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 (N₂RR) appears to diminish the catalytic N₂-to-NH₃ conversion. We find that for an iron-based system bearing the trisphosphine ligand P₂P^Ph, a dramatic difference in yields is observed for N₂RR 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 N₂RR 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 N₂RR 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 H₂ elimination and formation of (P₂P^Ph)Fe(N₂)₂, which itself is unreactive with acids at low temperature. N₂ functionalization does occur with acids and silyl electrophiles for the reduced species [(P₂P^Ph)Fe(N₂)]^- and [(P₂P^Ph)Fe(N₂)]^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 N₂RR 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) N₂RR catalysts.