Proton-Coupled Electron Transfer in Nitrogen Fixation

Citation

Chalkley, Matthew J. (2020) Proton-Coupled Electron Transfer in Nitrogen Fixation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/FE9D-9K14. https://resolver.caltech.edu/CaltechTHESIS:02052020-203503014

Abstract

This thesis focuses on the management of protons and electrons in the formation of X−H bonds. In our pursuit of better understanding this process, we have been particularly interested in the nitrogen fixation reaction (N₂-to-NH₃) because of the high number of protons and electrons involved in this conversion (6) and the significant difficulty of functionalizing N₂. The first chapter introduces the important themes of this thesis: (i) multiple bonding, (ii) proton-coupled electron transfer, (iii) overpotential in N₂ fixation, and (iv) selectivity in N₂ fixation. The second chapter discusses the bonding of an iron complex with a small molecule (NO) and how this bonding is key to activating the small molecule for reactivity. The third chapter looks at how employing a new proton and electron source allows an Fe catalyst to achieve improved selectivity and turnover number for the reduction of N₂ to NH₃ despite a lowered overpotential relative to previous reactions. It also raises the hypothesis that this is possible due to proton-coupled electron transfer mediated by a metallocene. The fourth chapter studies the effect of acid strength on N₂ fixation selectivity and demonstrates circumstantial evidence for the involvement of a decamethylcobaltocene (Cp*₂Co) in the formation of N−H bonds via proton-coupled electron transfer. It also highlights how the addition of co-catalytic [Cp*₂Co]⁺ to electrochemical experiments with our Fe catalyst enabled truly electrocatalytic N₂ fixation for the first time. The fifth chapter provides both atomistic detail on the protonation reactivity of Cp*₂Co and experimentally verifies the prediction that this species would be an extremely strong hydrogen-atom donor. It also develops a conceptual framework to explain the uniquely weak C−H bonds both homolytic and heterolytic that result from metallocene protonation and discusses their potential to play a role in not only the hydrogen evolution reaction (HER), but also the N₂ fixation reaction. In the final chapter, we develop a synthetic route to a base appended cobaltocene. We demonstrate that this second-generation cobaltocene can, unlike the first generation, serve as a net hydrogen-atom donor under electrocatalytic conditions. As a demonstration of the utility of this, we use the base-appended cobaltocene for the selective, proton-coupled reduction of ketones to pinacols via a rate-determing concerted proton-electron transfer.

Thesis Files