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

A group of three dinuclear ruthenium(II) complexes of the type [(bpy)(2)Ru-II(L)Ru-II(bpy)(2)](ClO4)(4). 2H(2)O, 1a-1c [bpy=2,2'-bipyridine, L=bridging ligand, NpC5H4CH=N-i-(R)-N-i=CHC5H4Np; 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 la and the geometry of the complexes 1 in solution have been assessed by fast atom bombardment (FAB) mass spectrometry and H-1/C-13 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 d pi(Ru-II)-->pi*(L) transition. The other two higher energy MLCT transitions are believed to be d pi(Ru-II)-->pi*(bpy) transitions. Highly intense ligand-based pi-->pi* transitions are observed in the UV region. in acetonitrile solvent, complexes 1 show one quasi-reversible two-electron oxidation process near 1.5 Vvs. 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. (C) 1999 Elsevier Science Ltd. All rights reserved.