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) ] (bpy=2,2'-bipyridine) complex fragments through the imidazolate N and p-quinone O (I→1 ) or through the imidazolate N and pyridyl N donor atoms (II→2 ). Characterization by crystal structure analysis, H/ 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 1 and 2 . Whereas 1 is reduced to a radical complex with considerable semiquinone character, the reduction of 2 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 (1 ) or predominantly (2 ) centered on the metal atoms.