Fe-Mediated HER vs N₂RR: Exploring Factors That Contribute to Selectivity in P₃^EFe(N₂) (E = B, Si, C) Catalyst Model Systems

Mitigation of the hydrogen evolution reaction (HER) is a key challenge in selective small-molecule reduction catalysis. This is especially true of catalytic nitrogen (N₂) and carbon dioxide (CO₂) reduction reactions (N₂RR and CO₂RR, respectively) 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 suggested that the respective efficiencies of these systems for N₂RR trend with the predicted N–H bond strengths of two putative hydrazido intermediates of the proposed catalytic cycle, P₃^EFe­(NNH₂)⁺ and P₃^EFe­(NNH₂). Further, a mechanism is presented for undesired HER consistent with DFT studies, and previously reported experimental data, for these systems; bimolecular proton-coupled electron transfer (PCET) from intermediates with weak N–H bonds is posited as an important source of H₂, instead of more traditional scenarios that proceed via metal hydride intermediates and proton transfer/electron transfer (PT/ET) pathways. Wiberg bond indices provide additional insight into key factors related to the degree of stabilization of P₃^EFe­(NNH₂) species, factors that trend with overall product selectivity.