Elio Vianello

Solubility and electrochemical determination of CO2 in some dipolar aprotic solvents

Abstract The solubility of CO 2 in some solvents of electrochemical interest was measured both as a function of its partial pressure and as a function of temperature. Henry constants determined at 298.15 K are reported for DMF, AN, DMSO and THF. The solubility of CO 2 in these solvents is also reported at various temperatures in the range −10 to 25 ° C (20 to 50 ° C for DMSO). The Gibbs energy, enthalpy and entropy of solution at 298.15 K and 1 atm partial pressure of CO 2 were estimated from the temperature dependence of the solubility. The effect of base electrolytes on the solubility of CO 2 was also investigated. Voltammetric investigations of CO 2 reduction showed that, in spite of the large dependence of the peak potential on the nature of the electrode material, the solvent and the background electrolyte, the peak current is proportional to the CO 2 concentration, thus providing a means of quantitative CO 2 determination.

Mechanism of the electrochemical reduction of carbon dioxide at inert electrodes in media of low proton availability

Direct electrolysis of CO2 in DMF at an inert electrode, such as mercury, produces mixtures of CO and oxalate, whereas electrolysis catalysed by radical anions of aromatic esters and nitriles produces exclusively oxalate in the same medium. Examination of previous results concerning the direct electrochemical reduction and the reduction by photoinjected electrons reveals that there are no significant specific interactions between reactant, intermediates and products on the one hand, and the electrode material on the other, when this is Hg or Pb. These observations and a systematic study of the variations of the oxalate and CO yields with temperature and CO2 concentration, allow the derivation of a consistent mechanistic model of the direct electrochemical reduction. It involves the formation of oxalate from the coupling of two CO2 radical anions in solution. CO (and an equimolar amount of carbonate) is produced by reduction at the electrode of a CO2–CO˙–2 adduct, the formation of which, at the electrode surface, is rendered exothermic by non-specific electrostatic interactions.

Mechanism of the electrochemical reduction of benzyl chlorides catalysed by Co(salen)

Abstract The electrochemical reduction of benzyl and 4-(trifluoromethyl)benzyl chlorides catalysed by Co(salen) (H2salen, N,N′-bis(salicylidene)-ethane-1,2-diamine) was studied in acetonitrile. Electrogenerated (Co1(salen))− reacts with the halide to give an organocobalt(III) complex. Further one-electron reduction of the latter yields an unstable intermediate that undergoes rapid decomposition by cleavage of the Co C bond. The mechanism of bond breaking in the one-electron-reduced organocobalt(II) complex was investigated. The results of preparative-scale electrolysis on solutions containing Co(salen) and benzyl chloride, performed under different experimental conditions, in particular in the presence of radical or carbanion scavengers, indicate homolytic cleavage of the Co C bond.

Electrochemical reduction of Schiff base ligands H2salen and H2salophen

Abstract The electrochemical reduction of the Schiff base ligands N , N ′-1,2-ethylenebis( salicylideneimine ) and N , N ′-1,2-phenylenebis( salicylideneimine ) has been investigated in DMF by cyclic voltammetry, coulometry and controlled potential electrolysis. The process involves a self-protonation mechanism whereby the two-electron reduction product, a cyclic derivative, is formed together with the conjugate base of the substrate, as a consequence of proton transfer from the substrate itself to the basic intermediates.

Electrochemical behaviour of 4-nitroimidazole and 2-methyl-5-nitroimidazole: Autoprotonation of anion radical and redox-catalysed reduction of the supporting electrolyte cation

Abstract The electrochemical behaviour of the title imidazoles (HRNO 2 ) has been investigated in dimethylformamide and acetonitrile by polarography, cyclic voltammetry and controlled-potential electrolysis and coulometry. The above nitroimidazoles present similar behaviour, displaying two reduction waves paralleled by two reduction peaks in linear sweep voltammetry. The first peak is irreversible up to a sweep rate of 250 V s −1 , while the second one appears to be reversible even at low sweep rates. The experimental data in the first process are consistent with the hypothesis that the irreversibility of the first wave (peak) is due to a rapid decay of the primary anion radical HRNO 2 because of a fast protonation reaction by the starting nitroimidazole (a father-son type of reaction), with the formation of the conjugate base RNO 2 − and of the neutral radical HRNO 2 H. This last radical should give rise, as final reduction product, to the hydroxylamino derivative, the necessary protons being supplied by the starting HRNO 2 . The second process is attributable to the reduction of RNO 2 − to RNO 2 2 . As long as HRNO 2 has not been consumed, during controlled-potential electrolysis at second wave potentials, it is conceivable that the radical dianion RNO 2 2 is oxidized by the starting nitroimidazole to give its conjugate base together with the radical HRNO 2 , which follows the reaction path already described for the first wave. Once all the HRNO 2 has been consumed, the homogeneous reduction of the supporting electrolyte cation (C 2 H 5 ) 4 N + , catalysed by the redox system RNO 2 − /RNO 2 2 , takes place. Electrochemical and spectroscopic investigations on 1-methyl-4-nitro and 1-methyl-5-nitro derivatives of the above nitroimidazoles have provided both an indirect support to the above mechanism and information on the type of tautomer predominantly present in solution at equilibrium.

