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.