Reduction potentials for dioxygen specie

TABLE 9.3 Standard Reduction Potentials for Dioxygen Species in Water [02l 1 atm (1.0 mM>] (Formal Potentials for 02 at Unit Activity)... 

TABLE 9.4 Formal Reduction Potentials for Dioxygen Species in Acetonitrile [02 at 1 atm (8.1 mM>]... 

Figure 1 Standard reduction potentials for dioxygen species in water [1 atm O2, pH 7, V vs. NHE]...

Table 1 Standard reduction potentials for dioxygen species in water ...

The standard reduction potentials for the main species formed by the Group 17 elements in aqueous solution are given in Tables 6.16 and 6.17, for pH values 0 and 14, respectively. Irrespective of the pH of the solution, the halogen elements range from the extremely powerful F2 (which has the potential to oxidize water to dioxygen), through the powerful oxidants Cl2 and Br2, to 12, which is a relatively weak oxidant. 

Table 9.3 summarizes the redox potentials for the reduction of various dioxygen species in aqueous media at pH 0, 7, and 14. For those couples that involve dioxygen itself, formal potentials are given in parentheses for 02 at unit activity ( 105 atm [OJ 1 mM at 1 atm partial pressure). 

Reduction potentials (V) for dioxygen species in aqueous solutions with O Caq) as standard state... 

The reduction potentials for O2 and various intermediate species in H2O at pH 0, 7, and 14 are summarized in Table IV similar data for -02 in MeCN at pH —8.8, 10.0, and 30.4 are presented in Table V. For those couples that involve dioxygen itself, formal potentials are given in parentheses for -02- at unit molarity ( 10 atm [ O2 ] Ri 1 mM at 1 atm partial pressure). The reduction manifolds for O2 (Tables IV and V) indicate that the limiting step (in terms of reduction potential) is the first electron transfer to 02- and that an electron source adequate for the reduction of 02- will produce all of the other reduced forms of dioxygen (O2, HOO, HOOH, HOO, HO-, H2O, H0 ) via reduction, hydrolysis, and disproportionation steps. Thus, the most effective means to activate -02 is the addition of an electron (or hydrogen atom H O -b H ), which results in significant fluxes of several reactive oxygen species. 

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... 

A related series of mixed-metal face to face porphyrin dimers (192) has been studied by Collman et al.506 A motivation for obtaining these species has been their potential use as redox catalysts for such reactions as the four-electron reduction of 02 to H20 via H202. It was hoped that the orientation of two cofacial metalloporphyrins in a manner which permits the concerted interaction of both metals with dioxygen may promote the above redox reaction. Such a result was obtained for the Co11 /Co" dimer which is an effective catalyst for the reduction of dioxygen electrochemic-ally.507 However for most of the mixed-metal dimers, including a Con/Mnn species, the second metal was found to be catalytically inert with the redox behaviour of the dimer being similar to that of the monomeric cobalt porphyrin. However the nature of the second metal ion has some influence on the potential at which the cobalt centre is reduced. 

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