Multi-electron redox reactions

Metal-assisted cycloaddition reactions involving meso-perfluoroalkylated porphyrinic templates define a versatile methodology for the syntheses of both cofacial porphyrin structures and facially-functionalized (porphinato)metal species [62], and corresponding compositions that offer new opportunities for development of small molecule redox catalysts [63, 64]. Scheme 21 shows the versatile metal-mediated [2+2+2] cycloaddition step for the isolation of meso-perfluoroalkyl cofacial porphyrins that results in the purified indane complexes in 10-12 % yields. These cofacial complexes are directly applicable to multi-electron redox reactions such as oxygen or nitrogen reduction and even hydrocarbon oxidation. 

Polyoxometalates can undergo fast and reversible multi-electron redox reactions involving -> electron transfer between mixed-valence transition metal sites (e.g., or Mo ) according to the electron self-... 

Hydrogen generation requires a photosensitizer, an electron donor, an electron mediator and a multi-electron redox eatalyst. To obtain the polymer the reaction mixture must be refluxed in methanol for 20 hours. This yields a polymer wifli 1% viologen groups. In the system, water donates the protons and electrons are supplied by ethylenediamine tetraacetic acid disodium salt. The viologen groups transport the electrons to the multi-electron redox catalyst (2,2 -bipyridine)ruthenium(ll), Ru(bpy). ... 

Abstract Recent advances in the metal-catalyzed one-electron reduction reactions are described in this chapter. One-electron reduction induced by redox of early transition metals including titanium, vanadium, and lanthanide metals provides a variety of synthetic methods for carbon-carbon bond formation via radical species, as observed in the pinacol coupling, dehalogenation, and related radical-like reactions. The reversible catalytic cycle is achieved by a multi-component catalytic system in combination with a co-reductant and additives, which serve for the recycling, activation, and liberation of the real catalyst and the facilitation of the reaction steps. In the catalytic reductive transformations, the high stereoselectivity is attained by the design of the multi-component catalytic system. This article focuses mostly on the pinacol coupling reaction. 

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.

Spectrophotometry has been a popular means of monitoring redox reactions, with increasing use being made of flow, pulse radiolytic and laser photolytic techniques. The majority of redox reactions, even those with involved stoichiometry, have seeond-order characteristics. There is also an important group of reactions in which first-order intramolecular electron transfer is involved. Less straightforward kinetics may arise with redox reactions that involve metal complex or radical intermediates, or multi-electron transfer, as in the reduction of Cr(VI) to Cr(III). Reactants with different equivalences as in the noncomplementary reaction... 

Many redox reactions at electrodes involve transfer of more than one electron. It is agreed that such processes usually involve several consecutive one-electron steps rather than a simultaneous multi-electron transfer. The kinetics of the overall reaction (and hence the current flowing) are complicated by such factors as the lifetimes of the transient intermediate species. 

Electron tunneling may also be of significance for redox catalysis, including enzyme catalysis. In particular it may turn out to be a tool for carrying out catalytic reactions via multi-electron paths. For instance, according to the data of ref. 11, the two-electron reduction of molecular oxygen to hyd-... 

Multi-electron mechanisms of redox reactions. Switching molecular devices... 

As seen in Eq. 1, the water-splitting reaction has an overall energy requirement of 4.92 eV per O2 molecule formed (or +474.7 kj/mol O2 formed). The most abundant solar radiation to strike the earth s surface falls in the visible range (750-400 nm) and fortunately, these photons are energetic enough (1.65-3.1 eV)27 so that as little as two photons are required to drive this process thermodynamically. When broken down into redox half-reactions (5 and 6), the multi-electron nature of reaction 1 is readily apparent. 

Figure 4B shows the set of 31 produetive and 21 unproductive electron transfer reactions in multi-redox center oxidoreduetases with structures available in the Protein Data Bank (PDB). Both productive and unpro-duetive reactions have statistically indistinguishable distributions, which are in turn indistinguishable from the arbitrary protein paeking distribution. Thus Nature has not generally selected protein heterogeneity to assist pro-duetive and hinder counterproductive eleetron tunneling. 

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