An "inherently chiral" 1,1′-bibenzimidazolium additive for enantioselective voltammetry in ionic liquid media

A dialkyl-1,1′-bibenzimidazolium salt, consisting of an atropisomeric dication (i.e. featuring a stereogenic axis and thus "inherently chiral") and an achiral counteranion, is employed as a chiral additive in three commercial ionic liquids, providing successful enantiodiscrimination in voltammetry experiments on screen-printed electrodes (SPEs) with the enantiomers of N,N′-dimethyl-1-ferrocenyl-ethylamine as model chiral probes. Significant differences in redox potentials are observed for the probe enantiomers despite the low concentration (0.01 M) of the chiral additive. The nature of the achiral ionic liquid in which the additive is employed significantly affects the peak potentials and potential differences, but does not alter the enantiomer sequence. Keywords: Chiral electrochemistry and electroanalysis, Ionic liquids, Chiral additives, Inherent chirality, Enantiorecognitio

Electrocatalytic reduction of bromothiophenes on gold and silver electrodes: An example of synergy in electrocatalysis

The electroreduction of bromothiophenes on Au and Ag provides a striking model of synergy in electrocatalysis, when a second group specifically interacting with the catalytic surface is present besides the reacting one, providing an auxiliary anchoring effect. The high catalytic activity of Ag for bromobenzene reduction is enhanced in the bromothiophene case. Moreover, Au, having for bromobenzene a much lower and less reproducible catalytic effect than Ag on account of the repulsive effect of its very negative surface charge in the working potential range, approaches Ag activity in the case of 2-bromothiophene, where the anchoring S group is adjacent to the Br group to be cleaved. The beneficial anchoring effect is lower when it has to be shared between two Br leaving groups adjacent to the S group, and becomes negligible in the case of a bromide leaving group in 3-position. Keywords: Molecular electrocatalysis, Dissociative electron transfer, Anchoring groups, Bromothiophenes, Gold electrodes, Silver electrode

Cu(i) and Ag(i) complex formation with the hydrophilic phosphine 1,3,5-triaza-7-phosphadamantane in different ionic media. How to estimate the effect of a complexing medium

The complexes of Cu(i) and Ag(i) with 1,3,5-triaza-7-phosphadamantane (PTA) are currently studied for their potential clinical use as anticancer agents, given the cytotoxicity they exhibited in vitro towards a panel of several human tumor cell lines. These metallodrugs are prepared in the form of [M(PTA)4]+ (M = Cu+, Ag+) compounds and dissolved in physiological solution for their administration. However, the nature of the species involved in the cytotoxic activity of the compounds is often unknown. In the present work, the thermodynamics of formation of the complexes of Cu(i) and Ag(i) with PTA in aqueous solution is investigated by means of potentiometric, spectrophotometric and microcalorimetric methods. The results show that both metal(i) ions form up to four successive complexes with PTA. The formation of Ag(i) complexes is studied at 298.15 K in 0.1 M NaNO3 whereas the formation of the Cu(i) one is studied in 1 M NaCl, where Cu(i) is stabilized by the formation of three successive chloro-complexes. Therefore, for this latter system, conditional stability constants and thermodynamic data are obtained. To estimate the affinity of Cu(i) for PTA in the absence of chloride, Density Functional Theory (DFT) calculations have been done to obtain the stoichiometry and the relative stability of the possible Cu/PTA/Cl species. Results indicate that one chloride ion is involved in the formation of the first two complexes of Cu(i) ([CuCl(PTA)] and [CuCl(PTA)2]) whereas it is absent in the successive ones ([Cu(PTA)3]+ and [Cu(PTA)4]+). The combination of DFT results and thermodynamic experimental data has been used to estimate the stability constants of the four [Cu(PTA)n]+ (n = 1-4) complexes in an ideal non-complexing medium. The calculated stability constants are higher than the corresponding conditional values and show that PTA prefers Cu(i) to the Ag(i) ion. The approach used here to estimate the hidden role of chloride on the conditional stability constants of Cu(i) complexes may be applied to any Cu(i)/ligand system, provided that the stoichiometry of the species in NaCl solution is known. The speciation for the two systems shows that the [M(PTA)4]+ (M = Cu+, Ag+) complexes present in the metallodrugs are dissociated into lower stoichiometry species when diluted to the micromolar concentration range, typical of the in vitro biological testing. Accordingly, [Cu(PTA)2]+, [Cu(PTA)3]+ and [Ag(PTA)2]+ are predicted to be the species actually involved in the cytotoxic activity of these compounds. \ua9 2017 The Royal Society of Chemistry

