[16985-99-2]  · C8Co2O8Zn  · Octacarbonyl(zinc)dicobalt  · (MW 407.33) (Cd[Co(CO)4]2(2Co-Cd))

[16986-00-8]  · C8CdCo2O8  · Octacarbonyl(cadmium)dicobalt  · (MW 454.35) (Hg[Co(CO)4]2(2Co-Hg))

[13964-88-0]  · C8Co2HgO8  · Octacarbonyl(mercury)dicobalt  · (MW 542.54)

(air-sensitive yet readily available organometallic species capable of catalyzing the selective dimerization of norbornadiene;1 in an isolated example catalyzes the linear dimerization of a propargyl amine2)

Alternate Names: bis(tetracarbonylcobalt)zinc; di(cobalttetracarbonyl)zinc.

Physical Data: mp 71-72.5 °C; subl 60 °C/0.005 mmHg. X-ray crystal structure shows each Co to be trigonal bipyramidal (with CO and Zn occupying axial positions) and unbridged Co-Zn-Co.3

Solubility: sol heptane, hexane, benzene, toluene, THF, and in acetone to provide enhanced stability; forms complexes with THF from which solvent is difficult to remove.

Form Supplied in: commercially not readily available.

Purification: IR,4 59Co NQR,5 MS,4a and Raman.4b,c

Preparative Methods: most conveniently prepared from the analogous mercury compound, Hg[Co(CO)4]2, by reaction with Zn (granular or powder) at 25 °C under argon at atmospheric pressure (93% yield).4a,b A variety of procedures which utilize high pressures of CO with Octacarbonyldicobalt (or various Co salts) and Zinc are also known.1a,6 Preparation from Sodium Tetracarbonylcobaltate and Zinc Chloride at atmospheric pressure is disfavored due to the numerous manipulations of air-sensitive materials required.7 A less widely used method from Co2(CO)8 and powdered Zn in hexanes at atmospheric pressure has been reported8 as has a simple, yet lower yielding, electrochemical synthesis.9 The cadmium analog, Cd[Co(CO)4]2 (recrystallized from hexane at -60 °C;4a yellow/orange plate-like crystals mp 79-81 °C;4b IR,4a,c,9,10 59Co NQR,5 MS,4a,11 and Raman4c,12), is also available by the techniques described above,1a,4a,b,6,9 as well by an alternative route from Hg[Co(CO)4]2.13

Octacarbonyl(mercury)dicobalt (Hg[Co(CO)4]2, recrystallized from acetone/H2O13,14 or MeOH;4a orange air-stable crystals mp 81-82 °C,14 partially sublimed at 50 °C/0.1 mmHg;14 sol heptane, hexane, benzene, Et2O, acetone, insol H2O; IR,4a,c,10,15 59Co NQR,16 MS,4a,17 Raman,4c,12,18 and X-ray19) is a useful intermediate in the preparation of other cobalt carbonyl derivatives which, unlike Zn[Co(CO)4]2, Cd[Co(CO)4]2, and Co2(CO)8, is stable in air for prolonged periods (in excess of one year). This mercury derivative is conveniently and commonly prepared without the requirement for high-pressure equipment by two general routes. In the first technique, readily available cobalt(II) nitrate hexahydrate is used in a room temperature reaction with CO and mercury(II) cyanide (90% yield).13,14 Alternatively, Co2(CO)8 can be treated with either Hg (quantitative yield)20a or 1% Sodium Amalgam (62% yield),20 again at room temperature and atmospheric pressure. Procedures which utilize higher pressures of CO have also been reported.6

Purification: can be recrystallized from toluene at -80 °C,4a sublimed at 60 °C/0.005 mmHg;4b yellow prismatic crystals.4b

Handling, Storage, and Precautions: decomposes readily in air and has been reported as being pyrophoric.6 Can be handled and stored as a solid or solution in the organic solvents quoted above under inert gas (argon or nitrogen).

Dimerization of Norbornadiene.

