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A Covalent Organic Framework for Cooperative Water Oxidation

The future of water-derived hydrogen as the “sustainable energy source” straightaway bets on the success of the sluggish oxygen-generating half-reaction. The endeavor to emulate the natural photosystem II for efficient water oxidation has been extended across the spectrum of organic and inorganic combinations. However, the achievement has so far been restricted to homogeneous catalysts rather than their pristine heterogeneous forms. The poor structural understanding and control over the mechanistic pathway often impede the overall development. Herein, we have synthesized a highly crystalline covalent organic framework (COF) for chemical and photochemical water oxidation. The interpenetrated structure assures the catalyst stability, as the catalyst’s performance remains unaltered after several cycles. This COF exhibits the highest ever accomplished catalytic activity for such an organometallic crystalline solid-state material where the rate of oxygen evolution is as high as ∼26,000 μmol L–1 s–1 (second-order rate constant k ≈ 1650 μmol L s–1 g–2). The catalyst also proves its exceptional activity (k ≈ 1600 μmol L s–1 g–2) during light-driven water oxidation under very dilute conditions. The cooperative interaction between metal centers in the crystalline network offers 20–30-fold superior activity during chemical as well as photocatalytic water oxidation as compared to its amorphous polymeric counterpart.

For CV experiments, the scan rate was 100 mV/s unless stated otherwise. iR compensation at 85 % was applied for single CV and DPV experiments. Parameters in the DPV experiments were: ΔE = 4 mV, amplitude = 50 mV, pulse width = 0.05 s, sampling width = 0.0167 s and pulse period = 0.5 s.

Chemically driven water oxidation
The chemically driven water oxidation reactions were performed under ambient conditions in Schlenk reaction vessels, which were connected to pressure sensors from Honeywell (SSCDANN030PAAA5, absolute pressure, 0 to 30 psi). For each standard measurement, 1 g (1.82 mmol) of Cerium(IV) ammonium nitrate (CAN) was dissolved in 3 mL of a water (pH = 1, acid: trifluoromethane sulfonic acid, ratios varying) in a Schlenk flask with ultra-sonication. The dispersed catalyst solutions of various concentrations (400 μL in water (pH = 7, Mili-Q water)) were injected through a septum via a Hamilton syringe. Then, after certain time when there is no oxygen evolution (the curve gets flatten), the number of moles of oxygen evolution was calculated. Initially, Ru(bda)-COF/polymer was sonicated in 1 mL water (pH = 7, Mili-Q water) in a 4 mL vial followed by stirring until a uniformly dispersed solution is resulted. The dispersed solution was used as stock solution for the catalysis. The stock was diluted further to prepare the lower concentrations of catalyst solution.

Photocatalytic water oxidation
The light-driven water oxidation reactions were carried out in an Oxygraph Plus System of Hansatech Instruments Ltd. Irradiation of the sample with a 150 W xenon lamp by Newport equipped with a 380 nm cutoff filter and a water jacket to remove UV and IR radiation, respectively. The intensity with which the sample was irradiated was 270 mW/cm 2 . Before the measurement all components (catalyst, [Ru(bpy)3]Cl2 x 6 H2O and Na2S2O8 were dissolved in 3:7 CH3CN/phosphate buffer (pH = 7.2) solution in the dark ([catalyst] = varying). Afterwards the sample was transferred to the reaction chamber and kept in the dark. After the baseline of the measurement was constant, the sample was irradiated (at the 50 th second). During the measurement, the reaction temperature (20 °C) and stirring speed (100 rpm) were kept constant.

Kinetic isotope effect experiments
The kinetic isotope effect experiments were carried out in an Oxygraph Plus System of Hansatech Instruments Ltd. (Clark-type oxygen electrode for electrochemical oxygen detection; more details of the Oxygraph Plus System could be obtained from the Web site of Hansatech Instruments Ltd.). 1.5 mL of a freshly prepared CAN solution (c = 0.7 M, pH 1 solution (H2O or D2O with CF3SO3H)) was placed in the Clark electrode reaction chamber. 0.5 mL of the dispersed catalyst solution of Ru(bda)-COF/polymer (varying concentrations) were added after 25 seconds. During the measurement, the reaction temperature (20 °C) and stirring speed (100 rpm) were kept constant.
For the error estimation, the preparation of the individual solutions using Eppendorf pipettes and the catalyst injection by a Hamilton syringe (pressure sensor setup) or an Eppendorf pipette (Clarkelectrode) was assumed to be the largest error sources. During chemical water oxidation, the error also appears due to the irregular dispersibility at higher concentration of 4 g L -1 . At very low concentration, the error also comes from the pressure sensors. The error bars are represented as standard deviation from the average value (3 times) without considering additional instrumental errors.

