Schiff bases, metal chelates

These examples illustrate that biomolecules may act as catalysts in soils to alter the structure of organic contaminants. The exact nature of the reaction may be modified by interaction of the biocatalyst with soil colloids. It is also possible that the catalytic reaction requires a specific mineral-biomolecule combination. Mortland (1984) demonstrated that py ridoxal-5 -phosphate (PLP) catalyzes glutamic acid deamination at 20 °C in the presence of copper-substituted smectite. The proposed pathway for deamination involved formation ofa Schiff base between PLP and glutamic acid, followed by complexation with Cu2+ on the clay surface. Substituted Cu2+ stabilized the Schiff base by chelation of the carboxylate, imine nitrogen, and the phenolic oxygen. In this case, catalysis required combination of the biomolecule with a specific metal-substituted clay. 

Thus a metal ion can either labilize or stabilize a Schiff base via chelate formation. The latter property of a metal ion has been found to be advan-... 

Schiff base." The chelation of metals by amino-sugar derived... 

The Schiff base of acetylacetone with ethylenediamine BAEH2 gives stable quadratic chelates with several transition metals, whose P.E. spectra have been reported by CondorelU et al (99). The free ligand has P.E. bands at 7.71,7.90 sh, and 8.78 eV, at-... 

Gamovskii, A. D. Nivorozhkin, A. L. Minkin, V. 1. Ligand environment and the structure of schiff base adducts and tetracoordinated metal-chelates. Coord. Chem. Rev. 1993,126, 1-69. 

Mercaptoaldimes 258 and 259 form Schiff bases with amines which are of considerable interest as complexing agents, producing chelates 262 and 263 with metal salts. 

Eichhom and his co-workers have thoroughly studied the kinetics of the formation and hydrolysis of polydentate Schiff bases in the presence of various cations (9, 10, 25). The reactions are complicated by a factor not found in the absence of metal ions, i.e, the formation of metal chelate complexes stabilizes the Schiff bases thermodynamically but this factor is determined by, and varies with, the central metal ion involved. In the case of bis(2-thiophenyl)-ethylenediamine, both copper (II) and nickel(II) catalyze the hydrolytic decomposition via complex formation. The nickel (I I) is the more effective catalyst from the viewpoint of the actual rate constants. However, it requires an activation energy cf 12.5 kcal., while the corresponding reaction in the copper(II) case requires only 11.3 kcal. The values for the entropies of activation were found to be —30.0 e.u. for the nickel(II) system and — 34.7 e.u. for the copper(II) system. Studies of the rate of formation of the Schiff bases and their metal complexes (25) showed that prior coordination of one of the reactants slowed down the rate of formation of the Schiff base when the other reactant was added. Although copper (more than nickel) favored the production of the Schiff bases from the viewpoint of the thermodynamics of the overall reaction, the formation reactions were slower with copper than with nickel. The rate of hydrolysis of Schiff bases with or/Zw-aminophenols is so fast that the corresponding metal complexes cannot be isolated from solutions containing water (4). 

The very great stability of the iron (II) — N=C—C=N— chelate ring provides the driving force for the reaction. This is further illustrated by the formation of monomeric Schiff bases between Qj-diketones and methylamine (Equation 21), for in the absence of the metal ion (Fe+2, Co+2, Ni+2) polymeric condensation products are formed (30, 49). 

The family of complexes containing Schiff base ligands derived from a-diketones and /J-mercaptoethylamine [the 2,2 -dialkyl (ethanediylidenedinitrilo) -diethanethiol complexes, structure V, Ni(BE), Ni(PE), and Ni(OE)] are well designed to extend chelate ring-forming ligand reactions to their ultimate by forming complete macrocycles, that completely enclose the metal ion. The objective is shown in Equation 24. 

Zirconium and hafnium tetraalkoxides are highly reactive compounds. They react with water, alcohols, silanols, hydrogen halides, acetyl halides, certain Lewis bases, aryl isocyanates and other metal alkoxides. With chelating hydroxylic compounds HL, such as j8-diketones, carboxylic acids and Schiff bases, they give complexes of the type ML (OR)4 these reactions are discussed in the sections dealing with the chelating ligand. 

Decarboxylation of p-oxoacids. Beta-oxoacids such as oxaloacetic acid and acetoacetic acid are unstable, their decarboxylation being catalyzed by amines, metal ions, and other substances. Catalysis by amines depends upon Schiff base formation,232 while metal ions form chelates in which the metal assists in electron withdrawal to form an enolate anion.233 235... 

CHO group greatly enhances the catalytic activity Since certain metal ions, such as Cu2+ and Al3+, increase the rates in model systems and are known to chelate with Schiff bases of the type formed with PLP, it was concluded that either a metal ion or a proton formed a chelate ring and helped to hold the Schiff base in a planar conformation (Fig. 14-6). However, such a function for metal ions has not been found in PLP-dependent enzymes. 

The complex bis(ethylxanthato)(py) Zn contains zinc in a trigonal plane composed of the pyridine and a pair of sulfur atoms from the xanthate ligands (Zn—N = 2.03 A Zn— S = 2.29 A).873 In contrast, the xanthate ligands in bis(ethylxanthato)(l,10-phen)Cd are bidentate, thus conferring a pseudooctahedral configuration on the metal (Cd—N = 2.38 A Cd—S = 2.64, 2.72 A). Zinc chelates of the stoichiometry ZnL have been isolated (HL = several Schiff bases derived from S-methyldithiocarbazate).875... 

The most important reaction of this type is the formation of imine bonds and Schiff bases. For example, salicylaldehyde and a variety of primary amines undergo reaction to yield the related imines, which can be used as ligands in the formation of metal complexes. However, it is often more desirable to prepare such metal complexes directly by reaction of the amine and the aldehyde in the presence of the metal ion, rather than preform the imine.113 As shown in Scheme 31, imine formation is a reversible process and isolation of the metal complex results from its stability, which in turn controls the equilibrium. It is possible, and quite likely, that prior coordination of the salicylaldehyde to the metal ion results in activation of the carbonyl carbon to amine nucleophilic attack. But it would be impossible for a precoordinated amine to act as a nucleophile and consequently no kinetic template effect could be involved. Numerous macrocyclic chelate systems have been prepared by means of imine bond formation (see Section 61.1.2.1). In mechanistic terms, the whole multistep process could occur without any geometrical influence on the part of the metal ion, which could merely act to stabilize the macrocycle in complex formation. On the other hand,... 

Schiff bases having two nitrogen atoms as donors may be derived either from condensation of dialdehydes and diketones with two molecules of an amine, or from reaction of diamines with aldehydes or ketones. In Section 20.1.2.1, it has been pointed out that coordination through the N atom may occur only under particular circumstances. However, in the case of diimines the formation of chelate rings stabilizes the metal-nitrogen bond. Thus, they can form both mono-41 and bis-chelate42 complexes. 

Mercury, like zinc, appears in some chain-like structures, but apparently only in combination with other metals such as As, Sb, Nb, and Ta. Some of these have the characteristics of metallic conductors.15 Rather high molecular weights have been obtained in the preparation of 6-coordinate cobalt (III) chelate polymers with acetylacetonato and leucinato ligands, 7-coordinate dioxouranium-(VI) dicarboxylate polymers, and 8-coordinate zirconium (IV) polymers with Schiff-base ligands.77... 

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