Calixpyrrole-based ion pair receptors
Compared with simple ion receptors, ion pair receptors display significantly enhanced affinity to ions through allosteric effects and additional electrostatic interactions between the bound ions, as well as host-guest interactions. Taken in concert, these necessarily permit a higher level of control over ion recognition and transport than that obtainable from simple ion binding. However, in spite of their potential applications in various fields, such as salt solublization, extraction, and membrane transport, ion pair receptors, which are able to form simultaneous complexation with an anion and a cation, still remains in a relatively unexplored area in supramolecular chemistry. This dissertation describes efforts to develop such systems on the basis of calixarenes and calixpyrroles. Calixpyrroles and calixarene derivatives bearing crown ethers or ester groups are known to act as efficient receptors for anions and cations, respectively. Therefore, the synthetic combination or modification of these two macrocyclic subunits provides an entry into novel ion pair receptors. The focus of this dissertation is on matched systems that form strong and specific complexes with cesium or potassium salts, depending on the exact structure in question. The selectivity demonstrated by these receptors is ascribed to a tuning of the cation recognition sites and control of the calixarene conformation. Solid state structural and 1H NMR spectroscopic analyses reveal that potassium and cesium cations are bound to different sites within these ion pair receptors. A strong dependence on the counter anion (e.g., fluoride, chloride and nitrate) is also seen. In some cases this dependence is near-absolute, thus mimicking AND logic gates. Noticeably, the ion pair receptor consisting of a 1,3-alterate calixarene crown-5 and a calixpyrrole is able to extract various cesium and potassium salts from a water phase into an organic phase in various binding modes, depending on the counter anions. Furthermore, the extraction behavior of this ion pair receptor towards such ion pairs can be controlled by cation switching and the use of different solvents.