A Multistep Oxidative Cascade Reaction from a Naphthalenediol-Based Pre-Ligand to a Tetranuclear Perylenequinone-Based Fe^III Complex

We have developed a family of dinuclear complexes using 2,7-disubstituted 1,8-naphthalenediol ligands that bind by molecular recognition to two neighboring phosphate diesters of the DNA backbone with the dinuclear Cu^II and Ni^II complexes exhibiting a severe cytotoxicity for human cancer cells. To increase the binding affinity, we intended to synthesize the corresponding dinuclear Fe^III complex. Surprisingly, we obtained a tetranuclear Fe^III perylene-based complex instead of the expected dinuclear Fe^III naphthalene-based complex. In order to establish a rational and reproducible synthesis, we carefully analyzed this reaction. This revealed a multistep oxidative cascade reaction including the pre-coordination of Fe^II ions in the N₃-binding pockets, the Lewis-acid assisted MOM-deprotection of the pre-ligand by the pre-oriented Fe^II ions, two oxidative aromatic C-C coupling reactions, oxidation of the perylene-based backbone and of Fe^II to Fe^III. The careful analysis of bond lengths, HOMA indices (harmonic oscillation model of aromaticity), FTIR and UV-Vis-NIR spectra supported by DFT calculations reveals the presence of an aromatic 18-electron oxidized perylenequinone ligand backbone. In summary, a multistep cascade reaction involving in total a 10-electron oxidation has been established for the straight-forward synthesis of an unprecedented perylenequinone-based ligand system.