Synthesis, structure and DNA interaction of cobalt(III) bis-complexes of 1,3-bis(2-pyridylimino)isoindoline and 1,4,7-triazacyclononane

The complex [CoL₂](ClO₄) • MeOH (<b>1</b>), where HL is the tridentate 3N ligand 1,3-bis(2-pyridylimino)isoindoline, has been isolated and its X-ray crystal structure successfully determined. It possesses a distorted octahedral structure in which both the ligands are coordinated meridionally to cobalt(III) via one deprotonated isoindoline (L⁻) and two pyridine nitrogen atoms. Interestingly, the average dihedral angle between pyridine and isoindoline rings is 25.9°, indicating that the ligand is twisted upon coordination to cobalt(III). The interaction of the complex with calf-thymus DNA has been studied using various spectral methods and viscosity and electrochemical measurements. For comparison, the DNA interaction of [Co(tacn)₂]Cl₃ (<b>2</b>), where tacn is facially coordinating 1,4,7-triazacyclononane, has been also studied. The ligand-based electronic spectral band of <b>1</b> and the N(σ) → Co(III) charge transfer band of <b>2</b> exhibit moderate hypochromism with small or no blue shift on interaction with DNA. The intrinsic binding constants calculated reveal that the monopositive complex ion [CoL₂]⁺ exhibits a DNA-binding affinity lower than the tripositive complex ion [Co(tacn)₂]³⁺. The steric clashes with DNA exterior caused by the second L⁻ ligand bound to cobalt(III), apart from the lower overall positive charge on the [CoL₂]⁺ complex, dictates its DNA-binding mode to be surface binding rather than partial intercalative interaction expected of the extended aromatic chromophore of deprotonated isoindoline anion. An enhancement in relative viscosity of CT DNA on binding to <b>1</b> is consistent with its DNA surface binding. On the other hand, a slight decrease in viscosity of CT DNA was observed on binding to <b>2</b> revealing that the smaller cation leads to bending (kinking) and hence shortening of DNA chain length. The electrochemical studies indicate that the DNA-bound complexes are stabilised in the higher Co(III) rather than the lower Co(II) oxidation state, suggesting the importance of electrostatic forces of DNA interaction.