Cobalt dicobalt compounds

Dicobalt octacarbonyl has been prepared by the thermal decomposition of cobalt tetracarbonyl hydride1 and by the reaction of suspensions of cobalt(II) compounds such as the carbonate in inert organic liquids with carbon monoxide under superatmospheric pressure.2,3 The procedure described here may be regarded as a modification of the second approach. It is superior to the first in giving a higher yield. 

Prior to 2005, only six crystal structures of dicobalt compounds with short Co-Co distances had been reported [7]. This section describes advances in the field of Co-Co bonding since this time. There is a significant amount of literature on cobalt clusters containing x-acid ligands, such as carbonyls, which have Co-Co bonds. However, these types of systems are outside the scope of this book so they will not be discussed in any detail. 

Cobalt.— The dimethylgermyl-bridged dicobalt compound (31) undergoes an intramolecular rearrangement process with an activation energy of 15 kcal mol". The process is thought to be that illustrated by (32) (33)... 

Unmodified Cobalt Process. Typical sources of the soluble cobalt catalyst include cobalt alkanoates, cobalt soaps, and cobalt hydroxide [1307-86 ] (see Cobalt compounds). These are converted in situ into the active catalyst, HCo(CO)4, which is in equihbrium with dicobalt octacarbonyl... 

Metal Hydrides. Metal hydrides generally react readily with acetylenes, often by an insertion mechanism. Cobalt hydrocarbonyl gives complicated mixtures of compounds with acetylenes. The only products which have been identified so far are dicobalt hexacarbonyl acetylene complexes (34). Greenfield reports that, under conditions of the hydroformy lation reaction, acetylenes give only small yields of saturated monoaldehydes (30), probably formed by first hydrogenating the acetylene and then reacting with the olefin. Other workers have identified a variety of products from acetylene, carbon monoxide, and an alcohol with a cobalt catalyst, probably cobalt hydrocarbonyl. The major products observed were succinate esters (74,19) and succinate half ester acetals (19). 

Polymerisation isomerism may be compared with polymerisation in organic compounds, and just as benzene may be regarded as a polymer of acetylene, so in the polynuclear cobalt-ammines the same type of isomerism may occur. For example, dodecammino-hexol-tetracobaltic salts are polymers of hexammino-triol-dicobaltic salts thus ... 

Compounds of this series are derived from the so-called melano-chloride, a substance formed by oxidising an ammoniacal solution of cobaltous chloride with air and precipitating a solid by means of concentrated hydrochloric acid. From the mixed solid obtained the sparingly soluble diaquo-hexammino-u-amino-ol-dicobaltic... 

In 1913 Werner 2 described a new series of optically active cobalt compounds containing two asymmetric cobalt atoms in the molecule, the tetraethylenediamino-/i-amino-nitro-dicobaltic salts. The resolution is effected by treating tetraethylencdiamino - /x - amino - nitro-dicobaltic bromide with silver d-bromo-camphor sulphonate and fractionally crystallising the product. 

In the present review we shall describe recent developments in the catalysis of reactions by dicobalt octacarbonyl. Although many of the reactions to be described do not necessarily involve dicobalt octacarbonyl directly in the catalytic cycle, but some derivative, there are several reasons for choosing this compound as a starting point. The most important reason being that dicobalt octacarbonyl is a reasonably stable, commercially available, fairly well characterized compound which easily gives active catalytic intermediates. Although by no means unique in their catalytic properties, the cobalt carbonyls do provide a particularly active and versatile example of metal carbonyl catalysis. Their catalytic reactions are also by far the most investigated and best understood. 

Heck (59) has suggested that the first step in the carboxylation reaction is the formation of cobalt hydrocarbonyl, which can be formed from dicobalt octacarbonyl and solvent (55). Alkylation and carbonylation then produce an acylcobalt carbonyl. Reaction of the acylcobalt carbonyl with the compound containing active hydrogen then regenerates cobalt hydrocarbonyl, e.g.,... 

The unique hydrogenating ability of a mixture of synthesis gas and a cobalt catalyst is intimately associated with the chemistry of the cobalt compounds formed under these conditions, namely dicobalt octacarbonyl and cobalt hydrocarbonyl. Before any mechanism for the hydrogenation reaction is discussed it is imperative to consider, if even briefly, the chemistry of the cobalt carbonyls. 

Cobalt analogues of biferrocene and bis(fulvalene)diiron have not been investigated as extensively as the iron compounds, despite the information on electronic interactions which could be gained from observing cobalt hyperfine splittings by ESR spectroscopy. The bis(fulvalene)dicobalt dication (M = Co) can be reduced in two successive one-electron steps at mild potentials ( = -0.07 V and -0.95 V), and [Co2(tj, t7 -CjoHg)Cp2], [1] " (M = Co), is reduced at -0.53 V and -0.88 V (3i) the preparation of the monocation [3]" (M = Co) has been briefly reported (32, 33). 

The species shown in the system 8.13 are those present under reaction conditions. At lower temperatures and pressures, a wealth of different complexes are present, including dicobalt octacarbonyl and cationic complexes of cobalt that differ depending on what cobalt compound was initially charged [17,23-25]. 

In general, the acetylenic triple bond is highly reactive toward hydrogenation, hydroboration, and hydration in the presence of acid catalyst. Protection of a triple bond in disubstituted acetylenic compounds is possible by complex formation with octacarbonyl dicobalt [Co2(CO)g Eq. (64) 163]. The cobalt complex that forms at ordinary temperatures is stable to reduction reactions (diborane, diimides, Grignards) and to high-temperature catalytic reactions with carbon dioxide. Regeneration of the triple bond is accomplished with ferric nitrate [164], ammonium ceric nitrate [165] or trimethylamine oxide [166]. 

Like the double bond, the carbon-carbon triple bond is susceptible to many of the common addition reactions. In some cases, such as reduction, hydroboration and acid-catalyzed hydration, it is even more reactive. A very efficient method for the protection of the triple bond is found in the alkynedicobalt hexacarbonyl complexes (.e.g. 117 and 118), readily formed by the reaction of the respective alkyne with dicobalt octacarbonyl. In eneynes this complexation is specific for the triple bond. The remaining alkenes can be reduced with diimide or borane as is illustrated for the ethynylation product (116) of 5-dehydro androsterone in Scheme 107. Alkynic alkenes and alcohols complexed in this way show an increased structural stability. This has been used for the construction of a variety of substituted alkynic compounds uncontaminated by allenic isomers (Scheme 107) and in syntheses of insect pheromones. From the protecting cobalt clusters, the parent alkynes can easily be regenerated by treatment with iron(III) nitrate, ammonium cerium nitrate or trimethylamine A -oxide. ° ... 

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