title: A calix[4]arene strapped calix[4]pyrrole: an ion-pair receptor displaying three different cesium cation recognition modes.
abstract: 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% CD(3)OD 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 2.CsF cation recognition takes place within the receptor cavity itself, as inferred from both single-crystal X-ray diffraction analyses and (1)H NMR spectroscopic studies. This binding mode is supported by calculations carried out using the MMFF94 force field model. In 10% CD(3)OD in CDCl(3) (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 (1)H 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.
authors.affiliation: Department of Chemistry and Biochemistry, 1 University Station-A5300, The University of Texas at Austin, Austin, Texas 78712-0165, USA.
authors.forename: Sung Kuk
authors.initials: SK
authors.lastname: Kim
authors.forename: Jonathan L
authors.initials: JL
authors.lastname: Sessler
authors.forename: Dustin E
authors.initials: DE
authors.lastname: Gross
authors.forename: Chang-Hee
authors.initials: CH
authors.lastname: Lee
authors.forename: Jong Seung
authors.initials: JS
authors.lastname: Kim
authors.forename: Vincent M
authors.initials: VM
authors.lastname: Lynch
authors.forename: Laetitia H
authors.initials: LH
authors.lastname: Delmau
authors.forename: Benjamin P
authors.initials: BP
authors.lastname: Hay