Reaction Intermediates in Artificial Photosynthesis with Molecular Catalysts

In nature, water oxidation is catalyzed by Mn4Ca clusters in the oxygen-evolving complex (OEC) of photosystem II (PSII), in which a manganese(V)-oxo species acts as an active reaction intermediate. Electrons and protons taken from water in PSII are used to reduce plastoquinone to plastoquinol via photoinduced charge separation in the photosynthetic reaction center. In photosystem I (PSI), NADP+ coenzyme is reduced by plastoquinol via photoinduced charge separation in the photosynthetic reaction center to produce NADPH, which is used as a reductant to reduce CO2 to carbohydrates in the Calvin cycle. Extensive efforts have so far been made to mimic functions of PSII and PSI for photocatalytic water oxidation and reduction to produce O2 and H2, respectively. Characterization and reactivity of high-valent metal-oxo, -hydroperoxo, -peroxo, and -superoxo intermediates have been investigated to clarify the mechanisms of water oxidation. Metal hydride complexes have also been studied in relation with the catalytic reactivity for water reduction to produce H2 as well as NAD+ reduction to NADH. This Review is intended to provide an overview on the functional model reactions of PSII and PSI for the photocatalytic water oxidation and reduction, respectively. The roles of high-valent metal-oxo, -hydroperoxo, -peroxo, and -superoxo complexes as the reaction intermediates in photocatalytic water oxidation are focused in relation with the catalytic mechanisms of water oxidation. The roles of metal hydride complexes are also discussed in relation with the catalytic mechanisms of hydrogen evolution and NAD+ reduction to NADH. The combination of functional model reactions of PSII and PSI leads to construct molecular artificial photosynthetic systems in which water is split to H2 and O2 in a 2:1 ratio, providing a way to realize artificial photosynthesis in molecular levels.