Cobalt electron transfer

Cobalt. Electron transfer from Co2+ to thionine has recently been suggested to occur upon flash excitation278. This process, however, is unlikely not only for the reasons mentioned before for Mn2+, but also because the lowest excited state of Co2+ (8.1 kK)15 is not expected to be a reductant strong enough to reduce thionine. 

Solid State Reactions.—There are three reports of kinetic and mechanistic studies on substitution in the solid state which may have some interest or relevance to cobalt(m) complex substitution mechanisms in solution. [Co(NH3)6](N3)3 decomposes to give cobalt nitride. The initial step, as in several solution mechanisms cited above, seems to be azide to cobalt electron transfer there is no evidence for nitrene intermediates. Decomposition of [Co(NH3)e]Cl3, which gives cobalt(ii) amongst the products, does not proceed via formation of [CoCl(NH3)5]Cl2, but for some nitrites, e.g. cis- and /ra j-[Coen2(NH3)2](N02)3, nitrite does enter the first co-ordination sphere of the cobalt in the course of reaction. Lastly, the mechanism of thermal and of photochemical decomposition of [Co(NH3)s(OH2)]X3 is said to be similar to the mechanism of reaction in solution, despite the ultimate formation of tetrahedral cobalt(ii) complexes in the solid state reactions. ... 

Metal-Catalyzed Oxidation. Trace quantities of transition metal ions catalyze the decomposition of hydroperoxides to radical species and greatiy accelerate the rate of oxidation. Most effective are those metal ions that undergo one-electron transfer reactions, eg, copper, iron, cobalt, and manganese ions (9). The metal catalyst is an active hydroperoxide decomposer in both its higher and its lower oxidation states. In the overall reaction, two molecules of hydroperoxide decompose to peroxy and alkoxy radicals (eq. 5). 

The superb elegance of this demonstration lies in the choice of reactants which permits no alternative mechani.sm. Cr" (d ) and Co" (d ) species are known to be substitutionally labile whereas Cr" (d ) and Co " (low-spin d ) are substitutionally inert, Only if electron transfer is preceded by the formation of a bridged internrediate can the inert cobalt reactant be persuaded to release a Cl ligand and so allow the quantitative formation of the (then inert) chromium product. Corroboration that electron transfer does not occur by an outer-sphere mechanism followed by los.s of CP from the chromium is provided by the fact that, if Cl is added to the solution, none of it finds its way into the chromium product. 

Cobalt trifluoride fluorination corresponds to the electron-transfer mechanism via a radical cation. RF groups attached to the ring enhance the stability of intermediate dienes and monoenes. Perfluoroalkyl pyridines, pyrazines, and pyrimidines were successfully fluorinated but pyridazines eliminated nitrogen. The lack of certain dienes was attributed to the difference in stability of FC=C and RFC=C and steric effects [81JCS(P1)2059]. 

Cobalt, hexaamminetrifluoroacetato-photothermography, 6,119 Cobalt, nitropentaammine-, 1, 3 Cobalt, octacarbonyldi-exchange reactions, 1,289 Cobalt, pentaammine-electron transfer, 1, 373... 

Cobalt, pentaamminecyano-isomerization, 1,186 Cobalt, pentaamminehydroxy-thiocyanate isomerization, 1,185 Cobalt, pentaammineisonicotinamido-electron transfer, 1,373 with hexaaquachromium, 1,369 reduction... 

Figure 9-6. The consequences of a self-exchange electron transfer between a ground state cobalt(ii) and a ground state cobalt(iii) complex.

Figure 9-7. The self-exchange electron transfer reaction between vibrationally excited cobalt(ii) and cobalt(iii) complexes.

On the other hand, if electron transfer does occur within this bridged complex, a bridged cobalt(ii) - chromium(iii) complex is generated (Eq. 9.36). The d cobalt(ii) center is labile whilst the d chromium(iii) center is inert. 

In the presence of bromide ion the slow one-electron transfer oxidation of the ArCH3 substrate is replaced by the rapid one-electron oxidation of bromide ion by cobalt(III) to afford a bromine atom. The latter, or rather its adduct with bromide ion, Br2 acts as the chain transfer agent in the reaction with the ArCH3 substrate (Fig. 10). 

Cobalt(II) complexes of three water-soluble porphyrins are catalysts for the controlled potential electrolytic reduction of H O to Hi in aqueous acid solution. The porphyrin complexes were either directly adsorbed on glassy carbon, or were deposited as films using a variety of methods. Reduction to [Co(Por) was followed by a nucleophilic reaction with water to give the hydride intermediate. Hydrogen production then occurs either by attack of H on Co(Por)H, or by a disproportionation reaction requiring two Co(Por)H units. Although the overall I easibility of this process was demonstrated, practical problems including the rate of electron transfer still need to be overcome. " " ... 

Thiocarbamate (tc, RHNCSO-) is a monodentate ambidentate ligand, and both oxygen- and sulfur-bonded forms are known for the simple pentaamminecobalt(III) complexes. These undergo redox reactions with chromium(II) ion in water via attack at the remote O or S atom of the S- and O-bound isomers respectively, with a structural trans effect suggested to direct the facile electron transfer in the former.1045 A cobalt-promoted synthesis utilizing the residual nucleophilicity of the coordinated hydroxide in [Co(NH3)5(OH)]2+ in reaction with MeNCS in (MeO)3PO solvent leads to the O-bonded monothiocarbamate, which isomerizes by an intramolecular mechanism to the S-bound isomer in water.1046... 

A number of metal porphyrins have been examined as electrocatalysts for H20 reduction to H2. Cobalt complexes of water soluble masri-tetrakis(7V-methylpyridinium-4-yl)porphyrin chloride, meso-tetrakis(4-pyridyl)porphyrin, and mam-tetrakis(A,A,A-trimethylamlinium-4-yl)porphyrin chloride have been shown to catalyze H2 production via controlled potential electrolysis at relatively low overpotential (—0.95 V vs. SCE at Hg pool in 0.1 M in fluoroacetic acid), with nearly 100% current efficiency.12 Since the electrode kinetics appeared to be dominated by porphyrin adsorption at the electrode surface, H2-evolution catalysts have been examined at Co-porphyrin films on electrode surfaces.13,14 These catalytic systems appeared to be limited by slow electron transfer or poor stability.13 However, CoTPP incorporated into a Nafion membrane coated on a Pt electrode shows high activity for H2 production, and the catalysis takes place at the theoretical potential of H+/H2.14... 

The cobalt(II)15 and zinc(II)16 complexes of phthalocyanine(Pc), octcyano-Pc, and tetrasulfon-ato-Pc incorporated in poly(4-vinylpyridine-co-styrene) or Nafion films coated on graphite have also been examined as catalytic devices for dihydrogen electrogeneration in phosphate buffer. These catalytic systems were strongly suggested to be dominated by the electron transfer within the polymer matrix. The best catalytic film is that constituted of the nonsubstituted Con-Pc complex in poly(4-vinylpyridine-co-styrene), giving a turnover number of 2 x 10s h-1 at an applied potential of —0.90 V vs. Ag Ag Cl. 

Further development of this aerobic oxidation was done by utilizing a quinone containing cobalt tetraphenyl porphyrin47. This gives a more efficient electron transfer between quinone and porphyrin and results in a faster aerobic 1,4-diacetoxylation of the diene. The... 

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