Simultaneous cation and anion receptors

Anion and ion-pair receptor chemistry highlights from 2000 

Zwitterionic receptors can be split into three classes (i) those that incorporate charge complementarity between the receptor and the zwitterions (ii) those in which the receptor binds one of fhe ionic parts of the substrate, which in turn causes a structural change (induced-fit) such that the other ion group then binds and (iii) betain receptors. Detains, for example, dioctanoyl-L-a-phosphaditylcholine (2.111), are nutrients that play a pivotal role in the cardiovascular system. Together with other nutrients, betains reduces the levels of toxic homocysteine (Hey) that may cause heart disease. Betains contain a tertiary trimethylammonium 

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P. A. Gale, Simultaneous cation and anion receptors, in Core concepts in Supramolecular Chemistry and Nanochemistry, eds J. W. Steed, D. R. Turner, and K. J. Wallace, John Wiley Sons, Inc., UK, 2007, 73. 

Dipolar electrostatic interactions have also been manipulated for the purposes of anion binding. Macrocyclic receptor 10 was shown to be capable of binding halide anions through interactions with the positive ends of the S = O and P = O dipoles (54). Evidence was also provided for the simultaneous binding of primary alkyl ammonium cations (to the oxygen atoms) and halide anions (to the dipoles). This topic of simultaneous cation and anion recognition is of considerable current interest. Further examples will be encountered during this chapter. 

The respective association constants of the latter anion to free 376 is only 10 M 1 while for the self-assembled complex it is equal to 2.5 104 M 1 [39]. Kubik and Goddard obtained a cyclic pseudopeptide ditopic receptor 378 for the simultaneous complexation of cations and anions [40]. Simultaneous encapsulation of N03 and PF6 ions by a highly charged (+12) anion receptor 379 was achieved by Schnebeck and coworkers [41]. By coordinating of 2,4,6-tris[(4-pyridyl)methylsulfa nyl]-l,3,5-triazine 380 with Ag, Hong and coworkers obtained nanosized tubes (Fig.7.8.1) that could host solvent and anionic molecules [42]. 

Dual host salt extraction involves the simultaneous extraction of a cation and anion by two separate receptors, one complementary to each ion. 

This system was prepared in 23% yield via the condensation reaction between the bis(acid chloride) form of sapphyrin 5.110 and bis(aminopropyl)diaza-18-crown-6 (5.128) (Scheme 5.6.5). It was hoped that this crown sapphyrin system might function as a ditopic ligand, capable of coordinating simultaneously both cationic and anionic substrates. To date, some preliminary evidence in favor of this proposal has been obtained. For instance, in work involving NH4F as the substrate, it was found that NH4" and F are simultaneously bound by the crown sapphyrin receptor. However, at present the structural nature of this proposed interaction remains indeterminate. 

A further modification saw the introduction of crown ethers at the end of the side arms creating a ditopic receptor (125) (238). This modification imparted the ability to bind cations and anions simultaneously, introducing an additional electrostatic component to the anion-binding interaction. This receptor exhibited significant transport of hydrophilic KH2P04 through a supported liquid membrane. Electrochemical studies indicated that H2PO perturbed the elec-... 

Pyrrole- and Polypyrrole-Based Anion Receptors, p. 1176 Simultaneous Binding of Cations and Anions, p. 1291... 

Beer, P.D. Chen. Z. Ogden, M.I. Voltammetric and NMR-studies of a l7A(ferrocenecarboxamide)-substituted diara 18-crown-6 receptor that simultaneously complexes and electrochemically recognizes both cations and anions. J. Chem. Soc,. Faraday Trans. 1995. 295-302. 

The following part focuses on a number of heterotopic ion-pair receptors reported in the literature that feature simultaneous complexation of both cationic and anionic species. The combination of structural motifs used to coordinate the ionic species is the interpretative key that is principle to this session. 

Coulombic considerations dictate that transport processes needs to be electroneutral, or nearly so. Therefore, in the case of carriers the diffusion of irais across the membrane is expected to occur via two limiting mechanism referred to as symport or co-transport, and antiport or exchange. Symport/co-transport occurs when both cations and anions (with the exact relative stoichiometry determined by charge considerations) are transported concurrently and in the same direction. In contrast, antiport/exchange processes involve the simultaneous transport of two ions in opposite directions (cf. Fig. 12.19). To date, most work has focused on anion transporters that do not allow for the concurrent complexation of a countercation. However, recently a few reports have appeared wherein ditopic receptors were employed as transporters. 

Cation-anion cotransport was effected by an optically active macrotricyclic cryp-tand that carried simultaneously an alkali cation and a mandelate anion and displayed weak chiroselectivity [4.23a], as did also the transport of mandelate by an optically active acyclic ammonium cation [6.39]. Employing together a cation and an anion carrier should give rise to synergetic transport with double selection by facilitating the flow of both components of a salt (see the electron-cation symport below). Selective transport of amino acids is effected by a convergent dicarboxylic acid receptor [4.24b]. 

The XH NMR study of receptors 70a-c revealed [90] that these compounds complex simultaneously sodium cation (lower rim amidic cavity) and anions (upper rim urea/thiourea), which markedly improves the solubility of the cor-... 

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