High-symmetry tubular Ta@B 18 3- , Ta 2 @B 18 , and Ta 2 @B 27 + as embryos of α-boronanotubes with a transition-metal wire coordinated inside.

Transition-metal doping leads to dramatic structural changes and results in novel bonding patterns in small boron clusters. Based on the experimentally derived mono-ring planar C-9v Ta (c) B-9(2-) (1) and extensive first-principles theory calculations, we present herein the possibility of high-symmetry double-ring tubular D-9d Ta@B-18(3-) (2) and C-9v Ta-2@B-18 (3) and triple-ring tubular D-9h Ta-2@B-27(+) (4), which may serve as embryos of single-walled metalloboronanotube alpha-Ta-3@B-48(3,B-0) (5) wrapped up from the recently observed most stable free-standing boron alpha-sheet on a Ag(111) substrate with a transition-metal wire (-Ta-Ta-) coordinated inside. Detailed bonding analyses indicate that, with an effective d(z2)-d(z2) overlap on the Ta-Ta dimer along the C-9 molecular axis, both Ta-2@B-18 (3) and Ta-2@B-27(+) (4) follow the universal bonding pattern of sigma + pi double delocalization with each Ta center conforming to the 18-electron rule, providing tubular aromaticity to these Ta-doped boron complexes with magnetically induced ring currents. The IR, Raman, and UV-vis spectra of 3 and 4 are computationally simulated to facilitate their future experimental characterization.