Synthesis, Spectroscopic Characterization, DFT Studies of Novel Praseodymium(III), Neodymium(III), and Samarium(III) Complexes of a Tridentate Schiff Base: Aspects of Their Antimicrobial, Antitumor, and Biological Profiles

A novel tridentate Schiff base ligand H₂B was synthesized and employed to prepare lanthanide(III) complexes with the general formula [LnBCl(H₂O)₂]·nH₂O, where Ln = (Pr, Nd, and Sm). The structural, spectroscopic, and biological profiles of these complexes were investigated. Comprehensive characterization was performed using ¹H NMR, infrared spectroscopy, elemental analysis, mass spectrometry, magnetic susceptibility, thermal and PXRD analyses, and conductivity measurements. The conductivity results confirmed the nonelectrolytic nature of the complexes, and magnetic studies indicated their paramagnetic behavior. The biological evaluation revealed that the metal complexes exhibited enhanced antimicrobial activity compared to the free ligand, particularly the Nd(III) complex, demonstrating the highest antibacterial efficacy with an inhibition zone of 28 mm against gram-positive strains. The activity trend was Nd(III) > Pr(III) > Sm(III) > H₂B for bacteria and Sm(III) > Nd(III) > Pr(III) > H₂B against fungi (Geotrichum candidum). Cytotoxicity assays against HepG-2 cancer cells demonstrated significantly higher activity of the complexes (IC₅₀ = 51.49–213.11 μg/mL) relative to the free ligand (IC₅₀ = 330.61 μg/mL) as compared to the reference drug vinblastine.

DFT calculations were carried out at the B3LYP level to understand further structural and electronic properties, and molecular docking studies revealed strong binding affinities of the complexes within the active sites of microbial receptors, supporting their biological efficacy. These findings highlight the potential of the synthesized Ln(III) complexes as promising candidates for antimicrobial and anticancer applications.