Experimental and Theoretical Evidence of an Axially Chiral Borospherene

Chirality plays an important role in chemistry, biology, and materials science. The recent discovery of the B-40(-/0) borospherenes marks the onset of a class of boron-based nanostructures. Here we report the observation of axially chiral borospherene in the B-39 nanocluster on the bases of photoelectron spectroscopy, global minimum searches, and electronic structure calculations. Extensive structural searches in combination with density functional and CCSD(T) calculations show that B-39(-) has a C-3 cage global minimum with a close-lying C-2 cage isomer. Both the C-3 and C-2 B-39(-) cages are chiral with degenerate enantiomers. The C-3 global minimum consists of three hexagons and three heptagons around the vertical C-3 axis. The C-2 isomer is built on two hexagons on the top and at the bottom of the cage with four heptagons around the waist. Both the C-3 and C-2 axially chiral isomers of B-39(-) are present in the experiment and contribute to the observed photoelectron spectrum. The chiral borospherenes also exhibit three-dimensional aromaticity, featuring s and p double delocalization for all valence electrons. Molecular dynamics simulations reveal that these chiral B-39(-) cages are structurally fluxional above room temperature, compared to the highly robust D-2(d) B-40 borospherene. The current findings add chiral members to the borospherene family and indicate the structural diversity of boron-based nanomaterials.