RESEARCH ARTICLE

 

The ligand substitution reactions of hydrophobic vitamin B₁₂ derivatives. Reaction of cobyric acid heptapropyl ester with heterocyclic N-donor ligands

 

 

Mohamed S.A. Hamza^{I, *}; Mohamed A. Elawady^II; Helder M. Marques^III

^IDepartment of Chemistry, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt
^IIDepartment of Radiation Protection, Nuclear Power Plants Authority, Cairo, Egypt
^IIIMolecular Sciences Institute, School of Chemistry, University of the Witwatersrand, P.O. WITS, Johannesburg 2050, South Africa

 

 

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ABSTRACT

The hydrophobic cobyrinic acid heptapropyl ester corrinoids XCbs-Pr (axial ligandX = CN^-,SO₃^2-,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^- >SO₃^2- > 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

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Received 17 October 2007
Revised 22 April 2008
Accepted 18 May 2008

 

 

* To whom correspondence should be addressed. E-mail: hamzamsa@yahoo.com