Question

Cobaloxime Synthesis Questions

1. Given this compound: chloro(pyridine)bis(dimethylglyoximato)cobalt(III)

a) Does this complex contain any chelating ligands? If so, which ones and what is their denticity?

b) Which ligands are monodentate?

c) Which ligands are neutral?

d) Which ligands are anionic?

2. The coordination complex you are making belongs to a family of compounds with a shorthand nickname of “cobaloxime”. Other derivatives of cobaloximes can be made by replacing chloro- and pyridine with other ligands. Cobaloximes have been used as model complexes of the naturally occurring cobalamin, otherwise known as vitamin B12.

a. What makes cobaloximes appropriate models for vitamin B12?

b. What are some differences between cobaloxime and vitamin B12?

Answer 1

1) a) This complex contains 2 chelating ligands, and they are classified as bidentate.

b) This complex contains two monodentate ligands, Cl^- and py (pyridine).

c) py is a neutral ligand.

d) Cl^- is an anionic ligand.

2) a) In the body, cobalt is present in the family of coenzymes called cobalamins, one derivative of which is cyanocobalamin or vitamin B₁₂. The structure of cobalamins are similar and consist of a cobalt ion situated in a tetrazamacrocyclic ligand with trans imidazole and alkyl groups. The molecular complexity of the cobalamins makes it difficult to study the coordination chemistry of the cobalt center. Inorganic chemists, however, have found that many properties of these enzymes can be simulated using model compounds derived from fairly simple ligands. The most successful model complexes are known as the cobaloximes.

b) The metal site of vitamin B₁₂ consists of a cobalt atom coordinated by a tetrapyrrole macrocyclic ligand called a corrin. One of the axial sites is occupied by a 5,6-dimethylbenzimidazole nucleotide. The N4 macrocycle in the cobaloxime model complexes is made up of two monodeprotonated dioxime molecules linked at two points by hydrogen bonding. The result is an essentially planar macrocyclic ligand. The most common cobaloximes utilize the dimethylglyoxime ligand.