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 N2 as well as 1,4‐hydrogen shift affords a carbene‐stabilized azadiboriridine 3. Abstraction of the H−B3mem 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 B2N 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 B2N3 ring with a pronounced 6π aromatic property. Moreover, the reaction of 4 with a diazo compound produces a cationic B2N3C pentafulvene derivative 7. [ABSTRACT FROM AUTHOR]