Thermal Properties of Macrocyclic Polyethers: Implications for the Design of Crown Ether-Based Ionic Liquids

The most commonly studied classes of ionic liquids (ILs) comprise relatively large and asymmetric heterocyclic cations (e.g., diakylimidazolium or N-alkylpyridinium) in combination with any of a wide variety of inorganic (e.g., BF4−, Cl−) or organic (e.g., bis[(trifluoromethyl- sulfonyl)imide], Tf2N−) anions. Recently it has been shown that ILs can also be formed by complexation reactions of metal cations (e.g., Li+, as its Tf2N− salt) with various neutral ligands (e.g., cyclohexano-15-crown-5 or alkylamines). Because the upper limit of the useful temperature range of any IL is governed by its thermal stability, and because the thermal stability of a neutral ligand (i.e., its propensity to either volatilize or decompose) is of obvious importance in determining that of an IL prepared from it, a systematic examination of the thermal properties of a series of macrocyclic polyethers of potential utility in the synthesis of new ILs has been undertaken. The results show that the temperature corresponding to the...