A Comparison of the Structure and Bonding in the Aliphatic Boronic R-B(OH)2 and Borinic R-BH(OH) acids (R=H; NH2, OH, and F): A Computational Investigation.

Boronic acids, R-B(OH)2, play an important role in synthetic, biological, medicinal, and materials chemistry. This investigation compares the structure and bonding surrounding the boron atoms in the simple aliphatic boronic acids, R-B(OH)2 (R = H; NH2, OH, and F) and the analogous borinic acids, R-BH(OH). Geometry optimizations were performed using second-order Møller-Plesset perturbation theory (MP2) with the Dunning-Woon aug-cc-pVTZ, aug-cc-pVQZ and aug-cc-pV5Z basis sets; single-point CCSD(FC)/aug-cc-pVTZ//MP2(FC)/aug-cc-pVTZ level calculations were used to generate a QCI density for Natural Bond Orbital analyses of the bonding. The optimized boron-oxygen bond lengths for the X-B-Ot-H trans-branch of the endo-exo form of the boronic acids and for the X-B-O-H cis-branch of the boronic and borinic acids (X = N, O, and F respectively) decrease as the electronegativity of X increases. The boron-oxygen bond lengths are generally longer in the endo-exo or anti forms of the boronic acids than in the corresponding borinic acids. NBO analyses suggest the boron-oxygen bond in H2BOH is a double bond; the boron-oxygen bonding in the remaining boronic and borinic acids in this study have a significant contribution from dative pπ-pπ bonding. Values for [Formula: see text] for the highly balanced reaction, R-B(OH)2 + R-BH2 → 2 R-BH(OH), suggest that the bonding surrounding the boron atom is stronger in the borinic acid than in the corresponding boronic acid.