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–Q2– of the
noninnocent ligand, the series 22+–20 and 32+–3– retain RuII. 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
RuII(Q0) or RuIII(Q• –).