Crown ethers coordination

Hydrocarbon crown ethers coordinate cations however, both the perfluoro crown ethers and the perfluorocryptands coordinate anions. For example, perfluoro crown ethers and perfluorocryptands tenaciously encapsulate O and F (60,61). 

When crown ethers coordinate with a metal cation, they thereby convert the metal ion into a species with a hydrocarbonlike exterior. 

X-Ray studies of crown ether complexes of Al reveal some variation in Al—O coordination with ring size. In particular, 15-crown-5 forms (Me3Al)4-crown with Al—O = 200.5 pm, whilst dibenzo-18-crown-6 gives (Me3Al)2-crown with Al—O = 196.7 pm.428 Other facets of crown ether coordination of aluminum are considered in Section 25.1.5.6. 

Crown ethers render many salts soluble in nonpolar solvents. For this reason they are called phase transfer catalysts. When a crown ether coordinates with a metal cation it masks the ion with a hydrocarbon-like exterior. 18-Crown-6 coordinates very effectively with potassium ions because the cavity size is correct and because the six oxygen atoms are ideally situated to donate their electron pairs to the central ion in a Lewis acid-base complex. 

Korber et al. [172] investigated the nature of the chemical bond in polypnictides. They synthesized novel compounds of the cyclotetrapnictide anion in a tripledeckerlike coordination between two crown ether-coordinated alkali metal cations. The structure of one of the complexes studied is shown in Fig. 35. 

We have also reported the first perfluorocryptand molecule, the perfluoro-cryptand[2.2.2] (see Figure 9) (11), The perfluorocryptand is a very stable, inert, high boiling clear oil. While hydrocarbon crown ethers coordinate cations, both the perfluoro crown ethers and the new perfluorocryptand coordinate anions. Two manuscripts have recently appear in collaboration with Professor Jennifer S. Brodbelt in which both perfluoro crown ethers and perfluorocryptands tenaciously encapsulate 02and F" (77-72). 

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