Chemical bonding and dynamic structural fluxionality of a boron-based AlB binary cluster: the robustness of a doubly 6π/6σ aromatic [B] molecular wheel.

Despite the isovalency between Al and B elements, Al-doping in boron clusters can deviate substantially from an isoelectronic substitution process. We report herein on a unique sandwich di-Al-doped boron cluster, AlB, using global structural searches and quantum chemical calculations. The cluster features a perfectly planar B molecular wheel, with two isolated Al atoms symmetrically floating above and below it. The two Al atoms are offset from the center of the molecular wheel, resulting in a symmetry for the cluster. The AlB cluster is shown to be dynamically fluxional even at far below room temperature (100 K), in which a vertical Al rod slides or rotates freely within a circular rail on the B plate, although there is no direct Al-Al interaction. The energy barrier for intramolecular rotation is only 0.01 kcal mol at the single-point CCSD(T) level. Chemical bonding analysis shows that the cluster is a charge-transfer complex and can be formulated as [Al][B][Al]. The [B] molecular wheel in sandwich cluster has magic 6π/6σ double aromaticity, which underlies the dynamic fluxionality, despite strong electrostatic interactions between the [Al], [B], and [Al] layers.