Fluoride ion receptors based on dipyrrolyl derivatives bearing electron-withdrawing groups: synthesis, optical and electrochemical sensing, and computational studies

Abstract

Two dipyrrolyl derivatives, 2,3-di(1H-2-pyrrolyl)-7,12-dihydronaphtho[2,3-f]quinoxaline-7,12-dione (1) and 5,6-di(1H-2-pyrrolyl)-2,3-pyrazine-dicarbonitrile (2), bearing electron-withdrawing quinone or dicyano subunits, have been synthesized and fully characterized by various spectroscopic and electrochemical methods. Both 1 and 2 are specific binders of F^- in organic solvents and show dramatic, binding-induced changes in their color (observable in the naked-eye experiments) and also optical and electrochemical signatures. These F^--induced color changes remain the same even in the presence of a large excess of Cl^-, Br^-, I^-, or ClO₄^-, thus rendering 1 and 2 to be efficient fluoride ion sensors. While K_a for F^- binding by receptor 1 is ~1.6 × 10⁴ M^-1, that for receptor 2 is an order of magnitude higher. ¹H NMR titrations were carried out to monitor the binding of 1/2 with F^-. These experiments not only provide evidence for the hydrogen-bonding interaction between the pyrrolic NH groups of these receptors and F^-, but also offer some key insights into the structures of the receptor-anion complexes. Further insights into the structures of the receptor-anion complexes and the observed binding discrimination have been obtained by density functional calculations. Both receptors 1 and 2 interact with a halide ion by forming two NH···X^- hydrogen bonds with the pyrrolic NH protons in a bidentate fashion. The predicted order of halide binding affinity for receptors is F» Cl > Br. The high selectivity for F^- among the halides is attributed mainly to the strength of the hydrogen bond and partly to the complementarity of the geometries between the receptor and anion. The higher F^- binding ability of 2 over 1 has been interpreted in terms of the greater electron deficiency and enhanced hydrogen-bond-donating character of the former derivative. Calculations of the NMR and UV-visible spectra support the experimental characterization of the receptor-anion complexes.