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Accessing Transition Metal Hydride Complexes Through Proton- Coupled Electron Transfer Processes

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Accessing Transition Metal Hydride Complexes Through Proton- Coupled Electron Transfer Processes ACCESSING TRANSITION METAL HYDRIDE COMPLEXES THROUGH PROTON- COUPLED ELECTRON TRANSFER PROCESSES Daniel A. Kurtz A dissertation submitted to the faculty at the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry. Chapel Hill 2019 Approved by: Jillian Dempsey Gerald Meyer Alexander Miller Joanna Atkin Erik Alexanian © 2019 Daniel A. Kurtz ALL RIGHTS RESERVED ii ABSTRACT Daniel A. Kurtz: Accessing Transition Metal-Hydride Complexes Through Proton-Coupled Electron Transfer Processes (Under the direction of Jillian Dempsey) The combustion of fossil fuels such as coal, natural gas, and oil is the primary source of energy to power our everyday lives. These energy sources are non-renewable and their continued consumption will only accelerate the already evident warming of the planet. One of the primary products of burning fossil fuels, carbon dioxide (fossil fuels are carbon-based fuels), has increased by 24% in the past 60 years which can be explained by the increased global energy demand and subsequent acceleration of fossil fuel consumption. It is unreasonable, though, to immediately stop our use of fossil fuels as an energy source without a viable replacement. Although it is 93 million miles away, the largest energy source available to humans is the sun. Nature has evolved to utilize the vast quantity of solar irradiation that illuminates the earth in order to perform complex photochemical reactions such as the biosynthesis of sugars. While there are researchers who are attempting to mimic the structure and function of these biological systems, the research discussed in this dissertation is focused on a more energy-relevant transformation: the conversion of solar energy into the simplest chemical fuel, hydrogen fuel. Transition metal complexes are capable of performing all the required steps needed to convert sunlight into hydrogen fuel: absorption of solar radiation, and catalytic orchestration of the combination of the two electrons and two protons required to form H2. While known technology exists for this conversion, the efficiency of all required steps are currently not at a level that makes the system economically/commercially viable. The work discussed herein is iii related to key steps in the path from solar energy to H2. The first chapter is focused on factors that influence the formation of a transition metal hydride complex, a key intermediate in catalytic hydrogen production. Chapter 2 then describes a model system in which to study hydride formation using two different methods is described, and Chapter 3 discusses systematic variation in structural parameters on the model system and how they influence hydride formation is then discussed. Finally, presented in Chapter 4 are efforts to improve the spectral absorbance of an existing class of chromophores, followed by efforts to directly form transition metal hydrides via light absorption in Chapter 5. iv ACKNOWLEDGEMENTS First and foremost, Jill. I can’t express how thankful I am to have been able to join your lab to pursue my PhD. You’ve been an amazing mentor throughout the last four and a half years. You’ve let me guide myself when I wanted to, and pushed me in areas where I need it. Every visitation weekend, the prospective students ask the question: “What’s Jillian like as a mentor?” and my answer is always the same: “She’s exactly the mentor you need her to be”, and you’ve been precisely that for me. I remember in our first meeting when I joined the lab, you told me that I was going to be your “no organic substrates” guy, and it seems like we’ve been on the same page and wavelength since then. I’ve gotten everything I wanted out of my PhD and more, and you’re the main reason for that. Thank you. Next, my other PhD “parent”, Alex. You’ve become like a second mentor to me, and I greatly appreciate that. I truly feel like I’ve been an adopted member of the Miller lab in my time here (I think being the only Dempsey lab member to attend a full “paper meeting” makes that official). From partaking in some of the (many) traditions, to being on the other side of the camera for a number of group photos, and of course the awesome science and publications that I’ve had the pleasure of working on with you and your lab. Thank you for the chemistry and basketball chats over the years. Jerry, thank you for being my orals and thesis committee chair and for all the help over the years. Your excitement during a conversation of ours at a visitation weekend with Matt Brady helped to spark the inspiration for Chapter 4 of my thesis, and hopefully a bright new direction for researchers down the road. I never understood your devotion to those cheese-heads v from the other side of Lake Michigan, but I did enjoy our friendly football rivalry over the years. To Joanna and Erik, I appreciate you being on my thesis committee and helping to guide me over this final hurdle. Your time, advice, and discussion is invaluable to my degree, thank you. As the 4th cohort of Dempsey lab, I have two very different groups of lab mates to thank. The first group are those that left before me: Thomas for infecting me with the UNC basketball bug, Robin for handing down her wisdom and knowledge of the laser lab, Brian for being my first desk mate (sorry Eric) and teaching me how to do my first reactions in lab, Eric for being a phenomenal friend both during and after you left, Chris for sharing my interest and excitement over “neat and weird” science facts, and Kelley for the introduction of “Disney Fridays” in lab. You all made me feel welcome in lab, and made it a fun place to work. The second group are those who will leave after me: Katherine for all the amazing baked goods over the years, Melody for the fun MCU talks at lunch whenever the new movies come out, Carolyn for all the laughs and for listening to my complaints and being comfortable enough to share yours, Michael for having such capable and meticulous hands to hand the laser lab over to, Tayliz for keeping the sass of A tower alive, and Aldo and Brittany for being bright lights in our lab’s future. I thank you all and am looking forward to keeping in touch and seeing how you all develop in your time remaining in the group and in the future. I’ve gotten to work with some phenomenal post-doctoral mentors during my time here. Having Noémie as my desk mate for over half of my time here was instrumental in my growth as a chemist. You and Matt really taught me to think critically about everything and to not get over- excited about data before checking everything about it. Thank you both for being great mentors and great friends, and I’m looking forward to seeing your respective groups flourish over the years. Tao and Debanjan, thank you for always being there to bounce ideas off of in lab and for vi setting an example on what hard work means in the Dempsey lab. Good luck in your futures, I’m sure they’ll be great. I’ve had the pleasure of working with some fabulous undergraduate researchers here. Thank you to Hui-Min and Kevin who both took charge of their projects and helped me push the rhenium chromophore project through. Kevin, thanks for all of the laughs over the last 3 years, the basketball and football talks, the chemistry we’ve done together, and for challenging me to become more creative in the lab. You’ve been a phenomenal colleague to work with, and I know you’re going to go on to have a great grad school experience and career. My friends outside of the Dempsey lab, and outside of UNC chemistry, have gotten me through some of the not so fun times in grad school. Thanks to Kyle and Nick for coming to visit me from Michigan, thanks to Earl for being an awesome roommate for the past few years, to Ali, Andreas, and Andrew for being great friends throughout the 5 years we’ve been here together, to Kelsey for always being there to talk about the good things and the bad even across many state lines, and thanks to all of the other people around the department that I don’t have room to list (otherwise this section would be twice as long as it already is). I wouldn’t be where I am today without all of my parents’ help and guidance both before I came to grad school and during it. They’ve always wanted what’s best for me from the time I was born until the time I’m going to be defending this thesis, and I can’t thank them enough for that. They say it takes a village, and my family being about the size of one has really helped me become the person I am today. I love you all for everything you’ve done and the sacrifices you’ve made over the years, thank you. Finally, thank you to Ann Marie. These last few years with you have been the best of my time here in Chapel Hill. You’ve been my support through everything, and I can’t thank you vii enough. From our advanced nature walks, to our trips to Michigan and Florida, to “raising” Wynnstan, to you offering and wanting to read every piece of writing I’ve done, your constant presence in my life is something I cherish.
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