Synthesis of Cytotoxic Iron-Containing Macrocycles Through Unexpected C(sp2)-C(sp2) Bonds Formation

In previous reports, it has been demonstrated that cyclometalated iron(II/III) complexes can be prepared by reacting iron(0) precursors and mercurated or brominated derivatives. However, in this report, the reaction between [Fe-3(CO)(12)] and mercurated 6-phenyl-2,2 '-bipyridine or brominated 2,6-diphenyl-pyridine pincer derivatives led to compounds in which C(sp(2))-C(sp(2)) bonds have been formed between two ligands. A 16-electron iron(II) complex (1Cl) bearing a tetradentate ligand originating from the dimerization of 6-phenyl-2,2 '-bipyridine was isolated, while a protonated 14-membered macrocycle with [FeBr4](-) as counterion (2) was obtained from 2,6-diphenyl-pyridine. Studies by X-ray diffraction crystallography, NMR, UV-vis and cyclic voltammetry confirmed the structures. Additionally, the cytotoxicity of the new compounds toward gastric cancer cell lines was evaluated, and it was established that the presence of the iron(II) center was crucial for an elevated activity.