Dioxygen species support metals

The data seem to support an alternative mechanism compared with the so-called Lyons mechanism in which O2 is activated forming a metal-oxo (M=0) species.1831 Via reaction of a second metalloporphyrin with a primarily formed superoxo (Me-OO0) or peroxo species, a metal-oxo is formed, reacting eventually with an alkane according to the oxygen rebound mechanism. Alternatively, radicals present in solution, upon reaction with dioxygen, may form alkyl hydroperoxides that are decomposed by metalloporphyrins.[83]... 

However, spectroscopic studies of activated BLM indicate that it is not an Fev=0 species. It exhibits an S - 1/2 EPR spectrum with g values at 2.26, 2.17, and 1.94 [15], which is typical of a low-spin Fe111 center. This low-spin Fem designation is corroborated by Mossbauer and x-ray absorption spectroscopy [16,19], Furthermore, EXAFS studies on activated BLM show no evidence for a short Fe—0 distance, which would be expected for an iron-oxo moiety [19], These spectroscopic results suggest that activated BLM is a low-spin iron(III) peroxide complex, so the two oxidizing equivalents needed for the oxidation chemistry would be localized on the dioxygen moiety, instead of on the metal center. This Fe(III)BLM—OOH formulation has been recently confirmed by electrospray ionization mass spectrometry [20] and is supported by the characterization of related synthetic low-spin iron(III) peroxide species, e.g., [Fe(pma)02]+ [21] and [Fe(N4py)OOH]2+ [22], The question then arises whether the peroxide intermediate is itself the oxidant in these reactions or the precursor to a short-lived iron-oxo species that effects the cytochrome P-450-like transformations. This remains an open question and the subject of continuing interest. 

The reduced species Fe2+ and Mn2+ have been detected electrochemically in anoxic waters (De Vitre et al., 1988). Vanadyl (V02 + ) is known to be incorporated in geoporphyrins in organic-rich sediments (Eckstrom et al., 1983). Moffett and Zika (1988) measured reduced Cu(I) photometrically in surface waters of the open ocean. The oxidation of these four metal species involves a simple one-electron transfer step. Haber and Weiss (1934) proposed a kinetic mechanism for the oxygenation of the ferrous ion, in which the first step in the four-electron reduction of the dioxygen molecule determines the rate. The redox potentials for the corresponding oxygen couples support this view they are plotted ill... 

Analogously, in the presence of silica-supported palladium catalysts, benzene is oxidized under ambient conditions to give phenol, benzoquinone, hydroquinone and catechol [37b]. Palladium chloride, used for the catalyst preparation, is believed to be converted into metallic palladium. The synthesis of phenol from benzene and molecular oxygen via direct activation of a C-H bond by the catalytic system Pd(OAc)2-phenanthroline in the presence of carbon monoxide has been described [38]. The proposed mechanism includes the electrophilic attack of benzene by an active palladium-containing species to to produce a a-phenyl complex of palladium(ll). Subsequent activation of dioxygen by the Pd-phen-CO complex to form a Pd-OPh complex and its reaction with acetic acid yields phenol. The oxidation of propenoidic phenols by molecular oxygen is catalyzed by [A,A"-bis(salicylidene)ethane-l,2-diaminato]cobalt(ll)[Co(salen)] [39]. 

---