Spectral, structural and quantum chemical computational and dissociation constant studies of a novel azo-enamine tautomer

[Display omitted] •A novel azo-enamine dye was synthesized and characterized by analytical and spectroscopic studies.•X-ray investigation of the dye showed that azo-enamine tautomer is favoured in the solid state.•Self-isomerisation via intramolecular proton transfer was investigated by UV–Vis. spectra and theoretical calculations.•Acid dissociation constants for the tautomer were determined potentiometrically. We report here the synthesis of (6Z)-4-[(E)-(4-ethylphenyl)diazenyl]-6-{[(2-hydroxy-5-methylphenyl)amino]methylidene}cyclohexa-2,4-dien-1-one by the condensation reaction between 2-amino-4-methylphenol and 5-[(E)-(4-ethylphenyl)diazenyl]-2-hydroxybenzaldehyde in equimolar ratio in MeOH and characterized by elemental analyses, infrared, electronic, mass,1H and 13C NMR spectroscopy. Molecular structure of the azo-enamine dye was also determined by single crystal X-ray diffraction technique. X-ray investigation of the dye showed that azo-enamine tautomer is favoured in the solid state. There is an intramolecular hydrogen bonding (N3–H···O1) in the molecule forming a S(6) graph set motif. Additionally, there is an intermolecular O2–H···O1 hydrogen bonding in the structure. The same intermolecular hydrogen bonding contacts are extended between the other symmetry-related molecules in their respective planes to form a 1D hydrogen bond chain. Self-isomerisation via intramolecular proton transfer was investigated by UV–Vis. spectra and theoretical calculations. Effects of polarity and temperature on UV–Vis. spectra were examined in detail. Moreover, acid dissociation properties of the polydentate compound was investigated at 25±0.1°C.