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 (EMImBF4), hexafluoroniobate (EMImNbF6) and hexafluorotantalate (EMImTaF6), 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 P21/cP21/c with a=8.653(5) Åa=8.653(5) Å, b=9.285(18) Åb=9.285(18) Å, c=13.217(7) Åc=13.217(7) Å, β=121.358(15) Åβ=121.358(15) Å, V=906.8(19) Å3V=906.8(19) Å3, Z=4Z=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)⋯π 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) Å. EMImNbF6 and EMImTaF6 are isostructural to each other and crystallize in the orthorhombic space group P212121P212121: EMImNbF6, a=9.204(4) Åa=9.204(4) Å, b=9.770(15) Åb=9.770(15) Å, c=12.499(13) Åc=12.499(13) Å, V=1124(2) Å3V=1124(2) Å3, Z=4Z=4 at 200 K; EMImTaF6, a=9.216(5) Åa=9.216(5) Å, b=9.763(2) Åb=9.763(2) Å, c=12.502(17) Åc=12.502(17) Å, V=1124.9(17) Å3V=1124.9(17) Å3, Z=4Z=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) Å. 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.

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