Toward Controlling Water Oxidation Catalysis: Tunable Activity of Ruthenium Complexes with Axial Imidazole/DMSO Ligands

Using the combinations of imidazole and dimethyl sulfoxide (DMSO) as axial ligands and 2,2′-bipyridine-6,6′-dicarboxylate (bda) as the equatorial ligand, we have synthesized six novel ruthenium complexes with noticeably different activity as water oxidation catalysts (WOCs). In four <i>C</i>_<i>s</i> symmetric Ru^II(κ³-bda)­(DMSO)­L₂ complexes L = imidazole (<b>1</b>), <i>N</i>-methylimidazole (<b>2</b>), 5-methylimidazole (<b>3</b>), and 5-bromo-<i>N</i>-methylimidazole (<b>4</b>). Additionally, in two <i>C</i>_2<i>v</i> symmetric Ru^II(κ⁴-bda)­L₂ complexes L = 5-nitroimidazole (<b>5</b>) and 5-bromo-<i>N</i>-methylimidazole (<b>6</b>), that is, fully equivalent axial imidazoles. A detailed characterization of all complexes and the mechanistic investigation of the catalytic water oxidation have been carried out with a number of experimental techniques, that is, kinetics, electrochemistry and high resolution mass spectrometry (HR-MS), and density functional theory (DFT) calculations. We have observed the in situ formation of a Ru^II-complex with the accessible seventh coordination position. The measured catalytic activities and kinetics of complex <b>1</b>–<b>6</b> revealed details about an important structure–activity relation: the connection between the nature of axial ligands in the combination and either the increase or decrease of the catalytic activity. In particular, an axial DMSO group substantially increases the turnover frequency of WOCs reported in the article, with the ruthenium-complex having one axial 5-bromo-N-methyl-imidazole and one axial DMSO (<b>4</b>), we have obtained a high initial turnover frequency of ∼180 s^–1. DFT modeling of the binuclear reaction pathway of the O–O bond formation in catalytic water oxidation further corroborated the concept of the mechanistic significance of the axial ligands and rationalized the experimentally observed difference in the activity of complexes with imidazole/DMSO and imidazole/imidazole combinations of axial ligands