Fe(III), Cu(II) and Zn(II) Complexes of the Rigid 9-Oxido-phenalenone Ligand—Spectroscopy, Electrochemistry, and Cytotoxic Properties

The three complexes [Fe(opo)(3)], [Cu(opo)(2)], and [Zn(opo)(2)] containing the non-innocent anionic ligand opo(−) (opo(−) = 9-oxido-phenalenone, Hopo = 9-hydroxyphenalonone) were synthesised from the corresponding acetylacetonates. [Zn(opo)(2)] was characterised using (1)H nuclear magnetic resonance (NMR) spectroscopy, the paramagnetic [Fe(opo)(3)] and [Cu(opo)(2)] by electron paramagnetic resonance (EPR) spectroscopy. While the EPR spectra of [Cu(opo)(2)] and [Cu(acac)(2)] in dimethylformamide (DMF) solution are very similar, a rather narrow spectrum was observed for [Fe(opo)(3)] in tetrahydrofuran (THF) solution in contrast to the very broad spectrum of [Fe(acac)(3)] in THF (Hacac = acetylacetone, 2,4-pentanedione; acac(−) = acetylacetonate). The narrow, completely isotropic signal of [Fe(opo)(3)] disagrees with a metal-centred S = 5/2 spin system that is observed in the solid state. We assume spin-delocalisation to the opo ligand in the sense of an opo(−) to Fe(III) electron transfer. All compounds show several electrochemical opo-centred reduction waves in the range of −1 to −3 V vs. the ferrocene/ferrocenium couple. However, for Cu(II) and Fe(III) the very first one-electron reductions are metal-centred. Electronic absorption in the UV to vis range are due to π–π* transitions in the opo core, giving Hopo and [Zn(opo)(2)] a yellow to orange colour. The structured bands ranging from 400 to 500 for all compounds are assigned to the lowest energy π−π* transitions. They show markedly higher intensities and slight shifts for the Cu(II) (brown) and Fe(III) (red) complexes and we assume admixing metal contributions (MLCT for Cu(II), LMCT for Fe(III)). For both complexes long-wavelength absorptions assignable to d–d transitions were detected. Detailed spectroelectrochemical experiments confirm both the electrochemical and the optical assignments. Hopo and the complexes [Cu(opo)(2)], [Zn(opo)(2)], and [Fe(opo)(3)] show antiproliferative activities against HT-29 (colon cancer) and MCF-7 (breast cancer) cell lines in the range of a few µM, comparable to cisplatin under the same conditions.