3-Hydroxyflavones belong to the naturally occurring class of flavonoids and have been extensively studied with regard to medicinal application. Moreover, it has been demonstrated that these compounds act as bioactive chelates to the ruthenium(II)–arene moiety. Such organometallic complexes have shown promising anticancer activity against tumor cells via a multitargeting mode of action, interacting with DNA and inhibiting topoisomerase IIα. In this paper, we present the synthesis and characterization of an extended series of 3-hydroxyflavone ligands and their corresponding ruthenium-p-cymene complexes to study the impact of substitution pattern as well as of electron-withdrawing and –donating substituents at the flavonol-phenyl group. The ligands and complexes were characterized by elemental analysis, ESI-MS, 1D as well as 2D NMR spectroscopy. The structures of four Ru(η6-p-cymene) complexes were determined in solid state by single-crystal X-ray diffraction, and the impact of the substitution pattern with regard to in vitro anticancer activity in human cancer cell lines is discussed. Structural differences, calculated octanol-water partition coefficients (clogP) of the flavonols and aqueous solubility were used to rationalize the finding that chlorido[3-(oxo-κO)-2-(3,5- dimethoxyphenyl)-chromen-4-onato-κO](η6-p-cymene)ruthenium(II) 2b exhibits the highest cytotoxicity with IC50 values in the low μM range in all tested cell lines.
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