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

• Published in: Molecules (Basel, Switzerland) Vol. 28; no. 7; p. 3276
• Main Authors: Han, Peng-Fei, Wang, Ying-Jin, Feng, Lin-Yan, Gao, Shu-Juan, Sun, Qiang, Zhai, Hua-Jin
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 06.04.2023; MDPI
• Subjects: Boron; boron-based alloy clusters; chemical bonding; Chemical bonds; dynamic fluxionality; Ligands; multifold π/σ aromaticity; three-layered sandwich cluster
• ISSN: 1420-3049; 1420-3049
• DOI: 10.3390/molecules28073276

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: [Na4]2+[B7]3−[Na]+. In this sandwich cluster, the [B7]3− core assumes a molecular wheel in shape and features in-plane hexagonal coordination. The magic 6π/6σ double aromaticity underlies the stability of the [B7]3− molecular wheel, following the (4n + 2) Hückel rule. The tetrahedral Na4 ligand in the sandwich has a [Na4]2+ charge-state, which is the simplest example of three-dimensional aromaticity, spherical aromaticity, or superatom. Its 2σ electron counting renders σ aromaticity for the ligand. Overall, the sandwich cluster has three-fold 6π/6σ/2σ 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 B7 wheel and the Na4 ligand. The Na5B7 cluster offers a new example for dynamic structural fluxionality in molecular systems.