FULL PAPER DOI: 10.1002/ejoc.201300338 Ionic Liquids Based on the 7-Azabicyclo[2.2.1]heptane Skeleton: Synthesis and Properties Nils De Vos, [a] Cedric Maton, [a] Peter De Vreese, [b] Neil R. Brooks, [c] Koen Binnemans, [c] and Christian V. Stevens* [a] Keywords: Ionic liquids / Nitrogen heterocycles / Cyclic voltammetry Based on a previously developed method for the synthesis of epibatidine analogues, a series of new ionic liquids, based on the 7-azabicyclo[2.2.1]heptane skeleton, have been syn- thesized. The chemical and physical properties of the ionic Introduction For many years, ionic liquids have attracted a great deal of attention because of their very interesting properties, such as non-volatility, good thermal and electrochemical stability and high ionic conductivity. This has led to various applications based on some of their unique properties, such as the modification of starch and other carbohydrates, [1–4] the electrodeposition of various metals [5–7] and their use as electrolytes in Li batteries, [8,9] lubricants [10,11] and (re-us- able) catalysts in various organic reactions. [12–15] The most popular choices for the cations of ionic liquids are based upon imidazolium, ammonium, piperidinium and pyridin- ium and popular anions are dicyanamide and bis(trifluoro- methylsulfonyl)imide. For electrochemical applications of ionic liquids, (cyclic) ammonium ionic liquids have the advantage of being more resistant to electrochemical oxidation and reduction, pos- sessing a large electrochemical window, and they show little thermal degradation. These ionic liquids have also been modified for specific applications to produce the so-called task-specific or functionalized ionic liquids. Ether-functionalized quaternary ammonium ionic li- quids have the advantage of having low viscosities due to the flexible ether side-chains, although they have been [a] SynBioC Research Group, Department of Sustainable Chemistry and Technology, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, 9000 Ghent, Belgium Fax: +32-264-6243 E-mail: chris.stevens@ugent.be Homepage: www.synbioc.ugent.be [b] Katholieke Hogeschool Sint-Lieven, Laboratory for Chemical Process Technology, Department of Industrial Engineering Division Chemistry, Gebroeders Desmetstraat 1, 9000 Ghent, Belgium [c] KU Leuven – University of Leuven, Department of Chemistry, Celestijnenlaan 200F, P. O. Box 2404, 3001 Heverlee, Belgium Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejoc.201300338. Eur. J. Org. Chem. 2013, 3741–3750 © 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3741 liquids with bis(trifluoromethylsulfonyl)imide (Tf 2 N) and di- cyanamide [N(CN) 2 ] anions were investigated and they were found to exhibit very good electrochemical and thermal sta- bilities. proven to be slightly less stable in electrochemical applica- tions. However, they can be used as electrolytes in lithium batteries. [16] Carboxylic acid functionalized ionic liquids possess the ability to dissolve large amounts of metal (hydr)oxides. However, their stability against reduction in electrochemical applications is rather poor thus limiting their use in, for example, batteries and electrochemical de- position. [17] Ionic liquids containing a cyano group bound directly to the cation are interesting salts because they pos- sess a high electrochemical stability and can interact with various cations, which makes electrolytical applications more feasible for this type of ionic liquid. [18,19] A drawback of the nitrile functionality, however, can be an increased viscosity due to hydrogen bonding with the cyano group, compared with their non-functionalized analogues. [20] One problem with ionic liquids containing quaternary ammonium cations is that they can be sensitive to Hofmann elimination. Therefore we created a new cationic core con- taining a bicyclic ammonium moiety, inspired by the struc- ture of epibatidine (1; Figure 1). Figure 1. Structure of epibatidine (1). This bicyclic structure should be more resistant to Hof- mann elimination due to the fact that there is no possibility of forming a double bond on the bridgehead of the central moiety (cf. Bredt’s rule). Little is actually known about these bicyclic cations. Most of them are based on bridged nitrogen-containing heterocycles, such as 1,4-diazabicyclo- [2.2.2]octane (DABCO), quinuclidine and azabicyclonon- ane moieties. [21] Other examples are based on bicyclic imid- azolium-based ionic liquids. [22,23]