Electron-transfer reaction, reversible

The two cyclic voltammograms shown in Fig. 13 of [Scm(LBu2)] (b) and Scln(LMe-)] (a) show an important feature. Whereas the cyclic voltammetry (CV) of the former compound displays three reversible one-electron transfer waves, the latter shows only two irreversible oxidation peaks. Thus methyl groups in the ortho- and para-positions of the phenolates are not sufficient to effectively quench side reactions of the generated phenoxyls. In contrast, two tertiary butyl groups in the ortho- and para-positions stabilize the successively formed phenoxyls, Eq. (5)... 

An irreversible chemical reaction interposed between two reversible one-electron transfers (case R-R). The so-called ErQEr process, if n — n2 = 1, can be written as ... 

Quinone Reduction This is a reversible, one-electron transfer reaction to the semi-quinone radical, followed by a second, reversible electron transfer that results in the formation of hydroquinone, as shown in Fig. 13.2. 

Some years later, at the beginning of the 1970s, first ECL system based on the luminescent transition metal complex tris(2,2 -bipyridine)ruthenium(II)-Ru (bipy)32 + -has been reported.11 It was shown that the excited state 3 Ru(bipy)32 + can be generated in aprotic media by annihilation of the reduced Ru(bipy)31 + and oxidized Ru(bipy)33 + ions. Due to many reasons (such as strong luminescence and ability to undergo reversible one-electron transfer reactions), Ru (bipy)32+ later has become the most thoroughly studied ECL active molecule. 

Several classes of coordination compound undergo several successive, reversible one-electron-transfer reactions. These comprise a so-called electron-transfer chain or series .8 Cyclic voltammetry is particularly useful for recognizing such behaviour and an example is illustrated by Figure 2. This shows the four members of the electron-transfer chain [Fe4S4(SPh)4]"-, n = 1-4.5 An electron-transfer series provides the coordination chemist with a means of examining the consequence of systematic addition (or removal) of electrons from a nominally fixed geometry thermodynamic, kinetic and spectroscopic relationships between members of a series can be explored.9... 

SEV is an effective means of probing homogeneous chemical reactions that are coupled to electrode reactions, especially when it is extended to cyclic voltammetry as described in the next section. Considerable information can be obtained from the dependence of ip and Ep on the rate of potential scan. Figure 3.20 illustrates the behavior of ip and Ep with variation in scan rate for a reversible heterogeneous electron transfer reaction that is coupled to various types of homogeneous chemical reactions. The current function j/p is proportional to ip according to the equation... 

The electrochemical oxidation of 2,5-diaryl-1,4-dithiins (50) has been studied using various voltametric techniques and all compounds were found to undergo quasi-reversible one-electron transfers to the radical cations and dications.126 The first formal redox potential and the lifetime of the radical cation were found to decrease with increasing electron donation from the aryl ring. The major products were the 2,2 -dimers, which result via reaction of two radical cations for which rate constants are given. Dibenzothiophene radical cations reacted with tetranitromethane under... 

In the general case, referred to as quasi-reversible, the electron transfer reaction in Equation 6.6 does not respond instantaneously to changes in . In other words, [R]x=o and [Ojx o are determined not only by the value of — °, but also by the magnitudes of k° and a through Equations 6.10 and 6.11. Typical voltammograms for quasi-reversible electron transfers are shown in Fig. 6.11. There are no simple analytical expressions for ped — °, z ped, ped — p/d, A p and — z°x/z ped for quasi-reversible electron transfers. Values for a given set of v, k° and a are, when needed, most conveniently obtained by digital simulation. 

The Qf E curve for a reversible two-electron transfer taking place in a monolayer is independent of time (i.e., it has a stationary character) and, therefore, is independent of the potential-time waveform applied to the electrode, as in the case of a reversible one-electron transfer reaction. It is also important to highlight that the normalized charge, has a identical expression to that for the normalized transient current 7 v N obtained for solution soluble species when the NPV technique is applied to an electrode with any geometry (see curves in Fig. 3.16, and Eq. (3.141)), and also to the normalized stationary current obtained for solution soluble species when any potential-time waveform is applied for ultramicroelectrodes with any geometry. 

