Interplay of Proton Transfer, Electron Transfer and Proton-Coupled Electron Transfer in Transition Metal Mediated Nitrogen Fixation

Citation

Matson, Benjamin David (2018) Interplay of Proton Transfer, Electron Transfer and Proton-Coupled Electron Transfer in Transition Metal Mediated Nitrogen Fixation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9MS3R0Z. https://resolver.caltech.edu/CaltechTHESIS:02232018-152758526

Abstract

Mitigation of the hydrogen evolution reaction (HER) is a key challenge in selective small molecule reduction catalysis, including the nitrogen (N₂) reduction reactions (N₂RR) using H⁺/e^- currency. Here we explore, via DFT calculations, three iron model systems, P₃^EFe (E = B, Si, C), known to mediate both N₂RR and HER, but with different selectivity depending on the identity of the auxiliary ligand. It is shown that the respective efficiencies of these systems for N₂RR trend with the predicted N–H bonds strengths of two putative hydrazido intermediates of the proposed catalytic cycle, P₃^EFe(NNH₂)⁺ and P₃^EFe(NNH₂). Bimolecular proton-coupled electron transfer (PCET) from intermediates with weak N–H bonds is posited as a major source of H2 instead of more traditional scenarios that proceed via metal hydride intermediates and proton transfer/electron transfer (PT/ET) pathways.

Studies on our most efficient molecular iron catalyst, [P₃^BFe]⁺, reveal that the interaction of acid and reductant, Cp*₂Co, is critical to achieve high efficiency for NH₃, leading to the demonstration of electrocatalytic N₂RR. Stoichiometric reactivity shows that Cp*₂Co is required to observe productive N–H bond formation with anilinium triflate acids under catalytic conditions. A study of substituted anilinium triflate acids demonstrates a strong correlation between pK_a and the efficiency for NH₃, which DFT studies attribute to the kinetics and thermodynamics of Cp*₂Co protonation. These results contribute to the growing body of evidence suggesting that metallocenes should be considered as more than single electron transfer reagents in the proton-coupled reduction of small molecule substrates and that ring-functionalized metallocenes, believed to be intermediates on the background HER pathway, can play a critical role in productive bond-forming steps.

Thesis Files