Crystal structures of frozen room temperature ionic liquids, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4), hexafluoroniobate (EMImNbF6) and hexafluorotantalate (EMImTaF6), determined by low-temperature X-ray diffraction

The crystal structures of three salts, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF(4)), hexafluoroniobate (EMImNbF(6)) and hexafluorotantalate (EMImTaF(6)), all of which form room-temperature ionic liquids (RTILs), have been determined by low-temperature X-ray diffraction studies of their single crystals. EMImBF4 crystallizes in the monoclinic space group P2(1)/c with a = 8.653(5) angstrom, b = 9.285(18) angstrom, c = 13.217(7) angstrom, 121.358(15) angstrom, V = 906.8(19) angstrom(3), Z = 4 at 100 K. EMImBF(4) exhibits a unique structure wherein EMIm cations form one-dimensional pillars facing the imidazolium ring to the next ring linked by H(methylene)(...)pi electron interaction. The BF4 anion also forms one-dimensional pillars along the same direction with the nearest (FF)-F-... contact distance of 3.368(3) angstrom. EMImNbF(6) and EMImTaF(6) are isostructural to each other and crystallize in the orthorhombic space group P2(1)2(1)2(1): EMImNbF6, a = 9.204(4) angstrom, b = 9.770(15) angstrom, c = 12.499(13) angstrom, V = 1124(2) angstrom(3), Z = 4 at 200 K; EMImTaF(6), a = 9.216(5) angstrom, b = 9.763(2) angstrom, c = 12.502(17) angstrom, V = 1124.9(17) angstrom(3), Z = 4 at 200 K. In EMImNbF(6) and EMImTaF(6), EMIm cations also form a one-dimensional pillar structure and the hexafluorocomplex. anions are located in a zig-zag arrangement along the same direction with the nearest (FF)-F-... distance of 3.441(12) angstrom. This structure (Type-B(MF6)) is different from the Type-A(MF6) structure previously reported for EMImPF(6), EMImAsF(6) and EMImSbF(6). Hydrogen bonds in the Type-A(MF6) (EMIMPF6 (333 K), EMImAsF(6) (326 K) and EMImSbF(6) (283 K)) crystal lattice are weaker than those in the Type-B(MF6) (EMImNbF(6) (272 K) and EMImTaF6 (275 K)) crystal lattice. This suggests that the strength of the hydrogen bond is not always a decisive and determining factor for the melting points of RTILs. The measurement of cell parameters for EMImBF4 between 100 K and its melting point revealed that EMImBF(4) essentially preserves the same structure in this temperature range and increases its volume by only 4% due to the melting. (c) 2006 Elsevier Masson SAS. All rights reserved.