Light Enhanced Fe-Mediated Nitrogen Fixation: Mechanistic Insights Regarding H2 Elimination, HER, and NH3 Generation

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 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 NH 3 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. Stoichiometric 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 thus lend further insight into the growing understanding of the selectivity, activity, and reductant strengths relevant to iron (and other) N 2 RR catalysts.