Effect of conformational preorganization of a three-armed host on anion binding and selectivity.

A set of three-armed urea-containing anion receptors was prepared. The receptors all have the same binding topology but differ in the level of conformational preorganization with respect to the arrangement of the side-arms relative to the platform and within the side arms themselves. This is mirrored in a specific increase (x 2.5) in the binding constant for chloride and in a 12-fold increase in the chloride/nitrate-selectivity.