Boron-based binary Be6B102− cluster: three-layered aromatic sandwich, electronic transmutation, and dynamic structural fluxionality

Abstract

Boron-based nanoclusters have unique structures, bonding, and dynamic properties, which originate from boron's electron-deficiency. We demonstrate here that pouring in extra electrons can alter such systems fundamentally. A coaxial triple-layered Be₆B₁₀²⁻ sandwich cluster is designed via global structural searches and quantum chemical calculations. It is well defined as the global minimum, which consists of a slightly elongated B₁₀ monocyclic ring and two Be₃ rings, the latter forming a Be₆ trigonal-prism albeit without interlayer Be–Be bonding. The B₁₀ ring shows structural and chemical integrity with respect to the Be₃ rings, and yet it differs markedly from the free B₁₀ cluster and closely resembles the C₁₀ cluster. The present data testify to the idea of electronic transmutation, in which a B⁻ is equivalent to C and a B₁₀ cluster, upon charge-transfer, is converted to and stabilized as a monocyclic ring analogous to C₁₀. Chemical bonding analyses reveal that the B₁₀ ring in the Be₆B₁₀²⁻ cluster has 10π and 10σ delocalization and each Be₃ ring is held together by 2σ electrons, collectively rendering four-fold π/σ aromaticity. The bonding pattern is in line with the formula of [Be₃]⁴⁺[B₁₀]¹⁰⁻[Be₃]⁴⁺, suggesting a highly charged electron-transfer complex. Furthermore, the Be₆B₁₀²⁻ cluster is dynamically fluxional with dual modes of revolution (orbiting) and rotation (twisting), being structurally robust at least up to a temperature of 1500 K.