Chelating agents, transition metal

Metal deactivators chelate the transition metal ions, which otherwise would catalyze the oxidation reaction. Some effective chelating agents for latexes are ethylenediaminetetraacetic acid and its salts, and citric acid. A list of commercial antioxidants is given in ref. 115. 

Pedersen on the effect of various chelating agents upon the autoxidation of petroleum (a useful model system) catalysed by various transition metals. The deactivating efficiency is the percentage restoration of the original induction period observed with a control sample of petroleum without metal or deactivator. Only iV" -tetrasalicylidenetetra(aminomethyl)methane was effective... 

Retard efficiently oxidation of polymers catalysed by metal impurities. Function by chelation. Effective metal deactivators are complexing agents which have the ability to co-ordinate the vacant orbitals of transition metal ions to their maximum co-ordination number and thus inhibit co-ordination of hydroperoxides to metal ions. Main use of stabilisation against metal-catalysed oxidation is in wire and cable applications where hydrocarbon materials are in contact with metallic compounds, e.g. copper. 

If one end of a chelate ring on an octahedral complex is detached from the metal, the five-coordinate transition state can be considered as a fluxional molecule in which there is some interchange of positions. When the chelate ring reforms, it may be with a different orientation that could lead to racemization. If the chelate ring is not symmetrical (such as 1,2-diaminopropane rather than ethyl-enediamine), isomerization may also result. For reactions carried out in solvents that coordinate well, a solvent molecule may attach to the metal where one end of the chelating agent vacated. Reactions of this type are similar to those in which dissociation and substitution occur. 

Bray KL (2001) High Pressure Probes of Electronic Structure and Luminescence Properties of Transition Metal and Lanthanide Systems. 213 1-94 Briicher E (2002) Kinetic Stabilities of Gadolinium(III) Chelates Used as MRI Contrast Agents. 221 103- 122... 

Chelex 100" is another example of a commercially available supported chelating agent. The material was available from Bio-Rad and was used to purify compormds, most notably transition metal ions. [1] The preference for transition metal ions, e.g., copper (II) or iron(II) ions, over such univalent metal ions as sodium or potassium is said to be about 5000 to 1 [1]. The material consists of a st3rrene-divlnylbenzene copol3rmer to which is attached iminodiacetic acid moieties. 

Phenolic acids and flavonoids can also act as chelating agents, complexing transition metals that are responsible of the initiation of peroxidative processes (Fenton and Haber-Weiss reactions). This property is much stronger in phenolics having a catechol, pyrogallol, or 3-hydroxy-4-carbonyl group [130]. 

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