Crystalline 2π Aromatic Azadiboriridinylium: A BN Analogue of Cyclopropenylium Cation.

N-Substitution of a thermally unstable diboratriazole 1 with a trimethylsilyl group affords a remarkably stable diboratriazole derivative 2. Ring contraction of 2 with an N-heterocyclic carbene accompanied by the release of N as well as 1,4-hydrogen shift affords a carbene-stabilized azadiboriridine 3. Abstraction of the H-B hydride in 3 with methyl trifluoromethanesulfonate leads to the isolation of a hitherto unknown azadiboriridinylium 4, the first BN analogue of cyclopropenylium cation. X-ray diffraction analysis and computational studies confirmed the delocalization of π electrons over the B N three-membered ring, indicating the 2π aromatic feature. Compound 4 undergoes ring expansion reactions with azobenzene and pyridazine to furnish triazadiborolidinylium species 5 and 6, the latter of which possesses a cationic B N ring with a pronounced 6π aromatic property. Moreover, the reaction of 4 with a diazo compound produces a cationic B N C pentafulvene derivative 7.