Chiral recognition based on the kinetics of ion transfers across liquid/liquid interface

Three-phase electrodes in combination with square-wave voltammetry are applied to study the transfer kinetics of chiral anions from water to the chiral 2-octanol. The experimental system used consists of a pyrolytic graphite electrode partly modified with a thin film of one of the enantiomers of 2-octanol, which was immersed into an aqueous solution containing anions of chiral 2-chloropropionic acid, 2-bromopropionic acid, or lactic acid. It is demonstrated that the kinetics of the ion transfer is a stereoselective. The rate of the ion transfer is higher when uncomplimentary transferring ion–solvent chiral isomers are used, i.e., (R)-ion and (S)-solvent, or (S)-ion and (R)-solvent. To the best of our knowledge this is the first evidence for the difference in the ion transfer kinetics of chiral isomers across water/chiral organic solvent interface. Keywords: Stereoselectivity, Chiral ions, Ion transfer kinetics, Three-phase electrode

Why Cobalt macrocyclic complexes are not efficient catalysts for the oxygen reduction reaction, under acidic conditions

International audienceThe activation of the oxygen reduction reaction (ORR) is of a primary importance for the development of fuel cells. Cobalt macrocyclic complexes, e.g. those of porphyrins, phthalocyanines, tetraazaannulenes (TAA), cyclam, have been tested in order to replace platinum, the best catalyst but a rare and expensive metal. However it has been noted in some studies that these complexes suffer from degradation in an acidic environment. Suspicions have already been expressed that such degradation are caused by hydrogen peroxide or demetallation, but the reason for the loss of activity of these catalysts has never been experimentally established. In the present work, it has been clearly demonstrated that cobalt complexes are stable in the presence of acids when the metal centre is at the Co(III) oxidation state but are demetallated when it is reduced to the Co(II) oxidation state, which occurs during the oxygen reduction reaction. Co2+ is a borderline Lewis acid in Pearson's classification and thus can be substituted by H+ in an acidic environment

Studying the kinetics of the ion transfer across the liquid|liquid interface by means of thin film-modified electrodes

The kinetics of the ion transfer across the liquid|liquid interface is studied by means of a thin film-modified electrode in combination with square-wave voltammetry. The thin film-modified electrode consists of an edge plane pyrolytic graphite electrode covered with a thin film of a water immiscible electroinactive organic solvent containing a neutral redox probe and a suitable electrolyte. The overall electrochemical process, proceeding as a coupled electron–ion transfer reaction, is controlled either by the electron transfer across the graphite electrode|organic solvent or by the ion transfer across the organic solvent|aqueous electrolyte interface. The theoretical model enabling to distinguish the rate limiting step is developed. The theory is used to measure the kinetics of ClO4-,NO3-, Cl−, SCN−, Na+, K+, (CH3)4N+, and (C4H9)4N+ across the water|nitrobenzene interface utilizing lutetium bis(tetra-tert-butylphthalocyaninato) as a redox probe. Keywords: Ion transfer kinetics, Liquid–liquid interface, Thin film-modified electrode, Quasireversible maximum, Square-wave voltammetr

Electrochemical study of hydrophilic ion transfers across cholesterol modified water–nitrobenzene interface by means of thin film electrodes

Electrochemically driven ion transfer reactions of ClO4-, NO3-, SCN−, and K+ across a cholesterol modified water–nitrobenzene (W–NB) interface have been studied by means of square-wave and cyclic voltammetry at thin organic film modified electrodes. It has been found that cholesterol exhibits a profound accelerating effect toward the kinetics of ClO4-, NO3-, SCN− transfers, whereas a retarding kinetic effect was observed for K+. Besides, the cholesterol layer alters significantly the mechanism of ion transfer, prompting adsorption of all studied ions as a consequence of specific interactions between cholesterol molecules and the transferring ion. Keywords: Cholesterol, Ion transfer, Liquid interface, Voltammetr

LuIII bisphthalocyanines as mediators for redox reactions at thin-organic-film modified electrodes

Lutetium bisphthalocyanines, dissolved in a thin-film of nitrobenzene covering a graphite electrode, have already been used for ion transfer studies across the liquid∣liquid interface. These sandwich complexes are also good candidates for the electron exchange with species in water, across the interface. Their one-electron oxidation and reduction are fully reversible, the oxidized and reduced forms being chemically stable. Compared with ferrocene derivatives and porphyrins that have been extensively used previously, these compounds offer very interesting properties for the mediation of electron exchanges with redox species in aqueous solutions. Keywords: Lutetium bisphthalocyanines, Modified electrode, Thin-organic-film, Electron transfer, Liquid∣liquid interface, Catalysi

Assisted Ion Transfer at Organic Film-Modified Electrodes

International audienceAn experimental and theoretical study of a complex electrochemical mechanism at three-phase and thin organic film-modified electrodes, where the coupled electron–ion transfer reaction is complicated by complexation reaction of the transferring ion, is reported. The transfer of monovalent and divalent cations across water|nitrobenzene interface, coupled with the complexation reactions with the ionophore valinomycin, is studied. Both types of electrodes are assembled of an edge plane pyrolytic graphite electrode modified with a nitrobenzene solution of lutetium bis(tetra-tert-butylphthalocyaninato) as a redox mediator and valinomycin as an ionophore. The reversible redox transformations of the redox mediator to either a monovalent hydrophobic anion or cation serve to drive the ion transfer across the liquid|liquid interface. In contact of the modified electrode with an aqueous electrolyte containing alkali or earth alkaline metal cations, significant partition of the aqueous electrolyte is taking place, due to the interfacial complexation of the cation with valinomycin. Thus, the thermodynamics and kinetics of the interfacial complexation–partition reaction at the liquid|liquid interface affect markedly the overall electron–ion transfer reaction at the modified electrodes under voltammetric conditions. Experiments are qualitatively compared with theoretical data collected by simulation of two different electrochemical mechanisms coupled with chemical reactions under conditions of square-wave voltammetry. It has been concluded that the overall electrochemical mechanism at three-phase electrodes can be described as a specific CrE reaction scheme, where Cr represents the reversible interfacial complexation–partition reaction of the transferring ion with valinomycin at the liquid|liquid interface