Alternating-current and frequency effects. With an AC rather than a DC voltage applied to the electrodes, the processes above reverse themselves with the period of the alternating voltage. But each process proceeds at a different rate (with a characteristic relaxation time) so that their relative contributions to energy dissipation vary with frequency. As the frequency is increased concentration-polarization can be reduced or eliminated, particularly if the electrode reaction is reversible (fast electron transfer in both directions). 

The reversible one-electron transfer to form an anion radical (R ) is followed by an irreversible chemical protonation to form /f H, which is subsequently reduced itself (the reduction potential of the species / H, has been shown to be more positive3 than that of the parent, R) and then undergoes another irreversible protonation reaction. In a protic solvent, the reactions proceed rapidly to the final product, / H2. In a rigorously purified aprotic solvent, the intermediate anion radical R , has an appreciable lifetime and reacts only slowly, principally with adventitious impurities in the solvent. Thus, the stability of aromatic anion radicals can be taken as a measure of the protic character of a solvent. 

The reaction of (NH4)SCN in aqueous solution with (NH4)2TcX 6 (X = Cl or Br) yields [Tc(NCS)6]2- together with the analogous Tc(III) complex131. The Tc(III) ion is octahedral with Tc-N bond distances averaging 2.045 A. Electrochemical studies in acetonitrile reveal a reversible one-electron transfer between the two oxidation states, so that solutions of [Tc(NCS)6]3 in air are quickly oxidized to the Tc(IV) form. 

There has been considerable interest in the chemistry and electronic structures of cobalt and iron complexes of a s-l,2-disub-stituted ethene-1,2-dithiol1 2 and their Lewis base adducts.3-5 The complexes, and their Lewis base adducts which may contain pyridine, phosphines, NO, dipyridyl (2,2/-bipyridine), etc., are capable of undergoing reversible one-electron transfer reactions, thereby generating a series of complex ions differing from each other by only one unit of electric charge. 

Cyclic voltammetry shows that [Nb3(/a-Cl)6(i7-C6Me6)3] undergoes reversible one-electron transfer ( = -0.21 V vs Ag/Ag", in DME), but anodic oxidation, or reaction with ceric ion, gave the hexanuclear product [Nb6Cli2(Tj-C5Me6)6r 334). Taken with the demonstrated stability of [Nb3(/i.-Cl)6(i7-C(,Me6)3], mentioned earlier, this seems to indicate that the trinuclear dication undergoes slow dimerization. The monocation is also reported to be reducible ( 1/2 = -1.8V vs Ag/Ag ), but the reduction products were not studied 134). 

The remarkable hexanuclear complex [ NiCp ] (81), prepared by the sodium naphthalenide reduction of nickelocene, undergoes an extensive series of reversible one-electron transfer reactions cyclic voltammetry shows waves relating the six species [ NiCp 6] (Z = -2 to 3). Chemical oxidation of 81, with Ag, gave the monocation whose structure shows only a small tetragonal distortion from the octahedral array of nickel atoms in the neutral precursor (198). 

An important application of combined electrochemistry and ESR spectroscopy is the characterization and identification of intermediates and products of electrode reactions [334,336,379-391]. For instance, the ESR technique is particularly useful to measure the degree of protonation under conditions where the radical ions take part in acid-base equilibria [380,381]. Such information may be obtained only with difficulty by other methods, but the coupling pattern of the ESR spectrum may often give the answer directly. An illustrative example is found in the anodic oxidation of 2,4,6-tri-rert-butylaniline, which, as expected, gives the radical cation as the initial electrode product [380]. In an aprotic solvent like MeCN or CH3NO2 the radical cation is stable and the ESR spectrum observed is in accordance with the reversible one-electron transfer indicated by CV. However, when the electrolysis is carried out in the presence of diphenylguanidine as a base, the ESR spectrum changes drastically and can be attributed to the presence of the neutral free radical formed by deprotonation of the radical cation. 

The above description of the redox reaction for Fe(CN)64- -/3 is a textbook example because the system nicely obeys Nemstian conditions and many cycles can be repeated without distortion of the voltammogram (we call this a reversible system) electron transfer is rapid and reversible at the electrode surface and complete concentration polarization is achieved under conditions of 1 m KNO3. We know this is a reversible system because ipa/ /pc 1. For a rigorous check on this condition, a plot of ip vs vl,/2 should be linear (where v — scan rate), equation (4.5). 

---