Tuning of Redox Potentials by Introducing a Cyclometalated Bond to Bis-tridentate Ruthenium(II) Complexes Bearing Bis(N-methylbenzimidazolyl)benzene or -pyridine Ligands

A series of asymmetrical bis-tridentate cyclometalated complexes including [Ru­(Mebib)­(Mebip)]+, [Ru­(Mebip)­(dpb)]+, [Ru­(Mebip)­(Medpb)]+, and [Ru­(Mebib)­(tpy)]+ and two bis-tridentate noncyclometalated complexes [Ru­(Mebip)2]2+ and [Ru­(Mebip)­(tpy)]2+ were prepared and characterized, where Mebib is bis­(N-methylbenzimidazolyl)­benzene, Mebip is bis­(N-methylbenzimidazolyl)­pyridine, dpb is 1,3-di-2-pyridylbenzene, Medpb is 4,6-dimethyl-1,3-di-2-pyridylbenzene, and tpy is 2,2′:6′,2″-terpyridine. The solid-state structure of [Ru­(Mebip)­(Medpb)]+ is studied by X-ray crystallographic analysis. The electrochemical and spectroscopic properties of these ruthenium complexes were studied and compared with those of known complexes [Ru­(tpy)­(dpb)]+ and [Ru­(tpy)2]2+. The change of the supporting ligands and coordination environment allows progressive modulation of the metal-associated redox potentials (RuII/III) from +0.26 to +1.32 V vs Ag/AgCl. The introduction of a ruthenium cyclometalated bond in these complexes results in a significant negative potential shift. The RuII/III potentials of these complexes were analyzed on the basis of Lever’s electrochemical parameters (EL). Density functional theory (DFT) and time-dependent DFT calculations were carried out to elucidate the electronic structures and spectroscopic spectra of complexes with Mebib or Mebip ligands.