Supramolecular networks in molecular complexes of pyridine boronic acids and polycarboxylic acids: synthesis, structural characterization and fluorescence properties

Abstract

3- and 4-pyridineboronic acids have been combined with trimesic and pyromellitic acids to give three molecular complexes of compositions [(3PBAH⁺)(H₂TMA⁻)] (1), [(3PBAH⁺)₂(H₂PMA²⁻)] (2), and [(4PBAH⁺)₂(H₂PMA²⁻)]·2H₂O (3) (3PBA = 3-pyridineboronic acid, 4PBA = 4-pyridineboronic acid, H₃TMA = trimesic acid, H₄PMA = pyromellitic acid). The products have been characterized by elemental analysis, infrared spectroscopy, single-crystal X-ray diffraction analysis and luminescence spectroscopy. The structural analysis revealed that the hydrogen bonding interactions of the resulting 2D and 3D assemblies involve at least one of the following charge-assisted synthons: –B(OH)₂⋯⁻OOCR, RCOOH⋯⁻OOCR or PyN⁺–H⋯⁻OOCR. The dimensions are enhanced by additional hydrogen bonds as well as π–π stacking interactions, which include B⋯π interactions in two crystal structures. Intermolecular close contacts have also been examined by Hirshfeld surface analysis, which by means of the associated 2D fingerprint plots enabled similarities and differences in the crystal structures to be revealed. Solid-state photoluminescence spectroscopy of compounds 1–3 revealed that variations of the π–π stacking in the molecular aggregates induced changes in the emission behavior.