Coaxial Triple‐Layered versus Helical Be₆B₁₁⁻ Clusters: Dual Structural Fluxionality and Multifold Aromaticity

Two low‐lying structures are unveiled for the Be 6 B 11 − nanocluster system that are virtually isoenergetic. The first, triple‐layered cluster has a peripheral B 11 ring as central layer, being sandwiched by two Be 3 rings in a coaxial fashion, albeit with no discernible interlayer Be−Be bonding. The B 11 ring revolves like a flexible chain even at room temperature, gliding freely around the Be 6 prism. At elevated temperatures (1000 K), the Be 6 core itself also rotates; that is, two Be 3 rings undergo relative rotation or twisting with respect to each other. Bonding analyses suggest four‐fold (π and σ) aromaticity, offering a dilute and fluxional electron cloud that lubricates the dynamics. The second, helix‐type cluster contains a B 11 helical skeleton encompassing a distorted Be 6 prism. It is chiral and is the first nanosystem with a boron helix. Molecular dynamics also shows that at high temperature the helix cluster readily converts into the triple‐layered one.