Cobalt, complexes quinone

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

The equilibrium between metal-quinone redox isomers has been found to be extremely sensitive to the properties of nitrogen-donor coligands. The redox isomers, reported in Ref. 159, can exist (5.14) in the cobalt complexes containing semiquino-late (SQ) and catecholate (Cat) ligands derived from 3,5-di-/-butyl-l,2-benzoquinone (3,5-DBBQ) ... 

Fig. 75 Catalytic cyclopropanation using a cobalt camphor quinone oximate complex...

A cobalt complex salcomine (7.39) oxidizes substituted phenols but unsubstituted at para position, such as 2,6-di-fert-butylphenol (7.40), to give the corresponding p-quinone, 2,6-di- tert-butyl-p-benzoquinone (7.41). 

The polarographic reduction of i/4-diene complexes was first noted for [M(i/4-l,4-quinone)Cp] (M = Rh or Ir, quinone = duroquinone or 2,6-di(t-butyl)quinone), which show one two-electron wave (M = Ir), or two one-electron waves (M = Rh) at potentials more negative than those of the free quinone (575). The cyclopentadienone compounds [M(i/4-C5R40)Cp] (M = CoorRh, R = Ph or C6F5) undergo reversible one-electron reduction to radical anions (e.g., M = Co, R = Ph, E° = — 1.46 V in thf). The cobalt complexes, more stable than those of rhodium, are generated by alkali metal reduction at 100 K and show ESR spectra characteristic of metal-based d9 species (374). 

A more efficient system was obtained if copper acetate was replaced by salen-type cobalt complexes as catalysts, operating together with hydroquinone or quinone. Finally, incorporation of a hydroqninone as part of the salen ligand gave an even more efficient catalyst that did not reqnire cocatalysis by quinone (Scheme 11). 

A cobaltphosphine complex (ClCo(PPh3)3) reacts with cyclobutenedione to afford a phthaloyl cobalt complex. As shown in eq. (17.33), the ligand is exchanged by adding dimethylglyoxime pyridine, and a quinone is obtained by reaction with acetylene [45,78,79]. 

The valence tautomerism of cobalt-quinone complexes in non-aqueous solvents... 

The carbon dioxide anion-radical was used for one-electron reductions of nitrobenzene diazo-nium cations, nitrobenzene itself, quinones, aliphatic nitro compounds, acetaldehyde, acetone and other carbonyl compounds, maleimide, riboflavin, and certain dyes (Morkovnik and Okhlobystin 1979). The double bonds in maleate and fumarate are reduced by CO2. The reduced products, on being protonated, give rise to succinate (Schutz and Meyerstein 2006). The carbon dioxide anion-radical reduces organic complexes of Co and Ru into appropriate complexes of the metals(II) (Morkovnik and Okhlobystin 1979). In particular, after the electron transfer from this anion radical to the pentammino-p-nitrobenzoato-cobalt(III) complex, the Co(III) complex with thep-nitrophenyl anion-radical fragment is initially formed. The intermediate complex transforms into the final Co(II) complex with the p-nitrobenzoate ligand. 

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). 

It has long been known (93) that cobalt(II) complexes of phthalocyanines interact with molecular oxygen. The water-soluble tetrasulfonato derivative of the parent phthalocyanine selectively and catalytically oxidizes 2,6-di-tert-butylphenol to the benzoquinone and the dipheno-quinone in both homogeneous solution (94) and when polymer-supported (95). The active intermediate in the catalytic cycle is proposed to be the (as expected) mononuclear dioxygen complex of the cobalt-tetrasulfonatophthalocyanine system (92). It has been proposed that the formation of a peroxo-bridged dinuclear complex is responsible for the deactivation of the cobalt(II)-tetrasulfonatophthalocyanine system, since such a dinuclear system would be unable to further bind and activate dioxygen (96). Such deactivation results, ultimately, in loss of the catalyst and low turnover ratios. 

Quinaldohydroxamic acid as metal precipitant, 506 Quinodimethane, tetracyano-metal complexes, 263 Quinoline, 8-(alkylthio)-metal complexes, 801 Quinoline, 8-hydroxy-metal complexes, 795 Quinoline, 8-hydroxy-2-methyl-cobalt(II) complexes, 795 Quinoline, 8-mercapto-metal complexes, 800 Quinoline, 8-mercapto-2-methyl-metal complexes, 800 Quinoline, 2-methyl-8-(methylthio)-metal complexes, 801 Quinoline, 8-methylthio-metal complexes, 801 8-Quinolineselenol metal complexes, 807 Quinones... 

