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

摘要

A rapid reversible inversion (epimerization) between the #DELTA#-(R, R, R) and #LAMBDA#-(R, R, R,) diastereomers for fac-#DELTA#- and #LAMBDA#-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 (#kappa#=#kappa#_#LAMBDA#+#kappa#_#DELTA#): 80 and 6 minutes at 20 and 40 deg C, respectively. The equilibrium constants, K=[#LAMBDA#-(R, R, R)]/[#DELTA#-(R, R, R)], are 1.2 at 20 deg C and 1.0 at 40 deg C, which means that the cobalt inversion rate constant #kappa#_#LAMBDA# nearly equals #kappa#_#DELTA#. Eight kinds of metal salts, M_3[Co(R-cys-N, S)_3] [M=Li~+, Na~+, K~+, Rb~+, Cs~+, M_(1/2)=Ca~(2+), Sr~(2+), Ba~(2+)], 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 Ba~(2+) salt by Arnold and Jackson. The phenomenon led to the #DELTA#-(R, R, R) diastereomer for the salts of Li~+, Na~+, K~+, Rb~+, Cs~+, Ca~(2+) and Sr~(2+), but to the #LAMBDA#-(R, R, R,) diastereomer for Ba~(2+). These results could be clearly explained by solubility measurements of each diastereomeric salt pair.