Dicobalt octacarbonyl

Dicobalt octacarbonyl is an organocobalt 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 parent member of a family of hydroformylation catalysts.[4] Each molecule consists of two cobalt atoms bound to eight carbon monoxide ligands, although multiple structural isomers are known.[5] Some of the carbonyl ligands are labile.

Dicobalt octacarbonyl
Dicobalt octacarbonyl, bridged C2v isomer

Co2(CO)8 soaked in hexanes
Names
IUPAC name
Octacarbonyldicobalt(Co—Co)
Other names
Cobalt carbonyl (2:8), di-mu-Carbonylhexacarbonyldicobalt, Cobalt octacarbonyl, Cobalt tetracarbonyl dimer, Dicobalt carbonyl, Octacarbonyldicobalt
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.030.454
EC Number
  • 233-514-0
RTECS number
  • GG0300000
UNII
UN number 3281
  • InChI=1S/8CO.2Co/c8*1-2;;/q;;;;;;;;2*+2 checkY
    Key: MQIKJSYMMJWAMP-UHFFFAOYSA-N checkY
  • InChI=1/8CO.2Co/c8*1-2;;/q;;;;;;;;2*+2
    Key: MQIKJSYMMJWAMP-UHFFFAOYAG
  • 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
Co2(CO)8
Molar mass 341.95 g/mol
Appearance red-orange crystals
Density 1.87 g/cm3
Melting point 51 to 52 °C (124 to 126 °F; 324 to 325 K)
Boiling point 52 °C (126 °F; 325 K) decomposes
insoluble
Vapor pressure 0.7 mmHg (20 °C)[1]
Structure
1.33 D (C2v isomer)
0 D (D3d isomer)
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
 Potential carcinogen
GHS labelling:
GHS02: FlammableGHS06: ToxicGHS07: Exclamation markGHS08: Health hazard
Danger
H251, H302, H304, H315, H317, H330, H351, H361, H412
P201, P260, P273, P280, P304+P340+P310, P403+P233
NFPA 704 (fire diamond)
NFPA 704 four-colored diamond
4
3
1
Flash point -23 °C (-9.4 °F)[1]
Lethal dose or concentration (LD, LC):
15 mg/kg (oral, rat)
NIOSH (US health exposure limits):
PEL (Permissible)
 none[1]
REL (Recommended)
 TWA 0.1 mg/m3[1]
IDLH (Immediate danger)
 N.D.[1]
Safety data sheet (SDS) External SDS
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 checkY☒N ?)
Infobox references

Synthesis, structure, properties

Dicobalt octacarbonyl an orange-colored, pyrophoric solid.[6] It is synthesised by the high pressure carbonylation of cobalt(II) salts:[6]

2 (CH3COO)2Co + 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 pentacyanocobaltate(II) complex that reacts with carbon monoxide to yield K[Co(CO)4]. Acidification produces cobalt tetracarbonyl hydride, HCo(CO)4, which degrades near room temperature to dicobalt octacarbonyl and hydrogen.[3][7] 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 exist as a mixture of rapidly interconverting isomers.[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 C2v 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–COterminal and Co–CObridge distances are 1.80 and 1.90 Å, respectively.[8] Analysis of the bonding suggests the absence of a direct cobalt–cobalt bond.[9]

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 D4d symmetry. It features an unbridged cobalt–cobalt bond that is 2.70 Å in length in the solid structure when crystallized together with C60.[10]

Isomers of dicobalt octacarbonyl
  • Bridged C2v isomer
    Bridged C2v isomer
  • non-bridged D3d isomer
    non-bridged D3d isomer
  • non-bridged D2d isomer
    non-bridged D2d isomer

Reactions

Reduction

Dicobalt octacarbonyl is reductively cleaved by alkali metals and related reagents, such as sodium amalgam. The resulting salts protonate to give tetracarbonyl cobalt hydride:[3]

Co2(CO)8 + 2 Na → 2 Na[Co(CO)4]
Na[Co(CO)4] + H+ → H[Co(CO)4] + Na+

Salts of this form are also intermediates in the cyanide synthesis pathway for dicobalt octacarbonyl.[7]

Reactions with electrophiles

Halogens and related reagents cleave the Co–Co bond to give pentacoordinated halotetracarbonyls:

Co2(CO)8 + Br2 → 2 Br[Co(CO)4]

Cobalt tricarbonyl nitrosyl is produced by treatment of dicobalt octacarbonyl with nitric oxide:

Co2(CO)8 + 2 NO → 2 Co(CO)3NO + 2 CO

Reactions with alkynes

The Nicholas reaction is a substitution reaction whereby an alkoxy group located on the α-carbon of an alkyne is replaced by another nucleophile. The alkyne reacts first with dicobalt octacarbonyl, from which is generated a stabilized propargylic cation that reacts with the incoming nucleophile and the product then forms by oxidative demetallation.[11][12]

The Nicholas reaction
The Nicholas reaction

The Pauson–Khand reaction,[13] in which an alkyne, an alkene, and carbon monoxide cyclize to give a cyclopentenone, can be catalyzed by Co2(CO)8,[3][14] though newer methods that are more efficient have since been developed:[15][16]

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.[13]

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.[17] 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.[18]

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[19]
Step 6: Oxidative addition of dihydrogen leads to a dihydrido complex
Step 7: Aldehyde product released by reductive elimination,[20] regenerating the active catalytic species
Step 8: An unproductive and reversible side reaction

