Chloroaluminate Liquid Clathrates: Is It the Cations or the Anions That Drive the Solubility of Aromatics?

Three chloroaluminate ionic liquids (ILs), triethylammonium tetrachloroaluminate ([HN222][AlCl4]), triethylammonium heptachlorodialuminate ([HN222][Al2Cl7]), and tetraethylammonium heptachlorodialuminate ([N2222][Al2Cl7]), were employed to investigate liquid clathrate (LC) formation and the relative role of the cation or anion in aromatic solubility. The trend in molar solubilities in all three ILs was found to be benzene > toluene > o-xylene > p-xylene ∼ m-xylene > mesitylene; however, the solubilities in the [AlCl4]−IL were 40–70% lower than those found for the two [Al2Cl7]−ILs, which were nearly equivalent. Attempts to crystallize analogues of the LC phases from aromatic solutions of [N2222][Al2Cl7] led to crystalline [N2222][Al2Cl7]·C6H6from benzene and [N2222][AlCl4] from all other aromatic solvents. NMR analyses of [HN222][AlCl4] and [HN222][Al2Cl7] LCs supported the crystallographic analyses, where 27Al NMR confirmed the dynamic speciation of the [Al2Cl7]−anions but not the [AlCl4]−anions, and 1H NMR supported the crystallographic observation that each benzene interacts with two cations; however, the solution data suggested a 2:1 benzene/IL ratio.