Metal complexes with organelle specificity and potent but selective cytotoxicity are highly desirable. In this work, we report the synthesis, characterization and cytotoxicity of six novel copper(ii) complexes of the formula [Cu(R-tpy)(N-O)]NO (1-6), where R-tpy is 4'-phenyl-2,2':6',2''-terpyridine (Ph-tpy; 1-3) or 4'-ferrocenyl-2,2':6',2''-terpyridine (Fc-tpy; 4-6), N-O is the anion of 8-hydroxyquinoline (HQ in 1, 4), 5-chloro-7-iodo-8-hydroxyquinoline (CQ in 2, 5) or 5-nitro-8-hydroxyquinoline (NQ in 3, 6). The complex [Cu(Fc-tpy)](ClO) (7) has also been prepared as a control and structurally characterized. The optimized geometries and the frontier orbitals of the complexes have been obtained from DFT calculations. The ferrocenyl appended complexes having the anticancer active CQ (in 5) and NQ (in 6) ligands show remarkable cytotoxicity, giving the respective IC values of 0.75 μM and 0.52 μM in HeLa and 1.3 μM and 2.6 μM in MCF-7 cancer cells. The phenyl appended complexes 2 and 3 are less active than their ferrocenyl analogues in both the cells while the complexes of HQ (in 1, 4) are the least active. Interestingly, complexes 4-6 are significantly less toxic to MCF-10A normal cells. The DCFDA, annexin-V-FITC and propidium iodide nuclear staining assays reveal an apoptotic mechanism of cell death which is attributable to the metal-assisted generation of reactive oxygen species. Imaging experiments on HeLa cells reveals that complex 5 accumulates primarily inside the mitochondria. The complexes bind to calf thymus DNA with moderate affinity giving K values in the range of 6.3 × 10-7.4 × 10 M and to HSA protein with significant affinity giving K values in the range of 8.6 × 10-1.3 × 10 M. Their affinity for DNA suggests that mitochondrial DNA could be the target while their affinity for HSA suggests that they could be transported by HSA in the blood. This work is the first report to show that the ferrocenyl appended copper(ii) complexes of hydroxyquinoline ligands are remarkably cytotoxic to cancer cells but significantly less toxic to normal cells.

Six novel mixed-ligand copper(II) complexes, namely, [Cu(R-tpy)(L)]NO 3 (1-6), where R-tpy is 4′-phenyl-2,2′:6′,2′′-terpyridine (Ph-tpy; 1-3) and 4′-ferrocenyl-2,2′:6′,2′′-terpyridine (Fc-tpy; 4-6), L is the bidentate O,O donor monoanion of plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone; plum in 1, 4), chrysin (5,7-dihydroxyflavone; chry in 2, 5) and curcumin (bis(4-hydroxy-3methoxyphenyl)-1,6-diene-3,5-dione; curc in 3, 6) have been synthesized and characterized and their in vitro cytotoxicity against cancer cells is evaluated.The energy optimized structures and the frontier orbitals of the complexes have been obtained from the DFT calculations. Complexes 4-6 with a conjugated ferrocenyl moiety and TCM anticancer ligands, namely, plum (in 4), chry (in 5) and curc (in 6) showed potent cytotoxicity giving respective IC 50 values of 1.2 μM, 0.62 μM and 0.21 μM in HeLa and 2.0 μM and 1.0 μM and 0.34 μM in MCF-7 cancer cells while being much less toxic to MCF-10A normal cells (IC 50 : 8.3-17.1 μM). In contrast, complexes 1-3 with a conjugated phenyl moiety were appreciably less toxic to HeLa cells with respective IC 50 values of 10.4 μM, 8.1 μM and 5.5 μM when compared with their ferrocenyl analogues 4-6. Mechanistic studies using Hoechst staining and Annexin-V-FITC assays on cancer cells revealed an apoptotic pathway of cell death induced by the complexes. Fluorescence imaging study showed that complex 6 having curcumin as ligand localized primarily in the mitochondria of HeLa cells. Thus, we demonstrate in this study that ferrocene conjugation to copper(II) complexes of TCM anticancer ligands significantly increases the selectivity and cytotoxicity of the resulting complexes towards cancer cells over normal cells.