Ancillary Ligand Control of Electronic Structure in o-Benzoquinonediimine-Ruthenium Complex Redox Series: Structures, Electron Paramagnetic Resonance (EPR), and Ultraviolet−Visible−Near-Infrared (UV-vis-NIR) Spectroelectrochemistry

The compounds Ru­(acac)₂(Q) (1), [Ru­(bpy)₂(Q)]­(ClO₄)₂ ([2]­(ClO₄)₂), and [Ru­(pap)₂(Q)]­PF₆ ([3]­PF₆), containing Q = N,N′-diphenyl-o-benzoquinonediimine and donating 2,4-pentanedionate ligands (acac^–), π-accepting 2,2^/-bipyridine (bpy), or strongly π-accepting 2-phenylazopyridine (pap) were prepared and structurally identified. The electronic structures of the complexes and several accessible oxidized and reduced forms were studied experimentally (electrochemistry, magnetic resonance, ultraviolet-visible-near-infrared (UV-vis-NIR) spectroelectrochemistry) and computationally (DFT/TD-DFT) to reveal significantly variable electron transfer behavior and charge distribution. While the redox system 1⁺–1^– prefers trivalent ruthenium with corresponding oxidation states Q⁰–Q^2– of the noninnocent ligand, the series 2²⁺–2⁰ and 3²⁺–3^– retain Ru^II. The bpy and pap co-ligands are not only spectators but can also be reduced prior to a second reduction of Q. The present study with new experimental and computational evidence on the influence of co-ligands on the metal is complementary to a report on the substituent effects in o-quinonediimine ligands [Kalinina et al., Inorg. Chem. 2008, 47, 10110] and to the discussion of the most appropriate oxidation state formulation Ru^II(Q⁰) or Ru^III(Q^• –).