The effect of counter cations on second-order asymmetric transformations in fac-Δ- and Λ-tris(R-cysteinato-N,S)cobaltate(III) complexes and the kinetics of mutarotation

Abstract

A rapid reversible inversion (epimerization) between the Δ-(R,R,R) and Λ-(R,R,R) diastereomers for fac-Δ- and Λ-tris(R-cysteinato-N,S)cobaltate(III) occurs at room temperature in an aqueous solution of each salt. The half-lives for the mutarotation were measured (k = k_Λ + k_Δ): 80 and 6 minutes at 20 and 40 °C, respectively. The equilibrium constants, K = [Λ-(R,R,R)]/[Δ-(R,R,R)], are 1.2 at 20 °C and 1.0 at 40 °C, which means that the cobalt inversion rate constant k_Λ nearly equals k_Δ. Eight kinds of metal salts, M₃[Co(R-cys-N,S)₃] [M = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺; M_1/2 = Ca²⁺, Sr²⁺, Ba²⁺], were prepared. It was found that all these salts show a second-order asymmetric transformation under a solid-solution equilibrium at room temperature of the kind first demonstrated for the Ba²⁺ salt by Arnold and Jackson. The phenomenon led to the Δ-(R,R,R) diastereomer for the salts of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Ca²⁺ and Sr²⁺, but to the Λ-(R,R,R) diastereomer for Ba²⁺. These results could be clearly explained by solubility measurements of each diastereomeric salt pair.