Synthesis of Ru(bda)-COF
The COF was synthesized by reacting tetra-(4-anilyl)methane 2 (5.7 mg, 0.015 mmol) with Ru(bda)-aldehyde 1 (16.5 mg, 0.03 mmol) in 5 mL N,N-dimethylacetamide (DMAc) and mesitylene solvent mixture (1:1) 5 . The reactants were taken in a 15 mL glass vial and ultrasonicated for 30 mins in order to make a homogeneous solution. Then, the mixture was kept in the preheated oven at 60 °C for 96 hours. The precipitate was washed with DMAc for multiple times (4-5 times) until addition of DMAc did not produce any colour. Then, the mixture was treated with anhydrous dioxane for three times followed by vacuum drying before further characterization (Yield: ~55%).

Synthesis of R(bda) diimine complex
The monomer was synthesized by taking Ru(bda)-aldehyde (200 μmol) and excess 4-tertbutylaniline (1 mmol) in a Schlenk flask in presence of catalytic amount acetic acid (10 μL). Afterwards, degassed methanol (10 mL) was added. The mixture was degassed again for 30 minutes. Then the mixture was refluxed for 24 h. The solvents were evaporated, and the solid product was purified by column chromatography over aluminium oxide with 3% MeOH in CH2Cl2 as eluent. (isolated yield = 83%).

Additional note on COF synthesis
During imine condensation, different monomers and molecular fragments are formed as byproducts and some smaller fragments are detected in HR-ESI mass spectra of the solvent residue after COF reaction ( Figure S9). To optimize the reaction temperature, COF synthesis was carried out at 120 °C, 90 °C, 60 °C and even at room temperature. For the reaction at 120 and 90 °C, initial formation of COF seeds is so fast that precipitation of mostly amorphous phases occurs within a couple of hours. Some crystalline COF domains are embedded with amorphous polymer after the completion of the reaction ( Figure S9b). The broad baseline confirms the presence of amorphous polymer phases besides crystalline domains. To slow down the rate of reaction and to allow a more ordered and slower growth of crystalline COF seeds, the best result was obtained for reaction at 60 °C. In the case of reaction at room temperature, formation of very less crystalline domains and a kinetically controlled formation of amorphous polymers as the main product is observed ( Figure  S9b). We further compared the residue solvents for each of the reaction mentioned above at different temperatures. From the color of the solution, it was obvious that in case of 60 °C there was lesser oligomers formation, thus indicating the highest yield for COF formation ( Figure S5). In all the aforementioned cases, the reaction was performed for 4 days in a 1:1 mixture of polar DMAc and nonpolar mesitylene. The thermodynamic and kinetic parameters (temperature, pressure, dilution, catalysts, and modulators) determine the growth of the crystalline phase by suppressing the amorphous phase. In case of Ru(bda)-COF the reaction propagates through direct crystallization process. Once the amorphous phase is formed, it does not convert to the crystalline phase. Thus, conversion of amorphous polymer in to crystalline COF is not possible ( Figure S10).    Figure S9. a) HR-ESI mass spectrum of the solvent residue for COF reaction at 60 °C. b) Powder X-ray diffraction pattern of as-synthesized semi-crystalline COF with embedded polymer at various temperatures. Figure S10. Comparison of powder X-ray diffraction patterns of amorphous polymer before and after recrystallization. After synthesizing the polymer, it was separated by filtration. Then the polymer was retreated with same solvent combination (N,N-dimethylacetamide:mesitylene = 1:1) used during COF synthesis for 4 days at 60 °C.

