Uncommon <i>cis</i> Configuration of a Metal–Metal Bridging Noninnocent Nindigo Ligand

Abstract

In contrast to several reported coordination compounds of <i>trans</i>-Nindigo ligands [Nindigo = indigo-bis­(<i>N-</i>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⁺ = <b>2</b>²⁺ exhibits edge-sharing five- and seven-membered chelate rings in the first documented case of asymmetric bridging by a Nindigo ligand in the <i>cis</i> configuration [L^2– = indigo-bis­(<i>N</i>-phenylimine)­dianion]. The dication in compound [<b>2</b>]­(ClO₄)₂ displays one Ru­(α-diimine)₃ site and one ruthenium center with three negatively charged chelate ligands. Compound [<b>2</b>]­(ClO₄)₂ is obtained from the [Ru­(bpy)₂]²⁺-containing <i>cis</i> precursor [(LH)­Ru­(bpy)₂]­ClO₄ = [<b>1</b>]­ClO₄, which exhibits intramolecular H-bonding in the cation. Four accessible oxidation states each were characterized for the <b>1</b>^<i>n</i> and <b>2</b>^<i>n</i> 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 <b>2</b>²⁺ is reflected by the electronic asymmetry (class I mixed-valence situation), leaving the noninnocent Nindigo bridge as the main redox-active site