Dicobalt octacarbonyl

Dicobalt octacarbonyl is the organometallic compound with composition Co₂(CO)₈. This metal carbonyl is used as a reagent and catalyst in organometallic chemistry and organic synthesis, and is central to much known organocobalt chemistry.[2][3] It is the precursor to a hydroformylation catalyst, cobalt tetracarbonyl hydride.[4] Each molecule consists of two cobalt atoms bound to eight carbon monoxide ligands, though multiple distinct structural arrangements are known.[5] Some of the carbonyl ligands are highly labile. The compound is highly reactive towards alkynes, and is sometimes used as an alkyne protecting group. As the cobalt-alkyne complex, it plays a role in promoting both the Nicholas reaction[3][6][7] and the Pauson–Khand reaction.[3][8][9]

Dicobalt octacarbonyl

Names
IUPAC name Octacarbonyldicobalt(Co—Co)
Other names Cobalt carbonyl, di-mu-Carbonylhexacarbonyldicobalt, Cobalt octacarbonyl, Cobalt tetracarbonyl dimer, Dicobalt carbonyl, Octacarbonyldicobalt
Identifiers
CAS Number	10210-68-1^Y
3D model (JSmol)	Interactive image Interactive image
ChemSpider	2007057^Y
ECHA InfoCard	100.030.454
EC Number	233-514-0
PubChem CID	25049
RTECS number	GG0300000
UNII	MDH533S43Q^Y
UN number	3281
CompTox Dashboard (EPA)	DTXSID10895040
InChI InChI=1S/8CO.2Co/c8*1-2;;/q;;;;;;;;2*+2^Y Key: MQIKJSYMMJWAMP-UHFFFAOYSA-N^Y InChI=1/8CO.2Co/c8*1-2;;/q;;;;;;;;2*+2 Key: MQIKJSYMMJWAMP-UHFFFAOYAG
SMILES O=C=[Co]1(=C=O)(=C=O)C(=O)[Co](=C=O)(=C=O)(=C=O)C1=O O=C=[Co-4](=C=O)(=C=O)(=C=O)[Co-4](=C=O)(=C=O)(=C=O)=C=O
Properties
Chemical formula	Co2(CO)8
Molar mass	341.95 g/mol
Appearance	red-orange crystals white crystalline solid when pure[1]
Density	1.87 g/cm^3
Melting point	51 to 52 °C (124 to 126 °F; 324 to 325 K)
Boiling point	52 °C (126 °F; 325 K) c.a. decomposes
Solubility in water	insoluble
Vapor pressure	0.7 mmHg (20 °C)[1]
Structure
Dipole moment	1.33 D (C2v isomer) 0 D (D3d isomer)
Hazards
Main hazards	Very toxic, evolves CO gas on decomposition,[1] pyrophoric, carcinogenic
Safety data sheet	External MSDS
GHS pictograms
GHS Signal word	Danger
GHS hazard statements	H228, H251, H302, H302+312, H315, H317, H319, H330, H334, H350, H351, H361, H412
NFPA 704 (fire diamond)	3 4 3
Flash point	Pyrophoric [1]
NIOSH (US health exposure limits):
PEL (Permissible)	none[1]
REL (Recommended)	TWA 0.1 mg/m^3[1]
IDLH (Immediate danger)	N.D.[1]
Related compounds
Related metal carbonyls	Iron pentacarbonyl Diiron nonacarbonyl Nickel tetracarbonyl
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is ^YN ?)
Infobox references

Synthesis, structure, properties

Dicobalt octacarbonyl is a white solid when of high purity, but more typically is an orange-colored, pyrophoric solid that is thermally unstable.[1] It is synthesised by the high pressure carbonylation of cobalt(II) salts. In the method patented by James Eli Knap, cobalt(II) acetate is heated to between 150 and 200 °C and exposed to hydrogen and carbon monoxide gases at pressures of 2000 to 6000 psi:[10]

