Oxidation of Iodide by a Series of Fe(III) Complexes in Acetonitrile

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Abstract
The oxidations of iodide by [FeIII(bpy)2(CN)2]NO3, [FeIII(dmbpy)2(CN)2]NO3, [FeIII(CH3Cp)2]PF6, and [FeIII(5-Cl-phen)2(CN)2]NO3 at 25 °C, ionic strength of 0.10 M in acetonitrile, are catalyzed by trace levels of copper ions. This copper catalysis can be effectively masked with the addition of 5.0 mM 2,2‘-bipyridine (bpy), which permits the rate law of the direct reactions to be determined: −d[Fe(III)]/dt = 2(k1[I-] + k2[I-]2)[Fe(III)]. According to 1H NMR and UV−vis spectra, the products of the reaction are I3- and the corresponding Fe(II) complexes, with the stoichiometric ratio (Δ[I3-] /Δ[Fe(II)]) of 1:2. Linear free-energy relationships (LFERs) are obtained for both log k1 and log k2 vs E1/2 with slopes of 16.1 and 13.3 V-1, respectively. A mechanism is inferred in which k1 corresponds to simple electron transfer to form I• plus Fe(II), while k2 leads directly to I2-• . From the mild kinetic inhibition of the k1 path by [FeII(bpy)2(CN)2] the standard potential (E°) of I•/I- is derived: E° = 0.60 ± 0.01 V (vs [Fe(Cp)2]+/0).

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