Ruthenium(II) Arene Anticancer Complexes with Redox-Active Diamine Ligands

The synthesis and characterization of ruthenium(II) arene complexes of the general formula [(η6-arene)Ru(XY)Z]+, where arene = p-cymene (p-cym), hexamethylbenzene (hmb), or biphenyl (bip), XY = o-phenylenediamine (o-pda), o-benzoquinonediimine (o-bqdi), or 4,5-dimethyl-o-phenylenediamine (dmpda), and Z = Cl, Br, or I, are reported (complexes 1−6). In addition, the X-ray crystal structures of [(η6-p-cym)Ru(o-pda)Cl]PF6 (1) and [(η6-hmb)Ru(o-bqdi)Cl]PF6 (3PF6) are described. The Ru−N distances in 3PF6 are significantly shorter [2.033(4) and 2.025(4) Å] compared to those in 1 [2.141(2) and 2.156(2) Å]. All of the imine complexes (3−5) exhibit a characteristic broad 1H NMR NH resonance at ca. δ 14−15. Complex 1 undergoes concomitant ligand-based oxidation and hydrolysis (38% after 24 h) in water. The oxidation also occurs in methanol. The iodido complex [(η6-p-cym)Ru(o-bqdi)I]I (4) did not undergo hydrolysis, whereas the chlorido complex 3 showed relatively fast hydrolysis (t1/2 = 7.5 min). Density functional theory calculations showed that the total bonding energy of 9-EtG in [(η6-p-cym)Ru(o-pda)(9-EtG-N7)]2+ (1EtG) is 23.8 kJ/mol lower than that in [(η6-p-cym)Ru(o-bqdi)(9-EtG-N7)]2+ (3EtG). The greater bonding energy is related to the contribution from strong hydrogen bonding between the NH proton of the chelating ligand and O6 of 9-EtG (1.69 Å). A loss of cytotoxic activity was observed upon oxidation of the amine ligand to an imine (e.g., IC50 = 11 μM for 1 and IC50 > 100 μM for 3, against A2780 ovarian cancer cells). The relationship between the cytotoxic activity and the solution and solid state structures of the imine and amine complexes is discussed.