2 Co(CH₃CO₂)₂ + 8 CO + 2 H₂ → Co₂(CO)₈ + 4 CH₃COOH

The preparation is often carried out in the presence of cyanide, converting the cobalt(II) salt into a hexacyanocobaltate(II) complex that is then treated with carbon monoxide to yield K[Co(CO)₄]. Acidification produces cobalt tetracarbonyl hydride, HCo(CO)₄, which can then be heated to form dicobalt octacarbonyl.[3][11] It can also be prepared by heating cobalt metal to above 250 °C in a stream of carbon monoxide gas at about 200 to 300 atm:[3]

2 Co + 8 CO → Co₂(CO)₈

It is known to exist in several isomeric forms, all with the same composition – [Co₂(CO)₈] – with two cobalt metal centres in oxidation state zero surrounded by eight carbonyl (CO) ligands. These ligands can be bridging between the two cobalt centres or bound to a single metal centre (a terminal ligand).[2][3] In solution, there are two isomers known that rapidly interconvert:[5]

The major isomer (on the left in the above equilibrium process) contains two bridging carbonyl ligands linking the cobalt centres and six terminal carbonyl ligands, three on each metal.[5] It can be summarised by the formula (CO)₃Co(μ-CO)₂Co(CO)₃ and has C_2v symmetry. This structure resembles diiron nonacarbonyl (Fe₂(CO)₉) but with one fewer bridging carbonyl. The Co–Co distance is 2.52 Å, and the Co–CO_terminal and Co–CO_bridge distances are 1.80 and 1.90 Å, respectively.[12] Analysis of the bonding suggests the absence of a direct cobalt–cobalt bond.[13]

The minor isomer has no bridging carbonyl ligands, but instead has a direct bond between the cobalt centres and eight terminal carbonyl ligands, four on each metal atom.[5] It can be summarised by the formula (CO)₄Co-Co(CO)₄ and has D_4d symmetry. It features an unbridged cobalt–cobalt bond that is 2.70 Å in length in the solid structure when crystallized together with C₆₀.[14]

Reactions

Nicholas reaction

The Nicholas reaction is a substitution reaction whereby an alkoxy group located on the α-carbon of an alkyne is replaced by another nucleophile.[6][7] The alkyne reacts first with dicobalt octacarbonyl, from which is generated a stabilized propargylic cation[15][16] that reacts with the incoming nucleophile and the product then forms by oxidative demetallation.[6][7]

The Nicholas reaction

Pauson–Khand reaction

The Pauson–Khand reaction,[8] in which an alkyne, an alkene, and carbon monoxide cyclize to give a cyclopentenone, can be catalyzed by Co₂(CO)₈,[3][9] though newer methods that are more efficient have since been developed:[17][18]

Co₂(CO)₈ reacts with alkynes to form a stable covalent complex, which is useful as a protective group for the alkyne. This complex itself can also be used in the Pauson–Khand reaction.[8]

Intramolecular Pauson–Khand reactions, where the starting material contains both the alkene and alkyne moieties, are possible. In the asymmetric synthesis of the Lycopodium alkaloid huperzine-Q, Takayama and co-workers used an intramolecular Pauson–Khand reaction to cyclise an enyne containing a tert-butyldiphenylsilyl (TBDPS) protected primary alcohol.[19] The preparation of the cyclic siloxane moiety immediately prior to the introduction of the dicobalt octacarbonyl ensures that the product is formed with the desired conformation.[20]

Catalytic cycle for the hydroformylation of a terminal alkene (RCH=CH₂) to an aldehyde (RCH₂CH₂CHO):[4]
Step 1: Dissociation of carbon monoxide from cobalt tetracarbonyl hydride to form HCo(CO)₃, the active catalytic species
Step 2: The cobalt centre forms a π bond to the alkene
Step 3: Alkene ligand inserts into the cobalt–hydride bond
Step 4: Coordination of an additional carbonyl ligand
Step 5: Migratory insertion of a carbonyl ligand into the cobalt–alkyl bond, converting the alkyl tetracarbonyl intermediate into an acyl tricarbonyl species[21]
Step 6: Oxidative addition of dihydrogen leads to a dihydrido complex
Step 7: Aldehyde product released by reductive elimination,[22] regenerating the active catalytic species
Step 8: An unproductive and reversible side reaction

