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 (N 2 ) reduction reactions (N 2 RR) using H + /e - currency. Here we explore, via DFT calculations, three iron model systems, P 3 E Fe (E = B, Si, C), known to mediate both N 2 RR 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 N 2 RR trend with the predicted N–H bonds strengths of two putative hydrazido intermediates of the proposed catalytic cycle, P 3 E Fe(NNH 2 ) + and P 3 E Fe(NNH 2 ). 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, [P 3 B Fe] + , reveal that the interaction of acid and reductant, Cp* 2 Co, is critical to achieve high efficiency for NH 3 , leading to the demonstration of electrocatalytic N 2 RR. Stoichiometric reactivity shows that Cp* 2 Co 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 p K a and the efficiency for NH 3 , which DFT studies attribute to the kinetics and thermodynamics of Cp* 2 Co 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.