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 -> 1(2+)) or through the imidazolate N and pyridyl N donor atoms (II -> 2(2+)). Characterization by crystal structure analysis, H-1/(CNMR)-C-13 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(n) and 2(n). Whereas 1(2+) is reduced to a radical complex with considerable semiquinone character, the reduction of 2(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(4+)) or predominantly (2(4+)) centered on the metal atoms.