The vital roles of metals in nutrients and medicines are not accessible to purely organic compounds.

The family of iodido OsII arene phenylazopyridine complexes [Os(η6‐p‐cym)(5‐R1‐pyridylazo‐4‐R2‐phenyl))I]+ (where p‐cym=para‐cymene) exhibit potent sub‐micromolar antiproliferative activity towards human cancer cells and are active in vivo. Their chemical behavior is distinct from that of cisplatin: they do not readily hydrolyze, nor bind to DNA bases. We report here a mechanism by which they are activated in cancer cells, involving release of the I− ligand in the presence of glutathione (GSH). The X‐ray crystal structures of two active complexes are reported, 1‐I (R1=OEt, R2=H) and 2‐I (R1=H, R2=NMe2). They were labelled with the radionuclide 131I (β−/γ emitter, t1/2 8.02 d), and their activity in MCF‐7 human breast cancer cells was studied. 1‐[131I] and 2‐[131I] exhibit good stability in both phosphate‐buffered saline and blood serum. In contrast, once taken up by MCF‐7 cells, the iodide ligand is rapidly pumped out. Intriguingly, GSH catalyzes their hydrolysis. The resulting hydroxido complexes can form thiolato and sulfenato adducts with GSH, and react with H2O2 generating hydroxyl radicals. These findings shed new light on the mechanism of action of these organo‐osmium complexes.

Half-sandwich Os-arene complexes exhibit promising anticancer activity, but their photochemistry has hardly been explored. To exploit the photocytotoxicity and photochemistry of Os-arenes, O,O-chelated complexes [Os(η 6 -p-cymene)(Curc)Cl] (OsCUR-1, Curc = curcumin) and [Os(η 6 -biphenyl)(Curc)Cl] (OsCUR-2), and N,N-chelated complexes [Os(η 6 -biphenyl)(dpq)-I]PF 6 (OsDPQ-2, dpq = pyrazino[2,3-f ][1,10]phenanthroline) and [Os(η 6 -biphenyl)(bpy)I]PF 6 (OsBPY-2, bpy = 2,2′-bipyridine), have been investigated. The Os-arene curcumin complexes showed remarkable photocytotoxicity toward a range of cancer cell lines (blue light IC 50 : 2.6−5.8 μM, photocytotoxicity index PI = 23−34), especially toward cisplatin-resistant cancer cells, but were nontoxic to normal cells. They localized mainly in mitochondria in the dark but translocated to the nucleus upon photoirradiation, generating DNA and mitochondrial damage, which might contribute toward overcoming cisplatin resistance. Mitochondrial damage, apoptosis, ROS generation, DNA damage, angiogenesis inhibition, and colony formation were observed when A549 lung cancer cells were treated with OsCUR-2. The photochemistry of these Osarene complexes was investigated by a combination of NMR, HPLC-MS, high energy resolution fluorescence detected (HERFD), Xray adsorption near edge structure (XANES) spectroscopy, total fluorescence yield (TFY) XANES spectra, and theoretical computation. Selective photodissociation of the arene ligand and oxidation of Os(II) to Os(III) occurred under blue light or UVA excitation. This new approach to the design of novel Os-arene complexes as phototherapeutic agents suggests that the novel curcumin complex OsCUR-2, in particular, is a potential candidate for further development as a photosensitizer for anticancer photoactivated chemotherapy (PACT).