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Computational Study of Pincer Iridium COMPUTATIONAL STUDY OF PINCER IRIDIUM CATALYTIC SYSTEMS：C-H, N-H, AND C-C BOND ACTIVATION AND C-C COUPLING REACTIONS by TIAN ZHOU A dissertation submitted to the Graduate School-New Brunswick Rutgers, The State University of New Jersey In partial fulfillment of the requirements for the degree of Doctor of Philosophy Graduate Program in Chemistry and Chemical Biology Written under the direction of Alan S. Goldman And approved by _____________________________ _____________________________ _____________________________ _____________________________ New Brunswick, New Jersey May 2017 ABSTRACT OF THE DISSERTATION COMPUTATIONAL STUDY OF PINCER IRIDIUM CATALYTIC SYSTEMS：C-H, N-H, AND C-C BOND ACTIVATION AND C-C COUPLING REACTIONS by Tian Zhou Dissertation Director: Alan S. Goldman Computational chemistry has achieved vast progress in the last decades in the field, which was considered to be only experimental before. DFT (density functional theory) calculations have been proven to be able to be applied to large systems, while maintaining high accuracy. One of the most important achievements of DFT calculations is in exploring the mechanism of bond activation reactions catalyzed by organometallic complexes. In this dissertation, we discuss DFT studies of several catalytic systems explored in the lab of Professor Alan S. Goldman. Headlines in the work are: (1) (R4PCP)Ir alkane dehydrogenation catalysts are highly selective and different from (R4POCOP)Ir catalysts, predicting different rate-/selectivity-determining steps; (2) The study of the mechanism for double C-H addition/cyclometalation of phenanthrene or biphenyl by (tBu4PCP)Ir(I) and (iPr4PCP)Ir illustrates that neutral Ir(III) C-H addition products can undergo a very facile second C-H addition, particularly in the case of sterically less-crowded Ir(I) complexes; (3) (iPr4PCP)Ir pure solid phase catalyst is highly ii effective in producing high yields of α-olefin products, since the activation enthalpy for dehydrogenation is higher than that for isomerization via an allyl pathway; higher temperatures favor the dehydrogenation/isomerization ratio; (4) (PCP)Ir(H)2(N2H4) complex follows a hydrogen transfer mechanism to undergo both dehydrogenation to form N2 and H2, as well as hydrogen transfer followed by N-N bond cleavage to form NH3, N2, and H2; (5) The key for the catalytic effect of solvent molecule in CO insertion reaction for RMn(CO)5 is hydrogen bond assisted interaction. The basicity of the solvent determines the strength of the hydrogen bond interaction during the catalytic path and determines the catalytic power of the solvent; and (6) Dehydrogenative coupling of unactivated C-H bonds (intermolecular vinyl-vinyl, intramolecular vinyl-benzyl) is catalyzed by precursors of the (iPr4PCP)Ir fragment. The key step for this mechanism is a Ir(III) vinyl hydride complex undergoing addition of a styrenyl ortho C-H bond to give an Ir(III) metalloindene plus H2 iii Acknowledgement My most sincere thanks go to my advisor, Prof. Alan Goldman, for his support, patience, and encouragement throughout my graduate studies. Alan is a great person and a great professor. As a foreign student, I consider myself amazingly fortunate to have a mentor such as Alan, who not only gives me most valuable advice in scientific problems, but also the freedom and encouragement to study, explore and master all kinds of knowledge. I might doubt about the choice of spending such a long time studying abroad away from my family sometime, but I would choose Alan as my mentor every time given that choice. Besides Alan, Professor Karsten Krogh-Jespersen is the mentor who taught me most if not all computational chemistry materials. Birds of a feather flock together, I do not think it is just coincidence that I get the best and nicest professors as mentors here at Rutgers. They make my time here much quicker than it seems to be, like happy life always flies. Besides my advisors, I would like to thank the rest of my dissertation committee, Prof. Fuat Celik, Dr. Yury Kissin, and Prof. Kai Hultzsch. Their advice and comments guided me through my research towards this dissertation. I thank the past and present members of Prof. Goldman's Group. Especially, I thank Dr. David Wang and Dr. Jason Hackenberg for their considerable help, when I just entered the group; Dr. Mike Haibach for all his wisdom and helpful discussions; Dr. Michael Blessent , Changjian Guan, Yang Gao, and Bo Li as great collaborators and iv great friends. I thank Dr. Katie Field, Dr. Akshai Kumar, and Dr. David Laviska for their support; I benefited a lot from working with them. Finally, I would like to thank my parents for always supporting me throughout my life. v Dedication To my family vi Table of Contents ABSTRACT OF THE DISSERTATION ii Acknowledgement iv Dedication vi Table of Contents vii Chapter 1: Introduction 1 Chapter 2: Computational Study of C-H Activation by(PCP)Ir and (POCOP)Ir: A Regioselectivity Study 4 Introduction 5 Results and Discussion 7 Conclusions 32 References 36 Computational