the 4-membered chelates formed upon coordination of amidate ligands would likewise be expected to generate interesting coordination complexes, possibly displaying novel and unexpected reactivity. 1.5 Amidate Ligands 1.5.1 Synthetic Methodology Amidate ligands are deprotonated organic amides, which in turn can be synthesized in a single step by combination of acyl chlorides with primary amines as illustrated in Scheme 1.2. 95 By incorporation of a diamine in the synthesis, tethered bis(amide) proligands can also be generated through the same procedure. Due to the wide range of commercially available acyl chlorides and amines/diamines, the amidate ligand system is highly modular. This makes this ligand system ideally suited for the study of structure-activity relationships in catalytic and stoichiometric reactivity of metal complexes bearing these ligands. 0^ 0 NEt3 CH2Cl2^R'^ N R" H ÷ R"NH2 R'^CI  2 +^H2N ^_... NH2 2 NEt3 R'^NH^HNR'CI CH2Cl2 Scheme 1.2 12^References begin on page 24 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals These proligands can be complexed to metals through two main routes, illustrated in Scheme 1.3. Addition of an alkali metal base to organic amides results in deprotonation of the amide to generate the alkali salt of the amidate ligand. 129 Transmetallation can then be performed with a metal halide starting material, resulting in the desired amidate complex. 129 Conversely, the pKa of an amide proton is approximately 15, making the amide proton acidic enough to participate in protonolysis reactions with metal amido or alkyl starting materials. 95 Installation of the amidate ligand is accompanied by the formation of amine or alkane byproducts, which are volatile and easily removed. While the syntheses presented in Scheme 1.3 are the most widely utilized routes into amidate complexes, select amidate complexes have also been generated through the insertion of isocyanates (RN=C=O) into the metal-carbon bonds of metal alkyl complexes. 13°-132 0 R" + MR R^N H - RH () Awl OM mix R"^ R" R'^N^-MX^R'^N  - RH ^0 ,INTI R" + [M'IR^ \ R" R^N Scheme 1.3 R = alkyl, amido [M'] = transition/main group metal, lanthanide, actinide M = alkali metal 0 R'^N H 1.5.2 Binding Motifs Scheme 1.3 highlights an important feature of the amidate ligand set. There are several distinct modes of coordination that amidate ligands can adopt when bound to metal centers. These binding motifs are summarized in Fig. 1.8. While the metal complexes illustrated in Scheme 1.3 are bound in a monodentate fashion through the oxygen donor (A), the amidate ligands can also coordinate through the nitrogen in a mondentate fashion (B). Furthermore, it is also possible for both donors to coordinate to metal centers at the same time. This can result in chelation to a single metal in a x 2 - fashion (C), or bimetallic species can be generated, where the amidate ligand acts as a 13^References begin on page 24 - 2+ H3N^NH Pt H3 N^NH N H3 H3N „.^,„ NH H 3N^NH3 H3 Ph3P \^N Pt Ph3 P N O Chapter 1: Amidates as New Modular Ligands for Group 4 Metals bridge between two metal centers, with the nitrogen donor coordinated to one metal, and the oxygen donor coordinated to the other metal (D). M O R^NR' M R.N  / M \ A B^ C^ D Figure 1.8 Coordination modes of amidate ligands While numerous amidate complexes have been isolated exhibiting these four coordination modes, in the interest of brevity, select examples will be given here. A recent microreview on amidate complexes has additional references for each of these coordination modes. 133 In general, amidate ligands bind in a monodentate fashion (A and B) to metal centers that are coordinatively saturated, and can not accommodate both donors in the coordination sphere. 134 The choice of N vs. 0 donation fits largely with the principles of hard-soft acid-base (HSAB) theory, where soft metals are bound through the N donor, and hard metals are bound through the 0 donor. 135 For example, the Pt complexes 1.19 136 and 1.20, 137 and the Co complex 1.21 134 have all been characterized as N-bound amidate species, in keeping with the soft nature of these metals (Fig. 1.9). 1.19^ 1.20^ 1.21 Figure 1.9 N-bound x'-amidate complexes 14^References begin on page 24 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals In contrast, harder metal centers like Ti tend to bind monodentate amidate ligands through the oxygen donor. 135 ' 138 One such complex (1.22) has been characterized by Stahl and coworkers, where one of the amidate ligands is bound in a x 3 -fashion to the metal center through the hard oxygen donor. 135 ' 138 This complex is illustrated in Fig. 1.1 0, and also features two bidentate chelating amidate ligands (coordination mode C, Fig. 1.8). While the donation of N vs. 0 is dictated largely by thermodynamics, the propensity of K2-amidate ligands to bridge between metal centers or chelate to a single metal center is controlled mainly by steric bulk. Steric properties of both the amidate ligand, and the metal center are important, as too much bulk at a small metal center will prevent chelation from occurring. In contrast, too little steric bulk at a larger metal center may promote bridging interactions. PhN Ph ' • • ..^.... N _.....:11' ..^ :\......... 0"...- I No.. ..> 1- --- NPh 1.22 Figure 1.10 Ti complex with K-1 -0-bound and bidentate x2-amidate ligands In Fig. 1.1 1, selected complexes are shown that illustrate how amidate ligands can bridge between metal centers. The aluminum complexes 1.23 and 1.24 indicate that even with relatively bulky substituents on the N donors of the amidate ligands, bridging interactions can still occur. 139 This can be rationalized by the lack of steric bulk provided by the methyl ligands in 1.23 and 1.24. In addition to illustrating bridging interactions, 1.23 also demonstrates that the oxygen donor of the amidate ligand can be part of a bidentate chelate, and bridge to a second metal center at the same time. Complex 1.25 is a so-called lantern-type complex, 14° which is structurally related to the paddlewheel complexes described in section 1.4. With this general knowledge in hand, the following section will highlight some areas where amidate ligands have proven useful in both structural studies and reactivity investigations. 15^References begin on page 24 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals NAVe X O—AI—N AI-0^Li /Me Me Ph N' . '0 Me..^/^\ .... , Me ^ .... AI Al '''....."" , ---,,Me^\^/^Me O -• ,-- N,,,1____ IPh P'^/ '''' N t I CI^H some amidate substituents removed for clarity CI N„„ .. I 1.23^ 1.24^ 1.25 Figure 1.11 Selected bridging bimetallic amidate complexes 1.5.3 Selected Applications of Amidate Ligands 1.5.3.1 Biological Models Amidate ligands have an important tie to biological chemistry and bioinorganic chemistry, as they are derived from organic amides, which are the building blocks of peptides and proteins. This makes these ligands important models for the study of interactions between transition metals and peptide units in metalloenzymes. 141 As such, amidate ligands have been utilized in the investigation of biological models of peptide- metal interactions in bio-inorganic chemistry. 142 While this is a large field, a few select model complexes are given in Fig. 1.12. Vanadium is an essential element, 143-145 and is known to be a key component of two vanadium-dependent enzymes, the nitrogenases, 146 ' 147 and the haloperoxidases. 148 ' 149 In biological systems, vanadium is known to exist as the vanadyl unit (V=0 )2+:45 Model systems, like 1.26 15° and 1.27, 151 have been developed to study protein-metal interactions to better understand the role of vanadium in metalloenzymes like the nitrogenases 146, 147 and haloperoxidases. 148' 149 Another biologically important metal is nickel, which is present in Ni/Fe hydrogenases. 152 ' 153 Models have been developed for these species in an effort to determine the redox properties of the hydrogenases. 152 Amidate ligands, like the sulfide terminated bis(amidate) ligand in 1.28, have proven useful in this area. 154 16^References begin on page 24 lsr OHZ 1.26 VIN N ^N NH 1.27 1.28 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals Figure 1.12 Selected amidate complexes for bioinorganic studies 1.5.3.2 Amidates as Structural Units Bimetallic complexes exhibiting metal-metal bonds are interesting targets for materials applications and supramolecular architectures. 123 The dirhodium carboxylate complex (1.18), shown in Fig. 1.7, illustrates how carboxylate ligands can effectively act as bridging units for late transition metals. 121 Amidate ligands can also bridge metals in the same fashion, as shown by Cotton and coworkers in Fig. 1.1 3, where the bis(amidate) ligand links two Mo2 units, with each Mo2 unit bridged by one N,0 unit of the bis(amidate) ligand.' 55 The remaining ligands utilized to generate the paddlewheel structures around each Mo2 group are formamidinate ligands, substituted with para- anisyl groups. It was found that subtle changes to the substituents on the N donors of the bis(amidate) ligand could be used to fine tune the redox potential of the Mo2 unit.' 55 Bridging carboxylate ligands like that found in 1.18 have been utilized to generate a wide range of supramolecular species, having diverse sizes and shapes. 123 Bis(amidate) ligands, like the one bridging the Mo2 units in 1.29, have the potential to generate similar supramolecular complexes, which may have novel electronic properties vs. the known carboxylate species. 17^References begin on page 24 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals 1.29 Figure 1.13 Amidate bridged Mo complex with Mo-Mo bonds (some para-anisyl units omitted for clarity) 1.5.3.3 Amidates as Reactive Intermediates While sections 1.5.3.1 and 1.5.3.2 have illustrated how amidate ligands can be successfully applied in a structural capacity, this section will shed light on some catalytic processes that involve amidate species as intermediates. 1.5.3.3.1 Intermediates in Transamidation Reactivity Transamidation is a process which exchanges the nitrogen substituent of an organic amide through reaction of primary amines with amides, as illustrated in Eq. 1.4. 156 Recently, Stahl and coworkers have reported that a variety of metals facilitate this transformation. 156 In particular, Ti 135 ' 138 and A1 157 complexes are highly effective for this process when the amide substituents are not very large. 156 Tris(amidate) aluminum complexes are believed to be the resting state for Al catalyzed transamidation. 157 H2NR 0 R"^N Catalyst H2NR' 0 (1.4) R"^NH  The ability of the amidate ligands to bind in a KJ -motif to the metal center allows for coordination of the primary amine, which can then transfer a proton to the K'-amidate ligand, generating a neutral amide, as shown in Scheme 1.4. 157 Subsequent nucleophilic 18^References begin on page 24 i< NHR N-R ,z R" IL Chapter 1: Amidates as New Modular Ligands for Group 4 Metals attack of the amido ligand on the coordinated organic amide generates a tetrahedral intermediate. Exchange of the coordinated nitrogen groups in this intermediate results in a new tetrahedral intermediate, which can collapse to generate an amido complex with a coordinated neutral amide. 157 Exchange of the new organic amide for an incoming one closes the catalytic cycle. A similar mechanism has been proposed for Ti-based catalysts. 138 Due to the ability of Ti imido complexes to form upon reaction with primary amines, amidine formation has also been observed as a side product for these species. 138 This results from cycloaddition of organic amides with the imido fragment, generating Ti oxo complexes, and liberating the amidine in the process. R' AI^O 0^H2NR -g r H2 NR K ,R R" N0^H R" 1, N  . R' NH2R H ^ ^[ 4 / N-R , -------- 0 NHR' [AI ,̂N-R 0=< R" 11 If HR [i< 0 .NHR' R" HR' [ 0 HR UNHR' [ AItA0^R" Scheme 1.4 NHR R" 19^References begin on page 24 „P Ph2P—Pd—PPh2 Ph — N/ )--0 1.30 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals 1.5.3.3.2 Intermediates in Amidation of Aryl Chlorides Transamidation is a facile route into a wide variety of secondary organic amides. Recent work by the Hartwig and Buchwald groups has shown that Pd complexes can also catalyze the amidation of aryl chlorides to generate N-aryl amides. 158 ' 159 This reaction works for both primary and secondary amides, and requires the reductive elimination of the product from mixed aryl amidate Pd complexes, such as 1.30 in Scheme 1.5. 158 Both Ki - and c2-amidate complexes of Pd have been isolated by Hartwig and coworkers, 158 and it was found that the K2-amidate complexes (1.31) were slower to reductively eliminate the product than the K1 -amidate species (1.30). This observation is consistent with the increased stability of 1.31 afforded by the chelate effect. It was possible to minimize the occurrence of the x2-amidate species through the use of bidentate phosphine ligands, resulting in more reactive catalysts for this process. 158 -1 tBu^ .. PN /Ph or^tBu ^Pdwu / \ 0, ,, N ph 1.31 Scheme 1.5 90 °C - 110 °C P, h I Phi 0 reductive elimination 1.5.3.4 Amidates as Ancillary Ligands for Group 4 Metals While the transamidation and aryl chloride amidation reactivity described in the previous section is synthetically useful, the possibility of these reaction pathways presents a problem if the amidate ligand set is to be utilized as a non-reactive ancillary for group 4 metals. Previous to the research presented in the following chapters, a single report detailed the synthesis and characterization of Ti complexes bearing a phenylene bridged bis(amidate) ligand. 129 While salt metathesis was problematic for this ligand, the 20^References begin on page 24 Me2N„ , ,NMe2 ..---Ti -----9 \ 9'--N' Ti Me2N.^NMe2 1.32 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals protonolysis methodology presented in Scheme 1.3 allowed for the isolation of a bis(amidate) bis(amido) complex (1.32) in modest yield (33 %). 129 Solid-state molecular structure characterization indicated that 1.32 was dimeric in nature, where the amidate ligands bridged the two Ti centers, as illustrated in Fig 1.14. Clearly, successful implementation of amidate ancillary ligands for group 4 metals is possible, as demonstrated by complexes 1.22 and 1.32.129, 135, 138 However, careful choice of steric bulk in the amidate ligand is critical in the selective formation of mononuclear species. Additionally, bis(amidate) ligands must be designed with careful consideration of the geometric constraints imposed by the tethering unit if mononuclear complexes are to be isolated. Figure 1.14 Bis(amidate) bis(amido) Ti complex 1.6 Scope of Thesis This introduction has shown how ligand design can influence the reactivity of transition metal complexes. In particular, dinitrogen activation, olefin polymerization, and alkyne hydroamination were selected as illustrative examples of how different ligand systems can facilitate various types of useful reactivity. It was also noted that small changes to individual ligand types can lead to large changes in reactivity, making ligand 21^References begin on page 24 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals tuning an important area of investigation. Evolution of new ligand architectures in the post-metallocene era has been fruitful, and many new non-Cp ligands have been developed for early transition metals. Nitrogen and oxygen donors have been especially successful for the group 4 metals, and several examples of these ligands were discussed. Novel structures and reactivity have been observed for group 4 amidinate and guanidinate complexes in which these ligands bind the metals in a bidentate fashion, forming 4- membered chelates. Structurally related amidate ligands, which feature an N,0 chelating motif, have not been widely studied in conjunction with group 4 metals, and are the subject of this dissertation. The fundamental coordination chemistry of amidate ligands is explored in Chapter 2, where homoleptic amidate complexes, tris(amidate) mono(amido), and bis(amidate) bis(amido) complexes of the group 4 metals are studied. Static solid-state molecular structures and dynamic solution phase behavior of these complexes is discussed, with an emphasis on the factors controlling the fluxionality of these species. Theoretical (density functional) studies explore the interconversion between the different possible geometric isomers. Preliminary protonolysis and insertion reactivity with the bis(amidate) bis(amido) complexes is also addressed. Chapter 3 details the synthesis and reactivity of organometallic complexes supported by amidate ligands. Fluxional processes of these species are described, and the influence of the amidate ligands on the hapticity of benzyl ligands is discussed. Insertion of isocyanides into the Zr-C bonds of the alkyl complexes is explored, and both solid and solution phase behavior of the resulting complexes are evaluated. Products of hydrolysis and alkyl abstraction reactions are also presented. Catalytic hydroamination is an important area of investigation, and the Schafer research group has made important contributions to this area in recent years. Chapter 4 describes the synthesis, characterization, and behavior of amidate supported imido complexes, which are believed to be key intermediates in catalytic hydroamination. The unique solid-state molecular structures observed for these imido complexes are mirrored by their reactivity, where catalytic cyclohydroamination of alkenes is described for the first time using group 4 imido complexes. This novel reactivity is supported by the unique electronic structure observed for these terminal imido species. 22^References begin on page 24 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals Chapter 5 explores the mechanism of catalytic cyclohydroamination of alkenes through a variety of kinetic studies, and the isolation of intermediates in the catalytic cycle. These studies demonstrate the importance of imido species in the catalytic hydroamination of primary aminoalkenes. The cationic benzyl complexes discussed in Chapter 3 are also examined as precatalysts for cyclohydroamination of secondary aminoalkenes. Control experiments and preliminary kinetic investigations suggest a contrasting mechanism for these complexes, involving o-bond insertion, rather than cycloaddition of imido species. The overall utility of amidate ligands for group 4 metal chemistry is summarized in Chapter 6, with a brief description of future areas of investigation. The work presented in this dissertation represents the initiation of a new research program focused on the development of a modular ligand system, and the exploitation of the reactivity of complexes bearing these ligands. The fundamental studies presented herein are meant to guide future studies in the areas of hydroamination catalysis and small molecule activation with amidate complexes. 23^References begin on page 24 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals 1.7 References 1. Willey, G. 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Soc. 2006, 128, 5177. 31^References begin on page 24 Chapter 1: Amidates as New Modular Ligands for Group 4 Metals 158. Fujita, K.-I.; Yamashita, M.; Puschmann, F.; Martinez Alvarez-Falcon, M.; Incarvito, C. D.; Hartwig, J. F. I Am. Chem. Soc. 2006, 128, 9044. 159. Ikawa, T.; Barder, T. E.; Biscoe, M. R.; Buchwald, S. L. I Am. Chem. Soc. 2007, 129, 13001. 32^References begin on page 24 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes CHAPTER 2 Coordination Chemistry of Group 4 Amidate Complexes 2.1 Introduction The utility of amidate ligands has been well documented for many transition and main group metals. 1-3° Amidates, or deprotonated amides, are an easily accessed ligand motif, which can be derived from the reaction of an acyl chloride and a primary amine. A general synthetic scheme for the synthesis of organic amide proligands is shown in Eq. 2.1, where the R' and R" groups of the resulting organic amide are easily modified by choosing the appropriate acyl chloride and amine starting materials. 0^ 0 NEt3 R'^CI ^R"NH2^ CH2Cl2^R ' ^ R" H r(NO) R '}H © Reproduced in part with permission from Thomson, R. K.; Zahariev, F. E.; Zhang, Z.; Patrick, B. 0.; Wang, Y. A.; Schafer, L. L. Inorg. Chem. 2005, 44, 8680. Copyright 2005 American Chemical Society. 33^References begin on page 98 (2.1) 2.7 Bu (2.2) t Bu Ti Me2Nf^NMe2 Ti(NMe2)4, 1/2 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes The key proligands that were utilized for metal chemistry in this project are ru(No)ph,—in (2.1), [DIPP(NO)Ph]H (2.2), [D1v1P(NO)P1H (2.3), [DmP(NO)tB1H (2.4), [Dipp(No)tB — (2.5), and Ad[02N2]1-12 (2.6). The abbreviated notation used throughout this thesis for these (pro)ligands is shown in Eq. 2.1, where R" denotes the substituent on the nitrogen on the amidate ligand, and R' denotes the substituent on the carbonyl of the amidate ligand. In the case of the [02N2] tethered bis(amides), the variable R group is in the carbonyl position. Although extensively utilized with late transition metals as a structural ligand for generation of multinuclear metal complexes, amidate ligands have not been widely studied with group 4 metals. 3" 8 A single report in 2001 detailed the synthesis and characterization of Ti complexes bearing a phenylene bridged bis(amidate) ligand. 39 It was found that salt metathesis for this ligand was problematic, and ill-defined metal complexes were formed through this process. A protonolysis methodology was found to yield clean amido metal complexes; however, the species generated were not monometallic in nature, but rather bimetallic with the amidate ligands bridging between the two Ti centers (Eq. 2.2).39 The importance of incorporation of steric bulk into the amidate ligand proximal to the metal center was quite apparent from this early communication. 34^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Amidate ligation can also be observed with some metal complexes upon insertion of isocyanates into metal alkyl bonds. 40 ' 41 The general formation of such species is outlined in Eq. 2.3. Although there is precedent for this reaction with group 4 metals as well, metal complexes formed through this reaction do not always have bidentate bound amidate ligands. 42 R' I [ Mi—R + 0=C=N—R'  (2.3) R,R' = alkyl, aryl Amidate complexes have also been postulated and observed as reactive intermediates in certain metal-mediated or metal-catalyzed organic transformations. 28 ' 43- 45 The most important of these processes being catalytic transamidation, where a secondary organic amide R I CONHR2 reacts with a primary amine H2NR3 resulting in the formation of a new amide R i CONHR3 (Eq. 2.4). 43 0 + H2 N—R3 N R3 + H2N-R2^(2.4) H While amidate ligands offer great potential as modular ancillary ligands for transition metals, the fundamental understanding of these ligands and their interactions with metal centers is very limited, particularly for early transition metals. This chapter will address the fundamental coordination chemistry of group 4 metal amidate complexes. In particular, investigations of homoleptic amidate species will be presented, as well as tris(amidate) mono(amido) and bis(amidate) bis(amido) complexes. Solution phase and solid-state molecular structure characterization of these compounds, as well as density functional theory (DFT) calculations, will shed light on exchange processes, amidate hemilability, and geometric isomerization. Exploratory reactivity studies of these complexes will only be introduced, as this will be more fully explored in Chapters 4 and 5. 35^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes 2.2 Homoleptic Amidate Complexes 2.2.1 Introduction Amidate ligands bearing both N and 0 donor atoms are ideally suited to bind hard, high valent early transition metals, such as Ti, Zr, and Hf. While a preliminary report by Arnold and coworkers illustrated that amidate ligands could be installed on Ti, no studies had been performed with the heavier congeners of group 4 (Zr and Hf). 39 The potential of amidate ligands to coordinate in three possible forms (N-K 1 , 0-K1 , or K2), as well as the possibility of geometric isomerism, made the study of fundamental coordination chemistry an important place to start investigations. Previous work in the Schafer research group has demonstrated that a maximum of two amidate ligands will coordinate to Ti in a bidentate K 2-fashion. Addition of a third equiv of amidate ligand to the small Ti center resulted in complexes that had two K2 -amidate ligands, with the third amidate ligand bound in a Kl -fashion through the oxygen donor. 46 Given these results, it was deemed that 8-coordinate homoleptic Ti complexes would not be accessible, and attempts at synthesizing such species were abandoned. The considerably larger Zr and Hf centers are well known to have expanded coordination numbers, allowing for up to 9- and 10-coordinate complexes. 47-49 The homoleptic complexes of Zr and Hf were thus targeted for preliminary investigations. In addition to their potential for interesting coordination chemistry, homoleptic amidate complexes of Zr and Hf may have applications as precursors for metal-organic chemical vapor deposition (MOCVD) to generate controlled metal oxide surfaces for materials applications. 50-52 2.2.2 Results and Discussion It was reported by Arnold and coworkers that salt metathesis of amidate ligands is problematic, resulting in products that are ill-defined and difficult to separate from their salt byproducts. 39 By comparison, protonolysis of the amidate ligands with reactive amido starting materials such as Ti(NMe2)4 is clean and results in easily isolable amido 36^References begin on page 98 Ph ./Ph N Ol N Hf /r.1 1 Ph^— 0 ,4,1(N Ph 2.8 4 Hf(CH2Ph)4 THF 2.3 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes complexes. For this reason, protonolysis routes into homoleptic species are preferable to more traditional salt metathesis protocols. Given the fact that the pK a of the amide proligands is approximately 15, protonolysis of amido and alkyl metal starting materials is extremely facile. 53 The homoleptic complex [D1v1P(NO)1 4Hf, 2.8, is formed through protonolysis of Hf(CH2Ph)4 with four equiv of [ DmP(NO)P1 ]H (2.3) in THF, and is isolated as a white microcrystalline solid in 94 % yield, Eq. 2.5. High symmetry is observed in solution for this complex, as evidenced by a single resonance for the aryl methyl protons at 8 2.51, consistent with all four ligands being equivalent. Electron impact mass spectrometry verifies formation of the homoleptic complex, with a parent ion peak appearing at m/z 1076. While the proposed structure shown in Eq. 2.5 matches the solution phase data, the geometric conformation of the amidate ligands could not be confirmed without solid-state molecular structure evidence. (2.5) Colorless single crystals of 2.8 can be grown in moderate yield from concentrated hexanes or pentane solutions, and the solid-state molecular structure for 2.8 was determined by single crystal X-ray diffraction, verifying the hypothesized dodecahedral (DD) structure, as shown in Fig. 2.1. Selected bond lengths and angles are given in Table 2.1, and summarized crystallographic data are located in Table A2.1 in Appendix A. The solid-state molecular structure of 2.8 exhibits pseudo-D2d symmetry, where one of the C2 axes bisects N1 and N1_1, and also bisects N1 _2 and N1_3. This axis also lies at the intersection of the two pseudo-mirror planes within the molecule. The two perpendicular C2 axes of symmetry lie between the aforementioned mirror planes, bisecting 01 and 01_3, as well as 01 and 012. The most notable feature of interest in this complex is the Tr-stacking interactions that stabilize the N-cis conformation, which would otherwise be sterically unfavorable. 37^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Ci C1 01 C5 Figure 2.1 ORTEP depiction Cellipsoids at 30% probability) of solid-state molecular structure of [ DmP(NO) 1"14Hf, 2.8 (hydrogens and non-ipso phenyl carbons omitted for clarity) Table 2.1 Selected Bond Distances (A) and Angles (°) for [DmP(NO)P1 ]4Hf, 2.8 Lengths Angles Angles Hf(1)-N(l) 2.325(3) 0(0-H41)-0(1_1) 159.43(14) N(1)-Hf(1)-N(1_1) 83.81(18) Hf(1)-O(1) 2.163(3) 0(1)-Hf(1)-0(1_2) 91.83(3) N(I_1)-Hf(1)-N(1_3) 123.64(11) C(1)-O(1) 1.306(5) 0(1)-Hf(1)-0(1_3) 91.83(3) O(1)-Hf(1)-N(1) 58.39(12) C(1)-N(1) 1.285(5) O(1_2)-Hf(1)-O(1_3) 159.43(14) O(1)-C(1)-N(1) 115.7(4) The inter-ring distance for the Tr-stacked groups, as measured from the ring centroids is approximately 4.03 A, however, the ipso carbons are only 3.56 A apart, falling near the range typically expected for it-stacking (— 3.6 A). 54 ' 55 The solid-state molecular structure in Fig. 2.1 appears to be maintained in solution as there is no change in the 'H NMR spectrum over a wide range of temperatures. Eight- coordinate complexes tend to exist in one of three polyhedral parent shapes: cubic, square antiprismatic, or dodecahedral. 56 Complex 2.8 is best described as derived from a dodecahedral parent shape. The simplified illustration in Fig 2.2 shows how a dodecahedron can be visualized from 2.8. 38^References begin on page 98 EChapter 2: Coordination Chemistry of Group 4 Amidate Complexes Figure 2.2 View of [DmP(NO)P14Hf (2.8) down 02 axis of symmetry (left), and view of dodecahedral core structure (right) Dodecahedra are classically defined as 12 faced Platonic solids. 57 ' 58 Whereas a regular dodecahedron is composed of 12 pentagonal faces, the dodecahedron presented in Fig. 2.2 is a truncated dodecahedron with triangular faces. 57 ' 58 The overall polyhedron is essentially composed of two pentagonal prisms, which share three of their five edges (AB, BC, and CD). A related Zr complex can be synthesized following the same method shown in Eq. 2.5, starting with Zr(CH2Ph)4 rather than Hf(CH2Ph)4, and utilizing proligand 2.4 emp(No)tBui-H) rather than proligand 2.3. While the size difference between Zr and Hf is negligible, the effect of ligand electronics can be explored by varying the backbone group of the amidate ligand from an electron withdrawing phenyl group to an electron donating tert-butyl group. The resulting complex, [D1'p(No)ii314—r (2.9), is isolated in high yield (85 %) as a white microcrystalline solid. 'H NMR spectroscopy of the crude product is extremely simple, with a diagnostic resonance at 6 1.02 for the tent-butyl methyl protons and a resonance at 6 2.40 for the aryl methyl protons. This suggests that the symmetry of 2.9 is analogous to 2.8, and the solid-state molecular structure verifies this as shown in Fig. 2.3. Selected bond lengths and angles are given in Table 2.2, and crystallographic data are located in Table A2.2 in Appendix A. 39^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Figure 2.3 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [DMP(NO)tBu14Zr, 2.9 (hydrogens omitted for clarity) Table 2.2 Selected Bond Distances (A) and Angles (°) for [Dmil)(NO) tB14Zr, 2.9 Lengths Angles Angles Zr(1)-N(1) 2.2911(16) 0(2)-Zr(1)-0(4) 93.13(6) N(3)-Zr(1)-N(4) 85.83(6) Zr(1)-N(2) 2.3236(16) 0(2)-Zr(1)-0(1) 159.14(5) N(1)-Zr(1)-N(2) 85.78(6) Zr( I )-N(3) 2.2954(16) 0(4)-Zr(1)-0(1) 87.89(6) N(3)-Zr(1)-N(2) 124.08(6) Zr(1)-N(4) 2.3203(16) 0(2)-Zr(1)-0(3) 88.01(6) N(4)-Zr(1)-N(2) 120.19(6) Zr(1)-0(1) 2.1221(14) O(4)-Zr(1)-O(3) 159.13(5) 0(1)-Zr(1)-N(1) 57.68(6) Zr(1)-0(2) 2.1121(14) 0(1)-Zr(1)-0(3) 98.38(6) 0(2)-Zr(1)-N(2) 57.39(5) Zr(1)-0(3) 2.1251(14) N(1)-Zr(1)-N(3) 121.55(6) 0(3)-Zr(1)-N(3) 57.62(6) Zr(1)-0(4) 2.1192(14) N(1 )-Zr(1)-N(4) 124.03(6) 0(4)-Zr(1)-N(4) 57.41(5) In comparison to 2.8, the it-stacking interactions in 2.9 are much stronger with centroid distances of 3.61 A. It is likely that this is due to crystal packing and is not dictated by the actual change in the ligand backbone. However, the additional steric bulk of the distally located tert-butyl may force the two n-stacked rings closer together. It is also possible to synthesize 2.9 via protonolysis with the tetrakisamido starting material Zr(NMe2)4 in very similar yield, demonstrating the compatibility of the protonolysis methodology with both alkyl and amido starting materials. While both 2.8 and 2.9 are stable in solution and do not appear to isomerize on the NMR timescale, the possibility 40^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes for multiple geometric isomers exists, and the application of a tethered bis(amidate) ligand can be utilized to control which isomer is observed. The tethered bis(amide) proligand Ad[02N2]-12, 2.6, can be synthesized in an analogous manner to the bidentate amide proligands 2.1 — 2.5, where 1,3-diamino-2,2- dimethylpropane is utilized as the diamine, and is reacted with two equiv of 1- adamantoyl chloride. The addition of two equiv of proligand 2.6 to Zr(NMe2)4 or Hf(CH2Ph)4 results in the formation of the homoleptic species Ad[02N2]2M (M = Zr (2.10) and Hf (2.11)). The 1 H NMR spectra of 2.10 and 2.11 are completely analogous to each other, and are consistent with D2d symmetry in solution, similar to 2.8 and 2.9, as illustrated in Eq. 2.6. M = Zr, Hf R = NMe2 , CH2Ph 2.6^ M = Zr (2.10), Hf (2.11) A single resonance is observed at 8 1.28 for the backbone methyls in 2.10, implying that they are equivalent by static symmetry, or by exchanging through a `wagging' process which occurs more rapidly than the NMR timescale. In addition, single resonances are observed for the adamantyl methylene and methine environments as well as the backbone methylene groups, indicating that the two coordinated ligands are chemically and magnetically equivalent. These complexes are isolated as white microcrystalline solids in high yields (80 % for 2.10 and 81 % for 2.11) and can be recrystallized as colorless prisms suitable for X-ray crystallographic analysis. The solid- state molecular structure of 2.10 is shown in Fig. 2.4 (core structure shown in Fig. 2.5), with selected bond lengths and angles given in Table 2.3, and crystallographic data summarized in Table A2.3 in Appendix A. 2 + MR4 Et20 -78 °C^r.t. (2.6) 41^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Figure 2.4 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of Ad [02N2]2Zr, 2.10 (hydrogens omitted for clarity) Figure 2.5 ORTEP depiction (ellipsoids at 30% probability) of core structure of Ad[02N2]2Zr, 2.10 (hydrogens and adamantyl groups omitted for clarity) 42^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Table 2.3 Selected Bond Distances (A) and Angles (°) for  Ad[02N2]2Zr, 2.10 Lengths Angles Angles Zr(1)-0(3) 2.202(4) 0(3)-Zr(1)-0(3_2) 87.3(2) 0(3)-Zr(1)-N(3) 58.09(15) Zr(1)-N(3) 2.260(4) N(4)-Zr(1)-N(4_2) 117.2(2) O(3)-C(3)-N(3) 113.3(4) Zr(1)-0(4) 2.252(4) 0(3)-Zr(1)-0(4_2) 84.16(15) O(4)-C(4)-N(4) 113.4(5) Zr(1)-N(4) 2.240(4) 0(3)-Zr(1)-0(4) 164.14(13) C(3)-N(3) 1.300(7) N(4)-Zr(1)-0(4) 58.17(15) C(3)-O(3) 1.293(6) 0(4)-Zr(1)-0(4_2) 107.1(2) As Fig. 2.4 illustrates, the tethered bis(amidate) ligand binds in a planar fashion to the Zr center, with the second bis(amidate) ligand adopting the same planar conformation, but at 90° to the first ligand. The complex is octa-coordinate with approximate D2d symmetry, matching the observed solution phase behavior. While the adamantyl groups offer substantial steric bulk, they only protect the Zr center in a planar conformation, where each of the four adamantyl groups in Fig. 2.4 lies approximately in the plane of the page. This arrangement leaves the metal center relatively exposed, and 2.10 still exhibits a high degree of air/moisture sensitivity, decomposing in less than an hour when exposed to air, similar to 2.8 and 2.9. A simplified core structure is shown in Fig. 2.5, with the adamantyl groups removed for ease of viewing. The amidate ligand backbone binds to the Zr in a planar fashion; however, the tethering unit between the amidate N donors is bent out of the plane to satisfy the geometric constraints of the sp 3 - hybridized three carbon tether. The presence of a single resonance for these methyl groups in the 1 1-1 NMR spectrum verifies the supposition that the tethering unit for the ligand is 'wagging' up and down rapidly in solution, making C(13) and C(14) equivalent on the NMR timescale. In general, the bond lengths and angles for 2.10 are unremarkable. The analogous Hf complex Ad [02N2]2Hf (2.11) is isostructural with its Zr congener. The simplified solid-state molecular structure of 2.11 as determined by X-ray crystallography is illustrated in Appendix A in Fig. Al, with selected bond lengths and angles located in Table A2.4, and crystallographic data given in Table A2.5. Although isostructural, the Hf complex 2.11 actually has marginally shorter M-N and M-0 bonds than the Zr complex 2.10. This is a trend that has been seen in the literature for Hf complexes, and is due to relativistic effects. 