Cobalt complexes slow electron

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... 

Similar considerations apply to the role of spin equilibria in electron transfer reactions. For many years spin state restrictions were invoked to account for the slow electron exchange between diamagnetic, low-spin cobalt(III) and paramagnetic, high-spin cobalt(II) complexes. This explanation is now clearly incorrect. The rates of spin state interconversions are too rapid to be competitive with bimolecular encounters, except at the limit of diffusion-controlled reactions with molar concentrations of reagents. In other words, a spin equilibrium with a... 

The reactions of both trisoxalato ferrate(III) and trisoxalatocobaltate-(III) with ascorbic acid have been examined. Evidence is presented in the reaction with Fe(III) for the formation of an intermediate prior to a relatively slow inner-sphere electron transfer reaction, which is independent of pH, ascorbic acid concentration, and ionic strength. For the cobalt complex, the pH (8-10) of the reaction medium leads to reaction with the doubly deprotonated ascorbate ion for which the rate constant is 20 s at 25 °C. Complexation of Fe(III) by... 

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

Macrocyclic N-Donors. Glick et al." have proposed that the greater difference in Co—X axial bond length between the cobalt(n) and cobalt(m) complexes of (16) compared with the corresponding complexes of (17) accounts for the unusually slow self-exchange rate of the former. The electronic spectra of the five-co-ordinate cobalt(n) complexes of the macrocycles (18) and (19) have been reported.100... 

In 2003, Cenini and coworkers reported (tetraarylporphyrin)cobalt(II) complexes 326 as efficient catalysts (1 mol%) for cyclopropanations. In the absence of air, styrenes 321 underwent an efficient cyclopropanation with ethyl diazoacetate 322 giving cyclopropanes 324 in 65-99% yield with 3-5 1 trans/cis ratios (Fig. 77) [348]. Simple olefins and more hindered diazoesters did not react. With diazoacetate and hydrocarbons, such as cyclohexane or benzene, C-H insertion took place furnishing cyclohexyl- or phenylacetate. In line with Ikeno s proposal the cyclopropanation reaction was considerably slowed down in the presence of TEMPO, though not completely inhibited. Based on a kinetic analysis a two-electron catalytic cycle with a bridged carbene unit was formulated, however. 

The rate of cobalt-catalyzed carbonylation is strongly dependent on both the pressure of carbon monoxide and methanol concentration. Complex 4.7, unlike 4.1, is an 18-electron nucleophile. This makes the attack on CH3I by 4.7 a comparatively slow reaction. High temperatures are required to achieve acceptable rates with the cobalt catalyst. This in turn necessitates high pressures of CO to stabilize 4.7 at high temperatures. 

All one-electron transfers involve paramagnetic molecules, but this aspect was not much considered until recently. An early observation was that reactions between cobalt(in) and cobalt(II) complexes are often unexpectedly slow, and this is evidently related to the necessity of spin change from the Aig (low-spin d ) cobalt(III) to the Tig (high-spin d ) cobalt(II) state. Endicott and coworkers have provided, and analyzed, a mass of experimental data on this question. The... 

Since the cobalt(II) complex is inert, its decomposition (114) even in acidic medium k = 1.2 x 10-2 mol T s ) is slow compared with the electron-transfer rate ()e = 5.0 mohU s" ). Therefore, process (113) dominates, and no photoeffect has been observed. The inertness of the [Co(sep)] + cation in photochemical processes is caused by complete encapsulation of the cobalt ion. In the system with the nonmacrocyclic hexamine [Co(NHs)6] trication, photoreduction was found to occur in relatively high quantum yield ( = 0.16). In this case, the photocomposition rate of [Co (NH3)5(NH3)+] + cation formed on irradiation k > 10 s >) is higher than that of electron transfer k = 10-5 mol-T s-i). 

The reactions of cobalt(III) in mixed-metal outer sphere electron transfer reactions are usually slow also, as a result of the influence of effects discussed above. The reaction (5.56) of the hexaamminecobalt(III) ion with the hexaaquachromium(II) complex... 

The biomethylation reaction between platinum and methylcobalamin involves both platinum(II) and platinum(IV) oxidation states. An outer-sphere complex is formed between the charged platinum(II) salts and the corrin macrocycle, which catalytically labilizes the Co—C o bond to electrophilic attack. A two-electron redox switch mechanism has been proposed between platinum(II) and platinum(IV). However, a mechanism consistent with the kinetic data is direct electrophilic attack by PtClg on the Co—C a bond in MeBu. Studies on [Pt(NH3)2(OH2)2] indicate that the bases on cobalt interact in the coordination sphere of platinum(II). Since both platinum(ll) and platinum(rV) are together required to effect methyl transfer from methylcobalamin to platinuni, Pt and C NMR spectroscopy have been used to show that the methyl group is transferred to the platinum of the platinum(n) reactant. The kinetics of demethylation by mixtures of platinum(II) and platinum(IV) complexes show a lack of dependence on the axial ligand. The authors conclude therefore that it is unlikely that the reaction involves direct attack by the bound platinum on the Co—C bond, and instead favor electron transfer from an orbital on the corrin ring to the boimd platinum group in the slow step, followed by rapid methyl transfer. ... 

In methanol, Cl and 7tH+ ions exhibit a retarding effect. The stabilities of nickel(o)-phosphine complexes have been assessed these seem to depend more on the size of the phosphine and electronic effects on bond strengths are of secondary importance. In the oxidative addition of aryl halides to nickel(o)-phosphine complexes the reaction appears to proceed via an initial slow dissociation step to give Ni(PR3)2 which then attacks the organic species. The mechanism of oxidative elimination of these nickel species thus contrasts with that for the platinum(o)-phosphines where the dissociation of the ligand is rapid and the rate-determining step is that involving the redox interaction. The oxidation of tetrahedral cobalt(i) complexes with carbon tetrachloride has been described ... 

An aerobic oxidation of tertiary amines catalyzed by Cobalt Schiff-base complexes afforded N-oxides in high yields [120]. The reaction was run at room temperature with 0.5 mol% of the cobalt catalyst 35 (Eq. (8.28)). The presence of molecular sieves (5 enhanced the rate of the reaction. With this procedure, various pyridines were oxidized to their corresponding N-oxides in yields ranging from 50 to 85%. Electron-deficient pyridines such as 4-cyanopyridine gave a slow reaction with only 50% yield. 

Reduction of cobalt(III) to cobalt(II) in vitamin B12 is slow and is ascribed to the lack of an electron transfer pathway through the corrin ring. Both axial positions are blocked by protein side chains preventing redox by this route. The [Co(I)] species Bi2s has been used in the 1 1 reduction of [(H20)5CrX] complexes. A marked dependence of the reaction rate on X suggests an inner-sphere mechanism to produce a [Co(II)-X] product which subsequently hydrolyzes. 

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