Crown ether simultaneous binding

Covalent binding of an anion and a cation receptor to obtain a ditopic receptor and subsequent application in an SLM has been reported by Rudkevich et al. (66). They synthesized a receptor (carrier 14) which is capable of binding a cation and an anion simultaneously. The crown ether part binds Cs, while the salophene moiety can complex Cl. For carrier 14 CsNOj-fluxes (J = 0.89 x 10 mol m s ) and CsCl-fluxes (J = 1.2 X 10 mol m s ) were measured. In the case of cation assisted transport, CsNOj is expected to give a higher flux than CsCl, due to the higher lipophilicity of the NO3 anion. This proves that both the anion and the cation are involved in the complexation of CsCl, and that selective transport of a hydrophilic salt over a lipophilic salt can be obtained with a ditopic receptor. 

The condensation reactions described above are unique in yet another sense. The conversion of an amine, a basic residue, to a neutral imide occurs with the simultaneous creation of a carboxylic acid nearby. In one synthetic event, an amine acts as the template and is converted into a structure that is the complement of an amine in size, shape and functionality. In this manner the triacid 15 shows high selectivity toward the parent triamine in binding experiments. Complementarity in binding is self-evident. Cyclodextrins for example, provide a hydrophobic inner surface complementary to structures such as benzenes, adamantanes and ferrocenes having appropriate shapes and sizes 12) (cf. 1). Complementary functionality has been harder to arrange in macrocycles the lone pairs of the oxygens of crown ethers and the 7t-surfaces of the cyclo-phanes are relatively inert13). Catalytically useful functionality such as carboxylic acids and their derivatives are available for the first time within these new molecular clefts. 

The presence of two crown ether moieties in the baskets allows the simultaneous binding of two ammonium groups. Organic diammonium salts of type H3N(CH2)nNH3 are bound very strongly, as is evident from the high association constants in Table 5 [19]. 

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-... 

We reported a bis-crown ether functionalized calix[4]arene (127) (241). The unmetalated form of this receptor shows negligible interaction with anions, while the potassium bound sandwich complex binds anions effectively (chloride, K = 3500 M CD3CN). H NMR investigations proved the involvement of amide hydrogen bonding in the anion-binding process. Simultaneous anion and... 

Another interesting development is the use of bicyclic guanidinium receptors as heteroditopic systems for the enantioselective recognition of amino acids. In this case the guanidinium fragment was combined via a flexible linker with a crown ether moiety in order to promote the simultaneous binding of the carboxylate anion and the ammonium cation belonging to a zwitterionic amino acid [45]. 

A bifunctional receptor constructed by coupling two crown ether fragments to a UO2-salen complex has been reported for the simultaneous complexation of anions and cations [69]. This compound, shown in Figure 49, showed high selectivity in binding potassium dihydrogen phosphate. Selectivity of the membrane transport of H2PO4" (over Cl ) has also been shown [70]. 

The thiadiazapentalenes 375 and 378 were studied in terms of the influence of several substituents toward Ag(I), Hg(II) and Na(I) in solution. The combination of thiadiazapentalenes 382 and 383 with benzo crown ether substituents resulted in a simultaneous binding of one hard and one soft metal ion by one molecule (01PS29, Scheme 128). 

Multi-crown dendrimers (Fig. 3.7-12) were found to exhibit good solubihty in solvents of low polarity, making them attractive as extractants for separation processes. The combination of protonated tertiary amine groups as anion binding sites together with cation-active crown ether moieties represents an approach for the simultaneous binding of cations and anions. Extraction studies performed with sodium pertechnetate and mercury(II) chloride have shown that the guest molecules are mainly bound in the interior of the protonated polyamine skeleton [18]. 

Anticrowns are peculiar antipodes of crown ethers and their thia and aza analogues. They contain several Lewis acidic centers in the macrocyclic chain and so are able to efficiently bind various anions and neutral Lewis bases with the formation of unusual complexes, wherein the Lewis basic species is simultaneously bonded to all Lewis acidic atoms of the macrocycle. This remarkable property of anticrowns, being reminiscent of the behavior of conventional crown compounds in metal cation binding, makes them prospective aids in the areas of molecular recognition, ion transport, as well as organic synthesis and catalysis. 

Cation-n Interactions, p. 214 Crown Ethers, p. 326 Fluorescent Sensors, p. 572 lonophores, p. 760 Organic Zeolites, p. 996 Platonic and Archimedean Solids, p. 1100 Protein Supramolecular Chemistry, p. 1161 Simultaneous Binding of Cations and Neutral Molecules, p. 1295... 

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