Mechanism of the Electrochemical Carboxylation of Aromatic Ketones in Dimethylformamide

The mechanism of the electrochemical carboxylation of several benzophenones (X-C6H4COC6H5; X = 4-OCH3, 4-CH3, H, 3-Cl, 3-CF3, 4-CF3 and 4-CN) and several ring-substituted acetophenones (Y-C6H4COCH3; Y = 4-OCH3, H, 3-OCH3, 3-Cl, 3-CF3, 4-CF3, 3-CN and 4-CN) has been investigated by cyclic voltammetry in dimethylformamide. In the presence of CO2, all compounds exhibit a single irreversible peak representing a 2e- reduction process. The reaction mechanism has been analysed using the dependence of the peak potential Ep on various experimental parameters such as the concentrations of the reacting species, the scan rate and the temperature as a mechanistic tool. Also the kinetics of the electrocarboxylation reaction has been examined. The whole set of results has been carefully analysed in the framework of an ECE-DISP mechanism. It has been found that, under the experimental conditions employed, the electrocarboxylation reaction is always under a mixed ECE-DISP1 kinetic control. The first step of the reaction is an oxygen centred attack of the electrogenerated ketyl radical anion RR'CO\u2022- at CO2. Further reduction of the carbonate-like adduct stemming from such an attack followed by a second carboxylation reaction gives a 2-arylcarboxylic acid

Electrochemical Reduction and Carboxylation of Chloroacetonitrile

Reduction of chloroacetonitrile in DMF or MeCN at a Hg or a glassy carbon electrode involves a protonation reaction between the electrogenerated carbanion NCCH2- and the starting halide. Such a self-protonation reaction can be avoided by adding good carbanion scavengers such as proton donors or CO2. Reduction of the halide in CO2-saturated solvents yields cyanoacetic acid. This process was investigated under various experimental conditions. The best results, in both solvents, were obtained when an undivided cell with aluminum sacrificial anode was used. The yields of the acid, under such conditions, were 73% and 93% in DMF and MeCN, respectively

Silver Cathodes as Electrocatalysts in the Carboxylation of Benzyl Chlorides

The electrochemical reduction of some benzyl chlorides at Ag, Hg and carbon electrodes has been studied in CO2-saturated CH3CN + 0.1 M Et4NClO4. Voltammetric investigations, conducted both in the absence and presence of CO2, have shown that Ag has a dramatic electrocatalytic effect on the reduction process. Controlled-potential electrolyses of solutions of the benzyl halides were carried out in an undivided cell with a sacrificial Al anode and a silver cathode. For the sake of comparison, a few experiments were also run at Ag-coated Pt, Hg and graphite cathodes. Although the principal product in all cases was carboxylic acid, the process at Ag takes place at potentials about 0.6 V less negative than at Hg or graphite. Also better current efficiencies (79-95%) are obtained at Ag as compared to Hg or graphite (< 50%)

Electrochemical synthesis of cyanoacetic acid from chloroacetonitrile and carbon dioxide

The electrochemical carboxylation of chloroacetonitrile was investigated in dimethylformamide (DMF) and acetonitrile\ue07e(MeCN) by cyclic voltammetry and controlled-potential electrolysis. Both direct electroreduction and mediated reduction of the halide in CO2-saturated solvents were used to achieve the electrocarboxylation process. Also the effects of cathode material and cell type (divided or undivided with dissolving anode) were examined. In DMF the electrolyses performed in the divided cell resulted in low to moderate yields of NCCH2CO2H (25-45%), independent of the electrode material and catalyst type. The process is remarkably more ef\ufb01cient in MeCN, in which acid yields of ca. 60% were obtained under similar conditions. Very good results were obtained in both solvents when an undivided cell with aluminum sacri\ufb01cial anode was used. In this case, the acid yield increased to 73 and 93% in DMF and MeCN, respectively

