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

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 bonds 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 H_2' 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.