Structural and Energetic Landscape of Fluorinated 1,4-Phenylenediboronic Acids

The results of X-ray crystallographic and computational studies of a series of fluorinated 1,4-phenylenediboronic acids (i.e., fluoro-1,4-phenylenediboronic acid, 2,6-difluoro-1,4-phenylenediboronic acid, 2,3-difluoro-1,4-phenylenediboronic acid, 2,5-difluoro-1,4-phenylenediboronic acid, and tetrafluoro-1,4-phenylenediboronic acid) are reported. The effect of fluorine substitution on crystal organization in the presence of strong and directional hydrogen bonds was studied. Comparison with the two previously reported forms of the unsubstituted 1,4-phenylenediboronic acid revealed a strong relation between a supramolecular network and the number of water molecules present in the crystal lattice. As indicated by the theoretical calculations performed in the CRYSTAL and PIXEL programs, the structures with greater amount of water are better stabilized (from about −170 kJ·mol–1 for anhydrous forms to about −420 kJ·mol–1 for tetrahydrate). The energy of hydrogen bonded dimers vary from −40 kJ·mol–1 to −50 kJ·mol–1. Contacts with fluorine atoms play rather a secondary role in the crystal packing. Fluorine substituents tend to interact with the electropositive boron atom. Furthermore, intramolecular interactions significantly affect the torsion angle of the B­(OH)2 group. The constrained energy scan revealed that stronger interactions with substituents stabilize the planar conformation and hamper the rotation of the boronic group. This in turn has a further impact on the interactions within selected crystal motifs and supposedly rules the proton disorder within boronic fragments. Besides the interactions with the fluorine atoms, other weak contacts such as C­(π)···B and O···B also influence the molecular organization. The energy of the corresponding dimers varies from −15 kJ·mol–1 to −25 kJ·mol–1.