Cyclic voltammetry aprotic solvents

FIGURE 2.34. a Reductive cyclic voltammetry of an aromatic hydrocarbon (e.g., anthracene) in an aprotic solvent (e.g., DMF) upon successive additions of a weak acid (e.g., phenol), b Thermodynamics of the combined addition of two electrons and two protons. 

Esters of aromatic acids in aprotic solvents form radical-anions detected by cyclic voltammetry on a short time scale [144]. Ethyl benzoate has E° = -2,19 V V5. see [145], Follow-up reactions of radical-anions from methyl and ethyl benzoate result from protonation by extraneous water. rm-Butyl benzoate radical-anion undergoes very rapid cleavage of the alkyl-oxygen bond to give benzoate ion and rerf-butyl radical. 

The characterization of the semiquinone radical anion species of PQQ in aprotic solvents was undertaken to provide information about the electrochemistry of coenzyme PQQ and to give valuable insight into the redox function of this coenzyme in living systems <1998JA7271>. The trimethyl ester of PQQ and its 1-methylated derivative were examined in aprotic organic solvents by cyclic voltammetry, electron spin resonance (ESR), and thin-layer UV-Vis techniques. The polar solvent CH3CN was found to effectively solvate the radical anion species at the quinone moiety, where the spin is more localized, whereas the spin is delocalized into the whole molecule in the nonpolar solvent CH2CI2. 

Most chemists are familiar with chemistry in aqueous solutions. However, the common sense in aqueous solutions is not always valid in non-aqueous solutions. This is also true for electrochemical measurements. Thus, in this book, special emphasis is placed on showing which aspects of chemistry in non-aqueous solutions are different from chemistry in aqueous solutions. Emphasis is also placed on showing the differences between electrochemical measurements in non-aqueous systems and those in aqueous systems. The importance of electrochemistry in non-aqueous solutions is now widely recognized by non-electrochemical scientists - for example, organic and inorganic chemists often use cyclic voltammetry in aprotic solvents in order to determine redox properties, electronic states, and reactivities of electroactive species, including unstable intermediates. This book will therefore also be of use to such non-electrochemical scientists. 

It is certain that, in the first reduction step in aprotic solvents, an electron is accepted by the LUMO of the organic compound. However, it was fortunate that this conclusion was deduced from studies that either ignored the influence of solvation energies or used the results in different solvents. Recently, Shalev and Evans [55] estimated the values of AG V(Q/Q ) for 22 substituted nitrobenzenes and nine quinones from the half-wave potentials measured by cyclic voltammetry. For quinones and some substituted nitrobenzenes, the values of AG V(Q/Q ) in a given solvent were almost independent of the EA values. Similar results had been observed for other aromatic hydrocarbons in AN (Section 8.3.2) [56]. If AG V(Q/ Q ) does not vary with EA, there should be a linear relation of unit slope between El/2 and EA. Shalev and Evans [55], moreover, obtained a near-linear relation between AG V(Q/Q ) and EA for some other substituted nitrobenzenes. Here again, the Ey2-EA relation should be linear, although the slope deviates from unity.8)... 

In aprotic solvents, the radical anion, RX , for aryl halides has been detected as intermediate. In cyclic voltammetry of aryl halides, though an irreversible two-electron reduction occurs at low scan rate, a reversible one-electron reduction occurs at high scan rate. Thus, it is possible to get the values of the standard potential ( °) for the RX/RX couple and the rate constant (k) for RX -> R (therefore, the lifetime of RX ). In Fig. 8.18, the relation between ° and log k for aryl bromides in DMF is linear with a slope of 0.5 [5If], It is apparent that the lifetime of RX , obtained by 1/k, increases with the positive shift of E0. In contrast, the existence of RX for alkyl monohalides has never been confirmed. With these compounds, it is difficult to say whether the two processes, i.e. electron transfer and bond cleavage, are step-wise or concerted (RX+e -> R +X ). According to Sa-veant [5le], the smaller the bond dissociation energy, the larger the tendency for the concerted mechanism to prevail over the step-wise mechanism. 

I11 aprotic solvents, the amalgam of R4N is somewhat stable at temperatures well below 0°C, forming a film on the surface of mercury electrode, and its re-oxidation reaction can be observed by cyclic voltammetry. However, at ambient temperatures or in the presence of water and other protic solvents, the amalgam soon decomposes. 

The effect of complexation on redox properties was studied by cyclic voltammetry. Unbound flavin, dissolved in an aprotic solvent (dichloromethane), undergoes a two electron reduction perfectly explained by the ECE mechanism. Upon addition of cyclene ligand and coordination of flavin to the zinc ion complex, the flavohydroquinone redox state was stabilised. 

