DFT characterization on the mechanism of sulfoxidation with H2O2 catalyzed by tetranuclear peroxotungstates [XO4{WO(O2)2}4](n-) (X = Si(IV), P(V), S(VI), As(V), and Se(VI)).

A thorough theoretical analysis was carried out on the sulfoxidation with H2O2 catalyzed by a tetranuclear peroxotungstate [SiO4{WO(O2)2}4](4-). The active species is the [SiO4{WO(O2)2}4(H2O2)](4-) (SiW4(H2O2)) complex rather than [SiO4{WO(O2)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 (ΔG°(‡)) of 17.1 kcal mol(-1). By comparing potential energy surfaces and condensed Fukui functions (ƒ(+)), 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 ΔG°(‡) value of 2.3 kcal mol(-1) by elongation of the peroxo O1-O4 distance and elimination of the product dimethylsulfoxide. Finally, the catalyst regeneration is found to occur via two successive proton transfers from H2O2 to the oxygen atoms of peroxotungstates with the ΔG°(‡) 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 ΔG°(‡) 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(O2)2}4(H2O2)](n-) in the order: Se(VI) ≈ S(VI) > As(V) ≈ P(V) > Si(IV).