Four-electron transfer

Practically nothing is known about the chemical behavior of [5]radialenes. An exception is the electrochemistry of systems 139-141. These potent electron-donating molecules show only one pair of reversible waves involving a net transfer of four electrons. In fact, these systems constitute the first examples of a single-wave four-electron transfer with only one macroscopic redox site in an organic redox system87,89. The corresponding tetracationic salts could be isolated. 

The kinetic models for these reactions postulate fast complex-formation equilibria between the HA- form of ascorbic acid and the catalysts. The noted difference in the rate laws was rationalized by considering that some of the coordination sites remain unoccupied in the [Ru(HA)C12] complex. Thus, 02 can form a p-peroxo bridge between two monomer complexes [C12(HA)Ru-0-0-Ru(HA)C12]. The rate determining step is probably the decomposition of this species in an overall four-electron transfer process into A and H202. Again, this model does not postulate any change in the formal oxidation state of the catalyst during the reaction. 

For the central violene rc-system, we chose benzene with acetylene spacers for decreasing steric hindrance between the large cationic centers. Therefore, the tetracation 254 salt was designed as a first example for a hybrid with a cyanine unit at the both termini (27). As illustrated in Figure 22, tetracation 254 exemplifies a new hybrid system, which produces another cyanine substructure 252 by two-electron transfer. In this case, overall, four electron transfer should afford a fully neutralized species such as a neutral diradical. 

Reduction of dibenzothiophene with sodium in liquid ammonia has been shown to be sensitive to the experimental methods employed however, the major product is usually 1,4-dihydrodibenzothiophene. 27 -28i The electrochemical reduction of dibenzothiophene in ethylene-diamine-lithium chloride solution has been shown to proceed via stepwise reduction of the aromatic nucleus followed by sulfur elimination. In contrast to the reduction of dibenzothiophene with sodium in liquid ammonia, lithium in ethylenediamine, or calcium hexamine in ether, electrolytic reduction produced no detectable thiophenol intermediates. Reduction of dibenzothiophene with calcium hexamine furnished o-cyclohexylthiophenol as the major product (77%). Polaro-graphic reduction of dibenzothiophene 5,5-dioxide has shown a four-electron transfer to occur corresponding to reduction of the sulfone group and a further site. ... 

Manecke et al.16s synthesized a semiconducting polymeric complex which possessed both bis(ethylene-l,2-dithiolato)Cu(II) and a phthalocyanine-Cu(II)-type structure 54. This Cu complex exhibited high catalytic activity in the oxidative polymerization of XOH, about 50 times higher than that of pyridine-Cu. A synchronous four-electron-transfer mechanism was proposed for the catalysis of 54. The phthalo-cyanine-Cu(II) type structure of 54 is presumed to form a complex with molecular... 

Such a mechanism might play an important role in a metal-ion-catalyzed enzymic oxidation in vivo, in which metal ions work cooperatively 166. A synchronous four-electron-transfer requires a specific spatial arrangement which should be posable in a macromolecular environment. 

The reactions of dioxygen have been amply documented The reduction can occur by a one-, two- or four-electron transfer reaction, the first of which is energetically unfavorable. The one-electron reduction of hydrogen peroxide yields the extremely reactive hydroxyl radical (Scheme 1). 

A tetraanion salt (84 )/4M has been formed by stepwise four-electron transfer from 4,7,12,15-tetrastyryl[2.2]paracyclophane (8) to lithium, sodium, and potassium metals in [2Hx]THF at 220 K a strong effect of the cyclophane hub on the charge distribution has been demonstrated and the influence of o1, o2, o3, o4-tetramethyl and px, p2,pi,pA-tetramethoxy substituents on the ease of reduction has been tested.9... 

A molecular conversion system based on a four-electron transfer to 02 was accomplished in the 02-oxidative polymerization of diphenyl disulfide (Figure 14) [116]. This is the first example of a multielectron mediator that is applied to molecular conversion systems. The multielectron transfer process from the reduced vanadium(III) complex (VOV+) to 02 not only revealed the 02 oxidation mechanism but also provided additional insight into the unique chemistry of vanadium with possible relevance to metal mono-oxygenases. 

The WOC is oxidized stepwise by a nearby tyrosine residue (Tyrz), which is itself oxidized by the chlorophyll cation radical P680+ (formed by light-induced charge separation). The electrons are eventually used by PSII for the reduction of plastoqui-none. After the WOC has lost four electrons, the accumulated oxidizing power drives the formation of molecular oxygen from two substrate water molecules, and the catalytic system is reset. The sequence of the four electron-transfer steps is summarized in the Kok cycle [32] of Figure 4.5.3, where the most probable spectroscopically derived oxidation states of the Mn ions [33] are shown for each of the five redox state intermediates S (n - 0-4). 

As an example, a four-electron transfer from two M+n metal atoms in a binuclear complex may be visualized ... 

The reduction of dioxygen to water (concomitant with the oxidation of other species) is a 4e process, and it is virtually impossible to have the four electrons transferred simultaneously without the formation of intermediates (like O - and H2O2). 

The electrodeposition of tellurium, Te, has been reported in a basic EMICl-AICI3 ionic liquid [31]. Tellurium tetrachloride, TeCLi, is soluble in the basic ionic liquid to form a tetravalent tellurium chlorocomplex anion, TeClg , which can be reduced to metallic Te by four electron transfer reaction ... 

Photosystem 11 (PS II) within higher plants represents a solar-energy-driven process that removes hydrogen atoms from water to form molecular oxygen (O2, dioxygen) through an overall four-electron transfer reaction (equation 20), ... 

Recently, it was reported that loading small amount of platinum onto tungsten(VI) oxide enhances the visible-light photocatalytic activity significantly and this is caused by the catalytic action of platinum to induce multiple-electron transfer to oxygen 44). Reactions of two and four-electron transfer processes are as follows (potential in parentheses is standard electrode potential versus standard hydrogen electrode at pH 0). 

In the context of low temperature fuel cell, it is important to reach a four electron transfer during the ORR in order to increase... 

It has been argued that heterotrophic nitrification involves enzyme systems that are quite different from those of the autotrophs (Wehrfritz et al, 1993) and that heterotrophic nitrification cannot serve as an energy generating mechanism (Castignetti, 1990), as the autotrophic process does. In the aerobic denitrifiers, which are also capable of nitrification, the initial enzyme, AMO, appears to be quite similar to the enzyme in autotrophic nitrifiers. However, HAO differs significantly and in the heterotrophs, is a smaller, simpler enzyme that performs a two electron transfer (instead of the four electron transfer of the autotrophic HAO) and... 

Conventional polarographic measurements are usually less definitive—one-, two-, three- or four-electron transfers have been associated with the first cathodic wave of various acetylenes (Table 7). In the case of diphenylacetylene,. several polarographic studies still do not permit a clear choice between a one- or a two-electron first wave . As for the dianions of equation (75), it is probable that they are present in some cathodic processes. Since the evidence for them generally depends on the interpretation of polarographic and product data, their intermediacy is less certain. 

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