Cobaloximes as Vehicles for College Teaching T. M. Brown and A. T. Dronsfield Derbyshire College of Higher Education Derby, DE3 IGB,United Kingdom
C. J. Cooksey and D. Crich University College London, London, WC1H OAJ. Unlted Kingdom Cobaloximes belong to a recently discovered class of organometallic compounds that contain a covalent cobaltcarbon bond. This species was thought to be elusive until it was shown in 1962 to be present in coenzyme B n ( I ) . In this molecule, the cobalt(II1) ion is stabilized by octahedral coordination to five nitrogen atoms, with sixth site being the CoC covalent bond. Much simpler model compounds, with similar chemical properties to coenzyme BIZ,may be synthesized in which two molecules of the monoanion of dimethylglyoxime provide four nitrogen atoms in a square plane and the fifth nitrogen is provided by pyridiue. The cobaltcarbon bond results from nucleophilic attack of a cobalt(1) intermediate on R B I (R= alkyl) or Ar-CH2Br. Cobalt(1) is an uncommon oxidation state for the metal and immediatelv oxidizes in the Dresence of oxveen. There are two widely ised methods for'the preparation of a suitable cobalt(1) comolex, both of which start from the corresponding cobalt(1i) c&plex and require an inert atmosphere. In his celebrated pioneering work Schrauzer (2)used sodium borohydride as a reducing agent a t low temperature t o achieve near-quantitative yields; however, for college use we prefer the cheaper and wicker alkaline disproportionation technique (3).Although the latter has the disadvantaee of ~ r o d u c i n eunwanted cobalt(I1I) . . comnlex in eaual amount, this does not complicate the workup procedure and adds little to the overall cost. Unlike Schrauzer's method. the disproportionation synthesis of cobaloximes may be achieved in a matter of minutes, making it particularly suitable for class use. The compound represented in the figure can be summarized as B ~ C o ( d m g H ) ~ and P y arises from the sequence:
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