Ion Pair Cooperative Binding of Potassium Salts by New Rhenium(I) Bipyridine Crown Ether Receptors

The new rhenium(I) bipyridine crown ether receptors 1-4 have been prepared and their ion pair recognition properties examined. The crystal structure of [1.KCl](2).2H(2)O demonstrates that potassium is coordinated by benzo-18-crown-6 and chloride is hydrogen bonded to the amide groups. Receptor 3 extracts solid KCl and KOAc into chloroform via ion pair complexation. NMR and emission titration studies with receptors 1-4 and KCl/KOAc show that cobound potassium enhances anion binding strength by electrostatic and conformational effects. Significant cooperative interactions are observed between the anion and cation sites for host 4 in CH(3)CN. This molecule coordinates potassium to form a 1:1 intramolecular sandwich complex, which preorganizes the host for acetate binding.