Fluxional Behavior and Stability of the Cu₂B₈^– Cluster: A Copper Borozene with a Freely Rotating Cu₂ Dimer

We demonstrate that Cu₂B₈^- exhibits distinct fluxional behavior, akin to that of a functional stirrer, with the Cu₂ dimer freely rotating on the B₈ molecular wheel. This behavior is confirmed by Born-Oppenheimer molecular dynamics simulations. The initiation of this dynamic motion is facilitated by an ultrasoft vibrational mode (less than 10 cm^-1), resulting in a negligible rotational barrier of 0.03 kcal/mol, as calculated at the single-point CCSD(T) level. The high stability of Cu₂B₈^- arises from the strong interlayer electrostatic interaction between Cu₂ and B₈, due to charge transfer from Cu₂ to B₈, along with additional covalent interactions from the delocalized π electrons of the B₈ wheel to the Cu₂ dimer. Notably, the Cu₂ dimer in Cu₂B₈^- features a two-center one-electron Cu-Cu single bond, while the B₈^2- moiety displays double aromaticity, characterized by the presence of six delocalized π electrons and six delocalized σ electrons.