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 (LiClO 4 ). All the reactions obey second-order rate law:[Formula: see text]The activation parameters for the reactions are: Fe(Phen) 3 2+ /Br 2 : ΔH ≠ = (64.2 ± 3.2) kJ mol −1 , ΔS ≠ = −(24.9 ± 1.5) J mol −1 K −1 . Fe(Phen) 3 3+ /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 X 2 − (X = Cl, Br) as reaction intermediates, since good linear free energy correlations for the primary step, resulting in the formation of X 2 − , 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.