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

Abstract

Two low-lying structures are unveiled for the Be B nanocluster system that are virtually isoenergetic. The first, triple-layered cluster has a peripheral B ring as central layer, being sandwiched by two Be rings in a coaxial fashion, albeit with no discernible interlayer Be-Be bonding. The B ring revolves like a flexible chain even at room temperature, gliding freely around the Be prism. At elevated temperatures (1000 K), the Be core itself also rotates; that is, two Be 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 helical skeleton encompassing a distorted Be 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.

Boron‐Based Chiral Helix Be6B102− and Be6B11− Clusters: Structures, Chemical Bonding, and Formation Mechanism

“…It is interesting to comment on the nature of Be 6 component in helix clusters 1 / 2 . Physically, six Be atoms in 2 do form a triangular prism (Figure (a)), which is no difference from the Be 6 prism in sandwich clusters 3 / 4 . However, they differ fundamentally in terms of bonding.…”

“…We shall address these issues in this paper, which offers key structural insights for boron helix clusters 1 / 2 , elucidates their chemical bonding, and sheds light on the folding mechanism. The present understanding of the B helices differs distinctly from a prior bonding model and may be applicable for other inorganic helix clusters as well.…”

“…Understanding the nature of bonding in highly distorted, twisting, and binary Be 6 B 10 2− ( 1 ) and Be 6 B 11 − ( 2 ) helix clusters is a demanding task, and indeed even a meaningful presentation of their geometric structures appears to be challenging . The CMOs are fundamental toward chemical bonding in a molecular system.…”

“…Our NBO calculation gives a charge‐transfer complex of [Be 6 ] +8.90 [B 11 ] 9.89− (Table ), in line with the unfolded model (Figure ). For reference, sandwich cluster 4 was well characterized previously as a formal [Be 6 ] 8+ [B 11 ] 9− complex due to unique four‐fold π/σ aromaticity, whose NBO data lead to a formula of [Be 6 ] +6.64 [B 11 ] 7.64− . Helix cluster 2 clearly facilitates intramolecular transfers of a larger number of electrons from Be to B than sandwich cluster 4 , which further justifies the formal complexes of [Be 6 ] 10+ [B 11 ] 11− for 2 and [Be 6 ] 8+ [B 11 ] 9− for 4 .…”

“…Since 2011, boron‐based molecular Wankel motors, nanoscale tank treads, molecular compass and propellers, and sandwich‐shaped binary clusters with dual‐mode dynamic fluxionality were reported; the latter mimics the revolution/rotation dynamics of the earth‐moon system at the subnanoscale. While studying dynamical fluxionality, the Zhai group also serendipitously encountered the first boron‐based helix clusters, Be 6 B 10 2− ( 1 , C 1 , 1 A) and Be 6 B 11 − ( 2 , C 2 , 1 A), which compete closely in energy with their three‐layered sandwich counterparts: Be 6 B 10 2− ( 3 , C 2v , 1 A 1 ) and Be 6 B 11 − ( 4 , C 2v , 1 A 1 ). The former helix species, Be 6 B 10 2− ( 1 ), was designed so that its sandwich counterpart fulfills exactly the 10π/10σ electron‐counting for double aromaticity, although the dianion nature makes 1 / 3 merely model systems.…”