Lanthanide complexes of the tridentate Schiff base ligand salicylaldehyde-2- picolinoylhydrazone: Synthesis, characterization, photophysical properties, biological activities and catalytic oxidation of aniline

The lanthanide Schiff base complexes: [La(H2L)2(NO3)2]NO3 (1), [Sm(H2L)2(NO3)2]NO3 (2), [Eu(H2L)2(NO3)2]NO3 (3), [Gd(H2L)2(NO3)2]NO3 (4), [Tb(H2L)2(NO3)2]NO3 (5) and [Dy(H2L)2(NO3)2]NO3 (6) were prepared by reactions between Ln(NO3)3.xH2O {where Ln, x: La, 6; Sm, 6; Eu, 5; Gd, 6: Tb, 5; Dy, 6} and salicylaldehyde picolinoylhydrazone (H2L) Schiff base ligand. These complexes were characterized by 1H and 13C NMR, UV–vis, and IR spectroscopy, elemental analysis, molar conductance and thermogravimetric analysis. The two H2L units are coordinated to the central lanthanide ion as tridentate ONO ligand through their carbonyl and phenolic oxygen atoms, and azomethine nitrogen atoms. Two nitrate groups acting as bidentate ligands are coordinated to Ln(III) ion which results in a coordination number of eight for Ln(III). The luminescence study on the free H2L and its corresponding complexes revealed that all complexes exhibit ligand-centered emission. Coordination of H2L to Ln(III) ions and as a result the LMCT explains the blue shift observed in the ligand-centered emission band in the complexes. The MIC values showed that 1–6 complexes exhibit moderate antibacterial activity against treated gram negative bacteria (S., Kp. and Ec.) and are inactive against treated gram positive bacteria (Sa., M. and Ef.). In contrast, H2L ligand is inactive against all treated bacteria. H2L and 1–6 complexes exhibit good antifungal activity against C. albicans fungal as compared to that of reference drug. The DPPH scavenging activity of H2L enhances on complex formation and complex 5 possess the highest activity. The catalytic activities of complexes 1–6 were investigated toward the oxidation of aniline and substituted anilines using hydrogen peroxide as the oxidant at room temperature. Complex 6 provided the highest conversion with the highest selectivity towards azoxybenzene, 79.9%. The oxidative reaction of aromatic aniline incorporated with 4-(4-chlorophenoxy) on the para position proceeded with the formation of azoxybenzene with excellent yield, 91%. Six new salicylaldehyde-2-picolinoylhydrazone lanthanide complexes have been synthesized and characterized. Complexes applicability as antioxidants, antimicrobials and catalysts for oxidation of aniline has been investigated. The luminescence properties investigation of free H2L and its corresponding complexes revealed that all complexes exhibit ligand-centered emission. [Display omitted] •Six lanthanide Schiff base complexes were