Peroxo complexes mononuclear

Dioxygen and its ions can bind in mononuclear and dinuclear structures in a number of ways,962 as illustrated in Scheme 1. The typical reaction of dioxygen with Co compounds involves a number of these binding forms, outlined in Scheme 2. Mononuclear Co111—peroxo complexes are relatively rare, but yellow trigonal bipyramidal complexes [Co(02)L2]+ (L = chelating phosphines dppe or dppp) have been characterized structurally where the 022 is bonded to the Co in the side-on r]2 form (Co—O 1.858(7) 1.881(4) A), with O—O stretching frequencies ( 870 cm-1) consistent with Coin-peroxo speciation.963... 

A substantial number of Mo peroxo complexes has been structurally characterized. These complexes all have nearly symmetrically side-on-bound peroxide. Among them are mononuclear, dinuclear, tetranuclear and heptanuclear compounds. The tetranuclear and hep-tanuclear complexes are related to the isopoly molybdates. Structural studies have identified... 

Mononuclear peroxo compounds of known crystal structure are listed in Table 6. Except for Mo(02)4 and Mo(02)2(ttp), all structurally characterized mononuclear peroxo complexes have one Mo—Ot in addition to one or two peroxo ligands. The Mo—O, bond lengths fall in the range 1.63 to 1.73 A. The Mo—O (peroxo) distances span from 1.83 to 1.96 A. The peroxo ligand is always found cis to the oxo group and two peroxo ligands are always found cis to each... 

Table 6 Structural Details for Mononuclear Peroxo Complexes...

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°. 

Some of the early transition metals are known to form mononuclear peroxo complexes. These complexes are not formed by reacting a metal complex in a low oxidation state with dioxygen, as this usually results in the formation of metal oxo species, but rather by reaction of a metal complex in a high oxidation state, eg. TiIV, Nbv or MoVI, with the peroxide anion. This frequently leads to more than one peroxide ligand per metal centre. 

Mononuclear peroxo complexes, such as MoO(02)2(HMPT), show a more electrophilic character in their reactions. This particular complex may be used for the epoxidation of alkenes.176 Much interest has been shown in the reaction of small molecules with dioxygen complexes of this type with a view to catalytic oxidation. These reactions and their products are sumarized in Table 10. The... 

The possibility of formation of an O—O bond at a single Mn site of the OEC, on the other hand, seems remote, but nevertheless, two mononuclear model Mn-peroxo complexes, a peroxo-Mnm porphinato 64 [157] and a peroxo-Mn111 pyrazolylborato complex 65 [158], have been structurally characterized. Both complexes contain side-on peroxo groups. Complex 65 has been crystallized in two forms, one brown and one blue. A peroxo-H—N (pyrazole) hydrogen bond is found only in the blue form (Figure 29). 

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. 

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... 

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