Water-soluble mononuclear cobalt complexes with organic ligands acting as precatalysts for efficient photocatalytic water oxidationElectronic supplementary information (ESI) available: Synthesis of cobalt complexes (Page S2), coexisting effect of Cl− ion on O2 evolution (Fig. S1), generation of [Ru(bpy)3]3+ (Fig. S2), H218O labeling experiment (Fig. S3), repeated O2 evolution at pH 8.0 (Fig. S4), 1H NMR spectra of 2 (Fig. S5), oxidation of organic ligands with [RuIII(bpy)3]3+ detected by UV-vis

The photocatalytic water oxidation to evolve O 2 was performed by photoirradiation ( λ > 420 nm) of an aqueous solution containing [Ru(bpy) 3 ] 2+ (bpy = 2,2′-bipyridine), Na 2 S 2 O 8 and water-soluble cobalt complexes with various organic ligands as precatalysts in the pH range of 6.0-10. The turnover numbers (TONs) based on the amount of Co for the photocatalytic O 2 evolution with [Co II (Me 6 tren)(OH 2 )] 2+ ( 1 ) and [Co III (Cp * )(bpy)(OH 2 )] 2+ ( 2 ) [Me 6 tren = tris( N , N ′-dimethylaminoethyl)amine, Cp * = η 5 -pentamethylcyclopentadienyl] at pH 9.0 reached 420 and 320, respectively. The evolved O 2 yield increased in proportion to concentrations of precatalysts 1 and 2 up to 0.10 mM. However, the O 2 yield dramatically decreased when the concentration of precatalysts 1 and 2 exceeded 0.10 mM. When the concentration of Na 2 S 2 O 8 was increased from 10 mM to 50 mM, CO 2 evolution was observed during the photocatalytic water oxidation. These results indicate that a part of the organic ligands of 1 and 2 were oxidized to evolve CO 2 during the photocatalytic reaction. The degradation of complex 2 under photocatalytic conditions and the oxidation of Me 6 tren ligand of 1 by [Ru(bpy) 3 ] 3+ were confirmed by 1 H NMR measurements. Dynamic light scattering (DLS) experiments indicate the formation of particles with diameters of around 20 ± 10 nm and 200 ± 100 nm during the photocatalytic water oxidation with 1 and 2 , respectively. The particle sizes determined by DLS agreed with those of the secondary particles observed by TEM. The XPS measurements of the formed particles suggest that the surface of the particles is covered with cobalt hydroxides, which could be converted to active species containing high-valent cobalt ions during the photocatalytic water oxidation. The recovered nanoparticles produced from 1 act as a robust catalyst for the photocatalytic water oxidation. Photocatalytic water oxidation was efficiently catalysed by Co(OH) x nanoparticles derived from mononuclear cobalt complexes with organic ligands during the reaction.