Chemical bonding and dynamic structural fluxionality of a boron-based Al2B8 binary cluster: the robustness of a doubly 6π/6σ aromatic [B8]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, Al2B8, 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 Al2B8 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 6 pi/6 sigma double aromaticity, which underlies the dynamic fluxionality, despite strong electrostatic interactions between the [Al](+), [B-8](2-), and [Al](+) layers.

Automated summary

Despite the isovalency between Al and B elements, Al-doping in boron clusters can lead to a unique sandwich di-Al-doped boron cluster, Al2B8. This cluster exhibits dynamic fluxionality with a vertically sliding or rotating Al-2 rod on a B-8 plate, despite no direct Al-Al interaction. The cluster shows charge-transfer complex behavior and has a magic 6 pi/6 sigma double aromaticity.