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-2pyridylbenzene, 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 (Ru-II/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 Ru-II/III potentials of these complexes were analyzed on the basis of Lever's electrochemical parameters (E-L). 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.