Typical salts are composed of positive and negative ions that appear alternatively, whereas decorated layered materials normally have ions anchored on the polygonal sites. In this way, the ions are spatially fixed and the system is stabilized on electrostatic grounds. Here we report on a unique boron-lithium cluster, B7 Li4 - , which contains a disk-like B7 core, being sandwiched by a Li3 ring and an isolated Li atom. All Li centers are stacked exactly on the B atoms from top or bottom, rather than being anchored on triangular B3 sites. The cluster shows dynamic fluxionality, whose Li3 ring rotates freely on the B7 disk even at below room temperature (200 K), akin to a subnanoscale propeller. The rotation barrier is only 0.37 kcal mol-1 at the single-point CCSD(T) level. The sandwich shape facilitates intramolecular charge-transfers, leading to a [Li3 ]+ [B7 ]3- [Li]+ salt complex. The [Li3 ]+ layer has 2σ aromaticity, while [B7 ]3- core is robust with both π and σ sextets. Three-fold π/σ aromaticity collectively stabilizes the system, as well as underlies its dynamic fluxionality. The interlayer bonding turns out to be strong, dominated by ionic interactions (of the order of 3-4 eV per Li3 /Li unit). The work demonstrates a propeller at the subnanoscale, which is dynamically fluxional despite strong covalent and ionic bonding.
Keywords: boron-based clusters; molecular dynamics; multifold π/σ aromaticity; structural fluxionality; subnanoscale propeller.
© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.