Ruthenium(II/III) bipyridine complexes incorporating thiol-based imine functions: synthesis, spectroscopic and redox properties

A group of five new ruthenium(II) bipyridine heterochelates of the type [RuII(bpy)2L]+ 1a-1e have been synthesized (bpy=2,2'-bipyridine; L=anionic form of the thiol-based imine ligands, HS-C6H4N=C(H)C6H4(R) (R=OMe, Me, H, Cl, NO2). The complexes 1a-1e are 1:1 conducting and diamagnetic. The complexes 1a-1e exhibit strong MLCT transitions in the visible region and intra-ligand transitions in the UV region. In acetonitrile solvent complexes show a reversible ruthenium(III)-ruthenium(II) couple in the range 0.2-0.4 ∨ and irreversible ruthenium(III)→ruthenium(IV) oxidation in the range 1.15-1.73 V vs. SCE. Two successive bipyridine reductions are observed in the ranges -1.43 to -1.57 and -1.67 to -1.78 V vs. SCE. The complexes are susceptible to undergo stereoretentive oxidations to the trivalent ruthenium(III) congeners. The isolated one-electron paramagnetic ruthenium(III) complex, 1c+ exhibits weak rhombic EPR spectrum at 77 K (g1=2.106, g2=2.093, g3=1.966) in 1:1 chloroform-toluene. The EPR spectrum of 1c+ has been analyzed to furnish values of distortion parameters (Δ=8988 cm-1; V=0.8833 cm-1) and energy of the expected ligand field transitions (ν1=1028 nm and ν2=1186 nm) within the t2 shell. One of the ligand field transitions has been experimentally observed at 1265 nm