A spectroscopic investigation of the self-association and DNA binding properties of a series of ternary ruthenium(II) complexes

Six ruthenium(II) complexes of the general form cis-α-[Ru(N4-tetradentate)(N2-bidentate)]Cl2 have been synthesized from the two related tetradentate ligands 1,6-di(2′-pyridyl)-2,5-dimethyl-2,5-diazahexane (picenMe2) and 1,6-di(2′-pyridyl)-2,5-dibenzyl-2,5-diazahexane (picenBz2) and the bidentate ligands 2,2′-bipyridine (bipy), 1,10-phenanthroline (phen) and dipyrido[3,2-f:2′3′-h]quinoxaline (dpq). Synthetic intermediate species of the general form cis-α-[RuII(N4-tetradentate)(DMSO)Cl][PF6] were isolated. The N4-tetradentate ligand picenMe2 formed only the cis-α stereoisomer, while picenBz2 formed both the cis-α and cis-β stereoisomers. These latter stereoisomers were resolved by fractional crystallisation. Dimer self-association constants, KD, were estimated from the concentration dependence of the 1H NMR shifts for some of these complexes in aqueous solutions at 25 °C. The values of KD ranged from 0.6 to 7.9 M−1 and a relationship was observed between the aromatic surface area of the bidentate component and the degree to which self-association occurred, whereby a greater level of self-association correlates with a larger surface area for the bidentate ligand. Some of these complexes demonstrate an ability to bind to DNA that is consistent with intercalation of the bidentate molecular component between the base pairs of the DNA molecule. Using calf-thymus DNA, the equilibrium binding constants, KB, were determined for some of the complexes using intrinsic methods and these ranged from 3.32 to 5.11 M−1, the intercalating abilities of the different bidentate ligands being in the order dpq > phen > bipy. This relationship between aromatic surface area of the bidentate ligand and the degree of DNA binding activity is the same as that observed in the self-association study.