Intramolecular dative bonds involving boron with oxygen and nitrogen in boronic acids and esters: a computational study

Abstract

The differential affinity of boron towards the oxygen and nitrogen lone pairs of electrons in 2-aminocarbonyl-phenylboronic acid (2-AC-PBA) and its corresponding ester, ethanediol (2-aminocarbonyl) phenylboronate (ED-2-AC-PB), has been investigated computationally using both density functional theory and second-order Møller–Plesset perturbation theory. In vacuo, the optimized boron–oxygen or boron–nitrogen distances in conformers of 2-AC-PBA, that involve either an intramolecular five-membered (:O C–C C–B) or (:N–C–C C–B) ring motif, are long, ∼2.4–3.0 Å, and the calculated charge distribution and 11B chemical shifts for these structures are not indicative of the presence of a B–O or B–N dative bond. In a variety of solvents, however, the optimized B–O or B–N distances from self-consistent reaction field calculations are much shorter, ∼1.7–1.8 Å, and the charge distribution and 11B chemical shifts are consistent with the formation of dative bonds. The results for the ester ED-2-AC-PB are similar, although a conformer with a B–N dative bond is also found in vacuo. The calculations show that conformers with the (:O C–C C–B) ring motif are consistently lower in energy than the corresponding conformers with the (:N–C–C C–B) ring motif, both in vacuo and in solution.