Cobalt complexes, peroxidation

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

The hexamine cobalt (II) complex is used as a coordinative catalyst, which can coordinate NO to form a nitrosyl ammine cobalt complex, and O2 to form a u -peroxo binuclear bridge complex with an oxidability equal to hydrogen peroxide, thus catalyze oxidation of NO by O2 in ammoniac aqueous solution. Experimental results under typical coal combusted flue gas treatment conditions on a laboratory packed absorber- regenerator setup show a NO removal of more than 85% can be maitained constant. 

Electrochemical (24) and chemical (25, 26) techniques have been utilized to investigate the kinetics and the mechanisms of the addition of dioxygen to a metal center, and to follow its subsequent reduction to hydrogen peroxide when catalyzed by cobalt(III) complexes of macro-cyclic amine ligands. Such complexes have also been involved in the general investigation of dioxygen addition to cobalt complexes (27,28). 

The [Con(bipy)2 ]2+ species has also been reported to activate hydrogen peroxide and ter -butyl hydroperoxide for the selective ketonization of methylenic carbons, the oxidation of alcohols and aldehydes, and the dioxygenation of aryl olefins and acetylenes (36). Later reports (37), however, while confirming that the cobalt complexes did indeed cata-... 

A cobalt complex of transferrin has been prepared by addition of Co(II) citrate to the apoprotein. Hydrogen peroxide was added to obtain the absorption spectrum of cobalt transferrin, and susceptibility measurements showed that the metal ion was incorporated as diamagnetic Co (III) (142). 

Unless otherwise mentioned in the text, all scouting for CCT reported here was carried out at 60 °C in MMA or in a methanol solution of MMA if LCo was not directly soluble in monomer. AIBN was used as the azo initiator because peroxides often decompose or poison the cobalt complexes. 

However, the similarity in bond strengths of the peroxide linkage to molecular 02, the ease with which the known -peroxo Cobalt complexes liberate 02 (in contrast to /x-oxo bipyridyl Mn dimers) on photolysis, kinetic barriers on ju-oxo to peroxo dimer conversions led Sawyer et al.47 -49) to suggest peroxo binuclear complexes as the most probable intermediates. More studies with model compounds are needed to elucidate this point. Various mechanisms proposed for water oxidations are variations of these two principal types. 

Even though most of the work reported has involved planar cobalt complexes nonplanar macrocylics also catalyze the reaction. For example, nonplanar cobalt tetrakis-(4-sulfonatophenyl)/ -octabromoporphyrin catalyzed the reduction of O2 via two electrons to give peroxide . Vitamin B12, which is also nonplanar, and its structure resembles that of a porphyrin (see Figure 2.4.), catalyzes the reduction of O2 via two electrons to give peroxide at low potentials and via four electrons to give water at higher overpotentials... 

With the aim of finding applications in polymer electrolyte fuel cells (PEFC) Co tetramethoxyphenylporphyrin (CoTMPP) and cobalt tetraazaannulene (Co-TAA) were tested for O2 reduction in acid media (IM H2SO4) with the catalysts bound with Nafion and deposited on a glassy carbon disk and rotating-disk measurements revealed high peroxide yields, as expected for cobalt complexes. 

The T) t) dinuclear complexes may be of the /Li-peroxo or p-superoxo type [26], and are very widespread among cobalt complexes. ESR evidence indicates that the unpaired electron is localized on the dioxygen moiety in the superoxo species [165]. The 0-0 bond distances are close to those observed in free peroxide and superoxide. 

Decomposition of hydroperoxide 20 in the presence of the cobalt complexes used and Mn(TPP) explains the formation of the quinol 15, and peroxides 29 and 30. During this decomposition, transient free radicals are also formed, which may in turn oxidize the phenol to the phenoxy radical. The role of 20 and its decomposition suggests that adventitious hydroperoxides in certain solvents may in principle also be involved. 

Methyl ethyl ketone peroxide (MEKP, MED, and peroxide) n. A complex peroxide mixture made by reacting hydrogen peroxide with MED, with the approximate formula (CH3C00C2H5)3. MEKP is an initiator for free-radical polymerization and a curing agent for polyester resins. In combination with an accelerator such as cobalt naphthenate, MEKP can bring about cure at room temperature. Because... 

Most mononuclear Co macrocyclics catalyze the reduction of dioxygen via two electrons to give peroxide [28, 82, 97, 99, 123]. The activity of Fe phthalocyanines in general is higher than those of Co phthalocyanines and the opposite is true for porphyrins, which reveals the importance of the nature of the ligand in determining the catalytic activity [96]. The opposite is true for heat-treated materials [133]. Cobalt complexes are more stable than iron complexes and this trend is maintained after heat treatment [72, 105]. However, iron complexes tend to promote the four-electron reduction of dioxygen and this will be discussed further on. 

A similar reaction occurs with low valent cobalt complexes with diarsines and tetra-arsines [38]. For example, 1 1 peroxo complexes can be prepared by reaction of cobalt(I) arsine complexes with molecular oxygen, equation (6) [38]. The same type of complex is formed by reaction of a Co(III) arsine complex with hydrogen peroxide [38]. Treatment of a Co(II) arsine complex with dioxygen, however, leads to the formation of a /x-peroxo species, equation (6) [38]. 

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