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 movem...