Dynamic fluxionality of ternary Mg₂BeB₈ cluster: a nanocompass

A series of fluxional planar boron and boron-based binary clusters have evoked considerable interest of chemists. Here we propose the first ternary nanocompass cluster Mg₂BeB₈ based on quantum chemical calculations. It possesses a half-sandwich structure with a Mg₂ dimer as the needle and a BeB₈ molecular wheel as baseplate, which is the global minimum on the potential energy surface. Mg₂BeB₈ can be viewed as a nanocompass, whose Mg₂ needle can rotate freely around the BeB₈ baseplate at 300 K. The calculated rotation barrier is only 0.1 kcal mol^-1 at the single-point CCSD(T)/6-311+G(d)//PBE0/6-311+G(d) level. Chemical bonding analyses indicate that Mg₂BeB₈ is a charge-transfer complex [Mg₂]²⁺[BeB₈]^2- in nature. There is localized covalent Mg-Mg bond for [Mg₂]²⁺ needle, while there are three delocalized π and three delocalized σ bonds for [BeB₈]^2- baseplate. The ionic bonding between the [Mg₂]²⁺ needle and the 6π/6σ double aromatic [BeB₈]^2- baseplate makes the Mg₂BeB₈ cluster fluxional. The current results suggest that altering the baseplate is an effective way to enrich the nanocompass' family.