The doubly deprotonated form L2- of indigo=H2 L can bind two [Ru(acac)2 ] complex fragments in the cis (1) and trans configuration (2), as evidenced from crystal structure analysis. While the latter type of N,O; N' ,O' coordination has been observed earlier, for example, with [Ru(bpy)2 ]2+ , leading to two equivalent six-membered ring chelates, the cis arrangement in 1 is observed here for the first time in a dinuclear complex, producing one five-membered ring chelate with N,N' coordination and one seven-membered chelate with O,O' coordination. The different structures of the isomers result in differing electrochemical and spectroelectrochemical (EPR, UV-Vis-NIR) responses for various accessible charge states 1n and 2n , n=-, 0, +, 2+. The associated electronic structures were analyzed by DFT (structures, spin density) and TD-DFT calculations (electronic transitions), revealing mainly metal-based reduction but largely indigo ligand-based oxidation of both neutral precursors.
Keywords: electron paramagnetic resonance spectroscopy; indigo; isomerism; ruthenium; spectroelectrochemistry; structure.
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