Abstract:  The unique structure, properties and potential application prospects of nanocluster molecular rotors have aroused extensive attention from researchers. The electron-deficient nature of boron makes boron-based clusters a fertile ground for designing nanocluster molecular rotors. In 2010, the discovery of the dynamic fluxionality of B₁₉⁻ cluster initiated the research on boron-based nonocluster molecular rotors. Metal doping is an effective strategy for expanding the family members of boron-based cluster molecular rotors. Among the currently reported boron alloy molecular rotors, the boron cluster units all possess aromaticity. Can boron alloy nanocluster molecular rotors with conflicting aromaticity be designed? The answer is yes. Herein, the first nano-rotor of Ca₃B₈ cluster with conflicting aromaticity has been theoretically predicted, based on computational global-minimum searches and quantum chemical calculations. It features a unique three-layer coaxial inverted sandwich structure: a slightly distorted B©B₇ molecular wheel serves as the middle layer, with a horizontal Ca₂ dimer above and a Ca atom below. Born-Oppenheimer molecular dynamics simulations reveal that the boron-based Ca₃B₈ cluster possesses novel dynamic fluxionality: the Ca₂ dimer can rotate freely on the umbrella-like CaB₈ base plate around the central axis at 300 and 600 K. The rotation barrier is only 0.25 kJ/mol at the single-point CCSD(T)/6-311+G(d)//PBE0/6-311+G(d) level. Ca₃B₈ can be approximatively formulated as [Ca₂]²⁺[B©B₇]^4–[Ca]²⁺, due to the weak B–Ca covalent bonding and obviously charge transfer from the Ca atoms to the boron motif. Chemical bonding analyses reveal that Ca₃B₈ has 8π and 6σ delocalized electrons on distorted B₈ wheel, leading to a conflicting-aromatic system. Ca₃B₈ represents the first boron alloy nano-rotor with conflicting-aromaticity, further expanding the research field of boron-based fluxional systems.

Key words: Molecular rotor, Boron alloy cluster, Global minimum, Fluxionality, Conflicting-aromaticity