Kinetics and mechanism of oxidation of iodide with a (μ‐oxo)diiron(III,III) complex in weakly acidic media

Abstract

AbstractThe complex ion [FeIII2(μ‐O)(phen)4(H2O)2]4+ (1) (phen = 1,10‐phenanthroline) and its hydrolytic derivatives [FeIII2(μ‐O)(phen)4(H2O)(OH)]3+ (1a) and [FeIII2(μ‐O)(phen)4‐ (OH)2]2+ (2a) coexist in rapid equilibria in the range pH 4.23–5.35 in the presence of excess phenanthroline (pKa1 = 3.71±0.03, pKa2 = 5.28± 0.07). The solution reacts quantitatively with I− to produce [Fe(phen)3]2+ and I2. Only 1 but none of its hydrolytic derivatives is kinetically active. Both inner and outer sphere pathways operate. The observed rate constants show second‐order dependence on the concentration of iodide, while the dependence on [H+] is complex in nature. Added Cl− inhibits the formation of adduct with I− and thus retards the rate of inner sphere path, leading to a rate saturation at high [Cl−], where only the outer sphere mechanism is active. Kinetic data indicate that simultaneous presence of two I− in the vicinity of diiron core is necessary for the reduction of 1. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 737–743, 2005