Cryptands, encapsulating

Figure 7.7. X-ray structure of anion 10 and the cryptand encapsulated lithium ion (Newman projection of the carbanion is shown in the box). (Adapted from reference 47.)...

Since the initial preparation of the cryptands (Section 2.3.2), many analogues have been synthesised using high-dilution techniques, but the most important of the cryptands is [2.2.2]cryptand, commercially available as an ion-sequestration agent under the name of Kyptoflx (2.13). The cavity size of the [2.2.2]cryptand is similar to the cavity size of [18]crown-6 and is also a good host for K+. Due to the three-dimensional nature of the host, the cryptand encapsulates the metal ion and shields it from the outside environment. The... 

Cryptands and rigid crossbridged macrocyclic zinc complexes have been characterized with varying degrees of encapsulation. 

Sargeson and co-workers have structurally characterized encapsulated zinc in hexaaza cryptands.742 743 Related cryptands (l-methyl-8-amino-3,13-dithia-6,10,16,19-tetraazabicyclo[6.6.6]-icosane and l-methyl-8-amino-3-thia-6,10,13,16,19-pentaazabicyclo[6.6.6]icosane) incorporating thioether donors also formed complexes with zinc which were structurally characterized. In both cases the zinc ion was encapsulated in the macrobicyclic cavity and the octahedral coordination geometry distorted to the mixed nitrogen and thioether donor atoms.744... 

It should be noted that the three-dimensional polyether cages (the cryptands) are usually most effective at producing naked anions . With these, the metal ion is completely encapsulated by the polyether network and thus better charge separation is achieved. In the case of the crowns, such complete encapsulation does not normally occur and hence the counter anion is more readily able to associate directly with the com-plexed metal cation. In such cases, the use of the term naked is somewhat of a misnomer. 

In macrobicyclic cryptate complexes where the cation is more efficiently encapsulated by the organic ligand these ion pair interactions are diminished and the reactivity of the anion is enhanced. This effect is seen in the higher dissociation constant, by a factor of 104, of Bu OK in Bu OH when K+ is complexed by [2.2.2]cryptand (12) compared to dibenzo[18]crown-6 (2). The enhanced anion reactivity is illustrated by the reaction of the hindered ester methyl mesitoate with powdered potassium hydroxide suspended in benzene. 

Various approaches have been taken to the synthesis of effective luminescent materials, using a variety of large encapsulating antenna-containing ligands, including podands, calixarenes, macrocycles, and macrobicycles (cryptands). These have been divided into acyclic (sub-section A) and cyclic (sub-section B). Representative ligands and complexes will be presented and discussed. 

The ligand shape has changed and it has inflated on encapsulating the cation. The structures of a large number of [2.2.2]cryptand alkali and alkaline earth metal complexes have been studied by X-ray crystallography.372 Using the criterion that the optimum complexation occurs when the cation and cavity radii are similar, it can be predicted that for this ligand potassium is... 

Nickel(II) complexes with cryptands are still rare. In general the encapsulation of nickel(II) in this type of macrocyclic ligand makes the complexes extraordinarily resistant to dissociation and substitution reactions. 

In both the chloride and bromide structures, the halide was also found to be encapsulated, but was positioned more centrally within the cavity, as explained by a better size match between the ligand and halide (Figure 7B) [48], In both cases the cryptand is hexaprotonated and all six of the protonated amines exhibit hydrogen bonding with the internal halide, with average NH—X distances of 3.30 A for chloride and 3.39 A for bromide. The distance between the bridgehead amines is 7.40 A and 7.50 A for the chloride and bromide complexes, respectively. 

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