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Novel Redox Non-Innocent Bis(Phenoxide) Pincer Complexes of Cobalt: Connecting Electronic Structures and Reactivity

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Novel Redox Non-Innocent Bis(Phenoxide) Pincer Complexes of Cobalt: Connecting Electronic Structures and Reactivity NOVEL REDOX NON-INNOCENT BIS(PHENOXIDE) PINCER COMPLEXES OF COBALT: CONNECTING ELECTRONIC STRUCTURES AND REACTIVITY A Dissertation Presented to The Academic Faculty by Caleb Frank Harris In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the School of Chemistry and Biochemistry Georgia Institute of Technology August 2017 COPYRIGHT © 2017 BY CALEB F. HARRIS NOVEL REDOX NON-INNOCENT BIS(PHENOXIDE) PINCER COMPLEXES OF COBALT: CONNECTING ELECTRONIC STRUCTURES AND REACTIVITY Approved by: Dr. Jake D. Soper, Advisor Dr. Z. John Zhang School of Chemistry and Biochemistry School of Chemistry and Biochemistry Georgia Institute of Technology Georgia Institute of Technology Dr. Joseph P. Sadighi Dr. Julia Kubanek School of Chemistry and Biochemistry School of Biology Georgia Institute of Technology Georgia Institute of Technology Dr. E. Kent Barefield School of Chemistry and Biochemistry Georgia Institute of Technology Date Approved: July 28, 2017 In loving memory of my father, Marvin F. Harris. TABLE OF CONTENTS LIST OF TABLES viii LIST OF FIGURES ix LIST OF SCHEMES xiii LIST OF SYMBOLS AND ABBREVIATIONS xv SUMMARY xvii CHAPTER 1. Introduction 1 1.1 Metal Catalyzed C-C Bond Forming Reactions and Their Significance 1 1.1.1 Cross-Coupling 1 1.1.2 Oxidative Coupling 3 1.2 Importance of Earth-Abundant 3d Metals for C–C Bond Forming Reactions 3 1.3 Introduction to Redox Non-Innocent Ligands 4 1.4 Determination of Ligand and Metal Physical Oxidations States 5 1.5 Electronic Isomers/Valence Tautomerism 6 1.6 Redox-Active Ligated Metal Complexes in Organic Transformations 8 1.7 Project Aims 9 1.8 References 10 CHAPTER 2. Redox-Active Bis(phenolate) N-heterocyclic Carbene Pincer Ligands Support Cobalt Electron Transfer Series Spanning Four Oxidation States 16 2.1 Note on Collaboration 16 2.2 Introduction 16 2.3 Results 20 2.3.1 Synthesis of the Diphenolate NHC Ligands 20 2.3.2 Synthesis and Characterization of the Cobalt Complexes 21 2.3.3 Electrochemistry 28 2.3.4 Synthesis and Structure of the [(SOCO)Co(THF)] Electron Transfer Series 30 2.3.5 EPR Spectroscopy 34 2.3.6 Computed Electronic Structures of 1, 1+, and 12+ 36 2.3.7 Electronic Spectra of the ET Series 40 2.4 Discussion 50 2.4.1 Physical Oxidation States in the (SOCO)Co Electron Transfer Series 50 2.4.2 Effects of NHC Unsaturation on Ligand-Centered Oxidations 51 2.4.3 (SOCO) as a Platform for Multielectron Transformations at Cobalt 53 2.5 Reactivity of the [(SOCO)Co(THF)] Series 54 2.5.1 Transmetallation to Complex 1+ 54 2.5.2 Reaction with One-Electron Halogen Source 54 2.5.3 Reaction with (Tosyliminoiodo)benzene 56 iv 2.6 Conclusions and Future Work 59 2.7 Experimental 60 2.7.1 General Considerations 60 2.7.2 Materials and Methods 62 S 2.7.3 Synthesis of [H3( OCO)]Cl 62 Ph 2.7.4 Synthesis of [H3( OCO)]Cl 63 2.7.5 Synthesis of Complexes 1 – 3 63 S 2.7.6 Synthesis of [( OCO)Co(MeCN)] (1) 64 S 2.7.7 Synthesis of [( OCO)Co(THF)] (1) 64 2.7.8 Synthesis of [(OCO)Co(MeCN)] (2) 65 Ph 2.7.9 Synthesis of [( OCO)Co(MeCN)] (3) 65 2.7.10 Synthesis of Complex 4 65 2.7.11 Synthesis of Complex 5 66 2.8 X-ray Crystallography 66 2.8.1 General Considerations 66 S 2.8.2 [( OCO)Co(MeCN)] (1) 67 2.8.3 [(OCO)Co(MeCN)] (2) 67 Ph 2.8.4 [( OCO)Co(MeCN)] (3) 68 S 2.8.5 [( OCO)Co(THF)] (1-THF) 69 S + 2.8.6 [( OCO)Co(THF)2](Ph4B) (1 ) 70 S 2+ 2.8.7 [( OCO)Co(THF)3](PF6)2 (1 ) 70 2.8.8 Complex 4 71 2.8.9 Complex 5 72 2.9 Computational Studies 73 2.10 References 74 CHAPTER 3. Photoinduced Radical Trifluoromethylation of (Hetero)Aryl C–H Bonds 83 3.1 Introduction 83 3.2 Results and Discussion 85 3.1.1 Synthesis and Characterization of the Complexes 85 S 3.1.2 Electronic Absorption Spectra of [( OCO)Co(CF3)(MeCN)] 90 3.1.3 Nuclear Magnetic Resonance 93 3.2 Electrochemistry 97 3.2.1 Stability Studies 100 S 3.2.2 Calculated Absorbance Spectrum of [( OCO)Co(CF3)(MeCN)] 101 3.3 Radical Trifluoromethylation 106 • 3.3.1 Radical Trapping of CF3 with TEMPO 106 3.3.2 Radical Trifluoromethylation of Organic Substrates 109 3.3.3 Attempts at Ligand Directed C–H Trifluoromethylation 112 3.4 Towards Catalytic Methods 115 3.4.1 Synthesis and Characterization of a Trifluoroacetate Complex 115 S 3.4.2 Electronic Absorption Spectra of [( OCO)Co(O2CCF3)] 117 S 3.4.3 Efforts Towards Catalytic Trifluoromethylation via [( OCO)Co(O2CCF3)] 118 3.5 Conclusions and Future Directions 119 3.6 Experimental 119 3.6.1 General Considerations 119 v 3.6.2 Materials and Methods 121 Ph Ph 3.6.3 Synthesis of [( OCO)Co( OCO-CF3)] (2) 121 S 3.6.4 Synthesis of [( OCO)Co(CF3)(MeCN)] (4) 122 S + 3.6.5 Synthesis of [( OCO)Co(CF3)(OH2)2] (4 ) 122 S 3.6.6 Syntehsis of [( OCO)Co(CF3)(MeCN)2] (5) 123 S 3.6.7 Synthesis of [( OCO)Co(CF3)(2-phpy)] (6) 123 S 3.6.8 Synthesis of [( OCO)Co(O2CCF3)] (7) 124 3.6.9 General Procedure for C-H Trifluoromethylation of (Hetero)Aryls 124 3.7 X-ray Crystallography 125 3.7.1 General Considerations 125 Ph Ph 3.7.2 [( OCO)Co( OCO-CF3)] (2) 125 S 3.7.3 [( OCO)Co(CF3)(MeCN)] (4) 126 S + 3.7.4 [( OCO)Co(CF3)(OH2)2][OTf] (4 ) 126 S 3.7.5 [( OCO)Co(CF3)(MeCN)2] (5) 127 S 3.7.6 [( OCO)Co(CF3)(2-phpy)] (6) 128 Ph Ph 3.7.7 [( OCO)Co( OCO-CF3)] (7) 128 3.8 Computational Methods 129 3.9 References 130 CHAPTER 4. Metal-ligand Cooperativity Supports CF3 Radical Generation from Electrophillic CF3 Sources 135 4.1 Note on Collaboration 135 4.2 Introduction 135 4.3 Results and Discussion 138 4.3.1 Radical Trapping of Electrophilic CF3 Reagents with TEMPO 138 • 4.3.2 Evaluation of High Valent Co–CF3 Complexes for CF3 Transfer 142 