Catalytic Reduction of N2 to NH3 by an Fe-N2 Complex Featuring a C-Atom Anchor

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 (CP3iPr3) 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 (CP3iPr3)FeN2- species can be functionalized by a silyl electrophile to generate (CP3iPr3)Fe-N2SiR3. (CP3iPr3)FeN2- also functions as a modest catalyst for the reduction of N-2 to NH3 when supplied with electrons and protons at -78 degrees C under 1 atm N-2 (4.6 equiv NH3/Fe).