Kinetics and mechanism of the hydrolysis of α-amino-acid esters in mixed-ligand complexes with copper(II) ethylenediaminemonoacetate
Abstract
Amino-acid esters (EH+) interact with [Cu(edma)]+ to give mixed-ligand complexes in accordance with the equilibrium, [Cu(edma)]++ EH+⇌[Cu(edma)E]++ H+, where EH+ is the protonated ester [graphic omitted]H3CH(R)CO2R′ and edma–= ethylenediaminemonoacetate. The ternary complexes are only formed over a narrow pH range (7.3–8.2) and 8.4–8.8 with methyl L-histidinate. At higher pH values there is kinetic evidence for the competing equilibrium, [Cu(edma)E]++ OH–⇌[Cu(edma)(OH)]0+ E. Rate constants have been obtained at I= 0.1 mol dm–3 and 25 °C for both base hydrolysis and solvolysis of the ester group of a series of α-amino-acid esters, while co-ordinated (probably chelated) in the ternary complexes. Base hydrolysis is some 103 times faster than for the unprotonated esters (E), and possible mechanisms for these reactions are considered.