Polyethers alkaline earth metal complexes

The findings that, both in ester and amide cleavage, an alkaline-earth metal ion is still catalytically active when complexed with a crown ether, and that a fraction of the binding energy made available by coordinative interactions with the polyether chain can be translated into catalysis, provide the basis for the construction of supramolecular catalysts capable of esterase and amidase activity. 

By studying a series of complexes, it is possible to observe the differences in structural type that occur with change of cation radius. Table 6 shows the ionic radii for the alkali and alkaline earth metal cations, together with the average ligand cavity radii for simple polyethers.33 From this information it can be seen that the predicted optimal fit situation for 1 1 complexes would arise for Li+ and 12-crown-4 (74) for Na+ and 15-crown-5 (75) and for K+ and Ba2+ and 18-crown-6 (76). For 24-crown-8 (77) all of the cations have smaller radii than that of the ligating cavity. 

The ion selectivities displayed by these antibiotics are lower than those of the neutral ionophores, and are given in Table 11. Many studies have been made on the properties of these ionophores, particularly with reference to the calcium-transporting abilities of A 23187 (146) and lasolocid (145). The search for new antibiotics is ongoing and there is constant addition to a list of about 50 distinct polyether antibiotics which have been isolated from various streptomycetes. Representative structures will be discussed here to illustrate the nature of complexation with alkali and alkaline earth metal cations. 

The THECOMAC database contains the stability constant, enthalpy, and entropy values for the complexations of the alkaline and alkaline earth metal cations with cyclic polyethers in water and organic solvents, and includes more than 3,500 records. Each record contains 22 textual and digital fields and 2D chemical structures of ligands. The database includes thermodynamic values for 330 cyclic polyethers containing oxygen coordination centers only.41... 

Complexation of inorganic cations such as alkaline or alkaline earth metals by macrocyclic polyethers produces large, lipophilic cationic metal-macrocycle complexes that are readily soluble in nonpolar solvents such as benzene, toluene and haloalkanes. In order to maintain charge balance, the cationic complex has an associated counter anion. In an immiscible two-phase liquid system, such as a mixture of chloroform and water, the anion is necessarily pulled into the organic phase as the cationic complex crosses the phase boundary. A simple illustration of this principle is obtained by addition of a chloroform solution of [18]crown-6 to an aqueous solution of potassium picrate (potassium 2,4,6-trinitrophenolate). The yellow colour of the picrate anion is transported rapidly into the contiguous (physically in contact) chloroform phase upon agitation (Figure 3.43). 

The lariat ethers comprise a snbset of the polyether macrocycles, and are identified by their pendant chains. They can be categorized as either N-pivot (19) or C-pivot (20), depending on which type of atom the chain is attached. As for their polyether parents, much of the focus on these macrocycles has been on complexation of alkali and alkaline earth metal ions. 

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). 

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. ... 

Most of the papers on cyclic polyethers are concerned with their complexation phenomena with common salts of alkali metal, alkaline earth metal or ammonium. Metal (M) itself can be regarded formally as a... 

Stability and selectivity. The pH-metric titration method is usually used to determine the stability constants of cryptates (47). For alkali and alkaline earth metal cations, high stability constants are generally observed. As with the monocyclic polyethers, the most stable complex results when the ionic radius of the metal cation best matches the radius of the cavity formed by the cryptand on complexation. Because the cryptand host cavity is three-dimensional and spheroidal in shape, it is well adapted for a ball-like guest metal cation. Hence they have more pronounced recognition receptor... 

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. 

Chi Y., Ranjan S., Chung P.W., Hsieh H.Y., Peng S.M., Lee G.H. Alkaline-earth metal fiuoroalkoxide complexes with multi-coordinated polyether appendage synthesis and charactoization. Inorg. Chim. Acta 2002 334 172-182 Comyns A.E., ed. Fluoride Glasses. Chichester Wiley, 1989... 

The second ligand type consists of a large group of cyclic compounds incorporating numbers of ether functions as donors. Structure (22) illustrates a typical example. Such crown polyethers usually show strong complexing ability towards alkali and alkaline earth ions but their tendency to coordinate to transition metal ions is less than for the above... 

Crown polyethers. Macrocyclic effects involving complexes of crown polyethers have been well-recognized. As for the all-sulfur donor systems, the study of the macrocyclic effect tends to be more straightforward for complexes of cyclic polyethers especially when simple alkali and alkaline earth cations are involved (Haymore, Lamb, Izatt Christensen, 1982). The advantages include (i) the cyclic polyethers are weak, uncharged bases and metal complexation is not pH dependent (ii) these ligands readily form complexes with the alkali and alkaline earth cations... 

Copolymers of methacrylic add and ethylene termed as ethylene ionomers have been used as the base polymer for binding alkali, alkaline earth and transition metal ions. Organic amines such as n-hexylamine, hexamethylene tetraamine, 2,2,6,6-tetramethyM-hydroxy piperazine, ethylene diamine and polymeric diamines such as silicone diamine, polyether diamine and polymeric diamines such as silicone diamine, polyether diamine and polyamide oligomers considerably enhance the complex formation characteristics of Zn(II) ethylene ionomers thereby enhancing the physico-chemical properties [13]. 

In the Ba2+ complex with (145), two anions coordinate to the cation in different ways (Figure 32b). The metal ion sits primarily in a cavity provided by one of the anions and is six-coordinated by two ether, two hydroxy, one keto and one carboxylate oxygen atoms. A nine-fold coordination is completed by further coordination to two oxygen atoms from the second anion and a water molecule. 73 A review of the structures of polyether antibiotic complexes is available and includes a compilation of structural data.372 The stoichiometries of alkali and alkaline earth complexes of (145) in methanol, have been determined potentiometri-cally and show 1 1 neutral complexes for the alkali metal cations, and high stability 1 1 (charged) and 1 2 (neutral) complexes for the alkaline earth cations.574... 

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