The dinuclear catalyst Zn[Co(CO)4]2 has two cobalt centers connected by zinc and is capable of catalyzing the dimerization of norbornadiene (eq 1).1 The product of this dimerization has been shown unequivocally to be Binor-S (1) and is obtained exclusively and quantitatively when a high catalyst to monomer ratio (1:28) is used in nonpolar solvents (e.g. heptane or toluene) under an inert atmosphere (N2 or Ar). At lower catalyst to monomer ratios (e.g. 1:238) a mixture of isomeric dimers of norbornadiene results. Furthermore, it was found that Lewis acids (as co-catalysts) activate the dimerization and enhance the formation of Binor-S (1). In particular, Boron Trifluoride Etherate and Aluminum Bromide were the most effective co-catalysts for the specific dimerization reaction. In the presence of Lewis acids, Cd[Co(CO)4]2, In[Co(CO)4]3, and Co2(CO)8 (all with BF3.OEt2) also gave quantitative yields of (1).

The use of Zn[Co(CO)4]2 has significant advantages over other metal-catalyzed dimerizations of norbornadiene (e.g. with Fe(CO)5, Fe2(CO)9, Fe3(CO)12 or, when used in isolation, Co2(CO)8), where an array of dienes, ketones, and saturated hydrocarbons result in relatively low yields under similar conditions.21 Additionally, in the reaction with acetylene, octacarbonyl(zinc)dicobalt produces benzene as the main volatile product and approximately 3% of cyclooctatetraene.1

By way of a comparison to the use of the title compound as a catalyst for the dimerization shown in eq 1, a range of alternative catalysts which are nearly as effective but which are more readily accessible have been reported.22 These include CoBr2.2PPh3, CoI2.2PPh3, and RhCl(PPh3)3, all being used with BF3.OEt2 as the co-catalyst. The complexes are air-stable and are readily obtained from the corresponding anhydrous halides and Triphenylphosphine. When compared to the original Zn[Co(CO)4]2, these mononuclear catalysts have an induction period prior to reaction. Following initial gentle heating, caution must be observed since the dimerization is exothermic and occasional cooling is required. As with the Zn system, metal-metal bonded intermediates are proposed as the catalytically active species. As part of independent studies, the CoBr2.2PPh3 catalyst has been utilized to prepare Binor-S (1) in 85% yield as part of an efficient synthetic route towards diamantane.23

The intermolecular condensation product of Zn[Co(CO)4]2, the mixed-metal cluster (2), can be prepared by heating the title compound in n-octane at 70 °C in the dark under a partial vacuum (eq 2).24 The orange needle-like crystals of bis[m-(tetracarbonylcobalt)zincio](m-carbonyl)hexacarbonyldicobalt(Co-Co) (2) when dissolved in toluene and heated with norbornadiene catalyze the dimerization to Binor-S (1) in 70% yield.24a This reaction proceeds without the induction periods observed for the previously discussed catalysts.

Propargyl Amine Linear Dimerization.

During a study, which generally shows the title compound to be a less efficient catalyst for the isomerization of allyl ethers and the cyclotrimerization of propargyl ethers and amines than alternative cobalt complexes, one unusual reaction is observed. In this isolated example, when 3-diethylaminopropyne is treated with Zn[Co(CO)4]2 a linear dimer is formed from reaction of the methylene carbon of an amino ethyl group (eq 3).2

Reactions of the Analogous Mercury Compound, Hg[Co(CO)4]2.

Cyclotrimerization reactions of alkynes in the presence of metal carbonyls is a well-known phenomenon in organometallic chemistry.25 In particular, octacarbonyl(mercury)dicobalt is an efficient catalyst for this process and, furthermore, when unsymmetrical alkynes are used, good selectivities for the 1,2,4-trisubstituted products are observed (eq 4).26 In addition, partial intramolecular cyclization resulted when terminal 1,7- or 1,8-diynes were used.27 The zinc species has not been shown to be an efficient catalyst for the alkyne cyclotrimerization process.

Two further Hg[Co(CO)4]2 catalyzed reactions which have not been reported as being mediated by the analogous zinc species are illustrated in eqs 5 and 6. In the former case, symmetrical diaryl ketones can be formed from the corresponding diarylmercury compound under photolysis conditions.28 Despite good to high yields being observed for diaryl ketones, this process is not applicable to the synthesis of dialkyl ketones. In the latter procedure, gem-dihalides can be coupled to give tetrasubstituted alkenes (eq 6).29 When Hg[Co(CO)4]2 is employed as the catalyst, higher yields are observed when the reaction is carried out photolytically. A number of alternative metal-cobalt carbonyl species also mediate this transformation.

Related Reagents.

Octacarbonyldicobalt; Sodium Tetracarbonylcobaltate.

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William J. Kerr

University of Strathclyde, Glasgow, UK

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