Preparation, cation-anion interactions and physicochemical properties of ether-functionalized imidazolium ionic liquids

A set of 1-alkyl ether (and 1-alkyl)-3-methylimidazolium ionic liquids 2-4 ([C(x)O(y)MIm](+)[Anion](-) or [C(x)MIm](+)[Anion](-), where MIm = 3-methylimidazolium; CxOy = 1-alkyl ether, C7O3 = -(CH2)(2)O(CH2)(2)O(CH2)(2)OCH3 (A), C3O1 = -(CH2)(2)OCH3 (B) C-x = 1-alkyl, C-10 = C10H21 (C), C-4 = C4H9 (D); and [Anion](-) = H3CSO3- (2), BF4- (3) or PF6- (4)) was prepared and characterized. The cation-anion hydrogen bonding strength showed to be mainly anion dependent and decreased in the order H3CSO3- > BF4- > PF6-. All methanesulfonate ionic liquids 2 possessed a strongly deshielded H-2 imidazolium ring proton. 1-Alkyl ether functionalized ionic liquids showed higher densities in comparison to their 1-alkyl equivalents. The salts 2a-b, 3a-d and 4a-b are room-temperature ionic liquids. All 1-alkyl ether functionahzed ionic liquids (except 4b) are completely amorphous. The widest liquid ranges were obtained with the tetrafluoroborate ionic liquids due to their late solidification and excellent thermal stability. These data provide important information for the understanding of their application scope and the preparation of task-specific ionic liquids.