Uncommon cis Configuration of a Metal–Metal Bridging Noninnocent Nindigo Ligand

In contrast to several reported coordination compounds of trans-Nindigo ligands [Nindigo = indigo-bis­(N-arylimine) = LH2] with one or two six-membered chelate rings involving one indole N and one extracyclic N for metal binding, the new diruthenium complex ion [(acac)2Ru­(μ,η2:η2-L)­Ru­(bpy)2]2+ = 22+ exhibits edge-sharing five- and seven-membered chelate rings in the first documented case of asymmetric bridging by a Nindigo ligand in the cis configuration [L2– = indigo-bis­(N-phenylimine)­dianion]. The dication in compound [2]­(ClO4)2 displays one Ru­(α-diimine)3 site and one ruthenium center with three negatively charged chelate ligands. Compound [2]­(ClO4)2 is obtained from the [Ru­(bpy)2]2+-containing cis precursor [(LH)­Ru­(bpy)2]­ClO4 = [1]­ClO4, which exhibits intramolecular H-bonding in the cation. Four accessible oxidation states each were characterized for the 1n and 2n redox series with respect to metal- or ligand-centered electron transfer, based on X-ray structures, electron paramagnetic resonance, and ultraviolet–visible–near-infrared spectroelectrochemistry in conjunction with density functional theory calculation results. The structural asymmetry in the RuIII/RuII system 22+ is reflected by the electronic asymmetry (class I mixed-valence situation), leaving the noninnocent Nindigo bridge as the main redox-active site.