The semiquinone-ruthenium combination as a remarkably invariant feature in the redox and substitution series [Ru(Q)_n(acac)_3-n]^m, n = 1-3; m = (-2), -1, 0, +1, (+2); Q = 4,6-di-tert-butyl-n-phenyl-o-iminobenzoquinone

Abstract

Three new compounds, [Ru(Q^·-)(acac)₂] = 1, [Ru(Q^·-)₂(acac)] = 2, and [Ru(Q^·-)₃] = 3, were obtained and characterized as RuIII complexes with 4,6-di-tert-butyl-N-phenyl-o-iminobenzosemiquinone (Q^·-) ligands. All three systems show multiple electron transfer behavior, which was analyzed using electron paramagnetic resonance (EPR) and UV-vis-near-infrared (NIR) spectroelectrochemistry. ¹H NMR spectroscopy and a crystal structure analysis suggest antiferromagnetically spin-spin coupled Ru^III and Q^·- in 1, similar to that in the related compound 4 with unsubstituted o-iminobenzosemiquinone. However, in contrast to 4ⁿ (Remenyi, C.; Kaupp, M. J. Am. Chem. Soc. 2005, 127, 11399), the system 1^m exhibits unambiguously metal-centered electron transfer, producing ions [Ru^IV(Q^·-)(acac)₂]⁺ = 1⁺ and [Ru^II(Q^·-)(acac)₂]^- = 1^-, both with EPR-evidenced ligand-based spin, as also supported by DFT calculations. Compared with the related redox system [Ru(Q)(bpy)₂]^k (5^k) (k = 0-3), the spectroelectrochemical similarity suggests corresponding electronic structures except for the 1+/5³⁺ pair (RuIV(Q·-)(acac)2]+ (1+) versus [Ru^III(Q⁰)(bpy)₂]³⁺ (5³⁺)). Compound 2, a three-spin system [Ru^III(Q^·-)₂(acac)] obtained in the all-cis configuration, possesses a complicated magnetic behavior including strong intramolecular antiferromagnetic coupling (JRu-Q, on the order of -10³ cm^-1 and J_Q-Q, -10² cm^-1) and weak intermolecular antiferromagnetic and ferromagnetic interactions. Strong intramolecular coupling leads to one unpaired electron at low temperatures, as also supported by the radical-type EPR signal of the solid and of solutions, which diminishes at higher temperatures. The up-down-up spin arrangement for the ground state of {(Q^·-)-Ru^III-(Q^·-)} (S =½ ) is confirmed by DFT calculations for 2. Oxidation to 2⁺ leaves the UV-vis-NIR spectrum almost unchanged, whereas reduction to 2^- and 2^2- produces low-energy absorptions. The ligand-centered spin for 2^2- = [Ru^II(Q^·-)(Q^2-)(acac)]^2- suggests the [Ru^II(Q^·-)₂(acac)]^- formulation for 2^-. Compound 3, obtained as a structurally characterized mer isomer, has a predominantly ligand-centered highest occupied molecular orbital (HOMO), as evident from the EPR signal of the intermediate 3⁺ and as supported by DFT calculations. In contrast, electron addition proceeds to yield a metal/ligand mixed spin intermediate 3^- according to EPR, in agreement with ca. 25% calculated metal character of the lowest unoccupied molecular orbital (LUMO). The near-infrared absorption of 3 at 1280 nm corresponds to the HOMO-LUMO transition (ligand-to-metal/ligand-to-ligand charge transfer). Oxidation to 3⁺ produces a weak broad band at about 2500 nm, while the reduction to 3^- gives rise to an intense absorption feature at 816 nm. The valence state alternatives are being discussed for all spectroelectrochemically accessible species, and the individual results are compared across this unique substitution and redox series involving a highly noninnocent ligand/metal combination. All established oxidation state formulations involve the iminosemiquinone-ruthenium entity, illustrating the remarkable stability of that arrangement, which corroborates the use of this combination in water oxidation catalysis.