Aqueous Complexation of Morin and Its Sulfonate Derivative with Lanthanum(III) and Trivalent Lanthanides

The complexation of two flavonoids, morin and morin-5'-sulfonic acid (MSA), with rare earth metals is studied in aqueous solution using absorption spectroscopy. In this research, 50 conditional and 26 "true" equilibrium stability constants of monocomplex species were determined for various values of pH at constant ionic strength I = 1 mol·L⁻¹ (NaClO₄). The interactions have been investigated for La³⁺ and 12 lanthanides (Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺). The obtained "true" stability constants (log₁₀K) lay within 6.7–7.8 logarithmic units. DFT calculations and three full-electron basis sets functions (Def2-SVP, Def2-TZVP and Def2-TZVPD) were applied for evaluation of theoretical log₁₀K^calc and comparative analysis of La(III)–flavonoid interactions.