A Ruthenium(II) Water Oxidation Catalyst Containing a pH-Responsive Ligand Framework

The synthesis of a new Ru^II-based water oxidation catalyst is presented, in which a nitrophenyl group is introduced into the backbone of dpp via a pH-sensitive imidazole bridge (dpp = 2,9-di-(2'-pyridyl)-1,10-phenanthroline). This modification had a pronounced effect on the photophysical properties and led to the appearance of a significant absorption band around 441 nm in the UV-vis spectrum upon formation of the monoprotonated species under neutral conditions. Theoretical investigations could show that the main contributions to this band arise from transitions involving the imidazole and nitrophenyl motif, allowing us to determine the pK_a value (6.8 ± 0.1) of the corresponding, twofold protonated conjugated acid. In contrast, the influence of the nitrophenyl group on the electrochemical properties of the catalytic center was negligible. Likewise, the catalytic performance of Ru(dppip-NO₂) and its parent complex Ru(dpp) was comparable over the entire investigated pH range (dppip-NO₂ = 2-(4-nitrophenyl)-6,9-di(pyridin-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline). This allowed the original catalytic properties to be retained while additionally featuring a functionalized ligand scaffold, which provides further modification opportunities as well as the ability to report the pH of the catalytic solution via UV-vis spectroscopy.