SPECTROSCOPIC, ELECTROCHEMICAL, AND SPECTROELECTROCHEMICAL INVESTIGATIONS OF MIXED-METAL OSMIUM(II) RUTHENIUM(II) BIMETALLIC COMPLEXES INCORPORATING POLYPYRIDYL BRIDGING LIGANDS

The synthesis, electrochemical, spectroscopic, and spectroelectrochemical properties of [(bpy)2Os(BL)Ru(bpy)2](PF6)4 (where BL = 2,3-bis(2'-pyridyl)pyrazine (dpp), 2,3-bis(2'-pyridyl)quinoxaline (dpq), and 2,3-bis(2'-pyridyl)benzoquinoxaline (dpb) and bpy = 2,2'-bipyridine) are reported. Addition of the Ru(bpy)2(2+) moiety to the vacant coordination site on the bridging ligand in the [Os(bpy)2BL]2+ parent compounds results in a shift to lower energies of the metal-to-ligand charge transfer (MLCT) transitions terminating in the bridging-ligand-based pi* orbital as well as a shift to more positive potential of the bridging-ligand-based electrochemical reductions. The lowest energy electronic transitions in these mixed-metal systems have been assigned to overlapping Os(d-pi) --> BL(pi*) and Ru(d-pi) -->BL(pi*) 1MLCT transitions with the Os(d-pi) --> BL(pi*) transition occurring at slightly lower energies. On the basis of spectroelectrochemical experiments, the first and second reductions of the bimetallic complexes have been assigned to sequential reductions of the bridging ligand.