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

Abstract

Whereas 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 N_α 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.

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