Chemical Speciation and Coordination Behavior of 8-Hydroxyquinoline-2-carboxylic Acid with Divalent Cations in Aqueous Solution: An Irving–Williams Series Study

In this work, the coordination properties of 8-hydroxyquinoline-2-carboxylic acid (8-HQA, LH2) toward Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+ are discussed. Stability constants for Mn2+, Co2+, and Ni2+/8-HQA systems were determined by ISE-H+ (glass electrode) potentiometry, and those of Cu2+ and Zn2+/8-HQA by ultraviolet-visible (UV-vis) spectrophotometry, in KCl(aq) at I = 0.2 mol dm(-3) and T = 298.2 K. For all systems, three species are formed: MLH+, ML, and ML22-. 8-HQA proved a good sequestering agent of M2+ over a wide pH range, as also shown by the calculated pL(0.5) values. The stability of the formed metal complexes follows the expected Irving-Williams trend, especially concerning the ML22- species, with log beta(120): 12.45 +/- 0.01 (Mn2+) < 13.45 (Fe2+) < 15.90 +/- 0.04 (Co2+) < 17.17 +/- 0.05 (Ni2+) < 20.64 +/- 0.03 (Cu2+) > 18.78 +/- 0.02 (Zn2+). This trend is inversely correlated to the M-N bond length determined by quantum mechanical calculations. These studies, together with voltammetry and electron paramagnetic resonance spectroscopy, allowed us to derive information about the coordination modes, structure, and nature of the formed species. Results support the formation of ML22- complexes over possible ML(OH)(-), with 8-HQA acting as tridentate in all formed species, including the protonated MLH+.