Structural transition in metal-centered boron clusters: from tubular molecular rotors Ta@B-21 and Ta@B-22(+) to cage-like endohedral metalloborospherene Ta@B-22(-)

摘要

Inspired by the recent discovery of the metal-centered tubular molecular rotor C-s B-2-Ta@B-18(-) 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 B-3-Ta@B-18 (1) and C-3v B-4-Ta@B-18(+) (2) and smallest axially chiral endohedral metalloborospherenes D-2 Ta@B-22(-) (3 and 3 '), unveiling a tubular-to-cage-like structural transition in metal-centered boron clusters at Ta@B-22 (-) via effective spherical coordination interactions. The highly stable Ta@B-22(-) (3) as an elegant superatom, which features two equivalent corner-sharing B-10 boron double chains interconnected by two B-2 units with four equivalent B-7 heptagons evenly distributed on the cage surface, conforms to the 18-electron configuration with a bonding pattern of sigma + pi 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 D-2 Ta@B-22 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.