Investigations on the Influence of the Axial Ligand in [Salophene]iron(III) Complexes on Biological Activity and Redox Behavior

The [N,N′-disalicylidene-1,2-phenylenediamine]iron(III) ([salophene]iron(III)) derivatives 1–4 with anionic axial ligands (A = Cl(−), NO(3)(−), SCN(−), CH(3)COO(−)) and complexes 5 and 6 with neutral ligands (A = imidazole, 1-methylimidazole) as well as the μ-oxo dimer 7 inhibited proliferation, reduced metabolic activity, and increased mitochondrial reactive oxygen species. Ferroptosis as part of the mode of action was identified by inhibitor experiments, together with induction of lipid peroxidation and diminished mitochondrial membrane potential. No differences in activity were observed for all compounds except 4, which was slightly less active. Electrochemical analyses revealed for all compounds a fast attachment of the solvent dimethyl sulfoxide and a release of the axial ligand A. In contrast, in dichloromethane and acetonitrile, ligand exchange did not take place, as analyzed by measurements of the standard potential for the iron(III/II) redox reaction.