59 43^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes The coordination geometry observed for these homoleptic species is akin to that seen for mixed nitrato acetoacetonato (acac) complexes of Zr and Hf published by Fay and coworkers.56' 60, 61 In contrast, the homoleptic acac complexes adopt a square antiprismatic (SAP) geometry. 62 The six-membered chelate formed by acac is better suited to bridge the longer edges of a SAP than the shorter edges of a dodecahedron (DD). While acetylacetonato ligands have a bite angle of approximately 75-80° owing to their six-membered chelate ring, the amidate ligands have a much tighter bite angle, at approximately 60°.61, 63 Thus, the substitution of acac ligands with nitrato ligands, having four-membered chelates, results in a shift in coordination geometry from SAP to DD. 60 ' 61 It is therefore logical that the homoleptic amidate complexes, with bite angles closer to that of nitrato than acac ligands, exist in a DD geometry. 2.2.3 Summary The synthesis of homoleptic amidate complexes of Zr and Hf was accomplished through protonolysis of tetrakis(amido) and tetrabenzyl Zr and Hf starting materials with amide proligands. Due to the smaller size of Ti, and its inability to facilitate coordination numbers greater than six, homoleptic complexes of Ti were not pursued. Both bidentate and tetradentate amidate ligands were utilized successfully, and the resulting complexes were all coordinatively non-fluxional in solution. In the solid-state, these complexes are of pseudo-D2d symmetry, with similar core molecular structures regardless of the amidate ligand utilized. While the tetradentate bis(amidate) ligand enforces the observed coordination geometry through its three carbon bridging unit, the bidentate amidate ligands exhibit unique a-stacking interactions that reinforce the observed N-cis dodecahedral geometry. The tight bite angle of the amidate ligands distorts these complexes from the ideal geometry that has been seen with other similar complexes in the literature. While these homoleptic complexes exhibit interesting coordination geometries, and may have potential as MOCVD precursors, 5°-52 the focus of this project was to utilize amidate ligands to examine metal-element bond reactivities. As such, the synthesis of amido complexes was undertaken to examine the reactivity of M-N bonds, and is discussed in the following section. 44^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes 2.3 Amidate Amido Complexes 2.3.1 Introduction Amido complexes of the group 4 metals have been shown to undergo a diverse range of reactivity. 64 These reactive functionalities have been utilized in catalytic transamidation,44 hydroamination catalysis, 65-71 imine metathesis, 72 and selected insertion reactivity. 73 ' 74 Since the synthesis of 8-coordinate homoleptic amidate complexes resulted in novel coordination complexes exhibiting unique geometries, the 7-coordinate tris(amidate) mono(amido) complexes were targeted. Due to the strong 3T-donating character of amido ligands, the orbital interactions with the amidate ligands could be dramatically affected by the introduction of an amido unit, and the coordination chemistry for these species was not readily predicted. As was noted in the previous section, the small size of Ti precluded its use in the synthesis of 7-coordinate tris(amidate) mono(amido) complexes. Thus, these studies focused on Zr, where Hf was not investigated in the interest of time, and may be the subject of future work. Previous isolation and characterization of tris(amidate) mono(amido) complexes of Ti in the Schafer lab have shown these complexes to adopt a 6-coordinate pseudo-octahedral ligation motif, with two bidentate amidate ligands, and a third ligand coordinated in a monodentate fashion through the 0 donor. 46 While tris(amidate) mono(amido) complexes may offer interesting protonolysis and insertion chemistry, bis(amidate) bis(amido) complexes would offer the possibility of accessing multiply bonded species, where this is possible due to the presence of two reactive ligands at the metal center. Both Ti and Zr species of this type will be discussed, while the Hf congeners will be left as possible future work. 45^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes 2.3.2 Results and Discussion 2.3.2.1 Tris(amidate) Mono(amido) Complexes of Zr Due to the larger size of Zr and the possibility of expanded coordination numbers, the Zr complexes were anticipated to be much different than the previously isolated Ti species. Synthesis of tris(amidate) mono(amido) complexes can be readily achieved by simply combining three equiv of the amide proligand with one equiv of a tetrakis(amido) metal starting material.  R" Zr(NMe2)4 Et20 R' = Ph, R" = 2,6- i Pr2C6H3^2.12 R' = tBu, R" = 2,6-Me2C6 H3^2.13 Preparation of [DIPP(NO)P1 ] 3ZrNMe2, 2.12, and [DmANo.tBu,j 3ZrNMe2, 2.13, is accomplished as shown in Eq. 2.7. The Ill NMR spectrum of 2.13 is consistent with a C3 symmetric structure in solution, as would be expected for a complex with three equivalent ligands, or rapidly exchanging amidate ligands. All three amidate ligands in 2.13 are equivalent on the NMR timescale, due to rapid exchange. In contrast, 2.12 exhibits hindered rotation about the N-Cipso bonds. This constraint is manifested by three doublets for the isopropyl methyl groups at 8 0.88, 1.07, and 1.26. These peaks exist in a 18:6:12 ratio, which can be rationalized by free rotation about two of the N-Ci pso bonds, which are inequivalent, and hindered rotation about the N-Cip so bond of the third amidate ligand. This results in two isopropyl doublets, one of which overlaps with the signal for one of the other amidate ligands, giving rise to the observed ratio. A single broad septet resonance is observed at 8 3.75 for the isopropyl methine protons, and the expected singlet for the dimethylamido ligand is present at 8 3.29, indicating free rotation of this ligand in solution. In addition to I I-I NMR spectra of 2.12 and 2.13, mass spectral data and elemental analysis confirm their composition as tris(amidate) mono(amido) species. 46^References begin on page 98 (R (2.8) R" I (N2^R I^Zr-NMe2 H N (2.9) NH 2 —NMe2 ^ Et20 (Ph 0 3 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes In mixed ligand complexes, the possibility of disproportionation always exists. The tris(amidate) mono(amido) complexes 2.12 and 2.13 could undergo ligand exchange to generate the homoleptic complexes and bis(amidate) bis(amido) complexes as illustrated in Eq. 2.8. As demonstrated in section 2.2, homoleptic complexes such as the generic example B shown in Eq. 2.8 are readily accessible in high yields. Bis(amidate) bis(amido) complexes (A) will be discussed in the following section. Although 2.12 and 2.13 can be isolated readily, allowing the reaction shown in Eq. 2.7 to proceed for extended periods (greater than 24 hours) can result in formation of both products A and B, where 2.12 is especially prone to this, presumably due to the large degree of steric encumbrance about the Zr with the very bulky 2,6-diisopropylphenyl groups. A possible intermolecular ligand exchange mechanism may involve K l -bound amidate ligands bridging between metal centers prior to ligand transfer. Evidence for this proposal will be discussed at length in the following section. A ^ B The reactive amido ligands in 2.12 and 2.13 can undergo protonolysis reactions with acidic groups such as amines. For example, protonolysis of 2.12 with one equiv of aniline results in the formation of a new tris(amidate) mono(amido) complex, where the dimethylamido unit has been replaced with a monoanionic anilido unit, as illustrated in Eq. 2.9. The resulting complex rip (No s P ,) j 3ZrNHPh, 2.14, can be isolated as a yellow microcrystalline solid. 2.12 ^ 2.14 47^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Unlike precursor 2.12, which exhibits a relatively simple i H NMR spectrum, the spectrum of 2.14 is very complicated, where hindered rotation about all three amidate N- Cvo bonds is evident. In particular, 12 distinct doublets are seen between 8 -0.08 and 1.81. In addition, 6 septet resonances are observed between 8 2.72 and 4.55, where three of the resonances overlap and are difficult to distinguish, and the remaining three correspond to single methine protons. The aromatic region is necessarily complicated, and does not offer much useful information apart from stoichiometry verification. Single crystals of 2.14 suitable for X-ray diffraction can be isolated from a 1:1 toluene/hexanes mixture, and the solid-state molecular structure is given in Fig. 2.6, with selected bond lengths and angles given in Table 2.4, and crystallographic data summarized in Table A2.6 (Appendix A). The low symmetry of this complex is readily apparent, where 2.14 exists in the solid-state as a 7-coordinate complex, which is best described as a distorted pentagonal bipyramid, in which the axial positions are occupied by the anilido ligand, and the N(2) donor of one of the amidate ligands. The remaining amidate donors (0(2), N(1), 0(1), N(3), and 0(3)) define the pentagonal plane in 2.14. Figure 2.6 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [DIPP(NO) Phj3ZrNHPh, 2.14 (non-ipso phenyl carbons and hydrogens omitted) 48^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Table 2.4 Selected Bond Distances (A) and Angles (°) for [ DIPP(NO)Ph1 3ZrNHPh, 2.14 Lengths Angles Angles Zr(1)-0(1) 2.1411(16) N(4)-Zr(1)-0(2) 93.63(7) 0(3)-Zr(1)-N(3) 58.43(6) Zr(1)-0(2) 2.1383(15) N(4)-Zr(1)-0(1) 117.53(8) N(4)-Zr(1)-N(2) 151.03(8) Zr(1)-0(3) 2.1965(15) 0(1)-Zr(1)-0(2) 132.00(6) 0(1)-Zr(1)-N(2) 89.26(7) Zr(1)-N(1) 2.3586(19) N(4)-Zr(1)-0(3) 86.24(7) 0(3)-Zr(1)-N(2) 83.07(6) Zr(1)-N(2) 2.3367(19) 0(2)-Zr(1)-0(3) 82.24(6) N(3)-Zr(1)-N(2) 99.99(7) Zr(1)-N(3) 2.2642(19) 0(1)-Zr(1)-0(3) 131.76(6) N(4)-Zr(1)-N(1) 85.20(8) Zr(1)-N(4) 2.068(2) N(4)-Zr(1)-N(3) 97.13(8) 0(2)-Zr(1)-N(1) 92.61(6) 0(2)-C(2) 1.311(3) 0(2)-Zr(1)-N(3) 138.17(6) 0(3)-Zr(1)-N(1) 169.71(6) N(2)-C(2) 1.309(3) 0(1)-Zr(1)-N(3) 76.45(6) Zr(1)-N(4)-C(58) 137.66(17) While the monoamido complexes illustrate the solution fluxionality of the amidate ligands, and the ability of the amido ligands to undergo protonolysis reactions, the bis(amidate) bis(amido) complexes were deemed more desirable targets. Related bis(amidinate)75 and bis(guanidinate) 76-79 complexes of Ti and Zr have demonstrated unique stoichiometric and catalytic reactivity. Specifically, a bis(guanidinate) imido complex has been shown to facilitate alkyne hydroamination catalysis, a reaction which will be discussed in detail in Chapter 5. 76 In addition, for many catalytic transformations the availability of two reactive ligand sites is critical to the success of these reactions, and new and exciting reactivity may be gleaned from the study of bis(amidate) complexes. 2.3.2.2 Bis(amidate) Bis(amido) Complexes of Ti and Zr The combination of two equiv of the amide proligand r u(NO)P1H (2.1) with one equiv of the starting material Zr(NEt 2)4 in Et20 results in clean formation of the desired bis(amidate) bis(amido) complex ru(NO)P112Zr(NEt2)2 (2.15), as illustrated in Eq. 2.10. To prevent over-speciation by the amidate ligands, these reactions are performed at reduced temperature and allowed to warm to room temperature over a couple of hours prior to workup. Isolation of 2.15 is readily achieved in high yield (80 %) as a pale yellow microcrystalline solid. Typically the crude isolated material is very pure, but can be further purified by recrystallization from a concentrated hexanes solution. This reaction is general and can be applied to a wide range of amidate ligands on both Ti and 49^References begin on page 98 C26 2.1 / NEt2 Zr\ NEt2 2 2.15 (^)Zr.NEt2 4 Et20 C27 C25 N N3^ 4 01 Zr1^02 C12 C3 C7 C8^C4 C1C C11 C5 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Zr. The solid-state molecular structure of 2.15 is shown in Figure 2.7, with selected bond lengths and angles given in Table 2.5 and crystallographic details located in Table A2.7 (Appendix A). The complex is pseudo-C2 symmetric in the solid-state with the amido ligands cis disposed and the N-donors of the amidate ligands cis oriented. Although the tert-butyl substituents on the amidate ligands are relatively bulky, the tight bite angle of the amidate ligands (— 58°) pulls these groups away from each other such that this conformation is not sterically unfavorable. The amido ligands in 2.15 can be considered as formal 4 e - donors. The Zr-N bond distances of 2.043(3) A are on the short side expected for Zr-N single bonds, 8° and N(4) is planar, with the sum of angles about N equal to 360°. The sum of angles about N(3) is approximately 348°, which is likely an artifact of the large degree of disorder present in this diethylamido ligand. (2.10) Figure 2.7 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of r u(NO)Phj2Zr(NEt2)2, 2.15 (hydrogens omitted for clarity) 50^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Table 2.5 Selected Bond Distances (A) and Angles (°) for ru(NO)Phi (^)2Zr \IEt2,2, 2.15 Lengths Angles Angles Zr(1)-N(4) 2.043(3) N(4)-Zr(1)-N(3) 100.93(15) N(4)-Zr(1)-N(2) 95.15(12) Zr(1)-N(3) 2.043(3) N(4)-Zr(1)-0(2) 109.93(11) N(3)-Zr(1)-N(2) 144.08(16) Zr(1)-O(2) 2.186(3) N(3)-Zr(1)-0(2) 86.22(15) 0(2)-Zr(1)-N(2) 58.03(9) Zr(1)-0(1) 2.187(3) N(4)-Zr(1)-0(1) 85.55(12) 0(1)-Zr(1)-N(2) 102.61(11) Zr(1)-N(1) 2.318(3) N(3)-Zr(1)-0(1) 110.44(15) N(1)-Zr(1)-N(2) 87.48(11) Zr(1)-N(2) 2.327(3) 0(2)-Zr(1)-0(1) 155.19(11) N(1)-C(1)-0(1) 114.8(3) 0(1)-C(1) 1.310(5) N(4)-Zr(1)-N(1) 142.98(13) N(2)-C(12)-0(2) 114.4(3) 0(2)-C(12) 1.317(4) N(3)-Zr(1)-N(1) 98.09(13) C(29)-N(4)-Zr(1) 133.0(3) N(1)-C(1) 1.286(5) 0(2)-Zr(1)-N(1) 102.72(12) C(27)-N(4)-Zr(1) 112.5(3) N(2)-C(12) 1.292(5) 0(1)-Zr(1)-N(1) 58.01(12) C(27)-N(4)-C(29) 114.4(3) In solution, 2.15 retains C2 symmetry as evidenced by a single resonance in the 'H NMR spectrum for the tert-butyl methyl protons, at 8 1.26, which overlaps with a single triplet resonance for the diethylamido methyl protons. Likewise, a single quartet resonance for the diethylamido methylene protons is observed at 8 3.78. Synthesis of the Ti analogue of 2.15 is readily accomplished upon combination of 2 equiv of the amide proligand with Ti(NEt2)4 in Et 20. Protonolysis to generate the product is rapid, and the dark red solution generated indicates successful formation of the bis(amido) complex in 82 % yield. The use of hydrocarbon solvents, such as toluene, results in formation of the desired product; however, mixtures of geometric isomers are often formed, and need to be converted to a single isomeric product via heating. Thus, synthesis of the bis(amidate) bis(amido) complexes is typically performed in ethereal solvents, which promote isomerization of the complex. The 1 H NMR spectrum of ru(No) ph,1INNEt2)2 (2.16) is largely analogous to that discussed for 2.15. Crystals suitable for X-ray diffraction can be isolated from a toluene/hexanes mixture at room temperature. The solid-state molecular structure is shown in Fig. 2.8, and is completely isostructural to its Zr analogue, exhibiting pseudo-C 2 symmetry in the solid-state. Selected bond lengths and angles are located in Table 2.6, with crystallographic details listed in Appendix A in Table A2.8. Again, the amido ligands are planar indicating sp2 -hybridization at the N atoms, with significant lone pair donation to the Ti center. Interestingly, both 2.15 and 2.16 display unsymmetrical bonding of the 51^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes amidate ligands to the metal center. Clearly, the backbone of the amidate ligand is not fully delocalized with the larger contributor being the alkoxy-imine tautomer shown in Fig. 2.9, as illustrated by the longer C-0 bonds and shorter C=N bonds (C(1)-O(1) = 1.310(5) A vs. C(1)-N(1) = 1.286(5) A in 2.15, and C(1)-O(1) = 1.329(8) A vs. C(1)-N(1) = 1.283(8) A in 2.16). The M-N and M-0 bond lengths in 2.15 and 2.16 reflect this, with shorter Zr-O bond lengths (Zr(1)-O(1) = 2.187(3) A) versus the Zr-N amidate bond lengths (Zr(1)-N(1) = 2.318(3) A) in 2.15, and shorter Ti-0 bond lengths (Ti(1)-0(1) -- 2.035(4) A) versus the Ti-N amidate bond lengths (Ti(1)-N(1) = 2.234(5) A) in 2.16. Figure 2.8 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of r u (NO)PhhTi(NEt2)2, 2.16 (hydrogens omitted for clarity) Table 2.6 Selected Bond Distances (A) and Angles (°) for f tBu(NO)P112Ti(NEt2)2, 2.16 Lengths Angles Angles Ti(1)-N(1) 2.234(5) N(4)-Ti(1)-N(2) 98.6(3) N(4)-Ti(1)-N(3) 95.0(2) Ti(1)-N(2) 1.907(6) N(4)-Ti(1)-0(2) 104.9(2) N(2)-Ti( I)-N(3) 150.4(2) Ti(1)-N(3) 2.254(5) N(2)-Ti(1)-O(2) 89.6(2) 0(2)-Ti(1)-N(3) 61.43(19) Ti(1)-N(4) 1.892(6) N(4)-Ti(1)-O(1) 89.0(2) O(1)-Ti(1)-N(3) 100.52(19) Ti(1)-0(1) 2.035(4) N(2)-Ti(1)-O(1) 105.9(2) N(1)-Ti(1)-N(3) 85.5(2) Ti(1)-0(2) 2.027(4) O(2)-Ti(1)-O(1) 157.59(19) N(1)-C(1)-O(1) 113.9(6) O(1)-C(1) 1.329(8) N(4)-Ti(1)-N(1) 149.9(2) N(3)-C(16)-O(2) 114.3(6) O(2)-C(16) 1.327(8) N(2)-Ti(1)-N(1) 95.4(2) C(12)-N(2)-Ti(1) 122.3(5) N(1)-C(1) 1.283(8) 0(2)-Ti(1)-N(1) 101.68(19) C(14)-N(2)-Ti( I ) 124.1(5) N(3)-C(16) 1.286(8) OW-TIM-NW 61.50(19) C(12)-N(2)-C(14) 113.5(6) 52^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes e 0^ 0 R'^N R"^ R" R ' Am ido-ketone^Alkoxy-imine Figure 2.9 Tautomers of amidate ligand While the diethylamido groups result in clean crystalline material for complexes 2.15 and 2.16, in general, the presence of these ligands results in greasy isolated materials, which are highly soluble and difficult to recrystallize. The starting material Ti(NMe2)4 can be utilized in analogous protonolysis reactions to isolate more crystalline materials. The resulting dimethylamido complexes are less soluble in hydrocarbon solvents, and recrystallized material is more easily accessed for these complexes. As seen for 2.15 and 2.16, isolation of ru(No)ph-, 2,-i(NMe2)2 (2.17) can be achieved in yields as high as 75 %. The solid-state molecular structure of the dimethylamido analogue of 2.16 (ru(NO)P112Ti(NMe2)2 , 2.17) was determined by X-ray crystallography and is given in Appendix A in Fig. A2. This complex is isostructural with 2.16, and relevant bond lengths and angles are given in Table A2.9, and crystallographic data are located in Table A2.10 (Appendix A). It is obvious that the smaller methyl groups on the amido ligands lead to no appreciable difference in coordination geometry of 2.17 vs. 2.16. In addition to more favorable solubility characteristics, the NMR spectroscopic properties of the resulting material are simplified, with a single resonance for the tert- butyl protons at 8 1.28 and a single resonance at S 3.62 for the amido methyl protons. For the aforementioned reasons, most of the remaining amido chemistry was performed using dimethylamido Ti and Zr starting materials. The effect of an aryl substituent on the amidate N can also be probed for comparison with the alkyl substituted complexes discussed previously. Utilizing the D[ mp(NopItiproligand^(2.4), the same protonolysis reaction is performed with Zr(NMe2)4 rather than Zr(NEt2) 4, with an isolated yield of 75 %. The dimethylamido starting material has the added advantage of being a solid rather than a liquid, which allows for simple combination of the two solids in a single reaction flask, rather than 53^References begin on page 98 C26 C30 C19 N2^ 01 02^N1 C25^C7 C12 C18 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes cannula transfer of a solution of one reactant into the other. The solid-state molecular structure of rup(No)tuui2zr(Nme2.2) (2.18) was determined by X-ray crystallography and is CI symmetric, where the two amidate ligands are bound in a trans N,0 configuration rather than the trans 0,0 configuration observed previously for complexes bearing ligand 2.1 (r°(NO)Ph]).j  Figure 2.10 illustrates the solid-state molecular structure of 2.18, with selected bond lengths and angles located in Table 2.7, and crystallographic details found in Table A2.11 (Appendix A). cio Figure 2.10 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [ mP(NO)t312Zr(NMe2)2, 2.18 (hydrogens omitted) 54^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Table 2.7 Selected Bond Distances (A) and Angles (°) for [ DmP(NO) tBu ] 2Zr(NMe2)2, 2.18 Lengths Angles Angles Zr(1)-N(3) 2.038(3) N(3)-Zr(1)-N(4) 98.36(10) N(3)-Zr(1)-N(1) 95.62(9) Zr(1)-N(4) 2.044(2) N(3)-Zr(1)-O(1) 110.73(9) N(4)-Zr(1)-N(1) 152.61(9) Zr(1)-O(1) 2.1524(19) N(4)-Zr(1)-O(1) 95.66(8) 0(1)-Zr(1)-N(1) 57.25(8) Zr(1)-O(2) 2.201(2) N(3)-Zr(1)-O(2) 149.81(9) O(2)-Zr(1)-N(1) 87.12(9) Zr(1)-N(2) 2.3055(19) N(4)-Zr(1)-0(2) 92.40(9) N(2)-Zr(1)-N(1) 99.76(9) Zr(1)-N(1) 2.403(2) O(1)-Zr(1)-O(2) 96.04(8) N(1)-C(1)-O(1) 113.8(3) N(2)-C(2) 1.301(4) N(3)-Zr(1)-N(2) 92.48(10) N(2)-C(2)-O(2) 113.5(2) O(2)-C(2) 1.295(3) N(4)-Zr(1)-N(2) 103.03(9) C(28)-N(3)-Zr(1) 107.15(18) O(1)-C(1) 1.325(3) O(1)-Zr(1)-N(2) 147.72(10) C(27)-N(3)-Zr(1) 142.5(2) N(1)-C(1) 1.294(3) O(2)-Zr(1)-N(2) 57.53(8) C(27)-N(3)-C(28) 110.3(2) While the solid-state molecular structure of 2.18 exhibits C, symmetry, the solution phase behavior is consistent with a C2 symmetric structure. The 1 I-1 NMR spectrum of 2.18 is extremely simple with a single resonance for the tert-butyl methyl protons at 8 1.03. In addition, a single resonance is observed for the aryl methyl protons at 8 2.12, indicating free rotation about the N-C ipso bonds of the amidate ligands. The expected amido methyl resonance is present at 8 2.93, along with resonances for the aryl protons at 8 6.86-6.92. This high symmetry species suggests that the amidate ligands are labile enough to undergo rapid isomerization in solution. The possibility of x i - intermediates during the isomerization process is likely, given that the amidate ligand is bound in an alkoxy-imine motif. A potential isomerization pathway is illustrated in Scheme 2.1, where the K 1 -intermediate can rotate and the N donor can recoordinate in a new geometric conformation. If this process is sufficiently rapid in comparison to the NMR timescale, in solution an average structure should be observed for this transformation. 55^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes  Ci Scheme 2.1 To test this hypothesis for isomerization, a competitive donor was added to Zr complex 2.18. Upon addition of one equiv of pyridine (Py) to 2.18, the 1 H NMR spectrum becomes more complicated, with multiple resonances for both the tert-butyl protons and the aryl methyl protons. The appearance of these new signals is consistent with a C1 symmetric structure such as that shown in Eq. 2.11. Variable temperature 'H NMR spectroscopic studies reveal that the complex is highly fluxional, and the coordinated pyridine is labile. At room temperature or above, the signals for pyridine appear where free pyridine would be expected. However, the ligand based resonances are more complicated than expected, suggesting rapid exchange of pyridine. Heating this reaction mixture to 50 °C or greater liberates pyridine, and the spectrum is essentially identical to 2.18 prior to addition of pyridine. The solid-state molecular structure of x...2_[ump(No)tBu]_ici [DmP(NO)t31Zr(NMe2) 2(Py) (2.19) was determined and is shown in Fig. 2.11, with selected bond lengths and angles given in Table 2.8, and crystallographic details located in Table A2.12 in Appendix A. One of the amidate ligands isomerizes to a K t -form, where the amidate N donor is labile and is replaced with pyridine. The solid-state molecular structure of 2.19 definitively illustrates that the amidate ligands can undergo hemilability to a monodentate form. The Zr(1)-N(5) bond of the bound pyridine is long at 2.469(2) A, consistent with being located trans to a dimethylamido ligand, which is a strong Tr-donor. Additionally, the solid-state structure of 2.19 confirms the notion that the amidate ligands tend to bind in an alkoxy-imine form, as the K'-coordinated ligand is 56^References begin on page 98 2.18 2.19 C18 C19 C30 N4 C20^C29 N5 C22 C33 Chapter 2.. Coordination Chemistry of Group 4 Amidate Complexes bound through the oxygen rather than the nitrogen. Examination of the bond lengths in the xc -amidate ligand also support this interpretation, where the C(2)-N(2) bond length of 1.267(3) A is consistent with a C=N double bond, 81 and the C(2)-O(2) bond length of 1.321(3) A is best described as a C-0 single bond.81 (2.11) Figure 2.11 ORTEP dction (ellipsoids at 30% probability) of solid-state molecular structure of,PP(NO)131-x-1 -[DmP(NO)tBIZr(NMe2)2(Py), 2.19 (hydrogens omitted for clarity) 57^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Table 2.8 Selected Bond Distances (A) and Angles (°) for K2-[DmP(NO)tBul-K.i_rivip(No)tsuizr(Nme2)2 ---y, ,(F ) 2.19 Lengths Angles Angles Zr(1)-0(2) 2.0148(18) 0(2)-Zr(1)-N(4) 106.24(9) 0(2)-Zr(1)-N(5) 79.00(8) Zr(1)-N(4) 2.041(2) 0(2)-Zr(1)-N(3) 99.43(9) N(4)-Zr(1)-N(5) 91.03(9) Zr(1)-N(3) 2.057(2) N(4)-Zr(1)-N(3) 100.49(10) N(3)-Zr(1)-N(5) 168.31(9) Zr(1)-0(1) 2.1813(18) 0(2)-Zr(1)-0(1) 102.27(7) 0(1)-Zr(1)-N(5) 78.07(7) Zr(1)-N(1) 2.372(2) N(4)-Zr(1)-0(1) 146.86(9) N(1)-Zr(1)-N(5) 82.35(8) Zr(1)-N(5) 2.469(2) N(3)-Zr(1)-0(1) 91.06(9) N(2)-C(2)-0(2) 125.0(2) 0(2)-C(2) 1.321(3) 0(2)-Zr(1)-N(1) 154.84(8) N(2)-C(2)-C(15) 119.7(3) N(2)-C(2) 1.267(3) N(4)-Zr(1)-N(1) 90.75(9) 0(2)-C(2)-C(15) 115.3(2) 0(1)-C(1) 1.306(4) N(3)-Zr(1)-N(1) 95.43(9) N(1)-C(1) 1.296(4) 0(1)-Zr(1)-N(1) 57.05(7) Inspection of the extended solid-state structure of 2.19 demonstrates two distinct intermolecular non-covalent interactions within the lattice. The first interaction is a hydrogen bonding interaction between the N(2) atom of the K'-bound amidate ligand and the meta aryl proton of C(23) of an adjacent molecule of 2.19. This interaction is illustrated in Fig. 2.12 (A) by dashed red lines, which form a ladder structure throughout the lattice. The 2.72 A contact distance for this interaction falls within the sum of van der Waals radii of N and H (N = 1.55 A, H = 1.20 A, total = 2.75 A) indicating that this is a real interaction, and not simply the product of crystal packing. 82 The second non-covalent intermolecular interaction present in the lattice is a less intuitive hydrogen bonding interaction between the meta proton of the coordinated pyridine at C(32) and the a-system of the N-aryl group on the K 2-bound amidate ligand, where the para carbon C(10) has the closest contact with the proton at C(32). This interaction is shown in Fig. 2.12 (A) by dashed blue lines, where the contact distance of 2.88 A falls within the sum of van der Waals radii for C and H (C = 1.70 A, H = 1.20 A, total = 2.90 A). 82 Similar interactions between protons and a-systems are frequently seen in extended molecular structures. 55 The result of these two intermolecular forces is the formation of a 1D network defined by two columns of 2.19 related by a C2 screw axis parallel to the b-axis of the crystallographic unit cell. This extended structure is depicted in Fig. 2.12 (B) as viewed along the C2 crystallographic screw axis. This extended solid- 58^References begin on page 98 AB Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes state structure supports the notion that amidate ligand exchange/transfer may be possible through bridging interactions between metal centers via K l -amidate ligation. Figure 2.12 Extended structure of 2.19 in the solid-state (A), and viewed down crystallographic C2 screw axis (B) (C = gray, H = green, Zr = purple, N = blue, 0 = red; hydrogen bonds = dashed red and blue lines) While the tris(amidate) complexes 2.12 and 2.13 are readily synthesized in high yield by the route illustrated in Eq. 2.7, stepwise synthesis is also possible upon combination of a third equiv of the amide proligand with the bis(amidate) bis(amido) complexes, as shown in Eq. 2.12. While beyond the scope of this thesis, the stepwise synthesis of the tris(amidate) complexes implies that complexes bearing two or more different amidate ligands should theoretically be accessible. However, the ligand redistribution issues presented in the previous section suggest that these mixed ligand species would likely be unstable to disproportionation. 59^References begin on page 98 N N Ph .—^Z( '' / 0^NMe2 2.21 + (2.13) Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes N R" ^ H (2.12) Et20 R' = Ph, R" = 2,6-'Pr2C6H 3^2.20^ R' = Ph, R" = 2,6-'Pr2C6H3^2.12 R' = tBu, R" = 2,6-Me2C6H 3^2.18 R = tBu, R" = 2,6-Me2C6H 3^2.13 It is also of note that the anilido complex 2.14, synthesized by protonolysis of 2.12 with aniline, can be isolated by reaction of excess aniline with the bis(amidate) bis(amido) complex 2.20 as shown in Eq. 2.13. A potential mechanism for the formation of 2.14 from 2.20 can be envisioned through an intermolecular ligand exchange reaction between two Zr centers, involving a x'-interaction analogous to that seen for 2.19. The other product formed would be a mono(amidate) imido complex 2.21. No solution phase or solid-state evidence exists to substantiate the claim of formation of 2.21, but is inferred from reaction stoichiometry alone. Given the unsuccessful attempts to synthesize mono(amidate) tris(amido) complexes, it is postulated that 2.21 would likely not be monomeric, but rather dimeric or oligomeric in nature. /^N^NMe2 xs. ^NH2^H 2 Ph-4, ^ ^ N Zr\   (Ph—( Zr \^0 2 NMe2^0 3 2.20 2.14 2.3.2.3 Insertion Reactivity of Amido Ligands The reaction of 2.12 with aniline to form 2.14 through protonolysis is an important reaction which has implications for the catalytic hydroamination studies to be presented in Chapter 5. Another important fundamental reaction would be insertion of unsaturated organic species into the M-N bonds of the amido complexes presented in the preceding sections. One test reaction to probe the feasibility of such a process for these complexes is the insertion of 2,6-dimethylphenylisocyanide into Zr-N bonds. While isocyanide insertion into metal alkyls is well precedented, the analogous reactivity with 60^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes metal amido complexes has received far less attention. 73 ' 74 ' 83 The combination of two equiv^of^2,6-dimethylphenylisocyanide^with^bis(amido)^complex [D/vfl'a\ -..tBu-.u) j2Zr(NMe2)2, 2.18, results in formation of the re-iminocarbamoyl complex 2.22 ([DmP(NO) tB12Zr(i 2-ArN=C(NMe2))2 (Ar 2,6-Me2C6H3)) as illustrated in Eq. 2.14. Preliminary sealed NMR tube experiments show this reaction to be essentially instantaneous. Isolation of 2.22 can be accomplished in 65 % yield upon precipitation from Et20. The low solubility of this material facilitates its isolation by filtration. Na--"C  2 (2.14) 2.18 2.22 Verification of insertion and re-iminocarbamoyl ligation is most easily shown by 13 C NMR spectroscopic studies, which show a signal at 8 208.7, which is highly characteristic of such species. 74 This complex exhibits C2 symmetry in solution, with hindered rotation about the amidate N-Cips0 bonds as well as the iminocarbamoyl bonds. This is manifested by a single resonance for the tent-butyl groups in the I II NMR spectrum at 8 1.17, and four resonances of equal intensity for the aryl methyl groups at 8 1.66, 1.94, 2.42, and 2.73. Free rotation of the amido NMe2 groups is observed as a single resonance at 8 2.09. Mass spectrometry also verified the formation of this product with a parent ion at m/z 848, and a fragment at m/z 673, corresponding to loss of one of the iminocarbamoyl ligands. While some iminoacyl and iminocarbamoyl complexes of this type are known to undergo C=C coupling of the re-bound ligands,77' 83, 84 2.22 does not undergo such reactivity even at elevated temperatures It is important to note that insertion of the isocyanide moiety into the amidate Zr- N bond does not occur. The steric protection afforded by the bulky groups on the amidate N donors effectively protects these sites from undesirable side reactivity. Insertion of simple alkenes and alkynes into the metal amido linkages of 2.18 would have important implications in the mechanism of catalytic hydroamination; however, attempts 61^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes at insertion reactions with styrene, 1-hexene, and phenylacetylene resulted in no reaction. This suggests that these less polar C-C multiple bonds cannot insert into the Zr-N bonds of these amido complexes through o-bond insertion processes. 