Substituent effect analysis in organocobalt corrinoids: binding constants for cyanide, azide and the pendent 5,6-dimethylbenzimidazole nucleotide

Abstract

Binding constants for cyanide ion to eight alkylcobinamides (including four β-alkylcobinamides in which the organic ligand is in the ‘upper’ axial ligand position and four α-alkylcobinamides in which the organic ligand is in the ‘lower’ axial ligand position) have been determined. Unlike all other alkylcobinamides investigated, which undergo rapid cyanolysis to form dicyanocobinamide in the dark, these eight complexes were sufficiently stable towards cleavage by cyanide to permit accurate measurement of the binding constants. The values obtained vary by 6.4×10 4-fold from 4.33 M −1 for β-ethylcobinamide to 2.79×10 5 M −1 for β-cyanomethylcobinamide. In contrast, these alkylcobinamides are stable in the presence of excess azide ion, but bind this ligand much more weakly, the binding constants varying from <1 M −1 for β-ethylcobinamide to 15.8 M −1 for β-cyanomethylcobinamide. A multiparameter substituent effect equation, including inductive, resonance and steric terms, has been successfully used to correlate the intramolecular equilibrium constants for the coordination of the pendent 5,6-dimethylbenzimidazole nucleotide to the metal atom in a series of ten alkylcobalamins. The significance of this correlation with respect to similar correlations recently obtained for alkylcobaloximes is discussed.