Two Reversible One-electron Transfers

1 Two reversible one-electron transfers. The simplest case to treat consists of two successive one-electron transfers (EE mechanism). 

It is conceivable that from the molecular viewpoint there are some important differences between the occurrence of separate and simultaneous two-electron transfers. In fact, in principle, the addition of the first electron must make the second electron addition electrostatically 

It should be recalled that in the case of the EE mechanism, in solution one can have the comproportionation reaction  

In order to have a positive exponent, AE0 is given by E E ) for cathodic processes, or by (El1- E ) for anodic processes. In the present case, since nx = n2 = 1  

Such values are reported in Table 2, together with those of a current function . The latter parameter is correlated to the variation of the relative height of the peak current as a function of AE°, where one considers the first value of 0.619 as indicative of a one-electron process. 

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Cyclic voltammograms of DTT-TTF, 86a and 86b, exhibited two reversible one-electron transfer processes corresponding to the successive formation for the stable cation radical and dication <2003JMC1324>. 

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 ... 

Dendritic derivatives of these macrocycles can be placed in the wider context of studies on metalloporphyrins with sterically hindered faces which have been designed in attempts to mimic the properties of heme proteins and chlorophylls, and there are suggestions that steric isolation of the metalloporphyrin nucleus is important in certain biological functions, The redox properties of metalloporphyrins are well-documented they are dominated by two, reversible one-electron transfers involving both the metal and the ligand. The first dendritic porphyrins of general structure 47 and their Zn complexes were reported by Inoue et al. who... 

Comparison of the reduction process with that of the acyclic analogue [Cu(l)2]" allowed to discriminate topographical and topological contributions to the overall electrochemical behavior of Cu.5+. For Cu-S" " (as for [Cu(l)2] ), two reversible one-electron transfers are observed at very negative potentials (-1.650 and -1.860 V versus SCE for Cu-S ). This behavior is distinctly different from that of other complexes containing unsubstituted chelates, like bpy and phen (Figure 3). For instance, the following reductive dissociation takes place at -0.262 V [23] ... 

A series of A A iV lV -tetraaryM -dimethyl-TS-phenylenediamines (57) shows a substituent effect on the stability of electro-oxidation products95. Cyclic voltammograms of 57a and 57b in propionitrile exhibit two reversible one-electron transfers at ambient... 

Complexes with pyridine-2,6-diimine ligands, five-coordinate [NiX2(174)] (X = C1, Br) or six-coordinate [Ni(174)2]X2, were usually assumed to have innocent neutral ligands. The X-ray structure and spectroscopic characteristics of [Ni(174)2](PF6) confirm that the complex contains the neutral ligand ([174] ) and a divalent nickel ion.579 The cyclic voltamogram of this complex in CH2C12 shows three reversible one-electron-transfer processes at = 1.19 V, —1.30 V, and — 1.82V (vs. Fc+/Fc), of which the first is a one-electron oxidation, while the other two correspond to two successive one-electron reductions. The spectroscopic data allow one to assign these processes as follows ([174]1 is a one-electron reduced radical form of [174] ) [Nini(174)°2]3+ [NiII(174)02]21 - " [NiI(174)°2]+ = " [NiI(174)°(174)1 ]°. 

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)... 

In addition to ferrocene, the oxidative redox couple that has received the most attention in supramolecular chemistry is tetrathiofulvalene (TTF), 35. This compound undergoes two reversible one-electron oxidations, first to a radical cation and then to a dication (Eq. 1.21). TTF first came to prominence in the 1970s when it was discovered that the charge transfer complex between it and 7,7,8,8-tetracyanoquinonedimethane (TCNQ) shows metallic conductivity. As a result, a large variety of different TTF derivatives have been prepared and characterized. This rich synthetic chemistry, coupled with the electroactivity, has intrigued supramolecular chemists for some time, with the result that the TTF unit has been incorporated into a wide variety of... 

