Fluxional Behavior and Stability of the Cu 2 B 8 - Cluster: A Copper Borozene with a Freely Rotating Cu 2 Dimer.

We demonstrate that Cu 2 B 8 - exhibits distinct fluxional behavior, akin to that of a functional stirrer, with the Cu 2 dimer freely rotating on the B 8 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 Cu 2 B 8 - arises from the strong interlayer electrostatic interaction between Cu 2 and B 8 , due to charge transfer from Cu 2 to B 8 , along with additional covalent interactions from the delocalized π electrons of the B 8 wheel to the Cu 2 dimer. Notably, the Cu 2 dimer in Cu 2 B 8 - features a two-center one-electron Cu-Cu single bond, while the B 8 2- moiety displays double aromaticity, characterized by the presence of six delocalized π electrons and six delocalized σ electrons.