Organoiridium Complexes: Anticancer Agents and Catalysts

CONSPECTUS: Iridium is a relatively rare precious heavy metal, only slightly less dense than osmium. Researchers have long recognized the catalytic properties of square-planar Ir-I complexes, such as Crabtree's hydrogenation catalyst, an organometallic complex with cyclooctadiene, phosphane, and pyridine ligands. More recently, chemists have developed half-sandwich pseudo-octahedral pentamethylcyclopentadienyl Ir-III complexes containing diamine ligands that efficiently catalyze transfer hydrogenation reactions of ketones and aldehydes in water using H-2 or formate as the hydrogen source. Although sometimes assumed to be chemically inert, the reactivity of low-spin 5d(6) Ir-III centers is highly dependent on the set of ligands. Cp* complexes with strong sigma-donor (CC)-C-boolean AND-chelating ligands can even stabilize Ir-IV and catalyze the oxidation of water. In comparison with well developed Ir catalysts, Ir-based pharmaceuticals are still in their infancy. In this Account, we review recent developments in organoiridium complexes as both catalysts and anticancer agents. Initial studies of anticancer activity with organoiridium complexes focused on square-planar Ir-I complexes because of their structural and electronic similarity to Pt-II anticancer complexes such as cisplatin. Recently, researchers have studied half-sandwich Ir-III anticancer complexes. These complexes with the formula [(Cp-x)Ir((LL)-L-boolean AND')Z](0/n+) (with Cp* or extended Cp* and (LL)-L-boolean AND' = chelated (CN)-N-boolean AND or (NN)-N-boolean AND ligands) have a much greater potency (nanomolar) toward a range of cancer cells (especially leukemia, colon cancer, breast cancer, prostate cancer, and melanoma) than cisplatin. Their mechanism of action may involve both an attack on DNA and a perturbation of the redox status of cells. Some of these complexes can form Ir-III-hydride complexes using coenzyme NAD(P)H as a source of hydride to catalyze the generation of H-2 or the reduction of quinones to semiquinones. Intriguingly, relatively unreactive organoiridium complexes containing an imine as a monodentate ligand have prooxidant activity, which appears to involve catalytic hydride transfer to oxygen and the generation of hydrogen peroxide in cells. In addition, researchers have designed inert Ir-III complexes as potent kinase inhibitors. Octahedral cyclometalated Ir-III complexes not only serve as cell imaging agents, but can also inhibit tumor necrosis factor alpha, promote DNA oxidation, generate singlet oxygen when photoactivated, and exhibit good anticancer activity. Although relatively unexplored, organoiridium chemistry offers unique features that researchers can exploit to generate novel diagnostic agents and drugs with new mechanisms of action.