Novel {Cu4} and {Cu4Cd6} clusters derived from flexible aminoalcohols: synthesis, characterization, crystal structures, and evaluation of anticancer propertiest

Two new copper clusters, {Cu-4} and {Cu4Cd6}, with polydentate aminoalcohol ligands, diethanol propanolamine (H3L1) and bis-tris{2-[bis(2-hydroxyethyl)amino]-2-(hydroxymethyl)propane-1,3-diol} (H6L2), have been synthesized under mild conditions and characterized thoroughly by single-crystal X-ray diffraction (XRD), infrared spectroscopy, elemental analysis, powder XRD, magnetic and DFT studies, and absorption and fluorescence spectroscopy. The cluster {Cu-4} exhibits a rare tetranuclear copper cubane core whereas {Cu4Cd6} forms an unusual heterometallic cage owing to the introduction of the second metal Cd into the ligand. A hexapodal ligand (H6L2) with N and O donor atoms was chosen deliberately for the construction of a high-nuclearity cluster, i.e., {Cu4Cd6}. Interestingly, both the clusters displayed significant cytotoxicity towards human cervical (HeLa) and lung (A549) cancer cells as evident from the shallow IC50 values [15.6 +/- 0.8 mu M (HeLa), 18.5 +/- 1.9 mu M (A549) for {Cu-4}, and 11.1 +/- 1.5 mu M (HeLa), 10.2 +/- 1.3 mu M (A549) for {Cu4Cd6}] obtained after a 24 h incubation. However, moderate toxicity was observed toward immortalized lung epithelial normal cells (HPL1D) with IC50 values of 32.4 +/- 1.2 mu M for {Cu-4} and 27.6 +/- 1.7 mu M for {Cu4Cd6}. A cellular apoptotic study using HeLa cells revealed that the {Cu-4} cluster triggered apoptosis at both the early and late phases while the {Cu4Cd6} cluster facilitate apoptosis mainly at the late apoptotic stage. A standard 2',7'-dichlorodihydrofluorescein-diacetate (DCFH-DA) test affirms that both the clusters enhanced ROS production inside the cancer cells, responsible for promoting cell apoptosis. The decanuclear {Cu4Cd6} clusters demonstrated better anticancer activity compared to the tetranuclear {Cu-4} clusters, indicating the role of high nuclearity and additional Cd metal in the enhanced intracellular production of ROS.