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Master Document Template Copyright by Zhu-Lin Xie 2019 The Dissertation Committee for Zhu-Lin Xie Certifies that this is the approved version of the following Dissertation: Bio-inspired Iron Pincers: from [Fe]-hydrogenase Mimics to Hydrogen Activation Reactivity Committee: Michael J. Rose, Supervisor Emily L. Que Richard A. Jones Michael J. Krische Benjamin K. Keitz Bio-inspired Iron Pincers: from [Fe]-hydrogenase Mimics to Hydrogen Activation Reactivity by Zhu-Lin Xie Dissertation Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy The University of Texas at Austin May 2019 Dedication To my family and most importantly to Him who covered me along this path Acknowledgements It is difficult for me to imagine the multitude of people who have helped me along this path and encouraged me in my pursuit of this dream. Time would fail me to mention the occasions I benefitted from my PhD advisor, Dr Michael J. Rose. It would be impossible for me to reach this goal without your dedicated guidance, encouragement and patience, inside or outside academia. I must thank you for being my incredible source of knowledge and teaching me how to be a scientist. I would like to thank all my fellow group members — you guys created an encouraging, agreeable and supportive environment in the Rose group and made my PhD study an unforgettable journey with a lot of joy. Acknowledgement is deserved to Dr GDP for his mentorship and instruction when I first joined the Rose group. Thank you for your numerous helpful discussions and demonstrations. I am also grateful to Dr Feng Li for graciously sharing his expertise in organic synthesis with me as well as being my friend outside the lab who offered a lot of wonderful suggestions on living. To Dr Kuppu and Dr Seo, for their incredible discussions on my research and generously sharing chemicals with me. To Dylan, Doran and James, for their devoted assistance on ligand synthesis and tolerance for my “tough” leadership. Actually, they taught me how to be an advisor. To Chris, for being a caring “neighbor” and offering me ride after group meetings without charging me. To Patrick, for your sympathetic comforting words when I made an April-fool’s-day prank and sharing your incredible music knowledge with me. To Spencer, for being a great companion along the journey and helping with my PhD study. With a special mention to Keren, Dr Cho, Dr Manes, Dr Pekarek, Dr Taylor, Joey, v Brenna, Sean, Jordan, Brittany, Anh, Clarisse, Eileen, Dan, Azim, Chase to name a few, It was fantastic to have the opportunity to work with you. I would also like to thank the people who have also been incredible supportive in the chemistry department throughout this endeavor. To Vince, Ian, Kristin, Steve, Angela, Garrett and Betsy, for all your assistance with my graduate studies. Without you, it would be impossible for me to achieve this milestone. To my friends, Gang, Junpeng, Yan, Weiran, Yawei, Jiajie, Wandi, Meng, Da, Hongyu, Shichao, you guys are amazing and have made my graduate life full of wonderful memories. A very special gratitude goes out to my brothers and sisters in Christ, especially Pastor Chen and Ruth. Thank you for all your encouragement and inspiration, as well as every act of kindness and selflessness, which have motivated me to keep up and move forward. I am grateful to my parents who have offered me steadfast support and sympathetic ear. You were always there for me no matter how I was and what challenge I was going through. Thank you for your unconditional love and persistent prayers. Finally, I would like to thank someone who is very dear to me, an angel sent by God at the right time. To Pu, thank you so much for taking the brave step to embrace me even though that meant long term physical separation. However, I would like to say that oceans have never conquered the power of love and my heart has never been so close to you. Your words of comfort and strength have carried me though many challenging times, and I would not be able to make it without you. Thank you! Zhulin Xie Austin, May 6, 2019 vi Abstract Bio-inspired Iron Pincers: from [Fe]-hydrogenase Mimics to Hydrogen Activation Reactivity Zhu-Lin Xie, Ph.D. The University of Texas at Austin, 2019 Supervisor: Michael J. Rose The enzyme [Fe]-hydrogenase catalyzes the heterolytic cleavage of H2 and + hydride transfer to the substrate methenyl-tetrahydromethanopterin (methenyl-H4MPT ), a C1 carrier during the methanogenic carbon dioxide (CO2) reduction. This metalloenzyme (also called Hmd: H2-forming H4MPT dehydrogenase) plays an obligate role in the ‘nickel-free’ metabolism of CO2 to methane (CO2→CH4) in the absence of bio-available nickel ([NiFe] hydrogenase) and is the only known biological example of H2 