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

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, P3EFe (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, P3EFe(NNH2)+ and P3EFe(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, [P3BFe]+, 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 pKa 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.