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

Abstract

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/c with a = 8.653(5)Å, b = 9.285(18)Å, c = 13.217(7), β = 121.358(15)Å, V = 906.8(19)ų, 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)⋯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 P2₁2₁2₁: EMImNbF₆, a = 9.204(4)Å, b = 9.770(15)Å, c = 12.499(13)Å, V = 1124(2)ų, Z = 4 at 200 K; EMImTaF₆, a = 9.216(5)Å, b = 9.763(2)Å, c = 12.502(17)Å, V = 1124.9(17)ų, Z = 4 at 200K. 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.