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.