Density functional study of the structure of the FeMo cofactor with an interstitial atom and homocitrate ligand ring opening

The structure of the FeMo cofactor with the central X ligand (X = C4-, C2-, N3-, N-, or O2-) has been determined by the density functional approach. The FeMo cluster with one of the proposed central atoms C4-, C2-, N3-, or N- has an optimized geometry, comparable with the high-resolution X-Ray crystallographic structure of the nitrogenase FeMo cofactor. When the O2- species is present, the FeMo cofactor has an expanded cage. Calculations in the gas phase show that an Fe-4 facet of the FeMo cluster binds NO and CO in end-on coordination with an exothermicity of 25 and 8 kcal mol(-1), respectively, while the singlet O-2 coordinates to the FeMo cluster with an endothermicity of 12 kcal mol(-1). Deoxygenization of the bound NO and CO by the proton-electron addition is favored energetically, which leads to penetration of the C or N atom into the FeMo cage and yields the FeMo(mu(6)-X) cluster. Antiferromagnetic coupling between the Fe sites and vibrational properties of the FeMo(mu(6)-X) cluster, as well as the Mo-bound homocitrate ligand ring opening, have been explored theoretically. Present results suggest that O2- is unlikely as a central anion and the central ligands are identifiable by their IR spectra. Predicted energetics indicates that the protonation opening of the homocitrate ligand ring at the Mo site is feasible.