Hydroformylation

Hydrogenation of Co₂(CO)₈ produces cobalt tetracarbonyl hydride, [HCo(CO)₄]:[23]

Co₂(CO)₈ + H₂ → 2 HCo(CO)₄

This hydride is used as a catalyst for hydroformylation – the conversion of a terminal alkene, RCH=CH₂, to an aldehyde, RCH₂CH₂CHO.[4][23] The catalytic cycle for this hydroformylation is shown in the diagram.[4][21][22]

Reduction of Co₂(CO)₈ with sodium amalgam gives the conjugate base of HCo(CO)₄. This salt yields the hydride on acidification, providing an alternative synthetic pathway to that species.[3] Salts of this form are also intermediates in the cyanide synthesis pathway for dicobalt octacarbonyl.[11]

Co₂(CO)₈ + 2 Na → 2 NaCo(CO)₄

NaCo(CO)₄ + H⁺ → HCo(CO)₄ + Na⁺

The CO ligands can be replaced with tertiary phosphine ligands to give Co₂(CO)_8−x(PR₃)_x. These bulky derivatives are more selective catalysts for hydroformylation reactions.[3] "Hard" Lewis bases, e.g. pyridine, cause disproportionation:

12 C₅H₅N + 3 Co₂(CO)₈ → 2 [Co(C₅H₅N)₆][Co(CO)₄]₂ + 8 CO

Methylidynetricobaltnonacarbonyl, HCCo₃(CO)₉, an organocobalt cluster compound structurally related to tetracobalt dodecacarbonyl

Tricobalt nonacarbonyls

Heating causes decarbonylation and formation of tetracobalt dodecacarbonyl:[3][24]

2 Co₂(CO)₈ → Co₄(CO)₁₂ + 4 CO

Like many metal carbonyls, dicobalt octacarbonyl abstracts halides from alkyl halides. Upon reaction with bromoform, it converts to methylidynetricobaltnonacarbonyl, HCCo₃(CO)₉, by a reaction that can be idealised as:[25]

9 Co₂(CO)₈ + 4 CHBr₃ → 4 HCCo₃(CO)₉ + 36 CO + 6 CoBr₂

X-ray crystallographic analysis shows the product contains a triangle of cobalt atoms at distances near 2.48 Å, each bound to three terminal carbonyl groups, and with a methylidyne (CH) group forming the apex of a triangular pyramid. This product is structurally related to tetracobalt dodecacarbonyl,[26] theoretically by replacing the methylidyne group by a fourth Co(CO)₃ moiety. However, there has been disagreement between theory and experiment with the actual structure of tetracobalt dodecacarbonyl being shown to have three bridging carbonyl groups.[27][28][29]

Safety

Co₂(CO)₈ a volatile source of cobalt(0), is pyrophoric and releases carbon monoxide upon decomposition.[30] The National Institute for Occupational Safety and Health has recommended that workers should not be exposed to concentrations greater than 0.1 mg/m³ over an eight-hour time-weighted average, without the proper respiratory gear.[31]

