Fate of the Molecular Ru–Phosphonate Water Oxidation Catalyst under Turnover Conditions

The present work uncovers the oxidative transformations of a recently reported polypyridyl phosphonate–phenoxo Ru-based water oxidation catalyst [Ru^III(tPaO-κ-N²O_PO_C)(py)₂]^2–, 2^2– {tPaO^5– is the 3-(hydroxo-[2,2′:6′,2″-terpyridine]-6,6″-diyl)bis(phosphonate)}, under turnover conditions. We show how the catalyst 2^2– suffers from oxidative degradation during water oxidation catalysis and generates the phosphonate–carboxylate Ru complex [Ru^II(Hbpc)(py)₂], 3H, where bpc^3– is 6′-phosphonato-[2,2′-bipyridine]-6-carboxylate. Complex 3H has been prepared by three different methods, and its oxidative transformations were also studied in detail. Under turnover conditions, complex 3H undergoes a series of transformations that can be monitored by electrochemical techniques including the generation of catalytically active molecular water oxidation catalyst intermediates and RuO₂. In addition, catalytically inactive species such as [Ru^II(bpc-κ-N²O_P)₂] have also been detected.