Tautomerism and Supramolecular Patterns in the Crystals of Schiff Bases Derived from Condensation of 2,3-Dihydroxybenzaldehyde with Isomeric Aminobenzoic Acids.

Abstract

Schiff bases obtained by the condensation of 2,3-dihydroxybenzaldehyde with isomers of aminobenzoic acid, 2-aminobenzoic acid (1), 3-aminobenzoic acid (2), and 4-aminobenzoic acid (3), are structurally characterized using attenuated total reflectance-fourier transform infared (ATR-FTIR) spectroscopy, differential scanning calorimetry (DSC)/thermogravimetric (TG), and single crystal X-ray diffraction (SCXRD) methods. Crystallographic studies show that in the molecules of compounds 1 and 3, an intramolecular proton transfer O^-···H-N⁺ occurs between the hydroxyl group and the azomethine N-atom, resulting in both compounds adopting a zwitterionic imine form. In contrast, in compound 2, an intramolecular O-H···N hydrogen bond is present, stabilizing this compound in the enol-imine form. A Cambridge Structural Database database survey shows that for both tautomeric forms, there are relationships between the geometric parameters characterizing the hydroxyl and azomethine groups. An analysis of intermolecular interactions in the crystals of the title compounds indicates that the carboxyl and hydroxyl groups can form various supramolecular synthons, depending on the position of the carboxyl group in the aromatic ring, which have different interaction energies. This leads to distinct molecular packing patterns within crystal lattices of title compounds and influences their thermal stability.