Stereoselective formation of a meso-diruthenium(II,II) complex and tuning the properties of its monoruthenium analogues

A novel bis(bidentate) ligand dgpm (dgpm = diguanidylpyrimidine) was synthesized by a catalyst-free C-N bond forming reaction in high yield (90%) by microwave-assisted heating. The ligand was coordinated to two [Ru(bpy)(2)](2+) cores to afford a meso-di-Ru(II,II) complex (1-meso) with high diastereoselectivity over its homochiral form. Three mononuclear ether-functionalized Ru(II) complexes (2: ethoxyether; 3: butoxyether; 4: 2-hydroxy-1-ethoxyether) were also isolated. The ligand and complexes were fully characterized by a variety of techniques including X-ray crystallography. In cyclic voltammetric studies, the complexes exhibit a Ru-III/II couple, which is similar to 500 mV less positive than the Ru-III/II couple in Ru(bpy)(3)(2+). The (MLCT)-M-1 absorption maxima of all the complexes (510-550 nm) are considerably red-shifted as compared to that of Ru(bpy) 3 2+ (450 nm). The (MLCT)-M-3 emission maxima of complexes 1-meso and 3 are also red-shifted by about 120 nm compared to that of Ru(bpy)(3)(2+) (620 nm), whereas the corresponding maxima for complexes 2 and 4 are shifted by 75 nm and 25 nm, respectively. These relative trends in redox potentials and (MLCT)-M-1 maxima are in good agreement with DFT and TD-DFT calculations, performed for all complexes. Complexes 1-meso and 3 display emission from a Ru-II-to-bpy (MLCT)-M-3 state, which is rarely the emitting state at lambda > 700 nm in [Ru(bpy)(2)(N-N)](2+) complexes when the ancillary ligand is neutral.