a
Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
c
Research Center for Materials Science (RCMS), Nagoya University, Chikusa, Nagoya 464-8602, Japan
› Author AffiliationsThis work was supported by the Asahi Glass Foundation (Step-up-grant to S.S.), the Japan Society for the Promotion of Science (Scientific Research (B) 19H02713 to S.S. and Early-Career Scientists 21K14642 to J. J.), and partially by the Ministry of the Environment of the Government of Japan.
A novel mononuclear ruthenium (Ru) complex bearing a PNNP-type tetradentate ligand is introduced here as a self-photosensitized catalyst for the reduction of carbon dioxide (CO2). When the pre-activation of the Ru complex by reaction with a base was carried out, an induction period of catalyst almost disappeared and the catalyst turnover numbers (TONs) over a reaction time of 144 h reached 307 and 489 for carbon monoxide (CO) and for formic acid (HCO2H), respectively. The complex has a long lifespan as a dual photosensitizer and reduction catalyst, due to the sterically bulky and structurally robust (PNNP)Ru framework. Isotope-labeling experiments under 13CO2 atmosphere indicate that CO and HCO2H were both produced from CO2.
19Experimental Conditions
Photocatalytic reactions were performed at atmospheric pressure in 8 mL test tubes containing 4 mL of a DMA/TEOA (5:1 v/v) solution purged with CO2 for 10 min, a Ru complex (0.1 mM), and BIH (0.10 M). The solutions were irradiated in a turntable irradiation apparatus at room temperature using a Xe lamp (300 W) combined with a UV-cut filter having a cutoff wavelength of below 400 nm and a UV- and IR-cut filter ranging from 385 nm to 740 nm. The gaseous reaction products were analyzed using a micro-GC (Agilent 490) equipped with a thermal conductivity detector (column: MS5A 10-m BF column; isothermal at 80 °C; carrier gas: Ar), and the products in the solution were analyzed using Prominence Organic Acid Analysis System (SCR-102H column; column temp: 40 °C; cell temp: 43 °C). The reaction set-up is shown in Figure S11 (see the Supporting Information).
20 Pre-activation of Ru-1 with a 2–8 equiv of KOt-Bu gave UV/Vis absorption spectra almost identical with one another, indicating that the photoactive complex is stable in the presence of excess KOt-Bu (Figure S3). Although 1H NMR (CD3CN) measurements of Ru-1·2KOt-Bu showed a RuH signal (JP–H = 26.4 Hz, triplet) at –16.3 ppm, it was difficult to determine the exact structure due to the overall intractable/inscrutable spectrum (Figures S19). In Figure S2, the structure of the pre-activated Ru complex was proposed merely on the basis of ESI-(HR)MS measurement.
21
Zhang J,
Leitus G,
Ben-David Y,
Milstein D.
Angew. Chem. Int. Ed. 2006; 45: 1113
23 DFT calculations of the HOMO–LUMO gaps of Ru-1 and the proposed structure of the pre-activated Ru complex which lost Cl– and a bipyridyl CH2P hydrogen also suggest that the complex which has a quinoid structure could absorb longer wavelengths than Ru-1 (Figure S12).
24
Hatchard CG,
Parker CA.
Proc. R. Soc. London, Ser. A 1956; 235: 518
