Superoxo bridged species

The superoxo bridged species of types (b) and (c) are again found mostly in Coni and Rhm species by oxidation of peroxo complexes, for example ... 

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. 

Somewhat more recently, the reduction of the superoxo-containing species [(eda)2Co(/x-NH2)(/x-02)Co(eda)2]4+ and its peroxo-containing counterpart [(eda)2Co(/x-NH2)(jLt-02)Co(eda)2]3+ with sulfite, nitrite, and arsenite have been reported (174). When sulfite or nitrite are used as the reductants, the ultimate product formed contains bridging sulfate or nitrate, respectively, in place of the /x-02 group. In the case of arsenite, however, the arsenite is oxidized to arsenate (AsO3-), and a complex with a /x-OH group in place of the /t-02 group results. 

Cobalt(II) porphyrins bind dioxygen as would be expected by analogy with the wealth of cobalt(II) complexes which display this property. The initial addition product is invariably a superoxo-type species, confirmed by ESR, IR and X-ray studies.154 Subsequent reaction of the oxygenated complex with more Co11 porphyrin leads to a peroxo-bridged dimer. No X-ray data are available for cobalt porphyrin peroxo-bridged dimers but the formation of such dimers is well established in cobalt chemistry. 

The majority of reported Co complexes in oxidation states higher than 3-1- contain noninnocent ligands, and this hence renders the assignment of metal oxidation state difficult. This problem has existed since Werner s time indeed, he initially formulated the superoxo-bridged dimers [(NH3)4Co(02)(NH2)Co(NH3)4] + and [(en)2Co(02)(NH2)Co(en)2]" + as mixed-valence peroxo-bridged Co(III)/(IV) complexes. One-electron oxidation of the unusual square-planar Co(III) complex (31) using Ce(IV) gives a deep blue low-spin species. This is soluble and stable in benzene, and exhibits a reversible one-electron reduction... 

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. ... 

Marcus theory (15) has been applied to the study of the reductions of the jU,2-superoxo complexes [Co2(NH3)8(/u.2-02)(/i2-NH2)]4+ and [Co2(NH3)10(ju.2-O2)]6+ with the well-characterized outer-sphere reagents [Co(bipy)3]2+, [Co(phen)3]2+, and [Co(terpy)2]2+, where bipy = 2,2 -bipyridine, phen = 1,10-phenanthroline, and terpy = 2,2 6, 2"-terpyridine (16a). The kinetics of these reactions could be adequately described using a simple outer-sphere pathway, as predicted by Marcus theory. However, the differences in reactivity between the mono-bridged and di-bridged systems do not appear to be explicable in purely structural terms. Rather, the reactivity differences appear to be caused by charge-dependent effects during the formation of the precursor complex. Some of the values for reduction potentials reported earlier for these species (16a) have been revised and corrected by later work (16b). 

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. 

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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