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

Abstract

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 p K 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-NO 2 ) and its parent complex Ru(dpp) was comparable over the entire investigated pH range (dppip-NO 2 = 2-(4-nitrophenyl)-6,9-di(pyridin-2-yl)-1 H -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.

A Phosphonate Substituent in a 1,10‐Phenanthroline Ligand Boosts Light‐Driven Catalytic Water Oxidation Performance Sensitized by Ruthenium Chromophores

“…Additionally, the phosphonic acid moieties may act as proton acceptor acting as internal base during the catalytic cycle accelerating deprotonation of the aquo ligand during catalysis. [29][30][31] These findings highlight the importance of the photosensitizer in photocatalytic processes. Based on the presented results a check list for the utilization of photosensitizers in WOC reactions could be implemented.…”

“…The conditions were used based on a screening published earlier by our group. [30] The respective catalytic turnover numbers (TON) are given in Table 4. For compound 1 and [Ru(bpy) 3 ](PF 6 ) 2 (see Figure S9, SI) no catalytic oxygen evolution was observed under the given conditions.…”

General Kinetic Model for pH Dependence of Proton-Coupled Electron Transfer: Application to an Electrochemical Water Oxidation System

“…Thus, the potential dependence of p K a,Ox was modeled as normalp K normala , Ox ( E ) = prefix− 0.12 × E − 1.1 0.25em normalV 0.059 0.25em normalV + 1.2 Note that this potential dependence on ITO has not been experimentally verified. Anchoring a Ru II water oxidation catalyst with a pH-sensitive visible absorption spectrum to the surface and monitoring its spectral behavior with an applied potential could provide useful insights. We also investigated models where p K a,Ox is independent of the potential E and found that such models could only reproduce the experimental data at higher pH, such as pH = 5, but not for the entire pH range (Figure S5).…”

Ruthenium containing molecular electrocatalyst on glassy carbon for electrochemical water splitting

“…6 However, in practice, higher stability and catalytic efficiency have been observed for ruthenium (Ru) and iridium (Ir)-based catalysts, which have made them the most widely studied. 7 Earlier, we reported a highly active Ru-based water oxidation catalyst, [Ru(mcbp)(py) 2 ] (mcbp 2− = 2,6-bis(1-methyl-4-(carboxylate)-benzimidazol-2-yl)pyridine, py = pyridine, 1 in Fig. 1).…”