Kinetics and mechanism of the oxidation of tris(2,2′-bipyridyl)- and tris(1,10-phenanthroline)-iron(II) complexes with peroxodisulphate in acetate buffers

Abstract

A detailed kinetics study of the oxidation of tris(2,2′-bipyridyl)iron(II), [Fe(bipy)₃]²⁺, at 45 °C and tris(1,10-phenanthroline)iron(II)[Fe(phen)₃]²⁺ at 50 °C with peroxodisulphate in acetate buffers of pH 3.2–5.9 has revealed that the reaction is inhibited by the organic ligand and the order with respect to [S₂O₈^2–] is between one and zero. Reaction occurs by two mechanisms, both being dissociative and oxidative. One involves the complexation of [FeL₃]²⁺(L = bipy or phen) with [S₂O₈^2–] yielding [FeL₃·S₂O₈][step (1)], dissociation of this complex yielding [FeL·S₂O₈] and a molecule of organic ligand [(2)], and internal redox reaction of the latter complex (3). The formation constants (dm³ mol^–1) for step (1) and rate constants for the dissociation (2) for bipy and phen were 33 ± 2 and (1.4 ± 0.1)× 10^–2 s^–1, and 45 ± 4 and (5.5 ± 1.5)× 10^–4 s^–1 respectively. The other mechanism involves the dissociation of tris to the bis complex with rate constants of (1.12 + 0.12)× 10^–4 and 1.25 × 10^–5 s^–1 for bipy and phen respectively, and the oxidation of the bis complex with [S₂O₈]^2–.