Dynamic fluxionality of ternary Mg2BeB8 cluster: a nanocompass.

Abstract

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 300K. The calculated rotation barrier is only 0.1kcalmol-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.