Solvent Dependent C-H····F-C Pseudo Hydrogen Bonding in a 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 the substituents and the external environment, particularly the solvent. The presence of a fluorine atom(s) in the organic compound has a pronounced impact on their conformational and tautomeric preference. 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 prefer to adopt the enol tautomeric forms. Further, we observed that under the influence of the fluorine substituent and solvent, compounds prefer to adopt a specific conformation. Interestingly, in benzene, compounds adopted a conformation which formed an intriguing short intramolecular C-F····H-Csp2, 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 JH,F couplings of 2.0–2.4 Hz. These observations were further corroborated by quantum chemical calculations.