N-Salicylidene aniline derivatives based on the N′-thiophosphorylated thiourea scaffold

Abstract

A new family of N′-thiophosphorylated thiourea-containing N-salicylidene aniline derivatives (anils) of the common formula 6-{(2-OH-aryl)–CHN}-Py-2-NHC(S)NHP(S)(OiPr)₂ [aryl = C₆H₄ (2), 5-Cl–C₆H₃ (3), 5-Br–C₆H₃ (4), 3,5-Cl₂–C₆H₂ (5), or 3,5-Br₂–C₆H₂ (6)] has been synthesized by the condensation of N-thiophosphorylated thiourea 6-NH₂-Py-2-NHC(S)NHP(S)(OiPr)₂ (1) with the corresponding salicylaldehyde. Compound 2 was obtained by dissolving 1 in pure salicylaldehyde, while anils 3–6 were synthesized in EtOH. Synthesis in EtOH with aniline led to X-ray suitable crystals of 1, in which the crystal structure was challenging so far. All compounds were characterized by elemental analysis, NMR, diffuse reflectance and fluorescence spectroscopy. The crystal structures of 2–6, elucidated by X-ray diffraction, are stabilized by two intra- and two intermolecular hydrogen bonds and a broad network of intermolecular π⋯π stacking interactions. Compounds 2 and 6 trapped salicylaldehyde and ethanol molecules, respectively, in their crystal structures through the formation of intermolecular hydrogen bonds. Molecules 3–6 show the presence of a mixture of enol and cis- and trans-keto forms in the solid state at room temperature. Only two former forms are observed for 2. Compounds 2–6 are exclusively thermochromic, while no photochromism was observed regardless of the irradiation wavelength and time.