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.