MONONUCLEAR, DINUCLEAR, AND TRINUCLEAR RUTHENIUM(II) COMPLEXES OF A TRIS(BIPYRIDINE) BRIDGING LIGAND - SYNTHESES, ABSORPTION-SPECTRA, REDOX POTENTIALS, AND PHOTOPHYSICAL PROPERTIES

The tris(bipyridine) ligand 1, l,3,5-tris[(((5-(ethoxycarbonyl)-2,2′-bipyridyl-5′-yl)carbonyl)benzylamino)methyl]benzene, previously used to prepare a hemicaged Ru(II) complex, 1[Ru]2+, has now been employed as a ligand capable of coordinating one, two, or three Ru(bpy)2 2+ units. The three new complexes 1[Ru(bpy)2]2+, 1[Ru(bpy)2]2 4+, and 1[Ru(bpy)2]3 6+ have been synthesized as PF6 − salts, and their spectroscopic and electrochemical properties have been investigated. The absorption spectra of the complexes exhibit bpy and 1 ligand-centered bands in the UV region and Ru → bpy and Ru → 1 charge-transfer bands in the visible region, with the latter being at lower energy. In acetonitrile solution, all the complexes exhibit a reversible oxidation wave, centered on the metal, at +1.36 V and a reversible reduction wave, centered on the coordinated bpy-type arm of 1, at −0.91 V vs SCE. Other reduction waves are present at more negative potentials. All the waves are monoelectronic, dielectronic, and trielectronic for the mononuclear, dinuclear, and trinuclear complexes, respectively. The luminescence properties are the same for the three complexes, with a band at 654 nm (τ ~ 1.4 μs) in a propionitrile-butyronitrile rigid matrix at 77 K and a band at 700 nm (τ = 60 ns) in a fluid propionitrile-butyronitrile solution at room temperature, indicating that (i) the lowest MLCT excited state involves the bpy-type coordinated arm of 1 in each complex and (ii) there is no strong interaction between the metal-containing chromophoric units. However, the fluorescence of the bpy-type arms of 1, observable in the free ligand 1 at 77 K, is no longer present in 1 [Ru(bpy)2]2+, indicating the occurrence of an intramolecular quenching. Excitation spectra performed on l[Ru(bpy)2]2+ confirm that there is an efficient mechanism for intramolecular energy transfer from the two noncoordinated arms of 1 and the metal-containing chromophoric unit. © 1990, American Chemical Society. All rights reserved.