DFT characterization on the mechanism of sulfoxidation with H2O2 catalyzed by tetranuclear peroxotungstates [XO4{WO(O2)2}4]n− (X = SiIV, PV, SVI, AsV, and SeVI)

Abstract

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.