Copper-superoxo complexes

Schatz M. Raab V. Foxon S. P Brehm G. Schneider S. Reiher M. Holthausen M. C. Sundermeyer J. Schindler S. Dioxygen complexes combined spectroscopic and theoretical evidence for a persistent end-on copper superoxo complex. Angew. Chem., Int. Ed. 2004, 43, 4360-4363. 

Copper Superoxo Complexes 16.4.3 Copper Hydroperoxo, Alkylperoxo, and Acylperoxo Complexes 5 OXYGENATION OF C—H BONDS RELATED TO COPPER... 

Three isolable copper superoxo complexes have been reported, (55), (56), and (57), " which almost certainly contain different coordination modes of superoxide (Scheme 6). Low-resolution crystal structures of (55) and (56) showed copper centers that were best described as tetragonal, with a side-on coordinated rj -02 ligand occupying two basal coordination sites (Figure 12). The short O—O bond of 1.22(3) A shown by (55) is consistent with a superoxo, rather than a peroxo, ligand the O—O bond length derived for (56) was unreliable owing to... 

As a part of their ongoing investigations into the reaction of dioxygen with copper(I) complexes to identify copper-superoxo/peroxo intermediate species, Schindler and co-workers51-53 have also provided examples of a number of mononuclear copper(II) complexes (Table 1) (as well as copper(I) Section 6.6.4.2.1). 

Interaction of dioxygen species with Fe aq and with Fe " aq has been very briefly reviewed. In relation to 0x0-, peroxo-, and superoxo-complexes as models for intermediates in oxygenase activity, a brief report on a 2000 symposium on activation of oxygen summarizes the then-current situation in the search for a mechanism common to mono- and dinuclear iron sites, mono- and dinuclear copper sites, and copper-iron sites. The outline proposals comprise ... 

The superoxo-containing species [(NC)6Co(/u.-02)Co(CN5]5 can be reduced with thiols such as 2-aminoethanethiol or L-cysteine (175), and the reduction reaction is catalyzed by copper(II) ions in aqueous solution. When copper(II) is present, the role of the thiol is to reduce cop-per(II) to copper(I), which then reacts with the superoxo species through an inner-sphere mechanism. Conversely, when the superoxo complex [(H3N)5Co(/x-02)Co(NH3)5]5+ is reduced with thiol (176), the reaction follows an outer-sphere mechanism, as would be expected. Ascorbic acid also reduces both complexes (177), but only the reduction of the cyano-containing complex exhibits copper(II) catalysis. 

Sulfur ligands, 633-655 coordination ability, 516 Sulfur monoxide metal complexes instability, 636 Superoxide dismutase, 773 copper complexes, 772 Superoxo complexes, 315-330 binuclear, 323, 325 reactions, 330 bridged... 

These copper-mediated reactions very often involve dinuclear intermediates, but detailed mechanistic studies on stoichiometric systems are relatively few. The key features are the formation of p-peroxo or p-superoxo complexes by electron transfer from cop-per(i) to dioxygen. The co-ordinated oxygen may then act as an electrophile to the aromatic ring. A possible mechanism for the ortho-hydroxylation of phenol by dioxygen in the presence of copper catalysts is shown in Fig. 9-29. 

Figure 9-29. A possible mechanism for the oxidation of phenol to 1,2-benzoquinone by dioxygen in the presence of copper(i) salts. The key steps involve the formation of a peroxo or superoxo complex, followed by electrophilic attack upon the benzene ring.

In light of the accepted mechanism for cytochrome P-450 (97-100), a superoxo-Cu(II) intermediate is further reduced, leading to dioxygen activation. Accordingly, a monomeric peroxo or hydroperoxo copper(II) complex serves as a synthetic model for these intermediates of copper-containing monooxygenases. However, no well-characterized complexes of these types are available to date. Formation of a monomeric hydroperoxo or acylperoxo complex was reported to occur when a trans-/u-l,2-peroxo complex, [(Cu(TPA))2(02)]z+, was treated with H+ or RC(O)+, but no details of the structures and properties of the complexes were provided (101). A related complex, a monomeric acylperoxo cop-per(II) complex, was synthesized (102). Low-temperature reaction of a dimeric copper(II) hydroxide complex, [Cu(HB(3,5-iPr2pz)3)]2(OH)2, with 2 equivalents of m-CPBA (3-chloroperoxybenzoic acid) yielded a monomeric acylperoxo complex whose structure was characterized by... 

Oxidation Catalysis by Copper Peroxo and Superoxo Complexes. Copper ions and compounds participate in or catalyze a variety of oxidation reactions that consume 02. This is one of the several key biochemical roles of copper and much of the recent work on the subject has been done in efforts to model the biological systems. In some (non-biological) cases, e.g., the Wacker process, copper(II) itself may be the actual oxidant, but usually it serves as a carrier of oxygen. 

In contrast to iron and cobalt, end-on superoxo-copper(II) species do not dominate the field of copper-oxygen chemistry. In 1 1 copper-dioxygen adducts, an alternative side-on, t 2 coordination mode is sometimes observed these [(L)Cu11 (t 2-(02 ")] or [(L)Cum-(ri2-(022 )] complexes are discussed below. Mononuclear copper-dioxygen complexes easily react with the second molecule of the Cu(I) complex, forming peroxo- or dioxo-bridged dinuclear species (Section 4.4). For sterically unhindered... 

Figure 12 Single-crystal X-ray structure of (55), a copper(II) superoxo complex. All hydrogen atoms...

In an alternative approach to mimic tyrosinase activity a copper(I)-copper(n) redox couple and a hydroquinone-quinone redox couple were incorporated in one complex (scheme 17). The hydroquinone moiety should act as an electron shunt between an external reducing agent, i.e. ascorbic acid, zinc or electrochemical reduction, and the copper ions. Catalytic oxygenation by monooxygenases is usually accompanied by the formation of water, with the aid of an external electron and proton source.35 46 Activation of O2 by dinuclear copper(I) complex 58 results in superoxo- or p-peroxo-dicopper(II) complex 59, which oxygenates an external substrate molecule. Internal electron transfer to quinone dicopper(II) complex 60 is followed by quinone to hydroquinone reduction. The electron transfer system shown here is reminiscent of the quinone based systems found in the primary photochemical step of bacterial photosynthesis, and in (metallo)porph3nin-quinone electron transfer systems.In contrast to expectation, the hydroquinone dinuclear copper(II) complex 60 (L = (2-pyridylethyl)formidoyl, scheme 17), designed to mimic step c in this cycle, is a stable system in which the hydroquinone moiety is not oxidized to a quinone structure 61. 

Both mechanisms (40) and (42), applied to either aquacopper(I) or copper(I) complexed with either unidentate amines or chelates, imply the transient formation of copper(II) superoxo or a p-peroxo species. The evidence supporting this view had been primarily kinetic, i.e. no direct detection of such intermediates could be achieved [340]. The situation has changed with the introduction of Karlin s ligands [201,383], which afford copper(I) complexes that form relatively stable... 

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