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(1 H -2-pyrrolyl)-7,12-dihydronaphtho[2,3-f]quinoxaline-7,12-dione ( 1 ) and 5,6-di(1 H -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 ClO 4 -, thus rendering 1 and 2 to be efficient fluoride ion sensors. While K a for F - binding by receptor 1 is ∼1.6 × 10 4 M -1, that for receptor 2 is an order of magnitude higher. 1 H 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.