Spectroscopy and quantum-chemical calculations of nitro-bis-bipyridyl complexes of ruthenium(II) with 4-substituted pyridine ligands

The luminescence, absorption, and luminescence excitation spectra of complexes cis-[Ru(bpy)₂(L)(NO₂)]⁺ [bpy = 2,2′-bipyridyl, L = pyridine, 4-aminopyridine, 4-dimethylaminopyridine, 4-picoline, isonicotinamide, or 4,4′-bipyridyl] in alcoholic (4 : 1 EtOH–MeOH) solutions are studied at 77 K. A linear correlation is established between the energy of the lowest electronically excited metal-toligand charge transfer state d_π(Ru) → π*(bpy) of the complexes and the pK_a parameter of the free 4-substituted pyridines used as ligands L. The B3LYP/[6-31G(d)+LanL2DZ(Ru)] hybrid density functional method is used to optimize the geometry of complexes and calculate their electronic structure and the charge distribution on the atoms of the nearest environment of ruthenium(II) ions. It is shown that there exists a mutually unambiguous correspondence between the charge on the nitrogen atom of ligands L coordinated in the complex and the pK_a parameter of ligands. The calculated energies of the electronically excited metal-to-ligand charge transfer states of complexes linearly (correlation coefficient 0.99) depend on the charge on the nitrogen atom of ligands L, which completely agrees with the experimental data.