Ligand K-edge X-ray absorption spectroscopy and DFT calculations on [Fe3S4]0,+ clusters:: Delocalization, redox, and effect of the protein environment

Ligand K-edge XAS of an [Fe3S4](0) model complex is reported. The pre-edge can be resolved into contributions from the mu(2)S(sulfide), mu(3)S(Sulfide), and S-thiolate ligands. The average ligand-metal bond covalencies obtained from these pre-edges are further distributed between Fe3+ and Fe2.5+ components using DFT calculations. The bridging ligand covalency in the [Fe2S2](+) subsite of the [Fe3S4](0) cluster is found to be significantly lower than its value in a reduced (Fe2S2] cluster (38% vs 61%, respectively). This lowered bridging ligand covalency reduces the superexchange coupling parameter J relative to its value in a reduced [Fe2S2](+) site (-146 cm(-1) vs -360 cm(-1), respectively). This decrease in J, along with estimates of the double exchange parameter Band vibronic coupling parameter lambda(2)/k-, leads to an S = 2 delocalized ground state in the [Fe3S4](0) cluster. The S K-edge XAS of the protein ferredoxin 11 (Fd 11) from the D. gigas active site shows a decrease in covalency compared to the model complex, in the same oxidation state, which correlates with the number of H-bonding interactions to specific sulfur ligands present in the active site. The changes in ligand-metal bond covalencies upon redox compared with DFT calculations indicate that the redox reaction involves a two-electron change (one-electron ionization plus a spin change of a second electron) with significant electronic relaxation. The presence of the redox inactive Fe3+ center is found to decrease the barrier of the redox process in the [Fe3S4] cluster due to its strong antiferromagnetic coupling with the redox active Fe2S2 subsite.