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

Abstract

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°).