Tuning of Redox Potentials by Introducing a Cyclometalated Bond to Bis-tridentate Ruthenium(II) Complexes Bearing Bis(N- methylbenzimidazolyl)benzene or -pyridine Ligands Wen-Wen Yang, † Yu-Wu Zhong,* ,† Shinpei Yoshikawa, ‡ Jiang-Yang Shao, † Shigeyuki Masaoka, § Ken Sakai, § Jiannian Yao, † and Masa-aki Haga* ,‡ † Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan § Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan * S Supporting Information ABSTRACT: A series of asymmetrical bis-tridentate cyclo- metalated 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. ■ INTRODUCTION Polyazine transition-metal complexes, particularly ruthenium- (II) complexes, have attracted tremendous interest because of their distinguished electrochemical and photophysical proper- ties. 1 They intensely absorb visible light from the metal-to- ligand charge-transfer (MLCT) transitions, which makes them good candidates as light-harvesting dyes and sensitizers. 2 Some ruthenium complexes with bright emission and long excited- state lifetimes, e.g., [Ru(bpy) 3 ] 2+ (bpy = 2,2′-bipyridine; Φ = 9.5% in oxygen-free acetonitrile; τ = 1150 ns), 3 are benchmark emissive organometallic complexes. They have been widely used in photoinduced electron- or energy-transfer processes and photocatalysis. 4 Bis-tridentate octahedral complexes such as [Ru(tpy) 2 ] 2+ (tpy = 2,2′:6′,2″-terpyridine) can be readily incorporated into supramolecular architectures with well- defined structures via easy and reliable functionalization at the 4′ position of the tpy ligand, 5 and linear multimetallic coordination arrays as potential molecular wires could be produced. However, it should be noted that the coordination environment and nature of the supporting ligands play significant roles in determining the electrochemical and photophysical properties of these complexes. This, in turn, determines their suitability for specific applications such as biomediators for electron shuttles between active sites of oxidoreductases and the electrode. 6 Recently, cyclometalated ruthenium complexes have been the focus of many research activities. 7 These complexes contain a covalent Ru-C bond between the metal center and one supporting ligand. Because of the presence of the anionic cyclometalating ligand, the metal center of cyclometalated complexes is much more electron-rich than that of non- cyclometalated analogues. As a result, the metal-associated redox potentials of these complexes are much less positive than the noncyclometalated ones. For example, the Ru II/III process 8 of noncyclometalated complex [Ru(tpy) 2 ] 2+ occurs at +0.89 V vs Fc/Fc + (+1.3 V vs Ag/AgCl 9 ), while this process could take place around at +0.12 V vs Fc/Fc + (corresponding to +0.57 V vs Ag/AgCl) for the cyclometalated analogue [Ru(tpy)(dpb)] + (dpb = 1,3-di-2-pyridylbenzene). 8 In this context, we also note that bis(triazole)- or bis(tetrazole)pyridine, as reported by Vos and co-workers, could also act as σ-donor ligands, and Received: August 3, 2011 Published: December 28, 2011 Article pubs.acs.org/IC © 2011 American Chemical Society 890 dx.doi.org/10.1021/ic2016885 | Inorg. Chem. 2012, 51, 890-899