Impressive enantiodiscrimination ability of 1,1'-bibenzimidazolium and 3,3'-bicollidinium salts in achiral ionic liquids

In "inherently chiral" materials the stereogenic element responsible for chirality coincides with the molecular portion responsible for their specific properties. This concept was successfully tested in designing chiral organic semiconductors employed by us as highly stereoselective electrode surfaces [1] and it is now applied to the preparation of inherently chiral ionic liquids (ICILs) and transport electrolytes. According to this strategy, we synthesised the N,N'-dialkyl salts of bi-heteroaromatic systems characterized by atropisomeric 1,1'-biimidazole 1 and 3,3'-bipyridine 2 scaffolds. The enantiopure dialkyl salts were prepared by alkylation of the enantiopure antipodes of a series of 2-alkyl-1,1'-bibenzimidazoles and of 3,3'-bicollidine. They were obtained in an enantiopure state by resolution of the corresponding racemates by employing semi-preparative HPLC on CSP in the former case and by crystallization of the diastereomeric salts with the O,O-dibenzoyltartaric acids in the latter. [2], [3] In particular, the long alkyl chain salts 2 are liquid at room temperature, therefore, they are the first atropisomeric ICILs. The new ICILs have been employed as additives to achiral ILs in cyclovoltammetric experiments devoted to check their enantiorecognition ability towards the antipodes of several probes differing in structure and stereogenic element on achiral electrodes. In particular, the selected probes are the N,N'-dimethyl-1-ferrocenyl-ethylamine (FcA), the 2,2'-bis[2-(5,2'-bithiophenyl)]-3,3'-bithianaphthene (BT2T4), DOPA, DOPA Methylester, Ofloxacin and Thalidomide enantiomers, while BMIMPF6 and BMIMBF4 are the selected media. The new chiral bis-onium salts provide outstanding differentiation of the oxidation or reduction peaks of the enantiomers of all the probes. [3] These results appear indeed attractive on account of both the impressive enantiorecognition levels and the very small chiral inductor consumption. Stereoselection mechanistic hypotheses are discussed.