Dioxygen complexes with metal

The singlet (S = 0) state lies about 1000 cm above the ground state triplet (S = 1) in the EPR spectrum of free dioxygen. Transitions associated with triplet oxygen in solution are detectable by EPR at low temperatures, but dioxygen complexes with even electron metal centers (e.g., ferroheme) are not generally observable by this method. Usually, only odd electron systems (Kramers systems) are detectable by magnetic resonance. 

Due to the possibility of chain initiation by direct reaction of a metal-dioxygen complex with substrate, many of these complexes have been examined as autoxi-dation catalysts, particularly for the oxidation of olefins.136 139-141 172-179 Thus, Collman et al.172 reported that dioxygen complexes of Ir(I), Rh(I), and Pt(0) catalyzed the autoxidation of cyclohexene at 25° to 60°C in benzene or methylene chloride. Cyclohexene-3-one is the major product (together with water) and cyclohexene oxide a minor product ... 

More recent investigations have shown that these reactions involve metal-catalyzed decomposition of hydroperoxides via the usual redox cycles. Thus, inhibition, polymerization and product studies in the RhCl(Ph3 P)3-catalyzed autoxidation of cyclohexene,136 ethylbenzene,136 and diphenylmethane137 were compatible only with metal-catalyzed decomposition of the alkyl hydroperoxide and not a direct reaction of the metal-dioxygen complex with substrate. Complexes Rh(III) (acac)3, Rh(III) (2-ethylhexanoate)3, and Co(II) (2-ethylhexanoate)2, gave results that were almost the same as those obtained with RhCl(Ph3P)3. The redox cycle may involve Rh(II) and Rh(III) ... 

The existence of numerous oxygenases that catalyze the direct oxygenation of organic substrates continues to stimulate the search for atom transfer oxidations of hydrocarbons by simple metal-dioxygen complexes. (For a further discussion of reactions of metal-dioxygen complexes with organic substrates via heterolytic pathways, see Section III.C). 

Recent reviews on the chemistry of metal-dioxygen complexes with particular relevance to cobalt systems include a number dealing with general properties,636,634 1 binuclear superoxo and peroxo complexes,642,643 reversible oxygenation,644-646 complex stability,647,448 catalytic oxidation649,650 and electronic651 and EPR652 spectral properties. 

Hydroxyflavone and its derivatives form a-ketoenolate (L) complexes with metals (M) of ML, ML2 and ML3 stoichiometry (Section in.B.l.b). Since QD from various sources can be activated by different metal ions, the catalysis of QD has been mimicked with mononuclear Mn(II) and Fe(III) complexes of flavonols, Mn(II)(L)2(py)2 and Fe(ni)(L)3. Upon exposure of these complexes to dioxygen at 95 °C, CO production and the formation of the 0-benzoylsalicylic acid methylester can be detected with GC-MS, analogous to the products found with QD catalysis. Using ca 60% 02 in the thermal... 

Structural parameters of doubly bridged metal-dioxygen complexes with a metal to 0 ratio of 2 1... 

The rhodium cluster compound [Rhg(CO)i6], also catalyzes the oxidation of cyclohexanone to adipic acid at 100°C in homogeneous solution [194]. The reaction pathway was not investigated and in view of the reaction of some group VIII metal dioxygen complexes with ketones, a study of such complexes as catalysts for ketone oxidation could prove interesting. 

The reaction, known for a long time, is the one implying the addition of CO2 to a dioxygen-metal-complex [69-72]. Nevertheless, very little was known about the reaction mechanism. Only recently have smdies been carried out to shed light on the reaction of dioxygen complexes with CO2 [73-75] or of carbon dioxide complexes with O2 [76]. Two ways are possible, which imply the 0-0 (Scheme 4.13, routes la, 2a) or M-O bond opening (Scheme 4.13, routes lb, 2b) with subsequent CO2 insertion and ring closure to afford the peroxocarbonate structure. 

A number of investigations of the copper-group oxides and dioxygen complexes have been reported. The electronic spectra of CuO, AgO, and AuO were recorded in rare-gas matrices (9), and it was found that the three oxides could be formed effectively by cocondensation of the metal atoms with a dilute, oxygen matrix, followed by near-ultraviolet excitation. The effective wavelengths for CuO or AgO formation were X > 300 nm and for AuO was X > 200 nm. In addition, the laser fluorescence spectrum of CuO in solid Ar has been recorded (97). 

Studies of coadsorption at Cu(110) and Zn(0001) where a coadsorbate, ammonia, acted as a probe of a reactive oxygen transient let to the development of the model where the kinetically hot Os transient [in the case of Cu(110)] and the molecular transient [in the case of Zn(0001)] participated in oxidation catalysis16 (see Chapters 2 and 5). At Zn(0001) dissociation of oxygen is slow and the molecular precursor forms an ammonia-dioxygen complex, the concentration of which increases with decreasing temperature and at a reaction rate which is inversely dependent on temperature. Which transient, atomic or molecular, is significant in chemical reactivity is metal dependent. 

The preliminary formation of metal-dioxygen complex was postulated in the mechanisms discussed earlier. However, a series of metal complexes were synthesized that form stable complexes with dioxygen. These complexes were studied as individual compounds. A few of them are given in Table 10.7. 

Ketones are resistant to oxidation by dioxygen in aqueous solutions at T= 300-350 K. Transition metal ions and complexes catalyze their oxidation under mild conditions. The detailed kinetic study of butanone-2 oxidation catalyzed by ferric, cupric, and manganese complexes proved the important role of ketone enolization and one-electron transfer reactions with metal ions in the catalytic oxidation of ketones [190-194],... 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

One of the most fundamental questions when dealing with the activation of dioxygen by transition metal complexes is whether the process is controlled kinetically by ligand substitution or by electron transfer. A model system that involved the binding of dioxygen to a macrocyclic hexamethylcyclam Co(II) complex to form the correspond-... 

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