Base hydrolysis of amino-acid esters and amides in the co-ordination sphere of cobalt(III). Part II. Hydrolysis of 2-aminoethyl acetate

Abstract

Base hydrolysis of the complex cis-[Co(en)₂X(NH₂CH₂CH₂OCOMe)]²⁺(en = ethylenediamine; X = Cl or Br) has been studied by pH-stat and spectrophotometry at 25 °C and I= 0·1M. Two consecutive reactions occur in basic media. Following loss of halide ion (k_OH^Cl= 1·38 × 10³, k_OH^Br= 8·7 × 10³|mol^–1 min^–1), slower base hydrolysis of the ester function occurs (k_OH^ester= 1·44 × 10²|mol^–1 min^–1). Initial base hydrolysis of halide ion is consistent with the observations that (a) rate constants for the slow hydrolysis steps in the bromo- and chloro-complexes are identical, within experimental error, and (b) a single rate process is observed spectrophotometrically with k_OH= 1·42 × 10²|mol^–1 min^–1. In the pH-stat measurements 2 mol of base are consumed per mol of complex and the final product of the reaction is the penta-aminohydroxo-complex which has been identified by visible spectra and potentiometric titration. The pK_a^p value for ionisation (i) is 6·1 at 25 °C and I= 0·1M. [Co(en)₂OH₂(NH₂CH₂CH₂OH)]³⁺⇌[Co(en)₂OH(NH₂CH₂CH₂OH)]²⁺+ H⁺(i) Hydrolysis of the complex cis-[Co(en)₂Cl(NH₂CH₂CH₂OCOMe)]²⁺ in ca. 6M-hydrochloric acid gives the corresponding penta-aminechloro-complex of 2-aminoethanol. Base hydrolysis of the penta-aminechloro-complex. containing N-co-ordinated 2-aminoethanol requires 1 mol of base for complete hydrolysis (k_OH= 1·23 × 10³|mol^–1 min^–1 by pH-stat and spectrophotometry at 25 °C and I= 0·1M). It is estimated that base hydrolysis of 2-aminoethyl acetate in the dipositive cobalt(III) complex is some 18 times faster than that for the free ligand and some 2 times faster than that for the protonated ligand. Mercury(II)-catalysed aquations of the complex cis-[Co(en)₂Cl(NH₂CH₂CH₂OCOMe)]²⁺ and the corresponding 2-aminoethanol derivative have also been studied. The reactions follow a rate law, rate =k[Hg^II][Complex] in 0·57M-HClO₄. There is evidence for neighbouring-group participation (NGP–5OH) in aquation of the 2-aminoethanol derivative.