An indole–pyridine Schiff base and its cadmium(II) com­plex: crystallographic and biological insights

A novel asymmetrical bis-Schiff base, 3-({(E)-[bis­(pyridin-2-yl)methyl­idene]hydrazinyl­idene}meth­yl)-1H-indole, C20H15N5 (3-D), incorporating both pyridine and indole units, was synthesized and structurally characterized alongside its cadmium(II) com­plex bis­[3-({(E)-[bis­(pyridin-2-yl)methyl­idene]hydrazinyl­idene}meth­yl)-1H-indole]­dichlorido­cadmium(II) trihydrate, [CdCl2(C20H15N5)2]·3H2O (3-D-Cd). Single-crystal X-ray diffraction revealed that both com­pounds adopt discrete mol­ecular architectures, with the com­plex exhibiting pronounced π–π stacking inter­actions, as confirmed by density functional theory (DFT) calculations. Comprehensive spectroscopic characterization (elemental analysis, HRMS, NMR, UV–Vis and fluorescence) was performed. Cytotoxicity assays against A375, A549 and HeLa cancer cell lines, as well as normal HFF-1 cells, demonstrated that, while 3-D is minimally toxic, 3-D-Cd selectively inhibits tumour cell proliferation with significantly reduced toxicity toward normal cells com­pared to CdCl2. These findings suggest the potential of 3-D as a detoxifying ligand and of 3-D-Cd as a candidate for use as a low-toxicity metal-based anti­cancer agent.