Ion Pair-Induced Conformational Motion in Calix 4 Arene-Strapped Calix 4 Pyrroles
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In order to understand the still-poorly understood interplay between calixarene conformations and cation and anion recognition in multicomponent systems, the ion pair receptors 1 and 2 were synthesized. In solution and in the solid state, the calixarene subunit of receptor 1 adopts a cone conformation, while that of 2 interconverts between the cone and the partial cone conformation. These geometric features differ from previous systems where the calixarene moiety was locked in the 1,3-alternate conformation. A combination of H-1 NMR spectroscopic analyses and single crystal X-ray diffraction studies reveal that receptor 1 binds the fluoride and the chloride anion via significantly different binding modes, displaying, for instance, 1 : 1 and 2 : 3 binding stoichiometries with CsF and CsCl, respectively. In the case of 2, the conformation of the calixarene constituent of 2 is highly dependent on the size and quantity of anions present. For example, upon treatment of 2 with the fluoride anion (as both the TBA(+) and Cs+ salts), the calixarene unit coexists as cone and partial cone conformers that are inter-convertible. In the presence of excess CsF, the aromatic rings of the calixarene subunit becomes locked in the pinched cone conformation with the result that an ion pair-mediated coordination polymer is formed. In the presence of excess CsCl, the calixarene unit of 2 adopts only the partial cone conformation stabilized by aryl CH-anion hydrogen bonding interactions. The present systems constitute a rare set of related receptors wherein the effects of conformational changes are so tightly coupled with ion recognition.