Amino-acid complexes of cobalt(III). Preparation and hydrolysis reactions of trans-bis(1,2-diaminoethane)(amino-carboxylato)halogeno-cobalt(III) complexes containing the carboxylato-bonded amino-acids glycine, DL-alanine, and DL-aminobutyric acid

Abstract

A series of complexes of the type trans-[Co(en)₂X{^–O₂C–CH(R)–NH₃⁺}][ClO₄]₂(X = Cl^– or Br^–), containing the monodentate carboxylato-bonded amino-acids, glycine (R = H), DL-alanine (R = Me), and DL-aminobutyric acid (R = Et) have been prepared and characterised. All the complexes behave as monobasic acids and pK_a values for the equilibrium (1) have been determined potentiometrically at 25 ± 0.1 °C. The kinetics and steric course of trans-[Co(en)₂X{^–O₂C–CH(R)–NH₃⁺}]²⁺ [graphic omitted] trans-[Co(en)₂X{^–O₂C–CH(R)–NH₂}]⁺+ H⁺(1) base hydrolysis (X^– displacement) of the species trans-[Co(en)₂X{^–O₂C–CH(R)–NH₂}]⁺ have been examined. Values of ΔH^‡(298.15) lie between 92.9 and 96.6 kJ mol^–1 for all the complexes studied. Values of ΔS^‡(298.15) for the chloro-complexes lie in the region 76–87 J K^–1 mol^–1 and for the bromo-complexes in the region 92–101 J K^–1 mol^–1. Base hydrolysis of the DL-alanine and DL-aminobutyric complexes give 65 ± 5%trans- and 35 ± 5%cis-[Co(en)₂OH{^–O₂C–CH(R)–NH₂}]⁺. The products of base hydrolysis of the glycine complexes contain in addition minor quantities of the N-bonded complex [Co(en)₂OH(NH₂CH₂CO₂^–)]⁺. The kinetics and steric course of the Hg (II)-assisted aquation of the complexes trans-[Co(en)₂X{^–O₂C–CH(R)–NH₃⁺}]²⁺ have also been studied. For the chloro-complexes ΔH^‡(298.15) values of 80.9–82.2 kJ mol^–1 and ΔS^‡(298.15) values of 0.96–4.8 J K^–1 mol^–1 have been obtained, while for the bromo-complexes ΔH^‡(298.15) values lie in the range 74.6–75.4 kJ mol^–1 with ΔS^‡(298.15) values in the range 28–35 J K^–1 mol^–1. The products of this reaction are 85 ± 5%cis-, 15 ± 5%trans-[Co(en)₂OH₂{^–O₂C–CH(R)–NH₃⁺}]³⁺.