A dinuclear bis(bipyridine)ruthenium(II) complex, [(bpy)2Ru^II{L²-}Ru^II(bpy)₂]²⁺, incorporating an unusual non-innocent bridging ligand containing a p-benzoquinonediimine fragment: synthesis, structure, redox, and UV/VIS/NIR and EPR spectroelectrochemical properties

Abstract

The reaction of [Ru(bpy)₂(EtOH)₂]²⁺ (bpy = 2,2'-bipyridine) with the symmetrical binucleating phenolatodiimine ligand HOC₆H₄NCHC₆H₄CHNC₆H₄OH (H₂L¹) in ethanol under dinitrogen results in the unexpected formation of the diruthenium complex [(bpy)₂Ru^II{OC₆H₄NC ₆H₃(NH)O}Ru^II(bpy)₂](ClO₄)₂1. In this complex, the bridging ligand (L^sq,^sq)^2- contains two inequivalent o-iminophenolate N,O-chelating binding sites, each formally at the semiquinone oxidation level, linked such that there is a p-benzoquinonediimine bridge between the two Ru centres. The crystal structures of H₂L¹ and of 1 have been determined. Complex 1 is electrochemically active and undergoes two reversible oxidations and two reversible reductions which, on the basis of UV/VIS/NIR and EPR spectroelectrochemical evidence, comparison with related systems and consideration of the redox potentials, we assign as centred on the bridging ligand. The two oxidations (at +0.12 and + 0.35 V versus SCE) result in formation of a neutral, fully quinonoidal bridging ligand Lq,q; in the 'mixed-valence' mono-oxidised state, a near-IR transition (1570 nm) is ascribed to an intra-ligand charge transfer from the non-oxidised (semiquinone-like) to the oxidised (quinone-like) terminus. The two reductions (at -0.98 and -1.38 V versus SCE) are localised on the central p-benzoquinonediimine unit of the bridging ligand, affording the p-benzosemiquinone radical and then a p-diamide unit. In addition, at more extreme potentials, two oxidations at +1.49 and +1.70 V versus SCE are ascribed to Ru(II)/Ru(III) couples, and the reduction at -1.81 V versus SCE is bpy-based.