Chemical bonding and dynamic structural fluxionality of a boron-based Al 2 B 8 binary cluster: the robustness of a doubly 6π/6σ aromatic [B 8 ] 2- 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, Al 2 B 8 , using global structural searches and quantum chemical calculations. The cluster features a perfectly planar B 8 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 C 2v symmetry for the cluster. The Al 2 B 8 cluster is shown to be dynamically fluxional even at far below room temperature (100 K), in which a vertical Al 2 rod slides or rotates freely within a circular rail on the B 8 plate, although there is no direct Al-Al interaction. The energy barrier for intramolecular rotation is only 0.01 kcal mol -1 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 8 ] 2- [Al] + . The [B 8 ] 2- molecular wheel in sandwich cluster has magic 6π/6σ double aromaticity, which underlies the dynamic fluxionality, despite strong electrostatic interactions between the [Al] + , [B 8 ] 2- , and [Al] + layers.