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)<sub>2</sub>(Q) (<b>1</b>), [Ru­(bpy)<sub>2</sub>(Q)]­(ClO<sub>4</sub>)<sub>2</sub> ([<b>2</b>]­(ClO<sub>4</sub>)<sub>2</sub>), and [Ru­(pap)<sub>2</sub>(Q)]­PF<sub>6</sub> ([<b>3</b>]­PF<sub>6</sub>), containing Q = <i>N,N</i>′-diphenyl-<i>o</i>-benzoquinonediimine and donating 2,4-pentanedionate ligands (acac<sup>–</sup>), π-accepting 2,2<sup>/</sup>-bipyridine (bpy), or strongly <i>π-</i>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 <b>1</b><sup>+</sup>–<b>1</b><sup>–</sup> prefers trivalent ruthenium with corresponding oxidation states Q<sup>0</sup>–Q<sup>2–</sup> of the noninnocent ligand, the series <b>2</b><sup>2+</sup>–<b>2</b><sup>0</sup> and <b>3</b><sup>2+</sup>–<b>3</b><sup>–</sup> retain Ru<sup>II</sup>. 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 <i>o</i>-quinonediimine ligands [Kalinina et al., <i>Inorg. Chem</i>. <b>2008</b>, <i>47</i>, 10110] and to the discussion of the most appropriate oxidation state formulation Ru<sup>II</sup>(Q<sup>0</sup>) or Ru<sup>III</sup>(Q<sup>• –</sup>).