Dicobalt octacarbonyl can cyclotrimerize diphenylacetylene and its derivatives to hexaphenylbenzenes.[21] Symmetrical diphenylacetylenes form 6-substituted hexaphenylbenzenes, while asymmetrical diphenylacetylenes form a mixture of two isomers.[22] Symmetric diphenylacetylene cyclotrimerization using dicobalt octacarbonyl
Asymmetric diphenylacetylene cyclotrimerization using dicobalt octacarbonyl

Hydroformylation

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

Co2(CO)8 + H2 → 2 H[Co(CO)4]

This hydride is a catalyst for hydroformylation – the conversion of alkenes to aldehydes.[4][23] The catalytic cycle for this hydroformylation is shown in the diagram.[4][19][20]

Substitution reactions

The CO ligands can be replaced with tertiary phosphine ligands to give Co2(CO)−8x(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

Conversion to higher carbonyls

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

Safety

Co2(CO)8 a volatile source of cobalt(0), is pyrophoric and releases carbon monoxide upon decomposition.[26] 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.[27]

References
  1. NIOSH Pocket Guide to Chemical Hazards. "#0147". National Institute for Occupational Safety and Health (NIOSH).
  2. Pauson, Peter L.; Stambuli, James P.; Chou, Teh-Chang; Hong, Bor-Cherng (2014). "Octacarbonyldicobalt". Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons. pp. 1–26. doi:10.1002/047084289X.ro001.pub3. ISBN 9780470842898.
  3. Donaldson, John Dallas; Beyersmann, Detmar (2005). "Cobalt and Cobalt Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH. doi:10.1002/14356007.a07_281.pub2. ISBN 3527306730.
  4. Elschenbroich, C.; Salzer, A. (1992). Organometallics: A Concise Introduction (2nd ed.). Weinheim: Wiley-VCH. ISBN 3-527-28165-7.
  5. Sweany, Ray L.; Brown, Theodore L. (1977). "Infrared spectra of matrix-isolated dicobalt octacarbonyl. Evidence for the third isomer". Inorganic Chemistry. 16 (2): 415–421. doi:10.1021/ic50168a037.
  6. Gilmont, Paul; Blanchard, Arthur A. (1946). Dicobalt Octacarbonyl, Cobalt Nitrosyl Tricarbonyl, and Cobalt Tetracarbonyl Hydride. Inorganic Syntheses. Vol. 2. pp. 238–243. doi:10.1002/9780470132333.ch76. ISBN 9780470132333.
  7. Orchin, Milton (1953). "Hydrogenation of Organic Compounds with Synthesis Gas". Advances in Catalysis. Vol. 5. Academic Press. pp. 385–415. ISBN 9780080565095.
  8. Sumner, G. Gardner; Klug, Harold P.; Alexander, Leroy E. (1964). "The crystal structure of dicobalt octacarbonyl". Acta Crystallographica. 17 (6): 732–742. doi:10.1107/S0365110X64001803.
  9. 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. PMID 23047247.
  10. 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. PMID 19921010.
  11. Nicholas, Kenneth M. (1987). "Chemistry and synthetic utility of cobalt-complexed propargyl cations". Acc. Chem. Res. (Review). 20 (6): 207–214. doi:10.1021/ar00138a001.
  12. Teobald, Barry J. (2002). "The Nicholas reaction: The use of dicobalt hexacarbonyl-stabilised propargylic cations in synthesis". Tetrahedron (Review). 58 (21): 4133–4170. doi:10.1016/S0040-4020(02)00315-0.
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  14. Blanco-Urgoiti, Jaime; Añorbe, Loreto; Pérez-Serrano, Leticia; Domínguez, Gema; Pérez-Castells, Javier (2004). "The Pauson–Khand reaction, a powerful synthetic tool for the synthesis of complex molecules". Chem. Soc. Rev. 33 (1): 32–42. doi:10.1039/b300976a. PMID 14737507.
  15. Schore, Neil E. (1991). "The Pauson–Khand Cycloaddition Reaction for Synthesis of Cyclopentenones". Org. React. 40: 1–90. doi:10.1002/0471264180.or040.01. ISBN 0471264180.
  16. 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. PMID 12722067.
  17. 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. PMID 21751323.
  18. Ho, Tse-Lok (2016). "Dicobalt Octacarbonyl". Fiesers' Reagents for Organic Synthesis. Vol. 28. John Wiley & Sons. pp. 251–252. ISBN 9781118942819.
  19. 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.
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  21. Vij, V.; Bhalla, V.; Kumar, M. (8 August 2016). "Hexaarylbenzene: Evolution of Properties and Applications of Multitalented Scaffold". Chemical Reviews. 116 (16): 9565–9627. doi:10.1021/acs.chemrev.6b00144.
  22. Xiao, W.; Feng, X.; Ruffieux, P.; Gröning, O.; Müllen, K.; Fasel, R. (18 June 2008). "Self-Assembly of Chiral Molecular Honeycomb Networks on Au(111)". Journal of the American Chemical Society. 130 (28): 8910–8912. doi:10.1021/ja7106542.
  23. Pfeffer, M.; Grellier, M. (2007). "Cobalt Organometallics". Comprehensive Organometallic Chemistry III. Vol. 7. Elsevier. pp. 1–119. doi:10.1016/B0-08-045047-4/00096-0. ISBN 9780080450476.
  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. (2007). "μ3-Methylidyne and μ3-Benzylidyne-Tris(Tricarbonylcobalt)". Inorganic Syntheses. pp. 226–229. doi:10.1002/9780470132517.ch53. ISBN 9780470132517. {{cite book}}: |journal= ignored (help)
  26. Cole Parmer MSDS
  27. CDC - NIOSH Pocket Guide to Chemical Hazards
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