Diruthenium Complexes of p-Benzoquinone-Imidazole Hybrid Ligands: Innocent or Noninnocent Behavior of the Quinone Moiety.

After double deprotonation, 2,6-diaryl-p-benzoquinonodiimidazoles (aryl=4-tolyl (I) or 2-pyridyl (II)) were shown to bridge two [Ru(bpy)2 ]2+ (bpy=2,2'-bipyridine) complex fragments through the imidazolate N and p-quinone O (I→12+ ) or through the imidazolate N and pyridyl N donor atoms (II→22+ ). Characterization by crystal structure analysis, 1 H/13 C NMR spectroscopy, cyclic and differential pulse voltammetry, and spectroelectrochemistry (UV/Vis/NIR, IR, EPR) in combination with TD-DFT calculations revealed surprisingly different electronic structures for redox systems 1n and 2n . Whereas 12+ is reduced to a radical complex with considerable semiquinone character, the reduction of 22+ with its exclusive N coordination exhibits little spin on the now redox-innocent quinone moiety, compared with the electron uptake by the pyridyl-imidazolate chelating site. The first of two close-lying oxidation processes occurs at the bridging heteroquinone ligand, whereas the second oxidation is partly (14+ ) or predominantly (24+ ) centered on the metal atoms.

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