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 similar or equal to 2(k(1)[I-] + k(2)[I-](2))[Fe(III)]. According to H-1 NMR and UV-vis spectra, the products of the reaction are I-3(-) and the corresponding Fe(II) complexes, with the stoichiometric ratio (Delta[(-)(3)]/Delta[Fe(II)]) of 1:2. Linear free-energy relationships (LFERs) are obtained for both log k(1) and log k(2) vs E-1/2 with slopes of 16.1 and 13.3 V-1, respectively. A mechanism is inferred in which k(1) corresponds to simple electron transfer to form I-center dot plus Fe(II), while k(2) leads directly to I-2(-center dot). From the mild kinetic inhibition of the k(1) path by [Fe-II(bpy)(2)(CN)(2)] the standard potential (E degrees) of I-center dot/I- is derived: E degrees = 0.60 +/- 0.01 V (vs [Fe(Cp)(2)](+/0)).