Electrocatalytic carboxylation of benzyl chlorides at silver cathodes in acetonitrile

Silver exhibits powerful electrocatalytic activities towards the reductive carboxylation of benzyl chlorides (RCl): in CO2-saturated CH3CN, reduction of RCl occurs at potentials that are about 0.6 V more positive than those of the same process at Hg or carbon electrodes and gives carboxylic acids in good to excellent yields

Evaluation of the standard reduction potentials of some electrochemical processes of primary importance

The definition and possible determination of an absolute standard reduction potential, SRP, is an intriguing subject, which continuously attracts the interest of researchers both from theoretical and experimental points of view. Experimental values of half-cell reduction potentials are generally anchored to the standard hydrogen electrode, SHE, in water to which the conventional value of exactly 0 V has been assigned. Thus, the experimentally measured values are relative SRPs and these are adequate for many applications of standard potentials. However, the growing development of computational efforts and the need of making comparisons between theoretical and experimental values set the question of absolute SRPs into relevant actuality. Another important redox process for which reliable SRPs are not available is the reduction of the halogen atoms. EoX\u2022/X- plays a crucial role in the evaluation of the SRPs of many alkyl halides that are involved in several important processes such as reductive dehalogenation of recalcitrant pollutants, atom transfer radical polymerization and various other processes of synthetic importance. This paper reports on the calculation of the SRPs of H+, X\u2022 and RX. The absolute potential of the standard hydrogen electrode, SHE, was calculated on the basis of a thermodynamic cycle involving H2(g) atomization, ionization of H\u2022(g) to H+(g) and hydration of H+. The most up-to-date literature values on the free energies of these reactions have been selected and, when necessary, adjusted to the electron convention Fermi-Dirac statistics since both e- and H+ are fermions. Unlike almost all previous estimations of SHE, \uf044Goaq(H+) was used instead of the real potential, \uf061aq(H+). This choice was made to obtain an SHE value based on chemical potential, which is the appropriate reference to be used in theoretical computations of SRPs. More complicated thermochemical cycles were used for the calculation of EoX\u2022/X\uf02d (vs SHE) in water as well as in MeCN and DMF. Last, the SRPs of a series of alkyl halides of relevance to atom transfer radical polymerization and other processes such as pollution abatement have been calculated in MeCN and DMF. This has been done by using a thermochemical cycle involving gas phase homolytic dissociation of the C-X bond, solvation of RX, R\u2022 and X\u2022, and reduction of X\u2022 to X- in solution

Absolute Potential of the Standard Hydrogen Electrode and the Problem of Interconversion of Potentials in Different Solvents

The absolute potential of the standard hydrogen electrode, SHE, was calculated on the basis of a thermodynamic cycle involving H2(g) atomization, ionization of H(g)• to H(g)+, and hydration of H+. The most up-to-dateliterature values on the free energies of these reactions have been selected and, when necessary, adjusted to the electron convention Fermi-Dirac statistics since both e- and H+ are fermions. As a reference state for the electron, we have chosen the electron at 0 K, which is the one used in computational chemistry. Unlike almost all previous estimations of SHE, ∆G°aq(H+) was used instead of the real potential. This choice was made to obtain a SHE value based on the chemical potential, which is the appropriate reference to be used in theoretical computations of standard reduction potentials. The result of this new estimation is a value of 4.281 V for the absolute potential of SHE. The problem of conversion of standard reduction potentials (SRPs) measured or estimated in water to the corresponding values in nonaqeuous solvents has also been addressed. In fact, thermochemical cycles are often used to calculate SRPs in water versus SHE, and it is extremely important to have conversion factors enabling estimation of SRPs in nonaqueous solvents. A general equation relating E° of a generic redox couple in water versus the SHE to the value of E° in an organic solvent versus the aqueous saturated calomel electrode is reported