Mechanistic studies on the intramolecular electron transfer in an adduct species of the oxo-centred trinuclear iron(III) cation and l-ascorbic acid in aqueous solution

Abstract

The kinetics of the intra-molecular electron transfer of an adduct of l-ascorbic acid and the [Fe ^III₃ O(CH₃COO)₆(H₂O)₃]⁺ cation in aqueous acetate buffer was studied spectrophotometrically, over the ranges 2.55 ≤ pH ≤ 3.74, 20.0 ≤ θ ≤ 35.0 °C, at an ionic strength of 0.50 and 1.0 mol dm⁻³ (NaClO₄). The reaction of l-ascorbic acid and the complex cation involves the rapid formation of an adduct species followed by a slower reduction in the iron centres through consecutive one-electron transfer processes. The final product of the reaction is aqueous iron(II) in acetate buffer. The proposed mechanism involves the triaqua and diaqua-hydroxo species of the complex cation, both of which form adducts with l-ascorbic acid. At 25 °C, the equilibrium constant for the adduct formation was found to be 86 ± 15 and 5.8 ± 0.2 dm³ mol⁻¹ for the triaqua and diaqua-hydroxo species, respectively. The kinetic parameters derived from the rate expression have been found to be: k ₀ = (1.12 ± 0.02) × 10⁻² s⁻¹ for the combined spontaneous decomposition and k ₁ = (4.47 ± 0.06) × 10⁻² s⁻¹ (ΔH ^‡₁  = 51.0 ± 2.3 kJ mol⁻¹, ΔS ^‡₁  = −100 ± 8 J K⁻¹ mol⁻¹), k ₂ = (4.79 ± 0.38) × 10⁻¹ s⁻¹ (ΔH ^‡₂  = 76.5 ± 0.8 kJ mol⁻¹, ΔS ^‡₂  = 6 ± 3 J K⁻¹ mol⁻¹) for the triaqua and diaqua-hydoxo species, respectively.