Cu-based Polyoxometalate Catalyst for Efficient Catalytic Hydrogen Evolution

Copper-based complexes have been largely neglected as potential water reduction catalysts. This article reports the synthesis and characterization of a tetra-copper-containing polyoxotungstate, Na3K7[Cu4(H2O)2­(B-α-PW9O34)2]·30H2O (Na3K7-Cu 4 P 2 ). Cu 4 P 2 is a water-compatible catalyst for efficient visible-light-driven hydrogen evolution when coupled to (4,4′-di-tert-butyl-2,2′-dipyridyl)-bis­(2-phenylpyridine­(1H))-iridium­(III) hexafluorophosphate ([Ir­(ppy)2(dtbbpy)]­[PF6]) as a light absorber and triethanolamine (TEOA) as sacrificial electron donor. Under minimally optimized conditions, a turnover number (TON) of ∼1270 per Cu 4 P 2 catalyst is obtained after 5 h of irradiation (light-emitting diode; λ = 455 nm; 20 mW); a photochemical quantum efficiency of as high as 15.9% is achieved. Both oxidative and reductive quenching pathways are observed by measuring the luminescence intensity of excited state [Ir­(ppy)2(dtbbpy)]+* in the presence of Cu 4 P 2 or TEOA, respectively. Many stability studies (e.g., UV–vis absorption, FT-IR, dynamic light scattering, transmission electron microscopy, and scanning electron microscopy/energy-dispersive X-ray spectroscopy) show that catalyst Cu 4 P 2 undergoes slow decomposition under turnover conditions; however, both the starting Cu 4 P 2 as well as its molecular decomposition products are the dominant catalytically active species for H2 evolution not Cu or CuO x particles. Considering the high abundance and low cost of copper, the present work provides considerations for the design and synthesis of efficient, molecular, water-compatible Cu-based water reduction catalysts.