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 has a significant contribution from dative pπ–pπ bonding. Values for ΔH2980\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{H}}_{298}^{0}$$\end{document} 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.