Alkali metal cation cooperative anion recognition by heteroditopic bis(calix[4]arene) rhenium(I) bipyridyl and ferrocene receptor molecules

New heteroditopic bis(calix[4]arene) rhenium(I) bipyridyl (L-1, L-2) and ferrocene (L-4) receptor molecules have been prepared and shown to bind a variety of anions at the upper rim and alkali metal cations at the lower rim. Proton NMR anion titration studies reveal the strength and selectivity of anion binding was dependent upon the presence of the calix[4]arene moieties and nature of the bridging group with the preorganised receptors L-1 and L-2 forming strong complexes in deuterated DMSO solutions. With L-1, L-2 and L-4 co-bound lithium, sodium and potassium cations were found to significantly enhance the strength of iodide binding in acetonitrile solutions with the largest positive cooperative binding effect observed with L-2 and sodium cations. Cyclic voltammetric investigations revealed L-4 to electrochemically recognise carboxylate and halide anions. In the presence of lithium cations the electrochemical response of L-4 to bromide anions was significantly amplified.