Most fundamental are the cobalt complexes with only alkyl ligands. Examples include Co(4-norbornyl)4 and its cation.[3]
Alkylcobalt is represented by
vitamin B12 and related enzymes. 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. The weak cobalt(III)-carbon bond in vitamin B12 analogues can be exploited in a type of
Cobalt mediated radical polymerization of acrylic and vinyl esters (e.g.
vinyl acetate),
acrylic acid and
acrylonitrile.[4]
Carbonyl complexes
Dicobalt octacarbonyl is produced by the
carbonylation of cobalt salts. It and its
phosphine derivatives are among the most widely used organocobalt compounds. Heating Co2(CO)8 gives Co4(CO)12. Very elaborate cobalt-carbonyl clusters have been prepared starting from these complexes. Heating cobalt carbonyl with
bromoform gives
methylidynetricobaltnonacarbonyl. Dicobalt octacarbonyl also reacts with alkynes to give
dicobalt hexacarbonyl acetylene complexes with the formula Co2(CO)6(C2R2). Because they can be removed later, the cobalt carbonyl centers function 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.
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 rare example of low-spin Co(II) complex. This 19-electron metallocene is used as a reducing agent and as a source of CpCo. Other sandwich compounds are CoCp(C6Me6) and Co(C6Me6)2, with 20 electrons and 21 electrons, respectively. Reduction of anhydrous cobalt(II) chloride with sodium in the presence of
cyclooctadiene gives Co(cyclooctadiene)(cyclooctenyl), a synthetically versatile reagent.[5]
The half-sandwich compounds of the type CpCoL2 have been well investigated (L = CO, alkene). The complexes CpCo(C2H4)2 and CpCo(cod) catalyze
alkyne trimerisation,[6] which has been applied to the synthesis of a variety of complex structures.[7]
Applications
Dicobalt octacarbonyl is used commercially for
hydroformylation of alkenes. A key intermediate is
cobalt tetracarbonyl hydride (HCo(CO)4). Processes involving cobalt are practiced commercially mainly for the production of C7-C14 alcohols used for the production of
surfactants.[10][11] Many hydroformylations have switched from cobalt-based processes to rhodium-based processes, despite the great expense of that metal. Replacing H2 by water or an
alcohol, the reaction product is a
carboxylic acid or an
ester. An example of this reaction type is the conversion of
butadiene to
adipic acid. Cobalt catalysts (together with
iron) are relevant in the
Fischer–Tropsch process in which it is assumed that organocobalt intermediates form.
Cobalt complexes have been applies to the synthesis of
pyridine derivatives starting from alkynes and nitriles.
Aspirational applications
Although really only dicobalt octacarbonyl has achieved commercial success, many reports have appeared promising applications.[12][13][14] Often these ventures are motivated by the use of "earth abundant" catalysts.[15]
^Byrne, Erin K.; Theopold, Klaus H. (1987-02-01). "Redox chemistry of tetrakis(1-norbornyl)cobalt. Synthesis and Characterization of a Cobalt(V) Alkyl and Self-Exchange Rate of a Co(III)/Co(IV) Couple". Journal of the American Chemical Society. 109 (4): 1282–1283.
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10.1021/ja00238a066.
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^Gosser, L. W.; Cushing, M. A. Jr. (1977). "Π-Cyclooctenyl-π-L,5-Cycloocta-Dienecobalt". π-Cyclooctenyl-π-1,5-cyclooctadienecobalt. Inorganic Syntheses. Vol. 17. pp. 112–15.
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^Cobalt-Catalyzed Cyclotrimerization of Alkynes: The Answer to the Puzzle of Parallel Reaction Pathways Nicolas Agenet, Vincent Gandon, K. Peter C. Vollhardt, Max Malacria, Corinne Aubert J. Am. Chem. Soc.; 2007; 129(28) pp 8860 - 8871; (Article)
doi:
10.1021/ja072208r
^Hebrard, Frédéric; Kalck, Philippe (2009). "Cobalt-Catalyzed Hydroformylation of Alkenes: Generation and Recycling of the Carbonyl Species, and Catalytic Cycle". Chemical Reviews. 109 (9): 4272–4282.
doi:
10.1021/cr8002533.
PMID19572688.
^Boy Cornils,
Wolfgang A. Herrmann, Chi-Huey Wong, Horst Werner Zanthoff: Catalysis from A to Z: A Concise Encyclopedia, 2408 Seiten, Verlag Wiley-VCH Verlag GmbH & Co. KGaA, (2012),
ISBN3-527-33307-X.
^Liu, Weiping; Sahoo, Basudev; Junge, Kathrin; Beller, Matthias (2018). "Cobalt Complexes as an Emerging Class of Catalysts for Homogeneous Hydrogenations". Accounts of Chemical Research. 51 (8): 1858–1869.
doi:
10.1021/acs.accounts.8b00262.
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^Guo, Jun; Cheng, Zhaoyang; Chen, Jianhui; Chen, Xu; Lu, Zhan (2021). "Iron- and Cobalt-Catalyzed Asymmetric Hydrofunctionalization of Alkenes and Alkynes". Accounts of Chemical Research. 54 (11): 2701–2716.
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10.1021/acs.accounts.1c00212.
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