Lewis acidic ionic liquids and their catalytic applications have been studied for decades [1], with several processes implemented on the industrial scale [2]. Despite the breadth of investigated reactions, Lewis acidic ionic liquids have had very limited structural diversity, with most examples combining a ‘spectator’ organic cation with anions of a general formula [MxXy]-, where M = e.g. AlIII, ZnII, GaIII or FeIII, and X = Cl or Br [1]. Here, we present recently developed, structurally innovative ionic liquids based on Lewis acidic borenium cations [3]. In borenium cations, the acidity of the tricoordinate boron centre is additionally enhanced by the positive charge, leading to Lewis superacidity. Borocations are typically studied in solution, and sometimes characterised crystallographically. In this work an alternative approach was adopted, whereby tricoordinate borenium cations were incorporated, for the first time, into ionic liquids (Figure 1). This change of environment allowed for the direct study of Lewis acidity in 'naked' cations, thereby eliminating the solvent effect. Using the Gutmann Acceptor Number approach, extremely high acidities were recorded (up to AN = 182), demonstrating that these borenium ionic liquids are the strongest liquid Lewis superacids reported to date. The ionic liquid environment appears to be crucial for enhanced Lewis acidity. Synthetic approaches, speciation studies and catalytic applications will be discussed.

Lewis superacidic ionic liquids with borenium cations

255th National Meeting (2018)

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