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 (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.
We thank the National Institutes of Health (R01GM-075757) for support of this work; the Dow Next Generation Educator Funds and Instrumentation Grants for their support of the NMR facility at Caltech; the Resnick Water and Environment Laboratory at Caltech for the use of instrumentation. P.G.B. thanks the Ramón Areces Foundation for a postdoctoral fellowship. M.J.C. thanks the Resnick Sustainability Institute for a graduate fellowship. The authors declare no conflict of interest. Data Availability Statement. The data that support the findings of this study are available in the Supporting Information of this article.
Supplemental Material - anie202216693-sup-0001-misc_information.pdf