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A pentanuclear iron catalyst designed for water oxidation

Nature volume 530, pages 465–468 (2016)Cite this article

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Abstract

Although the oxidation of water is efficiently catalysed by the oxygen-evolving complex in photosystem II (refs 1 and 2), it remains one of the main bottlenecks when aiming for synthetic chemical fuel production powered by sunlight or electricity. Consequently, the development of active and stable water oxidation catalysts is crucial, with heterogeneous systems3,4 considered more suitable for practical use and their homogeneous counterparts more suitable for targeted, molecular-level design guided by mechanistic understanding5,6,7,8,9,10,11,12,13,14,15,16,17,18,19. Research into the mechanism of water oxidation has resulted in a range of synthetic molecular catalysts, yet there remains much interest in systems that use abundant, inexpensive and environmentally benign metals such as iron (the most abundant transition metal in the Earth’s crust and found in natural20,21 and synthetic22 oxidation catalysts). Water oxidation catalysts based on mononuclear iron complexes have been explored9,12,16,18, but they often deactivate rapidly and exhibit relatively low activities. Here we report a pentanuclear iron complex that efficiently and robustly catalyses water oxidation with a turnover frequency of 1,900 per second, which is about three orders of magnitude larger than that of other iron-based catalysts. Electrochemical analysis confirms the redox flexibility of the system, characterized by six different oxidation states between FeII5 and FeIII5; the FeIII5 state is active for oxidizing water. Quantum chemistry calculations indicate that the presence of adjacent active sites facilitates O–O bond formation with a reaction barrier of less than ten kilocalories per mole. Although the need for a high overpotential and the inability to operate in water-rich solutions limit the practicality of the present system, our findings clearly indicate that efficient water oxidation catalysts based on iron complexes can be created by ensuring that the system has redox flexibility and contains adjacent water-activation sites.

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Figure 1: Design of molecular catalysts for water oxidation.
Figure 2: Evaluation of electrochemical properties and catalytic activity of 1.
Figure 3: Mechanism of O2 evolution from water catalysed by 1.
Figure 4: Determination of the TOF value for electrocatalytic O2 evolution.

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Acknowledgements

This study was supported by MEXT/JSPS Grants-in-Aid as follows: for Young Scientists A (no. 25708011 (S.M.) and no. 15H05480 (M.K.)), for Challenging Exploratory Research (no. 26620160 (S.M.)), for Scientific Research on Innovative Areas (''AnApple'' (no. 15H00889 (S.M.) and no. 25107526 (S.M.)), for Photosynergetics (no. 15H01097 (T.Y.)), for Soft Molecular Systems (no. 26104538 (Y.K.)), for a JSPS Fellowship (no. 254037 (M.O.)), for Scientific Research B (no. 25288013 (T.Y.)), and for Scientific Research C (no. 25410030 (Y.K.)). This work was also supported by JST ACT-C (M.K.), JST PRESTO (Y.K. and S.M.), the NINS Program for Cross-Disciplinary Study (S.M.) and the Morino Foundation for Molecular Science (Y.K.). The computations were performed using the Research Center for Computational Science, Okazaki, Japan. We also thank S. Furukawa for discussions.

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Authors and Affiliations

  1. Department of Life and Coordination-Complex Molecular Science, Institute for Molecular Science, Higashiyama 5-1, Myodaiji, 444-8787, Okazaki, Aichi, Japan

    Masaya Okamura, Mio Kondo, Reiko Kuga, Vijayendran K. K. Praneeth, Masaki Yoshida & Shigeyuki Masaoka

  2. SOKENDAI (The Graduate University for Advanced Studies), Shonan Village, Hayama, 240-0193, Kanagawa, Japan

    Masaya Okamura, Mio Kondo, Yuki Kurashige, Takeshi Yanai & Shigeyuki Masaoka

  3. Research Center of Integrative Molecular Systems, Institute for Molecular Science, Nishigo-naka 38, Myodaiji, 444-8585, Okazaki, Aichi, Japan

