Multielectron transfer

For multielectron-transfer (reversible) processes, the cyclic voltammogram consists of several distinct peaks if the E° values for the individual steps are successively higher and are well separated. An example of such a mechanism is the six-step reduction of the fullerenes C60 and C70 to yield the hexaanion products and C7q. Such six successive reduction peaks are observed in Figure 2-4. 

Alonso-Vante N, Schubert B,TributschH (1989) Transition metal cluster materials for multielectron transfer catalysis. Mater Chem Phys 22 281-307... 

All these results indicate that one is just at the beginning of understanding the function of catalysts being deposited on a semiconductor. There is still quite a confusion in many papers published in this field. Therefore the catalytic properties depend so much on the procedure of deposition . It seems to be rather difficult to produce a catalyst for 02-formation, as shown by results obtained with Ti02 (see e.g.) . Rather recently new concepts for the synthesis of new catalysts have been developed applicable for multielectron transfer reactions. Examples are transition metal cluster compounds such as M04 2RU1 gSeg and di- and trinuclear Ru-complexes . 

In any fast multielectron transfer reaction all the electrons cannot be transferred in one step but only by a succession of single-electron transfer steps, whereas Eq. (6) was arrived at by Devanathan31 for the simple case in which it is assumed that the same step is rate determining in both directions, irrespective of the number of electrons involved in the reaction. 

Semiconductor electrodes seem to be attractive and promising materials for carbon dioxide reduction to highly reduced products such as methanol and methane, in contrast to many metal electrodes at which formic acid or CO is the major reduction product. This potential utility of semiconductor materials is due to their band structure (especially the conduction band level, where multielectron transfer may be achieved)76 and chemical properties (e.g., C02 is well known to adsorb onto metal oxides and/ or noble metal-doped metal oxides to become more active states77-81). Recently, several reports dealing with C02 reduction at n-type semiconductors in the dark have appeared, as described below. 

If multielectron transfer takes place, the potentials required thermodynamically for C02 reduction are much less negative than... 

It has been our goal for some time to run photochemical energy storage reactions without relay molecules or separate catalysts. We have concentrated on the photochemistry of polynuclear metal complexes in homogeneous solutions, because we believe it should be possible to facilitate multielectron transfer processes at the available coordination sites of such cluster species. 

The main goals of this research are (1) to characterize polynuclear rhenium complexes which are capable of multielectron transfer reactions (2) to come up with appropriate conclusions on the redox-initiated transformations of this synthetic analog through the use of a new design of spectroelectrochemical cell and (3) propose possible systems or investigations where infrared spectroelectrochemi-stiy can be very useful. 

The mechanism of substrate reduction by sulfite reductase has not been established. The close contact between the 4Fe 4S cluster and the siroheme could provide an efficient pathway for multielectron transfer from the enzyme to the substrate (McRee et al., 1986). Of special significance is the possibility that the cluster—siroheme overlap could stabilize high-oxidation states of the siroheme that might be involved in the catalytic mechanism. With the availability of genetic, biochemical, spectroscopic, and crystallographic approaches, it is anticipated that rapid progress will be made in working out the details of substrate reduction by sulfite reductase. 

In conclusion, the electrochemical data offer a fingerprint of the chemical and topological structure of the polynuclear compounds. Furthermore, made-to-OTder synthetic control of the number of electrons exchanged at a certain potential can be achieved. The presence of multielectron processes makes such polynuclear complexes very attractive in view of their possible application as multielectron-transfer catalysts. Examination over a more extended oxidation potential window (in a solvent like liquid SOj) should permit one to obtain an even larger variety of oxidation patterns. 

The interest in highly branched polynuclear metal complexes, and more generally in dendritic species, is related not so much to their size, but rather to the presence of different components. An ordered array of different components can in fact generate valuable properties, such as the presence of cavities having different size, surfaces with specific functions, gradients for photoinduced directional energy and electron transfer, and sites for multielectron transfer catalysis. Studies along these directions are underway in our laboratories. 

The incorporation of redox-active organometallic units within or on the periphery of dendritic structures is an especially challenging target because such molecules ate good candidates to play a key role as multielectron transfer mediators in electrocatalytic processes of biological and industrial importance. In particular, the organometallic ferrocene moiety is an attractive redox center to integrate into dendritic structures, not only because it is electrochemically well behaved in most... 

A much more positive value of E° = —0.88 V is estimated in aqueous solution at pH >11 by using short-time pulse techniques [7, 8], The aqueous electrochemistry of Tc04 is complex. Under alkahne conditions, it is proposed that the protonation of Tc04 and/or the expansion of its coordination shell follows Eq. (1) and produces a more easily reduced Tc(VI) species, resulting in a multielectron transfer [8]. The subsequent reduction can... 

Of particular interest are the proton-coupled multielectron transfers of high-valent oxorhenium (poly)pyridyl complexes [6, 15-18, 35]. This behavior is... 

Copper(III)-aquo and-amine species have been generated by pulse radiolysis [194] but these are transient species that undergo rapid decay. Electrochemical generation of Cu(III) complexes of macrocyclic amines have been reported in acetonitrile [195] but they are also unstable and undergo spontaneous reduction to Cu(II). Nonetheless, it is presumed that copper(III) intermediates are generated as transients in a number of reactions, particularly those involving copper catalysis of multielectron transfer reactions. 

Finally, we should note that the E value of a multielectron transfer half reaction is given by the average of the respective standard one-electron reduction potentials. This is easy to rationalize when recalling that the overall standard free energy of reaction of a sequence of reaction steps is given by the sum of the ArG° values of each step. Hence, we may write ... 

As will be discussed in Sect. 4.1, multielectron transfer reactions at electrodes are most likely to occur in a series of single one-electron steps. For the present discussion, a general single n-electron transfer reaction is considered (only one transition state) with n most probably one... 

A multielectron electrode reaction may also occur by a number of mechanistic routes including sequential and parallel pathways, which in complex electrokinetics may also be analysed individually in terms of elementary chemical and electrochemical steps. Figure 7 depicts plots of log j vs. Tj for (a) sequential and (b) parallel paths for multielectron transfer reactions. It is apparent that, at a given electrode potential, in... 

It is noteworthy that siroheme is present in the enzymes responsible for catalyzing two out of only three known six-electron processes, and accordingly it is of great interest to try and identify any feature in siroheme that makes it particularly suitable for the mediation of multielectron transfer. A comparison of octaethylporphyrin, octaethylchlorin and octaethylisobacteriochlorin complexes of iron shows that redox potentials and vco of Fe(P)L(CO) and Fe(P)L(CO)2 were nearly independent of the porphyrin. The property that was most dependent upon the macrocycle structure was the potential for ring-based oxidation which increased in the order OEiBC < OEC < OEP.734... 

For the sake of simplicity, only two-electron transfer processes are discussed in this section. The theory corresponding to multielectronic transfer processes can be easily generalized from that presented here and is discussed in detail in [60-62],... 

Multielectron Transfer and Catalytic Mechanisms in Oxidative Polymerization... 

The catalytic center is regenerated through four-electron oxidation by 02. Such a multielectron transfer process may play a role in a metal-catalyzed enzymatic oxidation in vivo, such as photosystem II, in which metal ions often work cooperatively [77],... 

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. 

Recent research reveals that the oxidative polymerization of phenol with 02 also obeys the multielectron transfer with the polynuclear copper complexes [74], The multielectron oxidation of substrates provides new active species,... 

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