Bielectronic process

The hexanuclear Ru6 species has four outer and two inner metal centers oxidation active. Both in acetonitrile at room temperature ( 1/2 at + 1.52 V) and in liquid S02 at low temperature ( 1/2 at + 1.46 V), an oxidation process involving the practically simultaneous one-electron oxidation of the four outer Ru(II) centers is evidenced (Fig. 5.9 and Table 5.1). This confirms that the electronic interaction between metal centers that are not directly connected via a bridging ligand is negligible from an electrochemical viewpoint in the metal-polypyridine dendrimers. At more positive potentials, only recordable in liquid S02 at low temperature (Fig. 5.9), a bielectronic process, related to the simultaneous one-electron oxidation of the two inner metal centers at + 2.11 V, is found. This result was at a first sight surprising, since the redox... 

The electrochemical behavior of rotaxane 76+ can be straightforwardly explained on the basis of the above discussion. On reduction, a first monoelectronic process, assigned to the first reduction of the free bipyridinium unit, is followed by a bielectronic process, assigned to the first reduction of the two bipyridinium units encircled by the ring. Even the second reduction of the three bipyridinium units, which occurs at more negative potentials, occurs with the same 1 2 pattern. On oxidation, the behavior of rotaxane 76+ is again similar to that of rotaxane 66+, with a more intense process in correspondence of oxidation of the DMB units. 

For instance, complex 246+ exchanges up to a total of nine electrons. On reduction, it shows two monoelectronic and one bielectronic processes involving the bipyridinium units, and three monoelectronic processes concerning the bpy moieties (Fig. 13.20). On oxidation, two monoelectronic processes are observed the first one, being reversible, is assigned to the oxidation of the metal center and the second one, not fully reversible, to the oxidation of the alongside DMN unit of the macrocycle interlocked with the cyclophane. 

In fact, direct reduction of Pb(II) to Pb(0) by a bielectronic process has been reported imder laser irradiation, where due to the high photonic frequency, accumulation of electrons may allow multielectronic injection (Rajh et al., 1996a, b Thumauer et al., 1997). 

Figure 10. Correlation diagram for the electrochemical reduction of cyclophane [2]rotaxane 3", and [2]catenane 8 + (MeCN, 25 °C, potentials versus SCE) bielectronic processes are indicated [23],...

Cyclopropane-1,2-diols 1 are readily prepared in high yield by the electrochemical 1,3-coupling reaction of 1,3-diones. This reaction is a bielectronic process, whereby the hydroxylic solvent serves as proton donor. 

The polarographic method, another technique to measure the decay of superoxide, does not require special and expensive apparatus such as a Van der Graaf generator. A dropping mercury electrode works both as a source of superoxide and as a detector of superoxide dismu-tation products 100). The electroreduction of O2 in aqueous solution can be a monoelectronic process that produces O2 (O2 + e" O2) or a bielectronic process, with reduction to H2O2. The latter happens in the presence of protons O2 + 2e" -I- 2H H2O2. 

Miscellaneous Methods.—The electrochemical reduction of 1,3-diketones, PhCOCR 2 = H or alkyl, = alkyl or aryl), leads to cyclopropane-1,2-diols by a bielectronic process. In some cases stereoselective cyclization favours cis-diol formation and product yields are generally satisfactory. Electrochemical cyclization of 1,1-bisbromomethylcyclopropane affords spiropentane. ... 

Diphenylbenzidine can be reversibly oxidized according to a bielectronic process to give a colored diquinonediimine (Fig. 16.5). 

Dumbbell 34+ exhibits two bielectronic and reversible processes that can be attributed to the simultaneous first and second reduction of the two bipyridinium units contained in its axle-like section. The bielectronic nature of the processes indicates, as expected, that the bipyridinium units are equivalent and behave independently. Also, model rotaxane 44+ shows two bielectronic and reversible processes that are straightforwardly assigned to the bipyridinium units contained in its dumbbell component they are, however, shifted to more negative potentials compared to dumbbell 34+. These shifts can be attributed to the CT interactions with the electron donor ring that make the electron acceptor bipyridinium units more difficult to reduce, whereas the bielectronic nature of the processes indicates the such units are noninteracting and equivalent—both of them are surrounded by a ring—in full agreement with the supramolecular structure of 44+. 

Many enzymatic oxidation processes proceed by a multielectron mechanism. For example, bielectron reduction of 02 to H202 and tetraelectron reduction to H20 produce short-lived... 

CV) experiments [9a, 9b], the bielectronic oxidation process occurs on the TTF unit at +0.80 V vs. saturated calomel electrode (SCE), and the oxidation of the DNP ring system is strongly displaced toward +1.60 V vs. SCE, compared to that of the free crown ether (+1.17 V vs. SCE). The dramatic increase in the oxidation potential of the DNP ring system indicates that it is encircled by the tetracationic cydophane after the oxidation of the TTF unit. 

Dendrimer 67 contains two naphthalene diimide electroactive units in the core [137]. Electrochemical investigation in DMF showed the presence of two reversible bielectronic reduction processes at the potential typically featured by other naphthalene diimide-containing compounds. Thus, the two units behave independently and can be readily accessed from the electrode surface. The lack of encapsulating... 

It is usual that the first reduction wave of commonly studied POMs be monoelectronic or, at the utmost, bielectronic [89]. As a consequence, energetically favorable catalytic processes that require larger numbers of electrons can only be accomplished at fairly negative potentials where the necessary number of charges is accumulated and delivered by the POM framework hence the search for strategic parameters that could favor apparently multiple electron uptake on the first wave of POMs. The present case deserves emphasis as a simple illustrative example that proved to be very beneficial in the electrocatalytic reduction of nitrite. 

The redox potentials of short-lived silver clusters have been determined through kinetics methods using reference systems. Depending on their nuclearity, the clusters change behavior from electron donor to electron acceptor, the threshold being controlled by the reference system potential. Bielectronic systems are often used as electron donors in chemistry. When the process is controlled by critical conditions as for clusters, the successive steps of monoelectronic transfer (and not the overall potential), of which only one determines the threshold of autocatalytical electron transfer (or of development) must be separately considered. The present results provide the nuclearity dependence of the silver cluster redox potential in solution close to the transition between the mesoscopic phase and the bulk metal-like phase. A comparison with other literature data allows emphasis on the influence of strong interaction of the environment (surfactant, ligand, or support) on the cluster redox potential and kinetics. Rela-... 

One should therefore introduce mono- and bielectronic diagonal terms which take into account both the spectroscopic state of the atom and the interatomic repulsion energies, which depend on the orbital occupancies. The second-order Anderson-type corrections would then concern all back-and-forth processes from a neutral to a neutral OVB determinant through a singly ionic VB determinant. For instance in C2, the second-order process... 

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