MONONUCLEAR AND DINUCLEAR COMPLEXES OF RUTHENIUM(II) AND OSMIUM(II) WITH A 3,5-BIS(2-PYRIDYL)-1,2,4-TRIAZOLE CYCLOHEXYL-BRIDGED SPACER - ABSORPTION-SPECTRA, LUMINESCENCE PROPERTIES, AND ELECTROCHEMICAL-BEHAVIOR

Two novel dinuclear complexes of Ru(II) and Os(II) with a bis-chelating bridging ligand based on 3,5-bis(2-pyridyl)- 1,2,4-triazole moieties have been synthesized, and their absorption spectra, electrochemical behavior, and luminescence properties have been studied. For comparison purposes, the parent mono-chelating ligand and its mononuclear complexes have also been synthesized and their properties have been studied. The investigated complexes are (bpy)2M(dpt-cy''-dpt)M(bpy)2(4+) and (bpy)2M(dpt-Cy')2+ (M = Ru2+ or Os2+; bpy = 2,2'-bipyridine; dpt-cy''-dpt = trans-1,4-cyclohexanebis(N4-3,5-bis(2-pyridyl)-1,2,4-triazole)amide; dpt-cy' = N4-cyclohexane-3,5-bis(2-pyridyl)-1,2,4-triazole). Their absorption spectra exhibit intense bands (epsilon in the range 10(4)-10(5) M-1 cm-1) in the UV region due to bpy-centered transitions and moderately intense bands (epsilon in the range 10(3)-10(4) M-1 cm-1) in the visible region that can be assigned to metal-to-bpy charge-transfer (MLCT) transitions. The extinction coefficients of the bands of the dinuclear complexes are about twice those of the corresponding bands of the mononuclear species. All the complexes exhibit reversible, metal-based oxidation processes and reversible, bpy-based reduction processes in the potential window investigated (-1.8 V/+1.7 V vs SCE). In particular, the differential pulse voltammograms of the dinuclear compounds exhibit only one oxidation peak at approximately the same potential of the oxidation processes of the corresponding mononuclear species (+1.30 V and +0.80 V for Ru and Os compounds, respectively). This indicates that the metal ions of the dinuclear compounds undergo simultaneous oxidation. Complexes and ligands are luminescent both at room temperature in acetonitrile fluid solution and at 77 K in 4:1 MeOH/EtOH (v/v) rigid matrix. The ligands fluorescence, attributed to a pi-pi* level, is completely quenched in the complexes. The luminescence of the complexes can be attributed to the lowest energy triplet M --> bpy charge-transfer excited state(s). Luminescence energies and lifetimes are practically unchanged on passing from mono- to dinuclear species. The results obtained indicate that only a weak electronic interaction is possible between the metal units across the cyclohexyl-bridged spacer, so that the metal components maintain their own excited-state and redox properties when they are arranged into the (supramolecular) dinuclear arrays.