Mössbauer Electron Paramagnetic Resonance and Theoretical Study of a Carbene-based All-ferrous Fe4S4 Cluster: Electronic Origin and Structural Identification of the Unique Spectroscopic Site

摘要

It is well established that the cysteinate-coordinated [Fe4S4] cluster of the Fe-protein of nitrogenase from A. vinelandii (Av2) can attain the all-ferrous core oxidation state. Mössbauer and 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 ⁵⁷Fe quadrupole interactions. Recently Deng and Holm reported the synthesis of [Fe4S4(Prⁱ2NHCMe2)4],² >1, (Prⁱ2NHCMe2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) and showed that the all-ferrous carbene-coordinated cluster is amenable to physico–chemical studies. Mössbauer 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 T2 distortion a minimum with spin S = 4 and a C3v core structure in which one iron is unique and three irons are equivalent. This minimum has all 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 [4Fe-4S]⁰ core both in complex >1 and in Av2.