SYNTHESIS, ABSORPTION-SPECTRA, AND PHOTOCHEMICAL BEHAVIOR OF MONONUCLEAR AND DINUCLEAR RUTHENIUM(II) COMPLEXES

Syntheses, characterization, absorption spectra (including their solvent dependence), and photochemical behavior of some novel mono- and dinuclear ruthenium(II) complexes containing 2,3-dpp and 2,5-dpp as bridging ligands (dpp = bis(2-pyridyl)pyrazine) are reported. The complexes are [Ru(2,n-dpp)(eta-6-C6H6)Cl]PF6 (1, n = 3;2, n = 5), [(mu-2,n-dpp)[Ru(eta-6-C6H6)Cl]2](PF6)2 (3, n = 3; 4, n = 5), and [Cl(eta-6-C6H6)Ru(mu-2,n-dpp)Ru(CO)2Cl2]PF6 (5, n = 3; 6, n = 5). For comparison purposes, the same properties of some closely related mono- and dinuclear ruthenium (II) complexes are also reported. The absorption spectra of all the complexes show intense ligand-centered (LC) bands in the UV region and broad, moderately intense, metal-to-ligand charge-transfer (MLCT) bands in the visible region. In the heteroligated dinuclear complexes two different MLCT transitions are expected, but only one absorption maximum is present in the spectra because the metal fragments exhibit similar chromophoric properties. The energy position of the MLCT band is practically unaffected by solvent polarity in the complexes that do not contain carbonyl ligands, whereas for the other complexes a small red shift is observed in low-polarity solvents. The two heteroligated complexes are strongly photosensitive in fluid solution, with release of a CO ligand. Quantum yields for the reactant disappearance are 0.2 and 0.01 for 2,3-dpp and 2,5-dpp complexes, respectively. The differences in the quantum yields are attributed to intramolecular energy transfer from the reactive Ru(CO)2Cl2 to the unreactive Ru(eta-6-C6H6)Cl moiety, which competes with the photoreaction in the 2,5-dpp complex.