Complexation crown ether complex, solvent effect

Ethers form Lewis acid Lewis base complexes with metal ions Certain cyclic polyethers called crown ethers, are particularly effective m coor dinatmg with Na" and K" and salts of these cations can be dissolved m nonpolar solvents when crown ethers are present Under these conditions the rates of many reactions that involve anions are accelerated... 

Since the crown ethers are very effective complexing agents, the amount of free M+ in solution, as in (33)—(36), is expected to be small the crown ether competes very well with Rh and X for M +. Indeed, it is found that the addition of excess salt causes a much lower degree of rate inhibition in [18]-crown-6 as compared to some other solvents. For example, Fig. 10 illustrates the differences between [18]-crown-6 and tetraglyme as the level of salt promoter is increased. The capability of using an excess of salt reduces the criticality of precisely controlling the salt concentration and is beneficial for the stability of the catalyst (92). 

The crown ether has two effects on KMn04 first, it makes KMn04 much more soluble in benzene second, it holds the potassium ion tightly, making the permanganate more available for reaction. Chemists call this a "naked anion" because it is not complexed with solvent molecules. 

The crown ethers and cryptates are able to complex the alkaU metals very strongly (38). AppHcations of these agents depend on the appreciable solubihty of the chelates in a wide range of solvents and the increase in activity of the co-anion in nonaqueous systems. For example, potassium hydroxide or permanganate can be solubiHzed in benzene [71 -43-2] hy dicyclohexano-[18]-crown-6 [16069-36-6]. In nonpolar solvents the anions are neither extensively solvated nor strongly paired with the complexed cation, and they behave as naked or bare anions with enhanced activity. Small amounts of the macrocycHc compounds can serve as phase-transfer agents, and they may be more effective than tetrabutylammonium ion for the purpose. The cost of these macrocycHc agents limits industrial use. 

Other measures of nucleophilicity have been proposed. Brauman et al. studied Sn2 reactions in the gas phase and applied Marcus theory to obtain the intrinsic barriers of identity reactions. These quantities were interpreted as intrinsic nucleo-philicities. Streitwieser has shown that the reactivity of anionic nucleophiles toward methyl iodide in dimethylformamide (DMF) is correlated with the overall heat of reaction in the gas phase he concludes that bond strength and electron affinity are the important factors controlling nucleophilicity. The dominant role of the solvent in controlling nucleophilicity was shown by Parker, who found solvent effects on nucleophilic reactivity of many orders of magnitude. For example, most anions are more nucleophilic in DMF than in methanol by factors as large as 10, because they are less effectively shielded by solvation in the aprotic solvent. Liotta et al. have measured rates of substitution by anionic nucleophiles in acetonitrile solution containing a crown ether, which forms an inclusion complex with the cation (K ) of the nucleophile. These rates correlate with gas phase rates of the same nucleophiles, which, in this crown ether-acetonitrile system, are considered to be naked anions. The solvation of anionic nucleophiles is treated in Section 8.3. 

Evidence that the cation plays an essential role, at least in some cases, is that when the Li was effectively removed from L1A1H4 (by the addition of a crown ether), the reaction did not take place. The complex 19 must now be hydrolyzed to the alcohol. For NaBH4 the Na" " does not seem to participate in the transition state, but kinetic evidence shows that an OR group from the solvent does participate and remains attached to the boron... 

An enormous variety of solvates associated with many different kinds of compounds is reported in the literature. In most cases this aspect of the structure deserved little attention as it had no effect on other properties of the compound under investigation. Suitable examples include a dihydrate of a diphosphabieyclo[3.3.1]nonane derivative 29), benzene and chloroform solvates of crown ether complexes with alkyl-ammonium ions 30 54>, and acetonitrile (Fig. 4) and toluene (Fig. 5) solvates of organo-metallic derivatives of cyclotetraphosphazene 31. In most of these structures the solvent entities are rather loosely held in the lattice (as is reflected in relatively high thermal parameters of the corresponding atoms), and are classified as solvent of crystallization or a space filler 31a). However, if the geometric definition set at the outset is used to describe clathrates as crystalline solids in which guest molecules... 

The crowns as model carriers. Many studies involving crown ethers and related ligands have been performed which mimic the ion-transport behaviour of the natural antibiotic carriers (Lamb, Izatt Christensen, 1981). This is not surprising, since clearly the alkali metal chemistry of the cyclic antibiotic molecules parallels in many respects that of the crown ethers towards these metals. As discussed in Chapter 4, complexation of an ion such as sodium or potassium with a crown polyether results in an increase in its lipophilicity (and a concomitant increase in its solubility in non-polar organic solvents). However, even though a ring such as 18-crown-6 binds potassium selectively, this crown is expected to be a less effective ionophore for potassium than the natural systems since the two sides of the crown complex are not as well-protected from the hydro-phobic environment existing in the membrane. 

The stability of crown-ether complexes depends on several factors these include cavity size of the ligand, cation diameter, spatial distribution of ring binding sites, the character of the hetero-atoms, the presence of additional binding sites and the type of solvent used. In apolar solutions it also depends on the nature of the anion. The effects of these parameters will be illustrated in the next sections. 

On the other hand, Hartman and Biffar (1977) reported that decomposition of arenediazonium tetrafluoroborates in dichloromethane in the presence of copper metal is catalysed by dicyclohexyl- 18-crown-6. Electron-withdrawing substituents in the aryl ring enhance the rate of the reaction. The main function of the crown ether is probably to solubilize the salt. The effect of the complexation on the rate was not investigated in detail. Similar enhanced solubilization of diazonium salts in apolar solvents was reported and used by Martin and Bloch (1971) in pyrolysis experiments aimed at the generation of the dehydrocyclopentadienyl anion. 

Determination of the Effect of Water on Poljnnerization. To a solution of 4.76 mmole of HFB and 4.76 mmole of Bis-A in 20 ml of the appropriate solvent of known water content was added 20.6 mmole of K2CO3 and 1.32 mmole of 18-crown-6 ether. The crown ether, when necessary was dried by complexation with acetonitrile. Water content of solvent and catalyst was determined in all runs by Karl Fischer titration. Known microliter quantites of water in increments of lOyl were then added to the reaction mixture. The magnetically stirred, heterogeneous mixture was heated in an oil bath and maintained under N2. Upon cooling to room temperature, the reaction mixture was slowly poured into ca. 300 ml of a nonsolvent vigorously stirred in a blender. The filtered solids were washed three times with 300-ml portions of distilled water. 

They enable inorganic salts to be used in nonpolar solvents, a media with which salts are typically incompatible. (2) The cation of the salt is complexed in the center of the crown ether, leaving the anion bare and enhanced in reactivity. These effects of crown ethers are similar to those achieved with phase-transfer agents. The bare" anion is also present when polar aprotic solvents are used. 

---