Metal-oxygen couples

Fig. 1 Theoretical energy densities for several metal-oxygen couples compared to a conventional Li-ion battery. The abscissa indexes the discharge product(s) associated with each metal...

Because metal-air batteries combine power of battery s anodes and fuel cell cathodes, their advantage over traditional batteries is high energy density. The theoretical energy densities for a number of metal-oxygen couples are shown in Table 4.1 [157]. 

TABLE 4.1 Energy Densities of Selected Metal/Oxygen Couples [157]... 

Although C NMR is a powerful technique for the study of metal carbonyl complexes, its utility is marginal when quadrupo-lar metal nuclei are involved due to metal-carbon spin-spin coupling which broadens resonances and distorts their intensities. Since two bond metal-oxygen couplings do not interfere in a similar fashion, 0 NMR alone can provide definitive structural and dynamic information for many quadrupolar metal carbonyls (17-21). The most complex molecule of this type which has been studied to date is the 03 HFeCo3(C0)j 2 cluster (20), 9. At... 

Table 22.1 Electrochemical parameters of metal-oxygen couples.

Table 23.1 Energy densities of selected metal-oxygen couples.

The above-mentioned results indicate the additive effect of protons. Actually, a catalytic process is formed by protonation of the metal-oxygen bond instead of silylation. 2,6-Lutidine hydrochloride or 2,4,6-collidine hydrochloride serves as a proton source in the Cp2TiCl2-catalyzed pinacol coupling of aromatic aldehydes in the presence of Mn as the stoichiometric reduc-tant [30]. Considering the pKa values, pyridinium hydrochlorides are likely to be an appropriate proton source. Protonation of the titanium-bound oxygen atom permits regeneration of the active catalyst. High diastereoselectivity is attained by this fast protonation. Furthermore, pyridine derivatives can be recovered simply by acid-base extraction or distillation. 

Alternative paths for decomposition of the metal carboxylate can lead to ketones, acid anhydrides, esters, acid fluorides (1,11,22,68,77,78), and various coupling products (21,77,78), and aspects of these reactions have been reviewed (1,11). Competition from these routes is often substantial when thermal decomposition is carried out in the absence of a solvent (Section III,D), and their formation is attributable to homolytic pathways (11,21,77,78). Other alternative paths are reductive elimination rather than metal-carbon bond formation [Eq. (36)] (Section III,B) and formation of metal-oxygen rather than metal-carbon bonded compounds [e.g., Eqs. (107) (119) and (108) (120). Reactions (36) and (108) are reversible, and C02 activation (116) is involved in the reverse reactions (48,120). 

If a metal-metal oxide couple (e.g., Me-MeO) is used for the reference electrode ( die cathode in the present example), Pox(C) must he replaced with die equilibrium oxygen... 

In the context of the present discussion, it is worth noting that virtually all the experimental systems that exhibit such "anomalous temperature-dependent transfer coefficients are multistep inner-sphere processes, such as proton and oxygen reduction in aqueous media [84]. It is therefore extremely difficult to extract the theoretically relevant "true transfer coefficient for the electron-transfer step, ocet [eqn. (6)], from the observed value [eqn. (2)] besides a knowledge of the reaction mechanism, this requires information on the potential-dependent work terms for the precursor and successor state [eqn. (7b)]. Therefore the observed behavior may be accountable partly in terms of work terms that have large potential-dependent entropic components. Examinations of temperature-dependent transfer coefficients for one-electron outer-sphere reactions are unfortunately quite limited. However, most systems examined (transition-metal redox couples [2c], some post-transition metal reductions [85], and nitrobenzene reduction in non-aqueous media [86]) yield essentially temperature-independent transfer coefficients, and hence potential-independent AS orr values, within the uncertainty of the double-layer corrections. 

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