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

Epindolidione (H2L), 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 mu-L2-. The dications in compounds meso-[1](ClO4)(2) and meso-[2](ClO4)(2), [X2Ru(mu-L)RuX2](ClO4)(2), contain five-membered chelate rings N-C-C-O-Ru-II with pi bridged metals at an intramolecular distance of 7.19 angstrom. 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 pi systems for molecular electronics applications, low-energy absorptions, and multiple electron transfers.

Automated summary

The dinuclear complexes of Epindolidione with [RuX2] have similar structures and electrochemical properties, suitable for molecular electronics applications, and capable of multiple electron transfers.