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

Abstract

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.

Diverse Coordination Modes and Bidirectional Noninnocence of Pyridyl-β-diketonate on Ruthenium Platforms as a Function of Coligands

“…The dimeric [ 7 ]­ClO 4 and [ 8 ]­ClO 4 (Scheme ) differed with respect to the mutual orientation of the two sets of N,O donors of coordinated L – , where the former consisted of metal fragments in the same direction and the latter represented an unprecedented form involving metal fragments in the different directions, possibly due to the impact of bulkier Ru­(PPh 3 ) 2 units around the metal ions. The impact of the electronic nature of the coligands was also reflected in stabilizing Ru III and Ru II states in acac- and bpy/pap/PPh 3 -derived complexes, respectively. , …”

Synthesis of Asymmetric Coupled Polymethines Based on a 7-Chloropyrido[1,2-a]benzimidazole-8,9-dione Core

“…Functionalization of the quinone molecules and tuning the bonds formed between quinone ligands and metal ions as well as delocalized electron distributions endow the metal–organic frameworks (MOFs) with chemical and electrochemical stability, which are essential for organic electrode materials to have the desirable properties. Thus, quinone structure isomerization or condensation with heterocycles can lead to a different metal–ligand binding mechanism as well as to new properties of the compound. , …”

“…Thus, quinone structure isomerization or condensation with heterocycles can lead to a different metal−ligand binding mechanism as well as to new properties of the compound. 9,10 Hence, studies of the relationship between quinone's molecular structure and its photophysical and electrochemical behavior are extremely important as they may provide further insights into the chemical engineering of quinone molecules for a specific application.…”

Neocuproine as a Redox-Active Ligand Platform on Iron and Cobalt

“…As archetypal redox-active units, aromatic heterocycles have featured prominently in this area, including phenanthroline, − bipyridine, − quinoline based ligands, − and conjugated N-donor type ligands. − Within this family, 2,9-dimethyl-1,10-phenanthroline, also known by its common name neocuproine, should be a similarly useful scaffold. Neocuproine is commercially available at low cost, making it an attractive ligand platform for investigations into catalyst development.…”