Light Alters the NH 3 vs N 2 H 4 Product Profile in Iron-catalyzed Nitrogen Reduction via Dual Reactivity from an Iron Hydrazido (Fe=NNH 2 ) Intermediate.

Whereas synthetically catalyzed nitrogen reduction (N 2 R) to produce ammonia is widely studied, catalysis to instead produce hydrazine (N 2 H 4 ) has received less attention despite its considerable mechanistic interest. Herein, we disclose that irradiation of a tris(phosphine)borane (P 3 B ) Fe catalyst, P 3 B Fe + , significantly alters its product profile to increase N 2 H 4 versus NH 3 ; P 3 B Fe + is otherwise known to be highly selective for NH 3 . We posit a key terminal hydrazido intermediate, P 3 B Fe=NNH 2 , as selectivity-determining. Whereas its singlet ground state undergoes protonation to liberate NH 3 , a low-lying triplet excited state leads to reactivity at N α and formation of N 2 H 4 . Associated electrochemical and spectroscopic studies establish that N 2 H 4 lies along a unique product pathway; NH 3 is not produced from N 2 H 4 . Our findings are distinct from the canonical mechanism for hydrazine formation, which proceeds via a diazene (HN=NH) intermediate and showcase light as a tool to tailor selectivity.