2.3.3 Summary Both bis(amidate) and tris(amidate) amido complexes of Ti and Zr can be synthesized in high yields. The tris(amidate) mono(amido) complexes of Zr are C3 symmetric in solution, with rapidly exchanging amidate ligands. Solid-state molecular structure analysis of 2.14 shows that these 7-coordinate complexes exist in heavily distorted pentagonal bipyramidal geometries. Large groups on the amidate ligands lead to ligand exchange and disproportionation reactions with these complexes. Intramolecular exchange processes of bis(amidate) bis(amido) complexes are likely facilitated by the hemilabile nature of the amidate ligands, where complexes bearing x 3 - bound amidate ligands are important intermediates as demonstrated by 2.19. The tris(amidate) mono(amido) complexes 2.12 and 2.13 can be synthesized in a single step from the precursor Zr(NMe2)4, or sequentially from the bis(amidate) bis(amido) species 2.18 and 2.20. This implies that complexes bearing two or more different amidate ligands can be synthesized should the need arise. This synthetic flexibility could prove useful in future high-throughput combinatorial screening for polymerization catalysis and other useful reactivity. The reactive amido ligands in the bis(amidate) bis(amido) and tris(amidate) mono(amido) complexes can undergo protonolysis reactions to form species like anilido complex 2.14, as well as insertion reactions to form iminocarbamoyl complexes (2.22). 2.4 Geometric Isomerization of Amido Complexes 2.4.1 Introduction The ability of metal complexes to adopt varied coordination geometries is one of their most important features. This geometric flexibility is crucial to the novel reactivity 62^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes that metal complexes promote; however, this flexibility also makes characterization of such complexes difficult. The amidate ligands investigated in this thesis are unsymmetrical due to their mixed N,O chelate motif. The potential for multiple geometric isomers makes solution phase characterization complicated. It was previously noted that solution phase isomerization of bis(amidate) bis(amido) complexes may occur through K'-intermediates as shown in Scheme 2.1. The factors controlling the geometric isomer observed, and the interconversion of different geometric isomers were investigated by solid-state molecular structural analysis and density functional theory (DFT) calculations. 2.4.2 Results and Discussion 2.4.2.1 Structural Studies While the hard N,O chelating motif of the monoanionic amidate ligand set is ideally suited to bind to group 4 metals, the unsymmetrical chelating unit gives rise to several possible geometric isomers. As was alluded to in the previous section, these isomers can be manifested in solution by multiple resonances in the 'H NMR spectra, or by solid-state structures inconsistent with solution phase behavior (complex 2.18). There are five isomers possible based on a pseudo-octahedral coordination geometry. These isomers are illustrated in Fig. 2.13. X = amido, alkyl, chioro Figure 2.13 Possible geometric isomers of bis(amidate) Ti complexes 63^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Given the presence of bulky substituents on the N donors of the amidate ligands, it could reasonably be predicted that the predominant geometric isomer observed would be the N-trans C2 variant. However, bis(amidate) bis(amido) complexes of ligand 2.1 ru(No)ph—j), 2.15 — 2.17, exist in the 0-trans C2 geometry in the solid-state. The observation of this unexpected isomer can be attributed to the tight bite angle of the amidate ligands at approximately 60°, which place the bulky tert-butyl substituents farther apart than would typically be expected. In addition, the orientation of the N donors of the amidate ligands trans to the amido ligands should be energetically more favorable than having the 0 donors trans to the amido ligands. The alkoxy-imine binding mode of the amidate ligands to the metal centers implies significantly stronger M-0 bonds than M-N bonds. Due to the strong trans influence of the amido ligands, the overall stability of the metal complex should be enhanced by ensuring that the strong donors are not trans to the amido ligands. Interconversion of the different geometric isomers is proposed to occur through a mechanism involving x2-to-K1 -ligand isomerization, as illustrated in Scheme 2.1. Systematic investigation of the factors controlling the prevalent geometric isomer was accomplished by X-ray crystallographic and density functional theory (DFT) studies. While Ti complexes 2.15 — 2.17 clearly illustrate the preference for the 0-trans C2 geometric isomer, Zr complex 2.18 demonstrates that the C 1 isomer is also possible for these bis(amidate) bis(amido) species. The difference in size of Ti and Zr is considerable, and to ensure appropriate comparisons of the effects of steric factors influencing coordination geometry of bis(amidate) bis(amido) complexes, a series of Ti complexes was synthesized with amidate ligands sharing a common phenyl carbonyl substituent. In collaboration with the Wang group at UBC, DFT calculations determined the relative energies of the five possible geometric isomers of each of three different bis(amidate) bis(amido) complexes utilized in this study. In addition to complex 2.16 (ru(No)ph,12Ti(NEt2)2), complexes 2.23 ([DmANO)Ph]2Ti(NEt2)2) and 2.24 ([DIPP(NO) P1 ]2Ti(NEt2)2 ) can be synthesized following the procedure shown in Eq. 2.10. Using the crystallographically determined solid-state molecular structure of 2.16 as a starting point, DFT models of the five possible geometric isomers were generated. Fig. 2.14 illustrates the computed structures of the isomers of 2.16 and their relative 64^References begin on page 98 E (eV) Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes energetic ordering. The metrical parameters for the crystallographically characterized isomers of 2.16, 2.23, and 2.24 and their calculated variants are in good agreement and are given in Tables B2.1, B2.2, and B2.3, respectively, in Appendix B. Figure 2.14 Relative energetic ordering of geometric isomers of 2.16 (Ti = white, N = blue, 0 = red, C = gray) The 0-trans C2 isomer is calculated to be the most energetically favorable, and matches the solid-state molecular structure for 2.16, which is logical since the crystallized material should represent a thermodynamic minimum. The next highest energy isomer is the C1 isomer, calculated to be 0.112 eV higher in potential energy. If the isomerization mechanism proposed in Scheme 2.1 is envisioned, K -2-to-K-1 -ligand isomerization of one of the amidate ligands of the 0-trans C2 isomer, followed by recoordination of the amidate ligand, would result in the formation of the C 1 isomer. Repeating the 65^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes isomerization process with the second amidate ligand would result in generation of the N- trans C2 isomer, which is located 0.107 eV above the C 1 isomer. The two highest energy isomers (C2h and C2,) have trans oriented amido ligands and are considerably higher in energy than the C1 and C2 isomers. The strong trans influence of the amido ligands explains their preference for cis orientation, and thus the high energy of the C2h and C2v isomers. Finally, the sterically unfavorable interaction of the tert-butyl groups in the C2v isomer places it at the highest relative energy. At room temperature, 2.16 does not appear to isomerize on the NMR timescale; however, high temperature I II NMR spectroscopic experiments suggest solution phase isomerization. At higher temperatures, other signals grow into the spectrum indicating the presence of multiple geometric isomers. As the temperature is increased, the intensities of the signals for the minor geometric isomers increase relative to the signals for the ground state 0-trans C2 isomer. In contrast, bond rotation processes can be observed and frozen out at low temperatures. For example, at -25°C the quartet for the diethylamido methylene protons broadens and eventually splits into two broad multiplets of equal intensity at even lower temperatures, indicating hindered rotation about the amido Ti-N bond, resulting in inequivalent ethyl groups. Accompanying this observation, the corresponding methyl protons on the diethylamido ligands separate into two multiplets of equal intensity that inconveniently overlap with the tert-butyl signal. The fact that isomerization at room temperature is not observed for 2.16 is reasonable given the relatively large difference in energy between the various geometric isomers. The solid-state molecular structure of complex 2.23 was determined by X-ray crystallography and is shown in Figure 2.15. Selected bond lengths and angles are located in Table 2.9, and crystallographic data are presented in Table A2.13 (Appendix A). In this case, the sterically larger 2,6-dimethylphenyl groups on the amidate N donors force the amidate ligands from the 0-trans C2 conformation seen in 2.16 to one in which the overall symmetry of the complex is C 1 . Comparison of the amidate N-Ti-N bond angles for 2.16 and 2.23 show that this rearrangement results in an increase in this bond angle from 85.5(2)° to 94.94(5)°. This C, geometric isomer was seen for the related Zr complex [Dmp(No)tBu,i2Zr(NMe2)2, 2.18, which likewise has 2,6-dimethylphenyl substituents on the amidate N donors, but has tert-butyl groups at the carbonyl positions 66^References begin on page 98 C38^C35 C22C33 C20^C23 C25 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes of the amidate ligands. While 2.18 does exhibit asymmetry of the binding of the two amidate ligands, due to one having a N donor trans to an amido ligand, and the other having an 0 donor trans to an amido ligand, the effect is more pronounced in 2.23. This is likely due to the fact that the Ti-0 and Ti-N bond lengths are contracted relative to the Zr-0 and Zr-N bond lengths in 2.18, owing to the smaller ionic radius of Ti(IV) vs. Zr(IV), thus resulting in greater steric repulsions within 2.23 vs. 2.18. For example, the Ti(1)-O(2) bond length in 2.23 is 2.004(1) A, whereas the Ti(1)-0(1) bond length is significantly longer at 2.076(1) A, as expected for a donor trans to a strong a-donor like diethylamido. Likewise, the Ti(1)-N(3) bond is considerably longer than the Ti(1)-N(1) bond (2.375(1) A vs. 2.211(1) A, respectively). It is also interesting to note that the normal alkoxy-imine binding mode for the amidate backbone is shifted to the amido- ketone form (Fig. 2.9) for the amidate ligand that is bound with the 0 donor trans to the amido ligand (C(1)-0(1) = 1.293(2) A and C(1)-N(1) = 1.310(2) A). Figure 2.15 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [ DmP(NO)PITi(NEt2)2, 2.23 (hydrogens omitted for clarity) 67^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Table 2.9 Selected Bond Distances (A) and Angles (°) for [ Div1P(NO)P1 ]2Ti(NEt2)2, 2.23 Lengths Angles Angles Ti(1)-N(1) 2.211(1) 0(1)-Ti(1)-0(2) 95.37(5) 0(1)-Ti(1)-N(4) 157.55(5) Ti(1)-N(2) 1.894(1) N(1)-Ti(1)-N(3) 94.94(5) 0(2)-Ti(1)-N(1) 148.94(5) Ti(1)-N(3) 2.375(1) N(1)-Ti(1)-N(2) 105.06(6) 0(2)-Ti(1)-N(3) 59.62(4) Ti(1)-N(4) 1.901(1) N(1)-Ti(1)-N(4) 97.15(6) 0(2)-Ti(1)-N(4) 102.58(5) Ti(1)-0(1) 2.076(1) N(2)-Ti(1)-N(4) 99.79(6) 0(2)-Ti(1)-N(2) 95.02(5) Ti(1)-0(2) 2.004(1) N(3)-Ti(1)-N(4) 95.83(6) 0(1)-C(1) 1.293(2) N(3)-Ti(1)-N(2) 152.73(6) N(1)-C(1) 1.310(2) 0(1)-Ti(1)-N(3) 81.63(5) 0(2)-C(20) 1.318(2) 0(1)-Ti(1)-N(2) 91.86(5) N(3)-C(20) 1.296(2) 0(1)-Ti(1)-N(1) 61.09(5) As was done for 2.16, DFT calculations were performed to determine the relative energies of the five possible geometric isomers of 2.23. The results are illustrated in Fig. 2.16. The most interesting observation with complex 2.23 is that the C1 isomer characterized in the solid-state is not the ground state isomer determined by computational methods. The ground state isomer for 2.23 is calculated to be the N-trans C2 isomer, which has not been observed crystallographically at this point. The expected C 1 isomer is very close in energy to the ground state, at only 0.036 eV higher energy. In comparison, the energy gap between the ground state and C I isomers in 2.16 is 0.113 eV (over three times larger). This small energy gap for 2.23 is manifested experimentally by room temperature geometric isomerization, where the major isomer (> 80%) has NMR spectral properties consistent with the N-trans C2 isomer, calculated to be the thermodynamically most stable isomer. The low energy barrier between the N-trans C2 and C1 isomers allows for their rapid interconversion at room temperature and higher. Minor isomeric forms (< 20%) are also present in solution, as evidenced by multiple quartets and triplets corresponding to the diethylamido groups. At lower temperatures, multiple isomeric forms can clearly be seen by the appearance of multiple peaks in the region of each expected resonance. As the temperature is raised, line broadening is observed, accompanied by the coalescence of the peaks. For example, the diethylamido quartets for all the different isomers coalesce into one broad hump at temperatures above 60 °C. However, there are no well resolved peaks for any of the possible isomers, even at 68^References begin on page 98 E (eV) Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes temperatures up to 100 °C. This observation suggests that the interconversion of the higher energy isomers is occurring slowly on the NMR timescale at temperatures up to and including 100 °C. If the proposed K l -intermediate isomerization pathway is occurring (Scheme 2.1), the presence of these species would serve to further complicate the NMR spectrum of 2.23 at higher temperatures. Given the observation that 2.18 and 2.23 both crystallize as C I symmetric species, but exhibit C2 symmetry in solution, it is possible that crystal packing interactions in the solid state compensate for the slightly higher calculated gas-phase ground state energy. Figure 2.16 Relative energetic ordering of geometric isomers of 2.23 (Ti = white, N = blue, 0 = red, C = gray) 69^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes While the N-trans C2 and C I isomers are relatively close in energy, the 0-trans C2 isomer is substantially higher in energy, at 0.142 eV above the ground state, which is logical given the degree of steric crowding present in this isomer. As was seen for 2.16, the C2v and C2h isomers are much higher in energy than the C2 and CI isomers; however, the C2v isomer is stabilized relative to the C2h isomer by a n-stacking interaction between the two 2,6-dimethylphenyl groups on the N donors of the amidate ligands. This is the same interaction that was seen earlier for the homoleptic complexes 2.8 and 2.9, verifying the stabilization imparted by this non-covalent interaction. By increasing the steric bulk at the N donors of the amidate ligands to the very large 2,6-diisopropylphenyl groups, the N-trans C2 isomer can be accessed, which is illustrated in the solid-state molecular structure of [ DIPP(NO)P1 ]2Ti(NEt2)2, 2.24, in Fig. 2.17. For selected bond lengths and angles see Table 2.10. The solid-state molecular structure of 2.24 was previously elucidated, and is rigorously C2 symmetric, with the C2 axis of symmetry passing through the Ti center and relating N(2) and N(2)*, N(1) and N(1)*, etc. 85 This conformation effectively forces the amidate N-aryl groups apart, with a N(1)-Ti(1)-N(1)* bond angle of 140.42(8)°. The relevant bond lengths are consistent with those seen for 2.16 and 2.23, where the major observable difference is seen in the amidate backbone distances, with the backbone existing in the amido-ketone tautomer shown in Fig. 2.9 (C(1)-0(1) = 1.283(2) A and C(1)-N(1) = 1.320(2) A). In contrast to 2.16 and 2.23, the amidate Ti-0 and Ti-N bond lengths are nearly identical (Ti(1)-N(1) =- 2.156(1) A and Ti(1)-0(1) = 2.146(1) A). With 2.24, the preference for the bulky groups to be trans to each other competes with the general preference for the amidate ligands to bind with the N donors trans to the amido ligands. The resulting geometry forces the amidate ligand into a different tautomeric form than is normally seen. Unfortunately, due to the highly sterically congested nature of 2.24, the I I-I NMR spectrum is very complicated, and variable temperature I HNMR spectroscopic experiments looking at solution phase isomerization could not be accurately interpreted. The complex nature of the I I-I NMR spectrum suggests that hindered rotation exists about the amidate N-Cipso bonds, as well as the amido Ti-N bonds. At elevated temperatures, solution phase isomerization occurs readily, as witnessed by the increased complexity of the 1 1-1 NMR spectra. 70^References begin on page 98 C15 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes C23 C19 C22^ C18 C17 C21 C13^C12 C11 C10 C7 C4 =̂ C5 Figure 2.17 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [ DIPP(NO)P12Ti(NEt2)2, 2.24 (hydrogens omitted for clarity) Table 2.10 Selected Bond Distances (A) and Angles (°) for [DIPP(NO)P1 ]2Ti(NEt2)2, 2.24 Lengths Angles Angles Ti(1)-0(1) 2.146(1) 0(1)-Ti(1)-0(1)* 81.92(7) 0(1)-Ti(1)-N(1)* 88.30(5) Ti(1)-N(1) 2.156(1) N(1)-Ti(1)-N(1)* 140.42(8) 0(1)-Ti(1)-N(2) 159.30(5) Ti(1)-N(2) 1.899(2) N(1)-Ti(1)-N(2) 99.21(6) 0(1)-Ti(1)-N(1) 61.21(5) 0(1)-C(1) 1.283(2) N(1)-Ti(1)-N(2)* 105.68(6) 0(1)-C(1)-N(1) 114.5(2) N(1)-C(1) 1.320(2) N(2)-Ti(1)-N(2)* 101.1(1) The relative energies of the geometric isomers of 2.24 were calculated using DFT methods, and are shown in Fig. 2.18. The ground state isomer is calculated to be the N- trans C2 isomer, matching the observed solid-state molecular structure of 2.24. Similarly to 2.16 and 2.23, the next highest isomer in energy is the C 1 isomer, which is at 0.096 eV relative to the ground state. This larger energy gap explains the less prevalent room temperature isomerization observed for 2.24 vs. 2.23. As predicted, the 0-trans C2 isomer is considerably higher in energy than the C 1 isomer (0.342 eV relative to the C I ) due to the extremely unfavorable steric interactions between the amidate N-aryl groups. Again, the C2h and C2v isomers are much higher in energy than the C2 and C I isomers (a consequence of the trans influence of the amido ligands). Unlike 2.23, the a-stacking C16 71^References begin on page 98 C2v R NEt2R N ,,,,,N Nor }0^0 NEt2 R NEt2 • \ I 'NJ^C2h wet2R R^.ANEt2 0-trans R N^NEt, C2 r'NR 0/„,, J .ANEt2 at Ire '410R N^NEt2 (ThR 04,4 I ,,,NEt2 N-trans I NEt2^C2 NR ,a # 4 4$4.0* -ell, 4 414^Ait, 4 Air* 4 4 E (eV) 1.559) Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes stabilization of the C2v isomer of 2.24 is not possible due to the isopropyl groups preventing close approach of the aryl rings. Thus, the C2v isomer is higher in energy than the C2h isomer, as seen for 2.16. Figure 2.18 Relative energetic ordering of geometric isomers of 2.24 (Ti = white, N = blue, 0 = red, C = gray) 2.4.2.2 Electronic Structure Analysis In an attempt to understand the bonding interactions holding the bis(amidate) bis(amido) complexes together, single point DFT calculations were performed on the geometry optimized structures of the five geometric isomers of complexes 2.16, 2.23, and 2.24. Similar interactions are observed for all of the isomers of each of the complexes 72^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes investigated; therefore, the following discussion will focus only on the lowest energy isomer of complex 2.16 (0-trans C2). Thermal stability experiments at elevated temperatures (> 110 °C) for periods as long as one week show no appreciable decomposition of the bis(amidate) bis(amido) complexes, indicating that the ancillary amidate bonding interactions are very strong. It was predicted that these interactions would lie well below the frontier orbitals. The frontier bonding orbitals (HOMO and HOMO-1) are shown in Fig. 2.19. These two energetically similar orbitals are both amido-based 7r-symmetry orbitals. These orbitals support the previous designation of the amido ligands as 4 e - donors, with the N lone pair of electrons being donated to the Ti center. The dot-pn interactions in the HOMO and HOMO-1 orbitals involve overlap of the vacant d-orbital with p-orbitals on both amido ligands. Since the amido ligands are the reactive sites for protonolysis reactivity that will be discussed in Chapters 4 and 5, it is logical that the frontier orbitals involve bonding interactions with these ligands.  ^...0 ,^ .)^.441 ...) *10 .4 HOMO HOMO-1 Figure 2.19 Frontier bonding orbitals of ground state isomer of 2.16 (Ti = white, N = blue, 0 = red, C = gray) In addition to the n-symmetry bonding interactions of the amido ligands, the requisite a-bonding interactions are also present. The a-bond at the HOMO-9 level is shown in Fig. 2.20, and exhibits effective overlap between the d-orbital and hybrid orbitals of both amido ligands. 73^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Figure 2.20 Amido o-bonding orbital of ground state isomer of 2.16 (Ti = white, N= blue, 0= red, C = gray) The LUMO for 2.16 is calculated to be a non-bonding, unoccupied d-orbital, and is shown in Fig. 2.21. The assignment of which d-orbital is the LUMO is difficult owing to the fact that these complexes are severely distorted from a simple octahedral geometry, resulting in hybrid bonding interactions. The vacant d-orbital allows for coordination of Lewis bases, such as amines and phosphines. In particular, the availability of this orbital for amine coordination plays an important role in a precoordination step for catalytic hydroamination. A detailed mechanistic discussion of catalytic hydroamination of alkynes and alkenes will be presented in Chapter 5. LUMO Figure 2.21 Frontier non-bonding orbital of ground state isomer of 2.16 (Ti = white, N = blue, 0 = red, C = gray) Bonding interactions involving the amidate ligands are considerably lower in energy than those for the amido ligands, as predicted by the stability of these complexes. The HOMO-10 orbital illustrated in Fig. 2.22 shows d-p o-interactions between Ti and 74^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes the amidate 0 donors, as well as interactions with the amido ligands. It was previously noted that the amido ligands' strong trans influence has a large impact on the nature of the amidate bonding interactions. This influence can be seen in the HOMO-13 orbital (Fig 2.22), where the amidate N donor and the trans disposed amido ligand are sharing opposite ends of the same d-orbital. 411  HOMO-13 Figure 2.22 Amidate bonding interactions of ground state isomer of 2.16 (Ti = white, N = blue, 0 = red, C = gray) The HOMO-LUMO gap for 2.16 is calculated to be 3.681 eV, which is slightly larger than that for 2.23 (3.512 eV) and 2.24 (3.543 eV). For comparative purposes, the UV/vis spectra of 2.16, 2.23, and 2.24 were collected. Since these complexes are all d° Ti species, the observed spectral features for these complexes are all LMCT bands. Calculated and experimental k rnax values for 2.16, 2.23, and 2.24 are presented in Table 2.11. Table 2.11 Theoretical and Experimental Xmax Values for 2.16, 2.23, and 2.24 Compound HOMO-LUMO Gap (eV) kmax Theoretical (nm) max Experimental (nm) 2.16 3.681 276 270 2.23 3.512 325 358 2.24 3.543 315 265 For 2.16, the agreement between the theoretical and experimental values is excellent (276 nm vs. 270 nm); however, the agreement for 2.23 and 2.24 is less impressive. The spectra of 2.23 and 2.24 are much broader, with far less defined features 75^References begin on page 98 Experimental UV/vis Spectrum of r BINOrj2Ti(NEt2 )2 (2.16) 0^300^400^500^600^700^800 Wavelength (nm) 0.1 0.09 0.08 0.07 8 0.06 rig 0.05 go 0.04 0.03 0.02 0.01 0 -0.012 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes than that seen for 2.16. More prevalent solution phase isomerization of 2.23 and 2.24 vs. 2.16 make the presence of multiple geometric isomers highly likely. This implies that the experimental spectra are a combination of 2 or 3 different isomers. In general, the experimental and calculated spectra have very similar features and lineshapes, but the theoretical spectra are shifted in wavelength from the experimental spectra. The experimentally determined UV/vis spectrum for 2.16 is shown in Fig. 2.23, and the theoretical spectrum is shown in Fig. 2.24 (experimental and computational spectra for 2.23 and 2.24 are given in Appendix B for comparative purposes). Figure 2.23 Experimental UV/vis absorption spectrum of 2.16 76^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Calculated UVNis Spectrum of pu(NO)P1 2Ti(NEt2 )2 (2.16) i^I 1 .2 - 1 . 0 - 0.8 _ , a)0cal_ - 0.6^-e 0 0 .0 - 0.4 I I^' - 0.2 100 150 200^250^300^350 400 450 500 Wavelength (nm) Figure 2.24 Calculated UV/vis absorption spectrum of 2.16 2.4.3 Summary While the bis(amidate) bis(amido) complexes of Ti and Zr exist as pseudo- octahedral species, due to the unsymmetric N,0 chelate of the amidate ligands, five possible geometric isomers exist for these complexes. Systematic variation of the steric bulk at the N donor of the amidate ligand demonstrates that the geometric isomer observed in the solid-state is largely dictated by the bulk at this position. Density functional theory (DFT) calculations on the five possible isomers of complexes 2.16, 2.23, and 2.24 verified the solid-state molecular structures of 2.16 and 2.24 as the energetic minima for these complexes. Complex 2.23 was calculated to be most stable as the N-trans C2 isomer, but was characterized as the C 1 isomer in the solid-state. DFT calculations on the isomers of 2.23 showed that the energy gap between the N-trans C2 and C1 isomers is very small, explaining the solution phase characterization, which is consistent with C2 symmetry. As expected, the C2v and C211 isomers, which have trans 77^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes located amido ligands, are highest in energy due to the strong trans influence of these ligands. Geometric isomerization can further be controlled by tethering the amidate ligands together, as with proligand 2.6. However, this ligand binds to group 4 metals in a planar fashion, as illustrated by the homoleptic complexes 2.10 and 2.11. This planar coordination is problematic for the formation of amido complexes, as the amido donors prefer to be cis, which would leave the metal center highly exposed with this ligand. Proligand 2.6 is useful for the synthesis of bis(alkyl) complexes of Zr and Hf, and is discussed in Chapter 3. Solution phase isomerization of the bis(amidate) bis(amido) complexes was more pronounced with bulkier groups on the N donors of the amidate ligands. Variable temperature 1 1-1 NMR spectroscopy of 2.16, 2.23, and 2.24 support the relative energetic spacing of the geometric isomers of these complexes, where 2.16 has the largest spacing between isomers and demonstrates the least solution phase isomerization. Bonding interactions determined by DFT calculations for 2.16 support the assignment of bis(amidate) bis(amido) complexes as 16 e - species, where the amido ligands are 4 e- donors, with sp2 -hybridized N centers. The it-bonding interactions of the amido ligands for all isomeric forms of 2.16, 2.23, and 2.24 are located at the HOMO and HOMO-1 energy levels. In all cases, the LUMO for these species is a vacant d-orbital, which allows for coordination of Lewis bases such as amines. This has important implications for catalytic hydroamination with these complexes, and is discussed in detail in Chapter 5. The amidate bonding interactions for these complexes are located very low in energy in comparison to the frontier orbitals, and suggest that the amidate ligands bind to the Ti center with more electrostatic or ionic bonding character, rather than strictly covalent bonding. This ionic character is instrumental for the unique reactivity that is discussed in Chapters 3 — 5, as it allows these extremely electrophilic metal complexes to approach levels of reactivity for some reactions that are typically only seen with cationic species. 78^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes 2.5 Conclusions The amidate ligand set is highly adaptable to the group 4 metals, with novel coordination geometries, dictated largely by the tight bite angle of the amidate ligands. While salt metathesis methods for installing these ligands have been described previously, the products are difficult to isolate, often with disappointing yields. 86 However, protonolysis methods using tetrakis(amido) and tetrakis(alkyl) starting materials are highly efficient, and are utilized in the synthesis of bis(amidate) bis(amido), tris(amidate) mono(amido), and homoleptic tetrakis(amidate) complexes. Solid-state molecular structure characterization of many of these complexes demonstrates that the amidate ligands are typically bound in a is-fashion to the metal center, through both the N and 0 donors. Given the unsymmetrical nature of the amidate ligand, multiple geometric isomers are possible, and disagreement between solution phase and X-ray crystallographic data suggest that isomer interconversion is possible, and can be observed via variable temperature 1 }1 NMR spectroscopy. The mechanism of interconversion likely involves K 1 -intermediates, and experimental evidence supporting this notion exists in the X-ray crystallographic characterization of the mixed K 1 , K2-bis(amidate) bis(amido) pyridine adduct, 2.19. In general, isomerization can be controlled by modulating the steric bulk on the N donors of the amidate ligands. Despite the hemilability of the amidate ligands, dissociation/exchange in the absence of other donors was not observed for the bis(amidate) bis(amido) complexes. Density functional theory (DFT) studies, in concert with X-ray crystallography of a series of Ti bis(amidate) bis(amido) complexes (2.16, 2.23, and 2.24), support sterically controlled geometric isomerization, and are in agreement with variable temperature 1 H NMR spectroscopic studies. The electronic structure of the bis(amidate)-bis(amido) complexes was determined by DFT calculations and supports a strongly ionic bonding character for the amidate ligand set. The amido complexes in this chapter are reactive precursors for protonolysis reactions to form new amido complexes, such as the anilido complex 2.14. The bis(amidate) bis(amido) complexes can also react with additional amide proligands to 79^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes form tris(amidate) mono(amido) complexes through protonolysis, suggesting the ability to generate mixed amidate complexes for future applications. However, the stability of these species towards disproportionation is limited with very bulky amidate ligands. Insertion of isocyanides into Zr-N amido bonds is also possible (2.22), and the amidate ligands are not affected by this reaction, indicating that these amido complexes are stable for both protonolysis and insertion reactions. Both of these types of reactivity play an important role in the findings reported in the remaining chapters of this thesis. 2.6 Experimental 2.6.1 General Considerations The general statements presented herein apply to all subsequent chapters in this thesis, and are not reproduced for the sake of space. Except where stated otherwise, all manipulations were performed under an inert atmosphere of dry, oxygen-free dinitrogen using either standard Schlenk or drybox techniques. The work presented was performed in either an MBraun MB150BG-II glovebox (equipped with a dual column purification system and a -35 °C freezer), or an MBraun Labmaster 130 glovebox (equipped with a dual column purification system, a -35 °C freezer, and a parallel synthetic apparatus, capable of controlling the temperature and stir rate of 5 different reaction chambers). Anhydrous hexanes and toluene were purchased from Aldrich, sparged with dry, degassed dinitrogen, and passed through a column of activated alumina and Ridox (or Q- 5) catalyst prior to use. Anhydrous benzene, diethyl ether, tetrahydrofuran, and pentane were purchased from Aldrich, sparged with dry, degassed dinitrogen, and purified by passage through an Innovative Technologies SP S-PureS olv-400-4 apparatus. Dimethoxyethane (DME) was purified by distillation from sodium/potassium alloy under reduced pressure, and collected in a sealed flask for later use. Deuterated NMR solvents were purchased from Cambridge Isotopes Ltd. Deuterated benzene (C6D6) and deuterated toluene (C7D8) were degassed by successive freeze-pump-thaw cycles, and stored overnight over molecular sieves under an atmosphere of dry dinitrogen prior to use. Deuterated bromobenzene (C6D5Br) was 80^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes distilled from CaH2 under vacuum, and stored over molecular sieves under an atmosphere of dry dinitrogen prior to use. Deuterated tetrahydrofuran (C4D80) was purified by distillation from sodium/potassium alloy under an atmosphere of dry dinitrogen and stored over molecular sieves prior to use. Deuterated chloroform (CDC13) was stored in a darkened flask over molecular sieves prior to use with organic molecules. Deuterated water (D20) was used as received. 1 H, 1 H{31 P}, 13 C{ 1 H}, 31 13, 31 P { 1 1-1}, 19F, 19F {1H}, two-dimensional, and NOE spectra were collected on three different Bruker instruments: AV-300 with a 5 mm BBI probe operating at 300.0 MHz for 1 H NMR spectroscopy; AV- 400 with a 5 mm inverse BBI probe operating at 400.0 MHz for 1 H NMR spectroscopy; AV-400 with a 5 mm BBI probe operating at 400.0 MHz for 1 H NMR spectroscopy. All three instruments were equipped with cryoprobes capable of variable temperature experiments, ranging between -150 °C and +180 °C. 1 H NMR spectra were referenced to residual protons in deuterated solvents as follows: C6D 5H (8 7.15), C 7D7H (8 2.09), C41)7110 (8 3.58), CHC13 (8 7.24) with respect to TMS (8 0.00). 31 P NMR spectra were referenced to external P(OMe) 3 (8 141.0 with respect to 85% H3PO4 at 8 0.0). 13 C NMR spectra were referenced to (8 128.39) in deuterated benzene or (8 77.44) in deuterated chloroform with respect to TMS (8 0.00). 19F NMR spectra were referenced to external trifluoroacetic acid (8 0.00). IR spectra were recorded as KBr pellets on a BOMEM Michelson Series MB-100 FTIR spectrophotometer. UV/vis spectra were recorded on a Perkin Elmer A, 5000 spectrophotometer using air-tight cuvettes. Mass spectra were either collected on an Agilent technologies GCMS equipped with 6890N GC column and a 5793 mass selective detector operating with an electron impact source and quadrupolar detector, or were collected by Mr. M. Lapawa at the University of British Columbia, Department of Chemistry (EIMS on a Kratos MS 50 using a 70 eV electron impact source, unless otherwise stated). Assigned mass clusters for specific ions in the mass spectra show the appropriate isotopic patterns as calculated for the atomic composition of the species. Elemental analyses were performed by Mr. M. Lakha at the University of British Columbia, Department of Chemistry. Due to the air and moisture sensitivity of the metal complexes analyzed, these compounds were handled using glovebox or glovebag techniques during analysis. 