The Acidity of the HBr/AlBr3 System: Stabilization of Crystalline Protonated Arenes and Their Acidity in Bromoaluminate Ionic Liquids

Bulk protonated mesitylene, toluene, and benzene bromoaluminate salts were stabilized and characterized in the superacidic system HBr/nAlBr(3) with NMR spectroscopy and X-ray analysis of [HC6H3(CH3)(3)](+)[AlBr4](-) (1), [HC6H5(CH3)](+)[AlBr4](-) (2), and [C6H7](+)[Al2Br7](-)center dot C6H6 (3). Protonation attempts in bromoaluminate ILs led to a complete protonation of mesitylene, and a protonation degree of up to 15% for toluene in the IL BMP+[Al2Br7](-). Benzene could only be protonated in the more acidic IL BMP+[Al3Br10](-), with a degree of 25 %. Protonation attempts on aromatics provide evidence that the bromoaluminate ILs tolerate superacidic environments. On the basis of the absolute Bronsted acidity scale, quantum chemical calculations confirmed the superacidic properties, and rank the acidities in ILs down to a pH(abs) value of 164 with an error of less than one pH unit compared with experimental findings. The neat AlBr3/HBr system even may reach acidities down to pH(abs) 163.