Lewis Superacidic Ionic Liquids with Tricoordinate Borenium Cations.

The first examples of ionic liquids based on borenium cations, [BCl2 L](+), are reported. These compounds form highly Lewis acidic liquids under solvent-free conditions. Their acidity was quantified by determining the Gutmann acceptor number (AN). Extremely high ANs were recorded (up to AN = 182, δ31P = 120 ppm), demonstrating that these borenium ionic liquids are the strongest Lewis superacids reported to date, with the acidity enhanced by the ionic liquid environment.