Discovering SnB₇^-: a half-sandwich structure with double aromaticity and pathways to molecular machines

Numerous boron-based molecular fluxional models, such as the Wankel motor, tank treads B₁₁^- and B₁₀C, and the Earth-Moon system Be₆B₁₁^-, have been widely recognized for their potential to develop molecular machines. From a series of tin-doped boron clusters SnB^- (n = 5-14), the half-sandwich structure SnB₇^- is found to possess high relative energy stability, and a HOMO-LUMO gap of 4.33 eV. This structure exhibits valence electron orbitals reminiscent of σ-π double aromatic compounds. The incorporation of tin effectively fills the doubly vacant π orbitals of its parent triplet B₇^-, thereby enhancing both magnetic shielding capabilities and range. Thermal bath tests demonstrate its significant dynamic stability, as the kinetic energy provided by thermal baths below 3800 K remains insufficient to disrupt its inherent elasticity. Additionally, transition state searches and intrinsic reaction coordinate analyses confirm that the tin atom migrates from the centre to the edge of the boron ligand surface, a phenomenon that can be observed in high-temperature thermal bath simulations. This fluxional behaviour provides insights for constructing novel molecular machine models.