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

DOI: 10.1039/dt9960001385

Source and publish data:

Journal of the Chemical Society, Dalton Transactions p. 1385 - 1391 (1996)

Update date:2022-08-17

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Authors:

Abdur-Rashid, KamaluddinAbdur-Rashid, Kamaluddin

Dasgupta, Tara P.Dasgupta, Tara P.

Burgess, JohnBurgess, John

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Article abstract of DOI:10.1039/dt9960001385

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.

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