Ternary iron(II) complex with an emissive imidazopyridine arm from schiff base cyclizations and its oxidative DNA cleavage activity

Abstract

The ternary iron(II) complex [Fe(L')(L")](PF₆)₃( 1) as a synthetic model for the bleomycins, where L' and L" are formed from metal-mediated cyclizations of N,N'-(2-hydroxypropane-1,3-diyl)bis(pyridine-2-aldimine)(L), is synthesized and structurally characterized by X-ray crystallography. In the six-coordinate iron(II) complex, ligands L' and L" show tetradentate and bidentate chelating modes of bonding. Ligand L is formed from an intramolecular attack of the alcoholic OH group of L to one imine moiety leading to the formation of a stereochemically constrained five-membered ring. Ligand L" which is formed from an intermolecular reaction involving one imine moiety of L and pyridine-2-carbaldehyde has an emissive cationic imidazopyridine pendant arm. The complex binds to double-stranded DNA in the minor groove giving a K_app value of 4.1 × 10⁵ M^-1 and displays oxidative cleavage of supercoiled DNA in the presence of H₂O₂ following a hydroxyl radical pathway. The complex also shows photo-induced DNA cleavage activity on UV light exposure involving formation of singlet oxygen as the reactive species.