Efficient water oxidation by cerium ammonium nitrate with [Ir ^III(Cp*)(4,4′-bishydroxy-2,2′-bipyridine)(H ₂O)] ²⁺as a precatalyst

Water oxidation by cerium(iv) ammonium nitrate, CAN, with [Ir ^III(Cp*)(4,4′-R ₂-2,2′-bipyridine)(H ₂O)] ²⁺ (R = OH, OMe, Me or COOH) to evolve oxygen has been investigated together with the possible oxidation of the ligands by CAN. The apparent catalytic reactivity is highly dependent on the substituent R and the highest catalytic reactivity was obtained when R = OH. The apparent turnover frequency (TOF) of the catalytic water oxidation by CAN with [Ir ^III(Cp*){4,4′-(OH) ₂-2,2′-bipyridine} (H ₂O)] ²⁺, which acts as a precatalyst, gradually increased during the reaction to reach the highest value among the Ir complexes. In the second run, the apparent TOF value was the highest from the beginning of the reaction. ¹H NMR and dynamic light scattering measurements for solutions after the first run indicated formation of insoluble nanoparticles, which exhibited a much higher catalytic reactivity as compared with iridium oxide prepared by a conventional method. The 4,4′-R ₂-2, 2′-bipyridine ligand was also efficiently oxidized by CAN up to CO ₂ only when R = OH. TG/DTA and XPS measurements of nanoparticles produced after the water oxidation suggested that the nanoparticles were composed of iridium hydroxide with a small amount of carbonaceous residue. Thus, iridium hydroxide nanoparticles act as an excellent catalyst for the water oxidation by CAN.