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 A2-) 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.