Efficient water oxidation with organometallic iridium complexes as precatalysts

Catalytic water oxidation has been investigated using five iridium complexes as precatalysts and NaIO₄ as an oxidant at various pH conditions. An increase in the activity of all complexes was observed with increasing pH. A detailed analysis of spectroscopic data together with O ₂-evolution experiments using Cp*Ir(6,6′-dihydroxy-2, 2′-bipyridine)(OH₂)²⁺ as a precatalyst indicate that the high catalytic activity is closely connected with transient species (A) that exhibits an absorption band at λ_max 590 nm. The formation of this active form is strongly dependent on reaction conditions, and the species was distinctly observed using a small excess of periodate. However, another species absorbing at 600 nm (B), which seems to be a less active catalyst, was also observed and was more prominent at high oxidant concentration. Dynamic light scattering analysis and transmission electron microscopy have identified species B as 120 nm nanoparticles. The ultrafiltration method has revealed that species A can be attributed to particles with size in the range of 0.5-2 nm, possibly small IrO_x clusters similar to those described previously by Harriman and co-workers (J. Phys. Chem., 1991, 95, 616-621).