DENSITY FUNTIONAL STUDY OF THE STRUCTURE OF THE FeMo COFACTOR WITH AN INTERSTITIAL ATOM AND HOMOCITRATE LIGAND RING OPENING

Abstract

The structure of the FeMo cofactor with the central X ligand ( X = C 4-, C 2-, N 3-, N -, or O 2-) has been determined by the density functional approach. The FeMo cluster with one of the proposed central atoms C 4-, C 2-, N 3-, or N - has an optimized geometry, comparable with the high-resolution X-Ray crystallographic structure of the nitrogenase FeMo cofactor. When the O 2- 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 (μ6- X ) cluster. Antiferromagnetic coupling between the Fe sites and vibrational properties of the FeMo (μ6- X ) cluster, as well as the Mo-bound homocitrate ligand ring opening, have been explored theoretically. Present results suggest that O 2- 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.