The one-pot preparation of a cobaloxime; 2-Phenylethyl (pyridine) bis(dimethylglyoximato)cobalt(III)
SyntheticPage 531
DOI:
10.1039/SP531
Submitted Jan 20, 2012, published Jan 23, 2012
Chemicals Used
Cobalt chloride hexahydrate
Pyridine
Dimethylglyoxime (2,3-Butanedione dioxime)
2-Bromoethylbenzene
Sodium hydroxide
Methanol
Procedure
A 3-neck 250 ml round bottom flask was equipped with a pressure equalising dropping funnel with a nitrogen inlet reaching into the liquid, a reflux condenser with a gas bubbler attached and magnetic stirring. The flask was charged with pyridine (0.80 g, 11 mmol) and methanol (100 cm3) and the solution stirred under nitrogen for 5 m. Dimethylglyoxime (2.32 g, 20 mmol) was added, followed by cobalt chloride (2.37 g, 10 mmol). The colour of the solution became red-brown but most of the dimethylglyoxime did not dissolve. Sodium hydroxide (1.6 g, 40 mmol) in water (5 cm3) was added dropwise over 20 s. When half of the NaOH was added, the formation of the cobalt(II) complex was complete, the solution became deep red-brown and sometimes some solid precipitated. After the end of the addition, the solution became blue-black (the cobalt(I) complex) after 5-8 minutes. 2-Bromoethylbenzene (0.92 g, 5 mmol) in methanol (3 cm3) was added dropwise over 20 s. After 2 m the colour had changed to yellow-brown and the solution was poured into water (300 cm3) and whisked with a glass rod to (a) encourage crystallisation and (b) incorporate oxygen to oxidise any cobalt impurities to give water soluble cobalt(III). The yellow precipitate was filtered and washed with portions, four or more, of water, stirring in air, until the filtrate was not deeply coloured. Drying in air gave a yellow solid (1.54 g, 65%).
Author's Comments
Crucial to the success of this reaction is the rigorous exclusion of oxygen before the addition of the halide. After the addition of 2 mols of NaOH, the formation of the cobaloxime(II) is complete. Further addition of NaOH causes the disproportionation of the cobaloxime(II) to cobaloxime(I) (blue-black) and cobaloxime(III) (brown, but not seen). If the intense blue-black cobaloxime(I) is not observed, then it is best to start again. In this method, half of the cobalt is lost, but if desired, all can be incorporated by adding NaBH4. The rate of reaction of cobaloxime(I) with halides is said to be 107 times faster than that of methoxide, leading to the suggestion that cobaloxime(I) is a super-nucleophile, but more likely it is a radical reaction. The reaction is successful for a wide range of halides or tosylates, even in the hands of inexperienced chemists (6th form pupils), giving typical yields of 60-80%. For observing the 1H NMR spectrum, it is advisable to filter the CDCl3 solution through flash silica gel to completely remove any paramagnetic Co(II) species. In this example, the 1H NMR resonance of the two dimethylglyoxime OH groups were not observed; they normally appear as a broad s at δH 18 ppm.
Data
δH (CDCl3) 1.80t (2H, J 7.4), 2.11s (12H), 2.20t (2H, J 7.4), 7.11-7.22m (5H), 7.33 ca. t (2H, J 7.6), 7.72tt (1H, J 7.6, 1.5), 8.61d (2H, J 7.6).
Found C 52.63; H 5.96; N 14.60, calc. for C
21H
28N
5O
4Co C 53.28; H 5.96; N 14.79%.
Lead Reference
[PhCH2Co(dmgH)2py] Brown TM, Cooksey CJ, Dronsfield AT and Wilkinson A-S, Appl. Organomet. Chem., 1996, 10(6): 415-420; DOI: 10.1002/(SICI)1099-0739(199608)10:6<415::AID-AOC493>3.0.CO;2-7
Other References
[4-NitrobenzylCo(dmgH)2py] Brown TM, Cooksey CJ, Dronsfield AT and Crich D, J. Chem. Educ., 1990, 67(11): 973-974; DOI: 10.1021/ed067p973
[CH3CH2Co(dmgH)2py] Brown TM, Cooksey CJ, Dronsfield AT, Educ. Chem. 1990, 27(2): 45-47.
[n-C10H21Co(dmgH)2py] Brown TM and Cooksey CJ, Educ. Chem. 1987, 24(3): 77-80.
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Keywords: organometallics
