Kinetics and mechanism of oxidation of tris-(1,10-phenanthroline)iron(II) by chlorine and bromine and of the reduction of tris-(1,10-phenanthroline)iron(III) by iodide ions

Abstract

The rates of the oxidation of tris-(1,10-phenanthroline)iron(II) by chlorine and bromine, and of the reduction of tris-(1,10-phenanthroline)iron(III) by iodide ions have been measured at ionic strength I = 1.0 mol dm−3 (LiClO4). All the reactions obey second-order rate law:[Formula: see text]The activation parameters for the reactions are: Fe(Phen)32+/Br2: ΔH≠ = (64.2 ± 3.2) kJ mol−1, ΔS≠ = −(24.9 ± 1.5) J mol−1 K−1. Fe(Phen)33+/I−:ΔH≠ = (39.8 ± 2.1) kJ mol−1, ΔS≠ = −(19.7 ± 0.8) J mol−1 K−1.The reactions of tris-(1,10-phenanthroline)iron(II) with chlorine and bromine are unaffected by chloride, bromide, and acid. The proposed mechanism for these reactions involves a series of one-electron changes, with the species X2− (X = Cl, Br) as reaction intermediates, since good linear free energy correlations for the primary step, resulting in the formation of X2−, are obtained. The reduction of tris-(1,10-phenanthroline)iron(III) by iodide ions is catalysed by bromide and chloride ions, whereas the reduction of aquoiron(III) by iodide ions is known to be inhibited by bromide and chloride ions. A mechanistic interpretation of this observation is suggested.