Dioxygen electron-transfer reduction

Electron-Transfer Reduction of 02. Within aqueous solutions the most direct means to the electron-transfer reduction of dioxygen is by pulse radiolysis. Irradiation of an aqueous solution by an electron beam yields (almost instantly 10-12 s) solvated electrons [e (aq)], hydrogen atoms (H-), and hydroxyl radicals (HO-)- If the solution contains a large excess of sodium formate [Na+ 0(0)CH] and is saturated with 02, then the radiolytic electron flux efficiently and cleanly reduces 02 to superoxide ion (O ) 21-25... 

The proposition of a one-electron mechanism for the electron-transfer reduction of dioxygen and the associated conclusions present significant ramifications relative to the development of improved fuel cells and metal-air batteries. To date the practical forms of such systems have used strongly acidic or basic electrolytes. Such solution conditions normally cause atom transfer to be the dominant reduction process for molecular oxygen at metal electrodes. Hence, the search for effective catalytic materials should be in this context rather than in terms of a one-electron-transfer process. 

The superoxide ion, O , produced by the electron-transfer reduction of dioxygen (O2 + e O ), is a strong Bronsted base and an effective nucleophile. Because of rapid hydrolysis and disproportionation, the lifetime of O in aqueous solution is limited. This has led to investigations of its reaction chemistry in dipolar non-HBD solvents [632]. Under these conditions, the superoxide ion attacks haloalkanes by Sn2 displacement of the halides to eventually give dialkyl peroxides in a multi-step reaction [632]. 

Honda T, Kojima T, Pukuzumi S (2012) Proton-coupled electron-transfer reduction of dioxygen catalyzed by a saddle-distorted cobalt phthalocyanine. J Am Chem Soc 134 4196 206... 

At present, new developments challenge previous ideas concerning the role of nitric oxide in oxidative processes. The capacity of nitric oxide to oxidize substrates by a one-electron transfer mechanism was supported by the suggestion that its reduction potential is positive and relatively high. However, recent determinations based on the combination of quantum mechanical calculations, cyclic voltammetry, and chemical experiments suggest that °(NO/ NO-) = —0.8 0.2 V [56]. This new value of the NO reduction potential apparently denies the possibility for NO to react as a one-electron oxidant with biomolecules. However, it should be noted that such reactions are described in several studies. Thus, Sharpe and Cooper [57] showed that nitric oxide oxidized ferrocytochrome c to ferricytochrome c to form nitroxyl anion. These authors also proposed that the nitroxyl anion formed subsequently reacted with dioxygen, yielding peroxynitrite. If it is true, then Reactions (24) and (25) may represent a new pathway of peroxynitrite formation in mitochondria without the participation of superoxide. 

Now, we may consider in detail the mechanism of oxygen radical production by mitochondria. There are definite thermodynamic conditions, which regulate one-electron transfer from the electron carriers of mitochondrial respiratory chain to dioxygen these components must have the one-electron reduction potentials more negative than that of dioxygen Eq( 02 /02]) = —0.16 V. As the reduction potentials of components of respiratory chain are changed from 0.320 to +0.380 V, it is obvious that various sources of superoxide production may exist in mitochondria. As already noted earlier, the two main sources of superoxide are present in Complexes I and III of the respiratory chain in both of them, the role of ubiquinone seems to be dominant. Although superoxide may be formed by the one-electron oxidation of ubisemiquinone radical anion (Reaction (1)) [10,22] or even neutral semiquinone radical [9], the efficiency of these ways of superoxide formation in mitochondria is doubtful. 

It should be mentioned that Spasojevic et al. [57] recently determined the two-electron reduction potential of lucigenin in water as —0.14 V. As this value is close to the one-electron reduction potential of dioxygen °[02 702] = — 0.16 V, these authors regarded their finding as a support for lucigenin redox cycling. However, it has been demonstrated long ago that two-electron reduction potentials cannot be used for the calculation of equilibrium for one-electron transfer processes [58]. 

The reduction of dioxygen to its fully reduced form, H20, requires the transfer of 4 electrons, and the transfer may proceed via a series of intermediate oxidation states, such as 02 /H00, HOO /HOOH, 0 /OH. These reduced forms of oxygen exhibit different redox properties and in the presence of substrate(s) and/or catalyst(s) may open different reaction paths for the electron transfer process. Fast proton transfer reactions between the corresponding acid-base pairs can introduce composite pH dependencies into the kinetic and stoichiometric characteristics of these systems. 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

Co(II) and 02 to form Coin-C>2, the superoxo species. It is the latter process that accounts for the large volume reduction en route to the reaction products. Thus during flash-photolysis, electron transfer in the reverse direction occurs due to irradiation into the CT band, which is followed by the rapid release of dioxygen. 

Studies of medium effects on hexacyanoferrate(II) reductions have included those of dioxygen,iodate, peroxodisulfate, - [Co(NH3)5(DMSO)] +, and [Co(en)2Br2]+. Rate constants for reaction with dioxygen depended strongly on the electron-donor properties of the organic cosolvent. Rate constants for reduction of peroxodisulfate in several binary aqueous media were analyzed into their ion association and subsequent electron transfer components. Rate constants for reduction of [Co(en)2Br2] in methanol water and dioxan water mixtures were analyzed by a variety of correlatory equations (dielectric constant Grunwald-Winstein Swain Kamlet-Taft). 

Electrocatalytic Reduction of Dioxygen The electrocatalytic reduction of oxygen is another multi-electron transfer reaction (four electrons are involved) with several steps and intermediate species [16]. A four-electron mechanism, leading to water, is in competition with a two-electron mechanism, giving hydrogen peroxide. The four-electron mechanism on a Pt electrode can be written as follows ... 

The question arises as to the actual sequence of events that end up in the reduction of dioxygen into water. Even though complementary investigation is needed before a complete answer could be proposed, several observations are worth mentioning provisionally. The reoxidation of reduced heteropolyanions by dioxygen after the catalytic processes in which they participate is a popular practice. Its mechanism was generally and indistinctly explained to go through an intermediate adduct formation accompanied by inner sphere electron transfers [164]. 

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