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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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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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.
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The X-ray crystal structure of 1(BF4)3 is deposited in the Cambridge Crystallographic Data Centre (CCDC 996195).
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This file contains Supplementary Text, Supplementary Figures 1-30, Supplementary Tables 1-13 and Supplementary References– see contents for details. (PDF 4699 kb)
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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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DOI: https://doi.org/10.1038/nature16529
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