Structure simulation
Geometries of interpenetrated diamondoid COF (dia-Cn) were generated using the Automatic Topological Generator for Framework Structures (AuToGraFS) and optimized using UFF4MOF. AuToGraFS generates structures without symmetry (i.e. in P1 space group) and so, in order to validate the calculated structures against the PXRD, symmetric structures with a simplified Ru linker were generated, by removing the (bda) unit, which is expected to be disordered. a = b lattice parameters were scanned, while c was allowed to optimize. All cell angles were held fixed at 90.0 o . Non-symmetric calculations were undertaken using the Amsterdam Modelling Suite, AMS, [AMS 2020, SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands, http://www.scm.com.] and symmetric calculations were undertaken using AMS and the General Utility Lattice Program, GULP. 6 Interpenetrated images were generated using CrystalMaker ® (www.crystalmaker.com). Geometry optimization was performed using Gaussian 16, Revision C.01. 7 For the simulation of the interpenetrated structure, we have first scanned the various unit cell parameters a by keeping c as constant (a = b) ( Figure S11). From experimental powder X-ray diffraction, it is obvious that the unit cell parameter 'a' must be less than 20 Å as the first peak appears at 6.8 Å experimentally. For closer inspection, then the unit cell parameter 'a' was scanned from 18 to 20 with 0.5 Å interval. And it has been observed that a = 19 produces the PXRD pattern which is the closest to the experimental one ( Figure S12). Although there is a shift observed for the first peak position, next three peaks are very close. Thus, the parameters corresponding to a = 19 Å (a = b= 19 Å and c = 19.7 Å) were chosen for further refinement ( Figure S13). Then, various degrees of interpenetration (from dia-C4 to dia-C7) were simulated and their corresponding PXRD patterns were compared with the experimental pattern of the COF. The result clearly reveals that our experimental PXRD pattern matches best with the dia-C5 interweaving structure ( Figure S13). The simulated pattern was then further refined with Pawley refinement ( Figure S14,15). The structural details have been included in Table S2.      Figure S15. Simulated crystal structure of Ru(bda)-COF: a), b) and c) are the view of the asymmetric unit along the b-, a-and c-axis respectively.

Additional notes on structural simulation
1) During structural simulation, molecular dynamics have been further performed to check for the possibility of structural changes after interpenetration. Molecular dynamics (MD) was undertaken for a total of 50 ps using a model of dia-C5 (5-fold interpenetrated diamond topology) using UFF4MOF-II as implemented in AMS2021.207. After 20 ps of equilibration, 50 ps of MD was undertaken in the NVT ensemble using a time step of 0.25 fs. A Nose-Hoover chain thermostat was used to maintain a temperature of 300 K. From the dynamics, the lowest (low-energy frame: 317) and highest energy frames (high-energy frames: 239, 1994) were extracted, as they are expected to be the most geometrically distorted from the previously optimised structure (lowest pattern). From the PXRD patterns, it is evident that there is no significant change in the diffraction pattern. The only differences are in the relative intensity of the peaks at ~4.5, 9.3, 11.1, 14.6 degree ( Figure S16). Thus, structural integrity is maintained over the framework backbone once the interpenetrated structure is formed.

S25
2) The rotation of one bda-unit (bda = 2,2′-bipyridine-6,6′-dicarboxylate) has also been considered. It has been found that the rotation does not lead to any other structure, as PXRD pattern remains unaltered after 180-degree rotation of the unit ( Figure S17). From the molecular packing, it has been observed that once COF is crystallized in a particular order, there is no opportunity of the (bda)-unit to rotate further due to the space constraint. Thus, disorder of the bda units could predominantly be generated during crystallization or packing process. Figure S17. Comparison of PXRD patterns of Ru(bda)-COF before (top) and after (bottom) 180° rotation of (bda)-unit.

IR spectroscopy
The IR spectra shows the absence of amine N-H stretching frequency (3100-3400 cm -1 ) and aldehyde C-H frequency (2920 cm -1 ) that confirms the purity of the product. The generation of new peak at 1620 cm -1 also signifies the formation of the imine bonding. This is as per the previous literature reports. 8 a b Figure S18. Comparison of the IR spectra for starting materials dialdehyde 1 and tetrakisamine 2, and imine frameworks a, Ru(bda)-COF and b, Ru(bda)-polymer.

