Cobalt systems

Ozin, Hanlan, and Power, using optical spectroscopy (49,121). In view of the marked temperature-effect observed for the cobalt system, we shall focus on this cluster system here. Evidence for cobalt-atom aggregation at the few-atom extreme first came from a comparison of the optical data for Co Ar — 1 10 mixtures recorded at 4.2 and 12 K (see Fig. 4). A differential of roughly 8 K in this cryogenic-temperature regime was sufficient to cause the dramatic appearance of an entirely new set of optical absorptions in the regions 320-340 and 270-280 nm (see Fig. 4). Matrix variation, from Ar, to Kr, to Xe, helped clarify atom-cluster, band-overlap problems (see Fig. 5). 

Although less active, the cobalt system CoCl2/PPh3/NaBH4 affords a 90/10 mixture of 1,2- and 1,4-addition products in 98% yield at room temperature [172]. Palladium catalysts are more efficient (Eq. 4.43) [173]. 

Amidocarbonylation converts aldehydes into amido-substituted amino acids, which have many important industrial applications ranging from pharmaceuticals to detergents and metal-chelating agents.588 Two catalyst systems have been developed, a cobalt-based system and, more recently a palladium-based system. In the cobalt system, alkenes can be used as the starting material, thus conducting alkene-hydroformylation, formation of hemi-amidal and carbonylation in one pot as... 

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... 

Studies on Model Cobalt Systems at Model FT Conditions (CO + Syngas)... 

For more general overviews of post-metallocene a-olefin polymerisation catalysts, the reader is referred to a series of reviews [8, 9, 10, 11, 12], while recent reviews pertaining to the importance of 2,6-bis(imino)pyridines and to iron and cobalt systems per se have also been documented [13, 14],... 

Use of less sterically hindered examples of 5 in combination with MAO allows for active catalysts for the linear (head-to-head) dimerisation of a-olefins such as 1-butene, 1-hexene, 1-decene and Chevron Phillips C20-24 a-olefin mixture (Scheme 4) [47], The mechanism for dimerisation is thought to involve an initial 1,2-insertion into an iron-hydride bond followed by a 2,1-insertion of the second alkene and then chain transfer to give the dimers. Structurally related cobalt systems have also been shown to promote dimerisation albeit with lower activities [62], Oligomerisation of the a-olefms propene, 1-butene and 1-hexene has additionally been achieved with the CF3-containing iron and cobalt systems 5j and 6j yielding highly linear dimers [23],... 

The use of two or more different catalysts in the same reactor, sometimes known as in situ reactor blending or tandem catalysis, has been widely employed industrially as means of controlling the properties of a polyolefin (e.g. molecular weight and the molecular weight distribution). Recent years have seen a variety of reports emerge on the use of bis(imino)pyridine iron/cobalt systems as one component of the process [169, 170, 171, 172, 173, 174, 175, 176, 177,178, 179],... 

The inclusion of styrene in Table IV is noteworthy. Styrene hydrofor-mylates easily with rhodium catalyst to give a mixture of 2- and 3-phen-ylpropionaldehyde in good yield (/). This is in contrast to the results reported for the cobalt system, in which hydrogenation to ethylbenzene was the principal reaction. 

Another significant and positive characteristic of phosphine-modified cobalt systems is that a high proportion of linear products can be obtained from internal olefins, with only a small sacrifice in reaction rates (58), as shown in Table X. 

Of the three catalytic systems so far recognized as being capable of giving fast reaction rates for methanol carbonylation—namely, iodide-promoted cobalt, rhodium, and iridium—two are operated commercially on a large scale. The cobalt and rhodium processes manifest some marked differences in the reaction area (4) (see Table I). The lower reactivity of the cobalt system requires high reaction temperatures. Very high partial pressures of carbon monoxide are then required in the cobalt system to... 

Of a number of Tj6-arene complexes subsequently tested for reactivity toward H2, i76-C6H5CH3—Ru6C(CO)14 was converted stoichiometrically at 150°C to methylcyclohexane, and the t76-C6(CH3)6Ru-i74-C6(CH3)6 complex (cf. 58) was found to be a long-lived homogeneous catalyst for arene hydrogenation (444) in contrast to the cobalt system, extensive H- D exchange occurred in the aromatic ring and in substituent methyls of xylene substrates. 

