Dioxygen species transition metal oxides

Many organic and inorganic compounds, fibers, and particles are capable of damaging nucleic acids by generating reactive oxygen species via the reduction of dioxygen. These stimuli include different classes of organic compounds, classic prooxidants (anticancer antibiotics, various quinones, asbestos fibers, and so on), and even antioxidants, which can be oxidized in the presence of transition metal ions. 

The olefin oxygenations carried out with dioxygen seem to be metal-centered processes, which thus require the coordination of both substrates to the metal. Consequently, complexes containing the framework M (peroxo)(olefin) represent key intermediates able to promote the desired C-0 bond formation, which is supposed to give 3-metalla -l,2-dioxolane compounds (Scheme 6) from a 1,3-dipolar cycloinsertion. This situation is quite different from that observed in similar reactions involving middle transition metals for which the direct interaction of the olefin and the oxygen coordinated to the metal, which is the concerted oxygen transfer mechanism proposed by Sharpless, seems to be a more reasonable pathway [64] without the need for prior olefin coordination. In principle, there are two ways to produce the M (peroxo)(olefin) species, shown in Scheme 6, both based on the easy switch between the M and M oxidation states for... 

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. 

The different oxygenated species (1)—(8) which are liable to play a role in oxidation processes are schematically represented in Figure 1. Dioxygen is a potential four-electron acceptor, and its interaction with reduced transition metals is expected to be complex.14... 

Electronic properties of intrazeolitic complexes of transition metal ions with oxygen are of Interest for elucidation of oxygen binding and its activation for oxidation reactions. In an earlier study, dioxygen and monooxygen chromium species were reported to be formed by specific interactions between the oxygen molecules and the Cr ions planted in the Type A zeolite (1). 

The aerobic oxidation of alcohols catalysed by low-valent late-transition-metal ions, particularly those of group VIII elements, involves an oxidative dehydrogenation mechanism. In the catalytic cycle (Fig. 5) ruthenium can form a hydridometal species by /1-hydride elimination from an alkoxymetal intermediate, which is reoxidized by dioxygen, presumably via insertion of 02 into the M-H bond with formation of H202. Alternatively, an alkoxymetal species can decompose to a proton and the reduced form of the catalyst (Fig. 5), either directly or via the intermediacy of a hydridometal intermediate. These reactions are promoted by bases as cocatalysts, which presumably facilitate the formation of an alkoxymetal intermediate and/or /1-hydride elimination. 

A topic we have investigated for some time is the reaction of olefins towards a O2/ CO2 mixture. We have shown that carbon dioxide acts as regulator of the oxidative properties of dioxygen [7]. In fact, transition metal catalysts (Rh, Cu) behave as "mono-oxygenases" when a O2/ CO2 mixture is used (III in Scheme 3 is formed only in traces), and as "dioxygenases" in the presence of O2 only (III is the major product). We have postulated the intermediacy in such reaction of peroxocarbonates, species known for a long time. 

The terminal oxidase in an energy-transducing, cytochrome-based electron-transport system maintains electron flow by coupling cytochrome oxidation to dioxygen (O2) reduction. Members of this protein class are referred to as cytochrome oxidases they carry out Oj-binding and redox chemistry at transition metal-containing active sites. Although iron is the most commonly used metal and may occur as a protoheme or iron-chlorin species in the protein, this section is concerned only with mitochondrial cytochrome oxidase, which contains 2 mol of Cn and 2 mol of heme a bound Fe per function unit. Biochemistry of the protein will not be considered here, instead the focus will be on the stmcture of the metal centers, on the reactions they catalyze and on models for these centers. 

---