Thermodynamics, optical properties, and coordination of lanthanoids with hydroxyquinolate functionalised receptor

Two new lanthanide complexes with the general formula [Ln(hqtsc)₂Cl], where Ln = Eu³⁺/Tb³⁺ and hqtsc is (E)-2-((8-hydroxyquinolin-2-yl)methylene)hydrazine-1-carbothioamide have been synthesized. The structures of the complexes have been elucidated through IR, ¹H NMR, ¹³C NMR and HR-mass spectroscopy. The coordination behavior of the ligand was investigated with proton and two trivalent lanthanides, Eu(III) and Tb(III), by potentiometric and spectrophotometric methods in a highly aqueous medium. The studies reveal that the two lanthanides form complexes of the type ML₂H₂, ML₂H₁, ML₂, ML₂H_−1, and ML₂H₋₂. The high formation constants of ML with log β = 26.55 and 27.13 indicate that these complexes will become promising candidates for chelation therapy, radioimmunotherapy, and other biomedical applications. Further, the change in colour and electronic spectra of the complexes in the presence of anions showed the selective colorimetric sensing ability towards H₂PO₄⁻ (orange/yellow→, colorless) and CN⁻ (orange/yellow → red). The DFT studies were also carried out to establish the structure, bonding, and sensing mechanism of the complexes.