A new coordination mode of the photometric reagent glyoxalbis(2-hydroxyanil) (H₂gbha): bis-bidentate bridging by gbha^2- in the redox series {(μ-gbha)[Ru(acac)₂]₂}ⁿ (n = -2, -1, 0, +1, +2), Including a Radical-Bridged Diruthenium(III) and a Ru^III/Ru^IV Intermediate

Abstract

The bis-bidentate bridging function of gbha^2- with N,O^-/N,O^- coordination was observed for the first time in the complex (μ-gbha)[Ru^III(acac)₂]₂ (1). Density functional theory calculations of 1 yield a triplet ground state with a large (ΔE > 6000 cm^-1) singlet-triplet gap. Intermolecular antiferromagnetic coupling was observed (J ≈ -5.3 cm^-1) for the solid. Complex 1 undergoes two one-electron reduction and two one-electron oxidation steps; the five redox forms {(μ-gbha)[Ru(acac)₂]₂}ⁿ (n = -2, -1, 0, +1, +2) were characterized by UV-vis-NIR spectroelectrochemistry (NIR = near infrared). The paramagnetic intermediates were also investigated by electron paramagnetic resonance (EPR) spectroscopy. The monoanion with a comproportionation constant K_c of 2.7 × 10⁸ does not exhibit an NIR band for a Ru^III/Ru^II mixed-valent situation; it is best described as a 1,4-diazabutadiene radical anion containing ligand gbha^·3-, whichbinds two ruthenium(III) centers. A Ru^III-type EPR spectrum with g₁ = 2.27, g₂ = 2.21, and g₃ = 1.73 is observed as a result of antiferromagnetic coupling between one RuIII and the ligand radical. The EPR-active monocation (K_c = 1.7 × 10⁶) exhibits a broad (Δ ν ½ = 2600 cm^-1) intervalence charge-transfer band at 1800 nm, indicating a valence-averaged (Ru^3.5)₂ formulation (class III) with a tendency toward class II (borderline situation).