Formation and stability of Sc(III), La (III), Dy(III), Pr(III) and Y(III) complexes with hydroxypyridine-carboxylic acids in aqueous solution

The complexation of three hydroxy-substituted pyridine carboxylic acids: 2-hydroxypicolinic acid (L1), 3-hydroxynicotinic acid (L2), and 2-hydroxy-6-methylnicotinic acid (L3) - with Sc³⁺, Y³⁺, La³⁺, Pr³⁺, and Dy³⁺ was systematically studied using spectrophotometric and potentiometric titrations. Stepwise ligand deprotonation allowed determination of complex stoichiometries and formation constants (log β = 4.63 - 11.51), revealing a strong dependence on metal ionic radius, charge density, and donor atom type. Oxygen-donor ligands L1 and L3 preferentially stabilize small, charge-dense cations (Sc³⁺, Dy³⁺), whereas the nitrogen-donor ligand L2 favors larger ions (Y³⁺, La³⁺). The 6-methyl substituent in L3 introduces steric hindrance that reduces complex stability for larger cations. These results demonstrate the combined influence of donor atom identity, steric effects, and metal-ion properties on rare-earth complex stability, providing a quantitative basis for the rational design of selective ligands and functional coordination materials.