Homo- and Heteronuclear Ruthenium and Osmium Complexes Containing an Asymmetric Pyrazine-Based Bridging Ligand

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.