Cryptand solvation complexes

Alkali metals can dissolve in solvating media such as ethers and amines to form blue solutions of solvated electrons. In the presence of strongly complexing ligands such as crown ethers or cryptands, electrides (complexed alkali cation and electron), or nuclides (complexed alkali cation and alkali metal anion) can be formed as shown in Scheme 7.3 [50]. Nuclides have been shown to react with monomers such as styrene and methyl methacrylate... 

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). 

In the complexation reaction cryptand must compete with solvent molecules for the cations in solution. Thus solvents such as methanol with low dielectric constant and solvating power offer a preferrable reaction environment but we have achieved quantitative yields in water. The main problem encountered in syntheses of cryptates has been the presence of other cations such as Na and KT competing for the cryptand. Care is taken to minimize the concentration of competing cations of size similar to the cation intended for complexation by using lithium salts for buffering solutions.-... 

The number of donor atoms can be influential in complex formation. Ideally, as the incoming ligand is comparable to an inner solvation sphere, the number of donor atoms should match the preferred coordination number of the cation. An example of this factor comes from a comparison of two similarly sized ligands, [2.2.2]cryptand and [2.2.C8]cryptand, the latter having one dioxoether chain replaced by a C8 unit.478 A reversal of the Ba2+/K+ selectivity of the order of 106 is found, the ratio being 104 for [2.2.2]cryptand and <10 2 for [2.2.C8]cryptand the preferred coordination numbers are six for potassium and eight for barium. 

The data obtained for [2.2.2]cryptand in acetonitrile solutions were further investigated.479,480 Silver ions are strongly solvated by acetonitrile and a competition was found to exist between complexation of the ligand and the solvent. This was claimed to be predominantly responsible for the lower stability of Ag[2.2.2]+ in acetonitrile than in water and for the rapid decrease in the stability constant at low mole fraction of acetonitrile (xMccn)> This phenomenon was then studied by determining the rate of formation and dissociation of Ag[2.2.2]+ in acetonitrile-water mixtures.481... 

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. 

The first observation of the NMR spectrum of Na- in a metal solution without added cation-complexing agents has recently been reported (65a). The rationale behind this observation was that the solvent HMPA by itself appeared to fulfill many of the requirements generally sought from macrocyclic complexing agents high solubility via cation complexation, stability to electron reduction, weak anion solvation, etc. The NMR spectrum of Na- in fluid Na-HMPA solutions, shown in Fig. 26, exhibits precisely the same chemical shift as that observed for Na-in solutions of Na in anhydrous methylamine and ethylamine in the presence of 2,2,2-cryptand. The metal anion here is truly "gas-like in... 

The structure of mixed aggregates involving ester enolates is also of major interest to macromolecular chemists, since ionic additives are often introduced in the polymerization medium. The more stable arrangement between lithium 2-methoxyethoxide and MIB lithium enolate was thus calculated (at the DFT level) to be a 5 1 hexagonal complex with similar O—Li lateral coordinations212. The same team has recently extended this study to complexes formed between the same enolate in THF and a-ligands such as TMEDA, DME, 12-crown-4 and cryptand-2,1,1213. Only in the case of the latter ligand could a separate ion pair [(MIB-Li-MIB),2 THF]-, Li(2,l,l)+ be found as stable, still at the DFT level, as the THF solvated dimer [(MIB-Li)2,4 THF]. 

Completely unsolvated naked anions cannot be prepared in solution with coronands or even with cryptands as cation solvators. Even in this case ion-pairing still occurs leading to complexed ion pairs [646]. Totally unsolvated naked anions can exist only in the gas phase cf. Section 5.2. ). 

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