Thermodynamics of the homogeneous oxidation of L-ascorbic acid by molecular oxygen catalyzed by ruthenium ion and ruthenium chelates

Abstract

The homogeneous oxidation of ascorbic acid by molecular oxygen catalyzed by RuCl₂(H₂O)₄⁺, Ru(III)-IMDA and Ru(III)-EDTA has been investigated in the temperature range 278-308 K, μ=0.1 M KNO₃. The rate of oxidation of ascorbic acid in the presence of RuCl₂(H₂O)₄⁺ was found to be first order in RuCl₂(H₂O)₄⁺ and ascorbate concentrations and one-half order in molecular oxygen concentration. Based on the kinetic parameters, a μ-peroxoruthenium(IV)-ascorbate complex was suggested as an active intermediate in the RuCl₂(H₂O)₄⁺-catalyzed oxidation of ascorbic acid by molecular oxygen. The thermodynamic parameters corresponding to the formation of the μ-peroxoruthenium(IV)-ascorbate complex and the formation of various mixed ligand complexes with ascorbic acid were computed. The activation parameters for the oxidation steps corresponding to the rate constants were also calculated. The rate of oxidation of ascorbic acid in the presence of Ru(III) chelates was found to depend only on the metal chelate and ascorbate concentrations and was independent of the concentration of molecular oxygen. A mixed ligand complex of Ru(III)-EDTA and Ru(III)-IMDA with ascorbic acid is proposed as an active intermediate for electron transfer to the metal ion. The reduced metal ion is reoxidized in a fast step by molecular oxygen. As in the case of Fe(III) and Cu(II) aminopolycarboxylic acid chelate-catalyzed oxidation of ascorbic acid, the Ru(III)-IMDA and Ru(III)-EDTA chelates act as oxidase models in the oxidation of ascorbic acid by molecular oxygen. The thermodynamic parameters are discussed in terms of the oxidation potentials E½ of the metal complexes.