6-CO-ORDINATED VANADIUM-(IV) AND VANADIUM-(V) COMPLEXES OF BENZIMIDAZOLE AND PYRIDYL CONTAINING LIGANDS

A new series of vanadium-(IV) and -(V) complexes with ligands containi ng 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-ord inate even though previous complexes of the diethanolamine type have c ontained five-co-ordinate vanadium. The V-N (amine) bonds of the benzi midazole complexes are significantly longer than those of pyridyl comp lexes. 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 subst ituted vanadium complexes are less stable than corresponding acetate s ubstituted ones and the benzimidazole complexes are less stable than c orresponding pyridyl complexes. The combined knowledge concerning the properties of all these complexes provides an excellent platform to de sign new vanadium complexes pertinent to biological applications.