Identification of 1,3-Dialkylimidazolium Salt Supramolecular Aggregates in Solution

The nature of the interactions between 1,3-dialkylimidazolium cations and noncoordinating anions such as
tetrafluoroborate, hexafluorophosphate, and tetraphenylborate has been studied in the solid state by X-ray
diffraction analysis and in solution by 1H NMR spectroscopy, conductivity, and microcalorimetry. In the
solid state, these compounds show an extended network of hydrogen-bonded cations and anions in which
one cation is surrounded by at least three anions and one anion is surrounded by at least three imidazolium
cations. In the pure form, imidazolium salts are better described as polymeric supramolecules of the type
{[(DAI)3(X)]2+[(DAI)(X)3]2-}n (where DAI is the dialkylimidazolium cation and X is the anion) formed through
hydrogen bonds of the imidazolium cation with the anion. In solution, this supramolecular structural organization
is maintained to a great extent, at least in solvents of low dielectric constant, indicating that mixtures of
imidazolium ionic liquids with other molecules can be considered as nanostructured materials. This model is
very useful for the rationalization of the majority of the unusual behavior of the ionic liquids.