Self-protonation effects in the electrochemical reduction mechanism of p-nitrobenzoic acid

Abstract The electrochemical reduction mechanism of p -nitrobenzoic acid (NBA) in DMF has been investigated by cyclic voltammetry, polarography and controlled potential macroscale electrolysis. The stoichiometry of the reaction occurring at the first cathodic wave involves 0.8 Faraday and yields 0.2 moles of p -hydroxylaminobenzoic acid and 0.8 moles of the conjugate base of NBA, per mole of substrate consumed. This overall reaction is consistent with a mechanism involving four electron steps and four proton transfers from NBA to its basic reduction intermediates (self-protonation mechanism). Comparison with the voltammetric behaviour of the methyl ester of NBA supports the mechanism and allows to single out the species responsible for the successive reduction waves of NBA. The kinetic analysis of the self-protonation process in the conditions of cyclic voltammetry, allows to assign the rate determining steps of the overall reaction and to evaluate the rate constant of the proton transfer from NBA to the anion radical formed by reversible one electron transfer to the latter.

Electrochemical carboxylation of arylmethyl chlorides catalysed by [Co(salen)][H2salen =N,N′-bis(salicylidene)ethane-1,2-diamine]

The electrochemical carboxylation of some arylmethyl chlorides RCl [R = benzyl, 4-methoxybenzyl, 4-(trifluoromethyl)benzyl or diphenylmethyl] catalysed by [Co(salen)][H2salen =N,N′-bis(salicylidene)-ethane-1,2-diamine] was studied in acetonitrile. Comparable amounts of carboxylic acids and saturated hydrocarbons were obtained when R = PhCH2 or 4-MeOC6H4CH2 whereas carboxylic acids were the main products with R = 4-F3CC6H4CH2 or Ph2CH. Electrogenerated [CoI(salen)]– reacts with RCl to give an organometallic complex, [CoIII(salen)R], with a rate constant which, for all chlorides investigated, is of the order of 105 dm3 mol–1 s–1. The one-electron-reduced complex [CoII(salen)R]– is unstable and its decomposition in the presence of CO2 is the key step of the electrocatalytic process. Different decomposition pathways are considered and their mechanistic implications discussed. In the presence of proton donors [CoII(salen)R]– undergoes rapid hydrolysis to RH and [Co(salen)].

Electrochemical reduction of carbon dioxide catalyzed by [CoI(salophen)Li]

Abstract The electrochemical reduction of CO2 to CO and CO32−, in acetonitrile containing LiClO4 as background electrolyte, is catalyzed by [CoI(salophen)Li]. Although relatively slow, the reduction takes place at moderate overvoltages and the stability of the catalytic system, tested through long-term electrolysis, is fairly good. Mechanistic investigations point to the interaction Of CO2 with [CoI(salophen)Li], to give a precursor complex containing a head-to-tail CO2 dinner which is C-bonded to cobalt and further stabilized by Li+. Electron addition to the latter, followed by elimination of CO32− yields a cobalt—carbonyl complex, which slowly releases the final reduction product CO. Electrochemical studies of Co(salophen) under CO atmosphere have shown the intermediate cobalt—carbonyl complex to be [CoI(salophen)(CO)]−.

Kinetic analysis of a slow electron transfer coupled with a father—son reaction

Abstract The electrode reaction mechanism involving interaction of the products of a slow, rate-determining electron transfer with the parent molecule (father—son reaction) has been examined theoretically under the conditions of cyclic voltammetry. The kinetic analysis indicates how to determine the correct values of the kinetic parameters for both the heterogeneous charge transfer and the homogeneous chemical step. The irreversible cathodic reaction of diphenylmethylphenylsulphide in anhydrous DMF provides a good example of this mechanism, since the diphenylmethyl carbanion resulting from the irreversible two-electron reduction undergoes proton transfer from the parent molecule (self-protonation mechanism).

A STUDY OF THE ELECTROCHEMICAL REDUCTION-MECHANISM OF NI(SALOPHEN) IN DMF

The electrochemical reduction mechanism of Ni(II)(salophen), Ni(L), in DMF has been investigated by cyclic voltammetry and controlled potential electrolysis. The complex exhibits several redox processes. The first electron uptake, a ligand-based one-electron transfer at E° = −1.39 V vs sce, leads to the formation of a nickel(II) radical anion, [Ni(II)(L)]−, which rapidly dimerizes, giving rise to a product containing two Ni(L) units joined through a CC bond. The dimer [Ni(II)(L)]2−2 undergoes a nickel-centred reversible reduction process occurring at E° = −2.25 V vs sce to give [Ni(I)(L)]4−2. The dianion dimer can also be oxidized irreversibly at ca −0.8 V to regenerate the original Ni(L) complex. At the highest sweep rates dimerization is hampered thus allowing the reversible reduction of the primary radical anion to [Ni(I)(L)]2− to be observed.