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.

Abstract

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 B8 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 Al2 rod slides or rotates freely within a circular rail on the B8 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]+[B8]2-[Al]+. The [B8]2- molecular wheel in sandwich cluster has magic 6π/6σ double aromaticity, which underlies the dynamic fluxionality, despite strong electrostatic interactions between the [Al]+, [B8]2-, and [Al]+ layers.