Association of Pyramidal Boron Lewis Superacids with Pyridines: Bending 2,4,6-Collidine with the 10-Sulfonium-9-boratriptycene

The association of pyridine and of 2,4,6-trimethylpyridine (collidine) with a pyramidal triarylborane belonging to the 9-boratriptycene family is investigated by NMR spectroscopy and by X-ray diffraction analysis. Despite of the large size of the ortho-disubstituted collidine Lewis base and of the large steric hindrance of the boron Lewis superacid used (9-boratriptycene-10-sulfonium), the superacidity at its boron atom is enabling the formation of a B-N bond and is resulting in a very stabe Lewis adduct. Single- crystal X-ray diffraction analysis revealed an exceptional deformation of the collidine moiety with a deviation from planarity of nearly 41° between the pyridine C−N bonds and the ortho-methyl groups C−Me bonds, which is even resulting a small twist of the collidine six-membered heteroaromatic ring into a boat-like conformation. Computational investigations indicate that the steric repulsions are reducing the Gibbs free energy of Lewis adduct formation by approximately 50 kJ mol−1.