Analysis of Redox Series of Unsymmetrical 1,4-Diamido-9,10-anthraquinone-Bridged Diruthenium Compounds

The unsymmetrical diruthenium complexes
[(bpy)2RuII(μ-H2L2–)­RuIII(acac)2]­ClO4 ([3]­ClO4), [(pap)2RuII(μ-H2L2–)­RuIII(acac)2]­ClO4 ([4]­ClO4), and [(bpy)2RuII(μ-H2L2–)­RuII(pap)2]­(ClO4)2 ([5]­(ClO4)2) have been obtained by way of the
mononuclear precursors [(bpy)2RuII(H3L–)]­ClO4 ([1]­ClO4) and [(pap)2RuII(H3L–)]­ClO4 ([2]­ClO4) (where bpy =
2,2′-bipyridine, pap = 2-phenylazopyridine, acac– = 2,4-pentanedionate, and H4L = 1,4-diamino-9,10-anthraquinone).
Structural characterization by single-crystal X-ray diffraction and
magnetic resonance (nuclear magnetic resonance (NMR), electron paramagnetic
resonance (EPR)) were used to establish the oxidation state situation
in each of the isolated materials. Cyclic voltammetry, EPR, and ultraviolet–visible–near-infrared
(UV-vis-NIR) spectroelectrochemistry were used to analyze the multielectron
transfer series of the potentially class I mixed-valent dinuclear
compounds, considering the redox activities of differently coordinated
metals, of the noninnocent bridge and of the terminal ligands. Comparison
with symmetrical analogues [L2′Ru­(μ-H2L)­RuL2′]n (where L′ = bpy, pap, or acac–) shows that the redox processes in the unsymmetrical dinuclear compounds
are not averaged, with respect to the corresponding symmetrical systems,
because of intramolecular charge rearrangements involving the metals,
the noninnocent bridge, and the ancillary ligands.