Organocobalt chemistry

Organocobalt chemistry is the chemistry of organometallic compounds containing a carbon to cobalt chemical bond. Organocobalt compounds are involved in several organic reactions and the important biomolecule vitamin B₁₂ has a cobalt-carbon bond. Many organocobalt compounds exhibit useful catalytic properties, the preeminent example being dicobalt octacarbonyl.^[1] An early example of organocobalt chemistry is the carbonylation of azobenzene with dicobalt octacarbonyl as described by Murahashi & Horiie in 1956:^[2]

Carbonyl complexes

Dicobalt octacarbonyl reacts with hydrogen and alkenes to give aldehydes. This reaction is the basis of hydroformylation, the formation of aldehydes from an alkene, CO and hydrogen. A key intermediate is cobalt tetracarbonyl hydride (HCo(CO)₄). The original Ruhrchemie process produced propanal from ethene and syngas using cobalt carbonyl has been displaced by rhodium-based catalysts. Processes involving cobalt are practiced by BASF, EXXON, and Shell mainly for the production of C7-C14 alcohols used for the production of surfactants.^[3]

In hydrocarboxylations hydrogen is replaced by water or an alcohol and the reaction product is a carboxylic acid or an ester. An example of this reaction type is the conversion of butadiene to adipic acid.

Alkyne derivatives of Co₂(CO)₈

Dicobalt octacarbonyl also reacts with alkynes to give "tetrahedranes" of the formula Co₂(CO)₆(C₂R₂). Because the cobalt carbonyl centers can be removed later, it functions as a protective group for the alkyne. In the Nicholas reaction an alkyne group is also protected and at the same time the alpha-carbon position is activated for nucleophilic substitution.

Cyclization reactions

Cobalt compounds react with dialkynes and dienes to cyclic intermediates in cyclometalation. Other alkynes, alkenes, nitriles or carbon monoxide can then insert themselves into the Co-C bond. Reaction types based on this concept are the Pauson–Khand reaction (CO insertion) and alkyne trimerization (notably with cyclopentadienylcobalt dicarbonyl).

Cp, allyl, and alkene compounds

Sandwich compounds

Organocobalt compounds are known with alkene, allyl, diene, and Cp ligands. A famous sandwich compound is cobaltocene, a 19-electron metallocene that is used as a reducing agent and a source of CpCo. Other sandwich compounds are CoCp(C₆Me₆) and Co(C₆Me₆)₂, with 20 electrons and 21 electrons, respectively. Reduction of cobalt(II) compounds in the presence of cyclooctadiene gives Co(cyclooctadiene)(cyclooctenyl), a synthetically versatile reagent^[4]

Co(1,5-cyclooctadiene)(cyclooctenyl).

CpCo(CO)₂ and derivatives

The half-sandwich compound cyclopentadienylcobalt dicarbonyl (CpCo(CO)₂) is a particularly versatile reagent because the CO ligands can be replaced and olefin and alkyne derivatives undergo reactions. A well studied reaction is alkyne trimerisation,^[5] which has been applied to the synthesis of a variety of complex structures.^[6][7]

Vitamin B₁₂-type compounds

Cobalt is found in vitamin B₁₂ and related enzymes. These cofactors catalyze unusual reactions involving the intermediacy of Co-C bonds. In these reactions, the oxidation state of cobalt can vary from Co(III) to Co(I). In methylcobalamin the ligand is a methyl group, which is electrophilic. in vitamin B12, the alkyl ligand is an adenosyl group. Related to vitamin B12 are cobalt porphyrins, dimethylglyoximates, and related complexes of Schiff base ligands. These synthetic compounds also form alkyl derivatives that undergo diverse reactions reminiscent of the biological processes.

Cobalt-Mediated Radical Polymerization

The weak cobalt(III)-carbon bond in vitamin B12 analogues can be exploited in a type of controlled radical polymerization of acrylic and vinyl esters (e.g. vinyl acetate), acrylic acid and acrylonitrile. The reaction temperature is typically between 0 and 60 °C.^[8] A Co-C bond containing radical initiator breaks up (by heat or by light) in a carbon free radical and a cobalt(II) radical species. The carbon radical starts polymer chain formation with monomer for instance an alkene as in any ordinary radical polymerization. Cobalt reversibly reforms the bond with the carbon radical terminus of the growing chain, which minimizes the concentration of radicals and suppresses undesirable termination reactions by recombination of two carbon radicals. The cobalt trapping reagent is called a persistent radical and the cobalt-capped polymer chain is said to be dormant. CMRP can be regarded as a series of carbometalation reactions of vinyl monomers. When the monomer possesses protons that can be easily abstracted by the cobalt radical, (catalytic) chain transfer may occur. The concept was introduced independently by two groups in 1994.^[9][10]

Cobalt mediated radical polymerization can proceed by catalytic chain transfer or by degenerative transfer:

Fischer-Tropsch catalysis

Cobalt catalysts (together with iron) are relevant in the Fischer-Tropsch process in which synthesis gas is converted to hydrocarbons. In this process, it is assumed that organocobalt intermediates form. An idealized reaction sequence is depicted below:^[11]

M + CO → M-CO (M = Co, Fe)

M-CO + H
2→ M-CH
3

M-CH
3+ CO → OC-M-CH
3

OC-M-CH
3→ M-(CO)-CH
3

M-(CO)-CH
3+ H
2→ M-CH
2CH
3

See also

• Chemical bonds of carbon with other elements in the periodic table:

References