Enhanced Base Hydrolysis of Coordinated Phosphate Esters: The Reactivity of an Unusual Cobalt(III) Amine Dimer

Article abstract of DOI:10.1021/ja00337a026

The hydrolysis of the dimeric cation bis(μ-4-nitrophenyl phosphato)bis(2+) has been studied at pH 10 and over the hydroxide concentration range 0.05-1.0 M.Product distribution, kinetics (involving 4-nitrophenol release), and 31P NMR and 18O tracer studies were carried out to establish the course of the reaction.In a rapid first step, the eight-membered ring of the dimer is opened by rupture of one of the Co-O bonds (S_N1cB) to give a cis hydroxo complex.The ring-opened species reacts further via two competing pathways: (a) intramolecular attack of the coordinated hydroxide upon the bridging phosphate ester moiety and (b) further cleavage of the dimer by base-catalyzed (S_N1cB) rupture of some Co-O bonds.Route a results in ester hydrolysis with the concomitant formation of a chelated bridging phosphate species whereas route b yields cis- and trans-hydroxy(p-nitrophenyl phosphate)bis(1,2-ethanediamine)cobalt(III).The phosphate chelate ring in the initial product of path a is subsequently opened by Co-O bond rupture and the resultant dimeric bridging phosphato species slowly decomposes to cis- and trans-hydroxo(phosphato)bis(1,2-ethanediamine)cobalt(III).Comparison of the rate data for hydrolysis of the dimer and the cis-hydroxo(4-nitrophenyl phosphato)bis(1,2-ethanediamine)cobalt(III) ion indicates that the attack by the intramolecular nucleophile is largely responsible for the enhanced rate of ester hydrolysis (ca 1E5) with a smaller contribution from charge neutralization at the P center by the metal ion (ca 10-100).Parallel kinetic studies on the analogous dimer bis(μ-4-nitrophenyl phosphato)bis(2+), previously incorrectly formulated as a monomeric species containing chelated phosphate ester, indicate that ester hydrolysis in this complex proceeds by a similar mechanism to that for the 1,2-ethanediamine complex.In total, the results are rationalized in relation to a possible mechanism for the Zn²⁺ containing enzyme E. coli alkaline phosphatase.