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

Theoretical insights into the Electronic and Structural Properties of New, Low-band Gap Inherently Chiral Ethylendioxythiophene-based Oligothiophene

In the last years, conjugated oligothiophene macrocycles have attracted increasing scientific interest due to some peculiar properties related to their cyclic structure [1-3]. T. Benincori et al. synthesized the 2,2′-bis(2,2′-bithiophene-5-yl)-3,3′-bithianaphthene nicknamed BT2T4 (Figure 1) that represents the first member of a new class of chiral oligothiophenes in which chirality results from a tailored torsion produced in the polyconjugated backbone and not from the presence of stereogenic centres, external to it. Interestingly, the FeCl3 oxidation of the enantiopure BT2T4 produce a mixture of chiral macrocycles, like dimers and trimers. [4] Recently, also thanks to DFT and TD-DFT calculations, we have studied the new monomer BT2E4 in order to investigate the role of the insertion of 3,4-ethylenedioxythiophene (EDOT) units on the electronic and molecular properties of neutral and charged monomer and oligomer species. Furthermore, the electroactive films were evaluated by cyclic voltammetry (CV), UV/vis spectroelectrochemistry and CV coupled with in-situ conductance measurements. [5]Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

The solvent effect on the electrocatalytic cleavage of carbon-halogen bonds on Ag and Au

In recent years it has been shown in detail how the electrocatalytic cleavage of carbon-halogen bonds is modulated by (a) the stepwise or concerted nature of the dissociative electron-transfer mechanism, which is in\ufb02 uenced by the nature of the electrode surface, the type of halogen atom and the molecular structure of RX as a whole, and (b) the double-layer structure (as a function of the nature and bulkiness of the supporting electrolyte ions). In order to both complete and support the interpretative scheme thus developed, this work is focused on the solvent role. When one compares aprotic with protic organic solvents af ter appropriate intersolvental normalization, interesting peculiarities emerge, especially concerning protic media. Solvent proticity deeply affects both the reaction mechanism (on both noncatalytic and catalytic electrodes) and the extent of the catalytic effects. These items are discussed on the basis of a complete investigation carried out with a carefully controlled experimental protocol on two chloride and bromide couples, one aromatic and one aliphatic, representative of stepwise and concerted mechanisms, respectively, in four aprotic and four protic solvents, on both non catalytic GC and catalytic Ag and Au electrodes. The results are discussed in the framework of a recently developed interpretative scheme of the carbon-halogen cleavage mechanism

2,12-Diaza[6]helicene: An Efficient Non-Conventional Stereogenic Scaffold for Enantioselective Electrochemical Interphases

The new configurationally stable, unsymmetrical 2,12-diaza[6]helicene was synthesized as a racemate and the enantiomers were separated in an enantiopure state by semi-preparative HPLC on chiral stationary phase. Under selected alkylation conditions it was possible to obtain both the enantiopure 2-N-mono- and di-N-ethyl quaternary iodides. Metathesis with bis(trifluoromethanesulfonyl)imide anion gave low-melting salts which were tested as inherently chiral additives to achiral ionic liquids for the electrochemical enantiodiscrimination of chiral organic probes in voltammetric experiments. Remarkable differences in the oxidation potentials of the enantiomers of two probes, a chiral ferrocenyl amine and an aminoacid, were achieved; the differences increase with increasing additive concentration and number of alkylated nitrogen atoms

Relationship between supporting electrolyte bulkiness and dissociative electron transfer at catalytic and non-catalytic electrodes

Dissociative electron transfers (DETs) involving aryl halides in the presence of supporting electrolytes with tetraalkylammonium cations TAA+ of different diameters provide a first model case of rationalized relationship between supporting electrolyte bulkiness effect and DET mechanism. In non-catalytic conditions, with the molecule reacting at the OHP, increasingly bulkier TAA+ cations result in increasing double layer (DL) thickness and consequently in increasingly hindered electron tunnelling, and the reduction peak potentials Ep,DET linearly shift in the negative direction with increasing cation diameter dTAA+. The linear variation of Ep,DET with dTAA+ is consistent with the distance dependence of kET for reagents held at fixed distances from the electrode (ln k = ln k0 12 \u2c7x). The 02Ep,DET/ 02dTAA+ slope is not constant in the investigated model series, but linearly increases with decreasing parameter, accounting for the heterogeneous ET step becoming increasingly more determining with respect to the following C-X bond cleavage step (as a consequence of decreasing stability of the pi* orbital corresponding to the radical anion). Thus the beta parameter in the electron tunnelling equation appears to be strictly related to the ET activation barrier (in particular, to its relative weight in the overall DET kinetics). On catalytic Ag electrodes, with the molecule reacting at the IHP, the supporting electrolyte effect is assumed to depend on the increasingly smaller effective potential difference available to the molecule, reacting close to the electrode, with increasing double layer thickness. It appears more conspicuous than in non-catalytic conditions, in spite of the less extreme operating potentials, but consistently with the increased relative importance of the ET activation barrier at a catalytic electrode

Electrochemical activity of thiahelicenes: Structure effects and electrooligomerization ability

Thiahelicenes are polycondensed heteroaromatic molecules characterized by a chiral helix-like structure including multiple thiophene units, with a lowering effect on the oxidation potentials and a shrinking effect on the band gaps. As a consequence they can be regarded as electrochemically and optically active conducting materials, exhibiting interesting properties under electrical or magnetic polarization, and are under study for non-linear optics (NLO) applications. The present extensive investigation on 11 thiahelicenes with different chain length and functionalization (including the first example of a thiahelicene with perfluorinated alkyl chains) together with the precursor benzodithiophene provides a deep insight on the structure vs. electrochemical activity relationship within this attractive compound class, focusing on both electron transfer (ET) properties and oligomerization ability (hinging on free positions on terminal thiophene groups)

Inherently chiral electrodes: the tool for chiral voltammetry

0 -Bis[2-(5,2 0 -bithienyl)]-3,3 0 -bithianaphthene oligomers are a model case of electroactive films endowed with "inherent chirality", originating from a stereogenic element coinciding with the whole electroactive backbone, thus resulting in impressive manifestations. This study highlights their applicative potentialities as low-cost and easy-to-prepare artificial enantiopure electrode surfaces, which display an unprecedented ability to pronouncedly separate voltammetry peaks of enantiomers of quite different chiral probes of applicative interest, concurrently with linear dynamic ranges for peak currents, affording enantiomer excess determination. Thus inherently chiral enantiopure electrodes can indeed be regarded as a key to chiral voltammetry

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