The interaction of calixpyrrole with several chloride salts has been studied in the solid state by X-ray crystallography as well as in solution by isothermal titration calorimetry (ITC) and (1)H NMR spectroscopic titrations. The titration results in dimethylsulfoxide, acetonitrile, nitromethane, 1,2-dichloroethane, and dichloromethane, carried out using various chloride salts, specifically tetraethylammonium (TEA), tetrapropylammonium (TPA), tetrabutylammonium (TBA), tetraethylphosphonium (TEP), tetrabutylphosphonium (TBP), and tetraphenylphosphonium (TPhP), showed no dependence on method of measurement. The resulting affinity constants (K(a)), on the other hand, were found to be highly dependent on the choice of solvent with K(a)'s ranging from 10(2)-10(5) M(-1) being recorded in the test solvents used for this study. In dichloromethane, a strong dependence on the countercation was also seen, with the K(a)'s for the interaction with chloride ranging from 10(2)-10(4) M(-1). In the case of TPA, TBA, and TBP, the ITC data could not be fit to a 1:1 binding profile.

The development of ion-pair receptors, with the goal of achieving a higher level of control over recognition than that obtainable from simple ion binding, has intrigued researchers in supramolecular chemistry over the past decade.[1] Many reports have appeared during this period that describe the synthesis and study of very sophisticated receptor designs incorporating a range of electron-pair donor and acceptor groups. Most, if not all, of this work has been performed in the context of creating so-called ion-pair or salt hosts that will bind cation-anion pairs in homogeneous solution or enhance their extraction or transport under interfacial conditions. Whether the ditopic architecture in such systems confers real advantages over simpler combinations of single-ion receptors remains an open question. However, the utility of ion-pair

Two cleft-like anion receptors have been synthesised that contain indole hydrogen-bond donors and show fluoride selectively in a DMSO-water solution with crystallographic studies showing a 'twisted' binding mode for fluoride in the solid state.