Divide and Stack Up: Boron-Based Sandwich Cluster as a Subnanoscale Propeller

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.