Metal chelate formation

Strongly influences positions of equilibria involving complex ions and metal-chelate formation. 

Metabolism studies in dogs and rats with radiolabeled tetracycline showed that with the exception of metal-chelate formation, tetracycline was chemically unaltered by the rat (247). Organ extracts from dosed animals were not found to contain metabolic products of tetracycline. Dog urine also contained unchanged drug, indicating that metabolic transformation of tetracycline had not occurred. 

Several other tj s are available, which employ electrostatic or hydrophobic interactions or metal chelate formation including polyarginine (79), polyphenylalanine or polycysteine (80), and dipeptide His-Trp (81). 

A. E. Martell and M. Calvin. Chemistry of the Metal Chelate Compounds, Prentice Hall, 1952, New Jersey. Criteria of metal chelate formation. ... 

Williams 22) was the first to point out that decrease in enthalpy usually accounts for a considerable part of the stability increase associated with metal chelate formation. The thermodynamic data in Table III illustrate this effect for a series of ethylenediamine chelates relative to the corresponding metal ammines. The data show that in nearly all cases the increased stability of the metal diamine chelate over that of the amine is due largely to a more favorable heat of formation. This effect is considered due, at least in part, to a more positive heat of formation of the ligand... 

The above table contains a relatively complete list of the factors involved in determining enthalpies and entropies of metal chelate formation in aqueous solution. Of these, the effects designated by footnote a are either unique for metal chelate ring formation, as contrasted to coordination by unidentate ligands, or are factors which are considerably different when chelation is involved. 

The synthesis of several thioether pentadentate ligands (95) is described.149 Acid dissociation constants and metal-chelate formation for a series of divalent metals, as well as formation constants for dioxygen adducts of their cobaltous complexes, are also reported. 

A phenolic group is a useful ligand substituent in this type of investigation, since its pKa is usually higher than the pH range in which most metal chelate formation occurs. Several systems have been studied in which changes in the acidity of a phenolic group are caused by metal chelation. 

Deprotonation reactions have been known to take place upon metal chelate formation with a number of complex molecules, such as enzymes and proteins, and are undoubtedly of importance in biological systems (73). One such system that has been studied by a number of workers is the deprotonation of a number of glycine-containing dipeptides in the presence of copper(II) and nickel(II). Chelate formation occurs between the free a-amino group and the carboxylate ion and the peptide nitrogen, from which a proton is lost (68, 114, 116, 144, 164)-... 

The following example illustrates the manner in which changes that occur in the rate of a ligand reaction upon metal chelate formation can be used for synthetic purposes. Two halogen atoms are introduced into a molecule of 8-hydroxyquinoline rapidly and almost simultaneously to give the 5,7-dihalo derivative (118). Studies on the rate of bromination of the Cr(III) or Cu(II) chelates of 8-hydroxyquinoline showed that mono-bromination in the 5-position occurred extremely rapidly, whereas the subsequent introduction of another bromine atom in the 7-position was quite slow. Advantage was taken of this difference in reaction rates to prepare the 5-bromo-, 5-chloro-, and 5-iodo-substituted compounds of 8-hydroxyquinoline and its 2-methyl and 4-methyl derivatives in good yields (169). 

A reactive group in a ligand can be protected by metal chelate formation. For example, in the compound (XCV) one of the amino groups is protected and the terminal amino group is free to react (124, 126). The reactive nitroso group in o-nitrosophenol can be protected by the formation of a stable and water-insoluble copper(II) chelate (159). It is possible that other reactive groups, such as the mercapto group in 8-mercaptoquinoline, could be protected by metal chelation in the course of a synthetic procedure. 

An example of metal chelation formation can be found if one refers to Standard Methods for the Examination of Water and Wastewater for the recommended methods for determining the toxic metal cadmium. An atomic absorption method is hsted along with a dithizone method. Cd combines with dithizone (HDz) in aqueous media to form the neutral, colored molecule Cd(Dz)2. This neutral molecule can be subsequently extracted into a non-polar solvent thus providing an increase in the method... 

Identification and Quantification of Some Drugs in Body Fluids by Metal Chelate Formation... 

A comparison of the behavior of a series of ligands in which one of the bonding atoms is replaced by another in the same group of the periodic table, helps in the understanding of the properties of these ligands and their metal chelates. For these reasons we have made a comparative study of the acid dissociation and certain metal chelate formation equilibria of 8-quino-linol, quinoline-8-thiol and quinoline-8-selenol by spectrophotometric and potentiometric techniques (Tables I and II),... 

TABLE II. log kj METAL CHELATE FORMATION CONSTANTS IN 50% v/v AQUEOUS DIOXANE. 

Fransara et al. (1989) have been proposed that three basic reactions are involved during formation of the gel precursor in the EDTA solution process metal chelate formation. 

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