Metal-dioxygen complexes, with cobalt

Recent reviews on the chemistry of metal-dioxygen complexes with particular relevance to cobalt systems include a number dealing with general properties,636,634 1 binuclear superoxo and peroxo complexes,642,643 reversible oxygenation,644-646 complex stability,647,448 catalytic oxidation649,650 and electronic651 and EPR652 spectral properties. 

The use of cobalt and manganese carboxylates to initiate the oxidation of a large number of olefins such as the butenes [447, 448], propylene [449], oleic [450] linoleic [451], and stearic [452, 453] acids or their derivatives and a-methylstyrene [454, 455] is well known. The kinetics of oxidation of a-methylstyrene in the presence of cobaltous and manganous acetylacetonates as well as copper phthalocy-anine have been investigated [454, 455]. The results of this study led Kamiya to postulate a mechanism involving formation of radical species by a metal dioxygen complex, equation (270), concurrent with radical generation by hydroperoxide decomposition. 

This type of complex is derived from the mononuclear superoxo species via a further one-electron reduction of the dioxygen moiety. Cobalt is the only metal to form these complexes by reaction with dioxygen in the absence of a ligating porphyrin ring. Molybdenum and zirconium form peroxo-bridged complexes on reaction with hydrogen peroxide. In most cases the mononuclear dioxygen adducts of cobalt will react further to form the binuclear species unless specific steps are taken to prevent this. 

A number of transition metals are now known147-156 to form stable dioxygen complexes, and many of these reactions are reversible. In the case of cobalt, numerous complexes have been shown to combine oxygen reversibly.157 158 Since cobalt compounds are also the most common catalysts for autoxidations, cobalt-oxygen complexes have often been implicated in chain initiation of liquid phase autoxidations. However, there is no unequivocal evidence for chain initiation of autoxidations via an oxygen activation mechanism. Theories are based on kinetic evidence alone, and many authors have failed to appreciate that conventional procedures for purifying substrate do not remove the last traces of alkyl hydroperoxides from many hydrocarbons. It is usually these trace amounts of alkyl hydroperoxide that are responsible for chain initiation during catalytic reaction with metal complexes. 

The less favorable entropies of the polyamine ligand reactions are attributed to the greater loss of rotational freedom about the C N bonds of the aliphatic bases upon metal-ion coordination. It was observed that the coordinated polyamine of a cobalt dioxygen complex undergoes oxidative dehydrogenation under anaerobic conditions to form an imine with the double bond conjugated to the heterocyclic ligand.32... 

From the viewpoints of reaction mechanism and efficiency in organic synthesis, oxidation of phenols with dioxygen catalyzed by cobalt-, manganese- and related metal-amine complexes has been studied . In particular, much effort has been directed toward constructing new efficient catalysts by a combination of metals with... 

The mechanism of metal phthalocyanine catalysed oxidation by molecular oxygen -isobutyraldehyde system is not established at this stage. The iron[14], manganese[15] and cobalt tetrasulphonato-[16] phthalocyanines are known to form superoxo complexes with dioxygen and are known to catalyse autoxidation reactions[13]- The acyl radical formation thus can be initiated by interaction of metal phthalocyanine-dioxygen superoxo complex with isobutyraldehyde. The acyl radical in presence of oxygen can yield acylperoxy radical or peracid as the oxidising speceis[17]. 

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