Abstract
Development of catalytic transformations of ubiquitous small molecules (e.g., CO2, N2, and H2O) using renewable energy (especially solar energy) is one of the most important research topics for achieving a carbon-neutral society. In this context, we have developed various visible-light-driven catalytic transformations of small molecules using a combination of transition-metal complexes. In this paper, we review our recent achievements, particularly in photocatalytic CO2 reduction and nitrogen fixation. First, we have successfully synthesized multinuclear complexes, which function as effective photocatalysts (or electrocatalysts) for the reduction of low-concentration CO2. The reasons for their high catalytic activity and the reaction mechanism of photocatalytic CO2 reduction were investigated in detail using Ru(II)-Re(I) binuclear complexes, constructed with a Ru(II) photosensitizer unit and a Re(I) catalyst unit, by means of time-resolved spectroscopy. Second, molecular photocathodes, prepared by electrochemical polymerization of metal complexes on semiconductor electrodes, enabled durable photocatalytic CO2 reduction using water as an electron source. Third, we have successfully developed the following photocatalytic nitrogen fixation as the first examples: (1) uphilltype photocatalytic NH3 formation using dihydroacridine as both electron and proton sources, and (2) photocatalytic NH3 formation from N2 and water utilizing photochemical activation of water with a tertiary phosphine. These studies will provide important insights into the design of efficient photocatalytic systems for small-molecule transformations driven by renewable energy.
