Structure-bioactivity relationship in copper(II) complexes with new halogenated coumarin derivatives

This study reports the synthesis, characterization, and biological evaluation of copper(II) complexes with (3E)-3-(1-{[(pyridin-2-yl)methyl]amino}ethylidene)-3,4-dihydro-2H-benzopyran-2,4-dione (HL) and its newly designed derivatives bearing a halogen atom X in the position 6 (6-FHL, 6-ClHL, and 6-BrHL). The coumarin-based ligands were synthesized via a multistep organic synthesis route and characterized using IR, and NMR spectroscopy. A total of 9 new copper(II) complexes were prepared, divided in two distinct series: chloride-containing complexes [Cu(L/6-XL)Cl] (1-4) and nitrate-based complexes [Cu(L)(H₂O)(NO₃)] (5) and [Cu(6-XL)(NO₃)]_x (6-9). All complexes were characterised by IR spectroscopy, mass spectroscopy (1-8), and elemental analysis. Also structural characterization by single-crystal X-ray diffraction for complexes 5-9 was done. The stability of all the complexes in solution was confirmed by UV-Vis spectroscopy. All complexes and free ligands were screened for their cytotoxic activity against MDA-MB-231 (breast) and HCT 116 (colorectal) human cancer cell lines, with comparisons to healthy PDL-MSC cells. Based on IC₅₀ values, complexes 3, 4, and 7 emerged as the most promising candidates and were selected for further investigation. Detailed in vitro antitumor studies were carried out, including cell proliferation assays, quantification of cell number and viability, and flow cytometry analyses. Moreover, comprehensive evaluations such as Hirshfeld surface analysis, DNA-binding affinity (using fluorescence competitive dye displacement assay with ethidium bromide and viscometry), human serum albumin (HSA) interaction, antioxidant activity, and lipophilicity assessments were performed to further explore their biomedical potential.