Kinetics and mechanism of the copper(II) promoted hydrolysis of the methyl ester of ethylenediaminemonoacetate

Abstract

Base hydrolysis of methyl ethylenediaminemonoacetate has been studied at I=0.1 mol dm−3 (NaClO4) over the pH range 7.4–8.8 at 25 °C. The proton equilibria of the ligand can be represented by the equations, where E is the free unprotonated ester species. Values of pK1 and pK2 are 4.69 andca. 7.5 at 25° (I=0.1 mol dm−3). For base hydrolysis of EH+, kOH=1.1×103 dm3 mol−1 s−1 at 25 °C. The species E is shown to undergo lactamisation to give 2-oxopiperazine (klact ca. 1×10−3 s−1) at 25 °C. Formation of the lactam is indicated both by u.v. measurements and by isolation and characterisation of the compound. Base hydrolysis of the ester ligand in the complex [CuE]2+ has been studied over a range of pH and temperature, k OH 25 =9.3×104 dm3 mol−1 s−1 with ΔH≠=107 kJ mol−1 and ΔS 298 ≠ =209 JK−1 mol−1. Base hydrolysis of [CuE]2+ is estimated to be some 1055 fold faster than that of the free ester ligand. The results suggest that base hydrolysis occursvia a chelate ester species in which the methoxycarbonyl group of the ligand is bonded to copper(II).