Boron-based tubular BeB12+ and quasi-planar BeB120/− clusters: structural transformation and chemical bonding

Abstract

Boron is electron deficient and has strong bonding capacity, and thus it has given rise to several boron-based clusters with unique geometric structures and bonding properties. Inspired by the contemporary boron double-ring clusters, we are designing a tubular boron cluster doped with alkaline earth metals, which are considered promising ligands. Herein, we present a double-ring tubular BeB₁₂⁺ cluster, in which the Be atom is located above one side of a B₁₂ tube. It can be formally formulated as a charge transfer [Be]²⁺[B₁₂]⁻ complex, with minimal covalent interaction between the Be atom and B₁₂ tube. The [B₁₂]⁻ tube is more stable than the quasi-planar configuration because of three-fold π/σ aromaticity and optimal electrostatic interaction. After sequential reduction, a structural transformation occurs dramatically changing from tubular shape to quasi-planar. The neutral and anionic BeB₁₂^0/− clusters featured quasi-planar/planar geometries. The neutral BeB₁₂ cluster was structurally robust and dynamically nonfluxional with an insurmountable rotational energy barrier. Chemical bonding analysis revealed that anionic BeB₁₂⁻ has 7π and 10σ delocalization. In particular, this series of BeB₁₂^+/0/− clusters is isoelectronic with Li_nB₁₂ (n = 1–3) reported by Merino. However, they exhibit significantly different geometries due to the differences in electrostatic interaction. Therefore, optimal electrostatic interactions can tune the energetic preference between tubular and quasi-planar/planar structures.