Electron transfer-oxygen transfer oxygenation of sulfides catalyzed by the H₅PV₂Mo₁₀O₄₀ polyoxometalate

The oxygenation of sulfides to the corresponding sulfoxides catalyzed by H₅PV₂Mo₁₀O₄₀ and other acidic vanadomolybdates has been shown to proceed by a low-temperature electron transfer-oxygen transfer (ET-OT) mechanism. First, a sulfide reacts with H ₅PV₂Mo₁₀O₄₀ to yield a cation radical-reduced polyoxometalate ion pair, R₂^+·, H₅PV^IVV^VMo₁₀O₄₀, that was identified by UV-vis spectroscopy (absorptions at 650 and 887 nm for PhSMe ^+· and H₅PV^IVV^VMo ₁₀O₄₀) and EPR spectroscopy (quintet at g = 2.0079, A = 1.34 G for the thianthrene cation radical and the typical eight-line spectrum for V^IV). Next, a precipitate is formed that shows by IR the incipient formation of the sulfoxide and by EPR a VO²⁺ moiety supported on the polyoxometalate. Dissolution of this precipitate releases the sulfoxide product. ET-OT oxidation of diethylsulfide yielded crystals containing [V(O)(OSEt₂)_x(solv)_5-x]²⁺ cations and polyoxometalate anions. Under aerobic conditions, catalytic cycles can be realized with formation of mostly sulfoxide (90%) but also some disulfide (10%) via carbon-sulfide bond cleavage.