Mossbauer, Electron Paramagnetic Resonance, and Theoretical Studies of a Carbene-Based All-Ferrous Fe4S4 Cluster: Electronic Origin and Structural Identification of the Unique Spectroscopic Site

It is well established that the cysteinate-coordinated [Fe4S4] cluster of the iron protein of nitrogenase from Azotobacter vinelandii (Av2) can attain the all-ferrous core oxidation state. Mossbauer and electron paramagnetic resonance (EPR) studies have shown that the all-ferrous cluster has a ground-state spin S = 4 and an effective 3:1 site symmetry in the spin structure and Fe-57 quadrupole interactions. Recently, Deng and Holm reported the synthesis of [Fe4S4((Pr2NHCMe2)-N-i)(4)](2), (1; (Pr2NHCMe2)-N-i = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) and showed that the all-ferrous carbene-coordinated cluster is amenable to physicochemical studies. Mossbauer and EPR studies of 1, reported here, reveal that the electronic structure of this complex is strikingly similar to that of the protein-bound cluster, suggesting that the ground-state spin and the 3:1 site ratio are consequences of spontaneous distortions of the cluster core. To gain insight into the origin of the peculiar ground state of the all-ferrous clusters in 1 and Av2, we have studied a theoretical model that is based on a Heisenberg-Dirac-van Vleck Hamiltonian whose exchange-coupling constants are a function of the Fe-Fe distances. By combining the exchange energies with the elastic deformation energies in the harmonic approximation, we obtain for a T-2 distortion a minimum with spin S = 4 and a C-3v core structure in which one iron is unique and three irons are equivalent. This minimum has all of the spectroscopic and structural characteristics of the all-ferrous clusters of 1 and Av2. Our analysis maps the unique spectroscopic iron site to a specific site in the X-ray structure of the [Fe4S4](0) core both in complex 1 and in Av2.