Inclusion cryptand

Huang F, Zhou L, Jones JW, Gibson HW, Ashraf-Khorassani M (2004) Formation of dimers of inclusion cryptand/paraquat complexes driven by dipole-dipole and face-to-face [small pi]-stacking interactions. Chem Commun 2670-2671... 

Fig. 3. Crown compounds/cryptands and analogous inclusion hosts. (1 4) Crown macro rings bicyclic cryptands (5) [37095-49-17, (6) [31250-06-3J, (7) [31364-42-8] (8) [23978-09-8]-, (9) spherical cryptand [56698-26-1]-, (10) cylindrical cryptand [42133-16-4]-, (11) apodand [57310-75-5]-, and (12) a spherand...

Podates AcycHc analogues of crown ethers /coronands and cryptands (podands, eg, (11) (30) are also capable of forming inclusion compounds (podates) with cations and uncharged organic molecules, the latter being endowed with a hydrogen bond fiinctionahty. Podates normally are less stable than coronates and cryptates but have favorable kinetics. 

An investigation of the kinetics of formation of the Li+ and Ca2+ complexes of cryptand 2.1.1 using stopped-flow calorimetry suggests that complexation occurs initially at one face of the cryptand such that the metal is only partially enclosed (to yield an exclusive complex). Then follows rearrangement of this species to yield the more stable product, containing the metal ion inside the cryptand (the inclusive product) (Liesegang, 1981). X-ray diffraction studies have indeed demonstrated that exclusive complexes are able to be isolated for systems in which the metal is too large to readily occupy the cryptand cavity (Lincoln et al., 1986). 

The study of Lehn s cryptands has shown that a three-dimensional arrangement of binding sites leads to very stable inclusion complexes (cryptates) with many cations. For example, the stability constant for K+ in methanol/water (95/5) is five orders of magnitude higher with [2.2.2]-cryptand [37] (log K 9.75 Lehn and Sauvage, 1975) than with [2.2]-cryptand [38] (log... 

The next chapter, by Ronald T. Clarke, John H. Coates, and Stephen F. Lincoln (Adelaide) discusses inclusion complexes of the cyclomalto-oligosaccharides (cyclodextrins), a unique group of natural cryptands that has attracted great interest within and outside the carbohydrate fleld in recent years. The article updates the pioneering contribution by Dexter French in Vol. 12 (19S7) on these oligosaccharides, then known as the Schardinger dextrins. 

The X-ray crystal structures of the F", Cl , and Br" cryptates of 19-6H demonstrate the inclusion of one of the halide anions in an unsymmetrical fashion. In the case of the small fluoride ion complex a tetrahedral coordination environment is observed for the guest anion with a mean N(H) - P hydrogen-bonding distance of 2.72(8) A. The CP and Br" cryptates exhibit octahedrally coordinated halide ions situated more centrally within the host framework with N(H) - X" distances in the ranges 3.19-3.39 A (X = CP) and 3.33-3.47 A (X = Br ). It is noteworthy that the hydrogen-bonded distances for the anion within the cryptand host are longer by up to ca 0.15 A than those for the other anions in the lattice, suggesting a particularly... 

Fig. 1. The inclusion of a pentamethylene-diammonium guest cation in an uncharged cryptand 51 a formula, b crystal structure...

Pedersen used reactions of nucleophilic substitution to synthesize most of the crown ethers he has obtained. On the other hand, Lehn and his coworkers [17] (Fig. 7.1.4) carried out cyclization reactions involving amide formation under high dilution conditions in their quest for cryptands such as 54. Pedersen analysis of the selective inclusion of alkali metal cations into the crown ethers cavity... 

Results with Sr ] in Mice. While the results with Ag cryp-tate were encouraging, we sought further preliminary evidence of the potential value of labeled cryptates as blood-flow radiopharmaceuticals. There were several reasons for these studies the monovalent silver ion is very polarizable and thus may not be a general model for monovalent cations (5,17). In contrast, divalent cations form stronger inclusive cryptates than monovalent cations of the same ionic radii. On the other hand, the added charge of the divalent ion would require that the cryptand shield more charge if it is to result in an equally lipophilic complex. 

An exclusion complex of this type has been identified by X-ray crystallography for the KNCS complex of the smaller [2.2.1]cryptand,454 ([2.2.1]cryptand = 73, n = 1, m = 1, / = 0). The cation occupies a site in the 18-membered ring provided by the ligand rather than in the central cavity and so resembles the coordination of potassium by 18-crown-6 (Figure 21a). In contrast, the smaller Na+ cation is of optimal size for the [2.2.1]cryptand and so an inclusion complex occurs with the cation inside the cage (Figure 21b).454 Binuclear inclusion complexes have not yet been isolated for bicyclic cryptands. 

Cryptands (16) and (17) are macropolycyclic receptor molecules which provide a cavity for inclusion of a variety of substrates. Cryptate refers to the complexes. 

Multisite crown ethers (30) and (31) are polymacrocycles. These molecules are potentially like cryptands in view of the possibilities for forming inclusion-like species. The photoresponsive crowns provide an excellent example of this aspect, and consist of two crown ethers, as in (30a and 30b), attached via a photosensitive azo linkage. This molecule undergoes reversible isomerization (likened to a butterfly motion), shown in equation (13). The cis form gives a stable 1 1 cation ligand complex with the larger alkali cations (actually a 1 2 cation crown ratio). Concentrations of (30b) in solutions are thus noted to be enhanced by the addition of these cations.100,101 Other multisite crowns have been prepared from diphenyl- and triphenyl-methane dyes, e.g. (31).102... 

Indeed, macrobicyclic ligands such as 7-9 form cryptates [Mn+ c cryptand], 10, by inclusion of a metal cation inside the molecule [1.26, 1.27, 2.17, 2.24-2.26]. The optimal cryptates of AC and AEC have stabilities several orders of magnitude higher than those of either the natural or synthetic macrocyclic ligands. They show pronounced selectivity as a function of the size complementarity between the cation... 

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