The relevance of acid-base equilibria in the catalytic oxidations by tungsten and molybdenum peroxo complexes

The catalytic behaviour of the two peroxo complexes MO(O2)2HMPT· H2O (M = W(VI), Mo(VI); HMPT = hexamethylphosphoric acid triamide) in the oxidations by hydrogen peroxide in ethanol has been examined. Rate data, obtained in a model reaction, i.e. the oxidation of organic sulphides to the corresponding sulphoxides, indicate that in 'neutral' ethanol the Mo(VI)-catalyzed system is more efficient than the W(VI) one. Spectroscopic (1H NMR), potentiometric and kinetic experiments suggest that this is mainly due to the much stronger acidity of the W(VI)-peroxo complex which is almost completely dissociated into its anionic form, and which is a poor electrophilic oxidant. In fact, when the anions are neutralized upon addition of methanesulphonic acid, the neutral peroxotungsten complex appears to be a much more effective oxidizing agent than the structurally similar peroxomolybdenum compound. An additional advantage of the W(VI)-catalyzed system may be found in the reduced affinity of W(VI)-peroxo species, as compared with Mo(VI), for neutral ligands. Therefore HMPT, even at the highest catalytic concentrations employed, is completely removed from the coordination sphere of tungsten