MONONUCLEAR AND DINUCLEAR RUTHENIUM COMPLEXES WITH TRIAZOLE-CONTAINING LIGANDS - FINE-TUNING OF THE SPECTROSCOPIC PROPERTIES

The synthesis, characterization and properties of mononuclear and dinuclear Ru(bpy)2 complexes with bpzt- and mbpt- are reported (bpy = 2,2'-bipyridine, Hbpzt = 3,5-bis(pyrazin-2-yl)-1,2,4-triazole, and Hmbpt = 3-(6-methylpyridin-2-yl)-5-(pyridin-2-yl)-1,2,4-triazole). Both ligands deprotonate easily upon coordination to the Ru(bpy)2 moiety. The coordination mode of the complexes was determined by using H-1 NMR spectroscopy. For the mononuclear compounds, [Ru(bpy)2(L)]+, only coordination via N1 of the triazole ring and the pyridine or pyrazine nitrogen takes place. The two ruthenium centers in the dinuclear compounds with mbpt- and bpzt- are bound via N1 and N4 of the triazole ring. Electrochemical and resonance Raman measurements indicate that for [(Ru(bpy)2)2(bpzt)]3+ and [Ru(bpy)2(Hbpzt)]2+ the first reduction potential is bpzt- based. [Ru(bpy)2(bpzt)]+, [Ru(bpy)2(mbpt)]+, and [(Ru(bpy)2)2(mbpt)]3+ exhibit a bpy-based reduction. This suggests that in the dinuclear bpzt- compound and in the protonated compound, the pi* level of the bpzt- ligand is lower than that of the bpy ligand, while the LUMO (lowest unoccupied molecular orbital) is located on the bpy ligand in [Ru(bpy)2(bpzt)]+. The mixed-valence complexes [(Ru(bpy)2)2(bpzt)]4+ and [(Ru(bpy)2)2(mbpt)]4+ exhibit intervalence-transition bands at 5400 cm-1 (epsilon = 2200 M-1 cm-1) and 5400 cm-1 (epsilon = 2400 M-1 cm-1), respectively. The extent of electron delocalization is in both cases quite high (alpha-2 = 0.019 and 0.020 respectively), suggesting a fairly strong metal-metal interaction via the HOMO (highest occupied molecular orbital) of the triazolate bridge.