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

## TL;DR

This study explores how 8-HQA interacts with vanadium ions in solution, revealing stable complexes and their behavior under different pH conditions.

## Contribution

The paper presents a multitechnique analysis of vanadium(IV/V)-8-HQA complexes, including redox behavior and structural insights.

## Key findings

- Vanadium(IV) and vanadium(V) form stable ML complexes with 8-HQA in acidic conditions.
- Dioxidovanadium(V) remains stable at neutral pH, while vanadium(IV) oxidizes at higher pH.
- DFT calculations help clarify the structure of the major complex species in solution.

## Abstract

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 T = 298.2 K and an ionic strength of 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.