Cobalt model systems

Many model systems which mimic both the redox behaviour [for example, ready reduction to Co(i) species] and the alkyl binding ability of vitamin Bu derivatives have been investigated. The most studied of these has involved bis(dimethylgloximato)cobalt systems of type (307), known as the cobaloximes (Bresciani-Pahor et al., 1985). Other closely related... 

The oxidation of cobalt metal to inactive cobalt oxide by product water has long been postulated to be a major cause of deactivation of supported cobalt FTS catalysts.6 10 Recent work has shown that the oxidation of cobalt metal to the inactive cobalt oxide phase can be prevented by the correct tailoring of the ratio Ph2cJPh2 and the cobalt crystallite size.11 Using a combination of model systems, industrial catalyst, and thermodynamic calculations, it was concluded that Co crystallites > 6 nm will not undergo any oxidation during realistic FTS, i.e., Pi[,()/I)i,2 = 1-1.5.11-14 Deactivation may also result from the formation of inactive cobalt support compounds (e.g., aluminate). Cobalt aluminate formation, which likely proceeds via the reaction of CoO with the support, is thermodynamically favorable but kinetically restricted under typical FTS conditions.6... 

Another SIMS study on model systems concerns molybdenum sulfide catalysts. The removal of sulfur from heavy oil fractions is carried out over molybdenum catalysts promoted with cobalt or nickel, in processes called hydrodesulfurization (HDS) [17]. Catalysts are prepared in the oxidic state but have to be sulfided in a mixture of H2S and H2 in order to be active. SIMS sensitively reveals the conversion of Mo03 into MoSi, in model systems of MoCf supported on a thin layer of Si02 [21]. 

What is the structure of this Co-Mo-S phase A model system, prepared by impregnating a MoS2 crystal with a dilute solution of cobalt ions, such that the model contains ppms of cobalt only, appears to have the same Mossbauer spectrum as the Co-Mo-S phase. It has the same isomer shift (characteristic of the oxidation state), recoilfree fraction (characteristic of lattice vibrations) and almost the same quadrupole splitting (characteristic of symmetry) at all temperatures between 4 and 600 K [71]. Thus, the cobalt species in the ppm Co/MoS2 system provides a convenient model for the active site in a Co-Mo hydrodesulfurization catalyst. 

Unsupported, or bulk cobalt catalysts are commonly used as model systems to avoid the influence of support interactions. Bulk cobalt (also known as cobalt black) is typically produced by reduction of Co304, leading to a porous solid with a low... 

We have used the reaction of m-chloroperbenzoic acid with Co/Mn/Br as a model system to attempt to understand the nature of this important autoxidation catalyst. Using stopped-flow and UV-VIS kinetic techniques, we have determined the step-wise order in which the catalyst components react with each other. The cobalt(II) is initially oxidized to Co(III) by the peracid, the cobalt(III) then oxidizes the manganese to Mn(III), which then oxidizes the bromide. The order of these redox reactions is the opposite to that expected from thermodynamics. Suggestions will be made of the relationship of this model to the known characteristics of autoxidation processes. 

Complexes of other amino acids or their derivatives with cobalt(II) that have been investigated include dipeptides (120) these complexes have long been known to absorb dioxygen. For example, the mononuclear cobalt(II) complex of N, N,N", N "-diglycylethylenediaminete-traacetic acid (121) absorbs one mole of dioxygen per two moles of complex. This system has been proposed as a simple, convenient model system for the study of dioxygen complexes of cobalt(II) peptides in solution because of its relatively slow conversion to the irreversibly formed cobalt(III) dioxygen complex. 

As part of an attempt to model the tetramanganese center presumed to be responsible for the oxidation of water in photosynthetic green plants and the cyanobacteria, cobalt(III) systems were investigated... 

By applying an approach similar to the one taken with the unpromoted M0S2 nanoclusters, we recently managed to synthesize a model system for the promoted CoMoS catalyst 119). As a result of co-deposition of molybdenum and cobalt onto the Au(l 1 1) crystal during exposure to an H2S atmosphere and subsequent annealing, crystalline CoMoS clusters formed on the Au(l 1 1) terraces. As shown in Fig. 24, the main new finding is that the CoMoS nanoclusters now adopt a... 

Allylic amide isomerization, 117 Allylic amine isomerization ab initio calculations, 110 catalytic cycle, 104 cobalt-catalyzed, 98 double-bond migration, 104 isotope-labeling experiments, 103 kinetics, 103 mechanism, 103 model system, 110 NMR study, 104 rhodium-catalyzed, 9, 98 Allylnickel halides, 170 Allylpalladium intermediates, 193 Allylsilane protodesilylation, 305 Aluminum, chiral catalysts, 216, 234, 310 Amide dimers, NMR spectra, 282, 284 Amines ... 

