Catalytic Reduction of N_2 to NH_3 by an Fe−N_2 Complex Featuring a C‑Atom Anchor

Abstract

While recent spectroscopic studies have established the presence of an interstitial carbon atom at the center of the iron–molybdenum cofactor (FeMoco) of MoFe-nitrogenase, its role is unknown. We have pursued Fe–N_2 model chemistry to explore a hypothesis whereby this C-atom (previously denoted as a light X-atom) may provide a flexible trans interaction with an Fe center to expose an Fe–N_2 binding site. In this context, we now report on Fe complexes of a new tris(phosphino)alkyl (CP^(iPr)_3) ligand featuring an axial carbon donor. It is established that the iron center in this scaffold binds dinitrogen trans to the C_(alkyl)-atom anchor in three distinct and structurally characterized oxidation states. Fe–C_(alkyl) lengthening is observed upon reduction, reflective of significant ionic character in the Fe–C_(alkyl) interaction. The anionic (CP^(iPr)_3)FeN_2^– species can be functionalized by a silyl electrophile to generate (CP^(iPr)_3)Fe–N_2SiR_3. (CP^(iPr)_3)FeN_2^– also functions as a modest catalyst for the reduction of N_2 to NH_3 when supplied with electrons and protons at −78 °C under 1 atm N_2 (4.6 equiv NH_3/Fe).

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