| Summary: | Novel full-sandwich (<i>η</i><sup>5</sup>-Cp)-Ru-paraphenylene complexes with the general formula [(<i>η</i><sup>5</sup>-Cp)<sub>n</sub>Ru(<i>η</i><sup>6</sup>-L)](PF<sub>6</sub>)<sub>n</sub> where n = 1–3 and L = biphenyl, <i>p</i>-terphenyl and <i>p</i>-quaterphenyl, were synthesized and characterized by means of spectroscopic and analytical techniques. The structures of the complexes [(<i>η</i><sup>5</sup>-Cp)Ru(<i>η</i><sup>6</sup>-biphenyl)](PF<sub>6</sub>) (<b>1</b>), [(<i>η</i><sup>5</sup>-Cp)Ru(<i>η</i><sup>6</sup>-terphenyl)](PF<sub>6</sub>) (<b>3</b>) and [(<i>η</i><sup>5</sup>-Cp)<sub>2</sub>Ru(<i>η</i><sup>6</sup>-terphenyl)](PF<sub>6</sub>)<sub>2</sub> (<b>4</b>) was determined by X-ray single crystal methods. The interaction of the complexes [(<i>η</i><sup>5</sup>-Cp)Ru(<i>η</i><sup>6</sup>-quaterphenyl)]Cl, (<b>6</b>)Cl, and [(<i>η</i><sup>5</sup>-Cp)<sub>2</sub>Ru(<i>η</i><sup>6</sup>-quaterphenyl)]Cl<sub>2</sub>, (<b>7</b>)Cl<sub>2</sub>, with the DNA duplex d(5′-CGCGAATTCGCG-3′)<sub>2</sub> was studied using NMR techniques. The results showed that both complexes interacted non-specifically with both the minor and major grooves of the helix. Specifically, (<b>6</b>)Cl exhibited partial binding through intercalation between the T7 and T8 bases of the sequence without disrupting the C–G and A–T hydrogen bonds. Fluorometric determination of the complexes’ binding constants revealed a significant influence of the number of connected phenyl rings in the paraphenylene ligand (L) on the binding affinity of their complexes with the d(5′-CGCGAATTCGCG-3′)<sub>2</sub>. The complexes (<b>6</b>)Cl and (<b>7</b>)Cl<sub>2</sub> were found to be highly cytotoxic against the A549 lung cancer cell line, with complex (<b>6</b>) being more effective than (<b>7</b>) (IC<sub>50</sub> for (<b>6</b>)Cl: 17.45 ± 2.1 μΜ, IC<sub>50</sub> for (<b>7</b>)Cl<sub>2</sub>: 65.83 ± 1.8 μΜ) and with a selectivity index (SI) (SI for (<b>6</b>)Cl: 1.1 and SI for (<b>7</b>)Cl<sub>2</sub>: 4.8).
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