81^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Crystallographic details of all solid-state molecular structures are located in Appendix A. All measurements were made on either Rigaku ADSC, Rigaku AFC7, or Bruker X8 Apex CCD area detectors with graphite-monochromated Mo Ka radiation. Data sets that were not collected by the author were collected by Dr. Brian 0. Patrick. The data were processed87 and corrected for Lorentz and polarization effects. The structures were solved by direct methods, 88 and expanded using Fourier techniques. 89 All non-hydrogen atoms were refined with anisotropic thermal parameters. Neutral atom scattering factors and anomalous dispersion corrections were taken from the International Tables for X-ray Crystallography. 90' 91 All structures are visualized as ORTEP-3 depictions. 92 2.6.2 Starting Materials and Reagents The starting materials listed herein are also utilized in subsequent chapters, and are not reproduced for the sake of space. Wherever particular materials are relevant to individual chapters, details are given regarding the source, synthesis, and purification of these compounds. The following reagents were purchased from Aldrich and purified by distillation: aniline, 2,6-dimethylaniline; 2,6-diisopropylaniline; pyridine; tert- butylamine; trimethylsilylchloride; and benzylbromide. The following reagents were purchased from Aldrich and used as received: H2NCH2CMe2CH2NH2; benzoyl chloride; trimethylacetyl chloride; triethylamine; NaN(SiMe3)2; 1-adamantoyl chloride; benzyl magnesium chloride. Solutions of nBuLi (1.6 M in hexanes) were obtained from Acros Organics and used without further purification. ZrC14, TiCl 4, HfC14, Ti(NMe2)4, Ti(NEt2)4, Zr(NMe2)4, Zr(NEt 2)4, and 2,6-dimethylphenylisocyanide were purchased from Strem Chemicals and used without further purification. ZrC14(THF)2, 93 TiC14(THF)2 ,93 Ti(NMe2)2C12,94 Zr(NMe2)2C12(DME), 95 Zr(CH2Ph)4, 96 and Hf(CH2Ph)4, 96 were prepared via literature methods. 82^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes 2.6.3 Synthesis Synthesis of [ tINNO)P1 ]1-1, 2.1 A solution of tert-butylamine (7.1 mL, 67.8 mmol) and O cooled to -78 °C prior to dropwise addition of a CH2Cl2 triethylamine (9.4 mL, 67.8 mmol) in 100 mL of CH2C12 was N solution of benzoyl chloride (6.0 mL, 56.5 mmol). The clear colorless solution was allowed to warm to room temperature while stirring for approximately 2 h. The cloudy white suspension was then extracted with three portions of 1M HC1 (3 x 25 mL) until the aqueous washings were acidic. The organic fraction was then extracted with a single 25 mL portion of 1M NaOH, until the aqueous fraction was basic. Finally, the organic fraction was washed with 25 mL of brine and the organic fraction was dried over anhydrous MgSO4. Gravity filtration of the solution to remove MgSO4 was performed and excess CH2C12 was removed by rotary evaporation to give a white powder. The product was purified by recrystallization from a supersaturated toluene solution that was cooled to -15 °C. Colorless crystals of 2.1 were isolated by filtration, ground with a mortar and pestle to a fine white powder, and dried while heating to 75 °C under vacuum in a Schlenk flask to remove air and moisture. The purified product was isolated in 87 % yield (8.74 g). 1 H NMR (CDC13, 25 °C, 300 MHz): 8 1.30 (s, 9H, C(CH3)3), 5.50 (br, 1H, NH), 7.00-7.10 (m, 3H, m,p-Ar-I-1), 7.65 (d, 2H, 3411i = 6.6 Hz, o-Ar-H). 13 C { ifI} NMR (CDC13, 25 °C, 75 MHz): 8 29.1, 51.6, 127.6, 128.8, 131.2, 137.1, 166.7. IR (KBr/Nujol, cm -1 ): 1635 (m, C=0), 3326 (w, N-H). EIMS (m/z): 177 ([Mt]). Anal. Calcd for CI illi5NO (%): C, 74.54; H, 8.53; N, 7.90. Found: C, 74.77; H, 8.55; N, 7.98. 83^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Synthesis of [ DIPP(NO)P11}1, 2.2 A solution of 2,6-diisopropylaniline (26.6 mL, 141 mmol) and triethylamine (19.7 mL, 141 mmol) in 100 mL of CH2C12 was cooled to -78 °C prior to dropwise addition of a CH2C12 solution of benzoyl chloride (12.6 mL, 108 mmol). The clear colorless solution was allowed to warm to room temperature while stirring for approximately 2 h. The cloudy white suspension was then extracted with three portions of 1M HC1 (3 x 25 mL) until the aqueous washings were acidic. The organic fraction was then extracted with a single 25 mL portion of 1M NaOH, until the aqueous fraction was basic. The organic fraction was then washed with 25 mL of brine and the organic fraction was dried over anhydrous MgSO4. Gravity filtration of the solution to remove MgSO4 was performed and excess CH2C12 was removed by rotary evaporation to give a white powder. The product was purified by recrystallization from a supersaturated toluene solution that was cooled to -15 °C. Colorless crystals of 2.2 were isolated by filtration, ground with a mortar and pestle to a fine white powder, and dried while heating to 100 °C under vacuum in a Schlenk flask to remove air and moisture. The purified product was isolated in 53 % yield (16.0 g). 1 H NMR (CDC13, 25 °C, 300 MHz): 8 1.24 (d, 12H, 3JHH = 6.9 Hz, CH(CH3)2), 3.18 (sept, 2H, 3JHR = 6.9 Hz, CH(CH3)2), 7.24-7.95 (m, 9H, Ar-H, N-H). 13C { 1 H} NMR (CDC13, 25 °C, 75 MHz): 8 23.9, 29.1, 123.8, 127.4, 128.7, 129.0, 131.4, 131.9, 134.8, 146.6, 167.1. EIMS (m/z): 281 ([M+]). Anal. Calcd for C19H23NO (%): C, 81.10; H, 8.24; N, 4.98. Found: C, 80.62; H, 8.26; N, 4.92. Synthesis of rmiP(NO)Ph]H, 2.3 A solution of 2,6-dimethylaniline (11.5 mL, 92.8 mmol) and triethylamine (12.9 mL, 92.8 mmol) in 100 mL of CH2C12 was cooled to -78 °C prior to dropwise addition of a CH2C12 solution of benzoyl chloride (8.3 mL, 71.4 mmol). The clear, slightly pink solution was allowed to warm to room 84^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes temperature while stirring for approximately 3 h. The cloudy white suspension was then extracted with three portions of 1M HC1 (3 x 25 mL) until the aqueous washings were acidic. The organic fraction was then extracted with a single 25 mL portion of 1M NaOH, until the aqueous fraction was basic. Finally, the organic fraction was washed with 25 mL of brine and the organic fraction was dried over anhydrous MgSO4. Gravity filtration of the solution to remove MgSO4 was performed and excess CH2C12 was removed by rotary evaporation to give a white powder. The product was purified by recrystallization from a supersaturated toluene solution that was cooled to -15 °C. Colorless crystals of 2.3 were isolated by filtration, ground with a mortar and pestle to a fine white powder, and dried while heating to 70 °C under vacuum in a Schlenk flask to remove air and moisture. The purified product was isolated in 64 % yield (10.3 g). I fl NMR (CDC13, 25 °C, 400 MHz): 8 2.24 (s, 6H, Ph(CH3)2), 7.07-7.89 (m, 8H, Ar-H), 7.62 (br, 1H, N-H). 13C{ 1 fI} NMR (CDC1 3 , 25 °C, 75 MHz): 8 18.4, 127.2, 127.3, 128.1, 128.6, 131.6, 134.0, 134.4, 135.6, 165.9. EIMS (m/z): 225 ([M+]). Anal. Calcd for C151115NO (%): C, 79.97; H, 6.71; N, 6.22. Found: C, 80.24; H, 6.72; N, 6.20. Synthesis of [DiviP(NO) fi31H, 2.4 A solution of 2,6-dimethylaniline (5.1 mL, 41.5 mmol) and triethylamine (5.8 mL, 41.5 mmol) in 50 mL of CH2C12 was cooled to -78 °C prior to dropwise addition of a CH2C12 solution of trimethylacetyl chloride (5.1 mL, 41.5 mmol). The clear slightly pink solution was allowed to warm to room temperature while stirring for approximately 3 h. The cloudy white suspension was then extracted with three portions of 1M HC1 (3 x 25 mL) until the aqueous washings were acidic. The organic fraction was then extracted with a single 25 mL portion of 1M NaOH, until the aqueous fraction was basic. Finally, the organic fraction was washed with 25 mL of brine and the organic fraction was dried over anhydrous MgSO4. Gravity filtration of the solution to remove MgSO4 was performed and excess CH2C12 was removed by rotary evaporation to give a white powder. The product was purified by recrystallization from a supersaturated toluene solution that was cooled to -15 °C. Colorless crystals of 2.4 were isolated by 85^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes filtration, ground with a mortar and pestle to a fine white powder, and dried while heating to 100 °C under vacuum in a Schlenk flask to remove air and moisture. The purified product was isolated in 82 % yield (6.98 g). 1 H NMR (CDC13, 25 °C, 300 MHz): 8 1.30 (s, 9H, C(CH3)3), 2.14 (s, 6H, Ar(CH3)2), 7.04 (m, 3H, Ar-H), 7.13 (br, 1H, N-H). 13C{ 1 }1} NMR (CDC13, 25 °C, 75 MHz): 8 18.3, 27.8, 39.3, 127.0, 128.1, 134.3, 135.6, 179.7. EIMS (m/z): 205 ([M1). Anal. Calcd for C 1 3Hi9NO (%): C, 76.06; H, 9.33; N, 6.82. Found: C, 75.94; H, 9.11; N, 6.83. Synthesis of [DIPP(NO)'131H, 2.5 A solution of 2,6-diisopropylaniline (10.7 mL, 56.4 mmol) and triethylamine (8.0 mL, 56.4 mmol) in 50 mL of CH2C12 was cooled to -78 °C prior to dropwise addition of a CH2C12 solution of trimethylacetyl chloride (6.9 mL, 56.4 mol). The clear pale yellow solution was allowed to warm to room temperature while stirring overnight. The cloudy yellow suspension was then extracted with three portions of 1M HC1 (3 x 25 mL) until the aqueous washings were acidic. The organic fraction was then extracted with a single 25 mL portion of 1M NaOH, until the aqueous fraction was basic. The organic fraction was then washed with 25 mL of brine and the organic fraction was dried over anhydrous MgSO4. Gravity filtration of the solution to remove MgSO4 was performed and excess CH2C12 was removed by rotary evaporation to give a white powder. The product was purified by recrystallization from a supersaturated toluene solution that was cooled to -15 °C. Colorless crystals of 2.5 were isolated by filtration, ground with a mortar and pestle to a fine white powder, and dried while heating to 100 °C under vacuum in a Schlenk flask to remove air and moisture. The purified product was isolated in 58 % yield (8.54 g). 1 H NMR. (CDC13, 25 °C, 300 MHz): 8 1.21 (d, 12H, 3JHH = 6.9 Hz, CH(CH3)2), 1.38 (s, 9H, C(CH3)3), 3.03 (sept, 2H, 3JHH = 6.9 Hz, CH(CH3)2), 6.86 (br, 1H, N-H), 7.18 (d, 2H, 3JHH = 7.7 Hz, m-Ar-H), 7.29 (m, 1H, p-Ar-H). 13C { I fI} NMR (CDC13, 25 °C, 75 MHz): 8 23.6, 27.9, 28.8, 39.3, 123.4, 128.2, 131.6, 146.3, 177.3. EIMS (m/z): 261 ([M 1 ]). Anal. Calcd for Ci7H27NO (%): C, 78.11; H, 10.41; N, 5.36. Found: C, 78.22; H, 10.17; N, 5.42. 86^References begin on page 98 H N^•^N Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Synthesis of Ad[02N2]-12 , 2.6 A solution of 1,3-diamino-2,2-dimethylpropane (2.4 mL, 20.1 mmol) and triethylamine (5.6 mL, 40.3 mol) in 100 mL of CH2C12 was cooled to -78 °C prior to dropwise addition of a CH2C12 solution of 1-adamantane carbonyl chloride (7.98 g, 40.3 mmol). The clear colorless solution was allowed to warm to room temperature while stirring overnight. The cloudy white suspension was then extracted with three portions of 1M HC1 (3 x 25 mL) until the aqueous washings were acidic. The organic fraction was then extracted with a single 25 mL portion of 1M NaOH, until the aqueous fraction was basic. Finally, the organic fraction was washed with 25 mL of brine and the organic fraction was dried over anhydrous MgSO4. Gravity filtration of the solution to remove MgSO4 was performed, and excess CH2C12 was removed by rotary evaporation to give a white powder. The product was purified by recrystallization from a supersaturated toluene solution that was cooled to -15 °C. A white powder was isolated by filtration, ground with a mortar and pestle to a fine white powder, and dried while heating to 120 °C under vacuum overnight in a Schlenk flask to remove air and moisture. Due to the increased ability of 2.6 to hydrogen bond with water, higher temperatures were used to remove traces of moisture. The purified product was isolated in 67 % yield (5.75 g). 'H NMR (CDC13, 25 °C, 300 MHz): 8 0.80 (s, 6H, NCH2C(CH3)2CH2N), 1.70 (br, 12H, Ad- (CH(CH2)CH)3), 1.86 (d, 12H, 3JHH = 2.8 Hz, Ad-C(CH2)3), 2.02 (br, 6H, Ad-(CH)3), 2.91 (d, 4H, 3JHH = 6.7 Hz, HNCH2C(CH3)2CH2NH), 6.54 (t, 2H, 3 = 6.2 Hz, NH). 13C{ 1 1-1} NMR (CDC13, 25 °C, 75 MHz): 8 23.5, 28.2 , 36.5, 39.3, 40.8, 44.9, 178.7 (no signal observed for quaternary C of adamantyl group). CIMS (m/z): 427 ([M+11). Anal. Calcd for C27H42N202 (%): C, 76.01; H, 9.92; N, 6.57. Found: C, 75.69; H, 9.96; N, 6.70. 87^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Synthesis of [DmP(NO)P1 ] 4Hf, 2.8 In a foil wrapped 500 mL round-bottomed Schlenk flask equipped with a stir bar, 3.32 g (14.7 mmol) of 2.3 was combined with 2.00 g (3.68 mmol) of Hf(CH2Ph)4. To this flask was added 100 mL of THF which had been cooled to -78 °C. The cloudy white mixture was stirred for 2 hours while allowing to warm to room temperature. The clear, colorless solution was then concentrated to dryness in vacuo to give a white solid residue. The crude material was washed with approximately 50 mL of hexanes and dried in vacuo to yield 3.72 g of a white powder (94 % yield). Single clear colorless crystals suitable for X-ray crystallographic analysis were grown from a saturated hexanes solution at room temperature. 'H NMR (C6D6, 25 °C, 300 MHz): 8 2.51 (s, 24H, Ar(CH3 )2), 6.77-6.87 (m, 24H, Ar-H), 7.70 (d, 8H, 3JHH = 6.5 Hz, Ar-H). 13C {'H} NMR (C6D6, 25 °C, 75 MHz): 8 19.8, 125.3, 128.4, 128.8, 129.3, 132.0, 133.4, 134.5, 143.4, 179.2. EIMS (m/z): 1076 ([Mt]), 852 ([M+] — [DmP(NO)P1 ]). Anal. Calcd for C60E156N404Hf (%): C, 67.00; H, 5.25; N, 5.21. Found: C, 67.26; H, 5.37; N, 5.09. Synthesis of [Div113(NO)tB14Zr, 2.9 In a foil wrapped 250 mL round-bottomed Schlenk flask equipped with a stir bar, 770 mg (3.76 mmol) of 2.4 was combined with 250 mg (0.939 mmol) of Zr(NMe2)4. To this flask was added 50 mL of THF which had been cooled to -78 °C. The cloudy white mixture was stirred while allowing to warm to room temperature overnight. The clear, colorless solution was then concentrated to dryness in vacuo to give a white solid residue. The crude material was triturated with approximately 30 mL of hexanes, and the product was isolated by filtration and dried in vacuo to yield 720 mg of a white powder (85 % yield). Single clear colorless crystals suitable for X-ray crystallographic analysis were grown from a saturated hexanes solution at room temperature. 'H NMR (C6D6, 25 °C, 300 MHz): 8 1.02 (s, 36H, C(CH3)3) 2.40 (s, 24H, 88^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Ar(CH3)2), 6.62-6.69 (m, 12H, Ar-H). 13 C{ 1 fI} NMR (C6D6, 25 °C, 100 MHz): 8 20.5, 28.3, 41.6, 124.7, 128.0, 132.8, 143.3, 189.9. Synthesis of Ad[02N2]2Zr, 2.10 In a 100 mL round-bottomed Schlenk flask equipped with a stir bar, 250 mg (0.587 mmol) of 2.6 was combined with 78 mg (0.293 mmol) of Zr(NMe2)4. To this flask was added 30 mL of THF which had been cooled to -78 °C. The cloudy white mixture was stirred overnight while allowing to warm to room temperature. The clear, colorless solution was then concentrated to dryness to give a white solid residue. The crude material was triturated with 10 mL of hexanes dried in vacuo to give 220 mg of a white powder (80 % yield). Single clear colorless crystals suitable for X-ray crystallographic analysis were grown from a saturated toluene solution at room temperature. I li NMR (C6D6, 25 °C, 300 MHz): 8 1.28 (s, 12H, C(CH3)2), 1.60 (m, 24H, Ad-(CH(CH2)CH)3), 1.90 (br, 12H, Ad-(CH)3), 2.20 (br, 24H, Ad-C(CH2)3), 3.63 (s, 8H, NCH2C(CH3)2CH2N). 13C{ 1 El} NMR (C6D6, 25 °C, 75 MHz): 8 25.9, 29.1, 36.8, 37.5, 39.0, 43.0, 56.9, 190.2. EIMS (m/z): 938 ([Mt]), 803 ([Mt] — Ad). Synthesis of Ad[02N2]2Hf, 2.11 In a foil wrapped 100 mL round-bottomed Schlenk flask equipped with a stir bar, 750 mg (1.76 mmol) of 2.6 was combined with 478 mg (0.880 mmol) of Hf(CH2Ph)4. To this flask was added 50 mL of THF which had been cooled to -78 °C. The cloudy white reaction mixture was then stirred overnight while allowing to warm to room temperature. The clear, pale yellow solution was then concentrated to dryness in vacuo to give a pale yellow 89^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes solid residue. The crude material was washed with 25 mL of hexanes and filtered to isolate 730 mg of an off-white powder (81 % yield). Single crystals suitable for X-ray crystallographic analysis were grown from a saturated toluene solution at room temperature. 'H NMR (C6D6, 25 °C, 300 MHz): 8 1.27 (s, 12H, C(CH3)2), 1.56-1.66 (br, 24H, Ad-(CH(CH2)CH)3 ), 1.90-1.95 (br, 12H, Ad-(CH)3), 2.20 (br, 24H, Ad-C(CH2)3), 3.67 (s, 8H, NCH2C(CH3)2CH2N). 13C{ I H} NMR (C6D6, 25 °C, 75 MHz): 8 25.3, 28.3, 36.8, 38.3, 39.5, 42.6, 56.1, 188.9. EIMS (m/z): 1028 ([M-1), 893 ([Mt] — Ad). Anal. Calcd for C54H80N404Hf (%): C, 63.11; H, 7.85; N, 5.45. Found: C, 63.50; H, 7.72; N, 5.58. Synthesis of [DIPP(NO)P113ZrNMe2, 2.12 In a 250 mL round bottomed Schlenk flask, 3.15 g (11.2 mmol) of 2.2 was dissolved in approximately 100 mL of Et20 and cooled to -78 °C while stirring. In a separate flask, 1.00 g (3.73 mmol) of Zr(NMe2)4 was dissolved in about 30 mL of Et20 prior to addition to the solution of 2.2 via cannula. This solution was allowed to warm to room temperature overnight while stirring, resulting in a clear colorless solution. Excess Et20 was removed in vacuo to generate a white solid residue. The crude product was dissolved in 20 mL of pentane and filtered through Celite TM to remove traces of unreacted proligand. The solution was concentrated to dryness and the product was isolated in 86 % yield (3.13 g). NMR (C6D6, 25 °C, 300 MHz): 8 0.88 (d, 18H, 3JHH 6.4 Hz, CH(CH3)2), 1.07 (d, 6H, 3JHH = 6.6 Hz, C(CH3)2), 1.26 (d, 12H, 3JHH = 6.7 Hz, C(CH3)2), 3.29 (s, 6H, N(CH3)2), 3.75 (sept, 6H, 3JHH = 6.7 Hz, CH(CH3)2), 6.72-7.60 (m, 24 H total, Ar-H). 13C{ 1 11} NMR (C6D6, 25 °C, 75 MHz): 8 24.9, 25.5, 28.7, 43.7, 124.8, 126.4, 128.3, 130.8, 131.9, 132.9, 142.6, 143.2, 178.1. EIMS (m/z): 930 ([M4-] — NMe2). Anal. Calcd for C 59H72N4O3Zr (%): C, 72.57; H, 7.43; N, 5.74. Found: C, 71.47; H, 8.91; N, 5.20. 90^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Synthesis of [DmP(N0)'H13ZrNMe2, 2.13 In a 250 mL round bottomed Schlenk flask, 2.30 g (11.2 mmol) of 2.4 was slurried in approximately 120 mL of toluene, which was cooled to -78 °C. In a 50 mL Schlenk flask, 1.00 g (3.75 mmol) of Zr(NMe2)4 was dissolved in 25 / Zr-NMe2 mL of toluene, which was transferred via cannula into the \ slurry of 2.4. The reaction mixture was allowed to warm to \ room temperature overnight while stirring, resulting in a clear colorless solution. Excess solvent was removed in vacuo to give a slightly off-white waxy solid. The crude product was dissolved in 25 mL of pentane and filtered through Celite TM to remove traces of unreacted proligand. The solution was concentrated to dryness, to give 89 % yield (2.48 g) of 2.13. 1 1-1 NMR (C6D6, 25 °C, 300 MHz): 6 1.02 (s, 27H, C(CH3)3), 2.31 (br s, 18H, Ph(CH3)2), 2.96 (s, 6H, N(CH3)2), 6.78 (m, 9H, Ar-H). 13C { 1 1-1} NMR (C6D6, 25 °C, 75 MHz): 6 19.6, 27.8, 41.3, 43.2, 124.3, 127.8, 132.4, 143.7, 189.6. Synthesis of [DIPP(NO)P11 3ZrNHPh, 2.14 In a 250 mL round bottomed Schlenk flask, 1.00 g (1.35 mmol) of 2.20 was dissolved in 100 mL Et20 and cooled to -78 °C. To this flask was added via syringe 0.1 mL (1.35 mmol) of aniline which had been dissolved in 10N mL of Et20. This solution was allowed to warm to room temperature overnight while stirring, resulting in a bright yellow solution. Excess Et20 was removed in vacuo to generate a yellow solid residue. The crude product was dissolved in 15 mL of pentane and filtered through Celite TM to remove a small amount of pale yellow insoluble material. The solution was concentrated to dryness, resulting in 700 mg (51 % yield) of bright yellow 2.14. Single crystals were grown from a saturated toluene solution at room temperature. 1 1-1 NMR (C6D6, 25 °C, 300 MHz): 6 -0.08 (d, 3H, 3JHH = 6.7 Hz, CH(CH3)2), 0.18 (d, 3H, 3JHH = 6.5 Hz, CH(CH3)2), 0.24 (d, 3H, 3JHH = 6.6 Hz, CH(CH3)2), 0.48 (d, 3H, 3JHH = 6.8 Hz, 91^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes CH(CH3)2), 0.59 (d, 3H, 3JHH = 6.5 Hz, CH(CH3)2), 0.82 (d, 3H, 3JHH = 6.6 Hz, CH(CH3)2), 0.89 (d, 3H, 3JHH = 6.5 Hz, CH(CH3)2), 1.08 (d, 3H, 3JHH = 6.5 Hz, CH(CH3)2), 1.14 (d, 3H, 3JHH = 6.7 Hz, CH(CH3)2), 1.30 (d, 3H, 3JHH = 6.5 Hz, CH(CH3)2), 1.48 (d, 3H, 3JHH = 6.7 Hz, CH(CH3)2), 1.81 (d, 31-I, 3JHH = 6.8 Hz, CH(CH3)2), 2.69 (sept, 1H, 3JHH = 6.8 Hz, CH(CH3)2), 3.21 (sept, 1H, 3JHH = 6.8 Hz, CH(CH3)2), 3.58 (sept, 1H, 3JHH = 6.9 Hz, CH(CH3)2), 3.60 (sept, 1H, 3JHH = 6.7 Hz, CH(CH3)2), 3.69 (br, 1H, CH(CH3)2), 4.54 (sept, 1H, 3JHH = 7.6 Hz, CH(CH3)2), 6.57- 7.71 (m, 29H, Ar-H), 8.15 (s, 1H, Zr-NI-1). EIMS (m/z): 930 ([Mt] - NHPh). Synthesis of ru(NO)Ph12Zr(NEt2)2, 2.15 In a 50 mL round-bottomed Schlenk flask equipped with a stir bar, 4.18 mL (11.3 mmol) of Zr(NEt2)4 was dissolved in 10 mL of toluene. In a separate 50 mL round bottomed Schlenk flask, 4.00 g (22.6 mmol) of 2.1 was slurried in 25 mL of toluene at -78 °C. The solution of Zr(NEt2)4 was transferred by cannula into the solution of 2.1. The resulting yellow solution was stirred while warming to room temperature. The clear yellow-orange reaction mixture was evaporated to dryness in vacuo, resulting in an orange solid, which was dissolved in hexanes and filtered through Celite TM to remove contaminants. The hexanes soluble fraction was concentrated to dryness, and the oily yellow-orange solid obtained was subsequently dissolved and triturated in pentane. Removal of pentane in vacuo gave a yellow-orange powder in 80% yield (5.31 g). Single crystals suitable for X-ray crystallographic analysis were grown from a saturated 1:1 toluene/hexanes mixture at -37 °C. 'H NMR (C6D6, 25 °C, 400 MHz): 8 1.26 (s, 18 H, C(CH3)3), 1.26 (t, 12H, 3JHH = 6.7 Hz, N(CH2CH3)2), 3.78 (q, 8H, 3JHH = 6.7 Hz, N(CH2CH3)2), 7.03-7.06 (m, 6H, Ar-H), 7.39 (d, 4H, 3JHH = 9.5 Hz, Ar-H). 13C {'H} NMR (CDC1 3 , 25 °C, 75 MHz): 8 24.00, 37.50, 47.20, 127.50, 129.00, 132.00, 134.55, 168.50. IR (KBr/Nujol, cm -1 ): 1636 (m, CO). EIMS (m/z): 514 ([M1 — NEt2). Anal. Calcd for C30H48N4O2Zr (%): C, 61.28; H, 8.23; N, 9.53. Found: C, 58.12; H, 8.25; N, 9.69. 92^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Synthesis of [tau(NO)Ph]2Ti(NEt2)2, 2.16 In a 250 mL round-bottomed Schlenk flask equipped with a stir bar, 600 mg (3.39 mmol) of 2.1 was slurried in 60 mL of Et20 and cooled to -78 °C. In a separate 100 mL tube Schlenk 7 flask Ti(NEt2)4 (570 mg, 1.69 mmol) was dissolved in 50 mL \ Ti / \ NEt2 of Et20 and cooled to -78 °C. The solution of Ti(NEt2)4 was added via cannula transfer to the slurry of 2.1, and the resulting red-orange mixture was stirred and allowed to warm to room temperature for 5 hours. Excess Et20 was removed in vacuo and the crude product was taken up in hexanes, filtered through Celite TM, and excess hexanes was removed in vacuo. A dark red microcrystalline solid was isolated in 82% yield (755 mg). Single crystals suitable for X-ray crystallographic analysis were grown from a saturated 1:1 toluene/hexanes mixture at -37 °C. 1 1-1 NMR (C6D6, 25 °C, 300 MHz): 8 1.38 (s, 18H, C(CH3)3), 1.30 (t, 12H, 3JHH = 6.9 Hz, N(CH2CH3)2), 4.17 (q, 8H, 3JHH = 6.7 Hz, N(CH2CH3)2), 7.15-7.23 (m, 6H, Ar-H), 7.50 (d, 4H, 3JHH = 6.7 Hz, Ar-H). IR (KBr/Nujol, cm -1 ): 1637 (m, CO). EIMS (m/z): 472 ([M+] — NEt2), 400 ([Mt] — 2NEt2). Anal. Calcd for C30H48N4O2Ti (%): C, 66.16; H, 8.88; N, 10.29. Found: C, 65.76; H, 8.77; N, 10.39. Synthesis of ru(NO)Ph12Ti(NMe2)2, 2.17 In a 250 mL round-bottomed Schlenk flask equipped with a stir bar, 8.00 g (45.2 mmol) of 2.1 was slurried in 100 mL of Et20 and cooled to -78 °C. In a separate 100 mL tube Ti / NMe2 Schlenk flask Ti(NMe2)4 (5.07 g, 22.6 mmol) was dissolved in 50 mL of Et20 and cooled to -78 °C. The solution of 0 2 NMe2 Ti(NMe2)4 was added via cannula transfer to the slurry of 2.1, and the resulting red- orange mixture was stirred and allowed to warm to room temperature for 3 hours. Excess Et20 was removed in vacuo and the crude product was taken up in hexanes, filtered through CeliteTM, and excess hexanes was removed in vacuo. A dark red microcrystalline solid was isolated in 75% yield (8.25 g). Single crystals suitable for X- 93^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes ray crystallographic analysis were grown from a saturated 1:1 hexanes solution at room temperature. 1 H NMR (C6D6, 25 °C, 300 MHz): 8 1.28 (s, 18H, C(CH3)3), 3.62 (s, 12H, N(CH3)2), 7.04-7.10 (m, 6H, Ar-H), 7.36-7.39 (m, 4H, Ar-H). 13C {'H} NMR (CDC13, 25 °C, 75 MHz): 8 32.5, 47.1, 53.2, 127.4, 128.4, 129.8, 139.3, 175.3. EIMS (m/z): 488 ([Mt]), 444 ([M+] — NMe2), 400 ([Mt] — 2NMe2). Anal. Calcd for C26H40N4O2Ti (%): C, 63.93; H, 8.25; N, 11.47. Found: C, 63.72; H, 8.26; N, 11.10. Synthesis of [DivIP(NO) t1312Zr(NMe2)2, 2.18 In a 500 mL round-bottomed Schlenk flask equipped with a stir bar, 6.14 g (29.9 mmol) of 2.4 was combined with 4.00 g (15.0 mmol) of Zr(NMe2)4. To this flask was added 150 mL of THE which had been cooled to -78 °C. The cloudy white reaction mixture was then stirred overnight while allowing to warm to room temperature. The clear, colorless solution was then concentrated to dryness in vacuo to give a white solid residue. was dissolved in 70 mL of pentane and filtered through Celite TM to remove impurities. The pentane solution was concentrated to approximately 10 mL and cooled to -37 °C. The white precipitate was isolated in 75 % yield (6.55 g) by filtration and dried in vacuo. Single crystals suitable for X-ray crystallographic analysis were grown from a saturated pentane solution at room temperature. 1 H NMR (C6D6, 25 °C, 400 MHz): 8 1.03 (s, 18H, C(CH3)3), 2.12 (s, 12H, Ph(CH3)2), 2.93 (s, 12H, N(CH3)2), 6.86-6.92 (m, 6H, Ar-H). 13C{ 1 1-1} NMR (C7D8, 25 °C, 100 MHz): 8 18.6, 27.8, 41.1, 41.8, 124.3, 127.9, 131.7, 144.3, 187.1. EIMS (m/z): 586 ([M+]), 542 ([Mt] — NMe2), 498 ([Mt] — 2NMe2). Anal. Calcd for C30H48N4O2Zr (%): C, 61.28; H, 8.23; N, 9.53. Found: C, 61.14; H, 8.47; N, 9.43. N Z NMe2 ^ ,^rNz 0 /2 NMe2 The crude material 94^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Synthesis of K2-[nm(No)`Bu] Ki_ [Dmp(No , tBu) ]Zr(NMe2)2(Py), 2.19 In a 25 mL vial equipped with a stir bar, in a glovebox, 500 mg (0.853 mmol) of 2.18 was dissolved in approximately 15 mL of pentane and stirred at room temperature. In a 5 mL vial 67 mg (0.853 mmol) of Py was dissolved in approximately 3 mL of pentane and added to the stirring solution of 2.18. The clear colorless solution was filtered through CeliteTM and concentrated to approximately 3 mL, and an off-white microcrystalline solid was isolated by filtration and dried in vacuo. Yield = 79 % (450 mg). Colorless single crystals for X-ray analysis were grown from a saturated pentane solution. I li NMR (C6D6, 25 °C, 300 MHz): 8 1.03 (s, 18H, C(CH3)3), 2.12 (br, 12H, Ph(CH3)2), 2.78-2.93 (br, 12H, N(CH3)2), 6.63-6.89 (m, 9H, Ar-H , Py-m,p-.1-1), 8.46 (d, 2H, 3Jmi = 4.3 Hz, Py-o-II). 13C{ 1 1-1} NMR (C6D5Br, 25 °C, 100 MHz): 8 17.0, 17.3, 26.7, 28.0, 37.7, 39.7, 40.7, 119.6, 122.6, 122.9, 126.5, 126.8, 135.2, 142.8, 147.9, 148.8, 166.6, 182.7, 185.2. EIMS (m/z): 586 ([Mt] - Py), 542 ([Ml - Py - NMe2), 498 ([Mt] - Py - 2NMe2). Anal. Calcd for C35H53N5O2Zr (%): C, 63.02; H, 8.01; N, 10.50. Found: C, 61.93; H, 7.81; N, 10.00. Synthesis of [DIPP(NO)P1i]2Zr(NMe2)2, 2.20 In a 100 mL round-bottomed Schlenk flask 2.0 g (7.49 mmol) of Zr(NMe2)4 was dissolved in approximately 25 mL of Et20. In a separate 250 mL round-bottomed / Schlenk flask, 4.21 g (14.9 mmol) of 2.2 was slurried in^ Zr NMe2 approximately 75 mL of Et20. The ligand slurry was Nme2 2 stirred and cooled to -78 °C using a dry-ice isopropanol bath. The ethereal Zr(NMe2)4 solution was transferred via cannula into the ligand slurry. The reaction mixture was allowed to warm to room temperature overnight, during which time the solution became clear and turned a pale yellow color. Excess Et20 was removed 95^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes in situ to afford a pale yellow solid. The crude product was dissolved in hexanes and filtered through Celite TM to remove insoluble impurities. The pale yellow filtrate was concentrated to dryness, resulting in a pale yellow solid isolated in 92 % yield (5.09 g). Generally, no further purification was required; however, recrystallization from concentrated hexanes or pentane solutions resulted in analytically pure product in moderate yields. 'H NMR (C 6D6, 25 °C, 300 MHz): 8 0.91 (d, 12H, 3JHH = 6.8 Hz, CH(CH3)2), 1.21 (d, 12H, 3JHH = 6.8 Hz, CH(CH3)2), 3.11 (s, 12H, N(CH3)2), 3.49 (sept, 4H, 3JHH = 6.8 Hz, CH(CH3)2), 6.83-6.92 (m, 6H, Ar-H), 7.15 (br, 6H, Ar-H), 7.75 (d, 4H, 3JHH = 6.8 Hz, Ar-H). 13 C { 1 1-1} NMR (C7138, 25 °C, 75 MHz): 8 24.8, 28.7, 42.4, 124.5, 124.7, 126.3, 128.3, 130.8, 132.1, 133.1, 142.5, 177.7. EIMS (m/z): 738 ([M+]), 694 ([M+] — NMe2), 650 ([M+] — 2NMe2). Anal. Calcd for C42H56N4O2Zr (%): C, 68.16; H, 7.63; N, 7.57. Found: C, 68.22; H, 7.69; N, 7.23. Synthesis of ri"(NO)tBl2Zr(i 2-(2,6-Me2C6H3)N=C(NMe2))2, 2.22 _2 waxy white residue. The crude material was triturated with approximately 5 mL of Et20 and the white crystalline material was isolated by filtration, and dried in vacuo. Yield = 65 % after 3 crops of precipitation and filtration (420 mg). 'H NMR (C6D6, 25 °C, 300 MHz): 8 1.17 (s, 18H, C(CH3)3), 1.66 (s, 6H, Ph(CH3)2), 1.94 (s, 6H, Ph(CH3)2), 2.09 (s, 12H, N(CH3)2), 2.42 (s, 6H, Ph(CH3)2), 2.73 (s, 6H, Ph(CH3)2), 6.77-7.03 (m, 12H, Ar- H). 13 C{ I H} NMR (C6D6, 25 °C, 75 MHz): 8 18.8, 18.9, 19.8, 21.2, 28.8, 36.9, 41.5, 45.0, 124.0, 124.3, 128.1, 128.6, 131.3, 132.9, 133.8, 134.7, 148.5, 151.8, 186.5, 208.7. EIMS (m/z): 848 ([M1), 673 ([M+] — 2,6-Me2C6H3N=CNMe2). In a 25 mL vial, equipped with a stir bar, in a glovebox, 450 mg (0.768 mmol) of 2.18 and 200 mg (1.54 mmol) of 2,6-Me2C6H3NaC were dissolved in approximately 5 mL of pentane (approximately 1 mL of toluene added to aid solubility). The colorless reaction mixture was stirred overnight and concentrated to dryness to give a 96^References begin on page 98 NR2 Ti 0 2 NR2 R = Me, Et Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes Synthesis of [DmP(NO)Ph]2Ti(NEt2)2, 2.23 In a 250 mL round-bottomed Schlenk flask, a slurry of 2.3 (134 mg, 0.596 mmol) in toluene (40 mL) was cooled to -78 °C. In a separate Schlenk flask, a solution of Ti(NEt2)4 (100 / /^ N Et2 mg, 0.298 mmol) in toluene was cooled to -78 °C and LTi \ transferred via cannula into the solution of 2.3. The resulting \ 0 2 NEt2 red-orange mixture was allowed to warm to room temperature while stirring, and was subsequently heated to 100 °C for 7 hours (this reaction can also be performed in Et20, without heating to generate the same product). Excess toluene was removed in vacuo and the crude product was dissolved in pentane, filtered through Celite TM, and excess pentane was removed in vacuo. A red-orange microcrystalline solid was isolated in 84% yield (160 mg). I fl NMR (C6D6, 25 °C, 300 MHz): 8 1.19 (t, 12H, N(CH2CH3)2), 2.57 (s, 12H, Ph(CH3)2), 4.31 (q, 8H, N(CH2CH 3)2), 7.12-7.99 (m, 16H, Ar-H). EIMS (m/z): 568 ([M+] — NEt2). Anal. Calcd for C38H48N4O2Ti (%): C, 71.24; H, 7.55; N, 8.74. Found: C, 70.39; H, 7.55; N, 9.12. Synthesis of [ DIPP(NO)P1 ]2Ti(NR2)2, 2.24 This previously reported complex was synthesized from 2.2 and Ti(NEt2)4 following established literature procedures, and characterization data matched expected values. The synthesis of the dimethylamido variant (R = Me) was accomplished in the same manner as that published for 2.24. 85 1 1-1 NMR (R = Me) (C6D6, 25 °C, 300 MHz): 8 0.93 (d, 12H, 3JHH = 6.8 Hz, CH(CH3)2), 1.30 (d, 12H, 3JHH = 6.8 Hz, CH(CH3)2), 3.28 (s, 12H, N(CH3)2), 3.61 (sept, 4H, 3JHH = 6.8 Hz, CH(CH3)2), 6.81-6.88 (m, 6H, Ar-H), 7.15 (br, 6H, Ar-H), 7.69 (d, 4H, 3JHH = 7.8 Hz, Ar-H). 97^References begin on page 98 Chapter 2: Coordination Chemistry of Group 4 Amidate Complexes 2.7 References 1. Liu, F.; Chen, W. Eur. J. Inorg. Chem. 2006, 6, 1168. 2. Matsumoto, K.; Arai, S.; Ochiai, M.; Chen, W.; Nakata, A.; Nakai, H.; Kinoshita, S. Inorg. Chem. 2005, 44, 8552. 3. Chen, W.; Yarnada, J.; Matsumoto, K. Synth. Commun. 2002, 32, 17. 4. Chen, W.; Liu, F.; Nishioka, T.; Matsumoto, K. Eur. J. Inorg. Chem. 2003, 23, 4234. 5. Henderson, W.; Oliver, A. G.; Rickard, C. E. F. Inorg. Chim. Acta 2000, 307, 144. 6. Henderson, W.; Oliver, A. G.; Nicholson, B. K. Inorg. Chim. Acta 2000, 298, 84. 7. 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Chem. 1971, 26, 357. 103^References begin on page 98 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes CHAPTER 3 Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes 3.1 Introduction Generation of new C-C bonds is an extremely important process in synthetic organic chemistry. 1-5 The systematic development of methods to facilitate this process has consistently been an important goal for synthetic chemists since the beginning of rationally designed syntheses. 4 ' 5 Organometallic chemistry has played an extremely important role in this area of chemistry, due to the frequent requirement of transition metal-carbon bonded species as key intermediates in these processes. 6-1° Alkyl complexes of transition metals undergo a wide range of reactivity with many divergent pathways, resulting in the formation of new C-E bonds (E = C, N, 0, etc.). © Reproduced in part with permission from Thomson, R. K.; Patrick, B. 0.; Schafer, L. L. Can. J. Chem. 2005, 83, 1037. Copyright 2005 NRC Research Press Canada. 104^References begin on page 155 Migratory insertion into M-C bond Scheme 3.1 P = Polymer chain M = Ti, Zr, Hf Anion omitted for simplicity H2C=CH2MAOCI M SCI H2C CH2 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes Many industrially important processes involve organometallic species. The insertion of unsaturated organic molecules into M-C bonds is a fundamental process that is important to understanding metal-mediated reactions. The best known catalytic process for the formation of new C-C bonds in group 4 chemistry is olefin polymerization. 11-13 This reaction, first discovered in the 1955, is critical for the sustainability of our modern society, which is heavily dependent upon goods constructed from the plastics formed by this process. 14' 15 Heterogeneous Ziegler-Natta catalysts, composed of mixtures of titanium chloro species and aluminum alkyls, are responsible for the bulk of the polyethylene produced in the world. 16-19 Since this reaction is known to proceed at the surface of crystalline Ti chloro clusters, very little modification of the catalyst is possible, making product flexibility limited. 