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

The synthesis of a series of -heteroleptic paddlewheel diruthenium(II,II) complexes with various hydroxy-substituted benzoate ligands, [Ru((OH)PhCO)(2,6-(CF)PhCO)(THF)] ([RuII,II2]) as tetrahydrofuran (THF) adducts is reported, where (OH)PhCO stands for -hydroxybenzoate (o-OH), -hydroxybenzoate (m-OH), -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-(CF)PhCO represents 2,6-bis(trifluoromethyl)benzoate. In this heteroleptic series, the redox potential () of the [RuII,II2]/[RuII,III2] couple in THF varies over a wide range, from -18 mV ( Ag/Ag) for p-OH to 432 mV for 2,6-(OH)2. The redox properties are linearly dependent on the acidity (p) of the OH-substituted benzoic acids, but do not depend on the number of -substituted hydroxy (-OH) groups. This indicates that the steric effect of -substituents is irrelevant in the case of hydroxyl groups, owing to the formation of intramolecular hydrogen bonds between the -OH group and carboxylate oxygens. The value of the Hammett constant for the -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 -OH groups could be attributed to the direct effect of intramolecular hydrogen bonding on the charge density on the carboxylate oxygen.