Two- step one-electron reduction

Analogously, l,2-dicyano-l,2-diphenylethylene, which is free from steric strains, and its strained isomer l,l-dicyano-2,2-diphenylethylene are reduced at practically the same potentials (Ioffe et al. 1971, Todres and Bespalov 1972). In DMF, with the support of Et4NI, the reversible two-step one-electron reductions are characterized by the following potentials (mercury pool as a reference electrode) -0.48 and -0.98 V for 1,2-dicyanoethylene and -0.50 and -1.07 V for 1,1-dicyano isomer. Thus, electrochemical reduction does not fix the difference in isomer structures. 

Electrochemistry of the protonated compounds supported the reactions given in Scheme 2. The series of monoprotonated complexes exhibits a reversible two-step one-electron reduction of the protonated anthraquinone moiety (AqH) in the cyclic voltammograms, whose potentials are largely shifted in the more positive direction than those of nonprotonated forms (Table 1). The reversible oxidation waves of non-and monoprotonated complexes are derived from the metal-centered oxidation of the ferrocenyl and ftilvene complex moieties. In [l,5-Fc FvAqH2 ], the redox reaction... 

Suppose a compound, M, is observed to undergo two successive one-electron reduction steps (equation 5). The comproportionation constant Kc for reaction (6) is simply given by expression (7). At 298 K, with °, — E°2 = A ° expressed in mV, then expression (7) reduces to Kc = exp... 

It is now generally understood that the mechanism of hydroxylation by cytochrome P-450 proceeds by two successive one-electron reduction steps of the heme center, transforming dioxygen into a peroxide species bonded to iron (Scheme l).73,78 Well-defined steps (25) - (31) involve (a) the formation of a high-spin Fein-enzyme-substrate complex (26) (b) one-electron reduction of (26) to a high-spin ferrocytochrome (27) (c) addition of dioxygen to form a superoxo-Fe111 complex (28) and (d) one-electron reduction of the superoxo complex to a peroxide complex (29). 

Aprotic solvents mimic the hydrophobic protein interior. However, a functional artificial receptor for flavin binding under physiological conditions must be able to interact with the guest even in competitive solvents. As found by spectroscopic measurements with phenothiazene-labeled cyclene, the coordinative bond between flavin and Lewis-acidic macrocyclic zinc in methanol was strong enough for this function. Stiochiometry of the complex was proved by Job s plot analysis. Redox properties of the assemblies in methanol were studied by cyclic voltammetry which showed that the binding motif allowed interception of the ECE reduction mechanism and stabilisation of a flavosemiquinone radical anion in a polar solvent. As a consequence, the flavin chromophore switched from a two-electron-one-step process to a two-step-one-electron-each by coordination. 

Electrochemical interconversion of homo- and heteronuclear gold cluster compounds remains an area that has received scant attention, despite the potential for changing the electron count and hence the metal cage geometries of these clusters by electrochemical methods. The electrochemical redox reactions of [Pt(AuPPh3)8]2+ have been studied, using pulse, differential pulse, and cyclic voltammetric techniques (124, 242) and two reversible, one-electron reduction steps have been... 

Recently, a series of triruthenium clusters containing allenyl or alkynyl ligands have been investigated electrochemically, and two, subsequent one-electron reduction steps have been observed (471). Further studies of this kind should provide a new insight into the electronic processes which take place in alkyne-substituted cluster systems. 

In principle, these complexes undergo a one-electron oxidation and two successive one-electron reductions. The potential and reversibility of the oxidation step depend very much on the axial ligand E. Oxidation of halide and cyanide complexes is followed by rapid loss of the oxidized axial ligand [130, 131] ... 

The most widely studied examples are cyclooctatetraene (COT,I) and its derivatives. In such conventional aprotic solvents as DMF, dimethyl sulfoxide (DMSO), or acetonitrile containing tetraalkylammonium salts, two distinct one-electron reduction waves are observed at approximately —1.64 V and —1.80 V versus SCE, with AE separations varying from —130 V to —240 mV [43,53-56]. In THF and NH3, this separation reduces further [57-59], and in the presence of all alkali salts [60,61] even two-electron reduction waves with positive AE differences were obtained, indicating large disproportionation constants. The unusually small separation of the two redox steps in comparison to the... 

As mentioned in Chapter 5, Qa in native reaction centers is reduced only to the semiquinone form while Qb can undergo two successive one-electron reduction steps to form the quinol (after protonation). In this chapter we discuss the electron-transfer reactions between Qa and Qb, exchange between reduced Qb and the quinone pool, and protonation coupled to electron transfer. 

Reaction mechanism Based on the observation of reaction intermediates in the crystal structure and on quantum chemical calculations Einsle et al. [148] propose an outline of the first detailed reaction mechanism of the cytochrome c Nir from W. succinogenes. Nitrite reduction starts with a het-erolytic cleavage of the weak N-O bond, which is facilitated by a pronounced backbonding interaction between nitrite and the reduced active site iron. The protons come firom a highly conserved histidine and tyrosine. Elimination of one of both amino acids results in a significant reduced activity. Subsequently, two rapid one-electron reductions lead to a FeNO form and, by protonation, to a HNO adduct. A further two-electron two-proton step leads to hydroxylamine. The iron in the hydroxylamine complex is in the Fe(III) state [149], which is unusual compared to synthetic iron-hydroxylamine complexes where the iron is mainly in the Fe(II) state. Finally, it readily loses water to give the product, ammonia. This presumably dissociates firom the Fe(III) form of the active site, whose re-reduction closes the reaction cycle. 

Fig. 5 Cyclic voltammetry (500 mV s ) of 1 mM 2-nitrobenzylchloride in 0.1 M Klin liquid ammonia at —60 °C. (1) and (2) correspond to the first and second scans. Peak (a) is associate with the dechlorination step, while (b), (b ) and (c), (c ) correspond to the two successive one-electron reductions of the formed dimer.

The cyclic voltammogram of polymer lid containing an octamethylene bridge is shown in Figure 12 This CV shows the two sequential one-electron reduction steps that the iron centers pendent to the polymer backbone undergo at a scan rate of 5 V/s. It was foimd that at low scan rates, the second reduction step was irreversible however, die reversibility increased with higher scan rates. The Eia values corresponding to formation of the neutral 19-electron, and anionic 20-electron iron species occurred at -1.07 and -1.77 y respectively. 

One aspect that reflects the electronic configuration of fullerenes relates to the electrochemically induced reduction and oxidation processes in solution. In good agreement with the tlireefold degenerate LUMO, the redox chemistry of [60]fullerene, investigated primarily with cyclic voltammetry and Osteryoung square wave voltammetry, unravels six reversible, one-electron reduction steps with potentials that are equally separated from each other. The separation between any two successive reduction steps is -450 50 mV. The low reduction potential (only -0.44 V versus SCE) of the process, that corresponds to the generation of the rt-radical anion 131,109,110,111 and 1121, deserves special attention. 

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