Transition metal salts/complexes inhibition

Other miscellaneous compounds that have been used as inhibitors are sulfur and certain sulfur compounds (qv), picryUiydrazyl derivatives, carbon black, and a number of soluble transition-metal salts (151). Both inhibition and acceleration have been reported for styrene polymerized in the presence of oxygen. The complexity of this system has been clearly demonstrated (152). The key reaction is the alternating copolymerization of styrene with oxygen to produce a polyperoxide, which at above 100°C decomposes to initiating alkoxy radicals. Therefore, depending on the temperature, oxygen can inhibit or accelerate the rate of polymerization. 

It is well known that transition-metal salts and metal complexes, unlike non-Werner-type ferrocene compounds, act as inhibitors in the polymerization of vinyl monomers. For example, the radical polymerization of vinylpyridine is strongly inhibited in the presence of Cu(II) or Fe(III)32 However, vinylpyridine with Cu(I)... 

Another situation is observed when salts or transition metal complexes are added to an alcohol (primary or secondary) or alkylamine subjected to oxidation in this case, a prolonged retardation of the initiated oxidation occurs, owing to repeated chain termination. This was discovered for the first time in the study of cyclohexanol oxidation in the presence of copper salt [49]. Copper and manganese ions also exert an inhibiting effect on the initiated oxidation of 1,2-cyclohexadiene [12], aliphatic amines [19], and 1,2-disubstituted ethenes [13]. This is accounted for, first, by the dual redox nature of the peroxyl radicals H02, >C(0H)02 and >C(NHR)02 , and, second, for the ability of ions and complexes of transition metals to accept and release an electron when they are in an higher- and lower-valence state. 

Thus, the mechanism of MT antioxidant activity might be connected with the possible antioxidant effect of zinc. Zinc is a nontransition metal and therefore, its participation in redox processes is not really expected. The simplest mechanism of zinc antioxidant activity is the competition with transition metal ions capable of initiating free radical-mediated processes. For example, it has recently been shown [342] that zinc inhibited copper- and iron-initiated liposomal peroxidation but had no effect on peroxidative processes initiated by free radicals and peroxynitrite. These findings contradict the earlier results obtained by Coassin et al. [343] who found no inhibitory effects of zinc on microsomal lipid peroxidation in contrast to the inhibitory effects of manganese and cobalt. Yeomans et al. [344] showed that the zinc-histidine complex is able to inhibit copper-induced LDL oxidation, but the antioxidant effect of this complex obviously depended on histidine and not zinc because zinc sulfate was ineffective. We proposed another mode of possible antioxidant effect of zinc [345], It has been found that Zn and Mg aspartates inhibited oxygen radical production by xanthine oxidase, NADPH oxidase, and human blood leukocytes. The antioxidant effect of these salts supposedly was a consequence of the acceleration of spontaneous superoxide dismutation due to increasing medium acidity. 

The synthesis of polyhalide salts, R4NX , used in electrophilic substitution reactions, are described in Chapter 2 and H-bonded complexed salts with the free acid, R4NHX2, which are used for example in acid-catalysed cleavage reactions and in electrophilic addition reactions with alkenes, are often produced in situ [33], although the fluorides are obtained by modification of method I.I.I.B. [19, 34], The in situ formation of such salts can inhibit normal nucleophilic reactions [35, 36]. Quaternary ammonium chlorometallates have been synthesized from quaternary ammonium chlorides and transition metal chlorides, such as IrClj and PtCl4, and are highly efficient catalysts for phase-transfer reactions and for metal complex promoted reactions [37]. 

Commonly utilized starting transition metal materials are oxides, halides, salts of fatty acids, complexes containing sulfur, etc. The role of the reducing agents is to lower the oxidation state of a metal and not to reduce the compound to an active metallic phase because this would, in most cases, lead to the inhibition of the reaction as shown by thermodynamic calculations. This point is also indicated by the formation of carbonyl derivatives possessing CO and other coordinating ligands as intermediates. 

The Ce + ion is one of the most active catalysts for peptide hydrolysis. Its activity is much higher than that of the trivalent lanthanide ions and other transition metal ions. In particular, Ce + is far superior to other tetravalent ions like Zr" or Hf +. Yashiro et al. (1994) reported that dipeptides and tripeptides were efficiently hydrolyzed under neutral conditions by the y-cyclodextrin complex of cerium(IV). Komiyama and coworkers (Takarada et al., 2000) studied the catalytic hydrolysis of oligopeptides by cerium(IV) salts. The hydrolysis is fast, especially when the oligopeptides contain no metal-coordinating side-chains. The hydrolysis rates of the dipeptides, tripeptides and tetrapeptides is similar. The hydrolysis reaction was performed at pH 7 and 50 °C and under these conditions, the half-life of the amide bond was only a few hours. The authors found that ammonium hexanitratocerate(IV) is more active than other cerium(IV) compounds like ammonium cerium(IV) sulfate, cerium(IV) sulfate and cerium(IV) hydroxide. The lower reactivity of ammonium cerium(IV) sulfate is ascribed to the competitive inhibition by sulfate ions, while the low reactivity of cerium(IV) sulfate and cerium(IV) hydroxide can be explained by their poor solubility in water. However, in the reaction mixtures at the given reaction conditions, most of the cerium(IV) consists in a gel of cerium(IV) hydroxides. No oxidative cleavage has been observed. 

Inorganic and organic alternate valence metal salts and their complexes inhibit O2 absorption and mass loss in PAl at heating in the presence of O2 at high temperature (for example, Figure 9). Under these conditions, non-transition metal compounds are inert. The effect of 0.05 - 1.0 wt.% additions of transition metal compounds on PAl thermal oxidation is... 

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