BET (Brunauer, Emmett and Teller) surface area measurement
Solid samples for both Ru(bda)-polymer and Ru(bda)-COF were activated under high vacuum at 80 °C for two hours. Argon sorption isotherms were measured at 87.3 K, on a Micro-300 sorption device from 3P Instruments. The measuring temperature of 87.3 K was realized by means of a cryoTune from 3P Instruments. Both materials did not show any significant porosity. The shape of the isotherms and the obtained BET surface areas of 30 and 139 m 2 g −1 for Ru(bda)-COF and Ru(bda)-polymer, respectively, indicate that both materials are nonporous and the measured surface areas correlate to coverage of the interface for the nanoparticles. Interestingly, a higher value for amorphous Ru(bda)-polymer has been observed as compared to crystalline Ru(bda)-COF, indicating a higher surface roughness for the amorphous polymer.

Scanning Electron Microscopy (SEM)
For SEM analysis of Ru(bda)-COF, a sample was casted using THF as solvent. On larger length scale (> 10 μm), Ru(bda)-COF possesses sheet-like morphology (Figs. S21a-c). However, a detailed look (Figs. S21c-d) indicates that individual sheets are formed from smaller spherical particles. Ultra-sonication of Ru(bda)-COF samples in water for 15 minutes leads to a partial disruption of the sheets (Figs. S21e-h). However, for complete disruption into the constituent spherical particles (Figs. S21i-l), sonication of a water suspension for 30 minutes in ultrasonication bath is necessary. The assembly of the smaller nanospheres (< 250 nm) can also lead to the larger nanospheres of 1-1.5 μm upon settling down for 5-6 hours ( Figure S21k). The smaller nanospheres can be separated through polymeric membrane filters ( Figure S23).    The electrochemical properties were investigated by cyclic and differential pulse voltammetry to check the various oxidation states of the Ru(bda) units within the framework system. Considering the poor electrical conductivity of the Ru(bda)-COF/polymer, we have deposited the WOCs on 10 wt% of multi-walled carbon nanotubes as described in the main manuscript. 9 Electrochemical Measurements of as-synthesized Ru(bda)-COF/polymer were performed with 10 wt% multi-walled carbon nanotubes (MWCNTs) considering their poor electrical conductivity. Firstly, MWCNT (1 mg) was sonicated in THF (1 mL) for 45 min. The freshly prepared dispersion of MWCNTs in THF (0.2 mg in 0.2 mL) was added to a glass vial containing a suspension of Ru(bda)-COF/polymer (2 mg in 1.8 mL THF). The mixture was again sonicated for another 15 minutes for proper mixing. Then, this mixture was used for the electrode preparation. All the measurements were performed in 1 M phosphate buffer (pH = 7). The buffer was prepared by dissolving different amounts of phosphate salts in highly pure Mili-Q water: pH 7, 1 M ionic strength: NaH2PO4 (23.1 g, 193 mmol) and Na2HPO4 (37.7 g, 266 mmol) in water (1000 mL). For CV experiments, the scan rate was 100 mV/s unless stated otherwise. iR compensation at 85 % was applied for single CV and DPV experiments. Parameters in the DPV experiments were: ΔE = 4 mV, amplitude = 50 mV, pulse width = 0.05 s, sampling width = 0.0167 s and pulse period = 0.5 s. Table S3. Summary of the redox properties for Ru(bda)-COF and Ru(bda)-polymer at pH = 7 at room temperature. c c These values are with respect to NHE.

Oxidation Ru(bda)-COF (V) Ru(bda)-Polymer (V)
Ru  The chemically driven water oxidation reactions were performed under ambient conditions in Schlenk reaction vessels, which were connected to pressure sensors from Honeywell (SSCDANN030PAAA5, absolute pressure, 0 to 30 psi). For each standard measurement, 1 g (1.82 mmol) of cerium(IV) ammonium nitrate (CAN) was dissolved in 3 mL of a water (pH = 1, acid: trifluoromethane sulfonic acid, ratios varying) in a Schlenk flask with ultra-sonication. The dispersed catalyst solutions of various concentrations (400 μL in water (pH = 7, Mili-Q water)) were injected through a septum via a Hamilton syringe. Then, after certain time when there is no oxygen evolution (the curve gets flatten), the number of moles of oxygen evolution was calculated. Initially, Ru(bda)-COF/polymer was sonicated in 1 mL water (pH = 7, Mili-Q water) in a 4 mL vial followed by stirring until a uniformly dispersed solution is resulted. The dispersed solution was used as stock solution for the catalysis. The stock was diluted further to prepare the lower concentrations of catalyst solution.

