The Indigo Isomer Epindolidione as Redox‐Active Bridging Ligand for Diruthenium Complexes

Epindolidione (H2 L), a heteroatom-modified analogue of tetracene and a structural isomer of indigo, forms dinuclear complexes with [RuX2 ]2+ , X=bpy (2,2'-bipyridine, [1]2+ ) or pap (2-phenylazopyridine, [2]2+ ), in its doubly deprotonated bridging form μ-L2- . The dications in compounds meso-[1](ClO4 )2 and meso-[2](ClO4 )2 , [X2 Ru(μ-L)RuX2 ](ClO4 )2 , contain five-membered chelate rings N-C-C-O-RuII with π bridged metals at an intramolecular distance of 7.19 A. Stepwise reversible oxidation and reduction is mainly ligand centered (oxidation: L2- ; reduction: X), as deduced from EPR of one-electron oxidized and reduced intermediates and from UV/Vis/NIR spectroelectrochemistry, supported by TD-DFT calculation results. The results for [1](ClO4 )2 and [2](ClO4 )2 are qualitatively similar to the ones observed with the deprotonated indigo-bridged isomers with their six-membered chelate ring structures, confirming the suitability of both π systems for molecular electronics applications, low-energy absorptions, and multiple electron transfers.