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

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

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

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

Homogeneous reduction of haloacetonitriles by electrogenerated aromatic radical anions: Determination of the reduction potential of (CH2CN)-C-center dot

The mechanism of homogeneous reduction of XCH2CN (X) Cl, Br, I) by organic radical anions (D•-) has been investigated in DMF. All three haloacetonitriles undergo a concerted dissociative electron transfer with formation of a fragment cluster in the solvent cage. The interaction energy Dp of the fragment cluster has been determined by applying the “sticky” dissociative electron-transfer model to the kinetic data obtained for the reaction between each XCH2CN and a series of donors. The interaction energies lie in the range from 0.19 to 1.67 kcal/mol and decrease from Cl to Br and to I. Both the smallness of Dp values and their dependence on the bulkiness of X- confirm the electrostatic character of these interactions. The intermediate radical stemming from the dissociative electron transfer to XCH2CN reacts with D•- either by radical coupling (kc) or by electron transfer(ket). Examination of the competition between these reactions, which can be expressed by a dimensionless parameter q) ket/(kc+ ket), as a function of E°D/D•- allows determination of the standard reduction potential of •CH2CN (E° = -0.69 V vs SCE) as well as the reorganization energy λ of the redox process. A significant contribution of internal reorganization to λ has been found, indicating a change of structure from •CH2CN to -CH2CN

Electrochemistry for Atom Transfer Radical Polymerization

Atom Transfer Radical Polymerization (ATRP) is the most powerful and most employed technology of Controlled Radical Polymerization (CRP) to produce polymers with well-defined architecture, that is, composition, topology, and functionality. Several hundreds of papers are published every year on ATRP processes, mainly based on empiric experimental procedures. Electrochemistry powerfully entered in the field of ATRP about 10 years ago, providing important contributions both to the further development of the process and to a better understanding of its mechanism. Five main issues took advantage of electrochemistry and/or its synergism with ATRP: i) understanding the mechanism of ATRP activation; ii) determination of thermodynamic parameters; iii) determination of activation and deactivation rate constants; iv) the SARA ATRP vs SET-LRP dispute: the role of Cu0; v) electrochemically-mediated ATRP