Nature and Strength of Lewis Acid/Base Interaction in Boron and Nitrogen Trihalides

• Published in: Chemistry, an Asian journal an Asian journal, 2020-12, Vol.15 (23), p.4043-4054
• Main Authors: Rodrigues Silva, Daniela, Azevedo Santos, Lucas, P. Freitas, Matheus, Fonseca Guerra, Célia, Hamlin, Trevor A.
• Format: Article
• Language: English
• Subjects: Activation strain analysis; Ammonia; Bond energy; Bonding strength; Boron; Chemical bonds; Chemistry; Density functional calculations; Energy decomposition analysis; Lewis acid; Lewis acid-base pairs; Molecular orbitals; Nitrogen
• ISSN: 1861-4728; 1861-471X
• DOI: 10.1002/asia.202001127

We have quantum chemically investigated the bonding between archetypical Lewis acids and bases. Our state‐of‐the‐art computations on the X3B−NY3 Lewis pairs have revealed the origin behind the systematic increase in B−N bond strength as X and Y are varied from F to Cl, Br, I, H. For H3B−NY3, the bonding trend is driven by the commonly accepted mechanism of donor−acceptor [HOMO(base)−LUMO(acid)] interaction. Interestingly, for X3B−NH3, the bonding mechanism is determined by the energy required to deform the BX3 to the pyramidal geometry it adopts in the adduct. Thus, Lewis acids that can more easily pyramidalize form stronger bonds with Lewis bases. The decrease in the strain energy of pyramidalization on going from BF3 to BI3 is directly caused by the weakening of the B−X bond strength, which stems primarily from the bonding in the plane of the molecule (σ‐like) and not in the π system, at variance with the currently accepted mechanism. That's strainful! The stability of Lewis pairs involving boron trihalides and ammonia is driven by the energy required to deform the Lewis acids upon complexation, while the stability of Lewis pairs between borane and nitrogen trihalides is determined by the strength of orbital interactions.