Investigating the DNA binding properties of two new RuII(dppz) complexes incorporating the facially coordinated ligands tris(pyridin-2-yl)methylamine and di(pyridin-2-yl)methylamine

By employing the facially coordinated tris(pyridin-2-yl)methylamine ligand, a route to the synthesis of the complex ([Ru(tpyma)(dppz)(py)]2+ (dppz = dipyrido[3,2-a:2′,3′-c]phenazine) py = pyridine) is reported. Detailed NMR spectroscopy and crystallographic studies reveal that tpyma coordinates to the metal ion through a tridentate (2py, NH2), not (3py), binding motif. Subsequently, the synthesis of an analogous complex containing di(pyridin-2-yl)methylamine, dipyma, confirmed that it too is a (2py, NH2) tripodal ligand. In non-aqueous solvents both complexes display emission from the RuII → (dppz)-based 3MLCT excited state. Computational studies reveal that the metal centre of the dipyma complex is more polar than its tpyma analogue and also provide insights into why tpyma does not coordinate to the RuII(dppz) through a (3py) motif. In aqueous solutions the complexes bind to duplex DNA through intercalation with affinities that are entirely comparable to [Ru(bpy)2(dppz)]2+. However, while the tpyma complex displays a DNA light switch effect, the DNA-induced increase in emission from the dipyma complex is significantly lower. This effect is attributed to the different electronic and steric properties of tpyma and dipyma.