Complex cryptand-cation

Novel anions stabilized by alkali-polyether cations The ability of a crown (such as 18-crown-6) or a cryptand (such as 2.2.2) to shield an alkali cation by complex formation has enabled the synthesis of a range of novel compounds containing an alkali metal cation coordinated to a crown or cryptand for which the anion is either a negatively charged alkali metal ion or a single electron (Dye Ellaboudy, 1984 Dye, 1984). Such unusual compounds are called alkalides and electrides , respectively. 

The addition of a cryptand to some polyelectrolytes leads to significant increases in conductivity and in some cases IR and Raman spectroscopy demonstrate that the cryptand breaks up the ion-ion interactions (Chen, Doan, Ganapathiappan, Ratner and Shriver, 1991 Doan, Ratner and Shriver, 1991). Apparently the reduction of ion association more than offsets the reduction in mobility of the cation-crypt complex, which has a larger effective radius than the simple cation. It is also possible that the cryptand-ion complex is rendered more mobile by the reduction of polymer-cation complex formation, but this point has not been investigated in any detail. 

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... 

Alkalides can be isolated if the counter cation is complexed with a cryptand... 

Because of their electrophilic nature, Li" cations accelerate the reduction of carbonyl compounds by LiAlH4 or NaBH4. Li -complexing agents, such as cryptands, crown ethers or polyamines decrease the rate of reduction. In the case of a,p-unsaturated ketones, this slow down is associated with altered regioselectivity. For example, L1A1H4 reduction of cyclohexenones in the absence of the cryptand proceeds predominantly with 1,2-reduction. In the presence of the cryptand, 1,4-attack is favored. This selectivity is more pronounced with LiAlHa than with NaBHa (Scheme 36) and is also dependent on solvent. For example, with diethyl ether the 1,2-attack prevails, whereas when the cation is complexed, 1,4-addition predominates. 

Crowns and cryptands are able to increase considerably the basicity of a given base, owing to their extraordinary cationic complexation power. Thus A M+ in the presence of crowns or cryptands will see its M+ cation strongly complexed leading to A- as a nacked ion . In fact, the anion is not completely nacked as it may be seen for example from the use of crowns as transfer agents in phase-transfer catalysis reactions107. ... 

Crown ethers and cryptands, complexes with metal cations 79AG613,... 

Ms clusters have 12 framework bonding electrons as has [BsHs] (p. 161) the anions are also isoelectronic with the well-known cation [Bis] ". Similarly, the alloy NaSn. 2.25 reacts with cryptand in ethylenediamine to give dark-red crystals of [Na(crypt)]4 [809] the anion is the first example of a C4j., unicapped Archi-median antiprism (Fig. 10.10c) and differs from the Djh structure of the isoelectronic cation [Bi9] " which, in the salt Bi "[Bi9] [HfCl6]3 (p. 591), features a tricapped trigonal prism, as in [B9H9] (p. 153). The emerald green species [Pfig]" , which is stable in liquid NH3 solution, has not so far proved amenable to isolation via cryptand-complexed cations. 

The ultimate in encapsulation of a metal cation occurs by hgands termed cryptands, for example, N(CEl2-CH2-0-CH2-CH2-0-CEl2-CH2)3N, which completely encapsulates the cation the complex with the cation is 1,000 times more stable than the corresponding Na complex. The preparation and study of these compounds, by Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen, earned them the Nobel Prize in chemistry in 1987. 

There are now several structural studies on cobalt(II) crown ether and cryptand complexes (Table 78), which show the coordination mode to be markedly sensitive to the macrocycle cavity diameter. Both 12-crown-4 and 15-crown-5 (cavity diameters 120-150 pm and 170-220 pm respectively) can include ions to form structures in which every ether oxygen atom is bound to the metal. In contrast Co—O (ether) bonding is destabilized in the larger 18-crown-6 and dicyclohexyl-18-crown 6 polyethers. In the blue complexes obtained from the reaction of these compounds with C0CI2 the role of the cyclic polyether is to solvate discrete [Co(H20)6] cations and [CoC ] " anions and there are no direct Co—O (ether) bonds.A similar effect is seen in the Co" complex of a 27-membered-ring macrocycle where the pentacoordinate structure (267) features only one long Co—O (ether) bond. " ... 

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