Zn5B3-: A Coaxial Double-Ring Molecular Rotor with Multifold Aromaticity.

The first coaxial double-ring C_s Zn₅B₃^- anion with dynamic fluxionality has been designed, consisting of a B₃ triangle and a pentagonal Zn₅ ring below. It is a true global minimum (GM) structure on the potential energy surfaces based on the extensive density functional theory (DFT) isomeric searches and high-level CCSD(T) calculations. Born-Oppenheimer molecular dynamics (BOMD) simulations reveal that the Zn₅B₃^- cluster is dynamically fluxional: the B₃ triangle can rotate freely on the Zn₅ ring around the central axis at 500 K. Bonding analyses indicate that the double-ring Zn₅B₃^- cluster possesses 2π/6σ/6σ/6σ 4-fold aromaticity, conforming to the (4n + 2) Hückel rule, facilitating its structural fluxionality. The unique double-ring structure, interesting dynamic fluxionality, and novel 4-fold aromaticity make the Zn₅B₃^- cluster a highly promising candidate for boron-based molecular rotors.