A multicomponent positive-imaging process using ammonia release has been described by Ricoh.211 The components of the system are (1) a cobalt(III) hexaammine complex, (2) a quinone photoreductant, (3) a chelating agent such as dimethylglyoxime, (4) a leuco dye (triarylmethane type), (5) a photooxidant (biimidazole) and (6) an organic acid (toluenesulfonic acid). 

Cobalt-Schifi base complexes catalyze the selective oxidation of phenols by dioxygen into quinols (equation 245561) or quinones (equations 246s62,563 and 247561) under mild conditions. 

Nishinaga and co-workers isolated a series of stable cobalt(III)-alkyl peroxide complexes such as (170) and (171) in high yields from the reaction of the pentacoordinated Co"-Schiff base complex with the corresponding phenol and 02 in CH2C12. Complex (170 R=Bu ) has been characterized by an X-ray structure. These alkyl peroxide complexes presumably result from the homolytic addition of the superoxo complex Co111—02 to the phenoxide radical obtained by hydrogen abstraction from the phenolic substrate by the CoUI-superoxo complex. The quinone product results from / -hydride elimination from the alkyl peroxide complex (172)561,56,565,566 The quinol (169) produced by equation (245) has been shown to result from the reduction of the CoIU-alkyl peroxide complex (170) by the solvent alcohol which is transformed into the corresponding carbonyl compound (equation 248).561... 

The electrophilic reactions of co-ordinated 1,10-phenanthrolines are not always as simple as might be expected. Thus, the nitration of cobalt(m) 1,10-phenanthroline complexes yields 5-nitro-1,10-phenanthroline derivatives at low temperature, but prolonged reaction in hot solution leads to further reaction and oxidation of the ligand to give excellent yields of 1,10-phenanthroline-5,6-quinone complexes (Fig. 8-40). Even after the formation of the quinone, the complexes may exhibit further reaction. For example, reaction of the l,10-phenanthroline-5,6-quinone complexes with base results in the formation of a complex of 2,2 -bipyridine-3,3 -dicarboxylic acid (Fig. 8-41)... 

The oxidation of the metal complexes of l,10-phenanthroline-5,6-quinone is thought to proceed in a similar manner, with the first step being a benzilic acid rearrangement. Rearrangements of this type may also be followed directly in nickel(u) and cobalt(m) complexes of 2,2 -pyridil. The first step of the reaction involves nucleophilic attack on an O-bonded carbonyl group to form a hydrate, followed by a benzilic acid rearrangement. In this case, the benzilic acid rearrangement products may be isolated as metal complexes (Fig. 8-43). 

Among other reported p-quinone adducts with N,N,-ethylene-/h.v(salicylidenimi-nate), there is a cobalt one. o-Quinone complexes with the same ligands have the composition 1 1 [Fe(salen)Q] (Q = 9,10-phenanthrenequinone and 1,2-naphtho-quinone) and [Co(salen)(py)]2Q [239]. 

Ruthenium compounds are widely used as catalysts for hydrogen transfer reactions. These systems can be readily adapted to the aerobic oxidation of alcohols by employing dioxygen, in combination with a hydrogen acceptor as a cocatalyst, in a multistep process. These systems demonstrate high activity. For example, Backvall and coworkers [146] used low-valent ruthenium complexes in combination with a benzoquinone and a cobalt-Schiffs base complex. Optimization of the electron-rich quinone, combined with the so-called Shvo Ru-cata-lyst, led to one of the fastest catalytic systems reported for the oxidation of secondary alcohols (Fig. 4.59). 

The aerobic oxidation of phenols in the presence of cobalt-Schiffs base complexes as catalysts is facilitated by (electron-donating) alkyl substituents in the ring and affords the corresponding p-quinones, e.g. the Vitamin E intermediate drawn in Fig. 4.87. When the para-position is occupied the reaction may be directed to the ortho-position [252, 253]. Copper compounds also mediate this type of oxidation, e.g. the Mitsubishi Gas process for the Vitamin E intermediate... 

Di(terf-butyl)phenol (23) underwent catalytic oxygenation with aqua[iV,iV -bis(2 -pyridinecarboxamido)-l,2-benzene]cobalt (II) (234) in DMF or DMSO (room temp., 1 h) to afford the corresponding quinone 74 in 100% yield The metal complex 234 shows high selectivity and ability to work under mild conditions stirring at room temperature under an atmosphere of molecular oxygen . ... 

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