A supramolecular ruthenium macrocycle with high catalytic activity for water oxidation that mechanistically mimics photosystem II

Journal name:
Nature Chemistry
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Published online


Mimicking the ingenuity of nature and exploiting the billions of years over which natural selection has developed numerous effective biochemical conversions is one of the most successful strategies in a chemist's toolbox. However, an inability to replicate the elegance and efficiency of the oxygen-evolving complex of photosystem II (OEC-PSII) in its oxidation of water into O2 is a significant bottleneck in the development of a closed-loop sustainable energy cycle. Here, we present an artificial metallosupramolecular macrocycle that gathers three Ru(bda) centres (bda = 2,2′-bipyridine-6,6′-dicarboxylic acid) that catalyses water oxidation. The macrocyclic architecture accelerates the rate of water oxidation via a water nucleophilic attack mechanism, similar to the mechanism exhibited by OEC-PSII, and reaches remarkable catalytic turnover frequencies >100 s–1. Photo-driven water oxidation yields outstanding activity, even in the nM concentration regime, with a turnover number of >1,255 and turnover frequency of >13.1 s–1.

At a glance


  1. Synthesis and characterization of macrocycle [Ru(bda)bpb]3.
    Figure 1: Synthesis and characterization of macrocycle [Ru(bda)bpb]3.

    a, Synthesis of macrocycle [Ru(bda)bpb]3. b, A 400MHz 1H NMR spectrum of [Ru(bda)bpb]3 in 5:1 CD2Cl2:MeOD (blue = [Ru(bda)], red = bpb). c, HR-ESI mass spectrum of [Ru(bda)bpb]3 (1 × 10−6M in 5:1 CHCl3:CH3CN). Inset: Experimentally observed ESI-MS (black, bottom) and simulated isotopic distributions (grey, top) of [Ru(bda)bpb]3+.

  2. Electrochemical and spectroelectrochemical investigation of [Ru(bda)bpb]3.
    Figure 2: Electrochemical and spectroelectrochemical investigation of [Ru(bda)bpb]3.

    Three Ru redox events are observed, which exhibit distinct pH dependencies, and all Ru oxidation states reveal characteristic UV–vis absorption spectra. Measurements were performed at a concentration of 0.25 mM in 1:1 2,2,2-trifluoroethanol/water (pH 1; acid = trifluoromethane sulfonic acid). a, Cyclic and differential pulse voltammograms of the macrocycle [Ru(bda)bpb]3 (black) with a corresponding blank measurement (grey). b, Pourbaix (E–pH) diagram of [Ru(bda)bpb]3. The potentials were obtained from differential pulse voltammetry at certain pH values, which were adjusted by stepwise addition of NaOH solutions to a 1:1 2,2,2-trifluoroethanol/water (pH 1) solution. Coloured and shaded areas correspond to the stability regions of different Ru oxidation states. c, Spectroelectrochemistry of [Ru(bda)bpb]3 (c = 0.1 mM). d, Plot of absorption changes at 372, 553 and 668 nm versus applied potentials in the electrochemical cell. Top: Differential pulse voltammogram of the macrocycle.

  3. Catalytic performance of macrocycle [Ru(bda)bpb]3.
    Figure 3: Catalytic performance of macrocycle [Ru(bda)bpb]3.

    Concentration-dependent measurements reveal a very high turnover frequency (TOF) and a linear dependence on catalyst concentration. a, Oxygen evolution versus time with [Ru(bda)bpb]3 as catalyst (5.9–94 μM) dissolved in 3.4 ml of aqueous solutions (pH 1) containing 59% CH3CN using CAN (1 g, 1.82 mmol) as a sacrificial oxidant. b, Plot of initial catalysis rates versus catalyst amount with corresponding linear regression fit. Individual reaction rates were obtained by a linear fitting procedure of the first 2 s of catalysis (Supplementary Fig. 15). Error bars were estimated from the maximum volumetric error during solution preparation and catalyst injection.

  4. Kinetic isotope experiments for [Ru(bda)pic2] and [Ru(bda)bpb]3.
    Figure 4: Kinetic isotope experiments for [Ru(bda)pic2] and [Ru(bda)bpb]3.

    The mononuclear reference catalyst exhibits no KIE, whereas the KIE for [Ru(bda)bpb]3 is larger than 2, which implies O–H/D bond breaking in the rate-determining step. a,b, Oxygen evolution versus time during water oxidation with [Ru(bda)pic2] (a) and [Ru(bda)bpb]3 (b) as catalyst at varying catalyst concentrations in 2.0 ml aqueous pH 1 solutions (H2O or D2O) with 59% CH3CN content using CAN (0.525 M) as sacrificial oxidant measured with a Clark-type electrode set-up. c, Plot of initial catalytic rates versus [Ru(bda)pic2] concentration. Individual reaction rates kH2O and kD2O were obtained by plotting the initial rate versus the square of catalyst concentration (Supplementary Fig. 21). d, Plot of initial catalytic rates versus [Ru(bda)bpb]3 concentration, with corresponding linear regression fits to determine reaction rates kH2O and kD2O. Error bars were estimated from the maximum volumetric error during sample preparation and catalyst injection.

  5. Potential mechanistic pathways for water oxidation catalysis and proposed hydrogen-bonding network in macrocycle [Ru(bda)bpb]3.
    Figure 5: Potential mechanistic pathways for water oxidation catalysis and proposed hydrogen-bonding network in macrocycle [Ru(bda)bpb]3.

    The mass spectrometric analysis of the evolved oxygen using an 18O labelled catalyst in the water oxidation experiment proves the WNA mechanism to be the operative one. a, Mechanistic representation of the WNA and I2M reaction pathways of catalytic water oxidation, including mechanism-dependent expectations and the results obtained from 18O labelling on the oxygen isotope distributions. b,c, DFT-optimized structure of [Ru4+–OH(bda)bpb]33+ without (b) and with (c) cavity-embedded water molecules. Colour code for DFT-optimized structures: carbon, grey; hydrogen, white; oxygen, red; nitrogen, purple; ruthenium, green.

  6. Photocatalytic water oxidation experiment using [Ru(bda)bpb]3 as catalyst.
    Figure 6: Photocatalytic water oxidation experiment using [Ru(bda)bpb]3 as catalyst.

    The WNA mechanism allows the macrocycle to be very active in photocatalytic experiments, even under highly dilute conditions. a, Scheme of light-induced water oxidation with [Ru(bda)bpb]3 as WOC, [Ru(bpy)3]Cl2 as photosensitizer (P) and Na2S2O8 as sacrificial electron acceptor. b, Oxygen evolution curve of photocatalytic water oxidation with varying [Ru(bda)bpb]3 and [Ru(bda)pic2] concentrations measured with a Clark-type electrode set-up in 2 ml 1:1 CH3CN/phosphate buffer (pH 7.2) solution ([Na2S2O8] = 37 mM, [[Ru(bpy)3]Cl2] = 1.5 mM, λirr > 380 nm at 230 mW cm–2). Irradiation of the sample started after 25 s.


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  1. Universität Würzburg, Institut für Organische Chemie and Center for Nanosystems Chemistry, Am Hubland, 97074 Würzburg, Germany

    • Marcus Schulze,
    • Valentin Kunz,
    • Peter D. Frischmann &
    • Frank Würthner


P.D.F. and F.W. conceived the concept of metallosupramolecular macrocyclic water oxidation catalysts. M.S. and V.K. established the experimental methodologies and performed the reported experiments. M.S. wrote the manuscript with support from all co-authors.

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