Chemical Bonding and Dynamic Structural Fluxionality of a Boron-Based Na5B7 Sandwich Cluster

Doping alkali metals into boron clusters can effectively compensate for the intrinsic electron deficiency of boron and lead to interesting boron-based binary clusters, owing to the small electronegativity of the former elements. We report on the computational design of a three-layered sandwich cluster, Na5B7, on the basis of global-minimum (GM) searches and electronic structure calculations. It is shown that the Na5B7 cluster can be described as a charge-transfer complex: [Na-4](2+)[B-7](3-)[Na](+). In this sandwich cluster, the [B-7](3-) core assumes a molecular wheel in shape and features in-plane hexagonal coordination. The magic 6p/6s double aromaticity underlies the stability of the [B-7](3-) molecular wheel, following the (4n + 2) Huckel rule. The tetrahedral Na-4 ligand in the sandwich has a [Na-4](2+) charge-state, which is the simplest example of three-dimensional aromaticity, spherical aromaticity, or superatom. Its 2s electron counting renders s aromaticity for the ligand. Overall, the sandwich cluster has three-fold 6p/6s/2s aromaticity. Molecular dynamics simulation shows that the sandwich cluster is dynamically fluxional even at room temperature, with a negligible energy barrier for intramolecular twisting between the B-7 wheel and the Na-4 ligand. The Na5B7 cluster offers a new example for dynamic structural fluxionality in molecular systems.

Automated summary

Doping alkali metals into boron clusters can compensate for boron's electron deficiency and lead to interesting boron-based binary clusters. We report on the computational design of a three-layered sandwich cluster, Na5B7, which exhibits charge-transfer complex and three-fold aromaticity. Molecular dynamics simulation shows that the sandwich cluster is dynamically fluxional even at room temperature. The Na5B7 cluster offers a new example for dynamic structural fluxionality in molecular systems.