Anion binding by fluorescent biimidazole diamides

Six 2,2'-biimidazoles with various amide groups at the 4- and 4'-positions were prepared from 5-propyl-1H-imidazole-4-carboxylic acid ethyl ester. In the final step of the synthesis, biimidazole C2-C2' bond formation was accomplished in 33-45% yield by palladium(0)-catalyzed homocoupling of the corresponding 2-iodoimidazoles. Four of the biimidazoles were studied by X-ray diffraction. In the solid state, all display coplanar imidazole rings, an anti relationship of amide groups, and intramolecular (NHamide...N-imid) and intermolecular (NHimid...O-amide) hydrogen bonding. In CH2Cl2, the emission intensity of the biimidazoles is quenched by the presence of dihydrogenphosphate and chloride anions, but no shifts in lambda(emiss) are observed. Binding constants for 1:1 biimidazole-anion complexation (K-assoc) are on the order of 10(4) M-1 for H2PO4- and Cl-. One of the receptors (bearing 3,5-difluorobenzylamides) is selective for chloride. The participation of the amide NH atoms in anion binding was established by H-1 NMR.