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

• Published in: Solid state sciences 2006-10, Vol.8 (10), p.1250-1257
• Main Authors: Matsumoto, Kazuhiko, Hagiwara, Rika, Mazej, Zoran, Benkic, Primoz, Zemva, Boris
• Format: Article
• Language: English
• ISSN: 1293-2558
• DOI: 10.1016/j.solidstatesciences.2005.12.018

The crystal structures of three salts, l-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4), hexafluoroniobate (EMImNbF6) and hexafluorotantalate (EMImNbF6), 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)A(ngstrom), b = 9.285(18)A(ngstrom), c = 13.217(7)A(ngstrom), beta = 121.358(15)A(ngstrom), V = 906.8(19)A(ngstrom)3, Z = 4 at 100 K. EMImBF4 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 F...F contact distance of 3.368(3)A(ngstrom). EMImNbFg and EMImTaF6 are isostruc-tural to each other and crystallize in the orthorhombic space group P2(1)2(1)2(1): EMImNbF6, a = 9.204(4)A(ngstrom), b = 9.770(15)A(ngstrom), c = 12.499(13)A(ngstrom), V = 1124(2)A(ngstrom)3, Z = 4 at 200 K; EMImTaF6, a = 9.216(5)A(ngstrom), b = 9.763(2)A(ngstrom), c = 12.502(17)A(ngstrom), V = 1124.9(17)A(ngstrom)3, Z = 4 at 200 K. In EMImNbF6 and EMImTaF6, 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 F...F distance of 3.441(12) A. This structure (Type-B(MF6)) is different from the Type-A(MF6) structure previously reported for EMImPF6, EMImAsF6 and EMImSbF6. Hydrogen bonds in the Type-A(MF6) (EMImPF6 (333 K), EMImAsF6 (326 K) and EMImSbF6 (283 K)) crystal lattice are weaker than those in the Type-B(MF6) (EMImNbF6 (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 EMImBF4 essentially preserves the same structure in this temperature range and increases its volume by only 4% due to the melting.