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]2·2H2O 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 CH3CN. This
molecule coordinates potassium to form a 1:1 intramolecular sandwich complex, which preorganizes the host for
acetate binding.