Phosphate and phosphonate ester hydrolysis promoted by dinuclear metal complexes

Abstract

The kinetics and mechanism of the hydrolysis of bis(4-nitrophenyl)phosphate and (4-nitrophenyl)phenylphosphonate were studied in the presence of the dinuclear Ni2+, Cu2+, and Zn2+ complexes of 1,1-[(1H-pyrazole-3,5-diyl)bis(methylene)]bis(octahydro-1H-1,4,7-triazonine) (LH). Whereas the dinuclear Cu2+ complex does not promote the hydrolysis of these substrates, the dinuclear complexes of Zn2+ and Ni2+ are good catalysts for it. It has been shown by kinetical and 31P NMR studies that the monohydroxo complex [M2(LH−2)]2+ binds the substrate to give an intermediate species, in which an intramolecular attack of the coordinated OH− onto the substrate takes place. Phosphate monoester or phenylphosphonate which are the products of the hydrolysis remain coordinated to the dinuclear complex, and, thus inhibit the further reaction. From the Michaelis–Menten type reactivity which we have observed at high substrate concentrations, KM and kcat values were obtained. The values of KM are very similar to each other as well as for the two substrates as for the two metal ions. In contrast, kcat values are distinctly different, the phosphonate ester being more reactive than the phosphate diester by a factor of 3–4. The major effect is however, due to the metal ion, the dinuclear Ni2+ complex being 50–60 times more reactive than the Zn2+ complex.