Studies on the effect of fluorine on the interaction of different metal ions with a fluorinated azobenzene-Schiff base: intramolecular C–F activation under polar solvent

Abstract

Herein, we report a comparative study on the interactions of different metal ions with fluorinated and non-fluorinated azobenzene-Schiff bases. Three new varieties of fluorinated Schiff bases were synthesized in high yields and changes in their UV-vis absorbance and fluorescence emission when they interacted with various metal ions in EtOH–H₂O were studied. It was observed that alkali metals in the presence of F-substituent significantly altered UV-vis absorbance, causing red shifts, whereas transition metal ions caused blue shifts. In all cases, metal interactions increased the fluorescence emission. Interestingly, alkali metal ions intensified the color of parent compounds, whereas transition metal ions turned the solution colorless. We observed that the presence and position of fluorinated substituent had a significant impact on the interaction of metal ions. Interestingly, in the presence of a polar solvent such as DMSO, the interactions increased significantly, the C–F bond appeared to be activated in the presence of alkali metal ions, and the F-substituent was removed very quickly at room temperature as evidenced by the appearance of a very interesting azobenzene–dibenzoazepine compound in ¹H-NMR spectra. To gain deep insights into the nature of interactions, time-dependent ¹H- and ¹⁹F-NMR spectroscopy were used, which confirmed the participation of M–F interactions and the formation of new (N and) O⋯M⁺⋯F complexes due to significant changes in the chemical shift of ¹⁹F resonance (shielding effect) with respect to free ligands. It was also confirmed that alkali metals interacted at the site containing F-substituents, whereas transition metals preferred a non-fluorinated site. The appearance of a new shielded peak of the F-substituent in ¹⁹F NMR was found to be in the decreasing order of the ionic size of metal ions. Interestingly, in the absence of weak bases, electron-withdrawing groups, and other activators and under the action of polar solvents and alkali metal salts, the C–F bond exhibited σ bond donor ability with alkali metal ions, which could be activated. Thus, this method allows for the modulation of a Schiff base to create interesting functional and useful materials.