Water-soluble mononuclear cobalt complexes with organic ligands acting as precatalysts for efficient photocatalytic water oxidation

Abstract

The photocatalytic water oxidation to evolve O₂ was performed by photoirradiation (λ > 420 nm) of an aqueous solution containing [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine), Na₂S₂O₈ 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 O₂ evolution with [Co^II(Me₆tren)(OH₂)]²⁺ (1) and [Co^III(Cp^*)(bpy)(OH₂)]²⁺ (2) [Me₆tren = tris(N,N′-dimethylaminoethyl)amine, Cp^* = η⁵-pentamethylcyclopentadienyl] at pH 9.0 reached 420 and 320, respectively. The evolved O₂ yield increased in proportion to concentrations of precatalysts 1 and 2 up to 0.10 mM. However, the O₂ yield dramatically decreased when the concentration of precatalysts 1 and 2 exceeded 0.10 mM. When the concentration of Na₂S₂O₈ was increased from 10 mM to 50 mM, CO₂ 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 CO₂ during the photocatalytic reaction. The degradation of complex 2 under photocatalytic conditions and the oxidation of Me₆tren ligand of 1 by [Ru(bpy)₃]³⁺ were confirmed by ¹H 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.