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

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 (:OC-CC-B) or (:N-C-CC-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 (:OC-CC-B) ring motif are consistently lower in energy than the corresponding conformers with the (:N-C-CC-B) ring motif, both in vacuo and in solution.