A Dialkyl Viologen Ionic Liquid: X‑ray Crystal Structure Analysis of Bis(trifluoromethanesulfonyl)imide Salts

An
X-ray crystal structure of a viologen type ionic liquid compound,
bis­(trifluoromethanesulfonyl)­imide (TFSI) salt of butyl heptyl viologen
[C4VC7·(TFSI)2], with a melting
point (mp) of 52 °C was examined. A cis conformer of TFSI (cis-TFSI),
known to be less stable than trans-TFSI, was found in the crystal
in a 1:1 coexistence with trans-TFSI. In contrast, all TFSI anions
found in dimethylviologen TFSI salts [C1VC1·(TFSI)2] were of trans conformation. The lower mp of C4VC7·(TFSI)2 compared to that of C1VC1·(TFSI)2 originates in a smaller
electrostatic lattice energy and disordered structure of the side
chains of C4VC7. Any charge transfer interaction
causing a visible region absorption band was not observed in the solids
of either C4VC7·(TFSI)2 (dihedral
angle of bipyridinium ϕ = 37.2°) or C1VC1·(TFSI)2 (ϕ = 9°).