Chelate-complex formation

Chelate complex formation as a basis for titration processes. Analytica... 

Separation is based on the reversible chelate-complex formation between the chiral selector covalently bonded to the chromatographic support, and the chiral solute with transition metal cations. Chelation properties of both the chiral selector and the chiral solute are required. Compounds therefore need to have two polar functional groups in a favorable arrangement to each other, like a )3-amino acids, amino alcohols and a-hydroxy acids, which can form rings membered with central chelating metal ions, like Cu(II), Zn(II), Cyclic... 

Based on preliminary results from Helfferich130, further developments by Davankov and co-workers5 131 133 turned the principle of chelation into a powerful chiral chromatographic method by the introduction of chiral-complex-forming synlhetie resins. The technique is based on the reversible chelate complex formation of the chiral selector and the selectand (analyte) molecules with transient metal cations. The technical term is chiral ligand exchange chromatography (CLEC) reliable and complete LC separation of enantiomers of free a-amino acids and other classes of chiral compounds was made as early as 1968 131. 

Figure 18. Schematic representation of bidentate (transition metal) - (amino acid) chelate complex formations.

In the case of transition metal ion complexation, on the other hand, bi-denate and/or chelate complex formation in addition to monodentate complex formation have been revealed. The complexation modes of the transition metal ions, such as Cu2+, Cd2+, and Pb2+, depend on the degree of PAA... 

Removal of color from dye solutions is a complex process and involves phys-iochemical mechanisms of coagulation and/or chelation-complexation type reactions. The dyes, due to the chelation/complex formation reaction with chemical coagulants, lead to the formation of insoluble metal dye complexes which may either precipitate from solution or may be removed by adsorption onto metal hydroxyl species [86]. 

There are special physicochemical constraints, such as chelation, complex formation, or crystallization to consider (see 21CFR320.33). 

A study in 5 healthy subjects given a single 1-g dose of myeophenolate alone or with polycarbophil calcium 2.4 g found that the AUC and peak serum levels of mycophenolic acid were redueed by about 51% and 69%, respectively. The authors suggest that this interaction was probably the result of reduced absorption due to chelate-complex formation between mycophenolate and the calcium ions, which they demonstrated in an in vitro study. It was concluded that mycophenolate and polycarbophil calcium should not be taken at the same time. A suitable interval was not specified, but a separation of 2 hours has been suggested with antacids , (p.l067), which interact by a similar mechanism. Further study is needed. 

If this is carried out in the presence of metal ions, it can be expected that all metal ions are distributed homogeneously into the organic polymeric network. When the process is a combination of metal-chelate complex formation and an in-situ polymerization, the process is termed the polymerized complex method (Fig. 8.11). 

The most important compound is 2-mercapto-pyridine-A -oxide = Pyrithione. According to Albert (1968) its mechanism of antimicrobial activity is based on chelation complex formation. But there are findings (Cooney Felix, 1972 Chandler Segel, 1978) which demonstrate that other modes of action are involved, too, for example, influence on ATP levels, nutrient transport, interference with protein synthesis. 

The Ni + cation shows little discrimination between ligands in propylene carbon-ate. 2 Kinetic parameters have been communicated for reactions of Ni-+ and Co + with ammonia in aqueous methanol and for Ni + with ethylenediamine or 2,2 -bipyridyl in aqueous dimethylformamide, and with 2,2 -bipyridyl derivatives in aqueous ethanol. The kinetics of reaction of Fe + with benzoylacetanilides have also been studied in aqueous ethanol the kinetic pattern for reaction of Fe + with thiocyanate in aqueous carboxylic acids is complicated by parallel complex formation with solvent-derived carboxylate. Copper(ii)-formazan (2) chelate complex formation has been shown to take place in discrete steps in aqueous ethanol. Carboxy-late anion effects on the copper(ii)-tetraphenylporphine reaction have been studied in dimethylformamide. ... 

Corey and Dawson have discussed the use of metal ion-sensitive protecting groups [149]. They find that the 8-quinolyloxy-carbonyl group is removed by brief treatment with copper or nickel salts in 50% aqueous acetone at 25°. The strong tendency of 8-quinolinol to chelate complex formation promotes cleavage under conditions where normal hydrolysis does not take place. The derivatives are easily prepared using the stable, crystalline bis(8-quinolinyl)carbonate in methylene chloride at 0°. 

This compound is solid at ordinary temperature (m.p. 43°C) and cannot, therefore, be used as a pure extract. For all solvents investigated, however, the distribution constants are much higher than those found in the case of acetylacetone (see Table 21.18). On account of the low solubility in water of both HTTA and the metal chelates, complex formation in water is incompletely known, and anyhow of limited interest. The extraction equilibria are, on the other hand, very important and will be discussed more fully in Section 21.5.2. Other /J-diketones used for the extraction of actinides are benzoylacetone, PhC(0)CH2C(0)CH3, and dibenzoylmethane, these have properties between those of HAA and HTTA [134] (cf. also Table 21.18). 

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