DFT study of the intrinsic conformations of [2Fe–2S–4(SCH3)]2− clusters and their influence on exchange coupling

We have determined the equilibrium conformations of the diiron(III) cluster [2Fe–2S–4(SCH3)]2− using density functional theory. The conformers have dihedral Fe–Fe–S–C angles of ∼0° and ±120°. The relative energies of the conformers can be accurately parameterized with a small number of side-chain repulsion parameters. Of the 17 conformers identified on the basis of the ideal values for the dihedrals, 10 conformers are stable in both the ferromagnetic and broken symmetry state for the cluster. The exchange coupling constants for the seven energetically lowest conformers are predicted to belong to a narrow range, 150 cm−1 ⩽ J ⩽ 178 cm−1. The cluster conformers found in proteins do not coincide with any of the intrinsic ones, due to distortion of one of the dihedral angles under the influence of the protein scaffold.