Barrier-Free Zinc Dimer Rotor on a Borozene-like BSi₅³– Stator: The Smallest Molecular Compass with Planar Pentacoordinate Boron

This work introduces the novel anionic cluster BSi5Zn2- as the smallest molecular "compass", featuring a unique two-layered architecture with a planar pentacoordinate boron (ppB) center. The cluster comprises a quasi-planar BSi53- stator─a silicon-based analogue of borozene with σ/π double aromaticity (6π + 10σ delocalized electrons)─and a Zn2 rotor dimer. High-level calculations (CCSD(T)//PBE0-D3) reveal an ultralow rotational barrier of 0.02 kcal mol-1 for the Zn2 dimer, enabling barrier-free 360° rotation on the stator surface, as confirmed by Born-Oppenheimer molecular dynamics (BOMD) simulations at 298-800 K. Bonding analysis attributes this exceptional fluxionality to a hybrid ionic-covalent interaction best described as [Zn2+][BSi52-], where charge transfer (Zn2 → BSi5) ensures electrostatic stability, while the delocalized electron cloud of the aromatic stator minimizes rotational resistance. This design successfully extends to BGe5Zn2- but fails for BSn5Zn2- due to size mismatch, highlighting the synergy of electronic delocalization and geometric constraints. The study pioneers the extension of nanorotor design from pure boron clusters to group 14 element-based platforms with benzene-like σ/π delocalization, establishing the smallest molecular compass based on a minimal stator (6-atom BSi53-) and establishing a new paradigm for dynamic molecular machines.