Comparative biological activity of manganese(II) and zinc(II) hydrazone complexes

This work comparatively investigates structurally related Mn(II) and Zn(II) hydrazone complexes within a unified structural, computational, and biological framework. Three Mn(II) complexes, whose crystal structures have been previously reported, are examined here in detail alongside a newly synthesized and fully characterized Zn(II) analogue. Single-crystal X-ray diffraction revealed that the Zn(II) complex adopts a distorted octahedral coordination environment with an NNS donor set. Spectroscopic methods (IR, NMR and UV-Vis) confirmed ligand coordination in solution. Density functional theory (DFT) calculations showed that all complexes are thermodynamically stable in DMSO, while TDDFT calculations not only reproduced the experimental absorption energy but also resolved the band into two close-lying singlet excitations and, through NTO analysis, established their ligand-centred π → π* nature. The cytotoxic activity of the complexes was assessed against five human cancer cell lines (HeLa - cervical adenocarcinoma, A549 - lung carcinoma, MDA-MB-231 - breast adenocarcinoma, K562 - chronic myelogenous leukemia and LS 174T - colorectal adenocarcinoma) and human normal lung fibroblasts (MRC-5). The Mn(II) complexes exhibited pronounced cytotoxic effects, associated with cell cycle perturbation, apoptosis induction, and modulation of intracellular ROS levels in normal MRC-5 cells. The Zn(II) complex displayed a distinct biological profile. Antimicrobial activity was also evaluated against selected bacterial and fungal strains, and it was found that metal coordination significantly enhances activity compared to the free ligands. Overall, the results highlight comparable biological activity of Mn(II) and Zn(II) hydrazone complexes, supporting their potential as promising scaffolds for further development as antitumor and antimicrobial agents.