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

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