Chloride binding by a polyimidazolium macrocycle detected via fluorescence, NMR, and X-ray crystallography

Herein, we describe a macrocyclic polyimidazolium receptor that is preorganized for the binding of anionic guests, and particularly chloride. Additionally, diphenylimidazolium units were incorporated into this structure to enhance photophysical properties that were exploited for signal transduction of binding. In subsequent fluorescence binding studies, this receptor was found to bind a range of halides as well as phosphate with high affinity (Ka=1.8×104, and 1.5×104 for phosphate and chloride, respectively) in a competitive solvent mixture (1:1 water/acetonitrile). Results under these conditions were fitted to 1:1 binding curves, and indicated modest selectivity of the host for phosphate and chloride over other halides. Binding studies were also performed using 1H NMR spectroscopy, during which the imidazolium C–H signal was observed to shift downfield upon titration with anions. These experiments were run in less polar solvent (1:9 water/acetonitrile), and could not be fitted to a 1:1 binding curve, suggesting higher order aggregates in this environment. Binding was further probed in the solid state by obtaining an X-ray crystal structure of receptor–iodide complex. In the resulting structure, two iodides were found to bind through interactions with two polyimidazolium hydrogens each. These results show that the described macrocycle is effective for anion-binding in competitive solvent, with modest selectivity for chloride over other halides, and that the nature of the binding interactions varies depending upon the solvent environment.