Superoxo complexes bonds

Finally, a well-characterized 02-insertion transforms Tp Cr(02)Ph into the paramagnetic 0x0 alkoxide Tp Cr(0)0Ph, see Scheme 12 [4]. This reaction proceeds below room temperature, and the starting material has only been characterized by in-situ IR spectroscopy. However, analogous O2 complexes were isolated and characterized by X-ray crystallography, so there can be little doubt about its assignment as a side-on bonded Cr(IH) superoxo complex. 

Another general method is based on oxygen insertion into metal-hydrogen bonds (50,72,79-81) by any of several known mechanisms. Hydrogen abstraction by superoxo complexes followed by oxygenation of the reduced metal, as in the catalytic reaction of Eqs. (3)-(4) (50,72), works well but is limited by the low availability of water-soluble transition metal hydrides and slow hydrogen transfer (equivalent of reaction (3)) for sterically crowded complexes. 

Crystal structure analysis has been carried out for several hydroperoxo complexes. The 0-0 bond length (1.40-1.48 A) (79,87,90,100,103,105) is significantly longer than that in superoxo complexes and close to the 1.49 A value for hydrogen peroxide (54). 

These complexes can exist in a triangular peroxo form (7a) for early d° transition metals, or in a bridged (7b) or linear (7c) form for Group VIII metals. They can be obtained from the reaction of alkyl hydroperoxides with transition metal complexes (equations 9 and 10),42-46 from the insertion of 02 into a cobalt-carbon bond (equation ll),43 from the alkylation of a platinum-peroxo complex (equation 12),44 or from the reaction of a cobalt-superoxo complex with a substituted phenol (equation 13).45 Some well-characterized alkylperoxo complexes are shown (22-24). 

It is now generally understood that the mechanism of hydroxylation by cytochrome P-450 proceeds by two successive one-electron reduction steps of the heme center, transforming dioxygen into a peroxide species bonded to iron (Scheme l).73,78 Well-defined steps (25) - (31) involve (a) the formation of a high-spin Fein-enzyme-substrate complex (26) (b) one-electron reduction of (26) to a high-spin ferrocytochrome (27) (c) addition of dioxygen to form a superoxo-Fe111 complex (28) and (d) one-electron reduction of the superoxo complex to a peroxide complex (29). 

Fig. 3. Molecular structures of the cations in the peroxo complex [(l)Co( r-02)Co(l)](S206)Cl2 6 H20 (28) (left) and superoxo complex [(l)Co ( >02)Co(l)](S206)2Cl 10 H20 (29) (right) broken lines indicate intramolecular hydrogen bonds.

The complex has a green color with a characteristic absorption peak at 670 nm. ( = 890 M-1 cm.-1). It is paramagnetic (one unpaired electron), and e.p.r. studies indicate the equivalence of the cobalt atoms and delocalization of the odd electron.7 Recent x-ray crystallographic studies 6 have shown that the bridging oxygen atoms are bond angle 118°) and that the 0—0 bond distance is 1.31 A. This is shorter than that normally found in peroxides (1.48 A.) and is close to that found for the superoxide ion in K02. Whereas in the peroxo complex there is a torsion angle of 146° about the O—O bond, the Co—0—O—Co atoms are coplanar in the superoxo complex. 6... 

Hydroperoxo complexes are prepared75 by protonation of peroxo complexes, by insertion of dioxygen into metal-hydrogen bonds, by hydrogen abstraction by metal dioxygen complexes, by reduction of superoxo complexes or by reaction of the metal ion with hydrogen peroxide. Well-defined stable species have been characterized for Cu,76 Ir, Pt, and other metals, for example, by syntheses of the type ... 

Werner correctly identified the dinuclear complexes that Vaska classifies as type II b as r-peroxo complexes of two Co(III) ions, but it was only with the advent of modem physical techniques that it was possible to show by E.P.R. that the unpaired electron in Vaska type I b complexes such as [(H3N)5Co02Co(NH3)5] is localised on the dioxygen ligand , leading to their classification as <-superoxo complexes. X-ray structural data (Table 5) show that the 0-0 bond lengths in type Ib complexes are significantly shorter (and closer to the value for free Of) than those in type lib complexes which lie close to the values obtained for Ol". 

Taken together, these results have led to the mechanistic model shown in Fig. 19. The cycle emphasizes the role of PCET activation for 0—0 bond breaking. Our results show that it is the basicity (pXa 12.5 in PhCN) of the superoxo complex (shown in the box of Fig. 19) that is the key determinant of the selectivity for O2 reduction. Targeted proton transfer to the [C02O2]... 

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