A calix[4]arene strapped calix[4]pyrrole: An ion-pair receptor displaying three different cesium cation recognition modes

An ion-pair receptor, the calix[4]pyrrole-calix[4]arene pseudodimer 2, bearing a strong anion-recognition site but not a weak cation-recognition site, has been synthesized and characterized by standard spectroscopic means and via single-crystal X-ray diffraction analysis. In 10% CD₃OD in CDCl ₃ (v/v), this new receptor binds neither the Cs⁺ cation nor the F^- anion when exposed to these species in the presence of other counterions; however, it forms a stable 1:1 solvent-separated CsF complex when exposed to these two ions in concert with one another in this same solvent mixture. In contrast to what is seen in the case of a previously reported crown ether "strapped" calixarene-calixpyrrole ion-pair receptor 1 (J. Am. Chem. Soc. 2008, 130, 13162 -13166), where Cs⁺ cation recognition takes place within the crown, in 2CsF cation recognition takes place within the receptor cavity itself, as inferred from both single-crystal X-ray diffraction analyses and ¹H NMR spectroscopic studies. This binding mode is supported by calculations carried out using the MMFF94 force field model. In 10% CD₃OD in CDCl₃ (v/v), receptor 2 shows selectivity for CsF over the Cs⁺ salts of Cl^-, Br^-, and NO ₃^- but will bind these other cesium salts in the absence of fluoride, both in solution and in the solid state. In the case of CsCl, an unprecedented 2:2 complex is observed in the solid state that is characterized by two different ion-pair binding modes. One of these consists of a contact ion pair with the cesium cation and chloride anion both being bound within the central binding pocket and in direct contact with one another. The other mode involves a chloride anion bound to the pyrrole NH protons of a calixpyrrole subunit and a cesium cation sandwiched between two cone shaped calix[4]pyrroles originating from separate receptor units. In contrast to what is seen for CsF and CsCl, single-crystal X-ray structural analyses and ¹H NMR spectroscopic studies reveal that receptor 2 forms a 1:1 complex with CsNO ₃, with the ions bound in the form of a contact ion pair. Thus, depending on the counteranion, receptor 2 is able to stabilize three different ion-pair binding modes with Cs⁺, namely solvent-bridged, contact, and host-separated.