Fe–N ₂ /CO complexes that model a possible role for the interstitial C atom of FeMo-cofactor (FeMoco)

We report here a series of four- and five-coordinate Fe model complexes that feature an axial tri(silyl)methyl ligand positioned trans to a substrate-binding site. This arrangement is used to crudely model a single-belt Fe site of the FeMo-cofactor that might bind N2 at a position trans to the interstitial C atom. Reduction of a trigonal pyramidal Fe(I) complex leads to uptake of N2 and subsequent functionalization furnishes an open-shell Fe-diazenido complex. A related series of five-coordinate Fe-CO complexes stable across three redox states is also described. Spectroscopic, crystallographic, and Density Functional Theory (DFT) studies of these complexes suggest that a decrease in the covalency of the Fe-C(alkyl) interaction occurs upon reduction and substrate binding. This leads to unusually long Fe-C(alkyl) bond distances that reflect an ionic Fe-C bond. The data presented are contextualized in support of a hypothesis wherein modulation of a belt Fe-C interaction in the FeMo-cofactor facilitates substrate binding and reduction.