Structural transition in metal-centered boron clusters: from tubular molecular rotors Ta@B₂₁ and Ta@B₂₂⁺ to cage-like endohedral metalloborospherene Ta@B₂₂

Inspired by the recent discovery of the metal-centered tubular molecular rotor C_s B₂-Ta@B₁₈^- with the record coordination number of CN = 20 and based on extensive first-principles theory calculations, we present herein the possibility of the largest tubular molecular rotors C_s B₃-Ta@B₁₈ (1) and C_3v B₄-Ta@B₁₈⁺ (2) and smallest axially chiral endohedral metalloborospherenes D₂ Ta@B₂₂^- (3 and 3'), unveiling a tubular-to-cage-like structural transition in metal-centered boron clusters at Ta@B₂₂^-via effective spherical coordination interactions. The highly stable Ta@B₂₂^- (3) as an elegant superatom, which features two equivalent corner-sharing B₁₀ boron double chains interconnected by two B₂ units with four equivalent B₇ heptagons evenly distributed on the cage surface, conforms to the 18-electron configuration with a bonding pattern of σ + π double delocalization and follows the 2(n + 1)² electron counting rule for spherical aromaticity (n = 2). Its calculated adiabatic detachment energy of ADE = 3.88 eV represents the electron affinity of the cage-like neutral D₂ Ta@B₂₂ which can be viewed as a superhalogen. The infrared, Raman, VCD, and UV-vis spectra of the concerned species are computationally simulated to facilitate their spectral characterizations.