Implementing inherent chirality in ionic liquids and supporting electrolytes: design strategies, electrochemical properties, enantioselectivity

The interest for Chiral Ionic Liquids (CILs) has been steadily increasing in the last years. Although their application to electrochemical processes is a field still requiring exploration, they should have a huge impact for instance in asymmetric electropolymerizations and in regioregular electrochemically activated polymerizations, as well as in preparative electrosynthetic processes of chiral compounds. Our group has recently presented a new class of heterocycle-based "inherently chiral" electroactive materials of unprecedented chirality manifestations and enantiorecognition ability [1-3]. The "inherent chirality" concept implies that both the stereogenic element and the electroactivity source coincide with the whole conjugated molecular backbone, which features a tailored torsion induced by an atropisomeric bithiophene scaffold. Now we are applying the same "inherent chirality" approach to the development of inherently chiral ionic liquids (ICILs) and/or supporting electrolytes, hopefully endowed with high enantioselectivity, like the formerly developed inherently chiral electrodes. In particular, we are working on two molecule families, 1,1'-bibenzimidazolium dialkyl salts [4] and 3,3'-bicollidinium dialkyl salts. In fact both the 1,1'-bibenzimidazole scaffold (unlike the 2,2' one) and the 3,3'-bicollidine one consist of two moieties separated by very high torsional barriers; therefore they exist into two stable enantiomers that can be separated and stored. The high torsional barriers also result in low overall conjugation efficiency, with first oxidation and reduction located near or beyond the background, ensuring a very wide potential window (a desirable feature for use as electrochemical reaction medium) albeit becoming narrower on the cathodic side upon conversion of the scaffolds into the corresponding alkyl salts. The number and length of the alkyl chains, as well as the anion choice, modulate the melting points. According to the latter being higher or lower than room temperature, the new salts can be used as chiral ionic liquids or/and chiral supporting electrolytes, to which. The 3,3'-bicollidine case is particularly interesting because (a) its enantiopure antipodes can be obtained by fractional crystallization of diastereoisomeric salts, without requiring expensive preparative HPLC, and (b) all the salts liquid at room temperature so far obtained belong to this family. The electrochemical properties of the new inherently chiral media will be presented, as well as the promising preliminary results concerning their enantioselection capability. With the contribution of Fondazione Cariplo, grant no. 2011-1851 References [1] F. Sannicolò, S. Arnaboldi, P.R. Mussini et al. Angew. Chem. Int. Ed. 2014, 53, 2623-2627. [2] F. Sannicolò, P.R. Mussini, S. Arnaboldi et al. Chem. Eur. J. 2014, 20, 15298-15302. [3] S. Arnaboldi, T. Benincori, R. Cirilli, W. Kutner, M. Magni, P.R. Mussini, K. Noworyta, F. Sannicolò, Chemical Science 2015, DOI: 10.1039/C4SC03713H [4] S. Arnaboldi, R. Cirilli, A. Forni, A. Gennaro, A.A. Isse, V. Mihali, P.R. Mussini, M. Pierini, S. Rizzo, F. Sannicolò, Electrochimica Acta 2015 doi:10.1016/j.electacta.2015.03.177