Fluxional\nBehavior and Stability of the Cu<sub>2</sub>B<sub>8</sub><sup>–</sup> Cluster: A Copper Borozene with\na Freely Rotating Cu<sub>2</sub> Dimer

We demonstrate that Cu<sub>2</sub>B<sub>8</sub><sup>–</sup> exhibits distinct fluxional behavior, akin to that of a functional\nstirrer, with the Cu<sub>2</sub> dimer freely rotating on the B<sub>8</sub> molecular wheel. This behavior is confirmed by Born–Oppenheimer\nmolecular dynamics simulations. The initiation of this dynamic motion\nis facilitated by an ultrasoft vibrational mode (less than 10 cm<sup>–1</sup>), resulting in a negligible rotational barrier of\n0.03 kcal/mol, as calculated at the single-point CCSD(T) level. The\nhigh stability of Cu<sub>2</sub>B<sub>8</sub><sup>–</sup> arises\nfrom the strong interlayer electrostatic interaction between Cu<sub>2</sub> and B<sub>8</sub>, due to charge transfer from Cu<sub>2</sub> to B<sub>8</sub>, along with additional covalent interactions from\nthe delocalized π electrons of the B<sub>8</sub> wheel to the\nCu<sub>2</sub> dimer. Notably, the Cu<sub>2</sub> dimer in Cu<sub>2</sub>B<sub>8</sub><sup>–</sup> features a two-center one-electron\nCu–Cu single bond, while the B<sub>8</sub><sup>2–</sup> moiety displays double aromaticity, characterized by the presence\nof six delocalized π electrons and six delocalized σ electrons.