Superoxo species

If [(NHi)5Co-02-Co(NH )5] + is treated with aqueous KOH another brown com-plex, [(NH3)4Co(p.-NH2)0J.-O2)Co(NH,),] is obtained and, again, a 1-electron oxidation yields a green superoxo species, [(NH3)4Co(tJL-NH2)-(tJL-02)Co(NH3). J The sulfate of this latter is actually one component of Vortmann s sulfate — the other is the red [(NH3)4Co(tx-NH2>-(tx-OH)Co(NH3)4](S04)2- They are obtained by aerial oxidation of ammoniacal solutions of coball(II) nitrate followed by neutralization with H2S0.1. 

It was presumed that the more flexible (CH2) linker and the absence of an obvious position for electrophilic attack such as is found in system Cu2(R-XYL-H)]2+, as seen in Figure 5.14, would lead to more rapid oxygenation reactions and a more stable complex than that formed from [Cu2(R-XYL-H)]2+. Instead the researchers found a more complex system in kinetically controlled oxygenation studies. Two different peroxo complexes form via a postulated open-chain superoxo species as shown in the scheme shown in Figure 5.17.41a... 

Vibrational frequencies of some titanium peroxo complexes and of solids containing peroxo and/or superoxo species are summarized in Table III. The three infrared vibrations of the triangular peroxo group in the Qv structure... 

Fig. 23. EPR spectra (at 210 K) of titanosilicates interacting with aqueous H202 the gzz region at higher gain (X 5) is shown. The peaks corresponding to Af, A, and B-type Ti-superoxo species are indicated [(from Srinivas et al. (52)].

A comparative value of this ratio was also computed from EPR measurements (52). The line labeled theoretical passing through the origin in Fig. 26 was computed on the assumption that all the Ti ions in the sample react with H202 forming only the paramagnetic superoxo species. The line labeled experimental in Fig. 26 shows that the intensity of the EPR signal varies linearly with... 

B.M. If all the Ti ions in the sample had formed superoxo species upon interaction with H2O2, the effective magnetic moment should have been 1.73-1.78 B.M. The concentration of superoxo-Ti species is, thus, about 45% of the total Ti, comparable to the values found by EPR (55%) and electronic spectroscopies. The remaining fraction is, presumably, the diamagnetic hydroperoxo-/peroxo-Ti species. 

The conversion of hydroperoxide/peroxide to superoxide is a one-electron redox reaction and requires the presence of transition metals having accessible multiple oxidation states as in biological iron or manganese clusters (e.g., Fe(II, III, IV) clusters of monooxygenase or the Mn(II, HI, IV) clusters of photosystems). Ti is usually not reduced at ambient temperatures. The various possibilities that could facilitate the transformation of hydroperoxo/peroxo to superoxo species are as follows ... 

The HO radicals, generated from the decomposition of H202, perhaps cause the hydroperoxo/peroxo to superoxo conversion. The superoxo species (with the 0-0 stretching absorption near 1120-1150 cm- ) could not be seen in the FTIR spectmm (63), perhaps because of the dominant stretching and bending modes of water in the same region. 

A similar conclusion was also reached by Sankar et al. (46), who used EXAFS/ DFT techniques. From the selective decrease in the EPR intensity of the A type superoxo species during the epoxidation of styrene and allyl alcohol (Fig. 52), Srinivas et al. (52) concluded that these types of oxo species are preferentially consumed during the reaction. 

The three TS-1 catalysts with similar Ti contents have cuboidal morphology with comparable particle sizes of 0.2-0.3 pum (as shown in SEM pictures, Fig. 53). The EPR spectra of the samples in contact with aqueous H202 (46%) (Fig. 54) indicate that the ratio of the A to B superoxo species in various TS-1 samples increases in the order TS-1 (fluoride) < TS-1 (with anatase) < TS-1 (without anatase). Catalytic activity for phenol hydroxylation and allyl alcohol epoxi-dation (Table LIII) was found to parallel the A/B ratio of the oxo-Ti species (TS-1 (fluoride) < TS-1 (with anatase) < TS-1 (without anatase)). 

Catalytic activity in benzene hydroxy lation (Table LIV), on the other hand, followed the total concentration of the various superoxo species, which increased in the order TS-1 (with anatase) < TS-1 (without anatase) < TS-1 (fluoride). The total concentration of the superoxo species was obtained from the integrated intensity of all the EPR signals representing superoxo species. This intensity in various solvents increases in the order acetone < methanol [Pg.156]

Fig. 54. EPR spectra showing the differences in the types of superoxo species generated on various TS-1 samples prepared by different methods after contacting with aqueous H202 [from Shetti etal. (93)].

EPR Labile, rhombic type spectrum corresponding to Ti-superoxo species spectral features sensitive to the type of silicate structure, temperature, solvent and pH... 

Co(II) and 02 to form Coin-C>2, the superoxo species. It is the latter process that accounts for the large volume reduction en route to the reaction products. Thus during flash-photolysis, electron transfer in the reverse direction occurs due to irradiation into the CT band, which is followed by the rapid release of dioxygen. 

