Use of 13C and 1H NMR to determine the structure of the diastereomeric cobalt corrinoids aquacyanocobinamide and aquacyanocobester

Abstract

The structures of the diastereomers of aquacyanocobinamide and aquacyanocobester were determined by 2-dimensional NMR and comparison with the known X-ray crystal structure of α-cyano-β-aqua-cobyric acid (COBYIC). The α-CN− ligand of α-cyano-β-aqua-cobinamide and α-cyano-β-aqua-cobester is bent away from the C20 methyl group. Hydrogen bonding takes place between the β-OH2 ligand and the c-acetamide side-chain of α-cyano-β-aqua-cobinamide and, to a lesser extent, in α-cyano-β-aqua-cobester, as with α-cyano-β-aqua-cobyric acid. The presence of different side-chains in aquacyanocobinamide and aquacyanocobester has a notable effect on the corrin ring. A cis-effect of the axial ligands CN− and H2O is observed for the delocalised carbon atoms of the corrin ring and for the hydrogen attached to C10 in both diastereomers of aquacyanocobinamide and aquacyanocobester. Two-dimensional 1H and 13C NMR spectroscopy provides a convenient substitute for X-ray crystallography in determining the structures of diastereomers of corrinoids with CN− and H2O as axial ligands.