Structural Studies on the Basic Ionic Liquid 1-Ethyl-1,4-diazabicyclo[2.2.2]octanium Bis(trifluoromethylsulfonyl)imide and Its Bromide Precursor

The structure of 1-ethyl-1,4-diazabicyclo[2.2.2]octanium bis(trifluoromethylsulfonyl)imide ([C(2)DABCO] [NTf2]) was determined by single crystal X-ray diffraction, and its behavior with temperature was examined by synchrotron powder diffraction, differential scanning calorimetry (DSC), and Raman spectroscopy. [C(2)DABCO][NTf2] under-goes a solid solid phase transition at 309 K and a solid liquid (melting) transition at 350 K, as reported previously in the literature. The crystal structure of [C(2)DABCO][NTf2] was determined at temperatures below the solid solid transition (solid-II phase), whereas in situ synchrotron powder diffraction enabled the unit cell of the solid phase observed above the solid solid transition temperature (solid-II phase) to be indexed. High-resolution synchrotron powder diffraction data indicated the coexistence of a metastable phase with the solid-I and solid-II phases around the temperature of the solid solid phase transition. A variable temperature Raman study suggested rotational movement of both [C(2)DABCO](+) and [NTf2](-) in the solid-II phase indicating the formation of a rotator phase. The structure of the [C(2)DABCO] [NTf2] precursor 1-ethyl-1,4-diazabicyclo[2.2.2]octanium bromide ([C(2)DABCO][Br]) was also determined by single crystal X-ray diffraction and its phase transitions were examined by differential scanning calorimetry. A comparison of the structures and phase transitions of the two analogous salts provides insights into the relative electrostatic attraction between the ions and the relative packing density and efficiency in the crystal lattices.