OSMIUM(II)/RUTHENIUM(II) TRIMETALLICS INCORPORATING POLYAZINE BRIDGING LIGANDS - ISOVALENT NEAR-IR ABSORBERS WITH UNIQUE ELECTROCHEMICAL-BEHAVIOR

A series of mixed-metal Os(II)/Ru(II) trimetallic complexes bridged by polypyridyl ligands have been prepared of general form ([(bpy)(2)Ru(BL)]2OsCl2)(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). The compounds have been characterized by UV/vis absorption and near infrared spectroscopy, cyclic and Osteryoung square-wave voltammetry, and spectroelectrochemistry. The complexes display absorptions throughout the visible region of the spectrum and exhibit complicated electrochemical behavior. All of the systems studied possess both osmium- and ruthenium-based oxidative processes with the ruthenium-based oxidation occurring at significantly more positive potentials. The lowest lying bridging ligand-based pi* orbital shifts to lower energies as the pi delocalization of the bridging ligand increases giving bridging ligand-based reductions which shift to more positive potentials on going from dpp to dpq to dpb (-756, -388, and -260 mV, respectively). Unlike previously prepared mono- and mixed-metal bimetallic complexes utilizing these bridging ligands, the metal-based Os(II/III) oxidation in these trimetallics varies dramatically depending upon the bridging ligand employed. The electrochemical behavior of these species is also unique for polymetallic systems in that the bridging ligands are only reduced by one electron each prior to reduction of the bpy ligands. In addition, the two equivalent terminal ruthenium metal centers in the dpq and dpb complexes oxidize at different potentials, although they are separated by a BL-Os-BL network. The lowest lying excited state of these molecules is an Os(d phi) --> BL(pi*) MLCT based on the central metal center. This transition shifts to the red as the easier to reduce dpq and dpb ligands are incorporated into the trimetallic system. In fact, this MLCT occurs in the near-IR region of the spectrum at 966 nm for the dpq-bridged system and 1059 nm for the dpb-bridged system.