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

Benzoate-bridged paddlewheel diruthenium(II,II) complexes ([RuII,II2(R_<i>n</i>ArCO₂)₄(L_ax)₂] (L_ax = axial ligand); [RuII,II2]) exhibit reversible redox activity involving the oxidized species [RuII,III2]⁺. The redox activity can be finely tuned over a broad potential range by altering the substituent R on the benzoate-bridging ligand R_<i>n</i>ArCO₂^-. The electronic contributions of the substituents R depend on their type and position, as was empirically demonstrated by Hammett for substituents at the <i>meta</i>- and <i>para</i>-positions. However, the substituent effect at the <i>ortho</i>-position is not solely determined by the electronic contribution of R but also by steric hindrance between the <i>o</i>-substituents and adjacent carboxylate groups. Nevertheless, an OH group at the <i>o</i>-position did not provide any steric hindrance, leading to a strong electron-withdrawing effect owing to intramolecular hydrogen bonding between the <i>o</i>-OH group and the adjacent carboxylate group, despite the electron-donating ability of the <i>m</i>- and <i>p</i>-OH groups. The OH group at the <i>o</i>-position induced a significant shift in the redox potential and HOMO energy levels of the [RuII,II2] complexes, thereby stabilizing the [RuII,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 <i>o</i>-position.