Cobalt electron transfer mediators

In another approach, a DO sensor was proposed by modifying a glassy carbon (GC) electrode with cobalt tetrasulfonate phthalocyanine (CoTSPc). The direct reduction of oxygen at a solid electrode is a slow process and also requires a high negative potential which can be lowered by electron transfer mediators that can shuttle the electrons between oxygen and electrode. Among these mediators, phthalocyanines acquired a lot of attention because of their catalytic ability and... 

Bromoacetals containing carbon-carbon triple bonds were cyclized to 5-membered heterocyclic rings (5- xo-exclusively) using [chloropyridine-bis(dimethylglioximato)] cobalt(I) (cobaloxime I) generated from cobaloxime III as an electron transfer reagent (mediator) in alkaline methanol solution at constant current. The most efficient molar substrate/mediator ratio was 2 1 with 70-87% yields of isolated products. The proposed... 

The rhodium(II) radical [Rh(dmgH)2PPh3], formed by laser photolysis of the corresponding dimer, is oxidized by a range of cobalt(II) complexes which have coordinated halides. " Electron transfer is believed to occur through a ligand-mediated pathway and the reaction rate is dependent upon the choice of halide. In an examination of the Ti(III)-induced cyclization of epoxyolefins, a mechanism has been invoked which involves a metal-centered radical. ... 

The formation of phenoxy radicals upon oxidation from substituted phenols has been studied by the ESR technique, using nitrosodurene as spin trap [52]. Chloro(5,10,15,20-tetraphenylporphyrinato)cobalt(III) and its Ti-cation radical were used to promote the reaction. The phenols studied reacted with the cation radical but some of them interacted only with the complex. The reaction is pictured as involving electron transfer from the phenolate, mediated by an axially coordinated chloro ligand. 

The rate of reduction of cytochrome c by the cobalt-substituted analogue of rubredoxin is a factor of 2.25 lower than the rate with rubredoxin itself. Both proteins mediate the reduction of cytochrome c in the presence of NADH and the decrease in the rate is attributed to the decreased efficiency in oxidation of cobalt(ii) compared with iron(ii). Reduction of cytochrome c by NADPH is catalysed by an adrenodoxin reductase-adrenodoxin complex in which the rate-determining step is electron transfer from the flavin (FAD) of the reductase to the FeaS2 centre of adrenodoxin. The pH dependence of the rate shows a pATa of 6.75, with the high-pH form 27.5 times more reactive than the low-pH form. Both NADPH reduction of the complex and cytochrome c oxidation of the complex were faster than the catalytic rate. Catalytic roles of four iron-sulphur centres in trimethylene dihydrogenase and ferredoxin nitrite reductase have also been examined. Synthetic analogues of four iron ferredoxins have also attracted much attention. - ... 

A con alent modification of electrode surfaces by mediators N,N -di-aminopropyl-4,4 -bypiridinium (DAPV) or cobalt diaminosarcophagine (CoD) makes it possible to observe a direct electron transfer between the electrode and a viologen accepting pyridine nucleotide oxidoreduc-tase (VAPOR) or dihydrolipoamide dehydrogenase (Lip-DH) [227]. In this case the electron transfer does not involve a true mediation step since the mediator is a component of the electrode. In this way it forms a molecular wire between the electrode and the electron accepting prosthetic group of the enzyme [227]. 

Besides 6-azulenyl-substituted benzenes, 2-azulenyl-substituted benzenes 12a,b were later synthesized by the same research group from di(2-azulenyl) acetylene 13a,b using cobalt-mediated cyclotrimerization, as shown in Scheme 4.4 [13, 14]. Compound 12a exhibited a reversible reduction wave at —2.29 V versus Fc+ZFc, which involves multiple electron transfer in one step to produce highly charged anionic species, and two irreversible oxidation waves. However, the exact number of transferred electrons could not be determined from the analysis of the reduction wave in the cyclic voltammogram. With... 

The kinetics of the electron-transfer reactions between oxalatocobalt(m) complexes and iron(n) have been described. For cationic complexes, the rates decrease in the order (ox = oxalate) [Co(ox)(phen)J+>[Co(ox)(bipy)2] > [Co(ox)(NH3)4]+> [Co(ox)(en)2]+> [Co(oxXtrien)]+ the variation in reactivity is attributed to changes in the enthalpy of activation, a linear relationship being observed between and log k (k = observed rate constant). The much faster reactions of the phenanthroline and bipyridyl complexes are also considered to derive from the eflSciency of these ligands as electron mediators in reactions of this type. A relationship has also been observed between log k and the half-wave potential of the polarographic reduction of the cobalt(iii)... 

Extensive studies of the reductions of nitro substituents in the cobalt(II) cage complex, [Co(sar)] ", have been reported. These groups do not mediate electron transfer in the reactions of the metal center. The complexes [Ru(Hedta)(NO )] and [Fe(Hedta)(NO )] are catalysts in the electrochemical reduction of NOf, and the mechanism of the reaction has been investigated. Whether the product is mainly NjO or NH/ can be determined by appropriate choice of catalyst and pH. The chromium(I) complex, [Cr(N0)(H20)5] ", is resistant to oxidation. Reactions with the powerful oxidants lO and BrO are thought to be inner-sphere, with attack on the nitrosyl nitrogen atom to give nitrite and chromium(III) as products in a net two-electron reaction. ... 

The mechanism of Co(acac)2-mediated polymerization of Vac is still an open question. On the basis of an early work by Wayland and coworkers on the controlled radical polymerization of acrylates by complexes of cobalt and porphyrins, Debuigne and coworkers proposed a mechanism based on the reversible addition of the growing radicals P to the cobalt complex, [Co(II)], and the establishment of an equilibrium between dormant species and the free radicals (equation 8). Maria and coworkers, however, proposed that the polymerization mechanism depends on the coordination number of cobalt . Whenever the dormant species contains a six-coordinated Co in the presence of strongly binding electron donors, such as pyridine, the association process shown in equation 8 would be effective. In contrast, a degenerative transfer mechanism would be favored in case of five-coordinated Co complexes (equation 9). 

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