Aprotic solvents mimic the hydrophobic protein interior. However, a functional artificial receptor for flavin binding under physiological conditions must be able to interact with the guest even in competitive solvents. As found by spectroscopic measurements with phenothiazene-labeled cyclene, the coordinative bond between flavin and Lewis-acidic macrocyclic zinc in methanol was strong enough for this function. Stiochiometry of the complex was proved by Job s plot analysis. Redox properties of the assemblies in methanol were studied by cyclic voltammetry which showed that the binding motif allowed interception of the ECE reduction mechanism and stabilisation of a flavosemiquinone radical anion in a polar solvent. As a consequence, the flavin chromophore switched from a two-electron-one-step process to a two-step-one-electron-each by coordination. 

The cleavage of primary amines in aprotic solvents, such as acetonitrile or DMF, was studied by cpe, controlled potential coulometry, and cyclic voltammetry 463 The results indicate that at low anode potential the primarily formed radical cation RCH2NH2 dissociates to an alkyl cation RCH2+ and an amino radical NH2 , which subsequently is oxidized to nitrogen. At higher anode potentials path a)ofEq. (219), leading to an aldehyde and an ammonium ion, preponderates. 

TABLE 9.9 Redox Potentials for the Reduction of Dioxygen (1 atm) at Cyclic Voltammetry with Four Different Electrodes in Four Aprotic Solvents (0.1 M Tetraethylammonium Perchlorate)... 

Cyclic voltammetry has shown that the Mo(VI)/Mo(V) couple is usually electrochemically irreversible 136,148,166,168,221,222) however, reversible one-electron reductions are observed in aprotic solvents for six-coordinate [Mo 02l complexes of iV-alkylated tetradentate N2S2 ligands 107, 121, 223) or tetradentate S4-ligands (124) and for (L-N3)Mo02X complexes. The reversibility of the electrochemical reduc-... 

For example, [Re (CN)(CO)3(bpy)] in strictly aprotic solvents at low temperature is reduced in two chemically reversible one-electron steps at —1.77 and —2.42 V, which correspond to formation of [Re (CN)(CO)3(bpy)] z = 1— and 2—, respectively [130]. The first reduction of [Re(CO)4(bpy)]+ or [Re(P(OEt3))(CO)3(bpy)]+ is chemically reversible even at room temperature [133, 135]. Halide complexes [Re (X)(CO)3(bpy)] (X = Cl, Br) have a complicated reduction pattern because of further reactions of the reduced products. Several reaction intermediates and products have been characterized spectroelectrochemically [131, 135-137] and rate constants of the main steps have been determined [130] by low-temperature cyclic voltammetry. 

It was in 1990 that Kratschmer et al. [217,218] reported the first macroscopic preparation of in gram quantities by contact-arc vaporization of a graphite rod in a 100 Torr atmosphere of helium, followed by extraction of the resultant soot with toluene. Fullerene ions could also be detected by mass spectrometry in low-pressure hydrocarbon flames [219]. The door was opened by, Kratschmer and co-workers preparative success to extensive studies of the electrochemical behavior of the new materials. Cyclic voltammetry of molecular solutions of Ceo in aprotic electrolytes, e.g., methylene chloride/quatemary ammonium salts, revealed the reversible cathodic formation of anionic species, the radical anion, the dianion, etc. (cf. [220,221]). Finally, an uptake of six electrons in the potential range of 1-3.3 V vs. SHE in MeCN/toluene at — 10°C to form the hexavalent anion was reported by Xie et al. [222]. This was in full accordance with MO calculations. A parametric study of the electroreduction of Cgo in aprotic solvents was performed [223]. No reversible oxidation of C o was possible, not even to the radical cation. However, the stability of di- and trications with special counterions, in the Li/PEO/C 3 MoFf cell, was claimed later [224]. 

Pyryliiim [181-183] and isobenzopyrylium [184] salts have been shown to be polaro-graphically reducible in a one-electron reduction. In cyclic voltammetry in aprotic solvents, 2,4,6-trisubstituted pyrylium salts show two peaks the shape of the peak depends on the rate of dimerization. This process occurs more rapidly at C-4 than at C-2 (C-6) [182, 183], and the dimerization takes place spontaneously for 4-unsubstituted pyrylium salts. The equilibrium between radicals and dimer is displaced in favor of the radicals on introduction of electron-withdrawing substituents such groups enhance the aromatic character of the radical [185]. If the reduction of pyrylium salts is made in the presence of an alkyl iodide a fair yield of the 4-alkylated 4i/-pyrane is isolated with the dimer [182]. 

In aprotic solvents such as acetonitrile, constant potential electrolysis yielded benzoquinone as final product whereas in cyclic voltammetry on a platinum anode a one-electron step can be observed the product here being quinhydrone. 

In the current study cyclic voltammetry was performed on two quinoline-type anticancer drugs, Dup 785 6 and camptothecin 9 to ascertain whether they possess the requisite characteristics of ET agents. Emphasis is placed on reduction potentials obtained for the quinolinium (iminium) forms. Several model compounds were examined in order to determine the effect of substituents. Reduction potentials were obtained in both protic and aprotic solvents to simulate environments which may pertain at the active site. In addition, Epp/2 and A Ep values were calculated (Table 2). The relationship of electrochemical characteristics to drug activity is addressed. 

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