Cobalt carbonyl, stability

An additional advantage of the intramolecular protocol stems from the opportunity to prepare easily the required polyfunctional precursors via cobalt carbonyl stabilized propargyl cations. The approach based on the tandem utilization of Co-mediated alkylation and Pauson-Khand annulation was developed in Schreiber s studies to elaborate short pathways for the synthesis of polycyclic compounds. An example of the efficiency of this protocol is the two-step transformation of the acyclic precursor 409 into the tricyclic derivative 410. The cobalt-complexed acetal 409 was first transformed into the cyclooctyne derivative 411 via intramolecular reaction of the in situ generated propargyl cation 409a with the allylsilane moiety. Cyclooctyne 411 underwent smooth cycloaddition in the presence of carbon monoxide to give the target compound 410 with excellent stereoselectivity. 

For the phosphine-substituted cobalt carbonyl hydroformylations, it is probable that the mechanism follows the pathway of Heck and Breslow (77, 18), although the possibility of an associative mechanism has been raised (7). The increased stability of the HCo(CO)3PR3 complexes toward loss of CO was cited as being suggestive of a nondissociative pathway. 

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. 

Fig. 4. Stability of cobalt carbonyl catalyst [Co2(CO)8 and HCo(CO)4] as a function of CO partial pressure and reaction temperature (57, 58). (Reproduced with permission of Ernest Benn Ltd. and Springer-Verlag.)...

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... 

Strained cycloalkynes can be stabilized by coordination to one or more transition metal centers (198). The unusual vicinal defluorination reaction of perfluoro-l,3-cyclohexadiene with [Co2(CO)8] to give the /i-alkyne complex 45 (see Section III,E) prompted a study of the reactions of OFCOT with cobalt carbonyl precursors. 

The hydroformylation of olefins is one of the largest and most prominent industrial catalytic processes, producing millions of tons of aldehydes annually [102]. Initially, cobalt-carbonyl species were used as catalyst, though rhodium complexes modified by special ligands, usually phosphines, are predominantly used nowadays. Over the last two decades, continued development of new phosphine and phosphite ligands has allowed significant advances in hydroformylation chemistry, especially with respect to catalyst selectivity and stability [103]. 

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 ... 

In the sixties it was recognized that ligand substitution on the cobalt carbonyl might influence the performance of the catalyst. It has been found that aryl phosphines or phosphites have little influence in fact they may not even coordinate to cobalt under such high CO pressures. Tertiary alkyl phosphines, however, have a profound influence [5] the reaction is much slower, the selectivity to linear products increases, the carbonyl complex formed, HCoL(CO)3, is much more stable, and the catalyst acquires activity for hydrogenation. This process has been commercialized by Shell. As a result of the higher stability of the cobalt complex, the Shell process can be operated at lower pressures and higher temperatures (50-100 bar vs 200-300 bar for HCo(CO)4, 170°C vs 140°C). 

CatalystY of cobalt-carbonyl reactions can ntiUze tbe water solnbiUty of the salt and the hydrocarbon solnbihty of the metal metal dimer. Ion pairing is important in the disproportionated metal complexes, and traces of water in organic solvents can not only serve as catalysts for this reaction but can also influence the equilibrium position by stabilizing the ionic products. 

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). 

Compared with cobalt carbonyl, the phosphine-modified cobalt catalyst introduced by Shell in 1966 leads to an increase of selectivity toward linear products, to an increase in the thermal stability and hydrogenation activity, but also to a lower reactivity. In order to compensate for the lower activity, reaction temperatures have to be kept at about 180 °C. With higher temperatures the n/i selectivity drops [130] as less coordinated cobalt species are involved in the catalytic cycle. The reduced steric demand around the metal center leads to increased formation of branched aldehydes. With respect to formation of by-products, modified cobalt catalysts behave similarly to their unmodified derivatives. 

Ligand modification and thus stabilization of cobalt carbonyl, product separation by distillation and recycling of the catalyst phase (Shell process). 

For Co, having one electron less than Ni, one finds that Co(CO)4 can accept one additional electron. This is the reason why dimerization occurs giving the stable [Co(CO)4]2 complex, but bonding of Co(CO)4 with an hydrogen atom can also occur. Because of the stability of the Co(CO)4 anion (18 electron rule), the hydrogen atom in the hydrogen cobalt carbonyl complex has an acidic character ... 

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]. 

Other non-precious metal chalcogenides centers containing cobalt have been also synthesized following the template depicted in Fig. 9. In this woik, the in sitn free-surfactant method for carbon supported COxXy (X = S, Se) based on the decomposition of cobalt carbonyls was obtained. To control the growth of the metal nanoparticles and to prevent agglomeration, the use of stabilizers, e.g., donor hgands, polymers and surfactants, i.e., oleic acid (OA), trioctylphosphine oxide (TOPO), trioctyl-phosphine (TOP) and triphenylphosphine (TPP) are necessary. ... 

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