Variation of concentration for sacrificial electron acceptor ceric ammonium nitrate (CAN)
To screen for the most suitable reaction conditions for chemical water oxidation and to evaluate catalyst stability, CAN concentration was varied while keeping the amount of COF catalyst constant. For each measurement, 1 g L −1 of Ru(bda)-COF was thoroughly dispersed in water (pH = 7). Afterwards, varying amounts of CAN (see Table S4) were added, and oxygen evolution was measured by a pressure sensor.  859 1581 d 0.4 mg COF was used for each batch of the measurement. Considering the molecular weight of the COF as 1459.435 g/mol, 0.274 mmol of COF was taken into consideration for TON calculation. The TON has been represented for each (per) mole of Ru center. e It is due to the very low oxygen evolution at lower CAN concentration of 45.5 μmol that we could not determine data for the oxygen amount vs time plot as it is close to the sensing limit of the instrument. However, we were successful to get the initial data for rate measurement.

Turnover number (TON) calculation
During a complete conversion of starting units to the COF, the below suggested structure should be the repeating unit for the framework. Thus, this structure with the empirical formula C73H48N12O8Ru2 and a molecular weight of 1423.378 g mol −1 can be considered as the smallest molecular fragment for the diamond like network. However, the molecular weight of COF would be integer multiple of the molecular weight of this fragment. So, the molar amount of COF we are using for the calculation purpose should be quite lower than our current consideration which should relate to higher catalytic activity. To quantify the amount of Ru within the materials, we can define the smaller fragment C36.5H24N6O4Ru and a molecular weight of 711.689 g mol −1 , which can be used to calculate the TON per individual Ru center for the as-synthesized COF. During the chemical water oxidation, it has been observed that the catalytic performance of Ru(bda)-COF is only limited by the amount of CAN. Thus, considering the 5-cycle recyclability opportunities of the COF, five times of the TON for the individual catalytic cycle (5 x 576 = 2880) should be a lower barrier for the intrinsic TON of Ru(bda)-COF ( Figure S30b). Scheme S5. Repeating unit responsible for network formation in COF.  Figure S28. Determination of rate constants for water oxidation catalysis with Ru(bda)-COF (left) and Ru(bda)-polymer (right) from linear fits of initial rates at varying catalyst concentrations. The slope for the first two seconds of catalysis was used to determine the initial rate for the particular concentration of Ru(bda)-COF. For Ru(bda)-polymer, the initial rate for each catalyst concentration was calculated from the slope between 150-155 s considering the sigmoidal nature of the oxygen evolution curves. Linear regression fits determined the rate constants of the reactions (second order kCOF = 1655 μmol L g -2 s -1 for Ru(bda)-COF and first order kpolymer = 211 μmol g -1 s -1 for Ru(bda)-polymer).

Recyclability test
To elaborate on the recyclability of heterogeneous Ru(bda)-COF, the catalyst was recovered and reused for further cycles multiple times. To demonstrate that there is no considerable decrease in catalytic performance, we have performed five consecutive catalytic cycles with one batch of Ru(bda)-COF catalyst. The facile recovery of the catalyst from the reaction solution by simple filtration is shown in Figure S29, as the COF is collected at the bottom of the reaction tube after the reaction. To avoid any degradation of the imine framework under highly acidic conditions, water oxidation catalysis was measured for five minutes followed by recovery of the catalyst from the reaction solution for the next cycle. Figure S29. Recovery of Ru(bda)-COF catalyst from the reaction mixture. For recycling, the catalyst suspension was transferred to a new and centrifuged. The COF material was further washed with water multiple times to remove traces of CAN, if any, from the surface followed by drying in vacuum before the next cycle.  . PXRD and IR spectra of Ru(bda)-COF before and after the catalysis. The measurements were performed after recollecting the COF material from the reaction tube and activation at 90 °C for ten hours in high vacuum.

