Transition metal oxides dioxygen adducts

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). 

As mentioned above, catalytic oxidation of olefins via coordination catalysis with an intermediate such as LnM (olefin) 02 seemed an attractive possibility, and Collman s group (45) tentatively invoked such catalysis in the 02-oxidation of cyclohexene to mainly 2-cyclo-hexene-1-one promoted by IrI(CO)(PPh3)2, a complex known to form a dioxygen adduct. Soon afterwards (4, 46, 47) such oxidations involving d8 systems generally were shown to exhibit the characteristics of a radical chain process, initiated by decomposition of hydroperoxides via a Haber-Weiss mechanism, for example Reactions 10 and 11. Such oxidations catalyzed by transition-metal salts such as... 

Two-electron reduction of dioxygen into coordinated peroxide can be easily performed by two metal centers undergoing concomitant one-electron oxidations, as shown in Equation 4.4 (Section 4.2.2). A variety of transition metal ions (cobalt, nickel, iron, manganese, copper, etc.) can form dinuclear peroxides. These complexes differ in structure (cA-p-1,2-peroxides, trans- l- 1,2-peroxides, p-r 2 r 2-peroxides), in stability and subsequent reactivity modes, and in the protonation state of the peroxo ligands (Figure 4.3). In certain cases, dinuclear p-r 2 r 2-peroxides and bis-p-oxo diamond core complexes interconvert, as discussed below for copper-dioxygen adducts. 

Apart from dioxygen, free radicals may also be oxidized by other oxidants, such as transition metal ions in higher valence states, mostly in complexed form [cf. reaction (7)] [34], or nitro compounds (see for instance [72, 73]). These reactions do not necessarily proceed via direct electron transfer but apparently mostly through the intermediacy of an adduct. The finer details remain to be elucidated. 

Molecular oxygen adducts of transition metal complexes arc of interest and importance to catalytic processes and commercial oxidation processes, as well as being intermediates in oxidation reactions. Vaska " has reviewed the nature of dioxygen bound to transition metal complexes. All known iridium dioxygen complexes possess the peroxo structure (140). Experimental data reveal that the formation of covalent Ir—(O2) bonds on dioxygen addition to IrL, is accompanied by extensive redistribution of electrons, and the electron transfer is from the iridium to dioxygen. SCF-X -SW calculations on [Ir(02)(Ph3)4] and [Ir(Ph3)4] " indicate peroxo -metal bonding. ... 

Dioxygen can be coordinated to transition metal complexes in low oxidations state. Depending on the ability of the metal to donate electrons, the adducts are formulated as superoxo or peroxo complexes, while in the case of bridging molecular oxygen mu-peroxo complexes can be formed. 

---