Methylidynetricobaltnonacarbonyl

Methylidynetricobaltnonacarbonyl is an organometallic cobalt cluster with the chemical formula Co₃(CO)₉CH that contains a metal carbonyl core with the methylidyne ligand, first discovered in the late 1950s. A variety of substituents can be added to the methylidyne group to form derivatives of the parent compound that have unique spectroscopic properties and reactivity. This page will explore the discovery and synthesis of methylidynetricobaltnonacarbonyl, the structure and bonding of the parent compound, as well as some examples reactivity and catalysis with the cluster.

Methylidynetricobalt­nonacarbonyl

Names
Other names methylidyne-tris(tricarbonylcobalt)
Identifiers
CAS Number	15664-75-2
3D model (JSmol)	Interactive image
PubChem CID	519159 wrong formula
InChI InChI=1S/9CO.CH.3Co/c9*1-2;;;;/h;;;;;;;;;1H;;;/q9*+1;;3*-3 Key: WPOHNFRRRVAWSE-UHFFFAOYSA-N
SMILES [O+]#C[Co-3]12(C#[O+])(C#[O+])[Co-3]3(C#[O+])(C#[O+])(C#[O+])[Co-3]1(C#[O+])(C#[O+])(C#[O+])C23
Properties
Chemical formula	C10HCo3O9
Molar mass	441.909 g·mol−1
Appearance	purple solid
Density	2.01 g/cm3
Melting point	105–107 °C (221–225 °F; 378–380 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Synthesis

Methylidynetricobaltnonacarbonyl and derivatives were discovered in the late 1950s by Markby and Wender by reactions of the alkyne complexes Co₂(CO)₆(R₂C₂) with acids. The structure of the cluster was however misformulated.^[1] In 1962, the class of compounds were properly formulated methylidynetricobaltnonacarbonyl as well as several derivatives.^[2]

The synthetic procedure developed by Bor and coworkers relied on the reaction of [Co(CO)₄]^- anion with chloroform, bromoform, or iodoform in a solution of acetone or THF. Dark violet crystals were obtained in ~20% yield with chloroform, ~18% yield with bromoform, and ~1-2% yield with iodoform. The chloride derivative Co₃(CO)₉CCl was obtained with CCl₄ and was isolated as shiny, lilac crystals. Several other functional groups could be installed, including C₆H₅ and CO₂CH₃, all of which afforded dark violet crystals. Solutions of these compounds are air stable in organic solvents. Treatment of the parent compound with 100 atm CO at 160 °C resulted in the destruction of the complex and the formation of dicobaltoctacarbonyl.^[2]

3NaCo(CO)₄ + CHCl₃ → HC[Co(CO)₃]₃ + 3 NaCl

Structure and Bonding

The Co-Co bond lengths are ~2.47 Å and the Co-C bond distances are ~1.88 Å.^[2][3] These bond lengths do not change significantly depending on the substituent on the carbon group. The Co-C-Co angle is approximately 80°, indicating a significant bending of the carbon tetrahedral structure, and the ligands are arranged to provide maximum cobalt-carbon interaction.^[4] The IR spectra show four absorption bands of varying intensities in the region of terminal C-O groups between 2111 and 2018 cm-1. The ¹³C NMR spectra have also been analyzed.^[5] The mass spectra demonstrate parent molecular ions (Co₃(CO)₉CH ^.+) in good abundance, which demonstrates the high stability of the Co₃(CO)₉ clusters towards oxidation. Initial loss of CO, which is characteristic of most metal-carbonyls, though rupture of the metal-metal bond also often occurs.^[4]

Series of mixed organic and inorganic tetrahedrane structures illustrating that Co(CO)₃ and methylidyne are isolobal.^[6]

The carbonium ion had the correct symmetry to accept electron density from the apex carbon via s-p stabilization. They explained that apex carbon stabilize the carbonium ion via s-p conjugation.^[7]

Visual representation of methylidynetricobaltnonacarbonyl isolobal analogy with the CH fragment

Reactivity

¹⁴CO labelling studies on the kinetics have been reported.^[8] The group concluded the exchange rate of the carbonyl ligands decreases according to the order F > Cl > Br > H, which they attributed to the electronegativity of the atom bound to the methylidyne carbon. Additionally, they demonstrated that the nine carbonyl groups are not kinetically equivalent, and that only 3 CO groups exchange within the 35–55 °C temperature range studied, which agrees with the calculated frontier molecular orbitals discussed above. In the case with the COOCH₃ group, the exchange occurs for only one CO group which they attributed to the more electron deficient nature of the complex.^[8]

Methylidynetricobaltnonacarbonyls reacts with variety of organomercury compounds of the R₂Hg and RHgX type; the side product Hg[Co(CO)₄]₂ was formed.^[9]

Azobisisobutryonitrile (AIBN), a reagent for the initiation of radical reactions, to a solution of Co₃(CO)₉CH and allyl acetate afforded a red crystalline solid that they believed formed via a radical mechanism.^[10] The radical reactivity of Co₃(CO)₉CX varies with X. The order of initiator activity was found to be X = Cl > H > Br > Ph > F > i-Pr > C₂F₅, which aligns with the trend seen for CO ligand exchange studies.^[11][12] These cobalt carbonyl clusters undergo reversible, one-electron reduction in the range of -0.7 to -0.9V versus the saturated calomel electrode.^[13]

