Anion receptors based on organic frameworks: highlights from 2005 and 2006

Abstract

This critical review covers advances in anion complexation chemistry related to receptors based on organic frameworks in the years 2005-2006. The review covers anion receptors that employ amides and thioamides, pyrroles and indoles, ureas and thioureas, ammonium, guanidinium, imidazolium, and receptors containing hydroxyl groups. There is a discussion of anion templated assembly, followed by a short section outlining modelling studies of these systems. (226 references.)

Calix[4]pyrrole-Based Ion Pair Receptors

“…[4][5][6][7][8][9][10][11] In contrast, the majority of ion pair receptors rely on lone pair electron donors including crown ethers 15 and π-electron donors, such as functionalized calixarenes, for cation recognition. 16,17 For ion receptors for either cations or anions, their binding constants have been reported to vary significantly, especially in low dielectric constant media, depending on receptor concentration, ionic guest and its counterion concentrations, and nature of ions because interactions between receptors or/and between guests often make a remarkable influence on the formation of host/guest complexes.…”

“…The binding mode was inferred from 1 H NMR spectroscopic analyses and semiempirical calculations carried out at the PM3 level. On this basis, it was proposed that the ammonium cation is bound to the calix [5]arene cavity, while the chloride anion is complexed by the three facing urea groups, as shown in Figure 1.18.…”

“…2 However, in spite of their potential utility in such areas as salt solublization, ion extraction, and through-membrane transport, relatively little effort has been devoted to the synthesis and study of so-called ion pair receptors, species that are able to complex concurrently and with specificity both an anion and a cation. [3][4][5][6] While a number of host systems are known that contain both an anion and cation binding site, enhanced binding of an ion pair, where binding of the cation enhances binding of the anion or vice versa, is generally seen only in systems wherein the two ion binding sites are held in close proximity. 7 This has the consequence that in most cases it is so-called contact ion pairs, rather than solvent or spatially separated ion pairs, that are bound thereby avoiding the presumably unfavorable separation of two oppositely charged ions.…”

“…7 This has the consequence that in most cases it is so-called contact ion pairs, rather than solvent or spatially separated ion pairs, that are bound thereby avoiding the presumably unfavorable separation of two oppositely charged ions. 5,6,8 In fact, we are aware of only two closely related examples of structurally characterized spatially separated ion pair complexes. 6a-c However, in neither case was strong ion pair binding observed in solution.…”

“…As a consequence, both cation and anion recognition are now well-established branches of supramolecular chemistry. [4][5][6][7][8][9][10][11] From this body of work it has become increasingly apparent that counterions can play a critical role in modulating the binding strength and selectivity of what appears to be the simplest of ion receptors. This appreciation has, in turn, led to the consideration that more elaborate systems, capable of forming simultaneously complexes with both a cation and an anion, might offer considerable advantages in terms of affinity or selectivity.…”