The bonds and aromaticities of [NgBxOy]+ (Ng = Ar, Kr and Xe; x = 3–5, y = 5–7)

Abstract

The bonds and aromaticities of [NgBxOy]+ (Ng = Ar, Kr and Xe; x = 3–5, y = 5–7) were studied by quantum chemistry methods. [NgBxOy]+ was confirmed to be aromatic by various methods, and the trends were proposed as follows: For compounds with the same boroxol ring, the order of aromaticity is [ArBxOy]+ > [KrBxOy]+ > [XeBxOy]+; For compounds with different boroxol ring, the order of aromaticity is [NgB3O5]+ > [NgB4O6]+ > [NgB5O7]+. The conjugation effects were found among the 3c-2e π bonds on the boroxol ring and the 3c-2e π bond on the exocyclic BOB branches as well as the lone electron-pair orbitals of Ng atoms. Such conjugation effects are favorable to the expansion of the delocalization region but unfavorable to aromaticity. AIM results show that the NgB (Ng = Kr and Xe) bonds are covalent, while the ArB bond except that of [ArB3O5]+ is covalent partially. The bond dissociation energy (BDE) order of NgB bond in compounds with the same boroxol ring is [XeBxOy]+ > [ArBxOy]+ > [KrBxOy]+, and the BDE order of Ng-B bond in compounds with different boroxol ring is [NgB3O5]+ > [NgB4O6]+ ≈ [NgB5O7]+. Therefore, [XeBxOy]+ are credible candidates for the experimental synthesis researches.