20 By comparison, the homogeneous metallocene catalysts first discovered by Natta 16 and Breslow' ? in 1957, aided by Kaminsky's fortuitous discovery of the methylalumoxane (MAO) cocatalyst in 1980, 21 offers flexibility in the design of these catalysts systems, due to the ability to modify both the cyclopentadienyl ligands as well as the cocatalytic species. The overall process for ethylene polymerization is illustrated in Scheme 3.1, where M is a group 4 metal (Ti, Zr, or Hf). A key step in this transformation involves the insertion of a C=C bond into a M-C bond. This process is repeated many times before chain termination occurs, resulting in polyethylene. 105^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes The above olefin polymerization example demonstrates the importance of controlled sequential insertion into M-C bonds, but also illustrates the unique reactivity facilitated by cationic group 4 species, which are accessed by alkyl abstraction reactions. Thus the fundamental abstraction of alkyl groups from group 4 metals is an important reaction to understand for a given ligand system. In section 2.4.2, the highly ionic nature of the amidate bonding to Ti was shown through DFT calculations. 22 This results in very electron deficient metal centers that promote reactivity that approaches that of cationic species. Cationic amidate complexes have been generated in situ and studied by the Arnold group. 23 In particular, catalytic ethylene polymerization was studied using a cationic Ti complex bearing a phenylene linked bis(amidate) ligand set. This complex was found to be modestly active towards olefin polymerization when dichloro and bis(amido) derivatives were activated with MAO.23 Chapter 5 will illustrate the unique alkene hydroamination reactivity that is promoted by neutral bis(amidate) complexes, which is seen more commonly with group 3/lanthanide complexes and cationic group 4 complexes. The two most closely related ligand systems to the amidate ligand set are the amidinate24-3I and guanidinate 32-38 ligands, which have been much more widely studied. While these ligands form 4-membered chelates with metal centers similar to the amidate ligand system, they are considerably more electron rich and have increased steric bulk. Insertion studies of C-E multiple bonds (E = N, 0, S) into these organometallic species have shown interesting differences to those of other group 4 complexes. For example, the triisopropyl guanidinate Zr benzyl complex studied by Richeson and coworkers (3.1) has demonstrated unique reactivity with aryl isocyanides to generate imido species. 39 While most alkyl complexes simply insert isocyanides to generate iminoacyl complexes, like 3.2,4° this complex undergoes a novel rearrangement to yield imido complex 3.3. This is proposed to occur through an interesting mechanism involving intramolecular iminoacyl alkylation, followed by n-hydride elimination/rearrangement and retrocyclization, as shown in Scheme 3.2. Unfortunately, the basic nitrogen donors in the backbone of the guanidinate system also undergo undesirable transamination and alkylation reactivity, which is deleterious to subsequent reactivity. 33 ' 34 ' 4' The absence of 106^References begin on page 155 ArNC r- Ph Ph/ 3.4 'PrHN I^ ,,, Bn Zr 'Pr—N- I 'Pr HN^'Pr 3.1 'PrHN Pr —N,, Zr=NAr 'Pr—N 'Pr HN^\'Pr 3.3 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes these groups in the backbone of the amidate ligand may reduce the tendency of such unwanted side reactions. 'PrHN^/1Pr I ,,,,,, Bn Zr.1.0--NAr 'Pr—N^I 'PrHN^N\ 'Pr Ph ^ 'PrHN^'Pr 1\1/ ,,,, NAr Pr—N' Ph- 'PrHN^'Pr 'PrHN ^/Pr N'Pr—N„,'-'=:,, ^ Ar ^• ..^C -/,,,i ■^‘ i P —N' I H -'"-Iph ^)'-,,'  \^ H 'PrHN^'Pr 1 PrFIN^/iPr Ni,,, AN r H ,, C 'Pr—N, I^Ph H Ph 'PrHN^'Pr 3.2 Ar = 2,6-Me2C6H3 Scheme 3.2 Chapter 2 demonstrated the fundamental properties of Ti and Zr amido complexes bearing amidate ligands, including the exchange and isomerization processes that are possible with these ligand systems. This chapter discusses the synthesis and characterization of alkyl complexes bearing amidate ligands. Preliminary insertion reactivity studies are also presented. Given the fact that no group 4 alkyl complexes bearing amidate ligands had been characterized prior to this work, initiation of alkyl reactivity studies began with the synthesis and characterization of simple benzyl derivatives of Zr and Hf amidate species. 42 Analogous Ti complexes were not targeted as these species are notoriously thermally and photosensitive. 107^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes 3.2 Synthesis and Characterization of Dibenzyl Complexes 3.2.1 Introduction The synthesis of amidate supported amido complexes and homoleptic amidate species via protonolysis with homoleptic tetrakisamido Ti and Zr starting materials was highly successful, and was described in detail in Chapter 2. The generation of homoleptic Hf complexes was also accomplished via an analogous protonolysis route, starting with Hf(CH2Ph)4. By extension, it was reasoned that dibenzyl complexes of Zr and Hf could be synthesized through protonolysis using only 2 equiv of proligand, while starting with M(CH2Ph)4 starting materials (M = Zr, Hf), which are easily prepared through literature procedures. 43 Exploratory alkyl chemistry is also often plagued by facile decomposition pathways, the most prevalent being (3-hydride elimination. In an attempt to generate more stable alkyl species, benzyl complexes were targeted as they lack problematic (3- hydrogens. In addition, benzyl ligands are capable of satisfying various electronic requirements by adopting different binding modes: 44-49 While structural evidence of 1 ,50-53 i2 ,54-61 1,1 3 ,46, 47 ,94 ,62, 63 1] 5 ,64 and even T1 7-bound benzyl ligands have been reported,65 the more prevalent hapticities for Zr are 1 1 , 1 2, and r1 3 , where the r1 1 - coordination mode is most common, and 11 2-benzyl ligands are often seen for very electron deficient or cationic complexes (Fig. 3. 1).48, 66-71 The following section deals with the synthesis of Zr and Hf amidate benzyl species. 108^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes 1 1^ 11 2^ 11 3 11 4 ^ 11 5 ^ rl 7 Figure 3.1 Hapticities of benzyl ligands 3.2.2 Results and Discussion 3.2.2.1 Bidentate Amidate Dibenzyl Complexes The combination of 2 equiv of proligand 3.5 ([DMP(NO)Ph]H) and one equivalent of Hf(CH2Ph)4 in THF in the dark at -78 °C results in a pale yellow slurry, which upon warming to room temperature, forms a deep red-orange solution. An intense red-orange solid can be isolated, which is stable to ambient light and heat in the solid-state. 1 H NMR spectroscopy of the resulting product 3.6 ([DmANO)P1 ]2Hf(CH2Ph)2(THF)), is consistent with a THF adduct of the desired bis(amidate) dibenzyl Hf complex as shown in Eq. 3.1. The presence of a single resonance for the aryl methyl groups at 8 2.34, and a single resonance for the benzyl groups at 8 2.55, suggest a highly symmetric species in solution. The coordination of THF is evidenced by resonances at 8 1.64 and 3.83. 2 +^Hf(CH2Ph)4 THF ^0.- (3.1)  3.5 ^ 3.6 109^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes The solid-state molecular structure of 3.6 as determined by X-ray diffraction is shown in Fig. 3.2. Selected bond lengths and angles are cited in Table 3.1, and crystallographic data are given in Table A3.1 in Appendix A. The structure is a slightly distorted pentagonal bipyramid, where the amidate ligands and THF are coordinated in the equatorial plane, and the two benzyl groups are trans disposed in the axial positions at 169.8(2)° from each other. The deviation of the equatorially placed amidate and THF ligands from the equatorial plane is nearly non-existent, with the sum of angles about Hf being 360.16°. As expected, the THF Hf(1)-O(3) distance of 2.236(4) A is considerably longer than the Hf-O bond lengths to the anionic amidate oxygen donors (Hf(1)-O(1) = 2.161(4) A and Hf(1)-O(2) = 2.174(4) A). In contrast to the bis(amido) complexes described in Chapter 2, this complex has the reactive benzyl groups trans. This is possibly due to the lack of at-donor character in the benzyl groups. With the amido ligands, they may be forced into a cis orientation to avoid unfavorable trans influences. The overall symmetry of 3.6 is pseudo-C2v, and the 2,6-dimethylphenyl groups of the amidate ligands are cis oriented, and stabilized by a 3T-stacking interaction, in which the ring planes are nearly perfectly parallel, with a deviation of only 3.99°. This interaction was seen previously for the homoleptic complexes [ DMP(NO)Ph]4Hf and [DIVW(No tB ]4Zr discussed in Chapter 2, where the amidate ligands bind in a planar fashion to the metal center, and the cis N-aryl groups are 3T-stacked in the solid-state. Intramolecular 7r-stacking interactions with these bulky substituents are relatively rare, but the distance between the ring centroids for the N-aryl groups at approximately 3.6 A is consistent with a previously reported at-stacking interaction in a Ti - late metal bimetallic species, where the interactions ranged between 3.51 and 3.77 A. 72 The obtuse benzyl bond angles (Hf(1)-C(38)-C(39) = 122.3(4)° and Hf(1)-C(31)-C(32) = 124.3(4)°) are consistent with typical 1I 1 -coordination and formal 2 e donation. 51-53 The amidate ligand set was shown by structural analysis and DFT studies in Chapter 2 to be a 4 e - monoanionic bidentate donor. 22 Thus 3.6 can be classified as a 14 e- complex. Given its low electron count, 3.6 is remarkably thermally and photo-stable in the solid-state. 110^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes Figure 3.2 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [ Dm"(NO)P12Hf(CH2Ph)2(THF), 3.6 (hydrogens omitted) Table 3.1 Selected Bond Distances (A) and Angles (°) for imP(NO)PhhHf(CH2Ph)2(THF), 3.6 Lengths Angles Angles Hf(1)-0(1) 2.161(4) 0(1)-Hf(1)-0(2) 156.60(15) C(39)-C(38)-Hf( I ) 122.3(4) Hf(1)-0(2) 2.174(4) 0(1)-Hf(1)-0(3) 77.61(15) 0(2)-C(16)-N(2) 114.9(5) Hf(1)-0(3) 2.236(4) 0(2)-Hf(1)-0(3) 78.76(15) C(32)-C(31)-Hf(1) 124.3(4) Hf( I )-N(1) 2.265(5) 0(2)-Hf(1)-N(2) 59.45(16) 0(1)-C(1)-N(1) 114.7(5) Hf(1)-N(2) 2.258(5) 0(1)-Hf(1)-N(1) 59.53(15) Hf(1)-C(31) 2.326(6) N(2)-Hf(1)-N(1) 84.81(17) Hf(1)-C(38) 2.330(5) C(31)-Hf( I )-C(38) 169.8(2) At temperatures below room temperature, 3.6 maintains its solid-state C2v structure in solution; however, warming the solution above room temperature results in a color change from deep orange to yellow. NIVIR spectroscopy of this new complex is very similar to that for 3.6, with the presence of additional signals for free THF. Recoordination of the THF can be observed at temperatures below room temperature by the reappearance of the deep orange color. This reversible thermochromism is presumably due to the large geometrical perturbation of the metal complex upon coordination/decoordination of THF. Isolation of the THF free dibenzyl species is il l̂ References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes accomplished by evacuating the yellow solution to dryness under vacuum to give a pale yellow solid. The resulting material, [D1v11)(NO)P112Hf(CH2Ph)2, 3.7 can also be synthesized independently by the same synthetic protocol shown in Eq. 3.1, but in toluene rather than THF. The 1 1-1 NMR spectrum of the product isolated via independent synthesis is identical to that of the material isolated from 3.6 after THF loss. Unfortunately, no solid-state molecular structural data could be obtained for 3.7, so it was consistently isolated as an amorphous solid. Hf(CH2Ph)2^+^THE^(3.2) Yellow 3.7 The kinetic parameters of the THF exchange process illustrated in Eq. 3.2 were determined by standard NMR line broadening experiments." A variable temperature stacked plot of the a-methylene THF region of the 1 11 NMR spectrum is shown in Fig. 3.3. Both bound THF peaks are observed to broaden in response to decreased temperature. The peak at 8 3.90 broadens and splits into two peaks at 8 4.12 and 3.50 that sharpen at -50 °C. The coalescence temperature for these two peaks, corresponding to bound and free THF, respectively, occurs at (-25 ± 5) °C. The rate constant (ksoiv) for the THF exchange process is calculated to be (560 ± 110) s -1 by standard methods." The activation energy (AG1) for this process is likewise found to be (47 ± 1.5) kJ mol -1 . This value is similar to that seen for benzyl 11 2-i 1 -i2 interconversion of a pyridine diamido dibenzyl Zr complex, where the activation energy (AGI) for this process was found to be 47 kJ mol -1 (TC = -30 °C). 74 112^References begin on page 155 4^e 4.5^ 1.5 — — 2 4.0^2.5 4.5^I 2. 5 4.5^4.0^3.5 2.5 2.5^3.0 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes -50 °C -45 °C -40 °C -35 °C -30 °C -25 °C -20 °C Figure 3.3 Stacked plot 1 H NMR spectrum showing THE exchange on [D P(NO)P12Hf(CH2Ph)2(THF), 3.6 While solid-state molecular structural analysis of the 6-coordinate Hf complex 3.7 was not possible, the proposed structure, based upon solution phase behavior, is a pseudo-octahedral species with cis arranged benzyl ligands, exhibiting overall C2 symmetry. While Zr complexes are known to be more thermally sensitive than Hf species, there are many reported cases of stable Zr alkyls that have been fully characterized."."' 75, 76 Here a 6-coordinate Zr dibenzyl complex [Dmp(Nc)tBu,12Zr(CH2Ph)2, 3.9, can be synthesized using the synthetic protocol presented in Eq. 3.1, with [Drop(No)tBu]ii, as the proligand employed. Given the propensity of the N-2,6-dimethylphenyl-substituents to interact via it-stacking interactions, it is anticipated that the Zr complex will exhibit solution and solid-state molecular structures analogous to 3.6. Dibenzyl complex 3.9 can be isolated in very high yield as a bright yellow powder, which can be readily recrystallized from a saturated pentane solution, although 113^References begin on page 155 C34 Zrl 141^ N2 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes the crude material is typically pure enough for subsequent reactivity studies. Solution phase behavior of 3.9 is consistent with C2 symmetry, as indicated by single resonances for the amidate tent-butyl and aryl methyl groups at 8 0.91 and 2.05, respectively. This suggests free rotation about the N-Cipso bonds of the 2,6-dimethylphenyl substituents. Free rotation of the benzyl groups is also observed by a single resonance at 8 2.24. It is important to note that even when THF is the solvent of preparation, 3.9 does not coordinate THF to form a 7-coordinate complex analogous to 3.6. Variable temperature NMR experiments on 3.9 do not indicate any slow fluxional processes that can be arrested at low temperature. In comparison to Hf complex 3.6, 3.9 is significantly more photochemically and thermally prone to degradation. However, controlled thermolysis and photolysis experiments do not result in any easily isolable decomposition products, such as benzylidene species. 77 ' 78 Bright yellow crystals of 3.9 can be grown from a saturated pentane solution at low temperature. The solid-state molecular structure of 3.9 is shown in Fig. 3.4, and selected bond lengths and angles are depicted in Table 3.2, with crystallographic data located in Table A3.2 (Appendix A). Figure 3.4 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [DmP(N0r12Zr(CH2Ph)2, 3.9 (hydrogens omitted for clarity) 114^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes Table 3.2 Selected Bond Distances (A) and Angles (°) for [Dmi (NO) tB12Zr(CH2Ph)2, 3.9 Lengths Angles Angles Zr(1)-O(1) 2.0988(13) O(1)-Zr(1)-O(2) 95.75(5) C(35)-C(34)-Zr(1) 105.08(12) Zr(1)-O(2) 2.1080(13) N(1)-Zr(1)-N(2) 142.16(5) O(1)-Zr(1)-N(1) 58.78(5) Zr(1)-N(l) 2.2836(14) N(1)-Zr(1)-C(27) 82.98(6) O(1)-C(1)-N(1) 112.55(14) Zr(1)-N(2) 2.2875(13) O(1)-Zr(1)-C(27) 139.85(5) Zr(1)-C(27) 2.2440(18) C(27)-Zr(1)-C(34) 97.18(8) Zr(1)-C(34) 2.2390(19) C(28)-C(27)-Zr(1) 103.79(11) In contrast to the C I symmetric solid-state molecular structure of the bis(amido) complex bearing this amidate ligand, [Dmr(No)tBu,j2Zr(NMe2)2 (3.10) discussed in Chapter 2 (2.18), the structure of 3.9 is C2 symmetric, with the N donors of the amidate ligands trans to each other. While r1 1 -coordination of the benzyl group is obvious for 3.6, the hapticity of the benzyl group in 3.9 is less clear. One typical parameter used to evaluate the hapticity of benzyl ligands is the bond angle formed by the ipso carbon of the benzyl group, the benzylic carbon, and the metal center. 55 Another parameter sometimes utilized is the chemical shift of the ortho protons of the benzyl group. 47 ' 79 Finally, the sum of the van der Waals radii of the metal center and the ipso carbon can be used to gauge the presence of an i 2-benzyl interaction. 20 ' 80 The bond angles at the benzylic carbon atoms in 3.9 are 103.79(11)° and 105.08(12)°, and are considerably more acute than the 122.3(4)° and 124.3(4)° bond angles for 3.6. However, r1 2-benzyl coordination is generally only designated when this angle is less than 90 0 . 56 ' 79, 80 In the literature, a wide variation for this bond angle is seen, which is highly dependent upon the steric and electronic requirements of the metal center. 49 The bond angles in 3.9 can be classified as intermediate between ri l - and 1 2 - hapticity for the benzyl group. 47 A similar 'intermediate' hapticity can be seen for a (3- diketiminate tribenzyl species (TTP)Zr(CH2Ph)3 (TTP =p-tolyl-NC(CH 3 )CHC(CH3)N-p- toly1), where two of the benzyl ligands are clearly r11 with bond angles of 110.7° and 117.6°, with the intermediate benzyl ligand having a bond angle of 99.1°. 5° Another complex exhibiting intermediate hapticity is the Cp complex (CpZr(CH2Ph)3), which has a benzyl ligand with a bond angle of 94.5 0 . 81 115^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes It is generally noted that complexes with multiple benzyl groups having different hapticities in the solid-state often have 1 H NMR spectra that suggest a single type of benzyl environment, usually due to rapid fluxional processes that make the benzyl groups equivalent on the NMR timescale. 47 ' 50, 59, 60, 74 Given the acute angles formed at the benzylic carbon atoms and the overall C2 symmetry of 3.9, it is surprising that the diastereotopic benzylic protons do not manifest themselves as two different signals. Fluxional processes are likely responsible for this observation. Interconversion between il l - and re-coordination modes of the benzyl ligands is possible; however, variable temperature 1 H NMR experiments do not indicate any observable changes at low temperature. With ri 2-benzyl ligands, it is often seen that the ortho protons are shifted to higher field, which can be attributed to significant at-donation to the metal center. 47 This reduces the ring current within the arene ring, and is manifested by greater magnetic shielding. For 3.9, no arene resonances are observed upfield of approximately 8 6.8, which is generally considered the cutoff for re-behavior, 47 supporting average r1 1 -binding in the solution phase, despite the intermediate hapticity seen in the solid-state. This implies that such an interconversion is extremely rapid for 3.9. It is also possible that the amidate ligands in 3.9 are undergoing isomerization in solution through a K 1 -0-bound amidate intermediate as discussed in Chapter 2 for bis(amidate) bis(amido) complexes. However, evidence for this process was not observed through low temperature 1 H NMR spectroscopic experiments. This isomerization process seems less probable for these dibenzyl complexes, as the benzyl ligands offer less electronic stabilization than the amido ligands in the bis(amidate) bis(amido) complexes discussed in Chapter 2. Inspection of the distances between Zr(1) and the ipso carbon atoms (C(28) — 2.97 A and C(35) — 2.98 A) shows that they fall well outside the distance expected for formal Zr-C 6-bonds, which are usually approximately 2.2 - 2.4 A. 47 ' 48 Weak non-covalent interactions between Zr and the ipso benzyl carbons are present, since the Zr(1)-C(28) and Zr(1)-C(35) distances fall within the sum of the van der Waals radii of Zr and C (—j 3.7 A). 82 These data taken together suggest a degree of re-character for the benzyl ligands in 3.9, particularly in the solid-state. However, the solution phase data are indicative of more traditional r1 1 -hapticity. 116^References begin on page 155 THE (3.3) C Ph + M(CH -78 °C--0-- rt. Chapter 3: Synthesis, Structure, and Reactivity of Zr and HfAmidate Benzyl Complexes 3.2.2.2 Tethered Tetradentate Amidate Dibenzyl Complexes In contrast to the Ti bis(amidate) bis(amido) complexes described in Chapter 2, the dibenzyl complexes 3.6, 3.7, and 3.9 do not appear to undergo amidate ligand isomerization as readily. This reduces the need for a tethered bis(amidate) ligand to control geometric isomerization. However, the ability to access a trans type pentagonal bipyramidal geometry for the Hf dibenzyl complex, 3.6, indicates that the tethered proligand Ad [02N2]H2 (3.11) should be suitable for stabilizing bis(alkyl) complexes of Zr and Hf, as this ligand was shown to preferentially bind in a planar fashion in the homoleptic complexes Ad [02N2]2M (M = Zr (3.12), Hf (3.13)). Synthesis of the Zr (3.14) and Hf (3.15) dibenzyl complexes of proligand 3.11 can be accomplished as shown in Eq. 3.3. 3.11 M = Zr (3.14), Hf (3.15) While this reaction is completely analogous to the synthesis of 3.6, 3.7, and 3.9, additional care must be taken when performing these protonolysis reactions with proligand 3.11. The extra entropic driving force provided by the chelate effect of the tetradentate ligand set results in the formation of the homoleptic species 3.12 and 3.13 as byproducts in these reactions, even when performed in a strict 1:1 stoichiometry. The extremely low solubility of 3.11 in nearly all common solvents makes the slow addition of a solution of 3.11 to a solution of M(CH2Ph)4 unfeasible, and addition of cold solvent to an intimate mixture of 3.11 and M(CH2Ph)4 still results in the formation of large quantities of 3.12 and 3.13. To circumvent this problem, the slow addition of 3.11 to an ethereal solution of M(CH2Ph)4 via solid addition funnel is required. This results in high 117 ̂References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes yields of the desired dibenzyl species as bright yellow (3.14, Ad[02N2]Zr(CH2Ph)2(THF)) and pale yellow (3.15, Ad[02N2]Hf(CH2Ph)2(THF)) powders. As illustrated in Eq. 3.3, these complexes are stabilized by THF in solution, with signals for coordinated THF appearing at 8 1.33 and 3.99 for 3.15. The 1 H NMR spectra for 3.14 and 3.15 are nearly identical, suggesting that they are isostructural, which was seen previously for homoleptic species 3.12 and 3.13 bearing this ligand (Chapter 2). Solution phase C2, geometry is suggested by the high symmetry of the 'H NMR spectra of 3.14 and 3.15, with a single resonance observed for the amidate ligand backbone methyl groups at 8 0.86, suggesting the backbone 'wagging' process described in Chapter 2 for 3.12 and 3.13. Likewise, single resonances for the benzylic and backbone methylene protons are observed at 8 1.73 and 3.19, respectively. The bulky adamantyl groups appear as three resonances in a 12:6:12 ratio at 8 1.66, 1.95, and 2.09, corresponding to the methylene and methine protons. The large number of methylene protons in these complexes required the use of COSY NMR experiments to assign all of the 1 H NMR resonances. Similar to complexes 3.6, 3.7, and 3.9, 3.14 and 3.15 do not exhibit any solution behavior suggestive of re-coordination of the benzyl groups. Free rotation of the benzyl groups about their M-C bonds in solution is facile, and can not be arrested during low temperature 'H NMR experiments. Interestingly, unlike 3.6, the coordinated THF ligand in 3.14 and 3.15 is not labile in solution. Since these THF adducts are formally 14 e - species, the coordination of THF helps fulfill the electronic requirements of the metal centers in these species. The lack of lability is likely due to the fact that this tethered bis(amidate) ligand does not offer sufficient flexibility to assume a more traditional pseudo-octahedral geometry upon loss of THF. 42 In the case of 3.6, the 7r-stacking interaction between the two amidate ligands is easily broken up, and allows for ligand reorganization, likely resulting in a pseudo-octahedral geometry for 3.7, which is analogous to that seen for 3.9.42 Verification of the planar binding geometry of the bis(amidate) ligand in complexes 3.14 and 3.15 is given in the solid-state molecular structure of 3.14 in Fig. 3.5, with selected bond lengths and angles in Table 3.3, and crystallographic data located in Table A3.3 (Appendix A). 118^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes Figure 3.5 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of A [02N2]Zr(CH2Ph)2(THF), 3.14 (hydrogens omitted) Table 3.3 Selected Bond Distances (A) and Angles (°) for  Ad [02N2]Zr(CH2Ph)2(THF),  3.14 Lengths Angles Angles Zr(1)-0(1) 2.2083(17) 0(1)-Zr(1)-0(2) 161.62(7) C(29)-C(28)-Zr(1) 113.52(18) Zr(1)-0(2) 2.1962(17) 0(1)-Zr(1)-0(3) 82.46(6) 0(2)-C(2)-N(2) 113.3(2) Zr(1)-0(3) 2.2887(17) 0(2)-Zr(1)-0(3) 81.29(6) C(36)-C(35)-Zr(1) 116.58(19) Zr(1)-N(1) 2.183(2) 0(2)-Zr(1)-N(2) 59.84(7) 0(1)-C(1)-N(1) 113.2(2) Zr(1)-N(2) 2.1867(19) 0(1)-Zr(1)-N(1) 59.68(7) Zr(1)-C(28) 2.358(3) N(2)-Zr(1)-N(1) 77.74(7) Zr(1)-C(35) 2.366(3) C(28)-Zr(1)-C(35) 165.57(10) The metrical parameters given in Table 3.3 are unexceptional, and show that 3.14 demonstrates very similar bond lengths and angles to 3.6 and 3.9, where the amidate unit bite angle in all of these complexes is approximately 60°. The bond angle between the trans benzyl groups is 165.57(10)°, which is slightly contracted in comparison to 3.6, where this angle is 169.8(2)°. The highly exposed nature of the Zr center in 3.14 is apparent in Fig. 3.5, where the bulky adamantyl groups effectively guard the metal center from the sides, but leave the metal center exposed from the top, bottom, and front. As was seen for 3.6, the THE coordinates in the equatorial position between the two benzyl groups, generating a pentagonal bipyramidal structure. The bond angles at the benzylic carbon atoms are obtuse indicating 1 -binding of the benzyl ligands to Zr (C(29)-C(28)- 119^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and HfAmidate Benzyl Complexes Zr(1) = 113.52(18)° and C(36)-C(35)-Zr(1) = 116.58(19)°). One final interesting feature to note is the anti disposition of the benzyl arene units, where the rings are not sandwiching the THF group as was seen in 3.6. Presumably this is due to reduced steric crowding at the Zr with the tethered ligand; however, crystal packing interactions may also influence the orientation of the benzyl groups in the solid-state. The overall symmetry of 3.14 is C s . Rapid backbone 'wagging' of the amidate backbone, and free rotation of the benzyl groups about the Zr-C linkages gives rise to the observed C2v solution phase structure. Single crystals of the Hf congener 3.15 were also grown from a saturated pentane solution. The solid-state molecular structure of 3.15 is illustrated in Fig. A3 of Appendix A, with relevant metrical parameters given in Table A3.4, along with crystallographic details in Table A3.5. Aside from a slight contraction of the bond distances between the ligand donor atoms and the metal center, 3.15 is completely isostructural to 3.14. These bond length reductions can be attributed to the relativistic nature of Hf vs. Zr, as discussed in Chapter 2.20 3.2.3 Summary The synthesis and characterization of dibenzyl complexes of Zr and Hf was accomplished via protonolysis with tetrabenzyl Zr and Hf starting materials. The Hf dibenzyl complex [ mP(NO)P1 ]2Hf(CH2Ph)2(THF) (3.6) was isolated as a 7-coordinate THF adduct. Reversible thermochromism was observed for this complex as a result of THF lability in solution. The activation parameters for THF exchange of this complex were determined by variable temperature NMR experiments. Distorted pentagonal bipyramidal geometry was confirmed in the solid-state for this complex. A 6-coordinate solvent-free Zr dibenzyl complex [Dmip(NostBu-.) j2Zr(CH2Ph)2 (3.9) was also isolated. The benzyl ligands in this complex were intermediate between il l - and ri 2-hapticity in the solid-state, with a propensity to behave as ri 1 -ligands in solution. Two dibenzyl complexes supported by a tethered tetradentate bis(amidate) ligand were prepared in an analogous fashion (Ad [02N2]M(CH2Ph)2(THF), M = Zr (3.14), Hf (3.15)). Both the Zr and Hf species are 7-coordinate THF adducts in the solid-state, but unlike 120^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes 3.6, the coordinated THE ligand is not labile in solution. With the development of reliable preparative methods for the synthesis of the metal alkyl complexes in hand, preliminary reactivity investigations were undertaken. 3.3 Insertion Reactions of Isocyanides with iump(No s tBu ,.) j2Zr(CH2Ph)2 (3.9) 3.3.1 Introduction The ability of M-C bonds to insert unsaturated organic molecules is the foundation of many catalytic cycles, the most relevant here being catalytic olefin polymerization. 11-13 En route to the development of catalytic reactions, the study of stoichiometric insertion reactions allows for a greater understanding of the factors controlling insertion, stereoselectivity, and subsequent reactivity of the resulting products. This section will address the reactivity patterns observed for the insertion of isocyanides into group 4 metal carbon bonds. Preliminary experiments with the tethered bis(amidate) dibenzyl complexes 3.14 and 3.15 indicated that insertion of unsaturated organic moieties into the M-C bond was possible; however, the very open coordination sphere resulted in a mixture of unidentified products. Fortunately, Zr complex 3.9 offers a good degree of steric stabilization in comparison to 3.14 and 3.15, and has an extremely simple 1 1-1 NMR spectrum, making it ideal for the exploration of stoichiometric insertion reactivity. 3.3.2 Results and Discussion The insertion of isocyanides is a well known process that has been extensively studied, and is an attractive place to begin exploration of amidate supported group 4 organometallic species. ° The insertion of isocyanides into group 4 metal carbon bonds is known to proceed through initial coordination of the carbon lone pair of electrons, followed by rapid insertion into the adjacent reactive M-C bond. 40 ' 76 This results in the formation of an iminoacyl complex, where the N=C bond is usually coordinated to the metal center in an re-fashion, as illustrated in Scheme 3.3. 75 ' 83-86 Unlike the reversible 121^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes insertion of CO to form acyl species, the formation of iminoacyl complexes is irreversible, resulting in complexes that are stable to thermally induced deinsertion processes.40 c / R [M1_ t N— R' R ' Scheme 3.3 The insertion of 2,6-dimethylphenyl isocyanide was investigated with Zr dibenzyl complex 3.9. Upon addition of two equiv of isocyanide to a solution of 3.9 in C6D6 in a J-Young NMR tube, the solution is observed to immediately change from bright yellow to colorless. The II-1 NMR spectrum of the reaction mixture indicates formation of a single product in quantitative yield, due to complete disappearance of the signals corresponding to 3.9. Successful insertion of the isocyanide moieties is suggested by the appearance of canted doublets at 8 3.35 and 3.59 corresponding to the benzylic protons, indicating diastereotopic methylene protons. This was not observed for 3.9, and was attributed to rapid isomerization processes. The bis(iminoacyl) complex 3.16 ({DivrF(NO) BIZr(i 2-2,6-Me2C6H3N=CCH2Ph)2), is non-fluxional in solution, and the benzyl ligands cannot undergo isomerization processes due to their distance from the Zr center. The presence of four singlets of equal intensity at 8 0.95, 1.92, 2.12, and 2.62 is suggestive of two unique 2,6-dimethylphenyl groups, each experiencing hindered rotation about the N-Cips. bond. This is possible in a C2 symmetric species, where the amidate ligand aryl groups experience hindered rotation, and the inserted isocyanide aryl groups also are prevented from rotating freely on the NMR timescale. Thus, the insertion product 3.16 is proposed to have the structure shown in Eq. 3.4. MF-R +  [ MR \ R' 122^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes N=C (3.4)  Ar = 2,6-Me2C6 H 3 3.9^ 3.16 The i2-binding mode in Eq. 3.4 is strongly suggested by 13 C NMR spectroscopic data, where the resonance at S 250.6 is characteristic of i 2 -coordinated iminoacyl carbons. 40' 76 The most commonly observed geometric isomers of bis 11 2-iminoacyls are best described as pseudo-tetrahedral in either a head-to-head (A) or head-to-tail (B) arrangement, as shown in Fig. 3.6. 4°' 83 Interconversion between these two forms can occur, and in some cases has been observed spectroscopically. 85 A solution phase head- to-tail arrangement of the two iminoacyl units is required by the observed C2 symmetry in the 1 H NMR spectrum. The solution phase structure of 3.16 appears static at room temperature, and is consistent with geometric isomer B in Fig. 3.6, where the ancillary ligands (L) are K2-amidate groups. Variable temperature 1 H NMR experiments suggest that interconversion between forms A and B is not particularly facile, as no change in the spectrum is observed over the range of temperatures (-40 °C to 90 °C). A Figure 3.6 Common geometric isomers of bis(i 2-iminoacyl) complexes The solid-state molecular structure of 3.16 is shown in Fig. 3.7. Selected bond distances and angles are presented in Table 3.4 and crystallographic details are located in Table A3.6 (Appendix A). The presence of 1 2-ligation of the iminoacyl moieties was verified as shown in Fig. 3.7, where the Zr-N and Zr-C bond lengths are nearly identical 123^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes for each of the iminoacyl groups (Zr(1)-N(3) = 2.243(6) A, Zr(1)-C(3) = 2.252(7) A and Zr(1)-N(4) = 2.246(6) A, Zr(1)-C(4) = 2.249(8) A). As expected, the iminoacyl N=C bond lengths are consistent with double bonds (C(3)-N(3) = 1.274(10) A and C(4)-N(4) =- 1.282(9) A),87 ' 88 matching the structure shown in Eq. 3.4. The head-to-tail arrangement of the iminoacyl ligands is also verified in the solid-state, where the overall structure matches that seen for isomer B in Fig. 3.6. In many cases where two i 2-iminoacyl ligands are bound to a group 4 metal, the L-M-L plane is not parallel to the iminoacyl N=C bonds, and is twisted substantially from the C-->N1 vectors (Fig. 3.6). 