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From the perspective of modeling metallohydrolases, we initiated a program to systematically investigate hydrolytic reactions of metal-coordinated hydroxide ions in a dinuclear environment toward an unactivated ester such as ethyl acetate. Phenoxo-bridged dinuclear complexes of MnII, CoII, NiII, CuII, and ZnII have been synthesized and characterized by X-ray crystallography. Terminal N-donor ligands and terminal/bridging acetate and solvent molecules complete six-coordination around MII ions. Because of the presence of coordinated solvent molecules, these complexes act as catalysts for the hydrolysis of a model phosphate ester such as 2-hydroxypropyl-p-nirophenylphosphate (HPNP). In each case pH-metric titrations were done in aqueous-methanol medium to determine the pKa values of MII-coordinated water molecules. Consequently, the hydrolysis experiments were performed as a function of pH to set the optimal conditions for hydrolysis. The catalytic activity of these complexes towards model substrate HPNP has been investigated under saturation kinetics conditions. Comparative results from pH-titration and kinetic experiments have allowed us to rationalize the observed transesterication reaction, at a molecular level.
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