Aromatic character of planar boron-based clusters revisited by ring current calculations

Abstract

The planarity of small boron-based clusters is the result of an interplay between geometry, electron delocalization, covalent bonding and stability. These compounds contain two different bonding patterns involving both σ and π delocalized bonds, and up to now, their aromaticity has been assigned mainly using the classical (4N + 2) electron count for both types of electrons. In the present study, we reexplored the aromatic feature of different types of planar boron-based clusters making use of the ring current approach. B₃^+/−, B₄²⁻, B₅^+/−, B₆, B₇⁻, B₈²⁻, B₉⁻, B₁₀²⁻, B₁₁⁻, B₁₂, B₁₃⁺, B₁₄²⁻ and B₁₆²⁻ are characterized by magnetic responses to be doubly σ and π aromatic species in which the π aromaticity can be predicted using the (4N + 2) electron count. The triply aromatic character of B₁₂ and B₁₃⁺ is confirmed. The π electrons of B₁₈²⁻, B₁₉⁻ and B₂₀²⁻ obey the disk aromaticity rule with an electronic configuration of [1σ²1π⁴1δ⁴2σ²] rather than the (4N + 2) count. The double aromaticity feature is observed for boron hydride cycles including B@B₅H₅⁺, Li₇B₅H₅ and M@B_nH_n^q clusters from both the (4N + 2) rule and ring current maps. The double π and σ aromaticity in carbon-boron planar cycles B₇C⁻, B₈C, B₆C₂, B₉C⁻, B₈C₂ and B₇C₃⁻ is in conflict with the Hückel electron count. This is also the case for the ions B₁₁C₅^+/− whose ring current indicators suggest that they belong to the class of double aromaticity, in which the π electrons obey the disk aromaticity characteristics. In many clusters, the classical electron count cannot be applied, and the magnetic responses of the electron density expressed in terms of the ring current provide us with a more consistent criterion for determining their aromatic character.