40, 83 In the solid-state, the twist angle between the plane defined by C(1)-Zr(1)-C(2) and the iminoacyl C—>/\1 . vectors for 3.16 is approximately 26.45°. This is more easily viewed in Fig. 3.8, which shows 3.16 as viewed down its C2 axis of symmetry. This twist angle is similar to the value seen for a bis(aryloxy) zirconium complex (Zr(0-2,6 2BuC6H3)2(1 2- `13uN=CCH2Ph)2), which exhibits a twist angle of 21.5°. 83 Figure 3.7 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of j umP(NO)tB12Zr(i2-2,6-Me2C6H3N=CCH2Ph)2, 3.16 (hydrogens omitted for clarity) 124^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes Table 3.4 Selected Bond Distances (A) and Angles (°) for [ DmP(NO) ffil 2Zr(11 2-2,6- Me2C6H3N=CCH2Ph)2, 3.16 Lengths Angles Angles Zr(1)-0(1) 2.214(5) 0(1)-Zr(1)-0(2) 79.3(2) C(37)-C(3)-Zr(1) 162.2(6) Zr(1)-0(2) 2.217(5) N(1)-Zr(1)-N(2) 129.2(2) C(37)-C(3)-N(3) 123.2(7) Zr(1)-N(1) 2.339(6) C(3)-Zr(1)-C(4) 110.2(3) N(3)-C(3)-Zr(1) 73.1(4) Zr(1)-N(2) 2.335(6) N(3)-Zr(1)-N(4) 108.0(2) C(52)-C(4)-Zr(1) 161.0(6) Zr(1)-N(3) 2.243(6) N(3)-Zr(1)-C(3) 32.9(2) C(52)-C(4)-N(4) 124.4(7) Zr(1)-N(4) 2.246(6) N(4)-Zr(1)-C(4) 33.2(2) N(4)-C(4)-Zr(1) 73.3(4) Zr(1)-C(3) 2.252(7) C(29)-N(3)-C(3) 127.3(6) C(4)-N(4)-Zr(1) 73.5(4) Zr(1)-C(4) 2.249(8) C(29)-N(3)-Zr(1) 158.0(5) C(44)-N(4)-C(4) 125.6(6) N(3)-C(3) 1.274(10) C(3)-N(3)-Zr(1) 73.9(4) C(44)-N(4)-Zr(1) 159.8(5) N(4)-C(4) 1.282(9) Figure 3.8 ORTEP depiction (ellipsoids at 30% probability) of simplified structure of 3.16 viewed down the C2 axis of symmetry (amidate substituents and hydrogens omitted for clarity) 125^References begin on page 155 3.16 Ar ^N tttttt ^S.^N ^- 0^0- Ar = 2,6-Me2C 6H3 3.17 A (3.5) Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes While 3.16 is stable in solution at room temperature, heating this complex to 110 °C for 24 hours results in the formation of a new bright yellow species having Cs symmetry in solution. 'H NMR spectroscopy of the product of this reaction shows the presence of two resonances for the aryl methyl groups, indicating free rotation about the N-Cips° bonds of the 2,6-dimethylphenyl groups of the inserted isocyanides and the amidate ligands. Additionally, the 13 C NMR spectroscopic signal at 8 250.6 characteristic of 11 2-iminoacyl species is replaced with a new signal at 8 115.6, which is indicative of an sp2-hybridized carbon center. These results are consistent with a well precedented coupling transformation (Eq. 3.5).40, 76, 89-92 Ph^Ph This process requires the iminoacyl ligands to rearrange prior to C=C coupling. This rearrangement is energy intensive for 3.16 as this reaction requires 24 h at 110 °C to go to completion. The formation of new C=C bonds by the coupling of isocyanides in the presence of early transition metals is dependent upon the steric and electronic properties of both the metal center and the isocyanides." This process is generally intramolecular in nature, and is dictated largely by the energy of the n * C1\1 orbita1. 93 Accordingly, C=C coupling is more readily achieved with electron withdrawing groups on the iminoacyl moiety, such as aryl substituents, and less readily achieved with electron donating alkyl groups on the iminoacyl unit. 93 The xylyl groups in 3.16 are slightly electron withdrawing, and thus fit this pattern of reactivity. Formation of a new C=C bond to generate the enediamido complex [Dmp(No)` Bu]2zr( ArNC(CH2Ph)=C(CH2Ph)NAr), 3.17 (Ar = 2,6-Me2C6H3), is confirmed in the solid-state molecular structure illustrated in Fig. 3.9, with relevant bond lengths and angles given in Table 3.5, and crystallographic details presented in Table A3.7 (Appendix A). 126^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes A Figure 3.9 ORTEP depictions (ellipsoids at 30% probability) of [DmP(N0) s 12Zr(r14-ArNC(CH2Ph)=C(CH2Ph)NAr), 3.17 (Ar = 2,6-Me2C6H3) (A - hydrogens omitted, B - hydrogens and amidate substituents omitted) 127^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes Table 3.5 Selected Bond Distances (A) and Angles (°) for [Divrp(No)tB12-r ,r1 4_L k ArNC(CH2Ph)=C(CH2Ph)NAr), 3.17 (Ar = 2,6-Me2C6H3 ) Lengths Lengths/ Angles Angles Zr(1)-O(1) 2.1344(19) Zr(1)-C(28) 2.548(2) C(37)-N(3)-C(28) 121.04(19) Zr(1)-O(2) 2.1645(19) C(27)-C(28) 1.381(3) C(28)-N(3)-Zr(1) 93.85(13) Zr(1)-N(1) 2.470(2) N(3)-C(28) 1.406(3) C(52)-C(28)-N(3) 117.1(2) Zr(1)-N(2) 2.346(2) N(4)-C(27) 1.410(3) C(52)-C(28)-C(27) 123.4(2) Zr(1)-N(3) 2.0328(19) 0(1)-Zr(1)-N(1) 56.04(7) C(27)-C(28)-N(3) 118.9(2) Zr(1)-N(4) 2.069(2) 0(2)-Zr(1)-N(2) 57.69(7) 0(1)-Zr(1)-0(2) 116.11(7) Zr(1)-C(27) 2.560(2) C(37)-N(3)-Zr(1) 142.25(17) N(1)-Zr(1)-N(2) 93.72(7) Fig. 3.9 shows that 3.17 is not Cs symmetric in the solid-state, indicating that solution fluxionality must be occurring to give rise to the observed 1 1-1 NMR spectrum. Solution phase amidate fluxionality was discussed at length in section 2.4, and the proposed K2-K'-K2 mechanism for this process must be considered. The new C=C bond formed falls within the range expected for a double bond (C(27)-C(28) = 1.381(3) A), and the C-N distances of the enediamido backbone are consistent with single bonds (N(3)-C(28) = 1.406(3) A and N(4)-C(27) = 1.410(3) A). 88 A significant fold angle of approximately 54.5° exists between the planes defined by N(4)-C(27)-C(28)-N(3) and N(4)-Zr(1)-N(3). Fold angles of this size are commonly seen for complexes of this type, where one related example is Richeson's bis(guanidinate) enediamido complex, having a fold angle of 44.8°. 39 The deviation from planarity has been rationalized by the need to relieve steric interactions, as well as the additional electronic stabilization offered by the rf4-interaction with the metal center. 39 ' 89 ' 92'93 While the Zr(1)-C(27) and Zr(1)-C(28) bond distances (2.560(2) A and 2.548(2) A, respectively) lie outside the range generally observed for Zr-C CI bonds, they are similar to the distances observed in Cp complexes of Zr, which exhibit Tr-bonding interactions (average Zr-i 5 -0 5H5 distances are - 2.48-2.55 A). 94 It has also been suggested through DFT calculations that the driving force for the folding of the enediamido ligand is the rotation of the nitrogen lone pairs towards the metal center to enhance drc-prr orbital overlap. 95 128^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes Although it would be desirable to couple different isocyanide units to generate novel diamido ligands, the high reactivity of 3.9 results in rapid insertion of two equivalents of isocyanide per Zr center, even with strict 1:1 stoichiometry. The selective insertion of two equivalents of isocyanide into Zr-C bonds in the presence of only 1 equivalent of isocyanide is not generally seen. In most cases, insertion of a single equivalent of isocyanide results in stable mixed alkyl i 2-iminoacyl complexes. 4° However, this observed reactivity is consistent with both the greater steric access and enhanced electrophilic character of Zr afforded by the amidate ligands in 3.9. With the knowledge that literature precedence for C-C coupling of iminoacyls is favored with electron withdrawing groups, such as 2,6-dimethylphenyl, and disfavored with electron donating groups, the possibility of divergent behavior was investigated by studying the insertion of tent-butyl isocyanide. 40' 93 Upon addition of two equiv of 43uNaC to 3.9, the solution immediately deepens in color from pale yellow to an intense golden yellow. The 1 1-1 NMR spectrum of insertion product 3.18 is consistent with a C2 symmetric complex, with one signal for the aryl methyl groups, and two signals for the tert-butyl groups of the amidate ligands and inserted isocyanide units. Splitting of the signal for the benzylic protons in 3.18 into two canted doublets is suggestive of isocyanide insertion; however, the chemical shift of these signals is shifted far downfield at 8 5.84 and 7.28. These chemical shifts suggest the presence of vinyl amido ligands as shown in Eq. 3.6.36, 40, 96-98 The 3JHH coupling constants of 13.5 Hz for these signals indicate a trans geometry about the double bond. 88 Pi h^Ph 2 +1•/.. --C • (3.6) 3.9^ 3.18 129^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes Vinylamido insertion products like [ D1P(NO) t1312Zr(N(Bu)CH=CHPh)2 (3.18) have been reported in the literature, but are quite rare for group 4 metals. 36 ' 96 ' 98 Rothwell and coworkers reported a vinylamido hafnocene complex which resulted from the insertion of 2,6-dimethylphenyl isocyanide into the Hf-C bonds of Cp2Hf(CH2-py-6- Me)2.96 The solid-state molecular structure of the insertion product clearly established the trans geometry of the vinyl group, and doublets for the trans vinyl protons were observed in the 1 H NMR spectrum at 8 4.6-4.8 and 8.5-9.5, with 3JHH coupling constants of 12-13 Hz. A related bis(guanidinate) dibenzyl Zr complex was shown to insert two equiv of xylyl isocyanide, resulting in the formation of a bis(guanidinate) bis(vinylamido) product, analogous to 3.18. 36 The close similarity in the diagnostic NMR spectroscopic signals between these complexes and those of 3.18 lend further support to the proposed structure in Eq. 3.6.36' 96 In addition to the 1 H NMR spectroscopic data, 13C NMR spectroscopy supports the presence of a vinyl unit, with signals at 8 106.2 and 134.4, consistent with sp2-hybridized carbon centers. Templeton and coworkers have also reported that vinylamido ligands can be accessed through primary amine additions to alkynes coordinated to W pyrazolyl borate complexes. 99 In these cases, the 1 H NMR spectroscopic data are very similar to those for 3.18, supporting the formulation in Eq. 3.6. Finally, mass spectrometry data confirm the insertion of 2 equiv of 13uNE-C into 3.9, with an parent ion of m/z = 846. While X-ray crystallographic verification of this structure was not possible, the data presented are strongly supportive of the designation given in Eq. 3.6. The isomerization of T1 2-iminoacyl groups to vinylamides can be explained by a 1,2-hydrogen atom migration as shown in Eq. 3.7. 96' 97 The difference in reactivity seen for insertion of tli3uNaC vs. 2,6-Me2C6H3N-aC can be explained by two factors. First, the presence of the electron donating tert-butyl group increases the energy of the Jt* CN orbital, making the C=C coupling of the r1 2-iminoacyl groups much less favorable. 40 130^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and HfAmidate Benzyl Complexes  (3.7) Secondly, steric bulk has been noted as a factor which can shut down this reactivity in Cp complexes of Hf, where the insertion of 2,6-Me2C6H3NE-C into Cp2Hf(CHPh-py-6-Me)2 results in the generation of the bis(i 2-iminoacyl) complex, and 1,2-hydrogen atom migration to the bis(vinylamido) complex can not be induced.97 It is possible that the steric bulk of the amidate ligands in 3.9 prevents this rearrangement from occurring with 2,6-Me2C6H 3NmC. However, the 13uNEC group is less bulky by comparison and can rearrange to form the bis(vinylamido) complex 3.18. While observation of the expected intermediate r1 2-iminoacyl species was not possible for this reaction with 3.9, an NMR tube scale reaction of Hf complex 3.6 with two equiv of 13uN-=-C resulted in the formation of the i 2-iminoacyl complex [DmP(NO)P1 ]2Hf(1 2 - tBuN=CCH2Ph)2 (3.19), as evidenced by 1 1-1 NMR spectroscopic data, with diastereotopic benzylic resonances at 8 2.88 and 3.68. When the NMR tube was heated to 80 °C over 24 h, the solution changes color from yellow-orange to emerald green, showing diagnostic resonances at 8 6.02 and 7.47 for the trans vinyl group, indicating formation of the bis(vinylamido) complex [DmAN,-„ Pho) ]2figN(1Bu)CH=CHPh)2 (3.20). Unfortunately, this reaction is not clean, and a number of other signals are observed in the 'H NMR spectrum, and this reaction will not be discussed in any further detail. While these results are promising, detailed investigations of insertion reactivity patterns with other unsaturated organics (ketones, isocyanates, carbodiimides, etc.) are beyond the scope of this thesis, and preliminary NMR tube scale exploratory insertion reactions are discussed briefly in Chapter 6, as the basis for future investigations. 131^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes 3.3.3 Summary Preliminary insertion reactivity studies of bis(amidate) dibenzyl complex 3.9 with isocyanides were undertaken. The insertion of 2 equiv of 2,6-Me2C6H3NaC into the Zr-C bonds of 3.9 resulted in the formation of the bis(r1 2-iminoacyl) complex 3.16 ([DnNO)tBu]2Zr(r1 2-2,6-Me2C6H3N=CCH2Ph)2), which was verified in the solid-state. Attempts to insert one equiv of 2,6-Me2C6H3NEC resulted in a mixture of 3.9 and the double insertion product 3.16, rather than the expected mono-iminoacyl complex. Complex 3.16 underwent thermally induced C=C coupling to generate the enediamido complex [Dmi)(NO)tBi2ZrO14-ArNC(CH2Ph)=C(CH2Ph)NAr), 3.17 (Ar = 2,6-Me2C6H3), which was also characterized in the solid-state. In the solid-state this complex exhibits r1 4-binding of the enediamido ligand to the Zr center, but the amidate ligands are fluxional in solution giving rise to a very simple 1H NMR spectrum. In contrast to these results, the reaction of 2 equiv of 113uNmC with 3.9 resulted in rapid formation of the bis(vinylamido) complex [ DmP(NO)1131 2Zr(1\1( t 3u)CH=CHPh) 2 (3.18), which was characterized in solution, and found to have trans oriented vinylamido ligands. Formation of the vinylamido ligands is proposed to occur through a 1,2-hydrogen atom migration mechanism, as illustrated by Rothwell and coworkers. 97 3.4 Hydrolysis of Dibenzyl Complex [DmP(NO)Ph]21-1f(CH2Ph)2(THF) (3.6) 3.4.1 Introduction It is well known that M-C bonds are susceptible to protolytic cleavage by acidic protons. Amines, 100 ' 101 phosphines, 1°2 and alcohols85 are among the most common reagents that have been shown to undergo this type of reactivity with metal alkyls. The synthesis of amidate benzyl species via protonolysis in section 3.2 demonstrated that organic amides can be installed through protonolysis reactions. 42 Protonolysis routes into metal speciation are advantageous as they do not generate salt byproducts, which can be difficult to separate from the desired products. Additionally, in the case of the dibenzyl 132^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes complexes 3.6, 3.7, 3.9, 3.14, and 3.15, the byproduct of protonolysis is toluene, which can easily be removed under vacuum. Protonolysis of organometallic species can be utilized to access many types of metal-element linkages. Metal-oxo species, bearing M=0 multiple bonds, are one example of the different types of complexes accessible via protonolysis. Oxo complexes have been extensively studied as they are important intermediates in biological oxidation processes, 1°3 as well as industrially relevant chemical oxidations. 1°4 Oxo complexes of the group 4 metals have not been studied extensively, due in large part to the difficulties associated with controlled isolation of these species. However, the synthesis of terminal Ti oxo complexes has been accomplished by a number of different routes. For example, Cummins and coworkers have demonstrated that anionic terminal Ti oxo complexes can be accessed upon deprotonation and decarbonylation of a complexed formate ligand, as illustrated in Scheme 3.4. 1°5 Et20, 0^H •, 0 tBu Ar. /01/41-t Bu /N ^--- / N----tBu Ar - • CMe3 0 tBu^I^tBu J ,...-Ti.. ,,,, / N ^ N Ar z  N\\^ArAr tBu Li[NiPr2] - HN iPr2 -CO G d tBu^tgu ,,,,,,, /N^, N Ar A r  \t Bu Ar = 3,5-Me2C6H3 Scheme 3.4 Stahl and coworkers have also shown that bridging Ti oxo dimers can be generated through transamidation of sterically unprotected amidate ligands with primary amines, to form amidines, as shown in Eq. 3.8.106 Oxidation of Ti(II) species, such as (C5Me5)2Ti(C 2H4), with nitrous oxide has also been utilized to generate terminal Ti(IV) oxo complexes.'" At elevated temperatures, these complexes decompose to tetranuclear oxo clusters. 107 Ti oxo complexes have also been synthesized through the addition of a small excess of H2O to a Ti(III) hydrido complex, 1°8 and a Ti(IV) 11 2 -imine complex.' °9 133^References begin on page 155 0 Li Me3NO THF - NMe3 H2 THF A FL,. 1/2^i^ZcO H THF (3.9) I Ph,,,,, N O/ ■ o YNPh C6D6, 50 °C Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes Ph*--;1 1/2„,,, ,,,,,,^ (3.8)9 ,,,^....v....”. ix\ NPh N'Pr NHPh Dimeric oxo complexes of Zr have been isolated and structurally characterized. Recent results from the Stephan group have shown that oxo transfer agents, such as Me3NO, can react with an anionic zirconium hydride complex to furnish a bridging Zr oxo dimer as shown in Eq. 3 . 9 . 110 Related work by Hillhouse and Bercaw demonstrated that careful hydrolysis of a neutral hydride complex of Zr results in the formation of a 1.1-oxo hydrido dimer, as demonstrated in Eq. 3.10.111 + H 2O  +^2 H2^(3.10) Terminal oxo complexes of Zr and Hf are less prevalent, given their tendency to oligomerize due to the greater size of Zr and Hf vs. Ti. However, a terminal zirconium oxo complex has been trapped by Bergman and coworkers through thermolysis of a phenyl hydroxyl zirconium complex as illustrated in Scheme 3.5. 112 The highly reactive oxo fragment was trapped through cycloaddition reactions with unsaturated organics, such as alkynes and nitriles, forming new C-0 bonds in the process. 112 134^References begin on page 155 RiC=--CR2 Zr--=0 PhCN Ph ,Ph Zr OH - PhH 160 °C Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes Scheme 3.5 This last route into terminal oxo complexes closely resembles the methodology used to access terminal imido complexes, which will be discussed in detail in Chapter 4. The possibility of using oxo complexes, such as the one in Scheme 3.5, in catalytic C=0 bond-forming transformations provided the motivation to analyze amidate complexes bearing the metal-oxygen linkage. The following section discusses the fortuitous isolation of a Hf oxo complex, as the result of slow hydrolysis of the Hf bis(amidate) dibenzyl complex 3.6. 3.4.2 Results and Discussion Early transition metal oxo complexes are difficult targets to access reliably, and are typically isolated as dimeric 106 ' 108, 110, 111 or oligomeric species, 113-115 due to the sterically unprotected nature of the oxo ligand. Slow diffusion of trace amounts of H2O into single crystals of 3.6 results in the fortuitous isolation of the hydrolysis byproduct [Dmp(No)phi2Hfol_o, , 54n 3.21. 42 This complex has a tetrameric formulation, and is the result of addition of 1 equiv of H2O to the dibenzyl species 3.6, as shown in Eq. 3.11. 135^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes H2O 1/4 - THF - 2 C 7 H8 3.6 ^ 3.21 The solid-state molecular structure of 3.21 is shown in Fig. 3.10. Selected bond lengths and angles are available in Table 3.6 and crystallographic data are located in Appendix A in Table A3.8. The most important feature to note regarding the solid-state molecular structure of 3.21 is that the amidate ligands are intact, and only the benzyl ligands are eliminated from the metal center. This speaks to the robust nature of the amidate linkages to the Hf centers. While this complex is partially hydrolyzed under mild conditions, the resulting product is still unstable to further hydrolysis in the presence of excess H20, making it unlikely that group 4 amidate complexes would be useful in the presence of moist air. Independent synthesis of 3.21 was attempted using a stoichiometric quantity of water with 3.6. 111 The resulting oxo complex is completely insoluble in all solvents making solution characterization impossible. Due to the high molecular weight of 3.21 (2572 g mol -1 ), conventional EI mass spectrometry is not possible for bulk structure analysis of molecular mass. However, fragments are observed in the mass spectrum, indicating that the resulting material still has coordinated amidate ligands (m/z = 1076, [DNIP(NO)P114Hf; m/z = 852, [Dmv(NO)Ph]3Hf; m/z = 643, [DmP(NO)P1 ]2Hf=0). In an attempt to characterize the bulk material, MALDI mass spectrometric analysis was undertaken on 3.21. The MALDI instrument in the chemistry department at UBC is not 136^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes compatible with air-free analysis, thus analysis was performed as quickly as possible under aerobic conditions. No signal was observed corresponding to the tetrameric oxo complex; however, the mass fragment at m/z = 643.3, corresponding to the monomeric oxo complex [ DmP(NO) P1 ]2Hf=0, was observed again, along with the fragment at m/z =- 852.2 ([DmP(NO)P1 ] 3H0. Figure 3.10 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of {[DmP(NO)P12Hf(R-0)}4, 3.21 (hydrogens omitted) A simplified structure of 3.21 is given in Fig. 3.11, which clearly shows the multiple binding motifs possible with the amidate ligands. While and K2-binding of the amidate ligands was shown in Chapter 2, this is the first example where these bulky amidate ligands have been observed to bridge between multiple metal centers. The Tr- stacking interactions observed in the dibenzyl precursor 3.6 are also present in the oxo cluster for four of the eight amidate ligands. Interestingly, in each of the two pairs of 7G- stacked amidate ligands, one amidate is bound in a standard K 2 -fashion, while the other is 137^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes bound x2 to one Hf and bridged through the oxygen to another Hf center. In addition to the 3T-stacked amidate ligands in the oxo cluster, there are two x2-amidate ligands bound to single Hf centers. Finally, there are two amidate ligands bridging between two Hf centers that are bound i1 to both metals. The four Hf atoms are arranged in a kite-like geometry, with each Hf bridging to two other Hf centers through a ti-oxo ligand. The four oxo ligands (0(9), 0(10), 0(11), and 0(12)) are arranged such that the overall symmetry of 3.21 is approximately C2 symmetric, with the C2 axis of symmetry lying along the vector connecting 0(9) and 0(11). This is most easily seen in Fig 3.11, in which 3.21 is viewed down the approximate C2 axis of symmetry, and has all of the amidate substituents omitted for clarity. In general, oxo clusters of Hf tend to exist as very large units, having more than a dozen Hf centers, making direct comparison of bond lengths in 3.21 difficult."" The oxo Hf-O distances range between 1.944(3) A and 2.259(3) A, and are similar to the Zr-0 and Hf-0 distances seen in the cluster compounds Zr30(0 113u)io and Hf602(0E020(Et0H)2, where the oxo ligands are bridging to three Zr or Hf centers. 114 Select Hf-Hf bonds are given as a visual guide, and do not represent actual bonding interactions. These distances range between 3.2423(5) and 3.5288(3) A, whereas the ionic radius of Hf(IV) is only 0.85 A (6-coordinate) or 0.97 A (8-coordinate). 20 As such Hf(2) and Hf(4) are best considered as 7-coordinate, with distorted pentagonal bipyramidal geometries. In contrast, Hf(1) and Hf(3) are better viewed as highly distorted octahedra. 138^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes N8 Figure 3.11 ORTEP depiction (ellipsoids at 30% probability) of core solid-state molecular structure of {[ DmP(NO)P12Hf([1.-0)}4, 3.21 (hydrogens and amidate substituents omitted) Table 3.6 Selected Bond Distances (A) and Angles (°) for {[DmP(NO)P112Hf(it-0)}4, 3.21 Lengths Lengths Lengths/ Angles Hf(1)-N(1) 2.327(4) Hf(2)-O(3) 2.177(3) Hf(4)-0(10) 1.948(3) Hf(1)-N(2) 2.474(4) Hf(3)-O(4) 2.196(3) Hf(1)-0(10) 2.000(3) Hf(2)-N(3) 2.272(4) Hf(4)-O(5) 2.196(3) Hf(3)-0(11) 1.969(3) Hf(3)-N(4) 2.296(4) Hf(3)-O(6) 2.206(3) Hf(1)-0(11) 1.978(3) Hf(4)-N(5) 2.265(4) Hf(4)-O(6) 2.244(3) Hf(2)-O(12) 1.944(3) Hf(4)-N(6) 2.358(3) Hf(4)-O(7) 2.086(7) Hf(3)-O(12) 2.011(3) Hf(3)-N(7) 2.457(4) Hf(1)-O(8) 2.230(3) Hf(4)-O(9)-Hf(2) 146.59(16) Hf(2)-N(8) 2.341(4) Hf(2)-O(8) 2.259(3) Hf(4)-0(10)-Hf(1) 110.40(14) Hf(1)-O(1) 2.182(3) Hf(4)-O(9) 2.115(3) Hf(3)-0(11)-Hf(1) 113.06(15) Hf(2)-O(2) 2.088(3) Hf(2)-O(9) 2.117(3) Hf(3)-O(12)-Hf(2) 109.73(13) 139^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes 3.4.3 Summary A tetrametallic Hf oxo cluster, {[ DmP(NO)P1 ]2Hf01-0)}4, 3.21, was isolated by adventitious hydrolysis of the Hf dibenzyl complex [ DmP(NO)PhhHf(CH2Ph)2(THF), 3.6, in the solid-state. The insolubility of this complex precluded solution phase characterization; however, solid-state molecular structure characterization was possible through X-ray crystallographic studies. The resulting cluster complex retains all of the amidate ligands present in the benzyl precursor, where three distinct amidate binding modes are seen, including two bridging modes that had not previously been observed with these bulky ligands. Although Zr and Hf oxo clusters of this type are desirable for materials applications such as MOCVD, the inability to synthesize this cluster reliably makes this a poor candidate for such studies. 116-118 This novel oxo complex demonstrates the robust nature of the amidate linkages, and the flexibility of the amidate ligand set in different coordination environments. 3.5 Abstraction Reactivity of Bis(Amidate) Dibenzyl Complexes 3.5.1 Introduction The ability to generate cationic metal complexes is an extremely important consideration when developing a new ligand system. In particular, cationic group 4 metals are well known to be excellent catalysts for ethylene polymerization: 1-13 The isolation of well defined cationic species is often extremely difficult or impossible, and thus generation of cationic complexes in situ is often the preferred method for studying these species. 11, 13, 18 Abstraction of alkyl ligands is most often performed by strong Lewis acids like Al(III) and B(III). 119 In most polymerization applications, partially hydrolyzed Al alkyl species (MAO) are utilized as co-catalysts to activate dichloro or bis(alkyl) group 4 metals for the rapid polymerization of ethylene. 21 ' 119 In contrast, studies that focus on the generation of cationic complexes for careful stoichiometric studies are generally performed with boron based abstraction agents: 19 Preliminary studies by Arnold and coworkers revealed that a phenylene bridged 140^References begin on page 155 3.6 3.23 Phi. [Ph3CHB(C6F5)4] Hf(CH2 Ph)2 (THF)^ C6D6 2 O [B(C6F5)41 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes bis(amidate) Ti complex was a modestly active catalyst for ethylene polymerization. 23 In a collaboration with Exxon-Mobil, the dibenzyl complexes 3.6, 3.9, 3.14, and 3.15 have been investigated for ethylene polymerization. The tethered systems 3.14 and 3.15 show particular promise for this process, and are being investigated in more detail. Furthermore, cationic group 4 complexes have been reported as alkene hydroamination precatalysts. 120 121 Thus, the generation of cationic amidate complexes was investigated. 3.5.2 Results and Discussion The isolation of the first amidate supported bis(alkyl) complex (3.6, [Dm1)(NO)P1 ]2Hf(CH2Ph) 2(THF)) enabled the study of alkyl abstraction reactions with amidate supported group 4 complexes. 42 The combination of 3.6 and [Ph3C][8(C5F5)4] in a 1:1 ratio in a J-Young NMR tube in C6D6 results in an immediate color change from light orange to deep red. The solution immediately separates into two immiscible phases, where a bright yellow solution floats above a red oil. 1 H NMR spectroscopy of the mixture is complicated; however, the two phases can be separated by decanting the yellow solution away from the red oil. The 1 H NMR spectrum of the yellow solution is simple, with a diagnostic signal at 8 3.80, which can be attributed to the methylene group of 1,1,1,2-tetraphenylethane (3.22), the expected organic product generated by benzyl abstraction, as shown in Eq. 3.12. Ph3C--"\^(3.12) Ph 3.22 Unfortunately, spectroscopic data on the resulting cationic hafnium species [DmP(NO)P12HfCH2Ph][B(C6F5)4], 3.23, are extremely broad and of limited usefulness. However, the clean spectrum of 3.22 suggests that generation of 3.23 is occurring, and isolation of the desired complex may be possible. It has been noted in the literature that THE is a suitable ancillary ligand for stabilizing cationic group 4 complexes. 122, 123 141^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes Combination of 3.6 and [Ph3C][(C6F 5)4] in THF at -78 °C, followed by warming to room temperature results in a bright yellow solution. After several hours, this solution solidifies upon polymerization of the THF solvent. Polymerization of THF has been noted for many different Lewis acidic metals and acids, though rarely reported for cationic group 4 complexes. 124-126 Cationic Zr complexes have recently been utilized in the catalytic hydroamination of aminoalkenes, and the mechanism of this reaction, promoted by neutral Ti and Zr complexes, is probed in detail in Chapter 5. 120 ' 121 In order to evaluate two contrasting mechanistic proposals for this reaction, the use of cationic amidate Zr complexes in the hydroamination of aminoalkenes is necessary. While the cationic Hf complex 3.23 is not cleanly accessible, the abstraction reaction illustrated in Eq. 3.12, when performed with Zr dibenzyl complex 3.9, cleanly yields the cationic complex [ [DivrAN0 . iBui) J2ZrCH2Ph][B(C6F5)4], 3.24. Due to the low solubility of 3.23 in C6D6, the reaction of 3.9 and [Ph3C] [B(C6F5)4] was performed in C6D5Br, instantly generating a blood red solution. Generation of 1,1,1,2-tetraphenylethane is observed by a single resonance at 8 3.93. In addition, the integration for the benzyl resonance is reduced in intensity from 4 protons to 2, and is downfield shifted to 8 2.60. Single resonances for the tert-butyl and aryl methyl protons at 8 0.83 and 2.00, respectively, indicate rapid exchange of the amidate ligands in solution. The ortho proton resonances for the benzyl group are shifted upfield to 8 6.55, suggesting that the benzyl ligand may be bound in an 1 2-fashion. A significant change is observed in the 19F NMR spectrum for the [B(C6F5)4] - anion of 3.24, where the para fluorine atoms appear at 8 -162.1 ( 3JFF = 22.2 Hz). The ortho and meta fluorine atoms appear at 8 -165.9 and -131.6. In [Ph3C][B(C6F5)4], the 19F NMR spectrum indicates that all fluorine atoms are equivalent on the NMR timescale. This indicates that the para fluorine atoms in 3.24 are potentially involved in a weak interaction with the Zr center, as illustrated in Figure 3.12. The presence of three signals in a 2:1:2 ratio in the 19F NMR spectrum indicate that the pentafluorophenyl groups of the anion are equivalent on the NMR timescale, likely due to rapid exchange at the metal center. 142^References begin on page 155 3.24 ,.---Z(--__°N ^.*--,^N )e--.6 "O--‘ ( Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes Figure 3.12 Proposed structure of 3.24 in solution Characterization of discrete cationic complexes in the solid-state is extremely difficult. These complexes are often too unstable to isolate, and in many cases are not crystalline, but are amorphous in nature. I27 The relative scarcity of well characterized cationic complexes in the literature is a testament to the difficulty associated with the isolation of such compounds 4s, 71, 127-132 In most cases the generation of cationic complexes in situ is sufficient, even preferable, for reactivity studies. An exciting application of cationic group 4 complexes is the catalytic hydroamination of aminoalkenes to generate N-heterocyclic products. 120, 121 Complex 3.24 and a closely related compound, will be revisited in Chapter 5 during the study of the mechanism of catalytic aminoalkene hydroamination, where the clean in situ generation of 3.24 facilitates these studies. 3.5.3 Summary The generation of amidate supported cationic Hf and Zr complexes was possible through the combination of the dibenzyl complexes 3.6 and 3.9 with [Ph3C][B(C6F5)4]• Generation of the cationic complexes [[ DmP(NO)P1 ]2HfCH2Ph][B(C6F5)4], 3.23, and [[DmP(NO) IBu]2ZrCH2Ph][B(C6F5)4], 3.24 was supported by the appearance of 1,1,1,2- tetraphenylethane as a result of benzyl abstraction by the trityl cation. NMR spectroscopic data of 3.23 are complicated, indicating a mixture of products and possibly unstable cationic species in solution. In contrast, the NMR spectroscopic data for 3.24 are well resolved, suggesting generation of a single cationic complex in 143^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes solution. 19F NMR data indicate that the tetrakis(pentafluorophenyl) borate anion is weakly coordinating to the Zr through the para F atoms of the pentafluorophenyl rings. While ethylene polymerization studies of these complexes is beyond the scope of this thesis, 3.23 was found to polymerize THY at room temperature, indicating that the Hf center is strongly Lewis acidic. 3.6 Conclusions Amidate supported Zr and Hf dibenzyl species can be reliably accessed in high yields through protonolysis of tetrabenzyl Zr and Hf starting materials with organic amide proligands. Both bidentate and tetradentate amidate ligands were successful at stabilizing these complexes. While the Hf complex 3.6 was stable to ambient light and heat, it underwent THE 'exchange in solution, oscillating between 7-coordinate pentagonal bipyramidal and 6-coordinate pseudo-octahedral species. The Zr complex 3.9 was less stable to heat and light than 3.6, and exhibited intermediate 1 1 /11 2 -hapticity of the benzyl ligands in the solid-state, but behaved as an ri l -benzyl species in solution. Migratory insertion of aryl isocyanides was observed for 3.9, resulting in i 2 - iminoacyl complex 3.16, which could undergo thermal coupling to form enediamido complex 3.17. Insertion of 43uNaC resulted in the formation of a vinylamido complex, 3.18, which was generated through a 1,2-hydrogen atom migration mechanism. The inability to controllably insert one equivalent of isocyanide is indicative of the extremely electrophilic nature of 3.9, further supporting the highly ionic character of the amidate bonding interactions discussed in Chapter 2. Although the amidate ligands are poor at donating electron density to the group 4 metals, they bind strongly to these metals, such that partial hydrolysis of 3.6 results in the formation of a tetrametallic Hf oxo cluster, where all of the amidate ligands are intact. This complex demonstrates the variety of bonding modes possible with the amidate ligand set, as x -1 - and 1(2 -binding modes are noted, and for the first time, these ligands are observed to bridge between metal centers. In addition to the insertion and protonolysis reactivity possible with these bis(amidate) dibenzyl complexes, benzyl abstraction is also readily accomplished. The 144^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes ability of cationic Hf complex 3.23 to polymerize THE at room temperature is strongly supportive of the potential of these complexes to polymerize ethylene and polar monomers in future investigations. The benzyl complexes discussed in this chapter are readily isolated in high yields, and undergo reliable insertion, protonolysis, and abstraction chemistry. While most of the chemical reactivity of the dibenzyl complexes follows patterns previously seen in the literature, some of the deviations noted can be attributed to the highly ionic nature of the amidate bonding to Zr and Hf. These highly electrophilic benzyl complexes thus exhibit reactivity which approaches levels seen more often for cationic group 4 complexes. 