Calix[4]pyrrole-Based Ion Pair Receptors

Abstract

Ion pair receptors, which are able to bind concurrently both a cation and an anion, often display higher selectivity and affinity for specific ion pairs than simple ion receptors capable of recognizing primarily either a cation or an anion. This enhancement in recognition function is attributable to direct or indirect cooperative interactions between cobound ions via electrostatic attractions between oppositely charged ions, as well as to positive allosteric effects. In addition, by virtue of binding the counterions of the targeted ion, ion pair receptors can minimize the solvation of the counterions, which can otherwise have a negative effect on the interactions between the receptors and the targeted ions. As a result of their more favorable interactions, ion pair receptors are attractive for use in applications, such as extraction and sensing, where control of the binding interactions is advantageous. In this Account, we illustrate this potential in the context of ion pair receptors based on the calix[4]pyrrole scaffold. Both simple ditopic ion pair receptors, containing sites for the recognition of a single anion and single cation, and so-called multitopic ion pair receptors will be discussed. The latter systems differ from conventional, so-called ditopic ion pair receptors in that they contain more than one binding site for a given targeted ion (e.g., a cation). This permits a level of selectivity and control over binding function not normally seen for simple ion or ion pair receptors containing one or two binding sites, respectively. Calix[4]pyrroles are macrocyclic compounds consisting of four pyrrole units linked via fully substituted sp 3 hybridized meso carbon atoms. They are effective receptors for Lewis basic anions (e.g., halides) in typical organic media and under certain conditions will recognize ion pairs containing charge diffuse cations, such as a small alkylammonium, imidazolium, or cesium cations. The calix[4]pyrrole framework is further attractive in that it is relatively easy to modify. In particular, functionalization of the β-pyrrolic carbon and meso-carbon atoms with simple crown ethers or calix[4]arene crown ethers can produce heteromultitopic ion pair receptors containing more than two cation binding sites. This allows the interactions between receptors and ions to be manipulated on a higher level than can be achieved using simple ion receptors or heteroditopic ion pair receptors and has made these systems attractive for use in ion transport, recognition, and extraction. Recent progress in developing calix[4]pyrroles as both multitopic and more conventional ion pair receptors is summarized in this Account. The emphasis will be on our own work.

Tetra-phosphonate Calix[4]pyrrole Cavitands as Multitopic Receptors for the Recognition of Ion Pairs

“…When four phenyl phosphonate groups are introduced at the upper rim of resorcin [4]arene 3 (Scheme 1), up to six tetraphosphonate diastereoisomers 4 can be produced. 17 18 Conversely, molecular modeling studies disfavored the convergence of two or more phenyl groups in the cavity of related phosphonito cavitands (iioo, ioio, and iooo isomers).…”

“…17 18 Conversely, molecular modeling studies disfavored the convergence of two or more phenyl groups in the cavity of related phosphonito cavitands (iioo, ioio, and iooo isomers). 20 Experimentally, the reaction of resorcin [4]arene 3 with dichlorophenylphosphane 5 in acetone solution and in the presence of triethylamine afforded the 4iooo isomer, with three phenyl groups oriented toward the cavity interior as the main component of the crude reaction. 18 The isomers with four (4iiii) and three phenyl groups (4iiio) outwardly oriented with respect to the cavity were not even detected.…”

“…These molecules exhibited excellent solubility in most organic solvents. 27 For this reason, we selected octol 6b as a suitable aryl-extended calix [4]pyrrole alternative for the installation of the bridging phosphonates. The room-temperature reaction of the lipophilic octol α,α,α,α-6b and dichlorophenylphosphane 5 (5 equiv) in THF solution in the presence of triethylamine for 2 h produced a crude mixture in which at least two of the six possible tetraphosphonate diastereoisomers 7 were present.…”

Pyrrole, imidazole, and triazole derivatives as ion-pair recognition receptors

“…8 These and related achievements have been recently been summarized in an excellent account and to which the interested reader is directed. 9 Therefore, the content shown below is exclusively focused to describe the latest achievements in the field of ion-pair receptors based on the calix [4]pyrrole scaffold.…”

SPECTROSCOPIC STUDY OF EFFECTS OF TETRAALKYLAMMONIUM CATIONS ON F−-SENSING PROPERTIES OF CALIX[4]PYRROLE BORADIAZAINDACENE DYE

“…[2][3][4][5][6] In general, calix [4]pyrrole derivatives are prepared to sense halide anions through hydrogen bonds 7,8 or the ion-pairing effect. 9,10 In an anion-sensing process, tetrabutylammonium (TBA + ) cation salts are readily utilized, yet the effects of various tetraalkylammonium cations have been largely ignored. Sessler and co-workers elucidated that cations of different sizes can affect the anion-binding affinity of calixpyrroles in the solid state.…”