Effect of solvent on reactions of coordination complexes. Part 12. Kinetics and mechanism of chloride substitution reactions ofcis-(chloro)(2-aminoethanol)bis(ethylenediamine) cobalt(III) andcis-(chloro)(1-aminopropan-2-ol)-bis(ethylenediamine) cobalt(III) ions in acidic and basic ethylene glycol-water media

Abstract

The loss of chloride ion from the title complexes resulted in the predominant formation of the chelated amino-alcohol productscis-[Co(en)₂(NH₂CH₂CH(X)O/H)]^2+/3+ (X=H or Me). The kinetics of chloride release were investigated in aqueous ethylene glycol (EG) media (0 to 80% by wt of EG) at 40°-65°C in acidic media and at 20°-35°C in basic media. The rate constants decreased linearly with increasing mol fraction of the cosolvent. The plots of log kversus D_s ^-1 (S_s=bulk dielectric constant, k=first order or second order rate constants) were essentially linear with negative slopes for the reactions in an acidic medium, and tended to be curved for the base catalysed reactions. The activation enthalpies and entropiesversus X_EG (X_EG=mol fraction of EG) plots indicated extrema which might be associated with the effects of the solvent structural changes on these thermodynamic parameters. The observed solvent isotope effect (k_H2O /k_D2O) = 1.12 at 50°C, [HClO₄]=0.010 mol dm^-3 for Cl^- release was lower than the value for the aquation ofcis-[Co(en)₂(alkylamine)Cl]²⁺ complexes (k_H2O /k_D2O = 1.38 - 1.44) reported in the literature. This is consistent with the lack of direct solvent molecule participation in the actual act of substitution at the cobalt(III) centre, as expected for a true intramolecular reaction.