Titanium-Substituted Heteropolytungstates as Model Catalysts for Studying the Mechanisms of Selective Oxidation by Hydrogen Peroxide

The 31P NMR method shows that four forms of titanium(IV)-monosubstituted Keggin-type heteropolytungstate (Ti–HPA) exist in MeCN: the dimer (Bu4N)7[{PTiW11O39}2OH] (in the abbreviated form, (PW11Ti)2OH or H1), its conjugate base (PW11Ti)2O (1), and two monomers, PW11TiO (2) and PW11TiOH (H2). The ratio between the forms depends on the concentrations of H+and H2O. Dimer H1is produced from 2in MeCN when H+(1.5 mol) is added, and monomer H2is the key intermediate in this process. The catalytic activity of Ti–HPA in the oxidation of thioethers by H2O2correlates with their activity in peroxo complex formation and decreases in the order H2> H1> 2. The reaction of 2with H2O2in MeCN occurs slowly to form the inactive peroxo complex PW11TiO2(A). The addition of H2O2to H1and H2most likely results in the formation of the active hydroperoxo complex PW11TiOOH (B). Complexes Aand Btransform into each other when H+or OH–(1 mol) is added per 1 mol of Aor B, respectively. The activity of Btoward thioethers in the stoichiometric reaction is proven by 31PNMR and optical spectroscopy.