P-Peroxo complexes

I know of no definitive evidence concerning the formation of a p-peroxo complex between a3 and CuB, and if the analogy between cytochrome oxidase and the other heme proteins, I have referred to, holds then I see no need to suggest such peroxo-bridging. 

Formation of metastable p-peroxo complexes (also adduct formation, nucleophilic substitution, radical coupling, etc.)... 

DR. HENRY TAUBE (Stanford University) Another interesting point about Collman s complex should be noted. Let us presume that a peroxide intermediate is generated in his case. In all of Dr. Endicott s examples, the remarkable observation is that it is very difficult to reduce the 0-0 bond kinetically in the binuclear p-peroxo complexes. In Collman s case, this reduction... 

All three reactions shown below (p-peroxo complex in deficiency)... 

A common rate-determining process must attend each reaction and involve only the p-peroxo complex. The products arising with edta suggest fragmentation (->2Co(en)2(NH3)H20 + -I- O2) is followed by more rapid reactions. 

Electrophilic Attack on Dioxygen. The nucleophilic character noted for coordinated dioxygen in mononuclear complexes is also found for rj - dioxygen in the peroxo complexes, but there is no evidence for nucleophilic character of 7 j dioxygen in the superoxo complexes. The best known example is the protonation of the /i-amido p-peroxo complexes [(NH3)4Co( -NH2)(u-02)Co(NH3)4] ° and [(en)2Co( -NH2)(M-02)Co(en)2] (Fig. 16 reaction B). 

The bound dioxygen can now coordinate to a second heme, forming a p.-peroxo complex ... 

Essentially no incorporation of deuterium label was observed when the same reaction was carried out in toluene-dg. Since the formation of more than one equivalent of CO(n) hydroxide requires the transfer of hydrogen atoms, we hypothesized that the mechanism of this transformation involved the formation of a dinuclear p-peroxo complex, followed by rapid cleavage of the 0-0 bond and hydrogen atom abstraction from the Tp-ligand. The inferred activation of the primary C-H bonds of the rm-butyl groups of the ligand requires a highly reactive species thus the "activation" of O2 seems to have been accomplished. 

This site isolation suppresses the formation of inactive dioxygen bridged p,-peroxo complexes. 

As an example of the first mechanism, the 2 1 p-peroxo complex Co202(OH)(bipy)43+ (bipy = 2,2 -bipyridyl) was shown to degrade in aqueous solutions to a Co(III) species and H2O2 with the relatively oxidation resistant bipyridyl ligand remaining unchanged.il... 

The most extensive kinetic studies on the formation of p-peroxo complexes have been performed on cobalt(II) derivatives. Stopped-flow or ordinary UV-VIS spectrophotometry is usually the method of choice for rate measurements. Generally, the kinetics of formation of p-peroxodicobaltClII) species is consistent with the 2-step reversible scheme of equations (27) and (28). 

The decomposition of doubly bridged p-peroxo complexes has been found to be proton catalyzed [349,350]. The deoxygenation of 2- 3+... 

A crucial point in the above scheme is the intermediacy of the p-peroxo complex 11. Its formation is supported by kinetic evidence [66,67], but it can also be observed as a transient at -80 °C [63]. 

Another group of cobalt(11) complexes that show catalytic activity in 2,6-DTBP oxidation to the corresponding BQ and DPQ has been described by Bedell and Martell [56]. They are 1 1 Co(II) polyamine chelates (with the ligands listed below), some of which form monobridged p-peroxo complexes with dioxygen, whereas others afford dibridged (p-peroxo)(p-hydroxo) derivatives with the ligands listed in Table II. 

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