Fluorinated calix[4]pyrrole and dipyrrolylquinoxaline: Neutral anion receptors with augmented affinities and enhanced selectivities

The use of the 3,4-difluoro-1H-pyrrole as a building block for the preparat ion of octamethyloctafluorocalix[4]pyrrole and 2,3-di(3',4'-difluoropyrrol- 2'yl)quinoxaline is described. These latter two entities act as neutral ani on receptors and were found to bind anions such as fluoride, chloride, or d ihydrogen phosphate with an enhanced affinity compared to their non-fluorin ated congeners as judged from H-1 NMR, F-19 NMR, and fluorescence emission spectroscopic analyses. The increase in affinity was especially high in cas e of chloride and dihydrogen phosphate anion, with the 2,3-di(3',4'-difluor opyrrol-2'-yl)quinoxaoline system, in particular, displaying an affinity fo r H2PO4- that was improved by 3 orders of magnitude as compared to its nun- fluorinated congener. This improvement in the affinity for the dihydrogen p hosphate is accompanied by change of color from pale yellow to orange, thus allowing the use of such compounds as naked-eye sensors for phosphate anio n. In the case of the octafluorocalix[4]pyrrole system X-ray diffraction an alyses revealed the presence of four different macrocyclic conformations in the solid state, as well as close intermolecular contacts mediated by appa rent CF- -HN hydrogen bonds.