Dioxygen complexes stability

Formation and stabilities of cobalt dioxygen complexes in aqueous solution. A. E. Martell, Acc. Chem. Res., 1982,15,155-162 (68). 

In abroad sense, the model developed for the cobaloxime(II)-catalyzed reactions seems to be valid also for the autoxidation of the alkyl mercaptan to disulfides in the presence of cobalt(II) phthalocyanine tetra-sodium sulfonate in reverse micelles (142). It was assumed that the rate-determining electron transfer within the catalyst-substrate-dioxygen complex leads to the formation of the final products via the RS and O - radicals. The yield of the disulfide product was higher in water-oil microemulsions prepared from a cationic surfactant than in the presence of an anionic surfactant. This difference is probably due to the stabilization of the monomeric form of the catalyst in the former environment. 

The examples summarized above demonstrate that organometalhc derivatives of early transition metals can and will form dioxygen complexes, even though the stability of these adducts varies widely. The availability of some d-electrons is required i.e., d°-complexes do not show this mode of reactivity, presumably because binding of O2 requires some degree of electron transfer (oxidation of the metal). 

The cyclopentadienyl group, or its substituted derivatives, might be expected to stabilize dioxygen complexes or to ultimately react with dioxygen under the influence of transition metal centers. In rare cases, other (uncharged) dienes can also be found to stabilize dioxygen complexes of rhodium. Also discussed here are some reports of metallaben-zene systems and their reactions with dioxygen. 

The Walsh diagram implies that MXY 12 species should be unstable. However, the levels were derived for the ML4 fragment (the ML5 fragment is very similar). If fewer ligands are coordinated to the metal then MXY 12 complexes are stabilized. This is the case for dioxygen complexes such as (Bu NC)2Ni(02)19 and (PPh,)2Pt(02)20 which contain tf dioxygen. 

Recent reviews on the chemistry of metal-dioxygen complexes with particular relevance to cobalt systems include a number dealing with general properties,636,634 1 binuclear superoxo and peroxo complexes,642,643 reversible oxygenation,644-646 complex stability,647,448 catalytic oxidation649,650 and electronic651 and EPR652 spectral properties. 

In summary, the Coll(bpy)2 /HOOH/(4 1 MeCN/py) system forms a reactive intermediate (20) that selectively ketonizes methylenic carbon, and as such is closely similar to the intermediate of the Fe KPA)2/HOOH/(2 1 py/HOAc) system.36 The ability of FeP(DPAH)2 to activate O2 to an intermediate that has the same unique selectivity for hydrocarbon ketonization is further support for a common stabilized-dioxygen reactive complex (see Chapter 6), Several cobalt-dioxygen complexes exhibit oxygenase reactivity with organic substrates, 0,4l which is consistent with the dioxygen formulation for species 20. 

Increasing steric bulk at tetradentate tripodal ligands prevented dimer formation and stabilized end-on Cu-02 complexes, one of which was crystallographically and spectroscopically characterized (cavity around the copper center was shaped by the ligand (TMG TREN) (Figure 4.13).42 This dramatic result provided additional support for earlier end-on superoxide formulations for the 1 1 copper-dioxygen complexes supported by tripodal ligands (such as TPA or TREN derivatives)... 

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