Ethers, crown binding

Changes in the Inter- and intramolecular interaction and in the structure of carbanion pairs on adding cation-binding ligands (ethers, crown ethers, poly-amines) is briefly reviewed. 

Other recent developments include crown ethers which contain potential switch mechanisms for complexation and transport. When photoresponsive chromophores are linked to crown ethers, ion binding can change on photoirradiation. The crown (120) is formed as its tram isomer with no alkali metal affinity, but photoirradiation gives a cis isomer capable of alkali metal complexation 500 this is an example of a reversible all or none ion-binding capability. A... 

When the pendant crown ether groups bind metal ions, the phase separation temperature is expected to rise because the hydrophilicity of the polymer increases. The concentration change of special metal ions in solution can be detected as the phase transition of the polymer solution, or the conformation change of the polymer chain. 

Number of donor atoms. In general, supramolecular interactions are additive, hence we would expect the larger crown ethers to bind more strongly to metal cations as long as all of the donor atoms can fit around the metal. This contributes to the plateau selectivity seen for most cations on the right-hand side of Figure 3.20. 

A molecular system showing OR logic is shown in Scheme 11.12. In this case the characteristic thermally reversible [4ji+4ji] photodimerisation of anthracene derivatives is promoted by the preorganisation brought about either by binding Na+ in the crown ether-like binding site or by binding the softer Hg(II) by the bipyridyl unit.60 The system has some similarities to the allosteric switch described in Section 1.5 (Scheme 1.2). 

Incidentally, C. J. Pedersen s first report on crown ethers and their complexes was published in the same year as the mechanism of the biological activity of valinomycin was clarified [2], Crown ethers are cyclic derivatives of polyethylene glycol of varying ring size, an example of which is also depicted in Figure 2.2.1. The structural relationship with the ionophores is clearly visible. It is thus not surprising that crown ethers also bind metal cations by coordination with the oxygen atoms [1, 3]. 

Moreover, intermacromolecular complex formation is applied to selective recovery of organic and metallic ions. For example, as shown in Table 27, Cu2+ ion is much more effectively precipitated by the polyelectrolyte complex than by one of its components520. Furthermore, polyelectrolyte complexes including some metal ions have been studied in recent years (see Sect. 3.2.). Crown ethers can bind certain cations they especially exhibit high affinity to K+. Smid et al.S21) synthesized poly(vinylbenzo-[18]-crown-6). Such polymers containing crown ether with K+ behave like polycations in solution and can interact with polyanions such as poly(carboxyHc acid) to generate a kind of polyelectrolyte complexes. Moreover, PAA may interact with the ether oxy-... 

The ability of crown ethers to bind selectively to particular Group IA and Group IIA metal ions, because of the relationship between hole size and metal ion radius, has led to considerable interest in them in relation to membranes (models for selective ion transport), antibiotics (similar polyether structure), organic synthesis [solubilization of inorganic reagents leading to milder routes for oxidation (122), nucleophilic substitution (123), fluoridation (90)] and extraction of alkali... 

The tren-based heteroditopic receptor 22, featuring a tripodal tetrahedral amide hydrogen-bond anion-recognition site in combination with benzo-15-crown-5 ether cation-binding moieties, has been found to cooperatively bind chloride, iodide, and perrhenate anions via co-bound crown ether-complexed sodium cations. It also can efficiently extract the radioactive sodium pertechnetate from simulated aqueous nuclear waste streams. The anion-binding affinity of 22 is considerably reduced in the absence of a co-bound cation <1999CC1253>... 

In manganese(II) compounds incorporating crown ether macrocycles, the metal is generally H-bonded to the crown via water molecules, although it has been possible to isolate some compounds such as [Mn(12-crown-4)2]+ and [Mn(15-crown-5)(CF3803)2], in which all the ether oxygen atoms of the crown bind directly to manganese(II). ... 

Macrocyclic ionophores are molecules whose atoms are organized to form a cavity into which metal ions fit and bind with high affinity. Such compounds are also called polycyclic ethers, crown ethers, cryptands, or cryptahemispherands. Different macrocyclics can be made with cavities tailored to fit the ionic radii of different elements. When chromogenic properties are imparted to these ionophores, spectral shifts... 

A cholacrown has been reported by us (Figure 9), which was synthesized in one step from cholic acid [19]. As expected, this crown binds alkali metal ions and primary ammonium cations. The rigid juxtaposition of a functionalizable hydroxy group to the crown ether moiety in this molecule makes it attractive for the design of potential catalysts. 

A crown ether specifically binds certain metal ions or organic molecules, depending on the size of its cavity, forming an inclusion compound. The ability of a host to bind only certain guests is an example of molecular recognition. The crown ether can act as a phase-transfer catalyst. 

The "lariat ethers" (crown ethers bearing a flexible side-chain) have been the source of much interest. Studies on the extraction constants and stability constants in CHgCl -l O for 15-crown-5 derivatives revealed that substituents which were sterically incapable of donation to a cation reduce the binding. 

Table III summarizes the efficiencies of selected crown ethers for binding Na and XUl- Efficiencies are measured in terms of log of association constants (Ka) in methanol and water. The correlation between the dieuaeter of the alkali ions and the cavity diameter of the crown ethers is evident from the enhanced association constants. For example, 18-crown-6, (cavity diameter 2.6-3.2 A) binds (diameter 2.66 A) more selectively than Na (diameter 1.9 A). On the other hand, 15-crown-5 (cavity diameter 1.7-2.2 A) is more selective for Na than The case of 21-crown-7 is interesting. Gokel et al. (12) have shown that the cavi of this ionophore (diameter 3.4-4.3 A) is too large for IT but larger still for Na. As a result, it is more selective for than 18-crown-6. The difference in log Ka values in methanol between K and Na for 21-crown-7 is larger (1.9) compared to 18-crown-6, for which the difference is 1.78. However, 21-crown-7 is less sensitive than 18-crown-6 in binding (log Ka of 4.35 versus 6.1).

Cryptands are basket-like blcyclic ionophores in which three strands of polyethers are tied together by two nitrogen atoms. They provide three-dimensional spaces for binding metal ions (11.13.14. They are several orders of magnitude more selective than crown ethers in binding alkali metal ions. Table IV provides data on dimensions and log Ka values for K, Na and Li in water for [222] [221] and [211] cryptands. For the [222] cryptand, the difference in the log Ka value between K and Na in water is 2.54. For 18-crown-6, the difference (Table III) is 1.83. 

Figure 11.5 Crown ether cyclophane binds squaraine dye only in the presence of sodium cations...

Figure 9 Ruorescence titration of a 1 2 equilibria, (a) The structure of the chiral ditopic oligo(p-phenylenevinylene) crown ether (COPY) binding to sodium (Na+). (b) Emission spectra = 453 nm) in chloroform of COPY (4.4 x 10 M ) on addition of NaPFe. (c) The corresponding binding isotherm showing the change in emission at 518 nm as a function of [Na+]/[COPY]. The resulting binding constants are also shown. (Reproduced from Ref. 18. Royal Society of Chemistry, 2007.)...

Figure 12 Crown ether-cation binding modes (a) good fit (b) perching (c) sandwich (d) small cation (e) multiple cations.

Most synthetic alternatives to valinomycin take advantage of ether oxygens that bind to K+. Earlier examples of monocyclic crown ethers have given way to more selective bis-15-crown-5 ethers that bind the K+ in a sandwichUke fashion. which provides for a 10 -fold selec-... 

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