Effects of solvent on the reactions of coordination complexes. Part 24. Kinetics of base hydrolysis of some (aminomonocarboxylato)(tetraethylene-pentamine)cobalt(III) complexes in methanol+water media: The role of substrate hydrophobicity and solvent structure

Abstract

The kinetics of base hydrolysis of some (aminomonocarboxylato)(tetraethylenepentamine)cobalt(III) complexes, [(tetren)CoO₂CR]²⁺ (R=-NH₂CH₂, pyridine-2—, —NH₂CH₂CH₂, — NH₂CH(CH₃) (α βS isomer); R=— NH₂CH(CH₃) (α βR isomer)), have been investigated in methanol-water media (0-80 vol % MeOH) at 15.0≤t°C≤40.0 (0.02 mol dm^-3 NaOH). The second-order rate constant at zero ionic strength, k₂°, increases nonlinearly with X_MeOH. The transfer free energy of the initial state and the transition state of the amido conjugate base ([Δ_tG (i)]_(s←w)) for the glycinato- and pyridine-2-carboxylato complexes have been calculated using the solubility data of their picrate salts, pKNH date of their N-protonated forms, and the k₂° values in mixed solvent media. The kinetic solvent effects have been interpreted in terms of preferential solvation of the initial state, transition state, and the solvent structure. The activation enthalpies and entropies varied nonlinearly with X_MeOH displaying extrema, which is attributable to the solvent structural effects on these thermodynamic parameters. It is also evident that the mutation process, α βR→α βS isomer for the α-alaninato complex, where this isomerisation refers to the arrangement of the tetren skeleton around the planar secondary NH is sensitive to the nature of the cosolvent molecules and solvent structure. The mutation process is generally more favorable for the five coordinate amido conjugate bases than the initial state.