Aromaticity switch of borabenzene: from aromatic when free or weakly aromatic when fused to 2D PAHs to non-aromatic when fused to 3D carboranes†

Abstract

Free borabenzene is aromatic, but when fused with 3D aromatic carborane, it loses its aromaticity. A new series of ortho-carborane-fused boracycles has been successfully synthesized through selective intramolecular C–H borylation, with claims of forming fused 3D/2D aromatic systems. However, our quantum chemical analysis shows that while the boron cage maintains its aromatic character, the boracycle loses its aromaticity. The limited overlap between the π molecular orbitals of the planar boracycle and the n + 1 molecular orbitals of the carborane hinders significant electronic delocalization between the two fused components. Our findings reveal that the peripheral σ-aromaticity of the carborane and the π-aromaticity of the boracycle are not fused, making a true 3D/2D aromatic system unattainable. In contrast, when the same boracycle is fused to a 2D aromatic polycyclic aromatic hydrocarbon (PAH), it retains partial aromaticity. Thus, the aromaticity of free boratabenzene is lost when fused with 3D aromatic carborane, i.e., it switches from aromatic to non-aromatic when fused to carborane or weakly aromatic when fused to PAHs.