Fluxional Behavior and Stability of the CuB Cluster: A Copper Borozene with a Freely Rotating Cu Dimer.

We demonstrate that CuB exhibits distinct fluxional behavior, akin to that of a functional stirrer, with the Cu dimer freely rotating on the B 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), resulting in a negligible rotational barrier of 0.03 kcal/mol, as calculated at the single-point CCSD(T) level. The high stability of CuB arises from the strong interlayer electrostatic interaction between Cu and B, due to charge transfer from Cu to B, along with additional covalent interactions from the delocalized π electrons of the B wheel to the Cu dimer. Notably, the Cu dimer in CuB features a two-center one-electron Cu-Cu single bond, while the B moiety displays double aromaticity, characterized by the presence of six delocalized π electrons and six delocalized σ electrons.