Anchoring a bow-shaped boron single chain in binary Be6B7− cluster: hybrid octagonal ring, multifold π/σ aromaticity, and dual electronic transmutation

Abstract

Elemental boron clusters do not form linear chain or monocyclic ring structures, which is in contrast to carbon. Based on computer global searches and quantum chemical calculations, we report on the viability of a curved boron single chain in binary Be₆B₇⁻ cluster. The boron motif assumes a bow shape, being anchored on a Be₆ prism. Such a motif, which appears to be highly strained in its free-standing form, is exotic in boron-based clusters and nanostructures. Chemically, the cluster is analogous to a “clam-and-pearl-chain” system at the nanoscale (about 1 nm in size), in which a Be₆ clam moderately opens its mouth, except that a B₇ pearl chain is too large to be encapsulated inside. The picture differs from a three-layered sandwich. This cluster features a hybrid Be₂B₇ monocyclic ring, which is octagonal in nature and supports double 10π/6σ aromaticity. The number of π bonds substantially surpasses that in bare boron clusters of similar sizes. Two Be₃ rings in the prism are also σ aromatic, albeit with effective 1σ/1σ electron-counting only. The unique multifold 1σ/10π/6σ/1σ aromaticity governs the geometry of the Be₆B₇⁻ cluster, which can also be rationalized using the concept of dual electronic transmutation.