KINETICS OF OXIDATION OF ASCORBATE BY TETRANUCLEAR COBALT(III) COMPLEXES (HEXOLS) IN AQUEOUS-SOLUTION

The kinetics of oxidation of L-ascorbic acid (H(2)A) by cobalt(III) he xols, [Co{CoL(4)(mu-OH)(2)}(3)](6+) [L(4) = (NH3)(4), (en)(2), or tren ; en = ethane-1,2-diamine, tren = tris(2-aminoethyl)amine], was studie d 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 t he concentration of each reactant and increases as [H+] decreases. The kinetic data indicate involvement of the monoprotonated and deprotona ted ascorbate species (HA(-) and A(2-)) in the redox process. For L(4) = (NH3)(4) the rate constants, k(2) and k(3) are 0.22 +/- 0.02 and (5 .51 +/- 0.09) x 10(5) dm(3) mol(-1) s(-1) respectively at 25 degrees C , and the corresponding activation parameters are Delta H-2(double dag ger) = 103 +/- 7 kJ mol(-1), Delta S-2(double dagger) = 89 +/- 22 JK(- 1) mol(-1) and Delta H-3(double dagger) = 46 +/- 3 kJ mol(-1) and Delt a S-3(double dagger) = 19 +/- 11 JK(-1) mol(-1). The variations in rat e constants and activation parameters for the series of complexes ment ioned above are discussed. The Fuoss theory was applied to the redox p rocess to estimate the ion-pair formation constant and the rate consta nt for the electron transfer.