Superoxo complexes mononuclear

Coordination of dioxygen can occur in four ways, two of which may be considered to correspond to one-electron reduction (superoxo complexes) and the others to two-electron reduction (peroxo complexes). The four possible coordination modes are illustrated in Figure 2. As can be seen, each type of complex may be further categorized as to whether it is a mono- or bi-nuclear complex. Whether a peroxo- or a superoxo-type complex is formed is dependent upon the metal involved. For mononuclear complexes, superoxo complexes would be expected with metals which readily undergo one-electron oxidation, e.g. Fe11 or Co", while peroxo complexes would be expected for metals with a preference for two-electron oxidation, e.g. Ir1, Rh1, Pt°. 

The earliest known dioxygen complex contained [(NH3)10Co2O2]4+, first described by Werner and Myelius in 1893.94 A surge of interest in dioxygen complexes arose after the Co11 chelate of bis(salicylaldehyde)ethylenediimine was shown to take up oxygen reversibly by Tsumaki in 1938.95 It is only recently, however, that the existence of mononuclear superoxo complexes has been established and structural studies performed upon them. A vast array of Co chelates based on Schiff bases such as (4) and (5) have been synthesized. 

IR studies on mononuclear superoxo complexes reveal a range of 1075-1220 cm-1 for O—O stretching (Table 5). This correlates well with r0 0 for K02 and Na02 which are 1145 and 1142 cm-1 respectively. 

Although there are no binuclear superoxo complexes formed simply by the interaction of a metal complex and dioxygen, they are worthy of discussion here if only to complement mononuclear... 

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... 

A reaction specific to mononuclear cobalt superoxo complexes is free radical abstraction. Much... 

After isolation of the microcrystalline precipitate which is the peroxo complex 25, there remains a reddish brown filtrate which contains further peroxo complex. Acidification of the latter with hydrochloric acid produces, after a few hours, the superoxo complex [(l)CO-02-Co(l)](Cl)5 (26) in the form of green microcrystals. As has already been mentioned, mononuclear chloro complex may finally be isolated from the red mother liquor of this preparation. If 25 is treated with a measured quantity of aqueous hydrochloric acid, the superoxo... 

The pH-dependent reactivity of 25 and 26 resembles that of related complexes with other polyamine ligands (55) and may be summarized as follows The peroxo complexes may be transformed into the dinuclear superoxo complexes plus a mononuclear halo species by addition of a suitable acid. In aqueous alkaline solution (pH > 12), the peroxo complexes are stable over several hours, whereas the superoxo complexes will react immediately even with dilute aqueous ammonia to reform the peroxo complexes. 

Table 54 Structures Mononuclear Cobalt(IJI)-Superoxo Complexes...

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. 

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 reaction of iodide with [(en)2Co( r-OH)( -02)Co(en)2] in acid solution gives [Co(en)2(OH2)2] and iodine. The rate has been shown to be the same as that of dissociation in acid solution (reaction 12) and the initial step is hydrolysis of the /i-hydroxo group followed by I" attack on the monobridged species. The monobridged complex [(H3N)5Co(02)Co(NH3)5] produced by iodide reduction of the superoxo complex does not react with iodide but decomposes to Co(II) and ammonia The reduction of the mononuclear -peroxo complex [(H3N)5Co(02)Co(NH3)s] by or... 

The appealing goal of binding dioxygen reversibly inspired numerous studies on mononuclear T 1 superoxo complexes that resemble 02 coordination mode in natural... 

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