Valence-state alternatives in diastereoisomeric complexes [(acac)₂Ru(μ-QL)Ru(acac)₂]ⁿ (QL^2- = 1,4-Dioxido-9,10-anthraquinone,n = +2, +1, 0, -1, -2)

Abstract

The title complexes were obtained in neutral form (n = 0) as rac (1) and meso isomers (2). 2 was crystallized for X-ray diffraction and its temperature-dependent magnetism studied. It contains two antiferromagnetically coupled ruthenium(III) ions, bridged by the quinizarine dianion QL^2- (quinizarine = 1,4-dihydroxy-9,10-anthraquinone). The potential of both the ligand (QL^o → QL^4-) and the metal complex fragment combination [(acac)₂Ru^II]₂ → {[(acac)₂Ru^IV]₂}⁴⁺ to exist in five different redox states creates a large variety of combinations, which was assessed for the electrochemically reversibly accessible 2+, 1+, 0, 1-, 2- forms using cyclic voltammetry as well as EPR and UV-vis-NIR spectroelectrochemistry. The results for the two isomers are similar: Oxidation to 1⁺ or 2⁺ causes the emergence of a near-infrared band (1390 nm), without revealing an EPR response even at 4 κ . Reduction to 1^- or 2^- produces an EPR signal, signifying metal-centered spin but no near-infrared absorption. Tentatively, we assume metal-based oxidation of [(acac)₂Ru^III(μ-QL^2-)Ru^III(acac)₂] to a mixed-valent intermediate [(acac)₂Ru^III(μ-QL^2-)Ru^IV(acac)₂]⁺ and ligand-centered reduction to a radical complex [(acac)₂RuIII(µ-QL.3-)Ru^III(acac)₂]^- with antiferromagnetic three-spin interaction.