Chemical Speciation of Vanadium(IV/V)/8-Hydroxyquinoline-2-Carboxylic Acid System in Aqueous Solution: A Multitechnique Study

Recent studies have highlighted the biological significance of 8-hydroxyquinoline-2-carboxylic acid (8-HQA) as a metal chelator. In this work, several techniques were applied for the study of the interaction of 8-HQA with vanadium(IV/V) oxidometal ions at a temperature of <i>T</i> = 298.2 K and an ionic strength of <i>I</i> = 0.20 mol dm^-3 in KCl_(aq). The redox behavior of the chemical system was defined, and the stability constants of the formed complexes were determined by H⁺-ion selective electrode potentiometry and UV-vis spectrophotometric titrations, while nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopies and mass spectrometry provided stoichiometric and structural information. The formation of both oxidovanadium(IV) and dioxidovanadium(V) ML complexes with 8-HQA was observed in aqueous solution, with both complexes being particularly stable in acidic conditions. The dioxidovanadium(V) compound is stable even at neutral pH, but dimeric or tetrameric hydrolytic species are predominant under alkaline conditions. In contrast, oxidovanadium(IV) complexes undergo oxidation as the pH increases. Nevertheless, under strictly anaerobic conditions, the complexation of oxidovanadium(IV) by 8-HQA can also occur at pH > 6.0. The nature of the oxidovanadium(IV) and dioxidovanadium(V) ML complexes, representing the major species formed in solution, was further investigated by DFT calculations.