4.3.3 Electronic Spectroscopy 142 4.3.4 Reactivity with Silyl Enol Ethers 146 4.4 Mechanistic Studies 150 4.5 Conclusions and Future Direction 154 4.5.1 Conclusions 154 4.5.2 Future Directions 156 4.6 Experimental 156 4.6.1 General Considerations 156 4.6.2 Materials and Methods 157 4.6.3 General Procedures for Radical Trifluoromethylation 157 4.7 References 158 CHAPTER 5. Conclusions and Future Directions 161 5.1 Conclusions 161 5.2 Future Directions 163 5.3 References 164 APPENDIX A. Mechanistic Studies on Nickel-Catalyzed Direct Oxidative Alkynylation of Azole Derivatives 165 A.1 Introduction 165 A.2 Synthesis and Characterization 167 vi A.3 Results and Discussion 171 A.3.1. Oxidative Coupling 171 A.3.2. Base Effects 172 A.3.3. Synthesis and Characterization of the (PhOCO)NiII Benzoxazole Complex 174 A.3.4. Attempted Synthesis of the (PhOCO)NiII Acetylene Complex 176 A.4 Conclusions and Future Directions 176 A.4.1. Activation of Benzoxazole. 177 A.4.2. H/D Exchange 177 A.4.3. Substrate Scope 178 A.4.4. Facile Synthesis of Enynimine Synthons 178 A.5 References 180 APPENDIX B. Towards Cobalt Catalyzed C–H Bond Amination and an (ONO) Ligand Scaffold 184 B.1. Introduction 184 B.2. Project Summaries 184 B.2.1. C–H Bond Amination of Aryl Sulfonyl Azides 184 S B.2.2. Synthesis and Characterization of an ( OCO) Cobalt Azide Complex 185 B.2.3. Synthesis and Characterization of (ONO) Cobalt Complexes 187 B.3. Experimental 191 B.3.1. General Considerations 191 B.3.2. Synthesis of the ONO Ligand 191 S B.3.3. Synthesis of [( OCO)Co(N3)(MeCN)2] 192 B.3.4. Synthesis of [(ONO)Co(MeCN)]2 192 B.3.5. Synthesis of [(ONO)Co(py)3] 193 B.4. X-ray Crystallography 193 B.4.1. General Considerations 193 S B.4.2. [( OCO)Co(N3)(MeCN)2] 194 B.4.3. [(ONO)Co(MeCN)]2 195 B.4.4. [(ONO)Co(py)3] 195 Vita 198 vii LIST OF TABLES Table 2.1 Redox potentials (V vs. Fc+/Fc) of complexes 1 – 3. 29 Table 2.3 Relevant MOs for 1. Generated with IboView, iso surface threshold set at 80.0. 43 Table 2.4 Relevant MOs for 1+. Generated with IboView, iso surface threshold set at 80.0. 48 Table 3.1 Redox potentials (V) in coordinating vs. non-coordinating solvent. 99 Table 3.2 Relevant calculated MOs for complex 4 generated with IboView with the iso surface set at 80.0. 103 Table 4.1 Radical trifluoromethylation of silyl enol ethers by complex 1 and 3b. Yields were determined by 19F yields against an internal standard. 149 Table A.1 Optimization of reaction conditions. 172 Table A.2 Possible substrates for future investigation. 178 viii LIST OF FIGURES Figure 1.1 Select bond metrics of an amidophenolate ligated Pd complex in three different oxidation states. 6 Figure 2.1 ORTEP plot of [(SOCO)Co(MeCN)], complex 1, with thermal ellipsoids are drawn at 50% probability. 22 Figure 2.2 ORTEP plot of [(OCO)Co(MeCN)], complex 2, with thermal ellipsoids are drawn at 50% probability.
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