Dinuclear ruthenium(II) bipyridine complexes having non-symmetric α,α'-diimine based neutral bridging ligands.: synthesis, spectroscopic and electrochemical properties

Abstract

A group of three dinuclear ruthenium(II) complexes of the type [(bpy)₂Ru^II(L)Ru^II(bpy)₂](ClO₄)₄.2H₂O, 1a-1c [bpy=2,2'-bipyridine, L=bridging ligand, N_pC₅H₄CH=N_i-(R)-N_i=CHC₅H₄N_p; R=none, 1a; R=-C₆H₄-, 1b; R=-CH₂-C₆H₄-CH₂-, 1c] have been synthesized and characterized. The complexes are essentially diamagnetic and behave as 1:4 electrolytes in acetonitrile solution. The mass of the molecular ion for the complex 1a and the geometry of the complexes 1 in solution have been assessed by fast atom bombardment (FAB) mass spectrometry and ¹H/¹³C NMR spectroscopy, respectively. Complexes 1 display three metal-to-ligand charge-transfer (MLCT) transitions in the visible region, where the lowest energy MLCT transition is considered to be a dπ(Ru^II)→π^∗(L) transition. The other two higher energy MLCT transitions are believed to be dπ(Ru^II)→π^∗(bpy) transitions. Highly intense ligand-based π→π^∗ transitions are observed in the UV region. In acetonitrile solvent, complexes 1 show one quasi-reversible two-electron oxidation process near 1.5 V vs. Ag/AgCl, due to simultaneous one-electron oxidations [ruthenium (III)⇋ruthenium(II)] of both of the ruthenium centers in 1 and multiple reductions in the range -0.5--2.7 V vs. Ag/AgCl, due to successive reductions of the coordinated bridging ligand, L, as well as bipyridine. The chemically and electrochemically generated oxidized trivalent congeners of 1 are unstable at room temperature.