Syntheses, crystal structures, aggregation-induced emissions and anticancer activities of two Cd(II) complexes with one novel indole–pyridine Schiff base

A novel asymmetrical bis-Schiff base, (E)-3-(((di(pyridin-2-yl)methylene)hydrazono)methyl)-1-methyl-1H-indole (3DM), incorporating both pyridine and indole units, was synthesized and structurally characterized alongside its two cadmium(II) complexes (3DMCd, p-3DMCd). Comprehensive spectroscopic characterization (elemental analysis, HRMS, NMR, fluorescence) was performed. Both cadmium(II) complexes exhibit visible fluorescence, and p-3DMCd in particular displays intense yellow emission readily observable to the naked eye, indicating its potential as a novel aggregation-induced emissive material. Cytotoxicity assays conducted against A375, A549, and HeLa cancer cell lines, as well as normal HFF-1 fibroblasts, show that the free ligand 3DM is minimally toxic, whereas both cadmium(II) complexes selectively inhibit tumor cell proliferation and display markedly lower toxicity toward normal cells relative to CdCl2. In HeLa cells specifically, both 3DMCd and p-3DMCd exhibit substantially higher cytotoxicity than CdCl2, while maintaining significantly reduced toxicity toward normal cells. These results highlight the potential role of 3DM as a detoxifying ligand and support the promise of its cadmium(II) complexes as low-toxicity, metal-based candidates for anticervical cancer therapy. Given the established significance of DNA in the mechanism of action of metal-based drugs, docking studies were conducted against a DNA target to elucidate the structural basis for the differential potencies exhibited by the three compounds.