Estimation of crystallite size
Crystallite size calculation using the Williamson-Hall (W-H) plot method has been performed considering a Gaussian fitting for the first four high intensity reflections in diffractrograms for Ru(bda)-COF from the recycling experiments. This analysis suggests that the COF crystallite size does not alter considerably even after five catalytic cycles.
In XRD data, the total broadening (βT) of the reflections is due to the broadening effect of crystallites size (βD) and micro strain (βɛ). βT = βD + βɛ From Scherer equation, we know Where βD is the FWHM in radians, K = 0.9 is the shape factor, λ = 0.15406 nm is the wavelength of the X-ray source, D is the crystallite size and θ is the peak position in radian. Again, the XRD broadening due to micro strain is given by, βɛ = 4 ɛ tanθ Thus, it could be written rearranging as βT cosθ = 4 ɛ sinθ + Kλ/D This equation is comparable as y = mx +c Where y = βT cosθ, m = ɛ, x = 4sinθ and intercept c = Kλ/D S41

Control experiment
To rule out any decomposition of the heterogeneous catalyst and leakage of active Ru species as homogeneous catalysts, Ru(bda)-COF was ultra-sonicated for 30 minutes followed by stirring for 6 hrs in water and filterated afterwards. The filtrate was then tested in water oxidation catalysis. However, no evolution of O2 was detected for 80 minutes, thus indicating the chemical integrity of Ru(bda)-COF in water, and ruling out any homogeneous phase reaction.  The light-driven water oxidation reactions were carried out in an Oxygraph Plus System of Hansatech Instruments Ltd. Irradiation of the sample with a 150 W xenon lamp by Newport equipped with a 380 nm cut-off filter and a water jacket to remove UV and IR radiation, respectively. The intensity with which the sample was irradiated was 270 mW/cm 2 . Before the measurement all components (catalyst, [Ru(bpy)3]Cl2 x 6 H2O and Na2S2O8 were dissolved in 3:7 CH3CN/phosphate buffer (pH = 7.2) solution in the dark ([catalyst] = varying). Afterwards the sample was transferred to the reaction chamber and kept in the dark. After the baseline of the measurement was constant, the sample was irradiated (at the 50 th second). During the measurement, the reaction temperature (20 °C) and stirring speed (100 rpm) were kept constant.

Additional note on photocatalytic water oxidation
During photocatalytic water oxidation, the photosensitizer (PS) is excited to PS* in presence of light, which subsequentially is oxidized by the persulphate to PS + ( Figure S36a). This transient species further interacts with the WOC in the ground state in consecutive cycles to generate the highly oxidized Ru(V) state, which ultimately proceeds through I2M pathway and generates oxygen. Thus, photocatalytic activity is predominantly limited by the rate of generation of PS + . Alteration of the sensitizer could decrease the driving force for excited state oxidative quenching by the persulfate anion thereby affecting the overall catalytic activity. Even the hole-transfer process from photogenerated PS + to the WOC could limit the catalysis. Herein, absorption and luminescence spectroscopy (quenching studies) have already been used to assess the energy states and the stability of PS. 10 From cyclic and differential pulse voltammograms we have assigned the various oxidation potentials and provided the energy scheme of photocatalytic water oxidation ( Figure S36b).    Plots of the initial rates versus the catalyst concentration with quadratic fit and versus square of catalyst concentration with corresponding linear regression fit determined the second order rate constant photocatalytic water oxidation for Ru(bda)-COF particles (k = 1169 μmol L s -1 g -2 for 0.5-1 μm particles). The as-synthesized COF was initially sonicated for 30 minutes to transform them into nano-spheres. Then, the spheres were settled down for 6 hours followed by separation of the smaller nano-spheres through membrane filtration (PES membrane cut off size = 450 nm). The large nano-spheres were collected back and used for catalysis.
S49 Figure S41. Unimodal distribution in dynamic light scattering for Ru(bda)-COF in water: Mean = 850 nm; angle 90°, PI = ~0.7. Figure S42. Photochemical water oxidation catalysis for Ru(bda)-COF particles with size < 500 nm. Photocatalytic oxygen evolution curves for varying amounts of Ru(bda)-3D-COF (2 mL of 7:3 MeCN/phosphate buffer (pH = 7) solution, c(Ru(bpy)3Cl2) = 1.66 mM, c(Na2S2O8) = 41 mM, oxygen evolution was measured with a Clark-type electrode set-up, irradiation of the samples started at t = 50 s). Linear fitting of oxygen evolution curves between 65-75 sec provided the initial rates. Plots of the initial rates versus the catalyst concentration with quadratic fit and versus square of catalyst concentration with corresponding linear regression fit determined the second order rate constant photocatalytic water oxidation for Ru(bda)-3D-COF particles (k = 1591 μmol L s -1 g -2 for particles < 500 nm). The as-synthesized COF was initially sonicated for 30 minutes to transform them into nano-spheres. Then, the smaller nano-spheres were separated through filtration with polymeric membrane.

