Single Side Strapping: A New Approach to Fine Tuning the Anion Recognition Properties of Calix[4]pyrroles

Three calix[4]pyrroles bearing m-orcinol-derived diether straps of different lengths on one side
of the tetrapyrrolic core have been synthesized and characterized. Structural information for an analogous
diester bridged strapped system reported previously (Yoon, D. W.; Hwang, H.; Lee, C. H. Angew. Chem.,
Int. Ed. Engl. 2002, 41, 1757−1759) is also provided as are bromide and chloride anion affinities for all
four systems determined by Isothermal Titration Calorimetry (ITC) in acetonitrile. Although both sets of the
strapped calix[4]pyrroles displayed enhanced affinities for chloride and bromide anion, differences were
seen among the various receptors that support the conclusion that the anion binding ability of calixpyrrole-type systems can be effectively tuned by modifying the length and nature of the bridging straps. In the
specific case of the diether systems, the largest chloride affinity was seen with the shortest strap, whereas
the largest affinity for bromide anion was recorded in the case of the longest strap. On the basis of these
findings, as well as supporting 1H NMR spectroscopic studies, it is postulated that not only cavity size per
se, but also the ability of the aryl portion of the strap to serve as a CH hydrogen bond donor site are
important in regulating the observed anion affinities.