Metal-dioxygen adducts

Concerted One-Electron Reductions. Reduction of 02 in the presence of excess zinc cation [Znu(bpy)2+], (tetraphenylporphinato)iron(III) ion [(H20)+FemTPP], and cuprous ion [Cu MeCN)/] results in formation of metal-dioxygen adducts. Figures 9.13 and 9.14 illustrate the cyclic voltam-mograms for (FemTPP)Cl and tCuI(MeCN)4]C104, respectively, in the absence and presence of 02. Reaction schemes for the three metal-02 systems are outlined ... 

The above scheme does not involve direct oxygen transfer from the metal-dioxygen adduct to the substrate. Trace protic impurities may make this mechanism operative in nominally aprotic solvents such as benzene [6,7,9]. The oxidation of PPh catalyzed by Ru(NCS)(NO)(PPh ) ... 

Most of the metal-dioxygen adducts previously discussed have limited stability and therefore serve as the starting point in our discussion of reactive intermediates. Although many of the complexes are difficult to work with under ambient... 

A full discussion of the reactions of molecular oxygen involved in dioxygen adducts would require a separate review. Moreover, the reactions of co-ordinated molecular oxygen are discussed elsewhere l-8) dioxygen adducts of biological systems (7) and synthetic group VIII metal complexes 1,3) being of particular interest. 

Type 1. Bridging adducts (see Fig. 2), where the dioxygen ligand is bridged between two metal atoms intermolecularly. Here, the metal dioxygen ratio is invariably 2 1. 

Further to section III(C), we shall see that type I dioxygen complexes can be formed in solution via intermediates of a type 11(F) structure. Consequently, sections IV( ) and IV(F) will tend to overlap in places. The dioxygen adducts of some new synthetic metal porphyrins belong to the 11(F) classification. However, a discussion of the dioxygen adducts of naturally occurring and synthetic metal porphyrins is postponed until section V. 

A large number of type 1 dioxygen adducts are reported in the literature (25, 76, 80). The vast majority of metal complexes which take up dioxygen to form type I adducts contain cobalt(ll) as the metal ion. Ochiai (81) considered the hypothetical reactions ... 

Much work has been devoted to the study of Schiff base complexes, in particular M(salen), where M = metal, has been the subject of extensive work 114). The early work by Calvin et al. (section 111(A)) suggested that the 2 1 (M O2) dioxygen adduct, type I, formed by Co(salen) in the solid state, contains a peroxo linkage. An X-ray analysis 115, 116) of the complex (Co Salen)202(DMF)2 supports this hypothesis see Fig. 5 for the pertinent results of this study. 

We saw previously that a major factor in inhibiting the bimolecular termination reaction was the presence of sufficiently bulky ligands so that a monomeric dioxygen adduct could be isolated 135). A number of synthetic metal porphyrins 239) have been prepared recently which satisfy the above requirement, and bind molecular oxygen we shall now proceed to discuss these. 

The reactions of metal bpy ions with oxygen have been studied (193). The bis(bpy) complex ions, [M(bpy)2]2+, of Cr, Ru, and Os formed the dioxygen adducts, whereas the similar complex ions of Mn, Fe, Co, Ni, and Cu were unreactive. The CID of the [M(bpy)202]2+ ions exhibit interesting differences. 

The reaction of dioxygen with transition metal complexes has stimulated the curiosity of scientists for decades. Seminal studies by Vaska in the 1960s revealed that dioxygen coordinates reversibly to the the Ir center in (Ph3P)2lr(CO)Cl, 14 (Eq. 15) [101-104], Following this report, numerous groups reported dioxygen adducts of other late transition metals [105],... 

Once the oxidative-addition reaction of dioxygen to metal d -ions has occurred, the essentially electrophihc dioxygen becomes a nucleophilic peroxide ligand. Since the oxidation of substrates is associated with electron transfer from the substrate to the oxidant, i.e. in this case the dioxygen adduct, effective oxygenations require a further activation to transform the nucleophihc peroxide into an electrophihc species prior to the oxygen transfer. 

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