Catalytic Effect of $MO_4^{2-_4}$ (M=Cr, Mo and W) on Hydrolyses of Carbon and Phosphorus Esters

Second-order rate constants have been measured spectrophotometrically for the hydrolysis of p-nitrophenyl acetate (PNPA) and p-nitrophenyl diphenylphosphinate (PNPDPP) with MO4 2- (M = Cr, Mo and W) in phosphate buffer (pH = 8.00) at 35.0 o C. These MO4 2- species exhibit large catalytic effect in the hydrolysis of PNPA and PNPDPP except WO4 2- in the reaction with PNPA. The catalytic effect of these MO 4 2- species has been observed to be much more significant in the hydrolysis of PNPDPP than in the hydrolysis of PNPA. Since the smallest CrO4 2- would be most highly solvated by H2O molecules, CrO4 2- is expected to exhibit the least catalytic effect, if solvation effect is the most important factor. However, in fact, CrO 4 2- shows the highest catalytic effect toward PNPA, indicating that solvation effect is not solely responsible for the catalytic effect. The most basic CrO4 2- shows the highest catalytic effect, while the least basic WO 4 2- is least reactive toward PNPA, indicating that the basicity of MO 4 2- might be an important factor. However, in the hydrolysis of PNPDPP, no correlation is observed between the basicity and catalytic effect, suggesting that basicity alone can not be responsible for the catalytic effect shown by the MO 4 2- species. Formation of a chelate is suggested to be responsible for the high catalytic effect of MO 4 2- in the hydrolysis reaction of PNPA and PNPDPP. The formation of chelate is considered to be more suitable for the reaction with PNPDPP than with PNPA based on the larger catalytic effect observed in the reaction with PNPDPP than with PNPA.