Peroxo bridges

Similarly, when both the Cp and arene ligands are permethylated, the reaction of 02 with the Fe1 complex leads to C-H activation of the more acidic benzyl bond [57]. When no benzylic hydrogen is present, superoxide reacts as a nucleophile and adds onto the benzene ligand of the FeCp(arene)+ cation to give a peroxocyclohexadienyl radical which couples with a Fe Cp(arene) radical. A symmetrical bridging peroxo complex [(Fe"Cp)2(r 5-C6H60)2] is obtained. The C-H activation reactions of the 19e Fe1 radicals BH can be summarized as follows... 

X-ray crystallography confirmed both the structure and the presence of the bridging peroxo ligand. The presence of an Ir—Ir bond was indicated by the short Ir—Ir separation [2.705(1) ]. 

Dioxygen binds to metal porphyrins in the three expected modes, i.e. u-superoxo, peroxo and bridging peroxo. In contrast to die simple complexes discussed previously, dioxygen coordination occurs with a wide range of transition metals from titanium and niobium through to the Group VIII metals. 

The oxygenation reactions are usually reversible only at low temperatures, since otherwise the complex is either irreversibly oxidized or bridged peroxo or superoxo species are formed ... 

Further oxidation with hydrogen peroxide gives peroxo antimony complexes (R2Sb0)4(02)2 (12) with quadruply bridging peroxo groups (Scheme 4). ... 

Our knowledge of dioxygen complexes has increased considerably since Vaska s review of 1976. Theoretical and spectroscopic studies have given a description of the bonding in these complexes which rationalises the structures observed experimentally, and which provides a useful basis for the discussion of the reactivity. Very few theoretical studies on dinuclear complexes have appeared however, and we know of only one investigation of the relative merits of the and rf bridging peroxo structures . ... 

The present view is that cytochrome a is the acceptor of electrons from cytochrome c, but that a simple linear electron-transfer sequence from cytochrome a to Cua and then to the cytochrome 03/Cub centre is unlikely. Instead the sequence shown in equation (63) holds, where cytochrome a is in rapid equilibrium with Cua. These views depend largely upon pre-steady-state kinetics of the redox half reactions of the enzyme with its two substrates, ferrocytochrome c and O2. However, these conclusions are not in accord with kinetic studies under conditions when both substrates are bound to the enzyme, and which show maximal rates of electron transfer from cytochrome c to O2. In particular some of the cytochrome c is oxidized at a faster rate than a metal centre in the oxidase. In contrast, at high ionic strength conditions, where the cytochrome c and the cytochrome oxidase are mainly dissociated, oxidation of cytochrome c occurs only slowly following the complete oxidation of the oxidase. These results for the fast oxidation of cytochrome c have been interpreted in terms of direct electron transfer from cytochrome c to the bridged peroxo intermediate involving 03 and Cub, or to a two-electron transfer to O2 from cytochromes a and 03 during the initial phase of the reaction. 

The complex [ 202(02)2(2 ,S-mand)2]2, where mand = mandelato, gives an IR band from vOO at 920 cm-1, with vC02 bands of unidentate carboxy-lates.328 vOO bands in the IR spectra of V202(02)3(L)3, where L = amino acid ligands, gave evidence for terminal and bridging peroxo ligands.329... 

L. Salles, F. Robert, V. Semmer, Y. Jeannin, J.-M. Bregeault, Novel di- and trinuclear oxoperox-osuUato species in molybdenum(Vl) and tungsten(Vl) chemistry The key role of pairs of bridging peroxo groups. Bull. Soc. Chim. Fr. 133 (1996) 319. 

An example of a singly-bridging peroxo complex is the dinuclear cation [ Co(pydicn) 2-Oa] where pydien is the pentadentate ligand... 

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