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Despite their proposed accumulation at the Fe sites of the FeMo-cofactor of MoFe-nitrogenase, the presence of hydride ligands in molecular model systems capable of the nitrogen reduction reaction (N 2 RR) appears to diminish the catalytic N 2 -to-NH 3 conversion. We find that for an iron-based system bearing the trisphosphine ligand P 2 P Ph , a dramatic difference in yields is observed for N 2 RR catalyzed by precatalysts with zero, one, or two hydride ligands; however, irradiating the three different catalysts with a mercury lamp results in similar yields. Although the efficacy for N 2 RR versus the hydrogen evolution reaction (HER) is modest for this system by comparison to certain iron (and other metal) catalysts, the system provides an opportunity to study the role of hydrides in the selectivity for N 2 RR versus HER, which is a central issue in catalyst design. Stochiometric reactions with hydride containing precatalysts reveal a hydrogen evolution cycle in which no nitrogen fixation occurs. Irradiation of the dihydride precatalysts, observed during turnover, results in H 2 elimination and formation of (P 2 P Ph )Fe(N 2 ) 2 , which itself is unreactive with acids at low temperature. N 2 functionalization does occur with acids and silyl electrophiles for the reduced species [(P 2 P Ph )Fe(N 2 )] - and [(P 2 P Ph )Fe(N 2 )] 2- , which have been characterized independently. The requirement of accessing such low formal oxidation states explains the need for strong reductants. The low selectivity of the system for functionalization at N β versus Fe creates off-path hydride species that participate in unproductive HER, helping to explain the low selectivity for N 2 RR over HER. The data presented here hence lends further insight into the growing understanding of the selectivity, activity, and required driving force relevant to iron (and other) N 2 RR catalysts.

Mitigation of the hydrogen evolution reaction (HER) is a key challenge in selective small molecule reduction catalysis. This is especially true of catalytic nitrogen (N 2 ) and carbon dioxide (CO 2 ) reduction reactions (N 2 RR and CO 2 RR, respectively) using H + /e - currency. Here we explore, via DFT calculations, three iron model systems, P 3 Fe (E = B, Si, C), known to mediate both N 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 suggested 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 ). 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 P 3 E Fe(NNH 2 ) species, factors that trend with overall product selectivity.