Ionic liquids containing boron cluster anions.

The combination of different boron cluster anions and some of the cations typically found in the composition of ionic liquids has been possible by straightforward metathetic reactions, producing new low melting point salts; the imidazolium cations have been systematically studied, [C(n)mim]+ (when [C(n)mim]+ = 1-alkyl-3-methylimidazolium; n = 2, 4, 6, 8, 10, 12, 14, 16, or 18). Melting points increase in the anionic order [Co(C2B9H11)2]- < [C2B9H12]- < [B10Cl10]2- < [B12Cl12]2-. Nevertheless, alkyl chain length dramatically influences the thermal behavior, suggesting that packing inefficiency is the main cause of the existence of room temperature ionic liquids. The salts [C(n)mim][Co(C2B9H11)2] (n = 4, 6, 8, 10, 12 or 14) are liquids at room temperature, presenting strikingly low glass transition temperatures (> or = -34 degrees C). The salts [C(n)mim]2[X] ([X]2- = [B10Cl10]2- or [B12Cl12]2-, n = 16 or 18) show liquid crystal phases between the solid and liquid states. Tetraalkylphosphonium salts of [B10Cl10]2- have also been prepared. Physical properties, such as thermal stability, density, or viscosity, have been measured for some selected samples. The presence of the perhalogenated dianion [B12Cl12]2- in the composition of the imidazolium salts renders highly thermally stable compounds. For example, [C2mim]2[B12Cl12] starts to decompose above 480 degrees C in a dynamic TGA analysis under a dinitrogen atmosphere. Crystal structures of [C2mim][Co(C2B9H11)2] and [C2mim]2[B12Cl12] have been determined. 1H NMR spectra of selected imidazolium-boron cluster anion salts have been recorded from solutions as a function of the concentration, showing trends related to the cation-anion interactions.