Collect. Czech. Chem. Commun. 2001, 66, 307-337

Syntheses, Spectroelectrochemistry and Photoinduced Electron-Transfer Processes of Novel Ru and Os Dyad and Triad Complexes with Functionalized Diimide Ligands

M. Delower Hossaina, Masa-aki Hagaa,*, Bobak Gholamkhassb, Koichi Nozakib, Minoru Tsushimab, Noriaki Ikedab and Takeshi Ohnob

a Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan
b Department of Chemistry, Faculty of Science, Osaka University, Machikaneyama, Toyonaka, Osaka 560, Japan


A series of mono- and dinuclear Ru complexes containing a bridging ligand, L-diimide-L (L = 2-(2-pyridyl)benzimidazole; diimide = benzene-1,2:4,5-bis(dicarboximide) (bdi; pyromellitimide) or naphthalene-1,8:4,5-bis(dicarboximide) (ndi), with either propane or xylene group as linkers, have been prepared. The mono- and dinuclear Ru complexes containing the bdi or ndi ligand, exhibit characteristic metal-to-ligand charge transfer (MLCT) transition at 458 nm. The mono- and dinuclear Ru/Os complexes exhibit a rich redox chemistry arising from both M(II) to M(III) oxidation and ligand-based consecutive reduction of diimide and 2,2'-bipyridine (bpy) ligands. The emission decays fit well with double- or triple-exponential decay models. The non-exponential decay curve reveals the existence of several conformers in solution due to the flexible propane or p-xylene linker. The much shorter emission lifetimes of the [M(bpy)2(L-diimide-L)] complexes compared with the parent [M(bpy)2L] indicates the intramolecular electron-transfer from the excited M(bpy)2 moiety to the diimide. The fastest rates of electron transfer (3 ยท 1010 s-1) are attributable to the folded conformers suitable for the electron donor/acceptor through-space interaction. A time-resolved absorption spectroscopic study of the dinuclear bdi and ndi complexes revealed appearance of the electron-transfer products, M(III) and the diimide radical anion, and their rapid disappearance. The effect of the linkers of the ligand L-diimide-L on the rates of electron transfer and the back electron transfer is also discussed.

Keywords: Electrochemistry; Photochemistry; Ruthenium complexes; Osmium complexes; Photoinduced electron transfer; Diimides; Bipyridines.

References: 45 live references.