Controlling Cesium Cation Recognition via Cation Metathesis within an Ion Pair Receptor

Calix[4]pyrrole-based ion pair receptors

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.

Boosting the salt recognition abilities of l-ornithine based multitopic molecular receptors by harnessing a double cooperative effect

Systematic development of the geometric and electronic properties of the amino acid based salt receptors leads to a very effective and selective NaNO₂ receptor.

Colorimetric barbiturate sensing with hybrid spin crossover assemblies

Iron-based spin crossover complexes can be used as selective, visible sensors for barbiturates in the presence of other biologically relevant hydrogen bonding species.

Probing the origin of opposite ion-pair binding behavior for two new calix[4]pyrrole bis-phosphonate receptors

The origin of opposite ion-pair binding behavior for two new calix[4]pyrrole bis-phosphonate receptors has been explored.

Anion receptor chemistry: highlights from 2011 and 2012

This review covers advances in anion complexation in the years 2011 and 2012. The review covers both organic and inorganic systems and also highlights the applications to which anion receptors can be applied such as self-assembly and molecular architecture, sensing, catalysis and anion transport.

Selective ammonium nitrate recognition by a heteroditopic macrotricyclic ion-pair receptor

The heteroditopic macrotricyclic molecular receptor 1, which bears a tripodal anion binding domain and 4,10,16-triaza-18-crown-6 cation recognition domain, proves to be an effective ion-pair receptor. In the absence of the cobound cation (TBA(+) salts) receptor 1 preferably binds nitrate and nitrite over other anions, including basic anions such as acetate or dihydrogenphosphate. Ammonium cation binding by the 4,10,16-triaza-18-crown-6 subunit significantly enhances the strength of the nitrate and nitrite complexation at the triamide recognition site of the receptor. In the presence of ammonium cations, the association constants of nitrate binding reach an impressive value of 1050 M(-1) in highly polar DMSO-d6. Interestingly, the binding of other anions such as chloride and bromide is not enhanced in the presence of a cobound NH4(+) cation. The increased affinity of [1·NH4(+)]PF6(-) for anionic species is attributed to a strong cooperative effect that arises from the properly positioned binding sites in the receptor 1 cavity, thus allowing for the formation of the ion pair. Under liquid/liquid conditions, receptor 1 is able to extract NH4NO3 from an aqueous to an organic phase, as inferred from (1)H NMR spectroscopic and nitrite/nitrate colorimetric analyses.

Lithium-selective phosphine oxide-based ditopic receptors show enhanced halide binding upon alkali metal ion coordination

Previous work on a ditopic receptor based on a tripodal phosphine oxide core demonstrated preferential enhancement of bromide binding over chloride or iodide in the presence of lithium cation. Current studies on an elongated receptor provide evidence that preferential bromide binding enhancement in the presence of lithium cation is common to this receptor class in general, and that lengthening of the receptor results in an overall increase in halide association. Furthermore, the extended receptor shows a strong preference for Li+ binding in solution.