A Diruthenium Complex of a “Nindigo” Ligand

The compound {(μ-Nindigo)­[Ru­(acac)2]2} = 1, H2(Nindigo) =
indigo-N,N′-diphenylimine
and acac– = 2,4-pentanedionate, has been structurally
characterized in the rac form, which exhibits two
edge-sharing six-membered chelate
rings involving ruthenium, and the former β-diketiminato functions
with a twist angle of 33.9° around the central C–C bond.
The metric parameters suggest a neutral π acceptor bridge containing
coupled s-trans configurated α-diimines,
which are coordinated by two ruthenium­(II) centers. DFT calculations
confirm the experimental structure and oxidation state assignment
of the rac form; both diastereoisomers are present
in solution according to 1H NMR spectroscopy. A very intense
long-wavelength MLCT absorption at 630 nm (ε = 66 800
M–1 cm–1) and a weaker near-IR
band at 1120 nm (ε = 3000 M–1 cm–1) are observed for the CH3CN solution. Reversible one-electron
reduction and oxidation steps were studied by cyclic voltammetry,
differential pulse voltammetry, EPR, and UV–vis–NIR
spectroelectrochemistry to exhibit metal-centered oxidation and mixed
metal/ligand-centered reduction. These results are supported by TD-DFT
calculations of the species rac- or meso-1n, n =
3+, 2+, +, 0, −, 2–.