Synthesis, characterization, and biological studies of some lanthanide pyridine Acylhydrazone Schiff Base complexes

A series of dinuclear lanthanide complexes with the general formula [NHEt₃]₂[Ln₂(L)₂(NO₃)₄]∙nH₂O (Ln = La (1, n = 4), Pr (2, n = 3), Nd (3, n = 4), Sm (4, n = 3), Eu (5, n = 4), Gd (6, n = 4), Tb (7, n = 4), Dy (8, n = 4); H₂L = (E)-N′-(2-hydroxy-5-methoxybenzylidene)picolinohydrazide; NEt₃ = triethylamine) were synthesized and characterized using a range of techniques, including UV–Vis, FTIR, ¹H and ¹³C NMR spectroscopy, elemental analysis, molar conductance, thermal analysis, scanning electron microscopy (SEM), and single-crystal X-ray diffraction. The structural formula indicates that each compound is a dinuclear lanthanide complex. In each case, the Ln³⁺ ion is coordinated by two bidentate nitrate anions, two alkoxide oxygen atoms, one azomethine nitrogen atom, one phenolate oxygen atom, and one pyridyl nitrogen atom. All Ln³⁺  complexes exhibit a nine-coordinate geometry, with the dinuclear core featuring two lanthanide centers bridged by two alkoxide groups from two ligand molecules. Single-crystal X-ray diffraction was carried out for complex 8, revealing that the asymmetric unit comprises a Dy³⁺ ion, one L²⁻ ligand, a bidentate nitrate anion, and one coordinated DMF molecule. Antioxidant activity, evaluated via the DPPH assay, showed that all complexes displayed enhanced radical scavenging ability compared to the free ligand H₂L, with complex 5 exhibiting the highest activity (58 %). Antibacterial assays against both Gram-positive and Gram-negative bacteria demonstrated moderate activity against Gram-negative strains and weak activity against Gram-positive strains.