Chemistry of ruthenium. 2. Synthesis, structure, and redox properties of 2-(arylazo)pyridine complexes

Article abstract of DOI:10.1021/ic50221a061

Two types of ruthenium(II) complexes RuX₂L₂ (green) and [Ru(bpy)₂L] (ClO₄)₂·H₂O (red) are reported (X = Cl, Br, I; L = 2-(phenylazo)pyridine or 2-(m-tolylazo)pyridine; bpy = 2,2′-bipyridine). The RuX₂L₂ species systematically displays single RuX and RuN(pyridine) stretches in the far IR, strongly suggesting a centrosymmetric configuration for the coordination sphere. Both types of complexes have intense t₂(Ru) → π*(L) transitions in the visible region. RuCl₂L₂ is unreactive to Ag⁺ and nitrogen bases; monodentate tertiary phosphines (P) however give RuClPL₂⁺. Both RuX₂L₂ and Ru(bpy)₂L²⁺ exhibit reversible or nearly reversible ruthenium(III)-ruthenium(II) couples at platinum electrode. The E°₂₉₈′ values in acetonitrile are ～+0.9 V for RuX₂L₂⁺/RuX₂L₂ and ～+1.60 V for Ru(bpy)₂L³⁺/Ru(bpy)₂L²⁺ vs. SCE. The azopyridine ligand system greatly stabilizes the +2 oxidation state in ruthenium. The complexes also display electrochemical responses on the negative side of SCE due to ligand-based reductions. Azo reduction occurs quasi-reversibly at ～-0.5 V in Ru(bpy)₂L²⁺. The existence of a blue isomer of RuX₂L₂ (X = Cl, Br) having cis-RuX₂ is briefly noted.