Noninnocence of Indigo: Dehydroindigo Anions as Bridging Electron-Donor Ligands in Diruthenium Complexes

Complexes of singly or doubly deprotonated indigo (H2Ind) with one or two [Ru­(pap)2]2+ fragments (pap = 2-phenylazopyridine) have been characterized experimentally [molecular structure, voltammetry, electron paramagnetic resonance (EPR), and UV–vis–near-IR spectroelectrochemistry] and by time-dependent density functional theory calculations. The compound [Ru­(pap)2(HInd–)]­ClO4 ([1]­ClO4) was found to contain an intramolecular NH---O hydrogen bond, whereas [{Ru­(pap)2}2(μ-Ind2–)]­(ClO4)2 ([2]­(ClO4)2), isolated as the meso diastereoisomer with near-IR absorptions at 1162 and 991 nm, contains two metals bridged at 6.354 Å distance by the bischelating indigo dianion. The spectroelectrochemical study of multiple reversible reduction and oxidation processes of 2n (n = 4+, 3+, 2+, 1+, 0, 1–, 2–, 3–, 4−) reveals the stepwise addition of electrons to the terminal π-accepting pap ligands, whereas the oxidations occur predominantly at the anionic indigo ligand, producing an EPR-identified indigo radical intermediate and revealing the suitability of deprotonated indigo as a σ- and π-donating bischelating bridge.