DFT characterization on the mechanism of sulfoxidation with H2O2 catalyzed by tetranuclear peroxotungstates [XO4{WO(O-2)(2)}(4)](n-) (X = Si-IV, P-V, S-VI, As-V, and Se-VI)

摘要

A thorough theoretical analysis was carried out on the sulfoxidation with H2O2 catalyzed by a tetranuclear peroxotungstate [SiO4{WO(O-2)(2)}(4)](4-). The active species is the [SiO4{WO(O-2)(2)}(4)(H2O2)](4-) (SiW4(H2O2)) complex rather than [SiO4{WO(O-2)(2)}(4)](4-) (SiW4). The catalytic cycle consists of three elementary processes: oxygen transfer, sulfoxide dissociation, and catalyst regeneration. The oxygen transfer occurs from the peroxo oxygen atom O1 of SiW4(H2O2) to the sulfur center of dimethyl sulfide with a moderate Gibbs activation energy (Delta G degrees) of 17.1 kcal mol(-1). By comparing potential energy surfaces and condensed Fukui functions (f(+)), the electrophilicity of the outer peroxo atoms in SiW4(H2O2) determines which oxygen transfers to the dimethyl sulfide. Then, the sulfoxide dissociation proceeds with a small Delta G degrees value of 2.3 kcal mol(-1) by elongation of the peroxo O1-O4 distance and elimination of the product di-methylsulfoxide. Finally, the catalyst regeneration is found to occur via two successive proton transfers from H2O2 to the oxygen atoms of peroxotungstates with the Delta G degrees values of 15.9 and 15.3 kcal mol(-1), which has been firstly examined in the present study. All of these steps occur easily with moderate Delta G degrees values, but the oxygen transfer is the rate-determining step of this catalytic reaction. In addition, the catalytic activity of peroxotungstates can be effectively tuned by changing the heteroatom X of [XO4{WO(O-2)(2)}(4)(H2O2)](n-) in the order: Se-VI approximate to S-VI > As-V approximate to P-V > Si-IV.