oa South African Journal of Chemistry - The ligand substitution reactions of hydrophobic vitamin B12 derivatives. Reaction of cobyric acid heptapropyl ester with heterocyclic N-donor ligands : research article
The hydrophobic cobyrinic acid heptapropyl ester corrinoids XCbs-Pr (axial ligand X = CN-, SO32-, CH3- and CH3CH2-) have been prepared from vitamin B12 by hydrolysis of the amide side chains and their conversion to propyl esters. Both the position of the y-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, K, for the reaction of five-membered heterocyclic nitrogenous bases (the azoles imidazole, pyrazole and 1,2,4-triazole) with displacement of coordinated H2O 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 K are dependent on the identity of the trans ligand (X = CN- > SO32- > CH3- > CH3CH2-); 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 < H2O). Molecular mechanics calculations suggest that these effects are largely electronic in origin.
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