Kinetics and mechanism of the cobalt(III) tetraammine complex-promoted hydrolysis of 4-nitrophenyl glycinate

The kinetics of the hydrolysis of 4-nitrophenyl glycine ester (PNPG) catalysed by [Co(OH)(trien)(OH2)](2+), [Co(OH)(tren)(OH2)](2+) and [Co(OH)(en)(2)(OH2)](2+) complexes has been studied spectrophotometrically in weakly basic aqueous media (pH = 6.5 to 7.4). Kinetic experiments were carried out as a function of the pH, complex concentration and temperature. The rate of hydrolysis increases linearly with the complex concentration with a trend to wards rate saturation, suggesting the formation of associative species in a pre-equilibrium step. The pseudo-first order rate constant, k(obs), increases rapidly with a decrease in the hydrogen-ion concentration. The complexes promote the hydrolysis of 4-nitrophenyl glycinate significantly, and the acceleration rate is about 400-600. An attack of external OH- on the chelated ester is suggested as a probable mechanism for the hydrolysis. The lower rate enhancements observed in the present study could probably be ascribed to a weaker cobalt(III) alkoxy carbonyl interaction in the chelate, owing to a decreased nucleophilicity of the carbonyl oxygen in the p-nitrophenyl ester. The activation parameters for all three complex-promoted reactions are found to be comparable, thus suggesting a common mechanism operative in all complex catalysed reactions.