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 (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.