Os2–Os4 Switch Controls DNA Knotting and Anticancer Activity

Dinuclear trihydroxido-bridged osmium–arene complexes are inert and biologically inactive, but we show here that linking dihydroxido-bridged OsII–arene fragments by a bridging di-imine to form a metallacycle framework results in strong antiproliferative activity towards cancer cells and distinctive knotting of DNA. The shortened spacer length reduces biological activity and stability in solution towards decomposition to biologically inactive dimers. Significant differences in behavior toward plasmid DNA condensation are correlated with biological activity.