Cryptands solvation effects

As well as increasing anion nucleophilicity, crown or cryptand complexation can enhance the basicity of the anion. Table 3 exemplifies this effect with 1-bromooctane where base-promoted elimination to 1-octene competes with nucleophilic substitution. Being small and poorly solvated, naked fluoride is a strong and hard base which causes, in the case of certain substrates (e.g. Scheme 6), the elimination product to predominate. As the naked anions increase in size they display less basic characteristics but retain high nucleophilic reactivity (74JA2250). 

Increasing die effective nucleophilicity of an ion allows S 2 substitution reactions to occur under milder conditions. An anion will become a better nucleophile when it is less effectively solvated and when it is further separated from its counterion. Methods that can achieve these changes include selection of a tetraafldammomum counterion [see Eqs. (6) and (6)], addition of a crown ether or a cryptand [see Eq. (7)], and use of a solvent that effectively solvates cations [see Eqs. (1) and (2)]. 

Solvation/desolvation effects in the cryptand also complicate the expected simple dependence of stability constant on host basicity. For example the aliphatic cryptand O-bistren shows lower formation constants than the less basic aromatic analogues such as R3F, which we attribute to the greater desolvation cost of complexation with the former, more hydrophilic host. 

Strong complexing agents for alkali metal cations such as crown ethers and cryptands, have large effect on the considered processes of generation of solvated electrons and metal electrodeposition loo iis-iw) ginjing the alkali metal cations by cryptands and crowns stabilizes the unusual solid compounds containing electrons and alkali metal anions at anionic sites of the crystal lattice... 

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. " ... 

This is exactly what occurs when, instead of the THF molecule, back-solvation of the cation is effected by a yet more powerful sodium-complexant, such as a crown ether or a cryptand. 

The practical and effective synthesis of photoactive lanthanide cryptates (1), capable of effective light conversion, has found application in the design of novel fluorescent immunoassay systems. The macrobicyclic bipyridyl cryptands prevent the normal solvation quenching of these cations. Remarkably effective DNA cleavage was accomplished using intercalators (2) based on 2,7-diazapyrenium cations. 

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