Ru(bda)-polymer
O2 10800 μmol L h -1 g -1 0.5 μmol L -1 s -1 Figure S45. Cartoon representation to illustrate the cooperative interaction in covalent organic framework during O-O bond formation. The periodic arrangements simultaneously bring multiple units together during the interaction of two metal centers on the surface.

Kinetic isotope effect measurements
The kinetic isotope effect experiments were carried out in an Oxygraph Plus System of Hansatech Instruments Ltd. (Clark-type oxygen electrode for electrochemical oxygen detection; more details of the Oxygraph Plus System could be obtained from the Web site of Hansatech Instruments Ltd.). 1.5 mL of a freshly prepared CAN solution (c = 0.7 M, pH 1 solution (H2O or D2O)) was placed in the Clark electrode reaction chamber. 0.5 mL of the dispersed catalyst solution of Ru(bda)-COF/polymer (varying concentrations) were added after 25 seconds. During the measurement, the reaction temperature (20 °C) and stirring speed (100 rpm) were kept constant.

Comparison of activity for Ru(bda)-COF) and molecular precursor Ru(bda) dialdehyde 1
Chemical water oxidation (3.4 mL aqueous solution at pH = 1 (CF3SO3H:H2O), 20 °C, CAN (1 g, 1.82 mmol) as sacrificial oxidant, 1000 rpm stirring speed) was measured for Ru(bda) dialdehyde 1 as molecular model compound and the 2 nd order rate constant k1 = 1.48･10 5 L mol −1 s −1 was obtained by a linear fit of the initial rate of O2 evolution versus the square of catalyst concentration ( Figure S52b). For comparison, we calculated the molar concentration of Ru in Ru(bda)-COF suspensions for chemical water oxidation (711.689 g mol −1 for C36.5H24N6O4Ru) and plotted the initial rate of O2 evolution (see Table S5) against the square of this concentration to obtain a 2 nd order rate constant kRu(bda)-COF = 8.50 ･ 10 2 L mol −1 s −1 . To take into account that for an I2M mechanism with an bimolecular O-O bond formation between two COF particles, only Ru centers at the surface of the nanoparticles are catalytically active, we estimated the ratio between surface and bulk Ru for spherical nanoparticles of different size. Based on SEM ( Figure S23), Ru(bda)-COF nanoparticles utilized for chemical water oxidation are primarily in this size regime. To estimate the activity of the catalytically active surface sites, we adjusted the effective concentration of Ru at the nanoparticle interfaces and plotted the initial rate of O2 evolution against the square of the concentration to obtain 2 nd order rate constants ksurface in the range of 0.94-3.78 ･ 10 6 L mol −1 s −1 . The approximately tenfold increase of the rate constant in comparison to the molecular model compound 1 clearly indicates the proposed cooperative effect of the ordered alignment of well-defined Ru catalysts in crystalline Ru(bda)-COF, as the molecular activity of Ru(bda) is significantly enhanced after immobilization in the heterogeneous framework. In reality, this effect might even be higher as the actual amount of accessible Ru catalysts at the interface will be lower due to grain boundaries (crystallite size of ~ 15 nm, see Figs. S32, S33) and other defects.  ) and e) Plots of initial catalysis rates (obtained by linear fit of O2 evolution curve for the first 5 seconds of catalysis) versus WOC concentration with corresponding quadratic regression fit for monomer and aldehyde respectively. c) and f) Plots of initial catalyst rate versus square of catalyst concentration with linear regression fit for monomer and aldehyde respectively.

X-ray photoelectron spectroscopy
According to the universal curve, the electron mean-free path, i.e., the probing depth of this experiment was on the order of 0.5 to 2 nm for the investigated core levels. From the spectrum of O1s, it could be observed the enhancement of oxygen content after the catalysis. This might come from the coordinated water with the Ru centre after the catalysis. UV−vis spectroscopy Figure S55. UV−vis diffuse reflectance (DR) spectrum of the Ru(bda)-COF before (red) and after (green) the catalysis measured in the solid state referenced to barium sulphate.