Boron-based ternary Rb6Be2B6 cluster featuring unique sandwich geometry and a naked hexagonal boron ring

Abstract

Computational evidence is reported on a boron-based ternary Rb₆Be₂B₆ cluster as the “Big Mac” sandwich on a subnanoscale with thickness of 0.58 nm. The core hexagonal B₆ ring, occurring in the naked form due to double 6π/6σ aromaticity, is capped by two tetrahedral BeRb₃ ligands. Such a B₆ motif is scarce in boron clusters. The sandwich cluster has four-fold 2σ/6π/6σ/2σ aromaticity and its tetrahedral BeRb₃ ligand is the simplest case of three-dimensional aromaticity (or spherical aromaticity). The sandwich can be formulated as a charge-transfer complex, [Rb₃Be]³⁺[B₆]⁶⁻[BeRb₃]³⁺, whose components are held together by robust electrostatics, facilitating dual-mode dynamic fluxionality.