Structurally strained half-sandwich iridium(III) complexes as highly potent anticancer agents.

Six complexes of formula [Ir(eta5:kappa1-C5Me4CH2py)(C,N)]PF6, where C5Me4CH2py is 2-((2,3,4,5-tetramethylcyclopentadienyl)methyl)pyridine, and C,N is 2-phenylpyridine (1), 7,8-benzoquinoline (2), 1-phenylisoquinoline (3), 2-(p-tolyl)pyridine (4), 4-chloro-2-phenylquinoline (5) or 2-(2, 4-difluorophenyl)pyridine (6), have been synthesized. The cyclopentadienyl bears a tethered pyridine that binds to the metal center resulting in an Ir(eta5:kappa1-C5Me4CH2pyN) tether-ring structure, as confirmed by the X-ray crystal structures of 1, 2, 4, 5 and 6. Non-tether versions of 1 and 2 were synthesized to aid unambiguous correlation between structure and activity. Whilst non-tether complexes are highly potent towards MCF7 cancer cells (similar to cisplatin), complexes bearing the tether-ring structure, 1-6, are exceptionally more potent (1-2 orders of magnitude). Additionally, 1-6 disrupt mitochondrial membrane potential (Deltapsim) and induce oxidative stress. Internalization studies strongly correlate intracellular accumulation and anticancer activity in tether and non-tether complexes. We present a new class of organo-iridium drug candidates bearing a structural feature that results in a leap in anticancer potency.