Cryoreduction of the [FeO2]6 (n = 6 is the number of electrons in 3d orbitals on Fe and pi* orbitals on O2) dioxygen-bound ferroheme through irradiation at 77 K generates an [FeO2]7 reduced oxy-heme. Numerous investigations have examined [FeO2]7 centers that have been characterized as peroxo-ferric centers, denoted [FeO2]per7, in which a ferriheme binds a dianionic peroxo-ligand. The generation of such an intermediate can be understood heuristically if the [FeO2]6 parent is viewed as a superoxo-ferric center and the injected electron localizes on the O-O moiety. We here report EPR/ENDOR experiments which show quite different properties for the [FeO2]7 centers produced by cryoreduction of monomeric oxy-hemoglobin (oxy-GMH3) from Glycera dibranchiata, which is unlike mammalian "globins" in having a leucine in place of the distal histidine; of frozen aprotic solutions of oxy-ferrous octaethyl porphyrin; and of the oxy-ferrous complex of the heme model, cyclidene. These [FeO2]7 centers are characterized as "superoxo-ferrous" centers ([FeO2]sup7), with nearly unit spin density localized on a superoxo moiety which is end-on coordinated to a low-spin ferrous ion. This assignment is based on their g tensors and 17O hyperfine couplings, which are characteristic of the superoxide ion coordinated to a diamagnetic metal ion, and on the absence of detectable ENDOR signals either from the in-plane 14N ligands or from an exchangeable H-bond proton. Such a center would arise if the electron that adds to the [FeO2]6 superoxo-ferric parent localizes on the Fe ion, to make a superoxo-ferrous moiety. Upon annealing to T > 150 K, the [FeO2]sup7 species converts to peroxo/hydroperoxo-ferric ([FeO2H]7) intermediates. These experiments suggest that the primary reduction product is [FeO2]sup7 and that the internal redox transition to [FeO2]per7/[FeO2H]7 states is driven at least in part by H-bonding/proton donation by the environment.

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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(R* 123,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 electronwithdrawing 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; Fe02 + e~; Fe(II) + K02; Fe(III) + H202; and Fe(III) + C6H5IO) in the basic medium, emphasizing its relevance both as a cytochrome P450 model and as example of 0-0 bond formation.Non-Porphyrin Iron(II) Dioxygen Carriers

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