COMPARATIVE-STUDY OF POLYOXOMETALATES AND SEMICONDUCTOR METAL-OXIDES AS CATALYSTS - PHOTOCHEMICAL OXIDATIVE-DEGRADATION OF THIOETHERS

The photochemical degradation of thioether substrates catalyzed by representative semiconductor metal oxides and sulfides (anatase TiO2, SnO2, cubic WO3, and CdS) and photoredox-active early-transition-metal polyoxometalates (W10O32(4-), PMo12O40(3-), PW12O40(3-), SiMo12O40(4-), PV2Mo10O40(5-), Cu(II)W11PO39(5-), and P2W18O62(6-)) have been examined under both anaerobic and aerobic conditions. Under anaerobic conditions, all the semiconductors are completely ineffective at photochemically oxidizing or degrading the exemplary thioether substrate tetrahydrothiophene (THT) in the oxidatively resistant solvent acetonitrile. In contrast, all the homogeneous polyoxometalate systems under the same reaction conditions, except the neutral tetra-n-butylammonium (Q) salt of PW12O40(3-), are quite effective. The latter systems generate products derived from the carbon-based radical alpha to the sulfur atom and not sulfoxide or sulfone, the usual products of thioether oxidation by oxometal species. The rate for the most active anaerobic system, that involving the photochemical degradation of THT by Q4W10O32, under optically dilute conditions, is first order in W10O32(4-) and light intensity and variable order in THT substrate. A rate law consistent with these data is given. Upon addition of O2, TiO2 (with or without Pt(0)) becomes highly active, SnO2 becomes active, but WO3 and CdS remain inactive. Reactivity in thioether oxidation is dominated by the interactions of the semiconductors with O2 and O2-derived intermediates; there is no correlation between reactivity and semiconductor band gap. Upon addition of O2, all the polyoxometalate systems become more active. Both the semiconductors and polyoxometalates under aerobic conditions oxidize thioethers further than the sulfoxides or sulfones to a range of products.