Methylidynetricobaltnonacarbonyl catalyzes the Pauson-Khand reaction, a [2+2+1] cycloaddition.^[14] Methylidynetricobaltnonacarbonyl is more air-stable than the parent dicobalt octacarbonyl, making it a more attractive catalyst. The clusters demonstrated no need for additives such as trimethylphosphite, as is necessary with the dicobalt octacarbonyl, and the best results were obtained with the parent methylidyne cluster.^[14]

Pauson-Khand reaction using methlylidynetricobaltnonacarbonyl as a catalyst

Methylidynetricobaltnonacarbonyl derivatives are precatalysts for the asymmetric intramolecular Pauson-Khand reaction. The chiral diphosphine Josiphos provides the asymmetry. Despite the unique approach, the clusters are inferior compared to other metal clusters with NORPHOS or Me-DuPHOS ligands, which gave higher yields and fewer side products.^[15]

References

1. Markby, Raymond; Wender, Irving; Friedel, R. A.; Cotton, F. A.; Sternberg, Heinz W. (1958). "The Association of an Acetylene with Three Metal Atoms; A New Trinuclear Cobalt Complex". Journal of the American Chemical Society. 80 (24): 6529–6533. doi:10.1021/ja01557a018.
2. Bor, György; Markó, László; Markó, Bernadett (1962). "Kobaltcarbonylderivate des Tetrachlorkohlenstoffs und organischer 1.1.1-Trihalogenverbindungen: Trikobalt-enneacarbonyl-kohlenstoff-Verbindungen". Chemische Berichte. 95 (2): 333–340. doi:10.1002/cber.19620950207.
3. Castellani, Michael P.; Smith, William G.; Patel, Nimish G.; Bott, Simon G.; Richmond, Michael G. (1999). "Reaction of ethyl diazoacetate with Co3(CO)9(μ3-CCL): Isolation and X-ray structures of Co3(CO)9(μ3-CCO2Et), Co3(CO)9(μ3-CCH2CO2Et), and [Co3(CO)9(μ3-CCHCO2Et)]2". Journal of Chemical Crystallography. 29 (5): 609–617. doi:10.1023/A:1009561205829.
4. Robinson, B. H.; Tham, W. S. (1968). "Chemistry of methinyltricobalt enneacarbonyls. Part I. Mass spectra". Journal of the Chemical Society A: Inorganic, Physical, Theoretical: 1784. doi:10.1039/J19680001784..
5. Aime, S.; Milone, L.; Valle, M. (1976). "13C NMR study of methynyltricobalt enneacarbonyls". Inorganica Chimica Acta. 18: 9–11. doi:10.1016/S0020-1693(00)95577-4.
6. Schilling, Birgitte E. R.; Hoffmann, Roald (1979). "M3L9(ligand) complexes". Journal of the American Chemical Society. 101 (13): 3456–3467. doi:10.1021/ja00507a006.
7. Seyferth, D.; Williams, G. H.; Hallgren, J. E. J. Am. Chem. Soc. 1973, 95 (1), 266-267. DOI: 10.1021/ja00782a060.
8. Cetini, N.; Ercoli, R.; Gambino, O.; Vaglio, G. Atti della Accademia delle Scienze di Torino, Clase di Scienze Fisiche, Matematiche e Naturali 1965, 99, 1123-1126.
9. Seyferth, Dietmar; Hallgren, John E.; Spohn, Ralph J. (1970). "The C-arylation and alkylation of methylidynetricobalt nonacarbonyls by organomercury compounds". Journal of Organometallic Chemistry. 23 (2): C55–C57. doi:10.1016/S0022-328X(00)92937-1.
10. Seyferth, Dietmar; Hallgren, John E. (1973). "Organocobalt cluster complexes X. Evidence for the methylidynetricobalt nonacarbonyl radical". Journal of Organometallic Chemistry. 49: C41–C42. doi:10.1016/S0022-328X(00)84929-3.
11. Pályi, G.; Baumgartner, F.; Czájlik, I. (1973). "Methynyltricobalt enneacarbonyl compounds". Journal of Organometallic Chemistry. 49 (2): C85–C87. doi:10.1016/S0022-328X(00)84216-3.
12. Seyferth, Dietmar; Hallgren, John E.; Spohn, Ralph J.; Williams, Gary H.; Nestle, Mara O.; Hung, Paul L.K. (1974). "Organocobalt cluster complexes". Journal of Organometallic Chemistry. 65: 99–118. doi:10.1016/S0022-328X(00)83893-0.
13. Kotz, John C.; Petersen, James V.; Reed, Richard C. (1976). "The electrochemistry of some alkylidynetricobalt nonacarbonyl cluster complexes". Journal of Organometallic Chemistry. 120 (3): 433–437. doi:10.1016/S0022-328X(00)98054-9.
14. Sugihara, Takumichi; Yamaguchi, Masahiko (1998). "The Pauson−Khand Reaction Catalyzed by the Methylidynetricobalt Nonacarbonyl Cluster". Journal of the American Chemical Society. 120 (41): 10782–10783. doi:10.1021/ja982635s.
15. Mottalib, M. Abdul; Haukka, Matti; Nordlander, Ebbe (2016). "Chiral diphosphine derivatives of alkylidyne tricobalt carbonyl clusters – A comparative study of different cobalt carbonyl (Pre)catalysts for (Asymmetric) intermolecular Pauson–Khand reactions". Polyhedron. 103: 275–282. doi:10.1016/j.poly.2015.04.021.