Carbonic anhydrase is a zinc(II) metalloenzyme which catalyzes the hydration and dehydration of carbon dioxide, C02+H20 H+ + HC03. 25 As a result there has been considerable interest in the metal ion-promoted hydration of carbonyl substrates as potential model systems for the enzyme. For example, Pocker and Meany519 studied the reversible hydration of 2- and 4-pyridinecarbaldehyde by carbonic anhydrase, zinc(II), cobalt(II), H20 and OH. The catalytic efficiency of bovine carbonic anhydrase is ca. 108 times greater than that of water for hydration of both 2- and 4-pyridinecarbaldehydes. Zinc(II) and cobalt(II) are ca. 107 times more effective than water for the hydration of 2-pyridinecarbaldehyde, but are much less effective with 4-pyridinecarbaldehyde. Presumably in the case of 2-pyridinecarbaldehyde complexes of type (166) are formed in solution. Polarization of the carbonyl group by the metal ion assists nucleophilic attack by water or hydroxide ion. Further studies of this reaction have been made,520,521 but the mechanistic details of the catalysis are unclear. Metal-bound nucleophiles (M—OH or M—OH2) could, for example, be involved in the catalysis. 

The cobalt center in MeCbl, one of the two important B12 coenzymes, is clearly involved in key steps in catalytic methyl transfer processes. Here, the Co center cycles between Co(I) and Co(III)CH3. In methionine synthase, the proposed mechanism involves direct nucleophilic attack on the C of the Co(III)CH3 group. In model reactions, the thiolate most frequently simply binds tram to the alkyl group to give a product recently established by an x-ray study of a model system. The protein may block access to the Co, thus preventing this reaction common in models. It is likely that the reactive form of the bound cofactor is five-coordinate in the key point in the catalytic cycle. This reactive form will lead to a four-coordinate Co(I) species. The axial coordination of the cofactor by a protein imidazole allows for a finer tuning of the Cbl chemistry and may permit control of the coordination number. Thus, recoordination of Co in the Co(I) state may facilitate attack on methyltetrahydrofolate and re-formation of Co(III)CH3. 

An important observation needs to be made about channel models and, indeed, model systems in general. Chemists can design molecules to have remarkable shapes and sizes. For a model system, however, it is the properties that determine whether the compound is relevant. A compound that looks like it should be a channel is, as Fitzmaurice has put it, only a long thin thing absent a demonstration of efficacy (Fitzmaurice, 2004). There is no rule that demands selectivity for the biologically relevant ions. Indeed, transport of divalent cobalt has been studied. A cobalttransporting channel is not, however, a biological mimic so far as is currently known. 

Although the cobalt corrinoids have been studied extensively in. the last two decades (J), the significance of corrin as an equatorial ligand is not well understood. To characterize coenzyme B12 as an organocobalt derivative, a search for model cobalt complexes that can form a Co-C bond axial to a planar equatorial ligand has been stimulated. Studies on model systems (2-13), particularly on the cobaloxime derivatives (2-7), characterized their respective chemistry, but it is still not easy to establish a general correlation between the structure of an equatorial ligand and the properties of cobalt complex... 

Bis[2,3-butanedione dioximato(l-)] cobalt derivatives (cobaloximes(I)) appear in most respects to be good model systems for the cobalamins of the vitamin B12 coenzyme.1 The central cobalt atom exhibits three stable oxidation states... 

In the case of transition metal complex formation reactions, model systems are established and the patterns of reactivity can be discussed in terms of model systems. For example, cobalt(III) and platinum(II) do serve as paradigm for octahedral and square planar... 

James, R. 0. and Healy, T. W. Adsorption of hydrolyzable metal ions at the oxide-water interface. I. Cobalt (II) adsorption on silicon dioxide and titanium dioxide as model systems II. Charge reversal of silicon dioxide and titanium dioxide colloids by adsorbed cobalt (II), lanthanum (III) and thorium (IV) as model systems III. 

Granted quasi-equilibrium of ligand exchanges and neutralizations and the fact that Co(CO)3Ph is catalytically inactive and accounts for only an insignificant fraction of total cobalt, the system 8.13 can be modeled with two of the three equilibrium constants linking the concentrations of HCo(CO)3Ph, HCo(CO)4, and Co(CO)4 . This will be shown in more detail in Example 8.11 in Section 8.8.2. 

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