Selective recognition of an alkali halide contact ion-pair.

For more than 30 years there has been an active and continued effort to develop synthetic receptors for anions and cations in organic solvents.1 We2 and others3 have shown that if both of the counterions in a target salt have localized charges then the consequent ion-pairing of the salt can dramatically lower receptor/ ion binding affinities and alter binding selectivities. One way to counter this problem is to develop ditopic receptors that can simultaneously bind both of the counterions.4 Recently, we investigated the salt binding properties of receptor 1 and found that the presence of 1 molar equiv of Na+ or K+ ion increases the 1/Classociation constant by slightly less than ten.5 An X-ray crystal structure showed that receptor 1 binds NaCl as a solvent separated ion-pair. We felt that the binding cooperativity would be improved if the salt were bound to the receptor as a contact ion-pair. Thus, we designed macrobicyclic receptor 2 as a saltbinding analogue of 1 but with a smaller distance between the anion and cation binding sites.6 Using NMR spectroscopy and X-ray crystallography we find that 2 is the first example of a ditopic salt-receptor that binds a contact ion-pair in solution more strongly than either of the free ions.