Synthesis and characterization of metal-diaminobipyridine complexes as low-cost co-catalysts for photo-sensitized hydrogen evolution

Abstract We synthesized novel 4,4′-diamino-2,2′-bipyridine (dabpy) complexes of Cu(II), Co(III), and Ni(II), and determined their molecular structures by single-crystal XRD analysis. As a result, we found that [Cu(dabpy) 2 ] 2+ had a four-coordinated Cu center and distorted square-planar geometry, while the Co(III) and Ni(II) complexes contained six-coordinated metal centers and exhibited an octahedral geometry. The prepared complexes were investigated as co-catalysts for H 2 evolution from aqueous media in the presence of erythrosin B as a photosensitizer. Notably, the Cu(II) complex demonstrated a much higher co-catalyst activity than the corresponding Co(III) and Ni(II) species. Additionally, the H 2 evolution mechanism and the redox potentials of the metal dabpy complexes were elucidated using electrochemical studies, and it was found that the reduction of protons to H 2 was accompanied by a Cu(II)/Cu(I) redox reaction. In addition, it was suggested that the high co-catalyst activity of the Cu(II) complex was due to the low protonation-induced overpotential compared to those of Co(III) and Ni(II). Furthermore, the photo-sensitized H 2 evolution activity of the metal complex/K 2 PtCl 4 mixed systems were shown to exceed that of the K 2 PtCl 4 -only control system. The Cu(II) complex was found to be low-cost and effective co-catalyst for homogeneous H 2 evolution system.