Peroxides peroxo 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. 

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. 

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. 

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. 

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]. 

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... 

Actinide(V) and (VI) ions form soluble complex ions with peroxide ion in slightly alkaline medium, whereas actinide(III) and (IV) ions precipitate as hydroxides. Actinide(VI) ions in slightly alkaline hydrogen peroxide solution precipitate upon addition of cobalt(III) complex salts. Figure 7 shows the precipitation behavior of U(VI) peroxo complex ion with the following kinds of cobalt(III) complex salts ... 

The reaction of cobalt(III) complexes with H2O2, the reverse of reaction D, is also known reaction of cw[Co(en)2(H20)2] with H2O2 gives [Co(en)2(OOH)(OH2)] at pH 4, and on increasing the pH the /t-hydroxo- t-peroxo complex [(en)2Co(u-02)Cu-OH)Co(en)2] " is formed The reverse of this mechanism would seem to be the most probable mechanism for the formation of H2O2 fi"om a /t-peroxo complex. It is perhaps significant that there are no reports of formation of hydrogen peroxide from the /<-amido jU-peroxo complexes where dissociation in acid solution via reaction 12 does not occur. 

Two-electron reduction of dioxygen into coordinated peroxide can be easily performed by two metal centers undergoing concomitant one-electron oxidations, as shown in Equation 4.4 (Section 4.2.2). A variety of transition metal ions (cobalt, nickel, iron, manganese, copper, etc.) can form dinuclear peroxides. These complexes differ in structure (cA-p-1,2-peroxides, trans- l- 1,2-peroxides, p-r 2 r 2-peroxides), in stability and subsequent reactivity modes, and in the protonation state of the peroxo ligands (Figure 4.3). In certain cases, dinuclear p-r 2 r 2-peroxides and bis-p-oxo diamond core complexes interconvert, as discussed below for copper-dioxygen adducts. 

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. 

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