KINETICS AND INTERMEDIATES IN THE AUTOXIDATION OF SYNTHETIC, NONPORPHYRIN IRON(II) DIOXYGEN CARRIERS

Studies are reported on the autoxidation, in the solvent system 3:1:1 acetone/pyridine/water (APW), of a series of iron(II) cyclidenes, of the general formula [Fe(R3,R2,m-xyl)Cl]+ with R3 = Me, Ph and R2 = Me, Bz, which have high relevance as dioxygen carrier model systems. Autoxidation rates can be modeled by an electron-transfer mechanism involving a parallel dioxygen adduct equilibrium, with the autoxidation reaction being driven by a subsequent reaction of the primary superoxide product. The deduced rate constants k' for the electron-transfer process decrease in the sequence MeMe > MeBz > PhMe > PhBz, which is the order of increasing electron-withdrawing power and/or steric bulk of the substituents. Products of the reaction were identified explicitly for one of the complexes (R2 = R3 = Me), employing mainly UV-vis and ESR spectroscopy. A remarkable product of the autoxidation of iron(II) cyclidenes in this specific solvent system, is a peroxoiron(III) species, which can even be observed at ambient temperatures. Its ESR properties (low-spin iron g = 2 signal with an extremely low anisotropy) are in close similarity to those of natural systems analogues, and in contrast to the first reported model systems. This species can be generated by five independent routes (autoxidation; FeO2 + e-; Fe(II) + KO2; Fe(III) + H2O2; and Fe(III) + C6H5IO) in the basic medium, emphasizing its relevance both as a cytochrome P450 model and as example of O-O bond formation.