Superoxo complexes, with cobalt

The structure of the active component, manganese pyrophosphate, has been reported in the literature (24). It is layer like with planes of octahedrally coordinated Hn ions being separated by planes of pyrophosphate anions (P20y ). Examination of models of this compound gave calculated Hn-Hn thru space distances of 3.26 and 3.45 angstroms, a metal-metal distance close to that found for binuclear dibridged peroxo- and superoxo- complexes of cobalt ( ). 

These complexes can exist in a triangular peroxo form (7a) for early d° transition metals, or in a bridged (7b) or linear (7c) form for Group VIII metals. They can be obtained from the reaction of alkyl hydroperoxides with transition metal complexes (equations 9 and 10),42-46 from the insertion of 02 into a cobalt-carbon bond (equation ll),43 from the alkylation of a platinum-peroxo complex (equation 12),44 or from the reaction of a cobalt-superoxo complex with a substituted phenol (equation 13).45 Some well-characterized alkylperoxo complexes are shown (22-24). 

Many of the structurally characterized -superoxo complexes are cobalt containing, or are iron complexes with sterically hindering porphyrins. Co compounds often react with dioxygen to form mononuclear superoxo complexes. 

The reaction of /r-superoxo complexes with reducing metal ions generally follows an outer sphere mechanism, and kinetic data have been reported for reduction by Fe and cobalt(II) chelates =>, Mo(V) ), [Ru(NH3)6] and... 

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]. 

The chemistry of cobalt dioxygen complexes is replete with nearly every type of reaction of relevance to catalysis. It is the only metal for which all four types of metal dioxygen species have been unambiguously identified [3]. In addition, peroxo and superoxo complexes of cobalt have been prepared both by direct oxygenation and by reactions of cobalt species with peroxide. 

Figure 14.5 (a) Reaction of Al,Al -ethylenebis(3-Bu -salicylideniminato)cobalt(II) with dioxygen and pyridine to form the superoxo complex [Co(3-Bu Salen)2(02)py] the py ligand is almost coplanar with the Co-O-O plane, the angle between the two being 18°.< (b) Reversible formation of the peroxo complex [Ir(C0)Cl(02)(PPh3)2]. The more densely shaded part of the complex is accurately coplanar. ... 

Cobalt(II) porphyrins bind dioxygen as would be expected by analogy with the wealth of cobalt(II) complexes which display this property. The initial addition product is invariably a superoxo-type species, confirmed by ESR, IR and X-ray studies.154 Subsequent reaction of the oxygenated complex with more Co11 porphyrin leads to a peroxo-bridged dimer. No X-ray data are available for cobalt porphyrin peroxo-bridged dimers but the formation of such dimers is well established in cobalt chemistry. 

It has been shown that mononuclear cobalt superoxo complexes react with 2,4,6-tri-t-butyl-phenol. The anion [Co(CN)5(02)]3 acts as a base in the oxidation of the phenol. The neutral complex Co(salptr)(Oz) forms a peroxy adduct with 2,4,6-tri-t-butylphenol. An X-ray study on this adduct has revealed the structure shown in (10).m... 

Superoxo complexes have been characterized by X-ray structural analysis for iron and cobalt only. Dioxygen coordination occurs through a bent end-on mode, with an 0—0 length ranging... 

Nishinaga and co-workers isolated a series of stable cobalt(III)-alkyl peroxide complexes such as (170) and (171) in high yields from the reaction of the pentacoordinated Co"-Schiff base complex with the corresponding phenol and 02 in CH2C12. Complex (170 R=Bu ) has been characterized by an X-ray structure. These alkyl peroxide complexes presumably result from the homolytic addition of the superoxo complex Co111—02 to the phenoxide radical obtained by hydrogen abstraction from the phenolic substrate by the CoUI-superoxo complex. The quinone product results from / -hydride elimination from the alkyl peroxide complex (172)561,56,565,566 The quinol (169) produced by equation (245) has been shown to result from the reduction of the CoIU-alkyl peroxide complex (170) by the solvent alcohol which is transformed into the corresponding carbonyl compound (equation 248).561... 

The complex has a green color with a characteristic absorption peak at 670 nm. ( = 890 M-1 cm.-1). It is paramagnetic (one unpaired electron), and e.p.r. studies indicate the equivalence of the cobalt atoms and delocalization of the odd electron.7 Recent x-ray crystallographic studies 6 have shown that the bridging oxygen atoms are bond angle 118°) and that the 0—0 bond distance is 1.31 A. This is shorter than that normally found in peroxides (1.48 A.) and is close to that found for the superoxide ion in K02. Whereas in the peroxo complex there is a torsion angle of 146° about the O—O bond, the Co—0—O—Co atoms are coplanar in the superoxo complex. 6... 

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. 

Table 53 gives a list of a number of cobalt(III)-superoxo complexes which have been isolated as crystalline solids. There is a marked preponderance of complexes of the type [Co(SB)(B)(02)] (SB = SchifF base). The base adducts of simple Co11 porphyrins have low affinities for dioxygen at room temperature and consequently their 1 1 adducts with 02 are not isolable. In contrast, exposure of a solid sample of Collman s picket fence porphyrin system [Co(TpivPP)(iV-Meim)] to 1 atm of dioxygen for 24 h produces [Co(TpivPP)(A(-Meim)(02)] (200).654 The pivalamido pickets in this compound exercise control of solvation about the coordinated dioxygen moiety and the stability is comparable with that of CoMb02,655 where the globin protein environment performs the same function. 

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