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>I</i> = 0.2 mol dm^-3 and <i>T</i> = 298.2 K. For all systems, three species are formed: MLH⁺, ML, and ML₂ ^2-. 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 ML₂ ^2- species, with log β₁₂₀: 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 ML₂ ^2- complexes over possible ML-(OH)^-, with 8-HQA acting as tridentate in all formed species, including the protonated MLH⁺.