Metal complexes of crown ethers

Ionic liquids based on metal complexes of crown ethers... 

Amaud-Neu, F. Delgado, R. Chaves, S. Critical evaluation of stability constants and thermodynamic functions of metal complexes of crown ethers (IUPAC Technical Report). PureAppl. Chem. 2003, 75 (1), 71-102. 

Lowe, N.D. Garner, C.D. Transition-metal complexes of crown ether benzodithiolenes. Part 2. The effects of alkali-metal cation binding. J. Chem. Soc., Dalton Trans. 1993, 3333-3340 and references therein. 

Substitution reactions of cyanide with secondary alkyl halides are often accompanied by the formation of elimination products in variable amounts (Cook et al., 1974). The same holds for reactions of metal acetate complexes of crown ethers (Liotta et al., 1974). 

Crown ether is the other important class of phase-transfer catalysts which critically differs from the onium salt, in that the whole inorganic salt is transferred into the organic phase. The reaction modes described above can generally be accommodated in such crown ether-catalyzed reactions, simply by replacing the onium cation by a metal cation complex of crown ether. 

An interesting recent development is the synthesis and sttuctural characterization of unusual cyclosiloxanolate metal complexes, " the crown ether-type complexation... 

Bribre, K.M. Detellier, C. Metal interchange of crown ether-alkali metal cation complexes in solution. Li Nuclear magnetic resonance study of the exchange kinetics of lithium 15-crown-5 and lithium monobenzo-15-crown-5 in nitromethane. J. Phys. Chem. 1992. 96 (5), 2185-2189. 

In a similar way collisional spin exchange between unlike species can be studied, as has been demonstrated for Cu and Ni complexes of crown ethers and the nitroxide radical TEMPO [40]. Comparison of the rate constant of spin exchange with the rate constant of diffusion collisions gave information on the steric factor, which characterizes the accessibility of the ligated transition metal ion to the nitroxide radical. 

The most remarkable property of crown ethers is their ability to form stable complexes with alkali and alkaline-earth metal ions and with ammonium ions. Numerous complexes of crown ethers with nonionic organic molecules are also known [187]. 

Crystalline complexes of crown ethers and metal ions are obtained by mixing two components in a common solvent [182]. The complexes have higher melting points than the free crown ethers. Stoichiometry of the complexes (crown ether salt = 1 1, 2 1, or 1 2) is largely determined by the fit of crown ether cavity and... 

When the organic solvent containing crown ether is used in the form of a liquid membrane between two aqueous solutions (see Section 5.4.4), an alkali metal salt can be selectively transferred through the organic liquid membrane from one aqueous feed solution to an aqueous strip solution. Illustrative treatment of the kinetics and mechanisms of formation and dissociation of the metal complexes with crown ethers is available in Burgess... 

A relationship between At/ and the free energy of formation has been reported for potassium complexes with 11 hex-adentate crown ether ligands. In this study At/ values were calculated as the steric energy of the potassium-crown ether complex minus the steric energy of the crown ether ligand. The free energy of formation was linearly correlated with At/(r = 0.991) yielding the first quantitative structure-reactivity relationship for metal complexation by crown ethers. This study clearly demonstrates the concept that metal complex stability is influenced by steric strain and that this influence can be quantified with MM. 

In this volume we have collected 10 review chapters from distinguished scientists who have contributed extensively to the study and development of supramolecular assemblies that contain metals and metal-like elements with unusual structures and morphologies and possess potentially useful (and applicable) physical and biological properties. The first chapter by K. Ariga et al. is a general discussion of supramolecular structures that contain inorganic building blocks for hybrid lipid thin films, layer-by-layer assemblies, structure transcription, and functional mesoporous hybrids. This is followed by two chapters, the first by M. L. Kistler et al., who describe the self-assembly of hydrophilic polyoxometalate (POM) macro-anions and examine the structure and behavior of POM macro-ions in solution. This is followed by a chapter by S. K. Das, who provides an overview of the supramolecular features of POM-supported transition metal complexes, POM-crown ether complexes with supramolecular cations, and supramolecular water clusters associated with POMs. 

The strength of this bonding depends on the kind of ether Simple ethers form relatively weak complexes with metal ions but Charles J Pedersen of Du Pont discovered that cer tain polyethers form much more stable complexes with metal ions than do simple ethers Pedersen prepared a series of macrocyclic polyethers cyclic compounds contain mg four or more oxygens m a ring of 12 or more atoms He called these compounds crown ethers, because their molecular models resemble crowns Systematic nomencla ture of crown ethers is somewhat cumbersome and so Pedersen devised a shorthand description whereby the word crown is preceded by the total number of atoms m the ring and is followed by the number of oxygen atoms... 

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

Phase-transfer catalysis succeeds for two reasons. First, it provides a mechanism for introducing an anion into the medium that contains the reactive substrate. More important, the anion is introduced in a weakly solvated, highly reactive state. You ve already seen phase-transfer catalysis in another fonn in Section 16.4, where the metal-complexing properties of crown ethers were described. Crown ethers pennit metal salts to dissolve in nonpolai solvents by sunounding the cation with a lipophilic cloak, leaving the anion free to react without the encumbrance of strong solvation forces. 

In this chapter we discuss host-guest complexes of arenediazonium salts with crown ethers and related compounds. Transition metal complexes of arenediazonium ions are treated together with those of dinitrogen and of diazoalkanes in our second book (Zollinger, 1995, Sec. 10.1). 

Heimann and Vogtle [38] synthesized triesters of glycerol with different ether carboxylic acids with a short alkyl chain. They have found that these hydrophilic lipids, in contrast with the fatty acid glycerol triesters, give complex-ation with alkali and alkali earth metal cations in an analogy of crown ethers. 

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