Superoxo-complexes, mechanism

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 ... 

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. 

Prior to the work described below (50), hydrogen transfer to superoxometal complexes has been proposed by some (124 126) and questioned by others (127) who introduced plausible alternative mechanistic pathways. The work with rhodium hydrides (50) sought to establish whether hydrogen abstraction by superoxo complexes is a feasible and reasonable mechanism for thermodynamically favorable cases. 

All the superoxo complexes and rhodium hydrides in this work can be handled under both aerobic and anaerobic conditions. The ability to work with superoxides in the absence of 02 and with the hydrides in the presence of 02 provides an exceptionally large range of reaction conditions and an opportunity to detect and identify various intermediates, and thus establish the mechanism with reasonable confidence. 

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. 

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). 

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.

The reaction of /r-superoxo complexes with reducing metal ions generally follows an outer sphere mechanism, and kinetic data have been reported for reduction by Fe and cobalt(II) chelates =>, Mo(V) ), [Ru(NH3)6] and... 

The mechanism of metal phthalocyanine catalysed oxidation by molecular oxygen -isobutyraldehyde system is not established at this stage. The iron[14], manganese[15] and cobalt tetrasulphonato-[16] phthalocyanines are known to form superoxo complexes with dioxygen and are known to catalyse autoxidation reactions[13]- The acyl radical formation thus can be initiated by interaction of metal phthalocyanine-dioxygen superoxo complex with isobutyraldehyde. The acyl radical in presence of oxygen can yield acylperoxy radical or peracid as the oxidising speceis[17]. 

In Mechanism I, which is favored for the SOD enzymes and most redox-active metal complexes with SOD activity, superoxide reduces the metal ion in the first step, and then the reduced metal ion is reoxidized by another superoxide, presumably via a metal-peroxo complex intermediate. In Mechanism II, which is proposed for nonredox metal complexes but may be operating in other situations as well, the metal ion is never reduced, but instead forms a superoxo complex, which is reduced to a peroxo complex by a second superoxide ion. In both mechanisms, the peroxo ligands are protonated and dissoeiate to give hydrogen peroxide. 

Co2(02)(CN)io] which seems to be quite stable towards reduction. The reduction of superoxo complexes occurs via a simple electron-transfer mechanism. However, attempts to establish redox potentials for these species have not always taken into account effects due to protonation or dissociation. 

Electron Transfer Photochemistry.—It has been suggested that electron transfer can bring about base hydrolysis of cobalt(m) complexes. Recent examples include the aquation of sulphite complexes, where detection of dithionate as one of the products is good evidence for an electron transfer mechanism, and of the dicobalt /M-superoxo complex [(H3N)6Co 02 Co(NH3)5] + in the presence of sulphite. ... 

Superoxo 1 1 (Co O2) complexes are usually unstable relative to formation of bridged /i-peroxo complexes. In fact the mechanism of formation of most /x-peroxo complexes involvesintermediacy of the 1 1 superoxo complex [45], Wilkins, ef a/. [46-48] have presented kinetic evidence for such a mechanism for the oxygenation of cobalt(II) amine complexes in water, equations (9), (10). 

The kinetics and mechanism of the O2 oxidation of2-aminophenol (ap) to 2-aminophenoxazin-3-one (apx) under ambient conditions, catalyzed by the recently synthesized tetrakis(3,5-di-teAt-butyM-hydroxyphenyl)-dodeca-chlorophthalocyaninatocobalt(ll), (R4PCC0), (Figure 41) have been studied by spectrophotometry . The rate of ap formation is first-oider in [R4PCC0] and obeys Michaelis-Menten t) e kinetics with respect to [ ]. The suggested mechanism involves rate-determining inner-sphere election transfer from coordinated ap to coordinated O2 in the superoxo complex. 

The crystal structure of the dioxygen complex Tp V(0)(02)(L) has been determined, and revealed a side-on bonded O2 ligand. Based on the O - O distance of 1.379(6) A and vo-o of 960 cm it was formulated as a V(V) peroxo complex, even though these values are on the borderline between the peroxo and superoxo designations. The mechanism of this reaction is curious. Reaction with 02 showed incorporation of solely in the O2 ligand and not in the 0x0 groups. It appears that the O2 binding step must be preceded by a disproportionation (2 V(IV) V(III) + V(V)) followed by reaction of V(III) with O2. 

The stepwise mechanism of Eqs. (3) and (4) draws further support from the results obtained under air-free conditions. The stoichiometry now increased to 3 1, and the products changed to a mixture of the p-superoxo dirhodium(III) complex (NH3)4(OH)Rh (p-02)Rh(H20)(NH3)4+ and (NH3)4(H20)Rh3 +. These results are easily rationalized by a scheme whereby rhodium-based oxidants take over as scavengers for (NH3)4(H20)Rh2 + produced in the initial step, Scheme 3. 

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... 

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