References

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8. Pauson, P. L.; Khand, I. U. (1977). "Uses of Cobalt-Carbonyl Acetylene Complexes in Organic Synthesis". Ann. N. Y. Acad. Sci. 295 (1): 2–14. doi:10.1111/j.1749-6632.1977.tb41819.x.
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13. Green, Jennifer C.; Green, Malcolm L. H.; Parkin, Gerard (2012). "The occurrence and representation of three-centre two-electron bonds in covalent inorganic compounds". Chemical Communications. 2012 (94): 11481–11503. doi:10.1039/c2cc35304k.
14. Garcia, Thelma Y.; Fettinger, James C.; Olmstead, Marilyn M.; Balch, Alan L. (2009). "Splendid symmetry: Crystallization of an unbridged isomer of Co₂(CO)₈ in Co₂(CO)₈^·C₆₀". Chemical Communications. 2009 (46): 7143–7145. doi:10.1039/b915083h.
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17. Schore, Neil E. (1991). "The Pauson–Khand Cycloaddition Reaction for Synthesis of Cyclopentenones". Org. React. 40: 1. doi:10.1002/0471264180.or040.01.
18. Gibson, Susan E.; Stevenazzi, Andrea (2003). "The Pauson–Khand Reaction: The Catalytic Age Is Here!". Angew. Chem. Int. Ed. 42 (16): 1800–1810. doi:10.1002/anie.200200547.
19. Nakayama, Atsushi; Kogure, Noriyuki; Kitajima, Mariko; Takayama, Hiromitsu (2011). "Asymmetric Total Synthesis of a Pentacyclic Lycopodium Alkaloid: Huperzine‐Q". Angew. Chem. Int. Ed. 50 (35): 8025–8028. doi:10.1002/anie.201103550.
20. Ho, Tse-Lok (2016). "Dicobalt Octacarbonyl". Fiesers' Reagents for Organic Synthesis. 28. John Wiley & Sons. pp. 251–252. ISBN 9781118942819.
21. Heck, Richard F.; Breslow, David S. (1961). "The Reaction of Cobalt Hydrotetracarbonyl with Olefins". Journal of the American Chemical Society. 83 (19): 4023–4027. doi:10.1021/ja01480a017.
22. Halpern, Jack (2001). "Organometallic chemistry at the threshold of a new millennium. Retrospect and prospect". Pure and Applied Chemistry. 73 (2): 209–220. doi:10.1351/pac200173020209.
23. Pfeffer, M.; Grellier, M. (2007). "Cobalt Organometallics". Comprehensive Organometallic Chemistry III. 7. Elsevier. pp. 1–119. doi:10.1016/B0-08-045047-4/00096-0.
24. Chini, P. (1968). "The closed metal carbonyl clusters". Inorganica Chimica Acta Reviews. 2: 31–51. doi:10.1016/0073-8085(68)80013-0.
25. Nestle, Mara O.; Hallgren, John E.; Seyferth, Dietmar; Dawson, Peter; Robinson, Brian H. (1980). "μ₃-Methylidyne and μ₃-Benzylidyne-Tris(Tricarbonylcobalt)". Inorg. Synth. 20: 226–229. doi:10.1002/9780470132517.ch53.
26. Leung, P.; Coppens, P.; McMullan, R. K.; Koetzle, T. F. (1981). "The Structure of Nonacarbonyl-μ₃-methylidyne-triangulo-tricobalt. X-ray and Neutron Diffraction Studies". Acta Crystallogr. B. 37 (7): 1347–1352. doi:10.1107/S0567740881005906.
27. Corradini, Paolo (1959). "Structure of tetracobaltdodecarbonyl". Journal of Chemical Physics. 31 (6): 1676–1677. doi:10.1063/1.1730674.
28. Wei, Chin Hsuan (1969). "Structural analyses of tetracobalt dodecacarbonyl and tetrarhodium dodecacarbonyl. Crystallographic treatments of a disordered structure and a twinned composite". Inorganic Chemistry. 8 (11): 2384–2397. doi:10.1021/ic50081a030.
29. Farrugia, Louis J.; Braga, Dario; Grepioni, Fabrizia (1999). "A structure redetermination of Co₄(CO)₁₂: Evidence for dynamic disorder and the pathway of metal atom migration in the crystalline phase". Journal of Organometallic Chemistry. 573 (1–2): 60–66. doi:10.1016/S0022-328X(98)00879-1.
30. Cole Parmer MSDS
31. CDC - NIOSH Pocket Guide to Chemical Hazards