· (MW 189.12)
(reducing agent; catalyst; source of other organocobalt complexes and of substituted cyclopentanes or cyclopentadienes)
Alternate Name: cobaltocene.
Physical Data: mp 173-174 °C; sublimes 40 °C/0.1 mmHg; paramagnetic.
Solubility: sol petrol and other common org solvents, but reacts with chlorocarbons and oxidizing agents (e.g. nitromethane).
Form Supplied in: black-purple cryst; commercially available; readily synthesized from CoII salts [preferably Co(NH3)6Cl2] + CpNa in THF.1
Handling, Storage, and Precautions: highly air- and somewhat moisture-sensitive and light-sensitive; indefinitely stable in vacuo or under dry inert atmosphere (N2, Ar) in the dark; storage in the cold is recommended; toxic.
As well as its very facile oxidation to cobaltocenium salts (see Bis(cyclopentadienyl)cobalt Hexafluorophosphate), cobaltocene is a source of CpCoL2 complexes, e.g. the dicarbonyl (L = CO) (eq 1)2 (see Dicarbonyl(cyclopentadienyl)cobalt(I)) and the bis(ethylene) complex (L = C2H4) (eq 2)3 (see also (1,5-Cyclooctadiene)(cyclopentadienyl)cobalt(I)). Like these diamagnetic complexes (CpCoL2), it catalyzes the reaction of alkynes with nitriles to give 2-substituted pyridines (eq 3),4 related reactions with isocyanates (eq 4) and carbodiimides,5 and the closely related alkyne cyclotrimerization.6 It also catalyzes hydroformylation of alkenes, e.g. 1-octene (eq 5),7 and is comparable in activity as
catalyst to both Dicarbonyl(cyclopentadienyl)cobalt(I) and Octacarbonyldicobalt, but the last one gives a higher n:i-nonanal ratio in this reaction.
In the presence of base, cobaltocene catalyzes the efficient addition of carbon dioxide to 2-methylbut-3-yn-2-ol (eq 6).8 In conjunction with copper chelates, it is reported to catalyze polymerization and block copolymerization of methyl methacrylate.9
As a 19-electron complex, cobaltocene acts as a powerful one-electron reducing agent. Its use as such has been largely confined to the reduction of a wide range of organometallic substrates. Little is known about its ability to reduce metal-free organic substrates apart from its reactions with alkyl halides, including polyhalides, which proceed according to eq 7 and possibly by the steps in eqs 8 and 9. The last step (eq 9) finds support from the addition of the radical &bdot;CMe2CN, generated from AIBN.10 The general reaction (eq 7) occurs efficiently with halomethanes and with allylic, benzylic, and propargylic halides, with a-halo esters and ketones and with benzoyl chloride.11 A related reaction occurs with active hydrogen compounds (e.g. CHCl3,
HC2Ph, HCH2CN, HCH2COMe) in the presence of O2, apparently via an oxygen adduct of cobaltocene, e.g. eq 10.12 Interaction of cobaltocene with organomercury13 and -cadmium14 reagents also leads to 5-exo-substituted cyclopentadiene(cyclopentadienyl)cobalt (eq 11) accompanied by products suggesting radicals (for another route to these 5-exo-substituted complexes and some aspects of their behavior, see Bis(cyclopentadienyl)cobalt Hexafluorophosphate).
All such complexes are potential sources of substituted cyclopentadienes, but little attention has been paid to such use. (Trichloromethyl)cyclopentadiene has been liberated by iodine oxidation (eq 12) of the complex prepared from Cp2Co with CCl4 or with CHCl3/O2.15 However, reactions in which the diene moiety is displaced by other ligands (e.g. Me2S in the presence of HBF416 or PhC2Ph17) have been carried out without isolation of the metal-free product(s).
The complexes with 5-exo-haloalkyl groups undergo ring enlargement on solvolysis (eq 13).18 In a closely analogous sequence, cobaltocene adds dihaloboranes, e.g. eq 14, to give products which undergo ring enlargement (eq 15) on hydrolysis or on treatment with Lewis acids.19
A one-step synthesis of azulene from cobaltocene is reported to proceed in 12-21% yield according to eq 16.20
- 1. Cordes, J. F. CB 1962, 95, 3084.
