Introduction of substituents for tuning the redox properties of benzoate-bridged paddlewheel diruthenium(ii,ii) complexes: what does the OH group bring?

Abstract

Benzoate-bridged paddlewheel diruthenium(II,II) complexes ([Ru^II,II₂(R_nArCO₂)₄(L_ax)₂] (L_ax = axial ligand); [Ru^II,II₂]) exhibit reversible redox activity involving the oxidized species [Ru^II,III₂]⁺. The redox activity can be finely tuned over a broad potential range by altering the substituent R on the benzoate-bridging ligand R_nArCO₂⁻. The electronic contributions of the substituents R depend on their type and position, as was empirically demonstrated by Hammett for substituents at the meta- and para-positions. However, the substituent effect at the ortho-position is not solely determined by the electronic contribution of R but also by steric hindrance between the o-substituents and adjacent carboxylate groups. Nevertheless, an OH group at the o-position did not provide any steric hindrance, leading to a strong electron-withdrawing effect owing to intramolecular hydrogen bonding between the o-OH group and the adjacent carboxylate group, despite the electron-donating ability of the m- and p-OH groups. The OH group at the o-position induced a significant shift in the redox potential and HOMO energy levels of the [Ru^II,II₂] complexes, thereby stabilizing the [Ru^II,II₂] state. The redox potential and HOMO can be adjusted by introducing additional substituents, such as F, Cl, Me, OMe, and CF₃ groups, to cover a wide range, in accordance with an extended Hammett law that considers the contribution of the o-position.