CBe₂H₅⁻: Unprecedented 2σ/2π Double Aromaticity and Dynamic Structural Fluxionality in a Planar Tetracoordinate Carbon Cluster

Abstract

A 14‐electron ternary anionic CBe2H5– cluster containing a planar tetracoordinate carbon (ptC) atom is designed herein. Remarkably, it can be stabilized by only two beryllium atoms with both π‐acceptor/σ‐donor properties and two hydrogen atoms, which means that the conversion from planar methane (transition state) to ptC species (global minimum) requires the substitution of only two hydrogen atoms. Moreover, two ligand H atoms exhibit alternate rotation, giving rise to interesting dynamic fluxionality in this cluster. The electronic structure analysis reveals the flexible bonding positions of ligand H atoms due to C‐H localized bonds, highlighting the rotational fluxionality in the cluster, and two CBe2 3c‐2e delocalized bonds endow its rare 2σ/2π double aromaticity. Unprecedentedly, the fluxional process exhibits a conversion in the type of bonding (σ bond ⇔ π bond), which is an uncommon fluxional mechanism. The cluster can be seen as an attempt to apply planar hypercoordinate carbon species to molecular motors.

InnTl4–nH+ (n = 0∼4): Tetracoordinate Hydrogen in a Planar Fashion?

“…However, the emergence of planar tetracoordinate carbon (ptC) challenges this fundamental paradigm, offering novel insights into unconventional bonding configurations. , Building upon the pioneering concepts of Hoffmann, Alder, and Wilcox in 1970, a systematic computational inquiry led by Schleyer, Pople, and collaborators culminated in the discovery of 1,1-dilithiocyclopropane, the seminal molecule favoring ptC geometry relative to the tetrahedrally coordinated alternative. To date, theoretical designs of ptC structures led to some emerging as thermodynamically favored global minima, − and a few were realized in the gas phase and laboratory. − These developments underscore the critical importance of meticulous adherence to geometric and electronic criteria. ,, Remarkably, the concept of a planar hypercoordinate atom extends beyond carbon, enveloping all main-group elements within the second period, except Ne, − and progressively extending into higher periods. − This robust expansion markedly enriches the planar hypercoordinate family, representing a pivotal leap forward in advancing our comprehension of atomic bonding. , …”