Effects of solvents on the reactions of coordination complexes. Part 20. Kinetics and mechanism of base hydrolysis of (αβS) (Salicylato) (Tetraethylenepentamine) Cobalt(III) in aquo-organic solvent media

Abstract

The kinetics of base hydrolysis of (αβSS) (salicylato) (tetraethylenepentamine) cobalt(III) have been investigated in aquo-organic solvent media at 15.0 < t, °C < 40.0, and I = 0.10 mol dm (ClO₄⁻) using propane-2-ol (~70% v/v), t-butanol (~60% v/v), acetone (~70% v/v), acetonitrile (~50% v/v), and ethylene glycol (~70% v/v) as cosolvents. Both the spontaneous and base-catalyzed hydrolysis of the phenoxide species [(tetren)CoO₂CC₆H₄O]⁺ were appreciably accelerated by the cosolvents PriOH, ButOH, Me₂CO, and MeCN. On the contrary the base hydroylsis (k₂) was retarded while spontaneous aquation (k₁) was accelerated to a small extent with increased EG content. Variation of log k₁ and log k⁰₂ (k⁰₂ = k₂ at I = 0) with mole fraction (X0.S) or reciprocal of the relative permitivity (D^s−1) of the media were nonlinear. The transfer free energy of the transition state relative to that of the initial state of the substrate for transfer of species from water to mixed solvents also varied nonlinearly with _X0.S, or D_s^s−1 indicating solvent specificity. The activation parameters, ΔH^≠ and ΔS^≠ varied nonlinearly with solvent composition exhibiting extrema. The preferential solvation and solvent structural effects mediated the kinetics and energetics of the reaction.