Cobalt-dioxygen complexes, aqueous

Formation and stabilities of cobalt dioxygen complexes in aqueous solution. A. E. Martell, Acc. Chem. Res., 1982,15,155-162 (68). 

Co(his)2] combined reversibly with dioxygen in aqueous solution. This was an important result, for [Co(his)2] was the first synthetic compound to undergo reversible oxygenation in aqueous solution and it was evident that oxygenation was a general rather than an isolated phenomenon. This early chemistry of cobalt-dioxygen complexes has been reviewed.630,633- 5... 

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). 

The cobalt(II) cyclidene product VI is reasonably stable toward autoxidation in the solid state, and a solid state IR spectrum can be obtained under aerobic conditions. The cobalt(II) complex is best identified by its ESR spectrum in an acetonitrile-1.5M M-methylimidazoIe solution, frozen to a glass at — 196 C a 2.3 and 2.00. Exposure of the thawed solution to O2 then causes replacement of this spectrum with that of the 1 1 Oj adduct % 2% 2.01 and 3% 2.08. At 20 °C in 2.5 M aqueous N-methylimidazole the complex binds dioxygen reversibly with an equilibrium constant Kq = 1.6 torr. An X-ray structure of the cobalt(II) complex confirms the presence of the commodious lacuna, which protects the coordinated dioxygen from autoxidation in polar sol vents.An X-ray structure determination of the dioxygen adduct, prepared by recrystallization of the complex from aqueous V-methylimidazole exposed to the atmosphere, confirms the existence and nature of the O, adduct. ... 

Aqueous dispersions of charged copolymer latexes are active supports of cobalt catalysts for autoxidations of tetralin by cobalt-pyridine complexes and of 2,6-di-rm-butylphenol and 1-decanethiol by CoPcTs. Since these reactants are insoluble in water, a simple explanation of the catalytic activity is that the organic polymer serves to solubilize the reactants in the phase that contains the cobalt catalyst. All three reactions have initial rates that are independent of substrate concentration, as determined by absorption of dioxygen from a gas buret. All three reactions appear to proceed by different mechanisms. Tetralin autoxidation is a free radical chain process promoted by the CoPy complex, whereas the CoPcTs reactions are not free radical chain processes. The thiol autoxidation is reported to involve hydrogen peroxide, whereas the 2,6-di-re/t-butylphenol autoxidation apparently does not. 

Water soluble cobalt(ll) 2,9,16,23-tetrasulfophthaloeyanine, zinc(II)-2,9,16,23-tetrasulfo-phthalocyanine, zinc(ll)tetracarboxyphthalocyanine, and non-metallic sulfophthalocyanine complexes are catalysts and photocatalysts for the oxidation of sulfide, sulfite and thiosulfate ions by dioxygen. Typical conditions are 293 K, 1 atm Oi and pH 9.24 in aqueous solutioaThe cobalt phthalocyanine complexes show high catalytic activity only in the... 

Cobalt(III) is a potent oxidant in an aqueous solution and, in the absence of a complexing agent that can stabilize cobalt(III), it is rapidly reduced to cobalt(II) with the concomitant oxidation of water to dioxygen ... 

Cobalt(II) 4,4, 4",4" -tetrasulfophthalocyanine, covalently linked to the surface of titanium dioxide particles, Ti02-CoTSP, was shown [207] to be an effective photocatalyst for the oxidation of sulfur (IV) to sulfur (VI) in aqueous suspensions. Upon bandgap illumination of the semiconductor, conduction-band electrons and valence-band holes are separated the electrons are channeled to the bound CoTSP complex resulting in the reduction of dioxygen, while the holes react with adsorbed S(IV) to produce S(VI) in the form of sulfate. 

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