Metal Complex-Peroxide Systems

Although previous mention has been made of the oxidation of hydrogen peroxide by transition-metal ions, several studies have been involved with the metal-ion-catalysed decomposition 

Halko and J. H. Swinehart, 158th Meeting of the American Chemical Society, New York, 1969, Inorganic Paper 183. 

Jadmicek and K. Vesely, Coll. Czech. Chem. Comm., 1970, 35, 358. 

Similar intermediates are invoked in the catalysis of the reaction between hydrogen peroxide and hydrazine or hydroxylamine,  

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So-called reverse ATRP has been described where a conventional radical initiator (e.g. AIBN) and a transition metal complex in its Higher oxidation state are used. 85"288 One of the first systems explored was ( uBr- 133 AIBN VI VIA. It is important that the initiator is completely consumed early in the polymerization. The use of peroxide initiators in reverse ATRP can be problematical depending on the catalyst used and the reaction temperature.286 289 The system CuBr2/133/BPO/MMA at 60°C was found to provide no control,286 In ATRP at lower temperatures (40 °C), the system CuCl/133/BPO/MMA was successful though dispersities obtained were relatively broadf89 Radicals are produced from the redox reaction between the catalyst in its reduced form and BPO. 

In 1988 Bast and Haenen [201] reported that both LA and DHLA did not affect iron-stimulated microsomal lipid peroxidation. However, Scholich et al. [202] found that DHLA inhibited NADPH-stimulated microsomal lipid peroxidation in the presence of iron-ADP complex. Inhibitory effect was observed only in the presence of a-tocopherol, suggesting that some interaction takes place between these two antioxidants. Stimulatory and inhibitory effects of DHLA have also been shown in other transition metal-stimulated lipid peroxidation systems [203,204]. Later on, the ability of DHLA (but not LA) to react with water-soluble and lipid-soluble peroxyl radicals has been proven [205], But it is possible that the double (stimulatory and inhibitory) effect of DHLA on lipid peroxidation originates from subsequent reactions of the DHLA free radical, capable of participating in new initiating processes. 

Several studies suggest that LA and DHLA form complexes with metals (Mn2+, Cu2+, Zn2+, Cd2+, and Fe2+/Fe3+) [215-218]. However, in detailed study of the interaction of LA and DHLA with iron ions no formation of iron LA complexes was found [217]. As vicinal dithiol, DHLA must undoubtedly form metal complexes. However, the high prooxidant activity of DHLA makes these complexes, especially with transition metals, highly unstable. Indeed, it was found that the Fe2+-DHLA complex is formed only under anerobic conditions and it is rapidly converted into Fe3+ DHLA complex, which in turn decomposed into Fe2+ and LA [217]. Because of this, the Fe3+/DHLA system may initiate the formation of hydroxyl radicals in the presence of hydrogen peroxide through the Fenton reaction. Lodge et al. [218] proposed that the formation of Cu2+ DHLA complex suppressed LDL oxidation. However, these authors also found that this complex is unstable and may be prooxidative due to the intracomplex reduction of Cu2+ ion. 

Other detection methods have been used in optical MIP sensing systems. An MIP-based chemiluminescent flow-through sensor was developed for the detection of 1,10-phenanthroline (Lin and Yamada 2001). A metal complex was used to catalyze the decomposition of hydrogen peroxide and form the superoxide radical ion that can... 

The reactivity of the peroxo metal complexes indicated in Scheme 1 is much higher than that of the peroxidic precursor, either H2O2 or ROOH. This fundamental feature is the direct link between the chemistry of metal catalyzed oxidations and that of peroxo metal complexes These are in fact, in the majority of the systems, well defined species in solutions and in numerous cases they can be isolated and fully characterized also in the solid state. As far as the structure of the active complexes indicated in Scheme 1 is concerned, both in the solid state and/or in solution, several pieces of evidence indicate that most of them share a triangular arrangement of the peroxo moiety around the metal center (see below). In principle, all the /z-peroxo complexes structures, named following... 

Selective oxidation of aromatic amines to nitroso-derivatives with hydrogen peroxide and a catalyst is also studied [68, 69], In kinetic systems with transition metal complexes substrate oxidation is accompanied by H202 dissociation to H20 and 02. Therefore, in this case, the occurrence of chemical induction would be expected. 

The discovery of the ds and d10 metal ir-bonded dioxygen complexes (formally d6- and d8-peroxide systems), and the accompanying chemistry exemplified in Scheme 1, showed the possibility of attaining net oxygen-atom transfer to both inorganic and organic substrates. 

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