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
([C2DABCO]­[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. [C2DABCO]­[NTf2] undergoes 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 [C2DABCO]­[NTf2] was determined at temperatures below the solid–solid
transition (solid-I 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 [C2DABCO]+ and [NTf2]− in the solid-II
phase indicating the formation of a rotator phase. The structure of
the [C2DABCO]­[NTf2] precursor 1-ethyl-1,4-diazabicyclo[2.2.2]­octanium
bromide ([C2DABCO]­[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.