Mixed-ligand complexes of ruthenium(II) containing new photoactive or electroactive ligands: synthesis, spectral characterization and DNA interactions

Mixed-ligand ruthenium(II) complexes of three photoactive ligands, viz., (E)-1-[2-(4-methyl-2-pyridyl)-4-pyridyl]-2-(1-naphthyl)-1-ethene (mppne), (E)-1-(9-anthryl)-2-[2-(4-methyl-2-pyridyl)-4-pyridyl]-1-ethene (mppae) and (E)-1-[2-(4-methyl-2-pyridyl)-4-pyridyl]-2-(1-pyrenyl)-1-ethene (mpppe), in which a 2,2'-bipyridyl unit is linked via an ethylinic linkage to either a naphthalene, an anthracene or a pyrene chromophore and three electroactive ligands, viz., 4-(4-pyridyl)-1,2-benzenediol (catpy), 5,6-dihydroxy-1,10-phenanthroline (catphen) and 1,2-benzenediol (cat), were synthesized in good to moderate yields. Complexes [Ru(bpy)<SUB>2</SUB>(mppne)]<SUP>2+</SUP> (bpy is 2, 2'-bipyridyl), [Ru(bpy)<SUB>2</SUB>(mppae)]<SUP>2+</SUP>, [Ru(bpy)<SUB>2</SUB>(mpppe)]<SUP>2+</SUP>, [Ru(bpy)<SUB>2</SUB>(sq-py)]<SUP>+</SUP>, [Ru(bpy)<SUB>2</SUB>(sq-phen)]<SUP>+</SUP> and [Ru(phen)<SUB>2</SUB>(bsq)]<SUP>+</SUP> (phen is 1,10-phenanthroline) were fully characterized by elemental analysis, IR, <SUP>1</SUP>H NMR, fast-atom bombardment or electron-impact mass, UV-vis and cyclic voltammetric methods. In the latter three complexes, the ligands catpy, catphen and cat are actually bound to the metal center as the corresponding semiquinone species, viz., 4-(4-pyridyl)-1,2-benzenedioleto(+I) (sq-py), 1,10-phenanthroline-5,6-dioleto(+I) (sq-phen) and 1,2-benzenedioleto(+I) (bsq), thus making the overall charge of the complexes formally equal to + 1 in each case. These three complexes are electron paramagnetic resonance active and exhibit an intense absorption band between 941 and 958 nm owing to metal-to-ligand charge transfer (MLCT, d<SUB>Ru</SUB>→<SUP>∗</SUP><SUB>sq</SUB>) transitions. The other three ruthenium(II) complexes containing three photoactive ligands, mppne, mppae and mpppe, exhibit MLCT (d<SUB>Ru</SUB>→<SUP>∗</SUP><SUB>bpy</SUB> ) bands in the 454-461-nm region and are diamagnetic. These can be characterized by the <SUP>1</SUP>H NMR method. [Ru(bpy)<SUB>2</SUB>(mppne)]<SUP>2+</SUP>, [Ru(bpy)<SUB>2</SUB>(mppae)]<SUP>2+</SUP> and [Ru(bpy)<SUB>2</SUB>(mpppe)]<SUP>2+</SUP> exhibit redox waves corresponding to the Ru<SUP>III</SUP>/Ru<SUP>II</SUP> couple along with the expected ligand (bpy and substituted bpy) based ones in their cyclic and differential pulse voltammograms (CH<SUB>3</SUB>CN, 0.1 M tetrabutylammonium hexafluorophosphate)-corresponding voltammograms of [Ru(bpy)<SUB>2</SUB>(sq-py)]<SUP>+</SUP>, [Ru(bpy)<SUB>2</SUB>(sq-phen)]<SUP>+</SUP> and [Ru(phen)<SUB>2</SUB>(bsq)]<SUP>+</SUP> are mainly characterized by waves corresponding to the quinone/semiquinone (q/sq) and semiquinone/1,2-diol (sq/cat) redox processes. The results of absorption and fluorescence titration as well as thermal denaturation studies reveal that [Ru(bpy)<SUB>2</SUB>(mppne)]<SUP>2+</SUP> and [Ru(bpy)<SUB>2</SUB>(mppae)]<SUP>2+</SUP> are moderate-to-strong binders of calf thymus DNA with binding constants ranging from 10<SUP>5</SUP> to 10<SUP>6</SUP> M<SUP>−1</SUP>. Under the identical conditions of drug and light dose, the DNA (supercoiled pBR 322) photocleavage activities of these two complexes follow the order:[Ru(bpy)<SUB>2</SUB>(mppne)]<SUP>2+</SUP>&gt;[Ru(bpy)<SUB>2</SUB>(mppae)]<SUP>2+</SUP>, although the emission quantum yields follow the reverse order. The other ruthenium(II) complexes containing the semiquinone-based ligands are found to be nonluminescent and inefficient photocleavage agents of DNA. However, experiments shows that [Ru(bpy)<SUB>2</SUB>(sq)]<SUP>+</SUP>-based complexes oxidize the sugar unit and could be used as mild oxidants for the sugar moiety of DNA. Possible explanations for these observations are presented