Effect of solvent on the reaction of coordination complexes. 10. Kinetics of solvolysis of cis (chloro)(1-aminopropane-2-ol)bis-(ethylenediamine)cobalt(III) in MeOH + water, propane-2-ol + water and t-Butanol + water media

Abstract

The solvolysis of cis (chloro)(1-amino-propan-2-ol)bis(ethylenediamine)cobalt(lll) in aqueous alcoholic media using methanol, propan-2-ol, t-butanol as cosolvents, resulted in the formation of the (N,O) chelated product cis[Co(en)₂(NH₂CH₂CHOHCH₃)]³⁺. The pseudo first order rate constant decreased with increasing molfraction (X_org) of alcohols, the decrease being less marked as the bulkiness of hydrophobic moiety of alcohol increased. The plots of log k_obs vs. reciprocal of the bulk dielectric constant of the solvents, log k_obs vs. Grunwald-Winstein solvent parameter and log k_obs vs. X_org under isodielectric condition (D_s = 50, at 50°C) for CH₃0H/H₂0, C₂HsOH/H₂0, (CH₃)₂CHOH/H₂0, (CH₂)₃C-OH/H₂0, (CH₂lz(OH)₂/H₂0, and (CH₂)₂ C=O/H₂0 in water rich media indicated that both solvent structural effects and presumably the hydrophobic interaction appreciably mediate the reaction. The calculated values of the relative transfer free energy at 25°C [ΔG_t(C³⁺)-ΔG_t(i.s)_(s←w)) where C³⁺ and i.s. denote the dissociative transition state {cis[Co(en)₂NH₂CH₂CHOHCH₃)]³⁺}^∗ and the initial state, respectively, indicated that the tripositive transition state is more effectively solvated by the mixed solvent media, than the dipositive initial state, the effect appeared to be more significant with increasing X_org. The plots of activation enthalpy and entropy against X_org exhibited maxima and minima indicating that enthalpy and entropy changes associated with the solvent shell reorganization of the reactant contribute to the overall activation parameters.