Structure and properties of N,N-alkylene bis(N'-alkylimidazolium) salts.

A series of N,N-alkylene bis(N'-alkylimidazolium) salts with various anions was prepared and characterized. The hydrogen-bonding abilities and ion-pairing strengths of the salts in solution were varied by changing the solvent and anion. Qualitatively, the extent of ion pairing of the 1,2-bis[N-(N'-butylimidazolium)]ethane salts with different anions was determined in acetone-d(6) by (1)H NMR spectroscopy. Thermal properties of the imidazolium salts were related not only to the nature of anions but also to the spacer length between imidazolium cations. Exceptionally high melting points of 1,2-bis[N-(N'-alkylimidazolium)]ethane bis(hexafluorophosphate)s can be explained by multiple hydrogen bonds observed in the X-ray crystal structures. Moreover, a trans conformation of the ethylene spacer of 1,2-bis[N-(N'-alkylimidazolium)]ethane bis(hexafluorophosphate)s allows good stacking structure in the crystals.