Light Alters the NH₃ vs N₂H₄ Product Profile in Iron‐catalyzed Nitrogen Reduction via Dual Reactivity from an Iron Hydrazido (Fe=NNH₂) Intermediate

Whereas synthetically catalyzed nitrogen reduction (N2 R) to produce ammonia is widely studied, catalysis to instead produce hydrazine (N2 H4 ) has received less attention despite its considerable mechanistic interest. Herein, we disclose that irradiation of a tris(phosphine)borane (P3B ) Fe catalyst, P3B Fe+ , significantly alters its product profile to increase N2 H4 versus NH3 ; P3B Fe+ is otherwise known to be highly selective for NH3 . We posit a key terminal hydrazido intermediate, P3B Fe=NNH2 , as selectivity-determining. Whereas its singlet ground state undergoes protonation to liberate NH3 , a low-lying triplet excited state leads to reactivity at Nα and formation of N2 H4 . Associated electrochemical and spectroscopic studies establish that N2 H4 lies along a unique product pathway; NH3 is not produced from N2 H4 . 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.