Achieving Chiral Electroanalysis on Achiral Electrodes in "Inherently Chiral" Ionic Liquid Media

Recently, "inherently chiral" electrodes have been introduced, of unprecedented enantiorecognition ability, able to both discriminate and quantify the enantiomers of chiral probes; their surfaces consist of heterocycle-based "inherently chiral" electroactive oligomers [1-3]. Now, to achieve chiral electroanalysis, an alternative strategy to using chiral electrodes is to work on achiral electrodes in a chiral medium. In this perspective, chiral ionic liquids CILs should perform much better than chiral organic solvents, on account of their higher intrinsic order; and, by analogy with the electrode case, "inherently chiral" ionic liquids ICILs should perform better than CILs. To obtain ICILs, the best strategy we found so far was to start from an alkylated bipyridine scaffold, 3,3'-bicollidine. Of easy synthesis, it exists in two stable enantiomers that can be separated (even, importantly, by fractional crystallization of diastereoisomeric salts, without expensive preparative HPLC) and stored. Its high torsional barrier ensures a wide electrochemical window. Its enantiopure antipodes can be converted by dialkylation into the corresponding enantiopure bicollidinium double salts. The length of the alkyl chains, as well as the anion choice, modulate the melting points, even below room temperature, yielding enantiopure room-temperature ICILs. Two of them have been already obtained, and we have already highlighted their huge enantioselectivity, even as low-concentration additives (e.g. 0.01 M) in commercial achiral ionic liquids like BMIMBF4. The tests were performed on commercial achiral screen printed electrodes SPEs, using the same commercial ferrocenyl-based chiral probes previously used for testing the enantioselectivity of inherently chiral surfaces. The enantiomer peak separation is huge (~0.12-0.17 V depending on the ICIL additive used), comparable to that obtained working with inherently chiral electrodes, and of course specular employing the (R) or (S) additive. Importantly, similar and even better performances as low-concentration additives can also be obtained with smaller terms in the bicollidinium double salt series, solid at room T but of much easier synthesis. Moreover, enantiomer peak separation is modulated by the additive concentration (even reaching ~0.35V), and the medium enantioselectivity holds with chemically different probes, even of applicative interest, like DOPA), or in the simultaneous presence of different probes. Such results point to the possibility to obtain outstanding enantiodiscrimination on achiral electrodes employing the new compounds even as minority components in a commercial achiral medium.