Transferring, redox

Hydroperoxides are more widely used as initiators in low temperature appHcations (at or below room temperature) where transition-metal (M) salts are employed as activators. The activation reaction involves electron-transfer (redox) mechanisms ... 

Kinetic Parameters of Some Single-Electron Charge Transfer Redox Couples at a Platnium Interface ... 

Fig. 18b.9. Example cychc voltammograms due to (a) multi-electron transfer redox reaction two-step reduction of methyl viologen MV2++e = MV++e = MV. (b) ferrocene confined as covalently attached surface-modified electroactive species—peaks show no diffusion tail, (c) follow-up chemical reaction A and C are electroactive, C is produced from B through irreversible chemical conversion of B, and (d) electrocatalysis of hydrogen peroxide decomposition by phosphomolybdic acid adsorbed on a graphite electrode.

Ni(02)(CNBu )2] reacts with a variety of compounds and the reactions which are outlined in Scheme 5 can be classified as either atom transfer redox reactions, atom transfer oxidation reactions, oxidative substitution reactions or metal assisted peroxidation reactions. An i.r. study of [Ni( 02)(CNBu )2]... 

Ilan, Y. A., G. Czapski, and D. Meisel, The one-electron transfer redox potential of free radicals 1. The oxygen/superoxide system , Biochim. Biophys. Acta, 430, 209-244 (1987). 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

Platinum(IV) is kinetically inert, but substitution reactions are observed. Deceptively simple substitution reactions such as that in equation (554) do not proceed by a simple SN1 or 5 2 process. In almost all cases the reaction mechanism involves redox steps. The platinum(II)-catalyzed substitution of platinum(IV) is the common kind of redox reaction which leads to formal nucleophilic substitution of platinum(IV) complexes. In such cases substitution results from an atom-transfer redox reaction between the platinum(IV) complex and a five-coordinate adduct of the platinum(II) compound (Scheme 22). The platinum(II) complex can be added to the solution, or it may be present as an impurity, possibly being formed by a reductive elimination step. These reactions show characteristic third-order kinetics, first order each in the platinum(IV) complex, the entering ligand Y, and the platinum(II) complex. The pathway is catalytic in PtnL4, but a consequence of such a mechanism is the transfer of platinum between the catalyst and the substrate. 10 This premise has been verified using a 195Pt tracer.2011... 

For enzymatic reductions with NAD(P)H-dependent enzymes the electrochemical regeneration of NAD(P)H always has to be performed by indirect electrochemical methods. Direct electrochemical reduction, which requires high overpotentials, always leads to larger or smaller amounts of enzymatically inactive NAD dimers generated in a one-electron transfer reaction. If one-electron transfer redox catalysts are used as mediators, the same problem occurs. 

The examples presented in this chapter illustrate that many molecules without metals undergo redox processes in which the voltammetric current is proportional to their concentration. Often these nonmetallic substrates give responses that are due to the facilitated electron-transfer reduction of H30+/H20 or oxidation of H0 /H20. Hence, any substrate that forms a strong bond with H- or HO1 (or has an HO—/ or an R—H group with weak bonds to yield H—OH) will facilitate these electron-transfer processes at less extreme potentials to give peak currents that are proportional to the substrate concentration. The next two chapters (on organic compounds and organometallic compounds) include many more examples of matrix-centered electron-transfer redox processes. 

The cis-[RuVI(tet-Me6)(0)2]2+ complex has also been reported to display proton-coupled electron-transfer redox couples in aqueous medium (134,136). The following electrode reactions for the couples have been observed (pH 1.0) ... 

The term ion pump, synonymous with active ion-transport system, is used to refer to a protein that translocates ions across a membrane, uphill against an electrochemical potential gradient. The primary pumps do so by utilization of energy derived from various types of chemical reactions such as ATP hydrolysis, electron transfers (redox processes), and decarboxylations, or from the absorption of light (Table 1). Secondary pumps are symport and antiport systems that derive the energy for uphill movement of one species from a coupled downhill movement of another species. The electrochemical gradient driving the latter movement is often created by a primary pump. 

In a related study [62], similar effects on conductivity of SWCNTs were reported, but here a comparison was also made between the effects of nitric acid reflux and air plasma treatment, and an attempt was made to relate the changes observed to the creation of defect sites. The authors did not offer a more concrete proposal regarding the nature of the sites involved in these treatments. After the acid treatment, Raman microscopy results indicated a dramatic change in SWNT electronic structure, and both treatments enhanced the electron transfer kinetics for the oxidation of inner-sphere dopamine. By contrast, both treatments had a negligible effect on the voltammetric response of a simple outer-sphere electron-transfer redox process Ru(NH3)63+/2+. ... 

Finally, while the new synthetic methods for technetium compounds in the oxidation states I, II and V will undoubtedly play an important role in the development of improved radioscintigraphic agents, there remains a need to understand the chemical mechanisms involved in these syntheses, particularly with regard to ligand substitution reactions and atom transfer redox processes. Mechanistic studies to complement the increasing body of structural knowledge is essential to the further development of technetium radiopharmaceuticals. 

The basic principles describing the efFects of CT complexes on the energy profile along the reaction coordinate stem from the theory of electron transfer. Redox processes may occur (i) as ground-state thermal reactions, (ii) by direct irradiation of the CT band, and (iii) upon photoexcitation of one of the redox partners followed by diffusional complex formation [4, 24], as depicted in Chart 3. 

Electrochemically irreversible electron transfer redox processes are indicated with an asterisk. 

Unfortunately, factors which decrease A have a tendency to increase AE° and vice versa, so that fine-tuning of electron-transfer redox properties requires a fair amount of purely empirical work. As nearly always true for organic reactions, we shall perhaps do best by imitating the ways biological electron-transfer systems work (Moore and Williams, 1976) ... 

Consider a reaction where two solid phases are involved. Thus, in addition to electron transfer (redox process), one of the solid phases decomposes while the other one forms. For example,... 

A prime feature of many of these systems and others with both Fe and Mo, are reversible electron transfer redox reactions and these give a clue to the importance of such polynuclear species in Nature. The compounds are made quite readily from FeCl2, FeCla, [FeClJ1- 2, or [Fe(SR)4]12 as shown in Fig. 17-E-5. 

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