Titanium-substituted heteropolytungstates as model catalysts for studying the mechanisms of selective oxidation by hydrogen peroxide

The P-31 NMR method shows that four forms of titanium(Iv)-monosubstituted K eggin-type heteropolytungstate (Ti-HPA) exist in MeCN: the dimer (BU4N)(7)[ {PTiW11O39}(2)OH] (in the abbreviated form, (PW11Ti)(2)OH or H1), its conju gate base (PW11Ti)(2)O (1), and two monomers, (PWTiO)-Ti-11 (2) and PW11TiO H (H2). The ratio between the forms depends on the concentrations of H+ and H2O. Dimer H1 is produced from 2 in MeCN when H+(1.5 mol) is added, and mo nomer H2 is the key intermediate in this process. The catalytic activity of Ti-HPA in the oxidation of thioethers by H2O2 correlates with their activi ty in peroxo complex formation and decreases in the order H2 > H1 > 2. The reaction of 2 with H2O2 in MeCN occurs slowly to form the inactive peroxo c omplex PW11TiO2 (A). The addition of H2O2 to H1 and H2 most likely results in the formation of the active hydroperoxo complex PW11TiOOH (B). Complexes A and B transform into each other when H+ or OH- (1 mel) is added per 1 mo l of A or B, respectively. The activity of B toward thioethers in the stoic hiometric reaction is proven by P-31 NMR and optical spectroscopy.