Extremely Active, Tunable, and pH-Responsive Iridium Water Oxidation Catalysts

The development of an efficient water oxidation catalyst is crucial in the framework of constructing an artificial photo­(electro)­synthetic apparatus for the production of solar fuels. Herein, new hydroxy–pyridine–carboxylate iridium complexes are reported exhibiting high activity in water oxidation with both cerium ammonium nitrate and NaIO4 as sacrificial oxidants. With the latter, the catalytic activity strongly depends on the pH and position of the OH-substituent in the pyridine ring, reaching a record turnover frequency of 458 min–1 and turnover number (>14 500) limited only by the amount of NaIO4. Kinetic experiments measuring O2 evolution paralleled by NMR studies on oxidative transformation with NaIO4 suggest that Cp* of the catalyst is readily degraded, whereas the hydroxy–pyridine–carboxylate ligands remain coordinated at iridium, tuning its activity.