Valence-state alternatives in diastereoisomeric complexes [(acac)2Ru(μ-QL)Ru(acac)2]n (QL2-=1,4-dioxido-9,10-anthraquinone, n =+2,+1, 0,-1,-2)

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(0) -> OL4-) and the metal complex fragment combination [(acac)(2)RUII](2) -> {[(acac)(2)Ru-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 K. 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)(2)Ru-II(mu-QL(2-))Ru-III(acac)(2)] to a mixed-valent intermediate [(acac)(2)Ru-III(mu- QL(2-))Ru-IV(acac)(2)](+) and ligand-centered reduction to a radical complex [(acac)(2)Ru-III(mu-QL(3-))Ru-III(acac)(2)](-) with antiferromagnetic three-spin interaction.