Metal redox couples

Table 14 Formation of HO-DMPO" by the oxidation of an aqueous solution of DMPO by metal redox couples."...

Tables I and II contain electrochemical kinetic and related thermodynamic parameters for several transition-metal redox couples gathered at the mercury-aqueous interface. These systems were selected since the kinetics can be measured accurately under experimental conditions where the diffuse-layer potentials, , are small and/or could be estimated with confi-...

A new electrolysis system comprising two metal redox couples, Bi(0)/Bi(III) and A1(0)/A1(III), has been shown to be effective for electroreductive Barbier-type allylation of imines [533]. The electrode surface structure has been correlated with the activity towards the electroreduction of hydrogen peroxide for Bi monolayers on Au(III) [578], Electroreductive cycliza-tion of the 4-(phenylsulfonylthio)azetidin-2-one derivative (502) as well as the allenecarboxylate (505) leading to the corresponding cycKzed compounds (504) and (506) has been achieved with the aid of bimetallic metal salt/metal redox systems, for example, BiCh/Sn and BiCh /Zn (Scheme 175) [579]. The electrolysis of (502) is carried out in a DMF-BiCh/Py-(Sn/Sn) system in an undivided cell by changing the current direction every 30 s, giving the product (504)in 67% yield. 

Table 3 provides entropies for those species that are needed to determine the temperature dependence of standard potentials for alkali metal redox couples. AU of these entropies were obtained from values published by NIST [11]. The resulting temperature dependences agree well with values tabulated by Bratsch [17]. 

Molecular Motions Driven by Transition Metal Redox Couples Ion Translocation and Assembling-Disassembling of Dinuclear Double-Strand Helicates... 

MOLECULAR MOTIONS DRIVEN BY TRANSITION METAL REDOX COUPLES... 

In the reversible process, the redox reagents are turned over several times and recycled. The reversible mediators of oxidation can be a metallic redox couple, such as Ag+/Ag2+(Farmer et al. 1992), Co2+/Co3+(Leffrang et al. 1995), Ce3+/Ce4+(Nelson 2002), Fe2+/Fe3+(Dhooge and Park 1983), or inorganic ions such as Cr/CICT (Comninellis and Nerini 1995 Szpyrkowicz et al. 1995 (Panizza and Cerisola 2003a) or Br /BrO (Martinez-Huitle et al. 2005) added to or present in the electrolyte. The main drawback of the use of a solution redox couple is the need to subsequently separate the oxidation products from the mediator. 

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. 

Several transition metal phosphate systems are found to catalyze mild oxidations of organic molecules. Catalytic activity is due to transition metal redox couples see Oxidation Catalysis by Transition Metal Complexes and Oxide Catalysts in Solid-state Chemistry), but the phosphate groups are important in determining the underlying and surface structures. 

Recently, Grochala and Peters have shown that the thermal decomposition temperature of many binary hydrides could be correlated with the standard redox potential for the metal cation/metal redox couple and with the standard enthalpy of decomposition [18, 83[. They also showed that, for multinary hydrides, the decomposition temperature could be tuned by a careful choice of the stoichiometric ratio and of the Lewis acid/base character of the constituent elements. 

Electrochemical rate constants and activation parameters for inorganic electrochemical reactions span wide ranges e.g., standard rate constants vary from ca. 10 to > 10 cm s. For transition-metal redox couples, values of AH vary - from 0 to 80 kJ mol". As for homogeneous redox processes, it is highly desirable to elucidate the underlying reasons that are responsible for the observed rate parameters. In the following sections, the theory of electrochemical electron-transfer kinetics is outlined, followed by a discussion of some pertinent experimental results. 

In this section we look at ways in which Nature carries out redox chemistry with reference to blue copper proteins, iron-sulfur proteins and C5fiochromes the redox steps in Photosystem II were outlined in the discussion accompanying equation 21.53. We have already discussed two topics of prime importance to electron transfer in Nature. The first is the way in which the reduction potential of a metal redox couple such as Fe /Fe + can be tuned by... 

Solvation effects are crucially important for data recorded in water phase wherein we can certainly expect different 5m, Im values for different net charges for the same metal redox couple. Thus, systems of different net charge, when studied in water, should be independently analyzed. 

Sahami S, Weaver MJ (1981) Entropic and enthalpic contributions to the solvent dependence of the thermodynamics of transition-metal redox couples Part I. Couples containing aromatic ligands. J Electroanal Chem 122 155-170... 

Hupp IT, Weaver Ml (1984) Solvent, ligand, and ionic charge effects on reaction entropies for simple transition-metal redox couples. Inorg Chem 23 3639-3644... 

Fig. 3.14 Situation of low conductivity realized for imposed potentials far from E° of the metal redox couple only one partner of the redox couple is present (a) redox matching situation (b)...

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