Electron transfer between ascorbic acid and a (μ-oxo) diiron(III,III) complex: An example of chloride inhibition

Abstract

The electron transfer reactions involving ascorbic acid as electron donor are in general very fast. The present work is an example of electron transfer between ascorbic acid and a (μ-oxo) diiron(III,III) complex, where the otherwise very fast process has been slowed down by the presence of large excess of chloride ion. The complex ion [ Fe 2 III ( μ - O ) ( phen ) 4 ( H 2 O ) 2 ] 4 + ( 1 ) (phen = 1,10-phenanthroline) and its H 2O acid dissociated derivatives [ Fe 2 III ( μ - O ) ( phen ) 4 ( H 2 O ) ( OH ) ] 3 + ( 1 a ) and [ Fe 2 III ( μ - O ) ( phen ) 4 ( OH ) 2 ] 2 + ( 2 a ) coexist in rapid equilibria in the range pH 3.95–5.09 in presence of excess phen (p K a1 = 3.71 ± 0.11, p K a2 = 5.28 ± 0.10). The complex solution reacts with ascorbic acid in presence of excess phenanthroline to produce tris(phenanthroline)iron(II) and dehydroascorbic acid. All the three forms of the complex ion are kinetically active, whereas the ascorbate ion and not the free ascorbic acid has been found to be the reactive species. The observed rate constants exhibit a first-order dependence on the total concentration of ascorbic acid and an inverse square dependence on chloride ion concentration, while the dependence on [H +] is complex in nature. Kinetic and stoichiometric results indicate that one ascorbate ion reacts with one complex molecule in an inner sphere pathway. The kinetic experiments were carried out at 25 °C in aqueous media. The ionic strengths of the reaction media were kept 1.0 M using NaNO 3.