Discovering SnB 7 - : 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 B ₁₁ ^- and B ₁₀ C, and the Earth-Moon system Be ₆ B ₁₁ ^- , have been widely recognized for their potential to develop molecular machines. From a series of tin-doped boron clusters SnB _n ^- ( n = 5-14), the half-sandwich structure SnB ₇ ^- 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 B ₇ ^- , 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.