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)(2)Ru(mu,eta(2):eta(2)-L)Ru(bpy)(2)](2+) = 2(2+) 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(alpha-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 1(n) and 2(n) 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 Ru-III/Ru-II system 2(2+) is reflected by the electronic asymmetry (class I mixed-valence situation), leaving the noninnocent Nindigo bridge as the main redox-active site.