Sulfur K-Edge X-ray Absorption Spectroscopy and Density Functional Calculations on Mo(IV) and Mo(VI)O Bis-dithiolenes: Insights into the Mechanism of Oxo Transfer in DMSO Reductase and Related Functional Analogues

Sulfur K-edge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations have been used to determine the electronic structures of two Mo bis-dithiolene complexes, [Mo(OSi)(bdt)<sub>2</sub>]<sup>1−</sup> and [MoO(OSi)(bdt)<sub>2</sub>]<sup>1−</sup>, where OSi = [OSiPh<sub>2</sub><sup><i>t</i></sup>Bu]<sup>1−</sup> and bdt = benzene-1,2-dithiolate(2−), that model the Mo(IV) and Mo(VI)O states of the DMSO reductase family of molybdenum enzymes. These results show that the Mo(IV) complex undergoes metal-based oxidation unlike Mo tris-dithiolene complexes, indicating that the dithiolene ligands are behaving innocently. Experimentally validated calculations have been extended to model the oxo transfer reaction coordinate using dimethylsulfoxide (DMSO) as a substrate. The reaction proceeds through a transition state (TS1) to an intermediate with DMSO weakly bound, followed by a subsequent transition state (TS2) which is the largest barrier of the reaction. The factors that control the energies of these transition states, the nature of the oxo transfer process, and the role of the dithiolene ligand are discussed.