Molecular\nCopper–Anthraquinone Photocatalysts\nfor Robust Hydrogen Production

The\ndevelopment of robust and inexpensive photocatalysts for H<sub>2</sub> production under visible light irradiation remains a significant\nchallenge. This study presents a series of square planar copper anthraquinone\ncomplexes (R<sub>4</sub>N)CuL<sub>2</sub> (R = ethyl, L = alizarin\ndianion (CuAA); R = <i>n</i>-butyl, L = purpurin dianion\n(CuPP), (2-hydroxyanthraquinone)formamide dianion (CuAHA)) as molecular\nphotocatalysts to achieve high long-term stability in visible-light-driven\nH<sub>2</sub> production. These complexes are self-sensitized by the\nanthraquinone ligands and serve as proton reduction photocatalysts\nwithout additional photosensitizers or catalysts. Under irradiation\nof blue light, complex CuAA produces H<sub>2</sub> in a mixture of\nH<sub>2</sub>O/DMF with undiminished activity over 42 days, giving\na turnover number exceeding 6800. Electrochemical and UV–vis\nstudies are consistent with an EECC mechanism (E: electron transfer\nand C: protonation) in the catalytic cycle. The initial photochemical\nsteps involve conversion of both anthraquinone ligands to hydroquinones.\nFurther light-driven reductions of the hydroquinones followed by two\nprotonation steps results in formation of H<sub>2</sub>. Dependence\nof the catalytic rate on the concentration of H<sub>2</sub>O suggests\nthat either the generation of a Cu<sup>II</sup>–H intermediate\nby protonation or heterocoupling between Cu<sup>II</sup>–H\nand H<sup>+</sup> to produce H<sub>2</sub> is the turnover-limiting\nstep in catalysis.