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 pi electrons over the B2N three-membered ring, indicating the 2 pi 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 6 pi aromatic property. Moreover, the reaction of 4 with a diazo compound produces a cationic B2N3C pentafulvene derivative 7. The first cationic boron-containing 2 pi aromatic species, azadiboriridinylium cation, has been synthesized by the ring-contraction of a diboratriazole followed by the hydride abstraction. This compound undergoes ring-expansion reactions with unsaturated nitrogen molecules leading to the cationic 6 pi aromatic triazadiborolidinylium as well as a B2N3C pentafulvene derivative.image

Automated summary

In this study, a series of boron-containing compounds were synthesized. A novel compound 4 was successfully prepared by a sequence of reactions including substitution, ring contraction, and hydride abstraction. Furthermore, compound 4 underwent reactions with azobenzene and pyridazine to afford two new compounds, 5 and 6, and also reacted with a diazo compound to yield compound 7.