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

Epindolidione (H 2 L), a heteroatom‐modified analogue of tetracene and a structural isomer of indigo, forms dinuclear complexes with [RuX 2 ] 2+ , X=bpy (2,2′‐bipyridine, [ 1 ] 2+ ) or pap (2‐phenylazopyridine, [ 2 ] 2+ ), in its doubly deprotonated bridging form μ‐L 2− . The dications in compounds meso ‐[ 1 ](ClO 4 ) 2 and meso ‐[ 2 ](ClO 4 ) 2 , [X 2 Ru(μ‐L)RuX 2 ](ClO 4 ) 2 , contain five‐membered chelate rings N‐C−C‐O‐Ru II with π bridged metals at an intramolecular distance of 7.19 Å. Stepwise reversible oxidation and reduction is mainly ligand centered (oxidation: L 2− ; 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 ](ClO 4 ) 2 and [ 2 ](ClO 4 ) 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.