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

Area of Science:

• Inorganic Chemistry
• Catalysis
• Photochemistry

Background:

• Ruthenium complexes are widely studied as catalysts for water oxidation.
• Functionalizing ligand scaffolds can tune catalyst properties and introduce new functionalities.
• Developing pH-reporting catalysts is crucial for understanding reaction mechanisms and optimizing performance.

Purpose of the Study:

• To synthesize a novel Ru(II)-based water oxidation catalyst incorporating a pH-sensitive imidazole bridge and a nitrophenyl group.
• To investigate the impact of this modification on the catalyst's photophysical and electrochemical properties.
• To evaluate the catalytic performance and pH-reporting capabilities of the new complex.

Main Methods:

• Synthesis of a new Ru(II) complex with a functionalized dpp ligand (2,9-di-(2'-pyridyl)-1,10-phenanthroline).
• UV-vis spectroscopy to study photophysical properties and pH-dependent spectral changes.
• Electrochemical measurements to assess catalytic center properties.
• Theoretical investigations to understand spectral contributions.

Main Results:

• A new absorption band around 441 nm was observed for the monoprotonated species, linked to the imidazole and nitrophenyl motif.
• The pKa value of the conjugated acid was determined to be 6.8 ± 0.1.
• The nitrophenyl group did not significantly affect the electrochemical properties or catalytic performance compared to the parent complex.
• The Ru(dppip-NO2) complex retained catalytic activity and offered pH-reporting via UV-vis spectroscopy.

Conclusions:

• The functionalized ligand scaffold allows for pH monitoring of the catalytic solution through UV-vis spectroscopy.
• The catalyst retains its water oxidation activity while providing a built-in pH sensor.
• This approach offers opportunities for further ligand modification and mechanistic studies.