Anion Binding by Fluorescent Biimidazole Diamides

Abstract

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 (NH(amide).N(imid)) and intermolecular (NH(imid).O(amide)) hydrogen bonding. In CH(2)Cl(2), 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 H(2)PO(4)(-) 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 (1)H NMR.