A Dinuclear Ruthenium(II) Complex with the Dianion of 2,5-Dihydroxy-1,4-benzoquinone as Bridging Ligand. Redox, Spectroscopic, and Mixed-Valence Properties

Complex 1 was prepared from [Ru(bipy)2Cl2]·2H2O and K2L [bipy = 2,2‘-bipyridine; H2L = 2,5-dihydroxy-1,4-benzoquinone]. It undergoes two reversible one-electron oxidations separated by 0.34 V (Kc ≈ 6 × 105 for the mixed-valence state), and a one-electron reduction. A molecular orbital analysis showed that the HOMO is delocalized between the metals and the bridging ligand, so the oxidations have both metal and ligand character. The LUMO in contrast is largely based on the bridging ligand, and the EPR spectrum of the reduced species is consistent with a largely ligand-based radical. UV/vis spectra recorded during stepwise oxidation of the dication to the +3 and +4 states indicate that the mixed-valence state is valence-trapped (class II).