Six-co-ordinated vanadium-(IV) and -(V) complexes of benzimidazole and pyridyl containing ligands

A new series of vanadium-(IV) and -(V) complexes with ligands containing functionalities including hydroxyethyl, carboxylate, amine, pyridyl and benzimidazole have been prepared. The crystal structures of three complexes have been determined. All the vanadium atoms are six-co-ordinate even though previous complexes of the diethanolamine type have contained five-co-ordinate vanadium. The V-N (amine) bonds of the benzimidazole complexes are significantly longer than those of pyridyl complexes. An empirical relationship between pK(a) value of the protonated ligand and the V-N (amine) bond length (trans to an oxo group) of the corresponding vanadium-(IV), -(V) and -(IV/V) complexes was observed. Spectroscopic studies (NMR, ESR and UV/VIS) showed that the complexes remain intact in aqueous solution. In general, the hydroxyethyl substituted vanadium complexes are less stable than corresponding acetate substituted ones and the benzimidazole complexes are less stable than corresponding pyridyl complexes. The combined knowledge concerning the properties of all these complexes provides an excellent platform to design new vanadium complexes pertinent to biological applications.