Molecular rotor in copper borozene Cu4B8: A freely rotating σ-aromatic tetrahedral Cu4 core atop a boron wheel

We identify two-layered Cu4B8 clusters featuring tetrahedral Cu4 units anchored on B8 boron wheels as global minima. A substantial charge transfer from Cu4 to the B8 wheel is observed, resulting in a charge distribution close to the idealized [Cu42+] [B82−] electron-counting scheme. AdNDP analysis reveals two delocalized σ-electrons within the quasi-tetrahedral Cu4 unit, identifying it as one of the simplest σ-aromatic motifs. The B82− wheels exhibit dual (6π + 6σ) aromaticity, serving as boron analogs of benzene (“borozene”). Born–Oppenheimer molecular dynamics simulations show pronounced fluxionality, with nearly barrierless Cu4 rotation (∼0.01 kcal/mol) enabled by an ultrasoft vibrational mode. The delocalized electronic structure of the B8 wheel contributes to the facile Cu4 rotation, while electrostatic and dative interactions stabilize the two-layer framework. These results identify electroneutral Cu4B8 as a prototypical copper–borozene cluster combining aromatic stabilization and dynamic fluxionality, offering a promising model for molecular rotor.