Contact ion pair receptors

Faced with the challenge of ion pair binding, there are three basic ion pair receptor designs contact ion pair receptors (including cascade complexes) in which the anion and cation are bound as a contact ion pair, ditopic receptors with individual, well-separated anion and cation binding sites, and zwitterion receptors, Figure 5.2. We will discuss some examples of each type of complex in the following sections. 

Ion pair receptors contain a cation and an anion binding region, often a combination of common binding motifs, and can bind both contact or separated ion pairs to give an overall neutral complex. 

NMR titration experiments showed that the presence of an alkali metal cation greatly enhances halide-binding constants. [21] For example, the 13/CF association constant in CDCl3 DMSO-t/6 (85 15) was increased from 80 M 1 to 2.5 x 104 M 1 by the presence of one molar equivalent of potassium tetraphenylborate. Additional NMR titration experiments showed that receptor 13 binds KC1 better than NaCl. X-ray analysis of the 13 KC1 complex uncovered two independent but structurally similar complexes in the unit cell. One of the stmctures is shown in Figure 2. The salt is clearly a contact ion-pair whose K-Cl distance of 2.989 A is slightly shorter than that observed in solid KC1. We also obtained the X-ray structures of receptor 13 complexed with LiCl, LiBr, NaCl, NaBr, Nal, KC1, KBr, and KI. [22,23] As expected, the larger anions do not fit perfectly inside the macrocyclic cavity, and so salt affinities are decreased. 

In the future, salt-binding receptors will be employed in various separation and sensing applications. The work described in this chapter demonstrates that ditopic receptors, with an ability to bind the salts as contact ion-pairs, have particularly attractive properties as extraction and transport agents. Another future direction is the utilization of salts as molecular glue to assemble complex supramolecular structures that have dynamic properties and the capability to behave as molecular machines. 

Crystallographic evidence of the contact ion pairing of sodium and chloride in the presence of the receptor 18 (Figure 18) clearly shows the two ions within contact distance (2.65 A) and interacting with the crown and the cleft respectively. 

Figure 17. Contact ion-pair salt receptor schematic and crystallographic representation (X-ray structure shows receptor with methanol and water bound. Non-acidic receptor hydrogen atoms removed for clarity).

Obviously, high binding affinity is expected via ditopic recognition when the salt is bound to the receptor as a contact ion pair. For this purpose, besides having crown ether moieties, aromatic pendant arms can also be tagged to the receptor to achieve ditopic recognition of ion pairs or salt. Consequently, in this case, the recognition of cations involves cation- n interactions due to the aromatic pendant arms. For example, uranyl receptors, 15 and 16, endowed with aromatic pendant arms, are used as ditopic receptors of contact ion pairs/salts such as alkali metal halides and quaternary ammonium haUdes. ... 

A contact ion pair mechanism was also observed by Jabin et al. in the complexation of alkylammonium chlorides by the heteroditopic receptor 81, in which the calix[6]arene... 

Cascade receptors are those in which the distance between the two binding functionalities is so short as to allow cooperative contact ion-pair binding. These receptors first bind one of the ions, often it happens to be the cation, which becomes the new binding site for the counterion, showing a higher affinity because of maximized coulombic attraction. 

Another interesting example of a heteroditopic receptor capable of cooperative recognition of ion pairs throngh a contact binding mechanism was recently reported by Beer and coworkers.Receptors 5 and 6 were designed to provide a good complementarity to contact ion pairs. 

Fig. 6.7 Host-guest properties of heteroditopic tris-amide and tris-urea-based receptors XCU and XCA toward organic contact ion-pairs. H-bonds are indicated by dashed lines. Inset H NMR spectra (CDCI3,300 MHz, 298 K) of (a) XCA (b) after addition of EtNHa Pic , TBA- CP, TBA N03 and TBA AcO (c) after addition of TBA F ...

These heteroditopic receptors display high affinities for eiflier neutral guests, zwitterions, anions, contact ion-pairs and metal ions. However, despites this versatility, remarkable selectivity is obtained within each class of guests ... 

In 2010 Kim and Sessler described three limiting ion pair association modes for a two site, ditopic receptor corresponding to (a) host-guest separated ion pair, (b) contact ion pair, and (c) solvent-separated ion pair, (cf. Fig. 12.4) [20]. This intellectual construct has facilitated the design of ion pair calix[4]pyrrole-based receptors in recent years. 

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