Role of intramolecular hydrogen bonding in the redox chemistry of hydroxybenzoate-bridged paddlewheel diruthenium(ii,ii) complexes

Abstract

The synthesis of a series of trans-heteroleptic paddlewheel diruthenium(II,II) complexes with various hydroxy-substituted benzoate ligands, [Ru₂((OH)_xPhCO₂)₂(2,6-(CF₃)₂PhCO₂)₂(THF)₂] ([Ru^II,II₂]) as tetrahydrofuran (THF) adducts is reported, where (OH)_xPhCO₂⁻ stands for o-hydroxybenzoate (o-OH), m-hydroxybenzoate (m-OH), p-hydroxybenzoate (p-OH), 2,3-dihydroxybenzoate (2,3-(OH)₂), 2,4-dihydroxybenzoate (2,4-(OH)₂), 2,5-dihydroxybenzoate (2,5-(OH)₂), 2,6-dihydroxybenzoate (2,6-(OH)₂), or 3,4-dihydroxybenzoate (3,4-(OH)₂), and 2,6-(CF₃)₂PhCO₂⁻ represents 2,6-bis(trifluoromethyl)benzoate. In this heteroleptic series, the redox potential (E_1/2) of the [Ru^II,II₂]/[Ru^II,III₂]⁺ couple in THF varies over a wide range, from −18 mV (vs. Ag/Ag⁺) for p-OH to 432 mV for 2,6-(OH)₂. The redox properties are linearly dependent on the acidity (pK_a) of the OH-substituted benzoic acids, but do not depend on the number of ortho-substituted hydroxy (o-OH) groups. This indicates that the steric effect of o-substituents is irrelevant in the case of hydroxyl groups, owing to the formation of intramolecular hydrogen bonds between the o-OH group and carboxylate oxygens. The value of the Hammett constant σ_o for the o-OH substituent was determined to be 0.667, indicating a strongly electron-withdrawing character, contrary to the expectation of electron-donating character for an OH group. The redox properties of the compounds were well explained in a framework of Hammett analyses and were also consistent with their HOMO energy levels evaluated by DFT calculations based on the atomic coordinates. The unexpected electron-withdrawing character of the o-OH groups could be attributed to the direct effect of intramolecular hydrogen bonding on the charge density on the carboxylate oxygen.