Details 38 Computational Section References 47 Chapter 3: Computational Study of Double C-H Activation of Biphenyl or Phenanthrene 48 Introduction 49 Results and Discussion 50 Conclusions 75 References 77 Computational Details 80 Computational Section References 94 Chapter 4: Computational Study of Solid-Phase Molecular Pincer-Iridium Catalysts 96 Introduction 97 Results and Disscussion 99 Conclusions 134 References 138 Computational Details 141 Computational Section References 147 Chapter 5: Computational Study of N-H Activation by (PCP)IrH2 148 Introduction 148 vii Results and Disscussion 149 Conclusions 153 References 154 Computational Details 155 Computational Section References 157 Chapter 6: Computational Study of the Solvent Catalysed CO Insertion Reaction for RMn(CO)5 Complexes 158 Introduction 158 Results and Discussion 159 Conclusions 167 References 168 Computational Details 168 Computational Section References 178 Chapter 7: Computational Study of Catalytic Dehydrogenative C-C Coupling by a Pincer- Ligated Iridium Complex 179 Introduction 180 Results and Discussion 182 Conclusions 204 References 208 Computational Details 209 Computational Section References 222 viii 1 Chapter 1 Introduction This dissertation covers computational mechanism studies of C-H, C-C, and N-H activation and CO insertion in organometallic chemistry and catalysis. Themes of the research are arranged in order of introduction, experimental study followed by DFT mechanism explanation. In the first theme, we have demonstrated that (R4PCP)Ir alkane dehydrogenation catalysts are highly selectivity for the terminal position of n-alkanes, whereas (R4PCOP)Ir and (R4POCOP)Ir catalysts are much less terminal-selective, or are even selective for the formation of internal olefins. DFT calculations reveal that the differences in selectivity between the catalysts investigated are not due to variations in the differences in energy between the pro-terminal and pro-internal TSs for any given reaction step. Instead, the different selectivities are attributable to different rate-/selectivity-determining steps for different catalysts. In particular, for (tBu4PCP)Ir the rate-/selectivity-determining step is β- H-elimination, whereas for (tBu4POCOP)Ir the rate-/selectivity-determining step is loss of the coordinated olefin. In the second theme, we discuss the mechanism of double C-H addition/cyclometalation of phenanthrene or biphenyl by (tBu4PCP)Ir(I) and (iPr4PCP)Ir(I) complexes. The rate-determining step with both pincer ligands is calculated to be addition of the sterically hindered ortho C-H bond of biphenyl or the analogue at the C4 position of phenanthrene, affording (tBu4PCP)Ir(III) and (iPr4PCP)Ir(III) aryl hydride complexes. A second C-H addition, to produce cyclometalated Ir(V) dihydrides, has a 2 much lower calculated free energy barrier. While C-H addition to iridium is generally associated with Ir(I), this work illustrates that neutral Ir(III) C-H addition products can undergo a very facile second C-H addition, particularly in the case of sterically less- crowded systems. This result may have significant implications for potential C-C coupling catalysts. In the third theme, we report that pure solid phase (pincer)Ir catalysts are highly effective for the dehydrogenation of n-alkanes in the gas phase. (iPr4PCP)Ir can produce high yield α-olefin products. As the calculated activation enthalpy for dehydrogenation is higher than that for isomerization via the allyl pathway, higher temperatures favor the dehydrogenation/isomerization ratio and therefore higher α-olefin yields. Thus the high α-olefin yields obtained from the gas-solid systems with (iPr4PCP)Ir are in large part simply a result of the conditions that lead to the gas/solid-phase state: use of highly volatile hydrogen acceptors and high temperature, both of which mitigate the hydride isomerization pathway. In the fourth theme, we demonstrate that the (PCP)Ir(H)2(N2H4) complex follows a hydrogen transfer mechanism to undergo both dehydrogenation to form N2 and H2, as well as hydrogen transfer followed by N-N bond cleavage to form NH3, N2, and H2. Small molecule assistance plays a key role in the hydrogen transfer step during the process. Three key steps (α-N-H activation step, β-N-H transfer step, N-N bond cleavage step) are our potential rate determining steps for the ammonia formation. The effect of vacuum in removing dihydrogen from the metal center is supposed to lead to the pure dehydrogenation reaction. 3 In the fifth theme, we show that the basicity of a catalyst molecule determines the strength of the hydrogen bond interaction during the catalytic path and determines its catalytic power. The key for the catalytic effect of solvent molecule in CO insertion reaction for RMn(CO)5 is hydrogen bond assisted interaction. Based on whether such an interaction exists, we can predict the solvent catalytic effect for similar metal complex CO insertion reactions as well. The mechanism proposed in this work applies to various catalytic molecules as well as different metal CO insertion systems.
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