Electron transfer between ascorbic acid and a (μ-oxo) diiron(III,III) complex: An example of chloride inhibition
Graphical abstract
The complex ion and each of its H2O acid dissociated derivatives 1a and 2a react with one ascorbate ion in presence of excess phenanthroline in an inner sphere pathway to produce [Fe(phen)3]2+.
Introduction
The Fe–O–Fe linkage has been identified as a binuclear unit in inorganic chemistry [1], [2]. The (μ-oxo)diiron(III) complexes provide useful models for a class of biological non-heme metalloprotein sites like hemerythrin and ribonucleotide reductase [3]. The present complex ion (1) (Fig. 1) is an excellent infrared, Raman and electronic absorption model for μ-oxo-bridged binuclear iron proteins. Definitive evidences for a μ-oxo bridge in the binuclear iron centre of hemerythrin have been provided by crystallography [4], [5], [6] and various spectroscopic studies [7], [8], [9], [10], [11], [12], [13], [14]. The Raman spectra of 1 matches well with both symmetric and antisymmetric Fe–O–Fe vibrations of hemerythrin and ribonucleotide reductase [14], [15].
The redox properties of complexes like 1 are of great importance in view of their close resemblance to biologically active proteins. Mechanistic studies on reduction of 1 by hydroxylamine [16], thiosulfate ion [17] and hydrazine [30] have already been reported.
Ascorbic acid, the presently chosen reductant, is of importance as a hydroxylase cofactor [18] and as an antioxidant [19]. Brown et al. [20] utilized ascorbic acid as a source of reducing equivalents in a multicomponent system that promotes the photo-reduction of water. Metal complexes generally oxidize ascorbic acid (H2A) to dehydroascorbic acid (A) with a net transfer of two electrons and this change is generally accomplished in one-electron steps [21], [22]. In one important exception, a ferriporphyrin dimer [23] oxidizes ascorbic acid by a two-electron pathway, thus avoiding free-radical generation.
Section snippets
Materials
Crystals of were prepared by a known method [24]. The prepared samples gave satisfactory electronic spectra [24] and elemental analyses (Anal. Calc. for C48H32Fe2O13N12 · 7H2O: C, 47.15; H, 3.8; Fe, 9.1; N, 13.75. Found: C, 47.1; H, 3.7; Fe, 9.0; N, 13.7%.). Stock solution of ascorbic acid was prepared by dissolving l-ascorbic acid (H2A, G.R., E. Merck) in freshly boiled water and was stored in the dark. 1,10-Phenanthroline monohydrate (Sigma) was used as
Equilibrium studies
The built-in program of Metrohm 736 GP Titrino autotitrator yielded two pKa values for the title complex (1) from the pH titration curve, pKa1 = 3.71 ± 0.11 and pKa2 = 5.28 ± 0.10, respectively. The difference (pKa1 − pKa2) for the binuclear complex 1 is quite different from those for the dinuclear iron(III)-hydroxamic acid complexes (pKa1 = 8.80 ± 0.15; pKa2 = 9.40 ± 0.18) [27]. A prominent electronic interaction of the two MIII centers via the oxo-bridge in 1 is thus indicated and is indeed expected from the
Acknowledgements
We gratefully acknowledge the financial help provided by the Department of Science and Technology (New Delhi, India) for the present work. One of he authors (B.B.D.) is grateful to UGC, India, for the senior research fellowship provided to him.
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