Electron Transfer-Oxygen Transfer Oxygenation of Sulfides Catalyzed by the H_5PV_2Mo_(10)O_(40) Polyoxometalate

摘要

The oxygenation of sulfides to the corresponding sulfoxides catalyzed by H_5PV_2Mo_(10)O_(40) and other acidic vanado-molybdates has been shown to proceed by a low-temperature electron transfer-oxygen transfer (ET-OT) mechanism. First, a sulfide reacts with H_5PV_2Mo_(10)O_(40) to yield a cation radical-reduced polyoxometalate ion pair, R_2~+,H_5PV~ⅣV~ⅤMo_(10)O_(40), that was identified by UV-vis spectroscopy (absorptions at 650 and 887 nm for PhSMe+ and H_6PV~ⅣV~ⅤMo_(10)O~(40)) 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 VO~2+ 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.