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

Abstract

The compounds Ru­(acac)₂(Q) (<b>1</b>), [Ru­(bpy)₂(Q)]­(ClO₄)₂ ([<b>2</b>]­(ClO₄)₂), and [Ru­(pap)₂(Q)]­PF₆ ([<b>3</b>]­PF₆), containing Q = <i>N,N</i>′-diphenyl-<i>o</i>-benzoquinonediimine and donating 2,4-pentanedionate ligands (acac^–), π-accepting 2,2^/-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>⁺–<b>1</b>^– prefers trivalent ruthenium with corresponding oxidation states Q⁰–Q^2– of the noninnocent ligand, the series <b>2</b>²⁺–<b>2</b>⁰ and <b>3</b>²⁺–<b>3</b>^– 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 <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^II(Q⁰) or Ru^III(Q^• –)