3.7 Experimental 3.7.1 General Considerations See Chapter 2, section 2.6.1. Reactions with Zr(CH2Ph)4 and Hf(CH2Ph)4 were performed in the absence of light to avoid photo-initiated decomposition. 3.7.2 Starting Materials and Reagents See Chapter 2, section 2.6.2. 145^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes 3.7.3 Synthesis Synthesis of [ 1)1\413(NO)Ph]2Hf(CH2Ph)2(THF), 3.6 In a foil wrapped Schlenk flask, 3.50 g (6.5 mmol) of Hf(CH2Ph)4 and 2.90 g (12.9 mmol) of [DMP(NO)Ph]H (3.5) were combined. To this mixture of solids, approximately 125 / mL of THF was added at -78 °C via cannula. The reaction mixture was allowed to warm to room temperature while \ old Ph stirring overnight. Excess THF was removed from the bright red-orange solution in vacuo, resulting in the isolation of 5.60 g of the crude product as an intense red-orange solid (98% yield). The crude product was dissolved in approximately 150 mL of hexanes at room temperature and filtered through a fritted disk. A small amount of a pale yellow hexanes insoluble product ([ DmP(NO)Ph]2Hf(CH2Ph)2, 3.7) was isolated by filtration, and the dark red-orange filtrate was concentrated and cooled to -37 °C overnight. Microcrystals of the red-orange product (3.6) suitable for X-ray crystallographic analysis were isolated in 34% yield from the first batch. Repeated concentration and crystallization of the mother liquor resulted in a combined yield of approximately 65%. The spectroscopic properties of the isolated microcrystals are identical to that seen for the crude isolated material. 1 1-1 NMR (C6D6, 25 °C, 300 MHz): 8 1.40 (t, 4H, 3JHH = 6.4 Hz, O(CH2CH2)2), 2.10 (s, 12H, Ph(CH3)2), 2.30 (s, 4H, Hf(CH2Ph)2), 3.59 (t, 4H, 3JHH = 6.4 Hz, O(CH2CH2)2), 6.78-6.92 (m, 14H total, Ar-H), 7.10-7.24 (m, 8H total, Ar-11), 7.61 (d, 4H, 3JHH = 7.4 Hz, Ar-H). 13C NMR (C6D6, 25 °C, 75 MHz): 8 18.7, 25.8, 68.0, 81.6, 122.1, 126.0, 128.6, 128.7, 128.9, 129.1, 132.0, 132.3, 133.1, 142.1, 144.7, 179.6. EIMS (m/z): 719 ([Mt] — THF — CH2Ph), 627 ([1‘4 4] — THF — 2 CH2Ph). Anal. Calcd for C48HSON203Hf (%): C, 65.41; H, 5.72; N, 3.18. Found: C, 65.15; H, 6.00; N, 3.54. Ph P 146^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes Synthesis of [DmP(NO)P1 ]2Hf(CH2Ph)2, 3.7 In a foil wrapped Schlenk flask, 0.93 g (1.7 mmol) of Hf(CH2Ph)4 and 0.77 g (3.4 mmol) of [Dmy(NO)Ph]H (3.5) were combined. To this mixture of solids, approximately 75 mL of toluene was added / Ph at -78 °C via cannula. The reaction mixture was allowed to warm to room temperature while stirring for approximately 4 h. Excess \ Ph toluene was removed, resulting in the isolation of a bright yellow solid residue. The crude product was dissolved in approximately 25 mL of hexanes at room temperature and concentrated until a yellow powder precipitated out of solution. This powder was isolated in 62 % yield (0.86 g) by filtration through a flitted disk, and dried in vacuo. 1 H NMR (C6D6, 25 °C, 300 MHz): 8 2.10 (s, 12H, Ph(CH3)2), 2.30 (s, 4H, Hf(CH2Ph)2), 6.78-6.94 (m, 14H total, Ar-H), 7.10-7.24 (m, 8H total, Ar-H), 7.61 (d, 4H, 3JHH = 7.3 Hz, Ar-H). 13 C { 1 1-1} NMR (C6D6, 25 °C, 75 MHz): 8 19.0, 82.9, 122.1, 126.4, 129.0, 128.7, 129.3, 129.5, 130.1, 132.3, 132.7, 133.5, 142.5, 144.7, 179.6. EIMS (m/z): 719 ([Mt] — CH2Ph), 627 ([Mt] — 2 CH2Ph). Anal. Calcd for C44H42N2O2Hf (%): C, 65.30; H, 5.23; N, 3.46. Found: C, 64.41; H, 5.36; N, 3.81. Synthesis of [DmP(NO)`131 2Zr(CH2Ph)2, 3.9 In a foil wrapped 250 mL round-bottomed Schlenk flask equipped with a stir bar, 0.90 g (4.4 mmol) of [ DmP(NO) tB°]H (3.8) was combined with 1.00 g (2.2 mmol) of Zr(CH2Ph)4. To / Ph this flask was added 75 mL of toluene which had been cooled to -78 °C. The reaction mixture was then stirred for 3.5 hours \ 2^Ph while allowing to warm to — 10 °C. Given the thermal and photosensitivity of Zr(CH2Ph)4, care was taken to ensure reaction occurred in the absence of ambient light, and the reaction was not allowed to exceed 10 °C. The clear bright yellow solution was then concentrated to dryness in vacuo to give a bright yellow solid residue. The crude material was dissolved in 30 mL of pentane and filtered through CeliteTM to remove impurities. The pentane solution was concentrated to approximately 147^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes 10 mL and cooled to -37 °C. The yellow precipitate was isolated in 57 % yield (0.85 g) by filtration and dried in vacuo. Successive concentration/filtration cycles allow access to > 80 % yield of pure 3.9. Single crystals suitable for X-ray crystallographic analysis were grown in the dark from a saturated pentane solution at -37 °C. NMR (C6D6, 25 °C, 300 MHz): 8 0.91 (s, 18H, C(CH3)3), 2.05 (s, 12H, Ph(CH3)2), 2.24 (s, 4H, Zr(CH2Ph)2), 6.86-6.89 (m, 8H, Ar-H), 7.11-7.15 (m, 8H, Ar-H). 13 C { 1 1-1} NMR (C6D6, 25 °C, 75 MHz): 8 19.4, 27.5, 41.2, 75.8, 123.1, 125.5, 128.6, 129.4, 129.9, 132.1, 142.9, 143.2, 190.7. EIMS (m/z): 589 ([Mt] — CH2Ph), 497 ([M+] — 2 CH2Ph). Anal. Calcd for C40HSON2O2Zr (%): C, 70.44; H, 7.39; N, 4.11. Found: C, 69.71; H, 7.60; N, 4.67. Synthesis of Ad[N202]Zr(CH2Ph)2(THF), 3.14 A foil wrapped 250 mL round-bottomed Schlenk flask equipped with a stir bar, was loaded with 0.75 g (1.7 mmol) of Zr(CH2Ph)4. To this flask was added 50 mL of THF via Phcannula, and the resulting solution was cooled to -78 °C. Using^1\1 a solid addition funnel, 0.70 g (1.7 mmol) of 3.11 was added to the THF solution of Zr(CH2Ph)4 over a period of 5 minutes.^ —. Ph The cloudy yellow reaction mixture was then stirred while allowing to warm to 0 °C over the course of 4 hours. The clear bright yellow-orange solution was concentrated to dryness in vacuo to give a bright yellow solid residue, while ensuring that the reaction temperature did not exceed — 10 °C. The crude material was washed with 25 mL of pentane and dried under vacuum to give a bright yellow powder in 73 % yield (0.92 g). Single crystals suitable for X-ray crystallographic analysis were grown in the dark from a saturated hexanes solution at -37 °C. 'H NMR (C6D6, 25 °C, 400 MHz): 8 0.72 (s, 6H, C(CH3)2), 1.38 (m, 4H, O(CH2CH2)2), 1.61 (br, 12H, Ad-(CH(CH2)CH)3), 1.90 (br, 6H, Ad-(CH)3), 2.04 (br, 12H, Ad-C(CH2)3), 2.25 (s, 4H, Hf(CH2Ph)2), 3.08 (s, 4H, NCH2C(CH3)2CH2N), 3.66 (m, 4H, O(CH2CH2)2), 6.86 (m, 2H, Ar-H), 7.19-7.24 (m, 8H, Ar-H). 13C NMR (C6D6, 25 °C, 100 MHz): 8 24.9, 26.0, 28.9, 35.8, 37.2, 38.5, 43.0, 56.3, 62.4, 68.8, 120.2, 127.9, 128.5, 148.2, 192.3. EIMS (m/z): 605 ([1‘41 — THF — CH2Ph). Anal. Calcd 148^References begin on page 155 \ ..,..^.- ef< Ph A vi r17 N--H"-s"\,..— :--d \, Ph Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes for C45H62N2O3Zr (%): C, 70.17; H, 8.11; N, 3.64. Found: C, 65.01; H, 7.99; N, 4.14. (incomplete combustion through possible carbide formation may be responsible for low carbon analysis). Synthesis of Ad [N202]Hf(CH2Ph)2(THF), 3.15 A foil wrapped 250 mL round-bottomed Schlenk flask equipped with a stir bar, was loaded with 3.00 g (5.5 mmol) of Hf(CH2Ph)4. To this flask was added 100 mL of THF via cannula, and the resulting solution was cooled to -78 °C. Using a solid addition funnel, 2.36 g (5.5 mmol) of 3.11 was added to the THF solution of Hf(CH2Ph)4 over a period of 5 minutes. The cloudy yellow reaction mixture was then stirred while allowing to warm to room temperature overnight. The clear yellow solution was then concentrated to dryness in vacuo to give a light yellow solid residue. The crude material was triturated with — 30 mL of pentane and filtered over a fritted disk to give 2.75 g (58 % yield) of 3.15 as a pale yellow powder. The pentane washings were concentrated until product began to precipitate. After cooling to aid precipitation, and additional 0.50 g (11 % yield) of 3.15 was isolated by filtration, resulting in an overall isolated yield of 70 %. Single crystals suitable for X-ray crystallographic analysis were grown in the dark from a saturated hexanes solution at -37 °C. I I-1 NMR (C6D6, 25 °C, 400 MHz): 8 0.83 (s, 6H, C(CH3)2), 1.33 (m, 4H, O(CH2CH2)2), 1.66 (br, 12H, Ad-(CH(CH2)CH)3), 1.72 (s, 4H, Hf(CH2Ph)2), 1.95 (br, 6H, Ad-(CH)3), 2.09 (br, 12H, Ad-C(CH2)3), 3.19 (s, 4H, NCH2C(CH3)2CH2N), 3.99 (m, 4H, O(CH2CH2)2), 6.76 (m, 2H, Ar-H), 7.04 (d, 4H, 3JHH = 7.6 Hz, Ar-H), 7.19 (m, 4H, Ar-H). 13C { i fI} NMR (C6D6, 25 °C, 75 MHz): 8 24.9, 25.0, 28.2, 35.8, 36.5, 37.8, 42.8, 55.3, 57.3, 70.0, 117.9, 125.4, 127.4, 153.9, 190.9. EIMS (m/z): 695 ([Mt] — THF — CH2Ph). Anal. Calcd for C45H62N2O3Hf (%): C, 63.03; H, 7.29; N, 3.27. Found: C, 61.96; H, 7.64; N, 3.50. 149^References begin on page 155 Ph^Ph Ar Zr N Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes Synthesis of D[ MP(No )tBu] 2 .-r (11 2_ArN=CCH2Ph)2 (Ar = 2,6-Me2C6H3), 3.16 In a 50 mL Schlenk flask, 150 mg (0.2 mmol) of 3.9 was dissolved in approximately 5 mL of toluene at room temperature. To this solution was added 58 mg (0.4 mmol) of 2,6-Me2C6H3NaC dissolved in 5 mL of toluene. The bright yellow solution was stirred at room temperature, and became colorless within minutes. Excess toluene was removed in vacuo to give a white residue. This material was triturated with 5 mL of pentane, and the insoluble white powder was isolated by filtration in 52 % yield (108 mg). Higher yields are possible, but due to static electricity, the product sticks to glass very well, and small scale reactions suffer from this. Single crystals were grown from a saturated pentane solution at -37 °C. 1 H NMR (C6D6, 25 °C, 300 MHz): 8 0.95 (s, 6H, Ph(CH3)2), 1.11 (s, 18H, C(CH3)3), 1.92 (s, 6H, Ph(CH3)2), 2.12 (s, 6H, Ph(CH3)2), 2.62 (s, 6H, Ph(CH3)2), 3.35 (d, 2H, 2JHH = 12.0 Hz, CH2Ph), 3.59 (d, 2H, 2JHH = 12.2 Hz, CH2Ph), 6.19 (d, 4H, 3JHH = 6.6 Hz, Ar-H), 6.83-7.09 (m, 18H, Ar-H). 13 C { 1 H} NMR (C6D6, 25 °C, 75 MHz): 8 18.2, 19.1, 19.4, 20.6, 28.5, 41.3, 44.3, 125.7, 125.8, 126.6, 129.0, 130.0, 130.2, 130.8, 133.5, 133.9, 135.8, 147.8, 149.1, 187.3, 250.6. EIMS (m/z): 942 ([Mt]), 720 ([M+] — PhCH2C=N(2,6-Me2C6H3)). Anal. Calcd for C58E168N402Zr1 (%): C, 73.76; H, 7.26; N, 5.93. Found: C, 70.62; H, 7.25; N, 5.63 (incomplete combustion through possible carbide formation may be responsible for low carbon analysis). Synthesis of [Div[P(N0)` 312Zr(i 4-ArNC(CH2Ph)=C(CH2Ph)NAr) (Ar = 2,6-Me2C6H3 ), 3.17 In a 100 mL Schlenk flask, 360 mg (0.5 mmol) of 3.9 and 139 mg (1.0 mmol) of 2,6-Me 2C6H3NaC was dissolved in approximately 35 mL of toluene at room temperature. The bright yellow solution was stirred and heated to 110 °C overnight, resulting in a yellow orange solution. Excess toluene was removed in vacuo to give a yellow-orange solid. This material was dissolved in 10 mL of pentane, and filtered through Celite TM to remove 150^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes 3.16. Excess pentane was removed in vacuo, resulting in 81 % yield (403 mg) of 3.17 as a yellow orange solid. This complex can also be synthesized by heating 3.17 to 110 °C overnight. Single crystals for X-ray analysis were grown from a saturated toluene solution at room temperature. 1 H NMR (C6D6, 25 °C, 400 MHz): 8 0.82 (s, 18H, C(CH3)3), 1.95 (s, 12H, Ph(CH3)2), 2.19 (s, 12H, Ph(CH3)2), 3.99 (s, 4H, CH2Ph), 6.65- 7.08 (m, 22H, Ar-H). 13 C{ 1 1-1} NMR (C6D6, 25 °C, 100 MHz): 8 20.2, 21.2, 28.1, 37.8, 41.4, 115.6, 124.1, 124.8, 126.3, 128.0, 128.3, 128.6, 130.5, 132.1, 133.3, 140.4, 143.9, 147.3, 187.6. Synthesis of [DMP(NO) t1312Z4NtBUCH=CHPh)2, 3.18 In a 100 mL Schlenk flask, 360 mg (0.5 mmol) of 3.9 was Ph^Ph dissolved in approximately 30 mL of toluene at room temperature. To this solution was added 88 mg (1.0 tBu—N N- tBummol) of IluNsC dissolved in 5 mL of toluene at room temperature. The solution immediately became an intense^ N^ Zr yellow color. Excess toluene was removed in vacuo to give a vibrant yellow solid. This material was triturated with 10 mL of pentane, and isolated by filtration in 86 % yield (385 mg). 1 H NMR (C6D6, 25 °C, 300 MHz): 8 1.02 (s, 18H, O=CC(CH3)3), 1.35 (s, 18H, NC(CH3)3), 2.31 (s, 12H, Ph(CH3)2), 5.84 (d, 2H, 3JHH = 13.5 Hz, NCH=CHPh), 6.91-7.12 (m, 16H, Ar- H), 7.28 (d, 2H, 3JHH = 13.5 Hz, NCH=CHPh). 13 C NMR (C6D6, 25 °C, 75 MHz): 8 20.9, 28.3, 30.4, 41.3, 58.4, 106.2, 124.7, 125.2, 125.9, 128.1, 129.0, 132.1, 134.4, 139.5, 144.7 (no signal seen for C=0). EIMS (m/z): 846 ([Mt]). Anal. Calcd for C50H681\1402Zr1 (%): C, 70.79; H, 8.08; N, 6.60. Found: C, 69.42; H, 7.73; N, 6.16. o' b 151^References begin on page 155 Ph^Phr ,,,u_N N_ti. y Fif,--- ----_, N / --; N 71=-' Oa^b----' Ph^Ph / 01-711 Excess toluene was N Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes NMR tube reaction of 3.6 and 113u1\1-C to generate [ DmP(NO)P1 ]2Hf(12-1BuN=CCH2Ph)2, 3.19 and [DivIP(NO)P1 ]2Hf(1\1 113uCH=CHPh)2, 3.20 In a small vial in a glovebox, 100 mg (0.1 mmol) of 3.6 was dissolved in approximately 0.5 mL C6D6 and transferred into a J-Young NMR tube. In another vial 19 mg (0.2 mmol) of 13uNmC was dissolved in approximately 0.5 mL of C6D6 and added to the NMR tube. An immediate color change from orange to yellow-orange occurred upon mixing the two solutions. While the 1 }1 NMR spectrum was complicated and indicated a mixture of products, signals characteristic of an i2-iminoacyl were observed. 1 H NMR (C6D6, 25 °C, 300 MHz): 8 2.88 (d, 21-1, 2./Flli = 13.3 Hz, I3uN=CCH2Ph), 3.68 (d, 2H, 2JHH = 13.7 Hz, 13uN=CCH2Ph). Heating the NMR tube to 80 °C for 24 h resulted in a gradual color change from yellow-orange to red-brown, and finally to emerald green. Again, the 1 1-1 NMR spectrum was complicated with signals for multiple products, signals characteristic of a trans vinylamido complex analogous to 3.18 were observed. 1 1-1NMR (C6D6, 25 °C, 300 MHz): 8 6.02 (d, 2H, 2JHH = 12.8 Hz, tBuN=CCH2Ph), 7.47 (d, 2H, 2JHH = 11.9 Hz, tBuN=CCH2Ph). Attempted synthesis of I[ DivrANO)Ph]2H4R-0)}4, 3.21 In a 100 mL round bottomed Schlenk flask 710 mg (0.8 mmol) of 3.6 was dissolved in approximately 50 mL toluene. This solution was frozen in a liquid nitrogen bath prior to addition of 15 1AL of H2O via microsyringe. The reaction mixture was slowly warmed to -78 °C using a dry- ice/isopropanol bath, and was left to gradually warm to room temperature while stirring overnight. The red-orange solution was observed to turn into a cloudy white suspension. removed in vacuo to give a white powder. The crude product was triturated with 10 mL 152^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes of hexanes and filtered over a fritted disk. This material was completely insoluble in all organic solvents, and was uncharacterizable in the solution phase. While microanalysis did not verify the composition of 3.21 as determined by X-ray crystallography, mass spectral data suggested formation of oxo species. EIMS (m/z): 1076 ([DmP(NO)P1 ]41-10, 852 ([DIVIANO)Phi3H0, 643 ([DiviP(NO)P1 ]2Hf=0). MALDIMS (m/z) = 643.3 ([Divil3(NO)P1 2Hf=0), 852.2 ([DinNO)P1 ]3flf). Synthesis of cationic species 3.23 and 3.24 In situ generation of 3.23 In a small vial in a glovebox 100 mg (0.1 mmol) of 3.6 was dissolved in approximately 0.5 mL of C6D6 at room temperature. In a separate vial 105 mg (0.1 mmol) of [Ph3C][B(C6F5)4] was dissolved in approximately 1 mL of C6D6. The two solutions were transferred into a J-Young e [B(C6F5)4] NMR tube, and an immediate color change to deep red was observed. The solution partitioned into two phases, with a dense red oil settling on the bottom of the tube, and light yellow solution rising to the top. NMR spectroscopy of the red oil was not informative. 1 11 NMR spectroscopy of the yellow solution indicated the formation of Ph3CCH2Ph as a result of benzyl abstraction by the trityl cation. 1 1-1 NMR (C6D6, 25 °C, 300 MHz): 8 3.93 (s, 21-1, Ph 3 CCH2Ph), 6.71-7.22 (m, 20H, Ar-H). Attempted isolation of 3.23 In a 250 mL round bottomed Schlenk flask 812 mg (0.9 mmol) of 3.6 and 850 mg (0.9 mmol) of [Ph3C][B(C6F5)4] were combined prior to addition of approximately 100 mL of THE which had been cooled to -78 °C. The yellow solution was allowed to warm to room temperature while stirring overnight, during which time the solution solidified. The reaction mixture was quenched with 6M HC1 and agitated with a spatula to break up the 153^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes polymeric material. The white solid was isolated by filtration and dried under vacuum. GPC data indicated generation of poly-THF (M,, = 106884, M,„ = 299222, PDI = 2.80). 1 H NMR (C6D6, 25 °C, 300 MHz): 8 1.58 (m, 4H, -(OCH2CH2CH2CH2),,-), 1.83 (m, 2H, O(CH2CH2)2), 2.33 (s, 0.2H, CH2Ph, benzyl end group), 3.39 (m, 4H, - (OCH2CH2CH2CH2)i,-), 3.72 (m, 2H, O(CH2CH2)2), 7.16-7.23 (m, 0.5H, Ar-H, benzyl end group). In situ generation of 3.24 In a small vial in a glovebox 30 mg (0.04 mmol) of 3.9 B(C6F5)3 was dissolved in approximately 0.5 mL of C6D5Br. The two solutions were added to a J-Young NMR tube at^-7 0 room temperature, and a deep red solution formed 0( immediately. 1 H NMR (C6D5Br, 25 °C, 300 MHz): 8 0.83 (s, 18H, C(CH3)3), 2.00 (s, 12H, Ar-H), 2.60 (s, 2H, ZrCH2Ph), 3.93 (s, 2H, Ph3CCH2Ph), 6.55 (d, 2H, 3JHH = 7.1 Hz, ZrCH2(o-Ph-H)), 6.73-7.39 (m, 29H, Ar-H (Ph3CCH2Ph and 3.24)). 19F NMR (C6D 5Br, 25 °C, 282 MHz): 8 -165.9 (br, 8F, o or m- Ar-F), -162.1 (t, 4F, 3JFF = 22.2 Hz, p-Ar-F), -131.6 (br, 8F, o or m-Ar-F). was dissolved in approximately 0.5 mL of C6D5Br. In a separate vial, 41 mg (0.04 mmol) of [Ph3C][B(C6F5)4] 154^References begin on page 155 Chapter 3: Synthesis, Structure, and Reactivity of Zr and Hf Amidate Benzyl Complexes 3.8 References 1. Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew. Chem. Mt. Ed. 2005, 44, 4490. 2. Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew. 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EncycL Polym. Sci. Eng. 1989, 16, 649. 126. Kozhevnikov, I. V. Chem. Rev. 1998, 98, 171. 127. Bochmann, M. Dalton Trans. 1996, 3, 255. 128. Guo, Z.; Swenson, D. C.; Guram, A. S.; Jordan, R. F. Organometallics 1994, 13, 766. 129. Mehrkhodavandi, P.; Schrock, R. R.; Pryor, L. L. Organometallics 2003, 22, 4569. 130. Mehrkhodavandi, P.; Schrock, R. R. I Am. Chem. Soc. 2001, 123, 10746. 131. Yang, X.; Stern, C. L.; Marks, T. J. Angew. Chem. Mt. Ed. 1992, 31, 1375. 132. Sishta, C.; Hathorn, R. M.; Marks, T. J. I Am. Chem. Soc. 1992, 114, 1112. 161^References begin on page 155 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes CHAPTER 4 Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes 4.1 Introduction Many stoichiometric and catalytic reactions promoted by transition metals are dependent upon an intermediate involving a metal-element multiple bond. The most famous example of this being the olefin metathesis reaction, which proceeds through an alkylidene intermediate, having a M=CR 2 double bond. 1-5 In 2005, the Nobel prize in chemistry was awarded to Yves Chauvin, Richard R. Schrock, and Robert H. Grubbs for "development of the metathesis method in organic synthesis," where the multiply bonded M=CR2 species were the key to this extremely important reaction. 6 ' 7 The M=C double bond allows for a switching of partners, or metathesis, through a cycloaddition reaction, resulting in a metallacyclobutane intermediate (Scheme 4.1). 8 ' 9 © Reproduced in part with permission from Thomson, R.K.; Bexrud, LA.; Schafer, L.L. Organometallics 2006, 25, 4069 (Copyright 2006 American Chemical Society). 162^References begin on page 222 M=0 M=NR + \^/ M=PR  M-0^M-NR )^"L ,L I, M-PR )^C M=0 M=---NR + M=PR M-0^M-NR^M-PR )-L J-L )--L Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes M=-CR2 + \ J .........NI.,..)-CLR2 (4' M\---\ + R2C--=\ Scheme 4.1 By comparison, the related oxo (m=0), 10-22 imido (M=NR), 23-35 and phosphinidine (m=pR)36-49 complexes have been less extensively studied than the alkylidenes. Similar cycloaddition pathways can be envisioned for these species, which would result in the formation of new C-E bonds, where E = 0, N, P, etc. While catalytic C-C bond formation via olefin metathesis and ring-closing metathesis (RCM) has proven extremely useful in organic synthesis,5°' 51 the analogous exploitation of other metal- mediated catalytic C-E bond formations in organic synthesis has only recently come to gain widespread attention."' 52-57 One particular catalytic process that has seen an explosion of interest in the last 5 years is catalytic hydroamination, which is the formal addition of an N-H bond across a C-C multiple bond. 55 ' 58-67 With certain early transition metal based catalysts, especially group 4 and 5 complexes, the mechanism of alkyne hydroamination is understood to proceed through a cycloaddition reaction between the alkyne and an imido complex, such as the reaction illustrated in Scheme 4.1. 58 ' 68 In order to develop a complete understanding of the olefin metathesis reaction, a comprehensive investigation of metal alkylidene species was required. Systematic investigation of the steric and electronic influences imparted by the ancillary ligands led to vastly improved catalysts for this process.2' 4, 69-73 An excellent example of this development process can be observed upon studying the evolution of the Grubbs'-type olefin metathesis catalysts. 4 The earliest commercialized catalyst developed by Grubbs and coworkers was a benzylidene species stabilized by two chloro ancillary ligands, and 163^References begin on page 222 PCy3 ,CI Ru — / CI PCy3 H3C(F3C)2C0/.„„ ,N Mo r„, ^is N Ru C ( PCy3 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes two phosphine ligands, to generate a square pyramidal complex, as shown in Fig. 4.1 (4.1). 7376 While this catalyst exhibited excellent functional group tolerance for the metathesis reaction in comparison to the Schrock catalyst (4.2), the activity of the Ru complex was far lower than that of the Mo complex. 3 ' 70, 71 By changing one of the ancillary phosphine ligands to an N-heterocyclic carbene (4.3), the activity of the resulting complex was vastly increased, while still maintaining excellent functional group tolerance.4 ' 77 4.1 ^ 4.2^ 4.3 Figure 4.1 Grubbs and Schrock olefin metathesis catalysts The Schrock catalyst (4.2) also illustrates the usage of imido ligands as ancillary groups for mid to late transition metals. Imido ligands have been lauded as new tunable alternatives to Cp ligands, due to the isolobal nature of their bonding interactions. 78-8° This has found important application in olefin polymerization catalysis."' 80-83 The dianionic nature of the imido ligand typically requires the utility of neutral ancillaries on group 4 metals to maintain the essential cationic monoalkyl reactive site. A couple of representative complexes developed by the Mountford group are shown in Fig. 4.2 (4.4 and 4.5), where the supporting ancillaries are derived from the triazacyclononane (tacn) framework.8° ' 84 These complexes exhibit high to very high activity in the polymerization of ethylene. 82 An exciting new development in this area has also been observed for a chiral ansa-titanocene imido complex capable of syndiospecific polymerization of methylmethacrylate, also highlighted in Fig 4.2 (4.6). 85 164^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes Me Me NI / Me N sC)'\ tBu H2 Me(71 ---NMe N\^Me ICI\ N^'•^z Me tBu/ Me■-.AI/ e Me 4.4 ^ 4.5 ^ 4.6 Figure 4.2 New Ti imido polymerization catalysts Whereas the modification of cyclopentadienyl rings can be laborious and difficult, steric and electronic modification of the imido ligand can be readily accomplished by starting with the appropriate amine.33' 78, 79, 82, 83, 86 For example, access to the metal center can be moderated by incorporation of large groups in the 2 and 6 positions of the aryl ring, as in the case of the Schrock metathesis catalyst (4.2). 1 ' 2 While imido ligands can be utilized as non-reactive ancillaries, a wealth of interesting reactivity can also be accessed with these ligands. 33 ' 87 For example, the enzymatic fixation of dinitrogen has been postulated to involve an imido intermediate en route to the formation of ammonia. Recent studies of a model Mo system (4.7) have characterized imido species as intermediates along the catalytic pathway for the reduction of N2 to NH3 by stepwise addition of protons and electrons. 88 This system, studied by Schrock and coworkers, was the first synthetic complex to facilitate catalytic conversion of atmospheric N2 to NH3 under mild conditions, and the overall process is illustrated in Scheme 4.2. 88 165^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes + N2 MO(N2) mo lIl - NH 3^ MO(NH3)^MOW H+ ,e"^ t e- moiv^Mo—N=NH^[Mo(NH3)1+ me/ H+ t H+ Mov^riMo=N—NH2r^Mo—NH2 Mov e i^t e- [Mo —NH 2 1 + MovMov^Mo=N—NH2 H + 1, t Fl+ Mov [Mo=N—NH3]+^Mo=NH^Mov e"^ t e Move^Mo-N + NH3 1-1 —0- [Mo=NH] + MoveAr = 3,5-(2,4,6-Pr3C6H2)2C6H 3 4.7 Scheme 4.2 While imido species are known for many transition metals, the group 4 imido complexes display the most notable reactivity. 32 ' 33 The first reported group 4 imido complexes were isolated by the Wolczanski and Bergman groups in 1988 (Fig. 4.3). 89 ' 9° The Bergman system (4.8) was stabilized by traditional Cp ligands,9° whereas the Wolczanski complex (4.9) was stabilized by the bulky tri(tert-butyl)silylamido ligands (I3u3SiNH). 89 These imido complexes were highly reactive and could not be characterized in the solid-state, but their molecular structures were inferred from subsequent reactivity. It was noted that both of these imido complexes were capable of reversible C-H bond activation. 89 ' 9° Particularly impressive was the observed activation of methane by the amido ligated imido complex. 89 166^References begin on page 222 85 °C g ,HN 'tH3 Zr=N -C H4 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes  tBu HN^tBu Zr=N-Si-Su HN^tBu tBu Si m tBu tBu 4.8^ 4.9 Bergman Wolczanski Figure 4.3 The first group 4 imido complexes In these activation reactions, the C-H bond adds across the Zr=N bond in a 1,2- addition type reaction, resulting in an alkyl-amido, or aryl-amido complex, which can reversibly eliminate the alkane or arene, and regenerate the transient imido complex as illustrated in Scheme 4.3. 90 Since these early Zr imido complexes were not stable isolable species, they were generated in situ by alkane or arene elimination from mixed alkyl-amido or aryl-amido species, also shown in Scheme 4.3. Scheme 4.3 While the activation of C-H bonds has many potential applications in the formation of new C-E bonds, the bulk of observed reactivity with imido complexes has involved cycloaddition reactions of unsaturated organic species with the M=N bond. 32 ' 33 ' 91 The cycloaddition of ketones,52' 92, 93 isocyanates, 94 ' 95 carbodiimides, 87' 96-98 imines, 99 ' 100 phosphaalkynes, 1°1 ' 1°2 alkynes 93, 103 alkenes,93' 104, los and allenes 1°6 with imido 167^References begin on page 222 R 1 HN C R11 M N N R2 L, 1--N HR^\H^R3 L,M=NR1 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes fragments have all been noted in the literature. This type of stoichiometric reactivity is important for the fundamental study of new N-C bond formation; however, application of such reactions to catalytic processes has seen relatively limited success. Three important applications of imido cycloaddition reactivity are catalytic hydroamination 53, 58, 60, 61, 68, 107 catalytic carboamination, 108-111 and catalytic transamidation. 112-114 The most widely studied application of group 4 imido complexes is catalytic 53, 58, 60, 61, 68, 107alkyne hydroamination.^The mechanism for this reaction was first described in detail by Bergman and coworkers, where the key step in this transformation involves the cycloaddition of a zirconocene imido fragment with the triple bond of the alkyne. 68 This results in the formation of an azametallacyclobutene complex, shown in Scheme 4.4. Addition of another equivalent of amine to the metallacyclic intermediate results in cleavage of the Zr-C bond, and formation of a new amido-eneamido complex. 68 Regeneration of the imido complex is accomplished by u-H abstraction, facilitating the elimination of the eneamine product. 68 This reaction has been widely exploited by a number of research groups, and considerable progress has been made in the development of new catalysts for this important reaction. 58 ' 115 ,R1 L,M—N R3 J-1, R2 H2 NR 1 Scheme 4.4 168^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes Catalytic carboamination has been observed recently for Ti and Zr complexes. This process results in a three component coupling of an amine, an alkyne, and an imine. 108-111 The necessity of an imido linkage has also been illustrated for this reaction, which proceeds through the mechanism illustrated in Scheme 4.5. 1 " The mechanism for catalytic carboamination is identical to that for hydroamination until the formation of the azametallacyclobutene species. In catalytic carboamination, an imine is inserted into the M-C bond of the metallacycle, generating a new diazametallacyclohexene complex: 68 Cycloreversion of this metallacycle results in the regeneration of the active imido catalyst, while eliminating the a, 13-unsaturated imine product. These products are analogous to a, I3-unsaturated ketones, and are useful as precursors for hetero-Diels- Alder type reactions for the formation of new N-heterocycles. 116 ' 117  R3 Scheme 4.5 Catalytic transamidation has been described recently by Stahl and coworkers with Al and Ti complexes. 112-114, 118 In the case of Ti-based catalysts, the mechanism is proposed to involve nucleophilic attack of primary amines on Ti-coordinated secondary organic amides, analogous to that presented for the Al catalysts in Chapter 1 (Scheme 169^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes 1.4).! 14, 118 The overall result of this process is the cleavage of a N-C bond, and formation of a new N-C bond, to generate a new secondary organic amide. A competing side reaction in this process is the cycloaddition of secondary organic amides with Ti imido species, resulting in the formation of an azaoxytitanacyclobutane complex, with an N,0 chelating motif. This complex can subsequently cyclorevert to generate a Ti oxo complex and eliminate the organic amidine as shown in Scheme 4.6. 113 ' 114 LnTi lv=NR 0  L,,Tilv=0 R' R"^NR"^N [2+2] L ,;?,q R O^VMHR' R" Scheme 4.6 retro-12+2] Amidate supported Ti complexes have recently been exploited for the catalytic hydroamination of terminal alkynes. 119-122 These complexes are highly reactive for this process and exhibit excellent regioselectivity for the anti-Markovnikov imine product, and this process is discussed at length in Chapter 5. 120 The unique reactivity patterns seen for these complexes, and the known intermediacy of imido species in this process, required the detailed study of amidate supported imido complexes. This chapter will discuss the synthesis, characterization, and fundamental reactivity of Ti and Zr imido complexes supported by amidate ligands. 170^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes 4.2 Synthesis and Structure of Terminal Imido Complexes 4.2.1 Introduction A number of synthetic routes into imido complexes have been developed for group 4 metals. The first terminal Ti imido complexes were structurally authenticated by the Roesky 123 and Rothwel1 124 groups in 1990. The complex isolated by Roesky was accessed by a TMS-Cl elimination route. By combining TiC14 with (Me3Si)2NP(S)Ph2 in the presence of pyridine, the tris(pyridine) imido adduct (4.10) shown in Scheme 4.7 was isolated. 123 A similar strategy was recently utilized by Mindiola and coworkers, where oxidatively induced TMS-Cl elimination was accomplished by addition of ferrocenium triflate to a Ti(III) precursor. 125 This resulted in the generation of the Ti(IV) imido complex (4.11). Ti(II) precursors have also been utilized to access Ti(IV) imido species through oxidative cleavage of azobenzene ligands, as illustrated by complex (4.12) in Scheme 4.7. 126 171^References begin on page 222 [Cp2Fe][OTf] N—Ti P(Pr)2 -TMSCI -Cp2 Fe P(iPr)2 SiMe I^/^3 I ,, S Me3 I P(Pr)2 P('Pr)2 SiMe 3 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes /1TiCI4^(Me3S02N—P,.. ,, V 'Ph Ph Py Ph S^PiPh N CI„ ^II^,,Py Py° Py -2 TMSCI 4.10 4.11 Me2 CI Me2 ,–N ,,,, Ti . Me2 I^Me2 CI Ph2N2 I h N^Me2 ^ CI ,,,,,,,,^ ,, N ...••'^. Me2  4.12 Scheme 4.7 More common strategies for accessing imido species involve N-H bond cleavage or salt metathesis reactions. As noted in section 4.1, the earliest Zr imido complexes were accessed via alkane or arene elimination from mixed amido-alkyl or amido-aryl complexes (Scheme 4.3).89' 90 These precursor species were accessed by salt metathesis reactions with metal chloro starting materials. While the low-coordinate complexes generated by the Bergman and Wolczanski groups were not isolable, trapping of the imido complexes was possible in the presence of Lewis bases, such as pyridine or THF. The most heavily exploited routes into imido complexes involve the utility of mixed chloro imido starting materials. In 1995, Mountford and coworkers reported the synthesis of the Ti dichloro imido complex (4.13) shown in Scheme 4.8. 127, 128 This complex allowed for the facile installation of a variety of ancillary ligands through salt metathesis, and a couple of select examples are given in Scheme 4.8. 129 ' 130 172^References begin on page 222 /Ti ,,, ,,,,,, ,,,CI N Py 4.13 4.14 4.15 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes tBu N ,,,,CI ,, 1 Ti .. Na[C5Me5] - NaCI Py Py I^CI - 2 Py tBu Scheme 4.8 4.2.2 Results and Discussion 4.2.2.1 Synthesis and Structure of Amido Anilido Complex As mentioned in Chapter 2, salt metathesis routes for the installation of amidate ligands are problematic, and protonolysis routes are much more successful. Thus, protonolysis routes into imido complexes, analogous to those utilized by the Bergman and Wolczanski groups, were utilized for the formation of isolable imido complexes. 89 ' 9° Unlike the mixed alkyl-amido precursor complexes used by the Bergman and Wolczanski groups, the bis(amidate) bis(amido) species described in section 2.3.2.2 were utilized as precursors for aminolysis routes into imido complexes. Upon addition of 1 equiv. of 2,6- dimethylaniline to the Zr bis(amido) complex [DiPp(No) Ph,.12Zr(NMe2 )2 (4.16) (introduced in Chapter 2 as 2.20), the mixed amido complex [DIPP(No , Ph,) j2Zr(NH-2,6- Me2C6H3)(NMe2) (4.17) can be isolated in high yield (Eq. 4.1). 