oa South African Journal of Chemistry - The ligand substitution reactions of hydrophobic vitamin B derivatives. Reaction of cobyric acid heptapropyl ester with heterocyclic -donor ligands : research article

Volume 61, Issue 1
  • ISSN : 0379-4350



The hydrophobic cobyrinic acid heptapropyl ester corrinoids XCbs-Pr (axial ligand X = CN-, SO2-, CH- and CHCH-) have been prepared from vitamin B by hydrolysis of the amide side chains and their conversion to propyl esters. Both the position of the -band and the general shape of the UV-visible spectra of these complexes show significant solvent dependence as the polarity of the solvent is varied. The equilibrium constants, , for the reaction of five-membered heterocyclic nitrogenous bases (the azoles imidazole, pyrazole and 1,2,4-triazole) with displacement of coordinated HO in aquacyanocobyrinic acid heptapropyl ester, and coordination by the predominantly five-coordinate complexes sulphitocobyrinic acid heptapropyl ester, ethylcobyrinic acid heptapropyl ester and methylcobyrinic acid heptapropyl ester, have been determined spectrophotometrically at 25°C in water, methanol, acetonitrile, ethyl acetate and toluene. Values of are dependent on the identity of the ligand (X = CN- > SO2- > CH- > CHCH-); they increase with the basicity of the azole (pyrazole < 1,2,4-triazole < imidazole); and they increase as the solvent polarity increases (toluene < ethyl acetate < acetonitrile < methanol < HO). Molecular mechanics calculations suggest that these effects are largely electronic in origin.

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