Self-assembly of ambidentate pyridyl-carboxylate ligands with octahedral ruthenium metal centers: Self-selection for a single-linkage isomer and anticancer-potency studies

The synthesis of six new [2+2] metallarectangles through the coordination-driven self-assembly of octahedral Ru^II-based acceptors with ambidentate pyridyl-carboxylate donors is described. These molecular rectangles are fully characterized by ¹H NMR spectroscopy, high-resolution electrospray mass spectrometry, and single-crystal X-ray diffraction. In each case, despite the possible formation of multiple isomers, based on the relative orientation of the pyridyl and carboxylate groups (head-to-head versus head-to-tail), evidence for the formation of a single preferred ensemble (head-to-tail) was found in the ¹H NMR spectra. Furthermore, the cytotoxicities of all of the rectangles were established against A549 (lung), AGS (gastric), HCT-15 (colon), and SK hep 1 (liver) human cancer cell lines. The cytotoxicities of rectangles that contained the 5,8-dihydroxy-1,4-naphthaquinonato bridging moiety between the Ru centers (9-11) were particularly high against AGS cancer cells, with IC₅₀ values that were comparable to that of reference drug cisplatin. Express yourself: Six new [2+2] metallarectangles were synthesized by the coordination-driven self-assembly of octahedral Ru^II-based acceptors and ambidentate donors. Their cytotoxicity against various human cancer cell lines was established (see figure); those with 5,8-dihydroxy-1,4-naphthaquinonato groups (1-3) were particularly high.