Use of 13C and 1H NMR to determine in detail the structures of the base-off vitamin B12 complexes dicyanocobinamide and dicyanocobalamin

Abstract

Despite many attempts over more than sixty years, it has not been possible to produce single crystals of any cobalt cobinamides to determine their structure by X-ray crystallography. This paper describes the use of 13C and 1H NMR and comparison with known X-ray crystal structures of similar molecules to determine the structure of cobinamides. The structure of dicyanocobinamide, the simplest cobinamide, in methanol solution, is described by comparison with dicyanocobester. The delocalised system of dicyanocobinamide, which is nearly planar, and the rest of the corrin ring, except for C8, are very similar to those of dicyanocobester. The environment of the axial CN− ligands, including the Co–CN bond lengths and the NC–Co–CN bond angle is similar to that of dicyanocobester and the α-CN− ligand is bent away from the C20 methyl group. The structure of the corrin ligand and coordination sphere of base-off dicyanocobalamin, which occurs only in solution, is similar to that of dicyanocobinamide.