Reactions between iron(III) and catechol (o-dihydroxybenzene). part I. Equilibria and kinetics of complex formation in aqueous acid solution

Abstract

The equilibria and kinetics of formation of catecholato(2–)iron(III) have been studied over the pH range 1·00–2·00 in aqueous media with the stopped-flow technique. At 25·0 °C and ionic strength 1·0M(NaClO₄), the equilibrium constantK_e for chelate-complex formation (i) is (4·35 ± 0·10)× l0^–2 mol l^–1 and ε[Fe(cat)⁺]=(1·25 ± 0·05)× 10³l mol^–1 at 700 nm. From the hydrogen-ion dependence of the reaction rate it can be shown that complex Fe³⁺+ H₂cat ⇌[Fe(cat)]⁺+ 2H⁺(i), formation occurs between [FeOH]²⁺ ion and catechol with a rate constant k=(3·1 ± 0·2)× 10³ l mol^–1 s^–1; estimated activation data are ΔH^‡= 9·6 ± 1·0 kcal mol^–1 and ΔS^‡=–11·1 ± 3·0 cal K^–1 mol^–1. The experimental data are consistent with the accepted l_d mechanism for ligand substitution.