Oxovanadium(IV) complexes of imidazole-4-acetic, imidazole-4,5-dicarboxylic and pyrazole-3,5-dicarboxylic acids.

Complex formation between oxovanadium(IV) and several diazole derivatives (imidazole-4-acetic, imidazole-4,5-dicarboxylic and pyrazole-3,5-dicarboxylic acid) was studied in aqueous solution by pH-potentiometric and spectroscopic (electron paramagnetic resonance and electronic absorption) techniques. The results show that the imidazole-4-acetate ligand, being suitable for bidentate coordination, forms monomeric or mixed hydroxo-bridged species. In the case of the dicarboxylic acids both having two bidentate sites for metal binding the formation of tetrameric or dimeric complexes is strongly favored. Pyrazole-3,5-dicarboxylic acid forms dinuclear, while imidazole-4,5-dicarboxylic acid forms cyclic tetrameric species, both types containing diazolate bridges. There is evidence for the VO2+-supported deprotonation of the imidazole N1 (pyrrole-type) group at pH as low as 5 in aqueous solution.