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.

Co(iii) complexes of curcumin and phenanthroline bases show remarkable visible-light induced cytotoxicity in HeLa cells but are much less toxic in dark and to normal cells. The complexes bind to HSA with significant affinity.

An iron(iii)–Schiff base complex derived from vitamin B6and thiosemicarbazide is significantly photocytotoxic to HeLa cancer cells in visible light (400 nm–700 nm) but non-toxic in the absence of light.

A series of four novel neodymium(iii) complexes of the formulation [Nd(R-tpy)(O-O)(NO3)2] (), where R-tpy is 4'-phenyl-2,2':6',2''-terpyridine (Ph-tpy; , ) and 4'-ferrocenyl-2,2':6',2''-terpyridine (Fc-tpy; , ); O-O is the conjugate base of acetylacetone (Hacac; , ) or curcumin (Hcurc; , ), are synthesized and characterized. The single crystal structure of shows that the complex is a discrete mononuclear species with the Nd(iii) centre in a nine coordinate environment provided by a set of O6N3 donor atoms. Complexes and having the simple acac ligand are prepared as control compounds. Complex , possessing an appended ferrocenyl (Fc) and the curcumin moiety, is remarkably photocytotoxic to HeLa and MCF-7 cancer cells in visible light giving respective IC50 values of 0.7 μM and 2.1 μM while being significantly less toxic to MCF-10A normal cells (IC50 = 34 μM) and in the dark (IC50 > 50 μM). The phenyl appended complex , lacking a ferrocenyl moiety, is significantly less toxic to both the cell lines when compared with . Complexes and , lacking the photoactive curcumin moiety, do not show any apparent toxicity both in light and in the dark. The cell death is apoptotic in nature and is mediated by the light-induced formation of reactive oxygen species (ROS). Fluorescence imaging experiment with HeLa cells reveals mitochondrial accumulation of complex within 4 h of incubation. The complexes bind to calf thymus (ct) DNA with moderate affinity giving Kb values in the range of 10(4)-10(5) M(-1). The curcumin complexes and cleave plasmid supercoiled DNA to its nicked circular form in visible light via(1)O2 and ˙OH pathways. The presence of the ferrocenyl moiety is likely to be responsible for the enhanced cellular uptake and photocytotoxicity of complex . Thus, the mitochondria targeting complex , being remarkably cytotoxic in light but non-toxic in the dark and to normal cells, is a potential candidate for photochemotherapeutic applications.

A mitochondria-targeted ferrocenyl iron(iii) coumarin complex shows red-light toxicity against cancer cells but remains harmless in the dark and to normal cells.