Crown polyethers, complexes synthesis

The compound known as 18-crown-6 is one of the simplest and most useful of the macrocyclic polyethers. Its synthesis in low yield was first reported by Pedersen. Greene and Dale and Kristiansen" have reported syntheses of the title compound from triethylene glycol and triethylene glycol di-p-toluenesulfonate. Both of these procedures use strong base and anhydrous conditions and achieve purification by more or leas classical methods. The combination of distillation and formation of the acetonitrile complex affords crown of high purity without lengthy chromatography or sublimation. ... 

Tummler, B. Maass, G. Weber, E. Wehner, W., Voegtle, F., (1977) Noncyclic crown-type polyethers, pyridinophane cryptands, and their alkali metal ion complexes Synthesis, complex stability and kinetics J. Am. Chem. Soc. 99, 4683-4689. 

Novel anions stabilized by alkali-polyether cations The ability of a crown (such as 18-crown-6) or a cryptand (such as 2.2.2) to shield an alkali cation by complex formation has enabled the synthesis of a range of novel compounds containing an alkali metal cation coordinated to a crown or cryptand for which the anion is either a negatively charged alkali metal ion or a single electron (Dye Ellaboudy, 1984 Dye, 1984). Such unusual compounds are called alkalides and electrides , respectively. 

There were not many new developments in the use of crown ethers for fluorination during the last decade, since the basicity of the fluoride anion is also enhanced along with the nucleophilicity. What is more, cheaper substitutes such as open-chain polyethers frequently led to similar results. However, there are still some reactions which are difficult to accomplish without these complexing agents and the synthesis of /-butyl fluoroformates... 

The ratio of the size of the metal ion and the radius of the internal cavity of the macrocyclic polyether determines the stoichiometry of these complexes. The stoichiometry of these complexes also depends on the coordinating ability of the anion associated with the lanthanide. For example, 12-crown-4 ether forms a bis complex with lanthanide perchlorate in acetonitrile while a 1 1 complex is formed when lanthanide nitrate is used in the synthesis [66]. Unusual stoichiometries of M L are observed when L = 12 crown-4 ether and M is lanthanide trifluoroacetate [67]. In the case of 18-crown-6 ligand and neodymium nitrate a 4 3 stoichiometry has been observed for M L. The composition of the complex [68] has been found to be two units of [Nd(18-crown-6)(N03)]2+ and [Nd(NCh)<--)]3. A similar situation is encountered [69] when L = 2.2.2 cryptand and one has [Eu(N03)5-H20]2- anions and [Eu(2.2.2)N03]+ cations. It is important to note that traces of moisture can lead to polynuclear macrocyclic complexes containing hydroxy lanthanide ions. Thus it is imperative that the synthesis of macrocyclic complexes be performed under anhydrous conditions. 

The complexes between lanthanoid salts and crown ethers are usually isolated from nonaqueous solutions, the Ln(III)/polyether interaction being very small in water due to unfavorable energetics in removing water molecules from the inner coordination sphere of the metal ion. We report here the synthesis and the properties of complexes with 12-crown-4, 15-crown-5, and 18-crown-6 ethers, having different metal/crown ratios, namely 1 1, 1 2, and 4 3. The singlecrystal X-ray structures of four complexes have been solved. In the 1 1 complexes Eu(N03)3 (12-crown-4) and Eu(N03)3-(15-crown-5), which crystallize in a chiral space group, and Nd(N03)3-(18-crown-6), the metal ion displays coordination numbers of 10, 11, and 12, respectively. The first two complexes have similar structures the polyether sits to one side of the Eu(III) ions and the three bidentate nitrate groups are coordinated on the opposite side. The structure of [Nd(N03)3]4-(18-crown-6)3 revealed that this complex has to be formulated as [Nd(N03)2-( 18-crown-6)]3[Nd(N03)3]6. 

Greene observed that the formation of 18-crown-6 from a ditosylate and a diol in the presence of f-butoxide salts was enhanced when a potassium cation was used (Greene, 1972). This template effect was operative for the synthesis of other polyether crown compounds using alkali or alkaline-earth metal cations. Template effects have also been observed for the preparation of aza-crown ethers, although the effect is less pronounced because the softer A-donor atoms form weaker complexes with the alkali metal cations (Frens-dorff, 1971). Richman and Atkins reported that high-dilution techniques were not required for the cyclization reaction of the disodium salt of a pertosylated oligoethylenepolyamine with sulfonated diols to form medium and large polyaza-crown compounds (Richman and Atkins, 1974 Atkins et al., 1978). 

A number of reports [10, 17, 22, 25-27] suggest that, in the case of condensation between aromatic diols and polyethylene glycols, the nature of the template exerts an influence on the rate of macrocyclisation. The templates form various series depending on the size of the synthesised crown ether, but lithium ion is an inhibitor in all cases. This must be attributed to the fact that Li+ forms the most stable ion pair with phenolate and simultaneously gives the least stable complexes with benzo crown ethers. It should be noted that alkaline earth metal ions, even in small concentrations, promote these reaction more effectively than alkali metal ions. In addition, it has been emphasised [12] that there is a definite correlation between the basicity of the substance used in the template synthesis of macrocyclic polyethers and the yield of final product. 

Weber, E., Polytropic cation receptors. 2. Synthesis and selective complex formation of spiro-linked multitrop crown compounds,/. Org. Chem., 47, 3478, 1982. Goldberg, 1., Geometry of the ether, sulphide and hydroxyl groups and structural chemistry of macrocyclic and noncyclic polyether compounds, in The Chemistry of the Ether Linkage, Patai, E., Ed., Suppl. E, Part 1, Wiley, London, 1981, 175. 

The synthesis of functionalized calix[4]pyrroles able to accommodate ion pairs within their cavities was first approached by using crown ether-like polyether straps to link two of the four meso-csxhon atoms of the calix[4]pyrrole framework. The first of such ditopic systems, receptor 7, reported by Kim, Sessler, and coworkers, was designed to contain a the calix[4]arene crown-6-moiety for cesium cation recognition, while retaining a calix[4]pyrrole subunit for anion recognition (cf. Fig. 12.6) [22]. In the solid state, the individual ions within the CsF ion pair complex, which are bound to 7, are separated from one another by a single methanol molecule that was presumably captured during crystallization. Proton NMR spectroscopic analyses, carried out in deuterated methanol/chloroform (1 9 vA ),... 

Introduction.—The ability of certain molecules, such as the macrocyclic crown ethers, e.g. 18-crown-6 (28), and the macrobicyclic cryptands, e.g. [2,2,2] cryptand (29), to form complexes with metal and ammonium cations has been extensively investigated in recent years. Since the original discovery by Pedersen, in 1967, of the crown group,some reviews and many papers have appeared on the syntheses and complexing properties of different classes of ligands, but it is not the intention here to go into detail concerning these aspects. Laboratory syntheses of the polyether class are dependent on the Williamson ether synthesis (Equation 11), but methods for production of commonly used compounds, such as (28), have been improved, and many representatives of both the crown and cryptand groups are now commercially available. 

It is known that the solubility of metal salts in nonpolar media is drastically increased if small amounts of crown ethers (cyclic polyethers) are used as complexation agents. Such a concept has been demonstrated in various areas of chemistry. For example, they are used as phase-transfer catalysts in organic synthesis. Moreover, Cheng27 and Schue28 have expanded this idea in the areas of anionic polymerization. 

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