Solution speciation and spectral studies on oxovanadium(IV) complexes of pyridinecarboxylic acids.

Complex formation between oxovanadium(IV) and several pyridinecarboxylic acids (picolinic acid, 3-hydroxypicolinic acid and 2-hydroxynicotinic acid) was studied in aqueous solution by pH-potentiometric and spectroscopic (electron paramagnetic resonance and electronic absorption) techniques. The results demonstrated that picolinic acid forms mono and bis complexes with the VO(IV) ion, coordinated through the pyridine nitrogen and the carboxylate group. The pyridine nitrogen is also involved in the complexation of VO(IV) by 3-hydroxypicolinic acid and 2-hydroxynicotinic acid in the acidic pH range, but as the pH is increased deprotonation of the phenolic hydroxy group occurs and complexes with salicylate-type (COO-,O-)-coordination are formed. In the VO(IV)-3-hydroxypicolinic acid system, the ligand coordinates to the metal ion in a tridentate fashion via the (N,COO-,O-) donor set, forming a tetrameric complex (VOA)4 in the pH range 6-8.