Crystal structure and theoretical studies of the keto-enol isomerism of N,N′-bis(salicylidene)-o-phenylenediamine (salophen)

The Schiff base N,N′-bis(salicylidene)-o-phenylenediamine (salophen) was prepared by the condensation of salicylaldehyde with o-phenylenediamine in ethanol solution. The compound was characterized by elemental analysis, infrared (IR), 1H, 13C and 1H15N HMBC nuclear magnetic resonance (NMR) spectroscopic measurements, and also by X-ray diffraction. The tautomerism of salophen was also studied by calculations using density functional theory (DFT). Two of the three tautomers were shown to coexist. A comparison of the DFT data of the three tautomers has shown that the most stable one is salophen 1, which is in accordance with experimental X-ray crystallographic data.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Synthesis and structural characterization of salophen are presented. ► The compound crystallizes in the monoclinic system with Z = 4. ► Tautomerism of salophen was studied by calculations using DFT. ► Experimental and simulated IR data support the presence of salophen 1 in the solid sample.