Oxidation of iodide by a series of Fe(III) complexes in acetonitrile

The oxidations of iodide by [Fe(III)(bpy)2(CN)2]NO3, [Fe(III)(dmbpy)2(CN)2]NO3, [Fe(III)(CH3Cp)2]PF6, and [Fe(III)(5-Cl-phen)2(CN)2]NO3 at 25 degrees 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 (delta[I3-]/delta[Fe(II)]) of 1:2. Linear free-energy relationships (LFERs) are obtained for both log k1 and log k2 vs E(1/2) 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 [Fe(II)(bpy)2(CN)2] the standard potential (Eo) of I*/I- is derived: Eo = 0.60 +/- 0.01 V (vs [Fe(Cp)2](+/0)).