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 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 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.