Structure and Properties of High Stability Geminal Dicationic Ionic Liquids

Thirty-nine geminal dicationic ILs were synthesized and characterized in terms of their surface
tensions, densities, melting points, refractive indices, viscosities, and miscibilities with a polar and nonpolar
solvent. Two imidazolium or pyrrolidinium cations were joined via different length hydrocarbon linkage chains
(from 3 to 12 carbons long). The various geminal dications were paired with up to four different anions.
The effect of the dication type, linkage chain, alkyl substituents, and anion type on the physicochemical
properties of these compounds was examined. Among the more interesting findings for this class of
compounds was that their liquid and thermal stability ranges generally exceeded those of the more
conventional, better known ILs. Indeed, this range was from −4 to >400 °C for one of the pyrrolidinium-based geminal dicationic liquids. X-ray crystallography of the smaller solid ionic compounds indicated that
there may be a correlation between the configurational degrees of freedom of the ILs and their melting
points/glass transition temperatures. In one case, the crystal structure showed that a dicationic moiety had
three distinct conformations in an asymmetric unit cell. The solvation properties of the geminal dicationic
ILs tend to be similar to those of their monocationic analogues.