activation by a mononuclear iron site. The active site of [Fe]-hydrogenase exhibits a distinctive array of non-proteinaceous ligands (except for Cys176), including the cis- dicarbonyl, the bidentate pyridone-acyl unit that presents a unique (to biology) organometallic Fe–C bond, a cysteine thiolate, and a substrate binding site weakly occupied by H2O in the resting state. Although computational studies of [Fe]- hydrogenase have shown that H2 splitting is achieved by metal-ligand cooperation between iron and the pyridone-oxygen, there are few synthetic models that have sufficiently investigated this process. vii In order to shed light on the mechanism of [Fe]-hydrogenase, we developed three families of models that mimic different aspects the enzyme active site. The first family consisted of carbamoyl thioether pincer complexes (O=CpyNSMe) wherein the carbamoyl group (-NH-C=O-) mimics the acyl unit in the enzyme. The reactivities of the O=CpyNSMe complexes with hydride sources and strong base were investigated. The H2 activation reaction with the pentacoordinate O=CpyNSMe complex revealed that the fac-C, N, S arrangement is a critical factor to reactivity with H2. The second family was Schiff base pyNC=NSH complexes, in which a variety of iron Schiff base thiol complexes were synthesized with the non-bulky thiolate ligands and bulky thiolate ligand. The thermal stability of the complexes and the role of anionic thiolate donors in stabilizing the cis- Fe(CO)2 unit were investigated. The third family includes the carbamoyl phosphine pincer complexes (O=CpyNPR2), in which a phosphine donor was incorporated in place of the biomimetic sulfur donor. The phosphine donor is ideal for stabilizing Fe(II) carbonyl core, and promises the best path forward to a functional catalyst. The reactivity toward H2 activation and catalytic efficacy of the carbamoyl Fe(II) phosphine complexes were investigated. viii Table of Contents List of Tables ................................................................................................................... xiii List of Figures ....................................................................................................................xv List of Schemes ................................................................................................................xxv Chapter 1: Mononuclear Iron Complexes for (De)hydrogenation: an Overview of Iron Pincer Chemistry and the Synthetic Mimics for [Fe]-Hydrogenase ..............................1 1.1 Introduction ...........................................................................................................1 1.2 Theory, Reactivity of H2 Activation .....................................................................6 1.2.1 Historical Background and Theoretical Aspects of Metal–H2 Ligation .....................................................................................................6 1.2.2 H2 Activation: Heterolytic Cleavage of H2 ....................................11 1.2.3 Metal-Ligand Cooperation towards H2 Activation ........................13 1.3 (De)hydrogenation in Organometallic Chemistry with Pincer-Type Iron Catalysts ..............................................................................................................17 1.3.1 Hydrogenation of C=O bonds ........................................................18 1.3.2 Hydrogenation of C=C and C≡C bonds .........................................26 1.3.3 (De)hydrogenation of C1 Substrates ..............................................28 1.4 Hydrogenation in Biology: [Fe]-Hydrogenase and the Relevant Synthetic Modelling ............................................................................................................31 1.4.1 Spectroscopy and Structure of [Fe]-hydrogenase ..........................34 1.4.2 Computational Mechanistic Studies on H2 Reactivity of [Fe]- hydrogenase ............................................................................................38 1.5 Concluding Remarks...........................................................................................46 ix Chapter 2: Carbamoyl Complexes with a mer-CNS Chelate as Structural Mimics for Mono-Iron Hydrogenase: Mechanistic Study of H2 Cleavage ....................................48 2.1 Introduction .........................................................................................................48 2.2 The Bromide-bound mer-CNS Complex ............................................................51 2.2.1 Synthesis of the Ligand and Bromide Complex ............................51 2.2.2 Structural and Spectroscopic Characterizations.............................52 2.3 Pentacoordinate Complexes Related to the Active Species
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