Diruthenium Complexes of p-Benzoquinone-Imidazole Hybrid Ligands: Innocent or Noninnocent Behavior of the Quinone Moiety
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Chemistry - An Asian Journal 13 (19): 2947--2955 (2018)

After double deprotonation, 2,6-diaryl-p-benzoquinonodiimidazoles (aryl = 4-tolyl (I) or 2-pyridyl (II)) bridge two Ru(bpy)22+ (bpy = 2,2'-bipyridine) complex fragments through the imidazolate N and p-quinone O (I$\rightarrow$12+) or through the imidazolate N and pyridyl N donor atoms (II$\rightarrow$22+). Characterization by crystal structure anal., 1H/13C NMR spectroscopy, cyclic and differential pulse voltammetry, and spectroelectrochem. (UV/visible/NIR, IR, EPR) in combination with TD-DFT calcns. revealed surprisingly different electronic structures for redox systems 1n and 2n. Whereas 12+ is reduced to a radical complex with considerable semiquinone character, the redn. 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 1st of two close-lying oxidn. processes occurs at the bridging heteroquinone ligand, whereas the 2nd oxidn. is partly (14+) or predominantly (24+) centered on the metal atoms. on SciFinder(R)
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