Kinetics of oxidation of ascorbate by tetranuclear cobalt(III) complexes (‘hexols’) in aqueous solution

Abstract

The kinetics of oxidation of L-ascorbic acid (H₂A) by cobalt(III) hexols, [Co{CoL₄(µ-OH)₂}₃]⁶⁺[L₄=(NH₃)₄, (en)₂, or tren; en = ethane-1,2-diamine, tren = tris(2-aminoethyl)amine], was studied as a function of pH, L-ascorbic acid concentration, temperature and ionic strength, using stopped-flow and conventional spectrophotometric techniques. The rate of the reaction is first order with respect to the concentration of each reactant and increases as [H⁺] decreases. The kinetic data indicate involvement of the monoprotonated and deprotonated ascorbate species (HA^– and A^2-) in the redox process. For L₄=(NH₃)₄ the rate constants k₂ and k₃ are 0.22 ± 0.02 and (5.51 ± 0.09)× 10⁵ dm³ mol^–1 s^–1 respectively at 25 °C, and the corresponding activation parameters are ΔH₂^‡= 103 ± 7 kJ mol^–1, ΔS₂^‡= 89 ± 22 J K^–1 mol^–1 and ΔH₃^‡= 46 ± 3 kJ mol^–1 and ΔS₃^‡= 19 ± 11 J K^–1 mol^–1. The variations in rate constants and activation parameters for the series of complexes mentioned above are discussed. The Fuoss theory was applied to the redox process to estimate the ion-pair formation constant and the rate constant for the electron transfer.