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) (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•-). © 2015 American Chemical Society.