A molecular ruthenium catalyst with water-oxidation activity comparable to that of photosystem II

Journal name:
Nature Chemistry
Volume:
4,
Pages:
418–423
Year published:
DOI:
doi:10.1038/nchem.1301
Received
Accepted
Published online

Abstract

Across chemical disciplines, an interest in developing artificial water splitting to O2 and H2, driven by sunlight, has been motivated by the need for practical and environmentally friendly power generation without the consumption of fossil fuels. The central issue in light-driven water splitting is the efficiency of the water oxidation, which in the best-known catalysts falls short of the desired level by approximately two orders of magnitude. Here, we show that it is possible to close that ‘two orders of magnitude’ gap with a rationally designed molecular catalyst [Ru(bda)(isoq)2] (H2bda = 2,2′-bipyridine-6,6′-dicarboxylic acid; isoq = isoquinoline). This speeds up the water oxidation to an unprecedentedly high reaction rate with a turnover frequency of >300 s−1. This value is, for the first time, moderately comparable with the reaction rate of 100–400 s−1 of the oxygen-evolving complex of photosystem II in vivo.

At a glance

Figures

  1. Catalytic performances of complexes 1 and 2.
    Figure 1: Catalytic performances of complexes 1 and 2.

    a, Structures of complexes 1 and 2. b, Kinetic plots of oxygen formation by 1 and 2 versus time (conditions: CF3SO3H aqueous solutions (3.7 ml) containing CeIV (0.48 M, 1.79 × 10−3 mol) and catalyst (2.16 × 10−4 M, 8 × 10−7 mol)). c, Kinetic plots of oxygen formation by 1 and 2 versus time (conditions: CF3SO3H aqueous solutions (3.5 ml) containing CeIV (0.51 M, 1.79 × 10−3 mol) and catalyst (1.14 × 10−4 M, 4 × 10−7 mol)). d, CVs of 1 and 2 (conditions: [catalyst] = 1 mM, solvent = mixed CF3CH2OH/pH 1.0 (v:v = 1:2), scanning rate = 100 mV s−1, working electrode = pyrolytic graphite electrode (basal plane)).

  2. Kinetic and spectral data for complex 2.
    Figure 2: Kinetic and spectral data for complex 2.

    a, Spectral changes versus time at 5.0 °C after addition of 2 equiv. of CeIV to 2 in 0.1 M CF3SO3H. Each graph represents the spectra at a different time. b, Kinetic trace and fitting at λ = 290 nm. c, Calculated spectra for the ruthenium-containing species based on SVD analysis performed by Specfit. d, Species distribution diagram.

  3. O2 generation pathways of complex 2 with stoichiometric and excess amounts of CeIV at pH 1 (the circular pathway in the middle and further to the right, respectively).
    Figure 3: O2 generation pathways of complex 2 with stoichiometric and excess amounts of CeIV at pH 1 (the circular pathway in the middle and further to the right, respectively).
  4. Calculated encounter complex EC(isoq) ([O1 · · · O2] = 3.22 Å).
    Figure 4: Calculated encounter complex EC(isoq) ([O1 · · · O2] = 3.22 Å).

    Matching isoquinolines are nearly parallel with respect to one another and are at a relative distance and geometrical arrangement that are indicative of a stabilizing stacking interaction. All distances are in ångstroms.

  5. Calculated potential energy profile of O–O bond formation combined with the reactions steps of the liberation of O2 from the RuIV-peroxo dimer.
    Figure 5: Calculated potential energy profile of O–O bond formation combined with the reactions steps of the liberation of O2 from the RuIV-peroxo dimer.

    The profile portrays the calculated energy of the two interacting mono-radical ruthenium complexes, formally Ruv=O, as a function of the decreasing distance between the terminal oxygen, allowing location of the transition state (TSoo(isoq)) of such a complex process. All energies are in kcal mol−1 calculated in water; see calculated peroxo- and superoxo-dimers in Supplementary Figs S12 and S13.

  6. Calculated transition state (TSoo(isoq); [O1−O2] = 2.038 Å) according to the potential energy scan from Fig. 5.
    Figure 6: Calculated transition state (TSoo(isoq); [O1−O2] = 2.038 Å) according to the potential energy scan from Fig. 5.

    In the transition state, O–O atoms are much close to one another than in EC(isoq); however, face-to-face alignment of the matching axial isoquinolines persists. All distances are in ångstroms (see Supplementary Fig. S10 for details).

Compounds

2 compounds View all compounds
  1. trans-[(Isoquinoline)2(bda)ruthenium(II)]
    Compound 1 trans-[(Isoquinoline)2(bda)ruthenium(II)]
  2. trans-[(4-Methylpyridine)2(bda)ruthenium(II)]
    Compound 2 trans-[(4-Methylpyridine)2(bda)ruthenium(II)]

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Author information

Affiliations

  1. Department of Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, 100 44 Stockholm, Sweden

    • Lele Duan &
    • Licheng Sun
  2. Institute of Chemical Research of Catalonia (ICIQ), Avinguda Països Catalans 16, E-43007 Tarragona, Spain

    • Fernando Bozoglian,
    • Sukanta Mandal &
    • Antoni Llobet
  3. Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University (SU), 10691 Stockholm, Sweden

    • Beverly Stewart &
    • Timofei Privalov
  4. Department of Bioinspired Science, Ewha Womens University, 120-750 Seoul, Korea

    • Antoni Llobet
  5. State Key Lab of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), 116024 Dalian, China

    • Licheng Sun

Contributions

L.S. and A.L. supervised the project. T.P. supervised the theoretical part of the project. L.D. synthesized all the complexes and carried out the characterization, catalysis and electrochemistry. S.M. carried out the electrochemistry and mass measurements. F.B. performed stopped-flow UV–vis measurements. B.S. and T.P. performed DFT calculations. L.S., A.L., L.D. and T.P. wrote the paper.

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The authors declare no competing financial interests.

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