Four-electron processes

Before considering the role of the electrode material in detail, there is one further factor which should be pointed out. The product of an electrode process may be dependent on the timescale of the contact between the electroactive species and the electrode surface, particularly when a chemical reaction is sandwiched between two electron transfers in the overall process. This was first realized when it was found that ir E curves and reaction products at a dropping mercury electrode were not always the same as those at a mercury pool electrode (Zuman, 1967a). For example, the reduction of p-diacetylbenzene at a mercury pool was found to be a four-electron process, giving rise to the dialcohol, while at a dropping mercury electrode the product was formed by a two-electron process where only one keto group was reduced (Kargin et al., 1966). These facts were interpreted in terms of the mechanism... 

The possibility that adsorption reactions play an important role in the reduction of telluryl ions has been discussed in several works (Chap. 3 CdTe). By using various electrochemical techniques in stationary and non-stationary diffusion regimes, such as voltammetry, chronopotentiometry, and pulsed current electrolysis, Montiel-Santillan et al. [52] have shown that the electrochemical reduction of HTeOj in acid sulfate medium (pH 2) on solid tellurium electrodes, generated in situ at 25 °C, must be considered as a four-electron process preceded by a slow adsorption step of the telluryl ions the reduction mechanism was observed to depend on the applied potential, so that at high overpotentials the adsorption step was not significant for the overall process. 

To summarise, for the scheme above, analysis of the ring and disc currents allows us unequivocally to establish whether or not a four-electron process... 

Obtained in acetonitrile solution containing 0.2 mol dm-3 Bu NBF4 as supporting electrolyte. Solutions were 1 x 10 3 mol dm-3 in receptor and potentials were determined with reference to an Ag/Ag+ electrode. Two-electron process. Four-electron process. Cathodic shift in redox wave produced by the presence of anions (up to 5.0 equiv) added as their tetrabutyl-ammonium salts. 

Aromatic Azo Compounds Reduction of aromatic azo compounds involves a four-electron process that proceeds through a short-lived intermediate, hydrazoben-zene, that ends with complete reductive cleavage of the azo hnkage and formation of aromatic amines. 

The two-electron oxidation of primary alcohols RCH OH to aldehydes RCHO is rarer with RuO than is the four-electron process to RCOOH (2.1, 2.4.1.1 Table 2.1). Examples include the reagents RuCyaq. Na(Br03)/( Bu N)Br/CH2Cl2... 

For compound (Scheme 1 and Table 1) the oxidation pattern is quite different the differential pulse voltammetry exhibits two peaks of equal height, both corresponding to a two-electron oxidation process (Figure 11). The first oxidation occurs at nearly the same potential as the four-electron process of compound 6F. This shows that, as expected, the two Os(bpy)2( i-2,3-dpp) units are the first to be oxidised (Table 2). The second process concerns the oxidation of the two Ru(bpy)2(p-2,5-dpp) units. Since such units lie far away from the previously oxidized Os-containing units, their oxidation occurs at a potential (Table 2) close to that of the equivalent peripheral units of 6D. As in the case of the compounds 6A-F the oxidation of the two inner units are displaced outside the accessible potential window. 

Rajca and co-workers have studied star-branched and dendritic high-spin polyradicals which are potential organic magnets. Representative data were obtained for the model tetra-anionic compound 55. Three redox waves were observed by cyclic voltammetry and differential pulse voltammetry for a four-electron process between the potentials of -2.00 and -1.20 V (vs. SCE). Electrochemical experiments with these materials have usually been performed at 200 K. The polyradicals, which are less stable for systems with more unpaired electrons, have been characterized by spectroscopic studies, ESR data, and SQUID magnetometiy. 

Further work by Anson s group sought to find the effects that would cause the four-electron reaction to occur as the primary process. Studies with ruthenated complexes [[98], and references therein], (23), demonstrated that 7T back-bonding interactions are more important than intramolecular electron transfer in causing cobalt porphyrins to promote the four-electron process over the two-electron reaction. Ruthenated complexes result in the formation of water as the product of the primary catalytic process. Attempts to simulate this behavior without the use of transition-metal substituents (e.g. ruthenated moieties) to enhance the transfer of electron density from the meso position to the porphyrin ring [99] met with limited success. Also, the use of jO-hydroxy substituents produced small positive shifts in the potential at which catalysis occurs. 

For water oxidation, a redox potential of E, = 2.33 V (at pH 7, vs. NHE) is needed in the first step to abstract one electron from a water molecule (Eq. (1)). When the intermediate is stabilized on a catalyst and four electrons of two molecules of H20 are oxidized without isolating the intermediates (so-called four-electron process), the required redox potential is only 0.82 V (Eq. (2)). 

The potential level of the 02 evolving site of the photosynthesis (see Fig. 1) ranging around 0.82 V shows that a four-electron process occurs in it. The water oxidation site of the photosynthesis contains more than four Mn ions interacting with each other, thus leading to the four-electron reaction of water to give 02, Such a multielectron reaction leads to the generation of H2 from proton reduction as described later in chapter 4 on water photolysis. 

Fig. 1. Estimated Latimer diagrams for the reduction of aqueous dioxygen calculated on the thermodynamic assumption of [02] = 1 M at pH = 0, 25°C. The overall four-electron process has the same potential (1.27 V) for both the chromium free and chromium mediated processes (14).

A combination of majority and minority carrier processes has been observed to produce quantum yields in excess of one at both n- and p-type interfaces. In all cases where this has been noted, the redox species employed have been capable of multiple-electron processes. This type of behavior is often seen for the oxidation of carboxylic acids at n-type semiconductors (a two-electron process). It has also been noted for hydrazine oxidation (a four-electron process) and the reduction of hydrogen peroxide. 

Four-electron processes can take place in the context of a [l,2]-shift (anionic),... 

Most of the work with these compounds employs voltammetry. The isomeric nitropyridines (73) are reduced by either a reversible two-electron or an irreversible four-electron process (Scheme 22).111 The four-electron product, the hydroxylamine, can be further reduced to the amine. The (V-oxide of 4-(73) was reduced to the aminopyridine (74) in good yield (Scheme 23).112 At... 

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