Another specific effect of some consequence arises from the use of aqueous organic solvent mixtures. For the cobalt system, dry solvents induce CO hydrogenations which are stoichiometric for water (13a) (within 2% material balance) according to the reaction ... 

In summary, the four chemical systems described in this paper demonstrate the versatility and selectivity of electrochemical methods for synthesis and characterization of metal-carbon a-bonded metalloporphyrins. The described rhodium and cobalt systems demonstrate significant differences with respect to their formation, stability and to some extend, reactivity of the low valent species. On the other hand, properties of the electroche-mically generated mono-alkyl or mono-aryl germanium and silicon systems are similar to each other. 

DR. SUTIN The cobalt systems that you mention differ from the iron and ruthenium systems I discussed in that the electron transfer is also accompanied by a spin change the cobalt(III) complexes are low-spin and the cobalt(II) complexes are high-spin. Thus, the electron transfer is spin forbidden and should not occur since =0. It becomes allowed through... 

Another factor in the cobalt systems is that the electron transfer involves the e antibonding orbitals. As a conse-... 

DR. HENRY TAUBE (Stanford University) These cobalt systems now appear to be less interesting than we had originally believed. Much of the literature data upon which our interest... 

Recently proof has been reported for a heterometallic bimolecular formation of aldehyde from a manganese hydride and acylrhodium species [2], Phosphine free, rhodium carbonyl species show the same kinetics as the cobalt system, i.e. the hydrogenolysis of the acyl-metal bond is rate-determining. Addition of hydridomanganese pentacarbonyl led to an increase of the rate of the hydroformylation reaction. The second termination reaction that takes place according to the kinetics under the reaction conditions (10-60 bar, 25 °C) is reaction (3). The direct reaction with H2 takes place as well, but it is slower on a molar basis than the manganese hydride reaction. 

As discussed earlier in the case of the chro-mium2 3,244 3j j cobalt systems, excess lithium can be incorporated into the layered structure through a solid solution of Li2Mn03 and LMO2, where M = Cr... 

I wish to point out that these reactions were studied in either neutral or acidic solutions where the cyanide cobalt system is really unstable thermodynamically. I raise the question about oxidation-reduction in the iodo complex. This wasn t mentioned in the paper. It seems to me it would provide an alternate path which might increase the reaction rates in the case of the iodide complex. 

The other complex is one that we prepared with this obscure ligand in the cobalt system, also with triphenylphosphine in the pole position. 

These results are presented here to emphasize the fact that selectivity and rates to various products can be subject to great variation as a result of secondary reactions. Any attempt to determine the fundamental responses of a catalytic system to changes in reaction variables must recognize the potential complications of such secondary reactions. Rathke and Feder have carried out calculations to determine the amounts of primary products actually produced by the cobalt system, assuming that these products are methanol, methyl formate, and ethylene glycol (38). The amounts of these primary products were estimated by the following relationships ... 

Several differences between the cobalt- and rhodium-catalyzed processes are noteworthy with regard to mechanism. Although there is a strong dependence in the cobalt system of the ethylene glycol/methanol ratio on temperature, CO partial pressure, and H2 partial pressure, these dependences are much lower for the rhodium catalyst. Details of the product-forming steps are therefore perhaps quite different in the two systems. It is postulated for the cobalt system that the same catalyst produces all of the primary products, but there seems to be no indication of such behavior for the rhodium system. Indeed, the multiplicity of rhodium species possibly present during catalysis and the complex dependence on promoters make it... 

The observation of glycerol triacetate as a trace product of CO hydrogenation by this ruthenium system in acetic acid solvent (179) suggests that glycolaldehyde (ester) can undergo further chain growth by the process outlined in (26) for the cobalt system. As with formaldehyde, however, a carboxylic acid is apparently necessary to promote formation of the metal-carbon bonded intermediate which can produce the longer-chain product. 

From the reaction of iron atoms with a solution of 1,5-COD in methylcyclohexane, brown crystals of Fe(l,5-COD)2 can be isolated in 40% yield (81). The thermal instability (rdec = -30°C) of this substance presents special problems in isolation of a pure product however, subsequent reactions can often be performed on solutions freed from iron metal by low-temperature filtration. The yellow-brown cobalt system, Co(l,5-COD)2, may be prepared similarly (134). 

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