Kinetics and mechanism of the oxidation of L-ascorbic acid by cis-diaqua cobalt(III) ammine complexes

Abstract

The kinetics of oxidation of L-ascorbic acid by cis-diaquacobalt(III) complexes, [CoL₄(H₂O)₂]³⁺(L₄=(NH₃)₄, (en)₂ or tren; en = ethane-1,2-diamine, tren = tris(2-aminoethyl)amine] was studied as a fuction of pH, L-ascorbic acid concentration, temperature, ionic strength and methanol content of the solvent using stopped-flow and conventional spectrophotometry. The results indicated that only the ascorbate monoanion, HA^–, is involved in the redox process with the cobalt(III) species. The rate constants for the [Co(tren)(H₂O)₂]³⁺ and [Co(tren)(H₂O)(OH)]²⁺ species (k₂ and k₅) are 0.26 ± 0.09 and 1.25 ± 0.03 dm³ mol^–1 s^–1 respectively at 30 °C, and the corresponding activation parameters are ΔH₂^‡= 124 ± 9 kJ mol^–1, ΔS₂^‡= 137 ± 30 J K^–1 mol^–1 and ΔH₅^‡= 82 ± 2 kJ mol^–1, ΔS₅^‡= 26 ± 6 J K^–1 mol^–1. The variations in the rate constants and thermodynamic parameters for the series of complexes is discussed. The Marcus cross-relationship for electron transfer has been applied to the redox process to confirm the outer-sphere mechanism and to estimate the self-exchange rate constant for the [CoL₄(H₂O)(OH)]^2+/+ couple.