Chemical bonding and dynamic structural fluxionality of a boron-based MgAlB8+ nanocompass cluster

Abstract

Incorporating metals into boron clusters via doping or alloying has proved effective in tailoring the structural, electronic, and bonding properties of boron-based alloy clusters. We present herein a quantum chemical study on a ternary MgAlB8+ cluster, which assumes a double-layered geometry between a quasi-planar B8 molecular wheel and an Al–Mg dimer. Chemical bonding analysis reveals that charge transfer from metals to B8 molecular wheel gives rise to a formal [Mg–Al]3+[B8]2– complex, although Al site participates moderately in global π bonding. The ternary system features double 6π/6σ aromaticity, conforming to the (4n + 2) Hückel rule. Molecular dynamics simulation demonstrates intriguing dynamic fluxionality of the cluster, which closely mimics a compass at the subnanoscale. The Al–Mg needle hovers above molecular wheel B8 baseplate. The intramolecular rotation barrier turns out to be negligible, amounting to 0.01 kcal mol−1 at the single-point CCSD(T) level, which is virtually barrier free. The dynamic structural fluxionality is governed by double 6π/6σ aromaticity of the system.