Base hydrolysis of amino-acid esters and amides in the co-ordination sphere of cobalt(III). Part I. Hydrolysis of methyl 6-aminohexanoate

Abstract

The base hydrolysis of cis-{Co(en)₂Cl[NH₂(CH₂)₅CO₂Me]}²⁺ has been studied by pH-stat and stopped-flow spectrophotometric techniques at 25 °C and I= 0·1M. Two consecutive reactions occur in base. Following loss of Cl^–(k_OH= 12·8 l mol^–1 s^–1 by pH-stat, k_OH= 13·6 ± 1 l mol^–1 s^–1 by stopped flow), a slower base hydrolysis of the ester function occurs (K_OH= 0·22 l mol^–1 s^–1). In the pH-stat measurements 2 moles of base are consumed per mole of complex and the final product of the reaction is the hydroxypenta-ammine which has been identified by visible spectra and potentiometric titration. Base hydrolysis of the complex cis-{Co(en)₂Cl[NH₂(CH₂)₅CO₂]}⁺ containing the N-co-ordinated amino-acid requires 1 mole of base for complete hydrolysis (k_OH= 9·5 l mol^–1 s^–1 for loss of chloride by pH-stat, k_OH= 9·2 l mol^–1 s^–1 determined spectrophotometrically in borax buffers at l= 0·1 M and 25 °C). For base hydrolysis of the unprotonated form of the free ester, NH₂(CH₂)₅CO₂Me + OH^–→ NH₂(CH₂)₅CO₂^–+ MeOH in the pH range 11·0–11·5 the value of k_OH= 0·148 l mol^–1 s^–1. Thus a slight rate enhancement (ca. 1·5 times) occurs for hydrolysis of the N-co-ordinated ester. Such an effect is understandable owing to the length of the alkyl chain separating the cobalt(III) centre from the ester function. The present results establish a further hydrolytic pathway for N-co-ordinated amino-esters in halogenopenta-ammine complexes of cobalt(III), quite different from the reactions of glycine esters where intramolecular pathways occur.