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

• Published in: Chemistry : a European journal 2015-05, Vol.21 (20), p.7489-7502
• Main Authors: Scholz, Franziska, Himmel, Daniel, Eisele, Lea, Unkrig, Wiebke, Martens, Arthur, Schlüter, Peter, Krossing, Ingo
• Format: Article
• Language: English
• Subjects: Chemistry; ionic liquids; Lewis acids; protonated arenes; quantum chemical calculations; superacids
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201405952

Bulk protonated mesitylene, toluene, and benzene bromoaluminate salts were stabilized and characterized in the superacidic system HBr/n AlBr3 with NMR spectroscopy and X‐ray analysis of [HC6H3(CH3)3]+[AlBr4]− (1), [HC6H5(CH3)]+[AlBr4]− (2), and [C6H7]+[Al2Br7]−⋅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 Brønsted acidity scale, quantum chemical calculations confirmed the superacidic properties, and rank the acidities in ILs down to a pHabs 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 pHabs 163. Bromoaluminate salts of protonated mesitylene, toluene, and benzene resulted from the superacidic system HBr/n AlBr3 and in the respective bromoaluminate ionic liquids. Their superacidic properties were confirmed on an absolute basis, and found to reach a pHabs value of 164, which is seven orders of magnitude more acidic than the boundary to superacidity (pHabs=171).