South African Journal of Chemistry
On-line version ISSN 1996-840X
Print version ISSN 0379-4350
HAMZA, Mohamed S.A.; ELAWADY, Mohamed A. and MARQUES, Helder M.. The ligand substitution reactions of hydrophobic vitamin B12 derivatives. Reaction of cobyric acid heptapropyl ester with heterocyclic N-donor ligands. S.Afr.j.chem. (Online) [online]. 2008, vol.61, pp.68-73. ISSN 1996-840X.
The hydrophobic cobyrinic acid heptapropyl ester corrinoids XCbs-Pr (axial ligandX = 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.
Keywords : Hydrophobic vitamin B12; cobalt corrinoids; equilibrium constants; solvent polarity; trans influence.