Dual structural fluxionality in the copper borozene complex Cu3B8-: A two-layered molecular rotor

Doubly aromatic B82-, a borozene analog of benzene (C6H6) due to their similar π bonding, can be considered an ideal base for multi-layered molecular rotors. Here, we theoretically constructed the copper borozene complex Cu3B8- to investigate its stability and structural fluxionality. The lowest energy isomers consist of two-layered configurations: a B8 molecular wheel and a triangular Cu3 motif that either stands upright or lies flat above the B8 wheel. Both configurations exhibit structural fluxionality, as indicated by the free rotation of Cu3 with respect to the B8 molecular wheel, confirmed by Born-Oppenheimer molecular dynamics simulations even at low temperatures. This fluxional behavior is associated with an ultra-soft vibrational mode of Cu3 (less than 10.0 cm-1) and a negligible rotational barrier of 0.01 kcal/mol. Notably, high simulated temperatures cause irregular interconversion between the standing and lying orientations of Cu3 without regularity. Chemical bonding analysis confirmed that charge transfer from Cu3 to the B8 wheel renders Cu3B8- a typical copper borozene complex, [Cu3+][B82-], where B82- has six delocalized π and σ electrons. This electron delocalization contributes to a dilute and continuous electron cloud that underpins the dynamic behavior of the Cu3 trimer.