Isomeric Diruthenium Complexes Bridged by Deprotonated Indigo in cis and trans Configuration

The doubly deprotonated form L2- of indigo=H2L 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 1(n) and 2(n), 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.