    Mio Kondo & Shigeyuki Masaoka

  4. Advanced Catalytic Transformation Program for Carbon Utilization (ACT-C), Japan Science and Technology Agency, Honcho 4-1-8, Kawaguchi, 332-0012, Saitama, Japan

    Mio Kondo

  5. Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Nishigo-naka 38, Myodaiji, 444-8585, Okazaki, Aichi, Japan

    Yuki Kurashige & Takeshi Yanai

  6. Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, Honcho 4-1-8, Kawaguchi, 332-0012, Saitama, Japan

    Yuki Kurashige

  7. Department of Chemistry, Graduate School of Science and Technology, Kumamoto University, Kurokami 2-39-1, Kumamoto, 860-8555, Japan

    Shinya Hayami

  8. Department of Chemistry and Applied Chemistry, Graduate School of Science and Engineering, Saga University, Honjo-machi 1, Saga, 840-8502, Japan

    Ko Yoneda

  9. Department of Chemistry, Faculty of Science, Fukuoka University, Jonan-ku 8-19-1, Nanakuma, 814-0180, Fukuoka, Japan

    Satoshi Kawata

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  1. Masaya Okamura
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Contributions

M.O., M.K. and S.M. conceived the project. M.O., M.K., M.Y., K.Y., S.K. and S.M. designed the experiments. M.O. and R.K. performed all synthesis and characterisation experiments. Y.K. and T.Y. performed the DFT calculations. S.H. measured the Mössbauer spectra. V.K.K.P. performed the isolation of reaction intermediates. M.O., M.K. and S.M. analysed the data and co-wrote the manuscript. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Shigeyuki Masaoka.

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The authors declare no competing financial interests.

Additional information

The X-ray crystal structure of 1(BF4)3 is deposited in the Cambridge Crystallographic Data Centre (CCDC 996195).

Supplementary information

Supplementary Information

This file contains Supplementary Text, Supplementary Figures 1-30, Supplementary Tables 1-13 and Supplementary References– see contents for details. (PDF 4699 kb)

Supplementary Data

This file contains crystallographic information files (ZIP 149 kb)

Video 1: Water attack reaction to the S4 state (LS state, side view)

This video shows the calculated water insertion reaction pathway to the four-electron oxidised (S4) state of the pentanuclear iron complex (S4 + H2O →A). The MS parameter (total spin quantum number) of 18 was used in this calculation. For the details of the calculation, see also Supplementary Information (P.S28-33) (MP4 2085 kb)

Video 2: Water attack reaction to the S4 state (LS state, top view)

This video shows the calculated water insertion reaction pathway to the four-electron oxidised (S4) state of the pentanuclear iron complex (S4 + H2O →A) along with the profile of relative energies. The MS parameter (total spin quantum number) of 18 was used in this calculation. For the details of the calculation, see also Supplementary Information (P.S28-33) (MP4 1593 kb)

Video 3: Water attack reaction to the S4 state (HS state, side view)

This video shows the calculated water insertion reaction pathway to the four-electron oxidised (S4) state of the pentanuclear iron complex (S4 + H2O →A). The MS parameter (total spin quantum number) of 26 was used in this calculation. For the details of the calculation, see also Supplementary Information (P.S28-33) (MP4 2060 kb)

Video 4: Water attack reaction to the S4 state (HS state, top view)

This video shows the calculated water insertion reaction pathway to the four-electron oxidised (S4) state of the pentanuclear iron complex (S4 + H2O →A) along with the profile of relative energies. The MS parameter (total spin quantum number) of 26 was used in this calculation. For the details of the calculation, see also Supplementary Information (P.S28-33) (MP4 1479 kb)

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Okamura, M., Kondo, M., Kuga, R. et al. A pentanuclear iron catalyst designed for water oxidation. Nature 530, 465–468 (2016). https://doi.org/10.1038/nature16529

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