Bis(cyclopentadienyl)cobalt Hexafluorophosphate

[12427-42-8]  · C10H10CoF6P  · Bis(cyclopentadienyl)cobalt Hexafluorophosphate  · (MW 334.09)

(source of (cyclopentadiene)(cyclopentadienyl)cobalt and its substitution products and, hence, of substituted cyclopentadienes and cyclopentanes)

Alternate Name: cobaltocenium hexafluorophosphate.

Solubility: sol acetone, hot ethanol; sparingly sol ether, water.

Form Supplied in: yellow crystalline solid; available commercially.

Preparative Method: readily synthesized from cobalt(II) salts + CpNa, followed by air + HPF6 or NH4PF6. Salts with a wide range of other counterions may be obtained similarly. The cobaltocenium ion1 may be precipitated from aqueous solution by a variety of large anions. In addition to PF6-, convenient counterions include Br3- and I3-. The simple halides are hygroscopic, but may be obtained in anhydrous form by removal of water under vacuum.

Handling, Storage, and Precautions: very stable; irritant.

The potential synthetic value of cobaltocenium salts depends on the facile addition of hydride (eq 1)2 and of (unstabilized) carbon nucleophiles (eq 2)3 to give (cyclopentadiene)(cyclopentadienyl)cobalt(I) complexes. The unsubstituted (C5H5)Co(C5H6) is similar in reactivity to (1,5-Cyclooctadiene)(cyclopentadienyl)cobalt(I). Exo substituted derivatives of the type formed in eq 2 are also available from Bis(cyclopentadienyl)cobalt.

They react sluggishly or not at all (depending on R) with triphenylmethyl cations, which in general only abstract exo- but in some cases also abstract endo-H.5 However, in many of the cases where they fail it has been found that the products of eq 2 may be converted to substituted cobaltocenium salts by the action of N-Bromosuccinimide (eq 3)4 or strong Sulfuric Acid.5c Alternatively, thermolysis causes 1,5-H migration (eq 4),5a,b making possible subsequent exo-H abstraction (eq 5). However, any further carbanion addition to the resultant monosubstituted cobaltocenium salts will result in isomer mixtures and hence becomes less attractive as a route to disubstituted cyclopentadienes.

Methyl and 1,1-dimethyl derivatives of the cobaltocenium ion are smoothly oxidized by strong oxidants, e.g. Potassium Permanganate, to the corresponding mono- and dicarboxylic acids.6 The latter, in turn, have been converted to the diacid chlorides,6b,d a source of polymers;7 further conversion by Curtius degradation yields the 1,1-diaminocobaltocenium ion, itself oxidizable to the dinitro compound.6b,d Oxidation of 1,1-diethylcobaltocenium yields the 1,1-diacetyl substituted cation8 and the 1,1,2,2-tetracarboxylic acid is formed by oxidation of the dibenzocobaltocenium [bis(indenyl)cobalt(III)] salt with KMnO4.6a Such processes are a potential source of substituted cyclopentadienes, as are transformations utilizing the enhanced acidity of a-CH groups. The principle is illustrated by the polysubstitution shown in eq 6.9

A rather remarkable, but low yield, synthesis of azulene is achieved in one step when cobaltocenium hexafluorophosphate is heated with Sodium Hydroxide (or Sodium Amide), probably via the intermediates included in eq 7.10

1. (a) Gmelin, Handbuch der Anorganischen Chemie, 8th ed; Verlag Chemie: Weinheim, 1973; Supplement, Vol. 5, part 1. (b) Sheats, J. E. JOM Libr. 1979, 7, 461.
2. Green, M. L. H.; Pratt, L.; Wilkinson, G. JCS 1959, 3753.
3. (a) Fischer, E. O.; Herberich, G. E. CB 1961, 94, 1517. (b) Malkov, A. V.; Leonova, E. V.; Kochetkova, N. S.; Sergeev, V. A. Metalloorg. Khim. 1988, 1, 357.
4. Pauson, P. L. JOM 1980, 200, 207.
5. (a) Knox, G. R.; Nutley, M.; Pauson, P. L.; Toma, S.; Watts, W. E.; Elder, P. A.; Griffiths, R. JCR(S) 1981, 151; (b) Knox, G. R.; Nutley, M.; Pauson, P. L.; Toma, S.; Watts, W. E.; Elder, P. A.; Griffiths, R. JCR(M) 1981, 1901. (c) Malkov, A. V.; Petrovskii, P. V.; Leonova, E. V.; Rukhlyada, N. N.; Sergeev, V. A. Metalloorg. Khim. 1988, 1, 779. (d) Leonova, E. V.; Malkov, A. V.; Kochetkova, N. S.; Sergeev, V. A. Metalloorg. Khim. 1988, 1, 137. (e) El Murr, N. JOM 1981, 208, C9.
6. (a) Sheats, J. E.; Rausch, M. D. JOC 1970, 35, 3245. (b) El Murr, N. JOM 1976, 112, 177. (c) El Murr, N.; Dabard, R. CR(C) 1971, 272, 1989. (d) El Murr, N.; Dabard, R. JOM 1972, 39, C82.
7. Pittman, C. U. Jr.; Ayers, O. E.; McManus, S. P.; Sheats, J. E.; Whitten, C. E. Macromolecules 1971, 4, 360.
8. Nesmeyanov, A. N.; Leonova, E. V.; Kochetkova, N. S.; Rukhlyada, N. N.; Bychkov, N. V. IZV 1973, 2791; BAU 1973, 2726.
9. Gloaguen, B.; Astruc, D. JACS 1990, 112, 4607.
10. Attridge, C. J.; Baker, S. J.; Parkins, A. W. Organomet. Chem. Synth. 1970/71, 1, 183.

Peter L. Pauson

University of Strathclyde, Glasgow, UK

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