Cobalt carbonyls, decomposition

The high pressure necessary can be understood to avoid cobalt-carbonyl decomposition at the temperatures apfdied (98),... 

The hypothesis that the cobalt carbonyl radicals are the carriers of catalytic activity was disproved by a high pressure photochemistry experiment /32/, in which the Co(CO), radical was prepared under hydroformylation conditions by photolysis of dicobalt octacarbonyl in hydrocarbon solvents. The catalytic reaction was not enhanced by the irradiation, as would be expected if the radicals were the active catalyst. On the contrary, the Co(C0)4 radicals were found to inhibit the hydroformylation. They initiate the decomposition of the real active catalyst, HCo(C0)4, in a radical chain process /32, 33/. 

THE DECOMPOSITION OF COBALT OCTACARBONYL AND SUBSTITUTED BINUCLEAR COBALT CARBONYL DERIVATIVES IN THE PRESENCE OF TRIPHENYL PHOSPHINE... 

DECOMPOSITION OF DIGLYME SOLUTIONS OF VARIOUS COBALT CARBONYL COMPLEXES IN THE PRESENCE OF TRIMETHYLOLPROPANE PHOSPHITE... 

ARRHENIUS PARAMETERS FOR THE DECOMPOSITION OF COBALT CARBONYL COMPLEXES... 

Acetylation occurs at the 2-position of allene systems (Scheme 8.14). The intermediate 7t-allyl complex breaks down via the nucleophilic displacement of the cobalt carbonyl group by the hydroxide ion to produce the hydroxyketone (7) [ 11 ]. An alternative oxygen-initiated radical decomposition of the complex cannot, however, be totally precluded. The formation of a second major product, the divinyl ketone (8), probably arises from direct interaction of the dicobalt octacarbonyl with the allene and does not require the basic conditions. 

As a result of the higher stability the process can be (and must be ) operated at lower pressure (25-100 bar versus 200-300 bar for HCo(CO)4). The higher stability can be explained by the electron donation of the phosphine to the electron deficient cobalt carbonyl thus strengthening the Co-CO bonds. The phosphine complex is less active than the tetracarbonyl complex and therefore the reaction is carried out at higher temperatures (170 °C versus 140 °C). The temperature is "dictated" by the rate required the high pressures in the tetracarbonyl system are needed to prevent decomposition of the carbonyls to metal and CO. 

The compound may be prepared in a similar way from cobalt(II) iodide. Also, it may be prepared by thermal decomposition of cobalt carbonyl hydride ... 

Additional support for the carbon monoxide-alkoxycobalt insertion reaction is found in the reaction of tert-butyl hypochlorite with sodium cobalt carbonyl. This reaction, if carried out at — 80°C. in ether solution under nitrogen or carbon monoxide, leads to about a 10% yield of /er/-butoxycarbonylcobalt tetracarbonyl which has been isolated as the monotriphenylphosphine derivative. The major product seems to be cobalt octacarbonyl, possibly formed by decomposition of an intermediate JerJ-butoxycobalt tetracarbonyl (34). This reaction appears to be a true insertion reaction although a direct attack of a coordinated carbonyl group upon oxygen cannot be ruled out. 

Compared with the analogous hydrogenation of aldehydes, the reaction requires somewhat more drastic conditions (about 200°C and 6 hrs), but the temperature is still within the stability range of the cobalt carbonyl phosphine complexes containing tertiary alkyl phosphines as ligands. If aryl phosphines are used, a more or less pronounced decomposition of the carbonyl complexes can be observed (as indicated by the IR... 

Caution. Decomposition of the cobalt carbonyl clusters produces CO, which is an odorless, toxic gas. The hexane solution of ZnEt2 is convenient to handle, but is a source of ZnEt2, which is a pyrophoric and volatile liquid. Handling these compounds in a well-ventilated hood and wearing eye protection are recommended. 

In spite of its instability in the liquid state, cobalt hydrocarbonyl can be carried from one vessel to another as a vapor highly diluted with carbon monoxide or any other inert gas at room temperature and pressure. A semiquantitative study of the rate of decomposition in both the vapor state and in hexane solution indicates that the half-life is dependent on the initial concentration (Sternberg, Wender, Friedel, and Orchin, 29). It can be absorbed in water at 0° and behaves as a strong mineral acid. The ionization provides a cobalt carbonyl anion ... 

A high CO pressure would shift equilibrium (4.3) to the left and the catalytic reaction would become slower. In this complex CO is a far better ligand than an alkene. On the other hand the reaction uses CO as a substrate, so it cannot be omitted. Furthermore, low pressures of CO may lead to decomposition of the cobalt carbonyl complexes to metallic cobalt and CO, which is also undesirable. Finally, the product alcohol may stabilize divalent cobalt species which are not active as a catalyst ... 

Cool the reactor to —196° and remove the noncondensable materials by pumping. Allow the reactor to warm to 0° and remove the excess trifluorosilane, tetracarbonyl(trifluorosilyl)-cobalt, and any cobalt carbonyl hydride formed by pumping the volatile products through a — 78° (Dry Ice-acetone mixture) trap into a —196° trap. A dark residue will remain in the reactor consisting of unreacted dicobalt octacarbonyl and decomposition products. Unreacted trifluorosilane will be in the —196° trap. 

