Crown ethers selective binding

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. 

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... 

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. 

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. 

In Pedersen s early experiments, the relative binding of cations by crown ethers was assessed by extraction of alkali metal picrates into an organic phase. In these experiments, the crown ether served to draw into the organic phase a colored molecule which was ordinarily insoluble in this medium. An extension and elaboration of this notion has been developed by Dix and Vdgtle and Nakamura, Takagi, and Ueno In efforts by both of these groups, crown ether molecules were appended to chromophoric or colored residues. Ion-selective extraction and interaction with the crown and/or chromophore could produce changes in the absorption spectrum. Examples of molecules so constructed are illustrated below as 7 7 and 18 from refs. 32 and 131, respectively. 

A good deal of work has been done on polymeric crown ethers during the last decade. Hogen Esch and Smid have been major contributors from the point of view of cation binding properties, and Blasius and coworkers have been especially interested in the cation selectivity of such species. Montanari and coworkers have developed a number of polymer-anchored crowns for use as phase transfer catalysts. Manecke and Storck have recently published a review titled Polymeric Catalysts , which may be useful to the reader in gaining additional perspective. 

Alcohols can be selectively bound to the same host type if they are combined with an amine and vice versa, considering that a cation and an anion will be formed through a proton transfer. The so-formed alkoxide anion will bind to the boron atom, while the ammonium ion will be complexed by the crown ether (147, Fig. 39). Competition experiments involving benzyl-amine have shown enhanced selectivity for the complexation of alcohols with... 

The isomerization takes place because the excited states, both 5i and T, of many alkenes have a perpendicular instead of a planar geometry (p. 311), so cis-trans isomerism disappears upon excitation. When the excited molecule drops back to the So state, either isomer can be formed. A useful example is the photochemical conversion of c/s-cyclooctene to the much less stable trans isomer." Another interesting example of this isomerization involves azo crown ethers. The crown ether (5), in which the N=N bond is anti, preferentially binds NH4, Li, and Na, but the syn isomer preferentially binds and Rb (see p. 105). Thus, ions can be selectively put in or taken out of solution merely by turning a light source on or off." ... 

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 crowns as model carriers. Many studies involving crown ethers and related ligands have been performed which mimic the ion-transport behaviour of the natural antibiotic carriers (Lamb, Izatt Christensen, 1981). This is not surprising, since clearly the alkali metal chemistry of the cyclic antibiotic molecules parallels in many respects that of the crown ethers towards these metals. As discussed in Chapter 4, complexation of an ion such as sodium or potassium with a crown polyether results in an increase in its lipophilicity (and a concomitant increase in its solubility in non-polar organic solvents). However, even though a ring such as 18-crown-6 binds potassium selectively, this crown is expected to be a less effective ionophore for potassium than the natural systems since the two sides of the crown complex are not as well-protected from the hydro-phobic environment existing in the membrane. 

For monocyclic crown ethers the data presented in Table 4 and the stability constants for glymes [43]—[46] determined by Chaput et al. (1975) can be combined to calculate the macrocyclic effect (Table 7). The data indicate that the gain in binding energy on ring closure shows the same pattern as the ion selectivity of the crown ether, being highest for Na+/15-crown-5, K+/18-... 

Sr2+ so well that it was selectively extracted from a bulk sample of a barium salt (Helgeson et al., 1973a). Binding constants for metal-cation complexes of 1,3-xylyl-crown ethers [66]—[69] carrying an additional carboxylate binding... 

---