Polyether complexes

Phase transfer catalysis. As well as their use in homogeneous reactions of the type just described, polyethers (crowns and cryptands) may be used to catalyse reactions between reagents contained in two different phases (either liquid/liquid or solid/liquid). For these, the polyether is present in only catalytic amounts and the process is termed phase transfer catalysis . The efficiency of such a process depends upon a number of factors. Two important ones are the stability constant of the polyether complex being transported and the lipophilicity of the polyether catalyst used. 

Polyether complexation. The solution of the above problem is to add a suitable crown ether or cryptand to the alkali metal solution. This results in complexation of the alkali cation and apparently engenders sufficient stabilization of the M+ cation for alkalide salts of type M+L.M" (L = crown or cryptand) to form as solids. Thus the existence of such compounds appears to reflect, in part, the ability of the polyether ligands to isolate the positively charged cation from the remainder of the ion pair. 

Polyether complexation. The kinetics of formation of polyether crown, cryptand and related complexes have received considerable attention. Since formation rates are often quite fast, techniques such as temperature-jump, ultrasonic resonance, and nmr have typically been used for such studies. 

Recently, the presence of two different ions in a quadruplex channel has been reported. Four Ca2+ ions and three Na+ ions were detected, 3-4 A apart, in a four-stranded DNA G-quad-ruplex ion channel assembled from d(TG4T). Both the Ca2+ and the Na+ were found to be coordinated by six guanine-oxygens (608). It is interesting to compare these quadruplexes with species such as the stacked multimacrocyclic polyether complexes with spacers arranged to keep the mean planes roughly parallel and sufficiently far apart for each to bind a cation (609). 

Mass spectrometry studies have demonstrated the formation of a number of 1 1 crown polyether complexes of manganese(II) as cationic species in the gas phase." " ... 

Impetus was given to work in the field of selective cation complex-ation by the observation of Moore and Pressman (5) in 1964 that the macrocyclic antibiotic valinomycin is capable of actively transporting K+ across mitochondrial membranes. This observation has been confirmed and extended to numerous macrocyclic compounds. There is now an extensive literature on the selective complexation and transport of alkali metal ions by various macrocyclic compounds (e.g., valinomycin, mo-nactin, etc.) (2). From solution spectral (6) and crystal X-ray (7) studies we know that in these complexes the alkali metal cation is situated in the center of the inwardly oriented oxygen donor atoms. Similar results are found from X-ray studies of cyclic polyether complexes of alkali metal ions (8) and barium ion (9). These metal macrocyclic compound systems are especially noteworthy since they involve some of the few cases where alkali metal ions participate in complex ion formation in aqueous solution. 

Crystalline macrocyclic polyether complexes may be prepared by several routes,5,381 depending upon the solubilities of the precursors and the products. The polyester and the salt may be dissolved in a small amount of solvent and warmed together the complex can crystallize either on cooling or after evaporation of some of the solvent. It is also possible to warm a suspension of the ligand and the salt to effect solubilization and subsequent crystallization. The reaction can also be carried out by mixing the components in the solid state and heating to melting. 

A considerable number of NQR studies have been made on zinc and cadmium complexes, for example, the 35C1, 81Br and 127I NQR spectra of a number of CdX2-polyether complexes have been reported and indicate that the compounds are dimeric in solution, with symmetrical halogen bridges between the metal atoms.69 A number of amino acids and peptide complexes of cadmium(II) have been investigated by 14N NQR spectroscopy.70... 

Macrocyclic Polyethers Complex Alkali Metal Ions, Chem. and Eng. News, March 2, 1970, p. 26. 

In the isoelectionic ruthenium series, nucleophilic substitution by the disodium salt of 4,4 -dihydroxy-benzophenone on the complex (32) of p-dichlorobenzene with a [RuCp]+ unit produced the aromatic polyether complex represented by (33). Displacement by DMSO at 160 C led to the free polymer and the recoverable complex [CpRu(DMSO)3]+, which can be recycled directly by complexation with p-dichlorobenzene (equation 26). While ruthenium is not attractive to use in stoichiometric processes, this example appears to allow easy recycling.80... 

Since the publication of 26, we have determined the crystal structures of three additional diarylmagnesium-polyether complexes. Instead of diphenylmagnesium, its 4,4 -bis(tm-butyl) derivative was used in the com-plexation experiments since this species as a rule gives higher quality crystals. Complexation with 1,3-xylyl-18 crown-5 resulted in the formation of a rotaxane complex (27), too. Bond angles and distances, and even the... 

Bright, D. and Truter, M. R,. Crystal structure of a cyclic polyether complex of alkali metal thiocyanate. Nature (London) 225, 176-177 (1970). 

For all systems, the lighter isotope Li is enriched in the organic phase compared with Li. Obviously, this also proved true for the Chinese work where the obtained e-value of 30 x 10" is comparable with the results of Jepson and Cairns. On the whole, a significant dependence of the isotopic separation on the polyether as well as on the anion of the lithium salt was found. This can be understood by means of the different interaction of the cryptated cation with the anion in chloroform. According to Jepson and Cairns, the enrichment of Li in the organic phase is attributed to a more stable Li polyether complex compared with the Li compound. 

Structures of polyether complexes V. Molecular structure of bis(8-quinolyloxyethyl) ether-rubidium iodide, a linear polyether circularly embracing a metal ion, W. Saenger and B. S. Reddy, Acta Cryst., 1979, B35, 56. 

Structural investigation into the steric control of polyether complexation in the lanthanide series - macrocyclic 18-crown-6 versus acyclic pentaethylene glycol, R. D. Rogers, A. N. Rollins, R. D. Etzenhouser, E. J. Voss and C. B. Bauer, Inorg. Chem., 1993, 32, 3451. 

The polyether complexes have been used as models for naturally occurring compounds that are involved in the transport of alkali and alkaline-earth ions across membranes and for the very high sclcctivilies towards Na 1 and K+ or Ca2+ and Mg21 shown by natural systems.1815... 

The separation of isotopes of alkaline earth metals by ion-exchange chromatography (Be and Ca), using the band elution technique, and by chemical exchange reactions (Ca), using macrocyclic polyether complexes, has been assessed. The separation factors for Be and Ca decrease with increase in mass of the isotopes, and were found to be of the same order as those determined previously. Enrichment of the heavier isotopes of Ca by reaction (1), where L represents a macrocyclic polyether (c.g. DCH18C6, DB18C6), has also been shown to be effective. ... 

Izatt. R.M. Hayinore, B.L. Christensen. J.J. Stable oxonium-cyclic polyether complex characterized by infrared spectroscopy. J. Chem. Soc., Chem. Commun. 1972, 1308-1309. 

Saenger, W. Suh. I.H. Weber. G. Structures of polyether complexes. Part XIII. Wrapping of metal cations by linear polyethers. Isr. J. Chem. 1980, 18, 253-258. (Volume Date 1979). 

Unlike simple inorganic ligands, polyethers and, in particular, cyclic polyethers complex alkali metal ions quite strongly. The crown ethers are cyclic ethers which include... 

Basic Stereochemistry of the 1,4-Dioxa Group in Polyether Complexes... 

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