Heteroditopic rhenium(I) and ruthenium(II) bipyridyl calix[4]arene receptors for binding cation-anion ion pairs

New heteroditopic ion pair receptors that contain rhenium(I) and ruthenium(II) bipyridyl amide anion recognition sites covalently linked to a lower rim calix[4]arene tetraester alkali metal cation binding site have been prepared and shown to bind alkali metal (Li+, Na+)-halide (Br-, I-) ion pair species. Proton NMR titration studies reveal the lower rim ester co-bound alkali metal cation significantly enhances the strength of bromide and iodide binding in acetonitrile solutions with the largest positive co-operative binding effect of sixtyfold observed with bromide and the lithium complex of one receptor. Solid/liquid extraction experiments show two of the receptors are capable of solubilising NaCl and NaOAc in dichloromethane solutions.