Anion receptors ditopic

The property of the [Fe (Por)Cl] complex to act as a ditopic receptor towards diatomic anions is very attractive for studying its reaction... 

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]. 

Tetrahedral tetraazonium compounds 5a and 5b, developed by Schmidt-chen, bind carboxylates, such as formiate, acetate, and benzoate, among a variety of other anions [8]. Although these receptors do not display hydrogen bond interactions, they are endowed with well localized polycationic centers, which are independent on the acidity of the medium. A ditopic receptor build up of both a 5a and a 5b subunits bridged by a p-xylylene spacer showed selectivity towards the largest members of a series of dianionic probes [9],... 

To come to a water-soluble, ditopic receptor model, Lehn etal. attached JV6-[24] crown-8 with an acridine-derived DNA-intercalator, so combining the anion binding strength of the first (in protonated state) [67] with the n-stacking capacity of the latter. Indeed, increased binding of ATP by 46 was observed as compared with the unsubstituted azacrown [68]. Additionally, the multifunctional host 46 catalyzes the hydrolysis of ATP [69] with a higher selectivity over ADP as compared with the parent crown but with a somewhat reduced eflectiv-ity. The reaction intermediate was shown to be the Af-phosphorylated crown. 

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. 

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). 

In this section, we discuss macrocyclic receptors prepared specifically with the aim of producing molecules with different binding motifs. Such systems, which will be considered mostly in the context of binding anions with multiple protonation sites (e.g., phosphates or sulfates) in solution, can be considered as a subset of the large class of compounds known as ditopic receptors. 

Hosseini have devoted a lot of effort to aza crown complexes with ATP 48, showing them to be true yet weak ATPase mimics [24], Schmidtchen reported tetrahedral zwitterions 49 to discriminate between spherical anions [25]. Reetz was able to show that ditopic receptor 50, which includes both Lewis-acidic and basic centers will complex a whole ion pair [26]. Incorporation of transition metals into anion receptors such as 51 renders neutral species positively charged and susceptible to spectroscopic and electrochemical manipulations [27]. 

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-... 

Kelly has also developed the ditopic receptor 21 which can bind guests through the formation of four hydrogen bonds. Bis-anionic guests form strong complexes (Table 3), but again neutral groups show no association. Determination of association in less polar solvents was impossible due to the insolubility of this receptor [19]. 

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. 

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... 

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