Increasing the Lewis Acidity of Pyramidal Boranes: Cationic 9-Bora-1-Azatriptycenes as Strong Boron Lewis Superacids

Bora-azatriptycenes have been synthesized and used as geminal pyridine–boranes pairs, which exhibit unusual binding modes with weakly coordinating anions, to unique aza-triptycene/bora-triptycene Lewis adducts, and to bis-triptycenes dimers with unprecedented architectural motifs. Embedding a boron atom at the edge of a triptycene scaffold, in a strongly pyramidalized and geometrically constrained environment, and protonation of the adjacent pyridine ring strongly enhances the Lewis acidity of the boron atom. High levels of Lewis acidity are reached at the boron atom of these cationic boron Lewis superacids. Experimental and quantum-chemistry screening of their Lewis acidity enables elucidation of the interplay between Lewis acidity, pyramidalization angle, reorganization energy, and charge position in the adjacent pyridinium ring, and identifies new dicationic Lewis superacids.