The first generation of hydroformylation processes (e.g., by BASF, ICI, Kuhlmann, Ruhrchemie) was exclusively based on cobalt as catalyst metal. As a consequence of the well-known stability diagram for cobalt carbonyl hydrides, the reaction conditions had to be rather harsh the pressure ranged between 20 and 35 MPa to avoid decomposition of the catalyst and deposition of metallic cobalt, and the temperature was adjusted according to the pressure and the concentration of the catalyst between 150 and 180 °C to ensure an acceptable rate of reaction. As the reaction conditions were quite similar, the processes differed only in the solution of the problem of how to separate product and catalyst, in order to recover and to recycle the catalyst [4]. Various modes were developed they largely yielded comparable results, and enabled hydroformylation processes to grow rapidly in capacity and importance (see Section 2.1.1.4.3). 

The use of Co2(CO)g as a hydrogenation catalyst in the absence of CO generally results in decomposition to metal. Phosphine derivatives of cobalt carbonyl complexes, which are more robust hydrogenation catalysts, will be discussed next. 

Both Co2(CO)g and Co4(CO),2 are oxidized by air even in the solid state, and by halogens. With the latter, complete decomposition to CO and Co(II) halides occurs. Reaction of cobalt carbonyl derivatives with I2 in pyridine is frequently used for analyses by gas volumetry of CO evolved. Octacarbonyldicobalt, Co2(CO)g, is attacked by organic substances that contain active protons and converted to the corresponding Co(II) salts , as in reactions for Fe(CO)j [equations (1) and (m), 14.6.2.3.2]. Included here is reaction of Co2(CO)g with cyclopentadiene to give dicarbonylcyclopentadienylcobalt(I) as a dark red oil (b.p. 75 C/22 torr) ... 

The carbonylation of a benzyl halide in the presence of iron pentacarbonyl to give a phenylacetic acid may serve to exemplify the interaction of a metal carbonyl, carbon monoxide, PT catalyst, aqueous sodium hydroxide, and the substrate [79]. Fe(CO)5 is attacked by QOH at the interphase, and the species formed is extracted into the depths of the oganic phase, where it reacts with CO and benzyl halide (Eqs. 13 and 14). This new anion 3 is the actual catalyst. It reacts with a second benzyl halide to give a non-ionic intermediate 4 (Eq. 15). By insertion of CO and attack of QOH, 4 is decomposed to the reaction product under regeneration of 3 (Eq. 16). Thus, the action of the PT catalyst is twofold. Firstly it transports the metal carbonyl anion. More important seems to be its involvement in the (rate-determining) decomposition step. A basically similar mechanism was proposed for cobalt carbonyl reactions [80], which have been modified somewhat quite recently (see below). 

The cobalt carbonyls typify complexes of cobalt in low oxidation states. Finely dispersed cobalt reacts with gaseous carbon monoxide under pressure to yield orange, sublimable, dinudear dicobalt octacarbonyl, Co2(CO)g (m.p. 51 °C, decomposition above 52°C), a derivative of Co(0). This carbonyl is soluble in organic solvents, and on... 

Thomas succeeded in forming 2-30nm particles of cobalt with a narrow size distribution by decomposing cobalt carbonyl compounds in the presence of suitable surface-active reagents [50]. By the same procedure, iron and nickel particles were prepared. Papirer et al. studied the decomposition of a toluene solution of Co2(CO)8 in the presence of a surface-active reagent and found that at least two factors are responsible for the formation of particles with an extremely narrow size distribution the division of the system into microreactors and a diffusion-controlled growth mechanism of the individual particles [51]. 

An alternative method of preparing cobalt nanopartides is based on the thermal decomposition of Co2(CO)g in the presence of a suitable surfactant [27]. A typical recipe involves the injection of a solution of Co2(CO)g in diphenylether (solution A) into the hot (200 °C) mixture of oleic acid and TOP or TBP, dissolved also in diphenylether (solution B). The mixture is then heated at 200 °C for 15-20 min. Decomposition of the cobalt carbonyl, under the conditions described above, results in Co nanopartides with a so-called multiply-twinned face-centered cubic (mt-fee) lattice (Figures 3.115b and (3.116)d). The multiply-twinned particles are composed of domains with a distorted fee lattice, this structure being similar to that observed in multiply-twinned, icosahedral gold particles [27]. Post-preparative size-selection... 

Synthesis of CoPtj Magnetic Alloy Nanocrystals The synthetic approach developed for the preparation of elemental nanopartides can be further extended to intermetallic compounds. Thus, high-quality CoPt3 nanocrystals can be synthesized via the simultaneous reduction of platinum acetylacetonate and the thermal decomposition of cobalt carbonyl in the presence of 1-adamantanecarboxylic add (ACA) and hexadecylamine (HDA) as stabilizing agents [65]. 

The preparation of core-shell -type magnetic nanopartides was reported [92,93] as a two-step synthesis in which nanopartides of one metal served as the seeds for growth of the shell from another metal. Thus, the reduction of platinum salts in the presence of Co nanopartides would allow the preparation of air-stable CocorePtsheii nanopartides [92]. The thermal decomposition of cobalt carbonyl in the presence of silver salt led to the formation of AgcoreCOgheii nanopartides [93]. The syntheses of these multicomponent magnetic nanostructures are discussed in Section 3.3.2.4. 

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