Homo- and Heteronuclear Ruthenium and Osmium Complexes Containing an Asymmetric Pyrazine-Based Bridging Ligand Ronald Hage, ² Hans E. B. Lempers, ‡ Jaap G. Haasnoot, and Jan Reedijk Leiden Institute of Chemistry, Gorlaeus Laboratories, P.O. Box 9502, 2300 RA Leiden, The Netherlands Frances M. Weldon and Johannes G. Vos* School of Chemical Sciences, Dublin City University, Dublin 9, Ireland ReceiVed October 22, 1996 X The synthesis, characterization, and electrochemical, photophysical, and photochemical properties of the compounds [Ru(bpy) 2 (L)] 2+ (Ru), [Os(bpy) 2 (L)] 2+ (Os), [(L)Os(bpy) 2 Cl] + (OsCl), [Ru(bpy) 2 (L)Ru(bpy) 2 Cl] 3+ (RuRuCl), [Os(bpy) 2 (L)Os(bpy) 2 Cl] 3+ (OsOsCl), [Ru(bpy) 2 (L)Os(bpy) 2 Cl] 3+ (RuOsCl), and [Os(bpy) 2 (L)Ru(bpy) 2 Cl] 3+ (OsRuCl) are reported (bpy ) 2,2′-bipyridine, L ) 1-methyl-3-(pyrazin-2-yl)-1,2,4-triazole). The Os(bpy) 2 and the Ru(bpy) 2 moieties are coordinated to the pyrazyltriazole ligand in two different ways, i.e. in a bidentate fashion Via the triazole ring and N1 of the pyrazine ring and in a monodentate fashion only Via N4 of the pyrazine ring. In the homonuclear dimers the monodentate bound metal has an oxidation potential that is approximately 400 mV lower than that of the bidentate bound metal. Spectroelectrochemical investigations suggest the presence of a weak interaction between the metal centers in the dinuclear species. The emission properties of the compounds are indicative of efficient energy transfer in the excited state, leading to emission from only one metal unit. In acetone both RuRuCl and the OsRuCl show photodissociation of the monodentate ruthenium moiety; however, RuOsCl and OsOsCl were found to be photostable. Introduction Extensive investigations have revealed that mononuclear and polynuclear ruthenium compounds exhibit interesting electro- chemical, photophysical, and photochemical properties. 1-7 The polynuclear compounds exhibit in many cases photoinduced energy- and electron-transfer processes that are strongly related to the properties of the bridging ligand employed. We have undertaken a systematic study of mononuclear and dinuclear ruthenium-bpy complexes containing various triazole ligands. An interesting feature of triazole ligands is that the position of substituents on the triazole ring determines which coordination mode prevails, i.e. N1′ vs N4′ coordination. 8 In addition, triazole-based ligands, being strong σ-donors, have been shown to promote electronic interaction between metal centers. 9 In a previous paper the synthesis and physical properties of a series of ruthenium complexes containing pyrazyltriazoles were presented. We have shown that for 3-(pyrazin-2-yl)-1,2,4- triazole the lowest unoccupied molecular orbital (LUMO) in these complexes can be located on the pyrazinyltriazole ligand or on the bpy ligands, depending on whether the triazole ring is protonated or not. 10 Pyrazines can also act as bidentate ligands. Protons or metal ions may coordinate to the uncoor- dinated N4 atom of pyrazine-based ligands for example as in [Ru(bpz) 3 ] 2+ (bpz ) 2,2′-bipyrazine), 11,12 and pyrazine-based dinuclear compounds have been studied in great detail. 13 In this contribution we report the properties of a series of homo- and heteronuclear ruthenium- and osmium-based dinuclear compounds with the pyrazyltriazole ligand 1-methyl-3-(pyrazin- 2-yl)-1,2,4-triazole (L), where one metal center is bound in a bidentate fashion and a second Via N4 of the pyrazine ring shown as follows: The effect of the asymmetric coordination geometry on the electrochemical, electronic, and photochemical properties will be discussed. The electronic properties of the intervalence compounds are investigated. Emission and electrochemical ² Current address: Unilever Research Laboratory, Olivier van Noortlaan 120, 3133 AT Vlaardingen, The Netherlands. ‡ Current address: Department of Organic Chemistry and Catalysis, Technical University Delft, Julianalaan 136, 2628 BL, Delft, The Nether- lands. X Abstract published in AdVance ACS Abstracts, June 1, 1997. (1) Seddon, E. A.; Seddon K. R. The Chemistry of Ruthenium; Elsevier: Amsterdam, 1984. (2) Krause, R. A. Struct. Bonding 1987, 67, 1. (3) Kalyanasundaram, K. Coord. Chem. ReV. 1982, 46, 159. (4) Kalyanasundaram, K.; Gra ¨tzel, M.; Perlizzetti, E. Coord. Chem. ReV. 1986, 69, 57. (5) Du ¨ rr, H.; Do ¨ rr G.; Zengerle, K.; Mayer, E.; Curchod, J. H.; Braun, A. M. NouV. J. Chim. 1985, 9, 717. (6) Crutchley, R. J.; Lever, A. B. P. J. Am. Chem. Soc. 1980, 102, 7129. (7) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. 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