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

Abstract

AbstractWhereas synthetically catalyzed nitrogen reduction (N2R) to produce ammonia is widely studied, catalysis to instead produce hydrazine (N2H4) has received less attention despite its considerable mechanistic interest. Herein, we disclose that irradiation of a tris(phosphine)borane (P3B) Fe catalyst, P3BFe+, significantly alters its product profile to increase N2H4 versus NH3; P3BFe+ is otherwise known to be highly selective for NH3. We posit a key terminal hydrazido intermediate, P3BFe=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 N2H4. Associated electrochemical and spectroscopic studies establish that N2H4 lies along a unique product pathway; NH3 is not produced from N2H4. 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.