Model complexes for Vitamin B12

Model Complexes for Cobalamins
The simplist model for Vitamin B₁₂ is the [MeCo(CN)₅]^3- ion. A recent preparation has also described the preparation of the methylpentamminecobalt(III) ion. An early observation, and the first example of a model reaction for Vitamin B₁₂ was that the methylpentacyanocobaltate(III) complex will transfer its methyl group to an Hg(II), so generating the highly poisonous [MeHg⁺] moiety. A mercury methylation forming this highly toxic ion, and also involving methylcobalamin, and the related F430 nickel metalloenzyme is believed to be behind the Minamata tragedy in Japan.
The principle models used for Vitamin B₁₂ are the alkylcobalt(III)dimethylglyoxinates, so called alkylcobaloximes, which have the advantages of easy preparation and simplicity as models. They also use relatively inexpensive chemicals! The methyl complex photolyses readily to form ethane, light in the range 300nm to about 450nm is effective. This demonstartes the ready homolytic fission of the Co-C bond, a process behind one of the cobalamin-enzyme reactions. The methyl radicals can be trapped with spin-traps such as PBN and detected with electron magnetic resonance (EMR). Reaction with Hg(II) again forms the highly toxic MeHg⁺ ion.
Of course none of these will ever provide substitutes for Vitamin B₁₂, but they do provide platforms on which various aspects of the mechanisms of the Vitamin B₁₂ catalysed reactions can be studied. The alkylcobaloximes also provide an excellent experimental platform for the undergraduate synthetic laboratory. They illustrate many fundamental principles of inorganic coordination chemistry, are fascinating materials, and can also be used to introduce more esoteric techniques such as EMR spin trapping. Thus, in Bristol we use such a preparation of various alkyl cobaloximes in our second year undergraduate laboratory. A recent preparative text has an alternative preparation, "Synthesis and Techniques in Inorganic Chemistry", by G.S.Girolami, T.B. Rauchfuss and R.J.Angelici, University Science Books, Sausalito, CA, USA. See Experiment 21.