- 2. King, R. B. IC 1966, 5, 2227.
- 3. Jonas, K.; Deffense, E.; Habermann, D. AG 1983, 95, 729.
- 4. (a) Bönnemann, H, AG(E) 1985, 24, 248. (b) Geiger, R. E.; Lalonde, M.; Stoller, H.; Schleich, K. HCA 1984, 67, 1274.
- 5. (a) Waratsuki, Y.; Yamazaki, H. S 1976, 26; (b) Hong, P.; Yamazaki, H.; TL 1977, 1333; (c) Hong; P.; Yamazaki, H. Nippon Kagaku Kaishi 1978, 730 (CA 1978, 89, 108 980).
- 6. Sergeev, V. A.; Shitikov, V. K.; Kurapov, A. S.; Leonova, E. V.; Antonova-Antipova, I. P.; Chernomordik, Yu. A. IZV 1988, 1629; BAU 1988, 1445.
- 7. (a) Magomedov, G. K.; Voskoboinikov, A. Z.; Beletskaya, I. P. Metalloorg. Khim. 1989, 2, 806. (b) Beletskaya, I. P.; Magomedov, G. K.-I.; Voskoboinikov, A. Z. JOM 1990, 385, 289.
- 8. Inoue, Y.; Ishikawa, J.; Taniguchi, M.; Hashimoto, H. BCJ 1987, 60, 1204.
- 9. (a) Mun, Y. U.; Sato, T.; Otsu, T. Makromol. Chem. 1984, 185, 1507. (b) Mun, Y. U.; Sato, T.; Otsu, T. J. Macromol. Sci., Chem. 1984, A21, 1535.
- 10. Herberich, G. E.; Schwarzer, J. AG(E) 1970, 9, 897.
- 11. (a) Fischer, E. O.; Herberich, G. E. CB 1961, 94, 1517. (b) Herberich, G. E.; Bauer, E. JOM 1969, 16, 301. (c) Herberich, G. E.; Bauer, E.; Schwarzer, J. JOM 1969, 17, 445. (d) Herberich, G. E.; Greiss, G. JOM 1971, 27, 113. (e) Herberich, G. E.; Schwarzer, J. JOM 1972, 34, C43.
- 12. Kojima, H.; Takahashi, S.; Yamazaki, H.; Hagihara, N. BCJ 1970, 43, 2272.
- 13. Mar'in, V. P.; Vyshinskaya, L. I.; Petrovskii, P. V. Metalloorg. Khim. 1990, 3, 1368.
- 14. (a) Razuvaev, G. A.; Mar'in, V. P.; Andrianov, Yu. A.; Vyshinskaya, L. I.; Smirnov, A. S. IZV 1987, 462; BAU 1987, 423. (b) Razuvaev, G. A.; Mar'in, V. P.; Andrianov, Yu. A.; Vyshinskaya, L. I.; Druzhkov, O. N. JOM 1988, 346, 403. (c) Razuvaev, G. A.; Yudenich, S. G.; Dodonov, V. A. ZOB 1985, 55, 613.
- 15. Dahl, T.; Moberg, C. ACS 1973, 27, 728.
- 16. Kuhn, N.; Zauder, E. JOM 1989, 362, 217.
- 17. Nakamura, A. Mem. Inst. Sci. Ind. Res., Osaka Univ. 1962, 19, 81 (CA 1963, 59, 8786).
- 18. (a) Herberich, G. E.; Greiss, G.; Heil, H. F. JOM 1970, 22, 723. (b) Herberich, G. E.; Schwarzer, J. CB 1970, 103, 2016.
- 19. (a) Herberich, G. E.; Greiss, G.; Heil, H. F. AG(E) 1970, 9, 805. (b) Herberich, G. E.; Greiss, G.; Heil, H. F.; Müller, J. CC, 1971, 1328.
- 20. Kayushina, E. N.; Levin, D. Z.; Mortikov, E. S.; Promonenkov, V. K. IZV 1982, 2180; BAU 1982, 1931.
Peter L. Pauson
University of Strathclyde, Glasgow, UK
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