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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-Cu4P2). Cu4P2 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 Cu4P2 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 Cu4P2 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 Cu4P2 undergoes slow decomposition under turnover conditions; however, both the starting Cu4P2 as well as its molecular decomposition products are the dominant catalytically active species for H2 evolution not Cu or CuOx 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.

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Photocatalytic reduction of hexavalent chromium [Cr­(VI)] is a promising technology approach to highly efficiently and environmentally tackle the problem of Cr­(VI) pollution, in which the key challenge is in the development of effective photocatalysts. In this work, highly reduced hourglass-type molybdophosphate hybrids with the formulas [Zn­(mbpy)­(H2O)2]2[Zn­(mbpy)­(H2O)]2{Zn­[P4Mo6O31H7]2}·9H2O (1), [Na­(H2O)2]2[Zn­(mbpy)­(H2O)]2[Zn­(mbpy)­(H2O)2]2{Zn­[P4Mo6O31H6]2}·15H2O (2), and (H2mbpy)­{[Zn­(mbpy)­(H2O)]2[Zn­(H2O)]2}­{Zn­[P4Mo6O31H6]2}·10H2O (3) (mbpy = 4,4′-dimethyl-2,2′-bipyridine) have been hydrothermally synthesized and used as photocatalysts for the reduction of Cr­(VI) under mild conditions. Structural analysis showed that the inorganic moieties in crystals 1–3 are composed of a unique 0D single cluster form, a 1D chainlike structure, and a 2D-layered structure, respectively, in which polyanions were constructed by hourglass-type molybdophosphates with one Zn­(II) ion as the central metal. These hybrids displayed good performance for the photocatalytic reduction of Cr­(VI) by virtue of their wide visible-light adsorption, suitable energy band structures, and specific spatial arrangements of polyanionic species. Among them, hybrid 2 exhibits the best photocatalytic performance with a Cr­(VI) reduction conversion rate of almost 94.7% within 180 min of reaction time. The photocatalysis mechanism investigation revealed that highly reduced hourglass-type molybdophosphate clusters can be illuminated by visible light. The photoinduced electrons induced by hourglass-type polyanions can directly reduce Cr­(VI) to Cr­(III), while the photogenerated holes are used to oxidize the sacrificial agent isopropyl alcohol to acetone. This work provides new guidance for the design and preparation of highly efficient photocatalysts for the reduction of Cr­(VI).