It has been postulated on the basis of the crystal structure of [Mn (tpp)(02 )], that the peroxide ligand in Fe(III)-peroxo porphyrin species is coordinated in a side-on bidentate manner (Scheme 2) (8). Interestingly, based on the deuterium NMR studies the [Mn tppXOa )] complex was characterized as the Mn(II)-superoxo species (55). This discrepancy has been explained by an alteration of the normal d orbital ordering, where the highest energy d orbital is a d 2- -02 hybrid not the (8). These examples show that... 

The complexes [LCo(p-02)(p-OH)CoL] [L = en, trien, dien, tetra-ethylenepentamine, or tris-(2-aminoethyl)amine] have been studied, and the new complexes [[Co(imidazole)(gly)2 202],4H20 [ Co2(imidazole)2-(gly)402 0H],3H20, and [Co(imidazole)(gly)2(02)H20] have been prepared The spectroscopic properties of various p-peroxo- and p-superoxo-cobalt(iii) complexes have been examined. The singly-bridged p-peroxo-compounds have a strong band at 300 nm, whereas this falls at 350 nm for p-peroxo-p-hydroxo-complexes and two peaks at 480 and 700 nm are observed for p-superoxo-species. The i.r. spectra of p-peroxo-bridged complexes of cobalt(iii)-cyclam have been reported. ... 

The novel pentadentate ligand (K NCI CI NHCI CXNF CHa, tamdn (l,5,9-triamino-5-methyl-3,7-diazanonane), could be shown to coordinate to cobalt(III), and the brown dicobalt peroxo- and green dicobalt superoxo-bridged dimers [(tamdn)Co(p-02)Co(tamdn)]4+/5+ could be prepared (19). An X-ray crystal structure of the superoxo species (redrawn in Fig. 2) showed the expected structure. In addition, spectroscopic studies supported an analogous structure for the peroxo species. 

It has long been known that, when bound to cobalt(II), the pyridine-based chelate ligands 2,2 -bipyridine (bipy), 1,10-phenanthroline (phen), and 2,2 6, 2"-terpyridine (terpy) form complexes that react with dioxygen in aqueous solution (32-34). The mixed-ligand complexes [Co(terpy)(bipy)]2+ and [Co(terpy)(phen)]2+ can act as oxygen carriers in aqueous solutions at pH values as low as 3.0 (34b), and the superoxo species thus formed are apparently dinuclear. In addition, the dinuclear bipyridine complex [(bipy)2Coin(/ 2-0 )(/ 2-02 )CoIn(bipy)2 ]3+ has been shown to catalyze the oxidation of 2,6-di-ter -butylphenol to the feri-butyl-substituted diphenoquinone and quinone (35). 

Infrared spectroscopy can be used to classify metal-dioxygen complexes as either superoxo species (p(O-O) from 1200 to 1070 cm-1) or as peroxo species (p(O-O) from 930 to 740 cm-1) (49). However, this system fails to accurately define the type of dioxygen species present in 8 and 15, as these complexes exhibit v(O-O) absorptions at 961 and 941 cm-1, respectively. Preparation of the complexes with 180-enriched dioxygen confirmed that the dioxygen was bound side-on (if) in these complexes Complex 8 exhibited isotopically shifted vibrations indicative of a side-on bound dioxygen 0(160-160) absorption at 961 cm-1, p(160-180) at 937 cm-1, and /(180-180) at 908 cm-1), as did complex... 

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. 

This type of complex is derived from the mononuclear superoxo species via a further one-electron reduction of the dioxygen moiety. Cobalt is the only metal to form these complexes by reaction with dioxygen in the absence of a ligating porphyrin ring. Molybdenum and zirconium form peroxo-bridged complexes on reaction with hydrogen peroxide. In most cases the mononuclear dioxygen adducts of cobalt will react further to form the binuclear species unless specific steps are taken to prevent this. 

Of greatest interest are those compounds that attempt to model hemoglobin directly. Simple iron(II) porphyrins are readily autoxidized first to superoxo species, then to //-peroxo dimers and finally to /x-oxo dimers, as represented in equation (60). Bridge formation must be prevented if carrier properties are to be observed. This has been achieved by the use of low temperature and sterically hindered or immobilized iron(II) porphyrins. Irreversible oxidation is also hindered by the use of hydrophobic environments. In addition, model porphyrins should be five-coordinate to allow the ready binding of 02 this requires that one side should be protected with a hydrophobic structure. Attempts have also been made to investigate the cooperative effect by studying models in which different degrees of strain have been introduced. 

Addition of a second electron to the superoxo species is followed by cleavage of the 0—0 bond, with one oxygen atom being lost as water. The other oxygen atom is inserted into the C—H bond of the substrate to give the corresponding alcohol, which is then released to complete the catalytic cycle. 

A second pathway involves the formation of a mixed-valent [Fe2+, Fe3+]-superoxo species that could react with a second molecule of 36 to form a peroxo-bridged tetranuclear [Fe +, Fe2+] cluster. Homolytic cleavage of the peroxo 0-0 bond followed by electron transfer and rearrangement would yield 37. These two pathways differ in their oxygen stoichiometry pathway 1 has a ratio of 02/reduced iron dimer of 1 1, whereas pathway 2 has a ratio of 1 2. Manometric measurements of 02... 

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