173^References begin on page 222 (4.1) NN NN / ,Zr O \ 2^Nive2 4.17 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes Reduced symmetry is apparent in the 1 1-1 NMR spectrum of 4.17, as indicated by four doublets for the isopropyl groups on the N aryl units of the amidate ligands. These signals exist in a 6:6:6:6 ratio, suggestive of chemically inequivalent aryl groups, where two of the arene rings exhibit hindered rotation about the N-Cips0 bond. As expected, a single resonance at 8 2.12 is observed for the aryl methyl groups, indicating free rotation of the anilido group on the NMR timescale. Finally, the dimethylamido ligand appears as a single resonance at 8 3.47. The presence of an N-H group is confirmed by IR spectroscopy, where a weak signal is observed at 3316 cm -1 . This is very similar to results seen by Zuckermann and Bergman, where the mixed alkyl-amido complex Cp2Zr(NH(2,6-'Pr2C6H 3))(CH3) exhibits a weak NH stretch in the IR spectrum at 3311 1 35 C111 Verification of the structure of 4.17 was accomplished by a single crystal X-ray diffraction study, which resulted in the solid-state molecular structure shown in Fig. 4.4. Relevant bond lengths and angles are listed in Table 4.1, and crystallographic details are presented in Table A4.1 in Appendix A. The pseudo-octahedral geometry about the Zr center is similar to that seen for the other bis(amidate) bis(amido) complexes discussed in Chapter 2. The overall symmetry of 4.17 is C1, with the two amido ligands oriented cis to each other, as expected for these strong n-donor ligands. Both the dimethylamido and 2,6-dimethylanilido ligands are sp2 -hybridized, with planar N donors (sum of angles about N4 = 359.74°), suggesting n-donation. The double bond character of the dimethylamido ligand is further supported by the Zr(1)-N(4) bond length of 2.031(3) A, which is similar to the analogous bond lengths in Zr bis(amido) complexes [ts u(No)ph]2zr(NEt2)2 4.18, [DiviP(NO)rBl2Zr(Nme2)2 4.19, and K2- romp(No)tBti_Ki_ [DmP(NO) tBiZr(NMe2)2(Py) 4.20, which were discussed in Chapter 2. A slightly longer bond length is observed for the Zr(1)-N(3) linkage of the anilido ligand (2.099(3) A), 174^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes which can be rationalized by delocalization of the nitrogen lone pair electrons into the arene ring, thereby weakening the Zr-N bond. The tris(amidate) anilido complex [DIPP(Nox Ph) ]3ZrNHPh (4.21), presented in Chapter 2 (2.14), has a similar Zr-NHR ligand, where the Zr-N-Cips° bond angle is 137.66(17)°. In 4.17, the Zr(1)-N(3)-C(39) bond angle is very similar at 140.9(2)°, significantly distorted from the 120° bond angle expected for an sp 2 -hybridized N center. The obtuse angle is likely a product of reduced steric repulsions achieved upon moving the aryl group further from the sterically congested metal center. Figure 4.4 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [DIP '"(N0)Ph12Zr(NH-2,6-Me2C6F13)(NMe2), 4.17 (non-N-H hydrogens omitted) 175^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes Table 4.1 Selected Bond Distances (A) and Angles (°) Me2C6H3)(NMe2), 4.17 for [DIPPK-ssPh, 2 ,,ru) L (NH-2,6- Lengths Angles Angles Zr(1)-0(1) 2.193(2) 0(1)-Zr(1)-N(1) 58.05(10) 0(2)-Zr(1)-N(4) 90.97(12) Zr(1)-0(2) 2.164(3) 0(2)-Zr(1)-N(2) 57.21(10) 0(1)-Zr(1)-N(3) 138.09(11) Zr(1)-N(1) 2.288(3) N(1)-Zr(1)-N(2) 105.26(11) 0(1)-Zr(1)-N(4) 95.68(11) Zr(1)-N(2) 2.385(3) N(3)-Zr(1)-N(4) 100.69(13) 0(2)-Zr(1)-N(1) 143.13(10) Zr(1)-N(3) 2.099(3) 0(1)-Zr(1)-0(2) 87.11(10) 0(1)-Zr(1)-N(2) 87.06(10) Zr(1)-N(4) 2.031(3) N(2)-Zr(1)-N(4) 147.95(12) C(47)-N(4)-C(48) 110.8(3) 0(1)-C(1) 1.299(5) N(2)-Zr(1)-N(3) 98.24(11) C(47)-N(4)-Zr(1) 128.9(3) N(1)-C(1) 1.313(5) N(1)-Zr(1)-N(3) 80.64(11) C(48)-N(4)-Zr(1) 120.0(3) 0(2)-C(2) 1.304(4) N(1)-Zr(1)-N(4) 103.17(12) C(39)-N(3)-Zr(1) 140.9(2) N(2)-C(2) 1.299(5) 0(2)-Zr(1)-N(3) 130.45(10) While 4.17 could potentially eliminate dimethylamine to generate a 5-coordinate imido complex, thermolysis of this species does not result in the formation of an imido complex. Although 5-coordinate imido complexes of Zr are known for the related guanidinate ligand system (4.22) (Fig. 4.5), the amidate ligand set has two features that make this unlikely: 131, 132 first, due to the nature of the amidate N,0 chelate vs. the N,N chelate of guanidinate ligands, the amidate system only has steric protection on one side of the amidate ligand, and this coupled with the tight bite angle of the four-membered chelate results in a relatively open coordination sphere; second, while guanidinate ligands have been described in the literature as 4 e donors, electronically similar to amidate ligands, the NR2 donor in the guanidinate backbone allows for additional electron donation to the metal center, 133 ' 134 which is not possible with amidate ligands. 135 The increased capacity for electron donation is exemplified by resonance structure B shown in Fig. 4.5. This increased electron donation could be important for stabilizing a coordinatively unsaturated 5-coordinate complex. 176^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes N^NO A 'Pr 'Pr^Nie°' - - N HN ^ 'Pr 'Pr 4.22 R' N  ,IT^ R'CIR'^ R' N ,R' IR^,,F2 IR^\ R N N AN/\\ e e^e B C Figure 4.5 Five-coordinate bis(guanidinate) imido complex and guanidinate resonance forms 131 4.2.2.2 Synthesis and Structure of a Pentagonal Pyramidal Imido Complex Considering the success seen by Bergman and coworkers in isolating THE adducts of zirconocene imido complexes, 32, 90, 93, 100 as well as the numerous imido complexes isolated by Mountford and coworkers having Py in their coordination sphere,33 79, 91, 127, 128, 136 the formation of imido species was attempted in the presence of Lewis bases. Attempts at isolating imido complexes in the presence of Py, as shown in Eq. 4.2, resulted in materials that have extremely complicated 'H NMR spectra and were resistant to crystallization from all typical organic solvents. The inability to characterize these products by NMR spectroscopy led to the choice of a phosphine oxide Lewis base donor. Triphenylphosphine oxide (TPPO) is an ideal stabilizing group as it has the advantages of a 31 P nucleus for an NMR spectroscopic handle, and a hard oxygen donor well suited to bind to high valent group 4 complexes. 177^References begin on page 222 NH2 Et20, Py -2 HNEt2 (4.2) 4.23 N\ NH2 ;Zr/ NMe2 0/2 \NMe2^Ph3P=O -2 HNMe2 Ph^Zr, 2^.„\ Ph P-`'Ph Ph Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes Isolation of a TPPO stabilized imido complex is possible upon combining [DIPP(NO)P1 ]2Zr(NMe2)2 (4.16) with one equiv. of 2,6-dimethylaniline in a solution of toluene, followed by addition of one equiv. of TPPO, as illustrated in Scheme 4.9. The initial reaction between 4.16 and H2N(2,6-Me2C6H3) results in a subtle deepening of the pale yellow color of the solution, presumably corresponding to the formation of 4.17. Upon addition of TPPO, the solution color changes dramatically from light yellow to a deep golden yellow. Isolation of the imido complex [DIPP(NO)P1 ]2Zr—NAr(TPPO), 4.25 (Ar = 2,6-Me2C6H3), is facilitated by its low solubility in pentane, allowing it to be collected by filtration in high yield (82%). Coordination of TPPO is indicated by a downfield shift of the signal in the 31P NMR spectrum to 8 42.7 from 25.4 for free TPPO. 4.16 4.25 Ph 3P=O - HNMe2 /N Ph <^Zr 0/2 NMe2  4.17 Scheme 4.9 178^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes In solution, 4.25 exhibits C 1 symmetry as shown by 1 H NMR spectroscopy, with three distinct isopropyl doublet resonances in a 12:6:6 ratio. This indicates that the two amidate N-aryl groups are chemically inequivalent on the NMR timescale, where one of the aryl groups can freely rotate about its N-Cip s0 bond, and the other is sterically hindered from rotating. This behavior is mirrored by the isopropyl methine signals, where two distinct resonances are seen at 8 3.57 and 4.00. The resonance at 8 4.00 is broad, suggesting that it corresponds to the isopropyl methine protons on the ring experiencing hindered rotation. Correlation of the two isopropyl signals of equal intensity to the broad methine resonance at 8 4.00 by COSY NMR spectroscopy confirmed the peak assignment for the 1 H NMR spectrum. The appearance of a single resonance at 8 2.47 verifies the installation of the 2,6-dimethylphenyl imido unit, which can freely rotate about its N-Cips. bond. A resonance at 8 142.4 in the 13C {'H} NMR spectrum is diagnostic of arene carbons adjacent to imido fragments. 32, 35, 95, 127, 129, 137-139 Variable temperature 1 H and 31 P NMR spectroscopic experiments suggest that two C1 symmetric isomers are interconverting in solution. Free TPPO is not observed by 31P NMR spectroscopy, indicating that it is not coordinatively labile, and this process occurs through an inner-sphere ligand exchange mechanism. While 4.25 can be synthesized directly from the bis(amido) precursor 4.16, it is also possible to access 4.25 via addition of TPPO to the mixed amido species 4.17, as illustrated in Scheme 4.9. This result suggests that imido formation occurs through an associative unimolecular pathway, where sterically induced a-H transfer of the anilido proton to the dimethylamido ligand results in dimethylamine elimination and concomitant imido formation. An alternative reaction pathway would involve the addition of 2 equiv. of 2,6-dimethylaniline to form a bis(anilido) complex, which would then eliminate 2,6- dimethylaniline through a sterically induced a-H transfer. Attempts to synthesize and isolate such a bis(2,6-dimethylanilido) complex have been unsuccessful, and suggest that this mechanism for imido formation is unlikely. Furthermore, addition of one equiv. of 2,6-dimethylaniline to imido complex 4.25 does not result in a bis(anilido) complex, suggesting that the back reaction to a mixed amido complex is not favorable once the imido complex is formed. It is possible that the sterically bulky 2,6-dimethylphenyl substituents are too large to support such a bis(anilido) complex. 179^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes Single crystals of 4.25 were grown from a saturated benzene solution and analyzed by X-ray crystallographic analysis. The solid-state molecular structure of 4.25 is shown in Fig. 4.6, with relevant bond lengths and angles listed in Table 4.2, and crystallographic data located in Table A4.2 (Appendix A). The solid-state molecular structure of 4.25 exhibits a unique distorted pentagonal pyramidal coordination geometry. This type of coordination geometry is very uncommon, and has not previously been observed for group 4 complexes. In this structure, the amidate and I PPO ligands are located in the pseudo-equatorial plane, with the imido ligand located in the axial position. The zirconium center is situated 0.571 A above the mean plane defined by the amidate and TPPO donors (N(2)-O(2)-N(1)-O(1)-O(3)). The pentagonal base of the complex is most easily seen in bottom view, as shown in Fig. 4.7 (outlined in green). The imido ligand is axially located, perpendicular to the mean plane defined by the amidate and TPPO ligands, which makes an 89.8° angle with the plane defined by C(57)- N(3)-Zr(1). Figure 4.6 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [DIPP(NO)P12Zr=N(2,6-Me2C61-13)(TPPO), 4.25 (hydrogens omitted for clarity) 180^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes Table 4.2 Selected Bond Distances (A) and Angles (°) for [DIPP(NO)P1 ]2Zr---N(2,6- Me2C6H3)(TPPO), 4.25 Lengths Angles Angles Zr(1)-O(1) 2.2034(15) 0(1)-Zr(1)-N(1) 57.93(6) 0(2)-Zr(1)-N(1) 78.72(6) Zr(1)-0(2) 2.2391(15) 0(2)-Zr(1)-N(2) 57.45(6) 0(1)-Zr(1)-N(3) 105.85(7) Zr(1)-0(3) 2.2086(14) N(1)-Zr(1)-N(2) 121.90(7) 0(2)-Zr(1)-N(3) 96.89(7) Zr(1)-N(1) 2.3124(18) N(1)-Zr(1)-N(3) 103.37(7) P(1)-0(3 )-Zr(1) 147.87(9) Zr(1)-N(2) 2.3114(18) 0(1)-Zr(1)-0(2) 134.33(6) 0(3)-Zr(1)-N(1) 134.26(6) Zr(1)-N(3) 1.8531(18) N(2)-Zr(1)-N(3) 116.92(8) 0(3 )-Zr(1)-0(2) 136.32(5) 0(1)-C(1) 1.294(3) 0(3)-Zr( I )-N(3) 100.07(7) N(1)-C(1) 1.310(3) C(57)-N(3)-Zr( I ) 173.54(16) 0(2)-C(2) 1.290(3) 0(1)-Zr(1)-0(3) 78.14(6) N(2)-C(2) 1.315(3) N(2)-Zr(1)-0(3) 79.01(6) Figure 4.7 ORTEP depiction (ellipsoids at 30% probability) of bottom view of solid-state molecular structure of [uIPP(NO)P12Zr=N(2,6-Me2C6H3)(TPPO), 4.25 (hydrogens omitted for clarity) The overall symmetry of 4.25 is C 1 , and the solid-state molecular structure is consistent with the solution characterization data. As can be seen in Fig. 4.6, the aryl group located on the N(1) donor of one of the amidate ligands is less sterically encumbered than the other amidate aryl group at N(2), which is adjacent to the bulky 181 ̂References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes TPPO ligand. This structure matches the 1 11 NMR spectrum for this complex, where one of the amidate N-aryl groups gives rise to two signals for the isopropyl groups, due to hindered rotation, and the other is observed as a single resonance. The imido group is best envisioned as a triply-bonded ligand, with an sp-hybridized N center. Triple bond character is supported by the very short Zr(1)-N(3) bond distance of 1.8531(18) A, and the nearly linear C(57)-N(3)-Zr(1) bond angle of 173.54(16)°. While the solution phase data are supportive of the pentagonal pyramidal structure observed in the solid-state, a C, symmetric pseudo-octahedral solution phase structure cannot be ruled out. Density functional theory (DFT) calculations have been undertaken to determine the electronic structure of 4.25 and assist in the interpretation of its unique geometry and reactivity. 4.2.2.3 Electronic Structure of Pentagonal Pyramidal Imido Complex 4.25 Geometric optimization of the structure of 4.25 was accomplished using the solid- state molecular structure determined by X-ray crystallography as the input, and computational details are given in Appendix B. The energetically minimized structure closely matches the experimentally determined structure, and a single point calculation was performed to determine the electronic structure of this complex. As was observed in the DFT calculations of the bis(amido) complexes in Chapter 2, the amidate bonding interactions in 4.25 are energetically well below the frontier bonding interactions, which are dominated by the imido ligand. The amidate ligands have poor orbital overlap with the metal center, and the interactions are largely electrostatic in nature. 135 The TPPO ligand is also located at relatively low energy, having poor orbital overlap with Zr. The HOMO and HOMO-1 orbitals of 4.25 are two orthogonal pat-da bonding interactions between the dxz and dyz orbitals of Zr and the px and py orbitals of the sp- hybridized imido nitrogen. These interactions are shown in Figs. 4.8 and 4.9. 182^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes Figure 4.8 HOMO of [DIPP(NO)P12Zr=N(2,6-Me2C6H3)(TPPO), 4.25 Figure 4.9 HOMO-1 of [DIPP(NO)Phj2Zr=N(2,6-Me2C6H3)(TPPO), 4.25 The required a-symmetry bonding interaction for the imido ligand is located at a substantially lower energy than the n-interactions (HOMO-59, 4.217 eV below relative energy of HOMO-1). The intervening orbitals are largely ligand based in nature. Overlap of the dz2 orbital of Zr with the sp-hybrid orbital of the imido nitrogen results in the u-bond illustrated in Fig. 4.10. 183^References begin on page 222 rChapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes Figure 4.10 HOMO-59 of [ DIPP(NO)Phj2Zr=N(2,6-Me2C6H3)(TPPO), 4.25 These bonding interactions verify the triple bond status of the imido linkage in 4.25, and reinforce the notion that the amidate ligand set binds to the metal center with largely electrostatic character, allowing for optimal imido overlap. The six electrons donated by the imido ligand effectively satisfy the bonding interactions in the axial direction of 4.25, which leads to the question as to whether the position trans to the imido group is accessible for further electron donors. While displacement of the TPPO ligand is observed for 4.25 upon addition of an excess of pyridine, no 7-coordinate adduct could be cleanly isolated upon addition of one equiv. of pyridine to 4.25. The lack of steric accessibility of the position trans to the imido ligand is illustrated in the space-filling models of 4.25 shown in Fig. 4.11. 184^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes Ph^Ar OPPh3 AP Arm -Zr--ON P^0'^ -‘ersi^Ph h ■1-' Ar Ph^ Ph Bottom view^Top view^Front view Figure 4.11 Space-filling models of 4.25 (imido ligand = purple balls, amidate ligands = green balls, TPPO ligand = gray balls) While the front and top views of 4.25 show that the imido ligand is relatively accessible, the bottom view illustrates how the 2,6-diisopropylphenyl groups offer significant steric protection, where very little of the Zr center appears accessible. As previously observed for bis(amidate) bis(amido) complexes of Ti, 135 the amidate ligands in Zr imido complex 4.25 have poor orbital overlap with Zr, thereby generating a very electrophilic metal center that maximizes orbital overlap with the imido fragment, resulting in a formal triple bond. 4.2.2.4 Ligand Lability Investigations While the TPPO ligand in 4.25 is not coordinatively labile in solution, the addition of an excess of a small donor such as THE or pyridine results in loss of TPPO, as observed by 31 P NMR spectroscopy, where the signal at 8 42.7 is replaced by the signal at 8 25.4 for free TPPO. This suggests that under catalytic hydroamination conditions, the 185^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes TPPO donor can be readily displaced by another donor, such as an amine. These results are consistent with an associative mechanism for ligand substitution, proceeding through a 7-coordinate intermediate. It has been noted in the literature that Ti complexes are much more reactive for alkyne hydroamination than Zr complexes ,58' 115, 140 and thus the Ti analogue of 4.25 was synthesized for comparison. Direct synthesis of [Dipp(No s ph, 2) I Ti—N(2,6- Me2C6H 3)(TPPO) (4.26) is possible by combination of [ DIPP(NO)P1 ]2Ti(NMe2)2, the dimethylamido variant of 4.23, with 2,6-dimethylaniline in the presence of TPPO. In solution, 4.26 behaves much the same as 4.25, with I I-1, 13 C, and 31 13 NMR spectra that are essentially identical to those for 4.25. While solid-state structural characterization of this complex was not possible, mass spectral data and elemental analysis support the composition of 4.26 as designated. While the isopropyl methyl region of the I I-I NMR spectrum for 4.26 is nearly identical to that of 4.25, suggesting a CI symmetric complex with hindered rotation about one of the N aryl groups and free rotation about the other, the smaller size of Ti may force the more traditional pseudo-octahedral geometry. The solid-state molecular structures of two bis(amidate) Ti imido complexes isolated in the Schafer lab exhibit distorted octahedral geometries, supporting this hypothesis for the smaller Ti complex. 122, 141 As seen previously for 4.25, the addition of amine donors to 4.26 results in the displacement of TPPO, presumably through an associative mechanism. 4.2.3 Summary Terminal Ti and Zr imido complexes were isolated in high yields through an aminolysis methodology, starting with bis(amidate) bis(amido) complexes. Generation of the imido complexes occurs through initial generation of a mixed amido species [DIPP(No.Phi) ]2Zr(NH-2,6-Me2C6H3)(NMe2), 4.17. This complex can undergo sterically induced a-hydrogen abstraction upon addition of the Lewis base TPPO, liberating dimethylamine and generating the desired imido complex [DIFT(N.-..Phu) ]2Zr=N(2,6- Me2C6H3)(TPPO), 4.25. In the solid-state, 4.25 exists in a rare pentagonal pyramidal geometry. Bond lengths and angles for the imido fragment in 4.25 are indicative of triple bond character, and DFT calculations support this bonding picture. Although the bound 186^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes TPPO is not coordinatively labile, it is possible to displace this ligand upon addition of an excess of donor ligand. An associative mechanism involving a 7-coordinate imido complex is postulated for this process, and the following section describes the synthesis and characterization of such a species. 4.3 Synthesis and Structure of a Seven-Coordinate Imido Complex 4.3.1 Introduction The structural results for 4.25 demonstrated that amidate supported terminal imido species are readily accessible, and adopt a pyramidal geometry in the solid-state. It was possible to observe ancillary ligand exchange of TPPO with other donors, and 7- coordinate imido complexes are postulated as intermediates in this process. However, characterization of a 7-coordinate imido complex was not possible with the bulky [DIPP(No) Ph ] amidate ligand. Space-filling models of 4.25 indicate that the position trans to the imido ligand is difficult to access due to the bulky 2,6-diisopropylphenyl substituents on the amidate ligands. By reducing the steric bulk of the amidate ligands it was anticipated that 7-coordinate imido species could be accessed. 4.3.2 Results and Discussion In an attempt to isolate a 7-coordinate imido complex, the steric bulk at the N of the amidate ligands was reduced from 2,6-diisopropylphenyl to 2,6-dimethylphenyl, to allow access to the site trans to the imido fragment. The 1 H NMR spectrum of the bis(amido) complex [DmANo s) trtu,i2Zr(NMe2)2 (4.19) is much simpler than that for [DIPP(No ) Ph,j2Zr(NMe2)2 (4.16), and it was hoped that this simplicity would facilitate solution phase characterization of the resulting imido complex. Two routes into a 7- coordinate imido complex are illustrated in Scheme 4.10. 187^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes Ar I^Doi 3 , N^'I- ' " : r \N N M M e e :^Ph3P=0, Py^ 0–;111\..,•'°' ,Ar i^1■1 63– 1.--N NH2/ ^/ Zr—N Ar 1 -2 HNMe2 Ar = 2,6-Me2C6 H3 4.19 ^ 4.27  N H2 Ph3P=O -2 HNMe2 4.20 Scheme 4.10 Direct synthesis of the 7-coordinate distorted pentagonal bipyramidal imido complex [DIVIANostBu,) j2Zr=1\1(2,6-Me2C6H3)(TPP0)(Py), 4.27, can be accomplished by combining the bis(amido) precursor [ DmP(NO) t1312Zr(NMe2)2, 4.19, with one equiv. of 2,6-dimethylaniline in the presence of one equiv. of TPPO and one equiv. of pyridine. In solution, a single resonance is observed at 8 1.08 corresponding to the tert-butyl protons, indicating two equivalent amidate ligands. However, the 2,6-dimethylphenyl groups of the amidate ligands appear as two overlapping broad signals at 8 2.13 and 2.22, suggesting hindered rotation of these groups. This was confirmed by variable temperature 1 H NMR spectroscopic experiments, which show coalescence and sharpening of the broad signals into a single sharp singlet at elevated temperatures. A singlet resonance at 8 2.37 indicates free rotation of the imido group at room temperature. Coordination of TPPO can be seen in the 31 P NMR spectrum of 4.27, where the downfield shifted signal at 8 41.2 is similar to that seen for 4.25 and 4.26. Finally, coordination of pyridine is confirmed by signals at 8 8.70, 7.61, and 6.59 in the 1 H NMR spectrum. The broadness of these signals suggests fluxional behavior and a relatively weakly coordinated pyridine moiety. This fluxionality is also inferred by the appearance 188^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes of free pyridine signals in the 1 1-1 . NMR spectrum at elevated temperatures. Generation of 4.27 is also possible through reaction of one equiv. of 2,6-dimethylaniline and one equiv. of TPPO with the bis(amidate) bis(amido) pyridine adduct 4.20 (introduced in Chapter 2 as compound 2.19), where the coordinated pyridine in 4.20 remains coordinated to the resulting imido complex. As was the case for 4.25 and 4.26, isolation of 4.27 is facilitated by its low solubility in pentane and hexanes, and high yields of the product can be isolated as a bright yellow microcrystalline material. Electron impact mass spectrometry of the crude material supports the formation of the imido complex, with a signal at m/z 895, corresponding to Mf - pyridine. Confirmation of pyridine coordination in the solid-state was possible through X-ray crystallographic studies of single crystals grown from a saturated toluene solution at -37 °C. The solid-state molecular structure of 4.27 is illustrated in Fig. 4.12, with relevant bond lengths and angles listed in Table 4.3, and crystallographic details given in Table A4.3 (Appendix A). Figure 4.12 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of [ mP(N0) tB12Zr=N(2,6-Me2C6H3)(TPP0)(Py), 4.27 (non-ipso phenyl carbons and hydrogens omitted for clarity) 189^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes Table 4.3 Selected Bond Distances (A) and Angles (°) for [ DmP(NO)`B°]2Zr=N(2,6- Me2C6H3)(TPP0)(Py), 4.27 Lengths Angles Angles Zr(1)-0(1) 2.213(3) 0(1)-Zr(1)-N(3) 103.64(14) N(3)-Zr(1)-N(4) 175.32(14) Zr(1)-0(2) 2.243(3) N(1)-Zr(1)-N(3) 98.08(14) 0(1)-Zr(1)-N(1) 56.95(11) Zr(1)-0(3) 2.223(3) 0(3)-Zr(1)-N(3) 98.36(13) 0(2)-Zr(1)-N(2) 56.92(11) Zr(1)-N(1) 2.391(3) 0(2)-Zr(1)-N(3) 101.97(13) P(1)-0(3)-Zr(1) 167.73(19) Zr(1)-N(2) 2.334(3) N(2)-Zr(1)-N(3) 93.66(14) 0(3)-Zr(1)-N(1) 82.80(11) Zr(1)-N(3) 1.880(4) 0(1)-Zr(1)-N(4) 81.02(12) 0(3)-Zr(1)-0(2) 80.09(10) Zr(1)-N(4) 2.601(4) N(1)-Zr(1)-N(4) 84.76(13) N(2)-Zr(1)-0(1) 79.23(12) P(1)-0(3) 1.504(3) 0(3)-Zr(1)-N(4) 78.25(12) C(27)-N(3)-Zr(1) 178.0(3) 0(1)-C(1) 1.300(5) 0(2)-Zr(1)-N(4) 74.37(12) N(1)-C(1) 1.317(5) N(2)-Zr(1)-N(4) 86.77(13) It is clear that the smaller amidate ligands in 4.27 allow access to the site trans to the imido ligand, with a N(3)-Zr(1)-N(4) bond angle of 175.32(14)°, but the Zr(1)-N(4) bond length of 2.601(4) A indicates that the bound pyridine is very weakly coordinated to the Zr center. A graphical representation of the steric accessibility of the site trans to the imido ligand is shown in Fig. 4.13, where the coordinated pyridine has been removed from the space-filling model to demonstrate the increased access to the bottom coordination site in 4.27. The imido bond length (Zr(1)-N(3) = 1.880(4) A) is very similar to the analogous distance in 4.25, indicating that the presence of the pyridine ligand results in very little perturbation of the imido ligand. Complementing the short Zr-N bond length of the imido ligand is the nearly linear C(27)-N(3)-Zr(1) bond angle of 178.0(3)°. Taken together, these observations indicate triple bond formation between the imido nitrogen and the zirconium center, as was seen in complex 4.25. The pentagonal plane defined by the amidate and TPPO ligands in 4.27 is considerably more planar than that for 4.25, where the sum of equatorial angles about Zr is 356.01° for 4.27 and only 351.2° for 4.25. As expected, the deviation of Zr from the mean plane defined by N(1)-O(1)-N(2)-O(2)- 0(3) in 4.27 is substantially less than that seen for 4.25 (0.359 A (4.27) vs. 0.571 A (4.25)). 190^References begin on page 222 Ar Ph3P0,,g1^Ar 0 / Bu BA N Ar Bottom view^Top view^Front view Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes Figure 4.13 Space-filling models of 4.27 with pyridine removed (imido ligand = purple balls, amidate ligands = green balls, TPPO ligand = gray balls) 4.3.3 Summary By judicious choice of the less bulky amidate ligand [ D (NO)tB"], a 7-coordinate terminal imido complex 4.27 was isolable, where the position trans to the imido fragment is sterically accessible to electron donors like pyridine. Solid-state molecular structure characterization verified a pentagonal bipyramidal geometry. The coordinated pyridine is labile in solution phase, allowing for rapid exchange of coordinated ligands at this site. This complex is the first 7-coordinate Zr imido complex structurally authenticated, and is strong supporting evidence for an associative mechanism for ligand substitution at Zr. This complex will be revisited in Chapter 5 as a transition state model in the catalytic hydroamination of aminoalkenes. 191^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes 4.4 Structural Characterization of Dimeric Zr Imido Complexes 4.4.1 Introduction With many early transition metals, terminal imido complexes are in equilibrium with their dimeric forms (Eq. 4.3).68, 90, 93, 129, 130, 142 Complexes exhibiting this type of behavior generally have low coordination numbers, are electron deficient, or have highly labile ancillary ligands in their coordination sphere. The presence of dimeric imido complexes is an important consideration in hydroamination catalysis, as these species are considered catalytically inactive and reduce the availability of the catalytically active terminal imido complex. 68, 143, 144 2 [MN—R  RI [ Mt N N >I R (4.3) A dimeric Ti imido complex supported by amidate ligands has been characterized by mass spectrometry, and has the empirical formula [triu(No)C6F5]2Ti(,_ NPh)2Ti ru(\10)C6F5,2i (4.28). 141 This complex could not be characterized in solution or by X-ray crystallography due to its extremely low solubility in all common organic solvents. While the fluorinated bis(amido) precatalyst ru(No)C6F5 12Ti(NEt2)2 (4.29) is highly reactive for alkyne hydroamination, the non-fluorinated congener ru(No)ph l ^ upps^119, 120Phi2Ti(NEt2)2 (4.23).^complex r (No) ^The difference in reactivity between 4.30 and 4.23 can be rationalized by the reduced propensity of imido complexes bearing the very bulky [DIPP(No) Ph,1 amidate ligand to dimerize in solution. In the hopes of determining the relative importance of dimeric imido species in hydroamination catalysis with amidate supported group 4 complexes, isolation of these complexes was attempted. 2n(NEt2)2 (4.30) is only modestly active in comparison to the much bulkier 192^References begin on page 222 4.16 Ph Ar = 2,6-Me2C6H 3 -2 HNMe2 Ph Ar = 2,6-Me2C6H 3 4.31 Scheme 4.11 N H2 Ai r N ZrZNzZr. Ar Ph —̂ Ph -2 HNMe2 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes 4.4.2 Results and Discussion Given the rich imido chemistry observed for Zr amidate vs. Ti amidate complexes, attempts to synthesize dimeric imido complexes focused on Zr species. It was previously noted that pyridine adducts of bulky Ti and Zr imido complexes were ill- defined and difficult to characterize. Solution characterization of any dynamic monomer- dimer equilibria was therefore impossible with these systems. While the TPPO stabilized imido complexes 4.25 — 4.27 are easier to characterize in solution, the strongly donating TPPO ligand is not sufficiently labile to permit dimer formation. As a result, no solution phase evidence for the existence of dimeric imido complexes could be obtained. However, attempts to form azazirconacyclobutene species in situ from bis(amido) precursor [DIPP(NO)P112Zr(NMe2)2 (4.16) fortuitously resulted in solid-state evidence for the formation of a dimeric Zr imido complex, as illustrated in Scheme 4.11. 193^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes The reaction of 4.16 with one equiv. of 2,6-dimethylaniline and one equiv. of diphenylacetylene resulted in a color change from pale yellow to yellow-orange. The 1 H NMR spectrum of this complex is extremely complicated and of limited usefulness. The product isolated is a waxy yellow-orange paste that is very difficult to manipulate. The crude material is soluble in toluene, and this solution was left to crystallize at room temperature for several months. Single crystals were isolable in extremely low yield after crystallization, and were analyzed by X-ray diffraction. It is unlikely that the crystals isolated are representative of the bulk material, and are likely only a small fraction of the products formed in this reaction. However, this experiment afforded the first crystallographically characterized example of bridging imido ligands with ancillary amidate ligands. The solid-state molecular structure of [ DIPP(NO)P12Zr(11-N(2,6- Me2C6H3)) 2Zr[DIPP(NO)P1 ] 2 (4.31) is shown in Fig. 4.14, with relevant bond lengths and angles listed in Table 4.4 and crystallographic data located in Appendix A (Table A4.4). Figure 4.14 ORTEP depiction (ellipsoids at 30% probability) of solid-state molecular structure of r ip (NO)P12Zr(11-N(2,6-Me2C6H3))2Zr[D P(NO)P12, 4.31 (hydrogens omitted) 194^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes Table 4.4 Selected Bond Distances (A) and Angles (°) for [ DIPP(NO)P1 ]2Zr (1-N(2,6-Me2C6H3))2Zr[ DIPP(NO)Ph]2, 4.31 Lengths Lengths Angles Zr(1)-0(1) 2.152(3) Zr(1)-N(5) 2.109(4) Zr(1)-N(5)-Zr(2) 99.03(16) Zr(1)-0(2) 2.139(3) Zr(1)-N(6) 2.121(4) Zr(1)-N(6)-Zr(2) 99.26(16) Zr(2)-O(3) 2.189(4) Zr(2)-N(3) 2.415(4) N(5)-Zr(1)-N(6) 79.01(15) Zr(2)-O(4) 2.197(3) Zr(2)-N(4) 2.416(4) N(5)-Zr(2)-N(6) 82.70(15) Zr(1)-N(1) 2.358(4) Zr(2)-N(5) 2.034(4) N(1)-Zr(1)-N(2) 125.94(12) Zr(1)-N(2) 2.387(4) Zr(2)-N(6) 2.039(4) 0(3)-Zr(1)-0(4) 163.85(13) The considerable steric congestion present in 4.31 is readily apparent in Fig. 4.14, where the 2,6-diisopropylphenyl groups on N(1) and N(2) of the amidate ligands impart steric protection to the bridging imido ligands. While having the N donors of the amidate ligands trans disposed is sterically favored, the amidate ligands bound to Zr(2) are forced into the 0-trans geometry. This is likely due to the extreme steric pressure that would be experienced by the complex if all of the N-2,6-diisopropylphenyl groups were adjacent to the imido core. The core interactions in 4.31 are illustrated in Fig. 4.15, with bond lengths listed in red. Figure 4.15 ORTEP depiction (ellipsoids at 30% probability) of core molecular structure of [DIPP(NO)P ]2Zr(µ-N(2,6-Me2C6F13))2Zr[ DIPP(NOr 12, 4.31 (bond distances in A) 195^References begin on page 222 Chapter 4: Synthesis, Structure, and Reactivity of Amidate Supported Imido Complexes The Zr2N2 core of 4.31 is planar with the sum of angles within the core being exactly 360°. In addition, both bridging imido ligands are planar, with the sum of bond angles around N(5) and N(6) being approximately 360° (ENS = 359.45° and EN6 = 359.81°). Due to the contrasting coordination geometries about the two different Zr centers, the overall symmetry of 4.31 is pseudo-C2, with the C2 axis of symmetry passing through both Zr centers. The different coordination geometries about the Zr centers result in differing imido ligand distances to Zr(1) and Zr(2). As illustrated in Fig. 4.15, the imido distances to Zr(2) are 2.034(4) and 2.039(4) A vs. the analogous distances to Zr(1), which are 2.109(4) and 2.121(4) A. These distances are all considerably longer than those seen in the terminal imido complexes 4.25 (1.8531(18) A) and 4.27 (1.880(4) A). While an electronic argument based on the trans influence can be given to rationalize the different Zr-N(imido) bond distances in 4.31, it is likely that the m