A reversible one-electron transfer at + 1.34 V was observed, assigned to the metal-centered Ru(II)/Ru(III) oxidation. The complex cathodic pattern of this species, comprising 10 successive reversible one-electron transfer, is the sum of that of its two subunits, the C60 core and the Ru(II) tris-bipyridyl complex, thus excluding any... 

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... 

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. 

Photochemistry of quinones (1,4-benzoquinone, duroquinone, 2,6-di-t-butylbenzo-quinone, etc.) radical anions was studied in detail [173a]. According to cyclic voltammetry data in various solvents, the quinones showed typical reversible two-wave voltammograms, corresponding to two successive one-electron transfers [(5.22), (5.23)] to the radical anion then to the dianion ... 

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. 

A- Alky In i tropy razo I cs are also reduced in two stages the first stage corresponds to reversible one-electron transfer (Scheme 3.35). In comparison with nitropyrazoles not substituted on nitrogen atom, the first half-wave potentials of A-alkylnitro-pyrazoles are essentially moved in cathodic region. Using the ESR method the signals of primary radical anions are recorded. 

A typical example in heteroaromatic chemistry is the preparative electrolysis of some 2,5-diaryl-1,4-dithiins 13, which give two well resolved quasi-reversible one-electron transfer waves to the radical cation and the di-cation, respectively, in cyclic voltammetry. The electrolysis gives low yields of the 2,2 dimers (see Scheme 12) via the radical cation coupling mechanism [46]. 

In the first step, an electron transfer and a homolytic cleavage at the cation radical stage of one of the C-S bonds to the methylthio group are proposed, leading to carboca-tion XLI, which in two successive 1,2-shifts followed by elimination of MeS" " is transformed into the tetrathioethylene derivative (XLII). It was shown [150, 151] that compound XLII, after being oxidized in two reversible one-electron steps to the dication XLIL, rearranges to the more stable dication XLIII. Reactions similar to those described have also been observed as result of nonelectrochemical oxidations [152]. 

The anodic oxidation of a series of 2,5-diaryl-1,4-dithiins (LVIII), as in Eq. (96), has been studied in detail [189]. Cyclic voltammetry (CV) experiments showed that all compounds undergo two quasi-reversible one-electron transfers to form the radical cations and the dications. Linear correlations between E° and a were observed. Preparative electrolyses gave the corresponding 2,2 -dimers (LIX) as the major products in yields up to 20%. The low yields are due to workup difficulties and formation of polymeric materials of unknown composition. 

Reversible oxidation potentials for two consecutive one-electron transfers were also observed for alkoxy-substituted thianthrene [192, 193]. 

V,7V,7V,7V -tetrakis(2-hydroxybenzyl)ethylenediamine forms a non-oxovanadium derivative.640 The electrochemical properties of this derivative have been determined in dimethylformamide and two well-defined reversible one-electron-transfer events were detected at +0.16V for E11/2 and at -0.84 V for E21/2 (vs. Fc+/Fc) these potentials correspond to the V5+/4+ and V4+/3+ redox couples, respectively.640... 

Figure 2. Calculated cyclic voltammograms for two stepwise reversible one-electron transfers with different E° values. A, AE = — 180 mV B, AE = — 90 mV C, AE = 0 mV D, AE° = 180 mV. [Reproduced with permission from D. Polcyn, 1. Shain, Anal Chem., 38, 370 (1966).]...

The electrode reduction mechanism of benzenodicarbonitrile isomers was examined by polarography, cyclic voltammetry and controlled potential electrolysis in DMF solutions at a Hg cathode. 1,2- and 1,4-dicyanobenzenes were reduced in two successive steps under polarographic conditions, where the first step corresponds to a quasi-reversible one-electron transfer. Cyclic voltammetric experiments provided more information on the electrode reduction mechanism and allowed one to suggest the mechanistic scheme for 1,2-and 1,4-dicyanobenzenes shown in Scheme 16. 

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