Crystal structures of frozen room temperature ionic liquids, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF 4), hexafluoroniobate (EMImNbF 6) and hexafluorotantalate (EMImTaF 6), determined by low-temperature X-ray diffraction

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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. EMImBF 4 crystallizes in the monoclinic space group P 2 1 / c with a = 8.653 ( 5 ) Å , b = 9.285 ( 18 ) Å , c = 13.217 ( 7 ) Å , β = 121.358 ( 15 ) Å , V = 906.8 ( 19 ) Å 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)⋯ π electron interaction. The BF 4 anion also forms one-dimensional pillars along the same direction with the nearest F⋯F contact distance of 3.368(3) Å. EMImNbF 6 and EMImTaF 6 are isostructural to each other and crystallize in the orthorhombic space group P 2 1 2 1 2 1 : EMImNbF 6, a = 9.204 ( 4 ) Å , b = 9.770 ( 15 ) Å , c = 12.499 ( 13 ) Å , V = 1124 ( 2 ) Å 3 , Z = 4 at 200 K; EMImTaF 6, a = 9.216 ( 5 ) Å , b = 9.763 ( 2 ) Å , c = 12.502 ( 17 ) Å , V = 1124.9 ( 17 ) Å 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 F⋯F distance of 3.441(12) Å. This structure (Type-B(MF 6)) is different from the Type-A(MF 6) structure previously reported for EMImPF 6, EMImAsF 6 and EMImSbF 6. Hydrogen bonds in the Type-A(MF 6) (EMImPF 6 (333 K), EMImAsF 6 (326 K) and EMImSbF 6 (283 K)) crystal lattice are weaker than those in the Type-B(MF 6) (EMImNbF 6 (272 K) and EMImTaF 6 (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 EMImBF 4 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.