Synthesis, structural characterization and Hirshfeld surface analysis of two positional isomeric Schiff bases

Two positional isomers of Schiff bases, namely (E)-2-(((2-hydroxy-4-methylphenyl)imino)methyl)-4,5-dimethoxyphenol (1) and (E)-2-(((2-hydroxy-5-methylphenyl)imino)methyl)-4,5-dimethoxyphenol (2), were synthesized and comparatively investigated. The structures of both compounds were characterized by FT-IR, 1H NMR, 13C NMR, UV–Vis spectroscopy, and single-crystal X-ray diffraction (SC-XRD). Compound 1 crystallizes in the orthorhombic chiral space group P212121, whereas compound 2 belongs to the monoclinic space group P21/c and contains two crystallographically independent molecules in the asymmetric unit. 1 adopts the keto-amine form with an intramolecular N–H···O hydrogen bond, whereas 2 occurs in the enol-imine form stabilized by intramolecular O–H···N hydrogen bonding. In both structures, the azomethine linkage adopts an E-type arrangement. The crystal packings are mainly supported by intermolecular O–H···O hydrogen bonds, together with weaker C–H···O hydrogen bonds leading to extended three-dimensional supramolecular architectures. The O···H/H···O and C···H/H···C contacts make the largest contributions, accounting for 25% and 24% of the Hirshfeld surface area of 1, and 20% and 19% of that of 2, respectively. Shifting the methyl substituent from the para position relative to the imine nitrogen in 1 to the meta position in 2 is associated with changes in the solid-state tautomeric form, crystallographic symmetry, asymmetric-unit content, and hydrogen-bonding network.