Molecular triskelions: structure and bonding in the perhalogenated analogues of boric acid, X₃BO₃ (X=F, Cl, Br, I)

Using density functional theory calculations, we investigate the geometries and electronic structures of the perhalogenated derivatives of boric acid, X₃BO₃ (with X = halogen). These molecules in their low-energy states are C_3h symmetric with a shape that resembles a triskelion, an ancient symbol that appears inside the flags and coat of arms of countries, regions, and cities throughout Europe. The molecular triskelions can form polar complexes with ammonia which are stabilized by a dative bond between nitrogen and boron, while a second type of adducts arises from the halogen bonding between nitrogen and the terminal halogen atom. By virtue of their different HOMO–LUMO gaps, the molecular triskelions and their complexes with ammonia might provide novel bulk materials where the band gap can be tuned simply by changing the type of halogen bonded to the BO₃ moiety.