Medium effects on the reactions of coordination complexes: Base hydrolysisof(αβS)(p-hydroxybenzoato)(tetraethylenepentamine) cobalt(III) in iso-propanol–water, tert-butanol–water and dimethyl sulfoxide–water media

Abstract

The base hydrolysis of (αβS)(p-hydroxybenzoato)-(tetraethylenepentamine)cobalt(III) has been investigated in aqueous–organic solvent media using i-PrOH, t-BuOH and dimethyl sulfoxide (DMSO) as cosolvents at 20.0 ≤ T (°C) ≤ 40.0 (I=0.02 mol dm -3) with 80% (v/v) of cosolvents. Only the base-catalysed path (kobs=kOH[OH-]) is observed. The relative second order rate constant kOHos/kOHow at I=0 increases nonlinearly with increasing mol fraction (xO.S.) of the cosolvents, the rate acceleration in alcoholic cosolvents being greater than in DMSO. The destabilization of -OH in mixed solvent media alone does not explain the observed rate acceleration. The solvent composition dependence, log kOHos = log kOHow + ∑aixosi[i=1,2,kOH0 denotes kOH at I=0 in mixed solvent(s) and water (w)] indicates specific solute–solvent interactions. The values of the relative transfer free-energy data [ΔTG(t.s.) - ΔTGo (i.s.)](s←w)(25 °C)(G′), where t.s. and i.s. denote the transition state and initial state of the substrates respectively, are positive for all substrates at all compositions, indicating a greater destabilizing effect of the mixed solvent on the transition state than on the initial state. The G′ values also correlate with GE(G′ = axO.S. + cGE) for all solvents, supporting the fact that solvent structural effects mediate the rates and energetics of the reaction. However, the solvent effects on the solvation components of ΔH‡ and ΔS‡ are mutually compensating, thus indicating that there is no change in the mechanism.