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

다국어 초록 (Multilingual Abstract)

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, 1H/13C 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.
A question of innocence: A change in the aryl substituents (4‐tolyl/2‐pyridyl) in heteroquinone bridging ligands results in significantly altered metal coordination and electronic structures (see figure; bpy=2,2′‐bipyridine).