Imidazolium-based ionic liquids with large weakly coordinating anions

Abstract

Eight imidazolium-based salts with the weakly-coordinating anions [BAr^F]⁻ and [Al(O^tC₄F₉)₄]⁻ ([BAr^F]⁻ = tetrakis{3,5-bis(trifluoromethyl)phenyl}borate) have been synthesized. Characterization by ¹H NMR spectroscopy shows that the salts are fully dissociated in solution. Six examples have been further characterized by X-ray crystallography, revealing that weak hydrogen bonds do occur in the solid phase between the imidazolium cations and weakly coordinating anions. Comparison with the bulky [IDiPPH] imidazolium cation (IDiPPH = 1,3-bis{2,6-bis(diisopropyl)phenyl}imidazolium) shows that large steric bulk close to the imidazolium protons can preclude hydrogen bonds from forming. Differential scanning calorimetry of the salts reveal that all are thermally stable up to 200 °C which renders them as potentially suitable background electrolytes for electrochemical processes which take place at elevated temperatures.