Kinetic and Thermodynamic Investigation into the Formation of Diol-Boric Acid Complexes Using NMR and Computational
Brandy L Davidson1, Elaina Manyin1, Nathan E Morris1
1Department of Chemistry and Biochemistry, James Madison University, Harrisonburg, Virginia 22807, United States.
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Boric acid (BA) readily forms reversible covalent B-O bonds with diols, yielding complexes with broad implications for drug delivery, materials science, and polymer chemistry. While this chemistry is well-established in aqueous environments, little is known about the formation of neutral boron complexes with BA in organic solvents. Here, we describe a spectroscopic and theoretical study of the complexation of diols with BA in DMSO. NMR spectroscopy was used to evaluate the formation of complexes for three structurally distinct diols: ethylene glycol (EG), 1,3-propanediol (PD), and 1,4-butanediol (BD). Results indicate that with BA, PD forms the most closed B-O complex, while EG forms a mixture of open and closed B-O species, and BD forms only small amounts of the open diol-BA complex. These findings were supported computationally for both PD and EG, where the ΔG between the open and closed B-O complexes was -6.86 and -1.45 kcal/mol, respectively. For BD-BA, the open and closed complexes were found to be similar in energy, which was not observed experimentally. We hypothesize that this is the result of a kinetic sink. The rates of formation and the impact of water were also evaluated using NMR techniques. Kinetic analysis determined that all reactions were first order with respect to diol, with rate constants of 0.057 min-1, 0.031 min-1, and 0.124 min-1 for EG-BA, PD-BA, and BD-BA, respectively. In combination, our results indicate that the closed 1,3-diol-BA complexes are more thermodynamically stable, while the 1,2-diol-BA complex is kinetically favored and dynamic. Additionally, theoretical methods were used to probe the impact of stabilizing interactions of polyol-BA complexes with glycerol (Glyc). Namely, an intramolecular hydrogen bond within 1,2-Glyc-BA was found to significantly stabilize this complex and lead to similar relative energies for both isomers (ΔΔG of -0.57 kcal/mol). This result corroborated our previously reported experimental data, which showed an ∼50:50 mixture of the 1,2- and 1,3-Glyc-BA isomers.
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