Cations ditopic receptors

In particular, for a ditopic receptor that can bind successively two cations (see previous section), the criterion for cooperativity is Kn/Ku > 1/4, i.e. complexation of a second cation is made easier by the presence of a bound cation. For instance, a cooperative effect was observed with fluoroionophore E-l (see Section 10.3.4). 

Development of ditopic receptors 58 for both an anion and a cation is a logical extension [19b]. It should be stressed that the recognition by cyclodextrins, calixarenes, hemicarcerands, and many other systems does not involve specific recognition sites, whereas tetrahedral recognition by 53 and 54, the adenine 56 selective binding, and multiple recognition like the one shown in formula 59 [20] require the presence of specific sites in receptor capable of directional interactions. 

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]. 

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. 

Fedorova, O.A., Fedorov, Y.V., Andryukhina, E.N., Gromov, S.P., Alfimov, M.V. (2003) Cation-dependent photochromic properties of die novel ditopic receptors, Pure Applied Chem., 1077-1084. 

Rhenium complexes 22a,b were designed as ditopic receptors for the cooperative complexation of ion pairs [21]. As proved by H NMR titration experiments in CD3CN,the receptors form complexes of 2 1 stoichiometry with alkali metal cations (interaction with both ethyl acetate cavities on the lower rim) and exhibit [16] a positive allosteric effect for iodide complexation compare... 

A ditopic receptor 71, comprising a semitubular calix[4]arene system [91] for cation recognition and urea moieties for anion recognition, was reported very recently. The complexation of sodium or potassium cations into the central ethylene glycol cavity invokes substantial enhancement of the binding strength (more than one order of magnitude) towards selected anions (halides, acetate). 

The ditopic receptor 119 has been shown to bind both inorganic anions and alkali metal cations simultaneously (233). Selectivity was observed for HS04-over CP and the binding of alkali metal cations in the crown ethers was proven to substantially enhance the magnitude of anion binding (through electrostatic interaction). 

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-... 

Crowns bridged on both sides form ditopic receptor molecules that form binuclear cryptate complexes (e.g. (30)). The preparation is normally accomplished by treatment of a diazacrown with a diacid chloride. Reduction of the tetralactam affords the tetraamine <73AG(E)579,77JA8502,79CC309). In particular, these structures have been probed by Sutherland and co-workers who assessed intramolecular diammonium cation binding by use of the NMR technique <79CC309>. 

The allosteric effect also plays a role in the cooperative binding of anions and cations by some ditopic receptors, such as in the case of Receptor 8, Fig. 2."" The positive cooperativity in the binding of Cl in the presence of K" partly results from the electrostatic attraction between oppositely charged ions. But, at the same time, binding of H2PO4 shows a negative cooperativity with K , and this specificity to the type of anion indicates a contribution of some nonelectrostatic. most probably, conformational effects. 

Fages. F. Desvergne, J.-P. Bouas-Laurent. H. Lehn, J.-M. Barrans. Y. Marsau, P. Meyer. M. Albrecht-Gary, A.-M. Synthesis, stmctural. spectroscopic, and alkali-metal cations complexation studies of a bis-anthracenediyl macrotricyclic ditopic receptor. J. Org. Chem. 1994. 59. 5264-5271. 

Another way to view salt binding is to accept the fact that salts prefer to form ion pairs and. thus, develop ditopic receptors that bind salts as associated ion pairs (Fig. 2). Two examples are the macrobicycles 14 and 15 with juxtaposed anion- and cation-binding sites. In the case of 14, the presence of Na cation enhances Cl binding by about a factor of 10. An x-ray crystal structure of the 14 NaCl complex shows that the salt is bound as a solvent-separated ion pair. The Na sits in the crown ring, and... 

Calixarenes may also be linked through covalent interactions to form ditopic receptors, such as in the symmetric linking of two p-rerr-butylcalix[4] arene molecules via ethylene linkages to form a calix[4]tube. The calix[4]tube shows remarkable selectivity toward cations, with bound within the central ethylether cage of the calix[4]tube. Molecular mechanics calculations suggest that the uptake of cations requires an... 

Another family of calixarene-based ditopic receptors are the l,3-calix[4]arene-Z w(crown ethers), where the calixarene is locked into the 1,3-alternate conformation, and substrate binding normally occurs at the crown ether loops. These may form mononuclear or dinuclear complexes with alkali inetal cations, or may bind neutral guest molecules. The unsymmetrical 1.3-alternate calix[4]ar-ene-l,3-crown-6 2,4-(l,2-phenylene)-crown-6 forms a 1 1 complex with Cs. The crystal structure shows that the cation is bound only within the adapted phenylene-crown-6 loop, and a molecule of acetonitrile is bound within the unmodified crown-ether loop. ... 

Compared to systems based on a single interaction, cooperative sensors based on multiple, concerted, inter-molecular interactions between the components exhibit higher discrimination capabilities and enhanced functions. As examples for this class of sensors, two systems, both based on the absorption/luminescence properties of the porphyrin nucleus are illustrated in Fig. 5. Structure (a) represents a ditopic receptor where cooperative binding of a cation by the amide-appended calixarene and of an anion by the Zn-porphyrin allows recognition of complete binary metal salts. Structure (b) incorporates two cyclodextrin cavities and relies on the modification of the Fe(III) complex spectroscopic properties by inclusion of guests in these hydrophobic sites.This species is able to sense the presence of benzylmercaptane and 1-adamantanecarboxylate in four... 

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