Peroxo-cobalt complex

Figure 26.2 O2 bridges in dinuclear cobalt complexes (a) peroxo (02 ) bridge, and (b) superoxo (02 ) bridge.

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. 

CoCl(PPh3)3], Reaction of [Co(TIMEN )]Cl 9 with oxygen in the presence of NaBPh leads to the formation of the peroxo-complex [Co(r -02)(TIMEN 5 )] BPh 10, which is a rare example of a side-on r -peroxo cobalt complex (the majority of Co-O -adducts are rj -O -complexes, i.e. end-on). The authors also showed that 10 is capable of converting molecular oxygen to benzoylchloride. 

DR. DAVID STANBURY (Rice University) With regard to the problem of the electrocatalytic reduction of oxygen, I have attempted to formalize some ideas regarding the constraints of thermodynamics in order to elucidate the probable character of cobalt complexes which may catalyze the oxygen electrode via binuclear peroxo-bridged intermediates. The following gross mechanism is presupposed ... 

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. 

Reversible oxygen binding has also been examined using poly(ethyleneimine)-cobalt complexes in aqueous solution. Cobalt(II) complexes of linear and branched poly(ethyleneimine) in aqueous solution are able to form with oxygen a p-peroxo adduct as is evidenced from stoichiometry and spectral properties 107). 

Table 55 Water Soluble Cobalt(III) Peroxo and Superoxo Complexes... 

Table 57 Structures Monomeric and Dimeric Cobalt(III)-Peroxo Complexes...

Apart from the unambiguous and useful classification of the dinuclear cobalt complexes as /i-peroxo or /i-superoxo species, we shall not use formal oxidation states frequently in this review, and we limit the discussion here to the following points ... 

Recent work by Sykes (33) deals with the kinetics of the reduction of the complex (p-2) by ferrous ion in aqueous solution, and gives some evidence for the protonation of the peroxo bridge in acid solution. Other kinetic work, dealing with the oxidation of Co (III) perchlorate by H2O2, has been interpreted in terms of a dimeric cobalt complex species bridged by oxygen (I). 

Another type of polynuclear cobalt complex is produced by aerial oxidation of ammoniacal Co(N03)2 solution. This is the / -peroxo-bis penta-amminecobalt(III) ion, [(NH3)5Co-0 0-Co(NH3)5] +. Further oxidation gives the ion [(NH3)5Co-0-0-Co(NH3)5] + which Ebsworth and Weil have shown, by electron-spin resonance, to contain cobalt atoms of identical oxidation number, not Co and Co as previously thought. 

While peroxo dimer formation is sometimes reversible for cobalt complexes, it inevitably leads to further oxidations in case of iron complexes. Steric hindrance about the metal center would prevent dimerization processes. 

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 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. 

Figure 3. Bonding orbitals in a dinuclear cobalt M-peroxo complex.

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