"Inherently Chiral" Ionic Liquid Media: Enantioselective Voltammetry on Achiral Electrodes

To achieve chiral voltammetry, an alternative strategy to using chiral electrodes is to work on achiral electrodes in a chiral medium. In this frame, chiral ionic liquids (CILs) should perform much better than chiral organic solvents, on account of their higher intrinsic order; and, by analogy with the recently introduced "inherently chiral" electrode surfaces of unprecedented enantiorecognition ability [1-4], "inherently chiral" ionic liquids (ICILs) should perform better than CILs. To obtain ICILs we started from 3,3'-bicollidine, an inherently chiral atropoisomeric bipyridine scaffold of easy synthesis from cheap reagents, converting by dialkylation its antipodes into enantiopure bicollidinium double salts. With appropriate choice of the alkyl chain length and of the counter anion the melting points were lowered below room temperature, yielding enantiopure ICILs. Of these we highlighted the huge enantioselectivity in chiral voltammetry tests, employing them as low-concentration additives in a commercial achiral ionic liquid. The tests were performed on commercial achiral screen printed electrodes SPEs, using the same chiral probes previously used for testing inherently chiral surfaces. The enantiomer peak separation is huge, 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-conventration additives can also obtained with smaller terms in the bicollidinium double salt series, solid at room temperature but of much easier synthesis. The enantiomer peak separation is modulated by the additive concentration (even reaching ~0.35 V) and by the achiral ionic liquid in which it is employed. The medium enantioselectivity holds with chemically different probes, even of applicative interest, like DOPA. Such results point to the possibility to obtain outstanding enantiodiscrimination on achiral electrodes even employing the new compounds as minority components in a commercial achiral medium.[5] The work is currently under further development, aiming to consolidate, widen and rationalize the first results; a selection of the most recent achievements will be presented and discussed. The support of Fondazione Cariplo, grant no. 2011-1851 "Inherently Chiral Ionic Liquids" and of Fondazione Cariplo/Regione Lombardia "Avviso congiunto per l’incremento dell’attrattività del sistema di ricerca lombardo e della competitività dei ricercatori candidati su strumenti ERC - edizione 2016” (Project 2016-0923) are gratefully acknowledged. References: [1] Angew Chem. Int. Ed. 53 (2014) 2623. [2] Chem. Eur. J. 20 (2014), 15298. [3] Chem. Sci. 6 (2015) 1706. [4] Anal. Bional. Chem. 408 (2016) 7243. [5] Angew Chem. Int. Ed. 56 (2017) 2079.