Solvent-dependent C–H⋯F–C pseudo-hydrogen bonding in fluorinated azobenzene Schiff bases: studies on structural and atropisomerism features

Abstract

The characteristic properties of any organic compounds, viz. physical, chemical, and biological, are strongly dependent on the type of conformational isomer and tautomer preferred under ambient conditions. The population of these isomeric forms depends significantly on the nature of substituents and the external environment, particularly the solvent. The presence of a fluorine atom(s) in the organic compound strongly influences its conformational and tautomeric preferences. In this study, we investigated a series of fluorinated azo Schiff bases to explore the effect of fluorine substituents on their structural properties and spectroscopic behaviour. UV-vis absorption spectroscopy revealed that all compounds preferred to adopt the enol tautomeric forms. Further, we observed that under the influence of the fluorine substituent and solvent, compounds preferred to adopt a specific conformation. Interestingly, in benzene, compounds adopted a conformation that formed an intriguing short intramolecular C–F⋯H–C_sp² interaction resembling a pseudo-hydrogen bond, with fluorine acting as an electron donor. This through-space interaction was supported by NMR spectroscopy, which exhibited characteristic J_H,F couplings of 2.0–2.4 Hz. These observations were further corroborated by quantum chemical calculations.