Superoxo copper

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). 

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. 

FIGURE 14.6 Copper superoxo mechanism for OPM and PMH. (Adapted from Klinman, 2006.)... 

The formation of a copper-superoxo intermediate appears to provide a working mechanism that is capable of rationalizing the voluminous amount of data available for D/3M and PHM. The expanded mechanism of Figure 20 also provides an answer to the long-standing question of how these enzymes catalyze ET across bulk water at a rate that is compatible with catalytic turnover. 

Figure 20 Copper-superoxo mechanism for D/3M and PHM. Reproduced from J. P. Klinman, J. Biol. Chem. 2006, 281, 3013-3016, with permission from The American Society for Biochemistry and Molecular Biology.

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... 

Copper superoxo and p-oxo species are believed to be active catalysts. The oxidation of 2,6-di-t-butyl-4-methylphenol by 0 in the... 

Ans See the paper by Que and Tohnan (2008). For iron, hydroperoxo and oxo for copper, superoxo, hydroperoxo, and alkoxo complexes, (b) Yes, with very high selectivity. See the paper by Chen and White (2007). 

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. 

Unlike Au+(02), which was green,224 both Ag+(02) and Ag+(04) were colourless, although weak absorptions at 275 and 290 nm respectively were observed in their UV spectra. Ag+(02) was formulated as a side-on -bonded superoxo molecular species and this was supported by IR spectra. Ag+(C>4) was described as a tetraoxygen species rather than a bis(dioxygen) complex. Neither copper nor gold gave a secondary reaction to produce a related M+(04-) complex. 

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. 

Another 3d-metal involved in biological and biomimetic dioxygen activation, copper, can also form end-on superoxo complexes39 11 ... 

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... 

Mechanisms and rates of dioxygen binding to copper(I) were determined for a series of tripodal complexes that afford end-on superoxo intermediates. Kinetic parameters for a these complexes proved to be rather similar, with activation enthalpies ranging about 3(MK) k.l/mol for reactions in coordinating solvents (nitriles). Activation entropies were relatively small for these processes both small positive and small negative values have been reported.40 This behavior is indicative of... 

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