Impact of substituents on molecular properties and catalytic activities of trinuclear Ru macrocycles in water oxidation†

Herein we report a broad series of new trinuclear supramolecular Ru(bda) macrocycles bearing different substituents at the axial or equatorial ligands which enabled investigation of substituent effects on the catalytic activities in chemical and photocatalytic water oxidation. Our detailed investigations revealed that the activities of these functionalized macrocycles in water oxidation are significantly affected by the position at which the substituents were introduced. Interestingly, this effect could not be explained based on the redox properties of the catalysts since these are not markedly influenced by the functionalization of the ligands. Instead, detailed investigations by X-ray crystal structure analysis and theoretical simulations showed that conformational changes imparted by the substituents are responsible for the variation of catalytic activities of the Ru macrocycles. For the first time, macrocyclic structure of this class of water oxidation catalysts is unequivocally confirmed and experimental indication for a hydrogen-bonded water network present in the cavity of the macrocycles is provided by crystal structure analysis. We ascribe the high catalytic efficiency of our Ru(bda) macrocycles to cooperative proton abstractions facilitated by such a network of preorganized water molecules in their cavity, which is reminiscent of catalytic activities of enzymes at active sites.

S4 samples were dissolved in anhydrous DCM (c = 0.25 mM) and tetrabutylammonium hexafluorophosphat was added as electrolyte (c = 0.1 M). A Platinum disk and wire as well as a Ag/AgCl electrode were used as working, auxiliary and pseudo-reference electrodes, respectively. Ferrocene (Fc) was added at the end of each experiment as an internal standard (Fc + /Fc = +0.63 V vs. NHE). 15 For measurements in aqueous media, glassy carbon was used as working electrode, a Platinum wire as counter electrode and a Ag/AgCl (3 M KCl) electrode as reference electrode (Ag + /Ag = +0.21 V vs. NHE). 16 Samples were dissolved in aqueous mixtures with acetonitrile or TFE as organic cosolvents (c = 0.25 mM). If not otherwise stated, CV and DPV were recorded at a scan rate of 100 mV s -1 and 20 mV s -1 , respectively.

Spectroelectrochemistry
Spectroelectrochemistry in reflexion mode was perfomed using a Agilent Cary 5000 spectrometer in combination with a home-built sample compartment consisting of a cylindrical PTFE cell with a sapphire window and an adjustable three-in-one electrode (6 mm Platinum disk working electrode, 1 mm Platinum counter and pseudo-reference electrode). All experiments were carried out at a sample concentration of c = 0.24 mM in 1:1 acetonitrile/water (pH 7, phosphate buffer) with a layer thickness of about 100 µm. The potential was referenced to the first oxidation event as it was determined by DPV.

UV/Vis absorption and emission spectroscopy
UV/Vis absorption spectra were recorded at 25 °C using a Jasco V-670 spectrometer. Samples prepared with spectroscopic grade solvents were measured in 1 cm quartz cuvettes.

Chemical water oxidation
Chemical water oxidation experiments were performed at 20 °C in reaction vessels connected to SSCDANN030PAAA5 pressure sensors (Honeywell, absolute pressure, 0 to 30 psi). For each measurement, 1 g (1.82 mmol) ceric ammonium nitrate (CAN) was dissolved in 3 mL of aqueous mixtures (pH 1, triflic acid) with acetonitrile or TFE as organic cosolvent. 400 µL of the catalyst stock solution was then injected through a septum using a Hamilton syringe. To determine the gas composition at the end of gas evolution, 500 µL of the gas head space was injected into a gas chromatograph GC-2010 Plus (Shimadzu, thermal conductivity detector at 30 mA, argon as carrier gas) using a gas tight Hamilton syringe. TON was calculated based on the total amount of oxygen evolved during catalysis divided by the amount of catalyst injected. The amount of evolved oxygen was determined by the pressure increase in the reaction vessel using the ideal gas law: Δp V = R T Δn,

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where T = 293.15 K, R = 8.314 J K -1 mol -1 , V = 20.6 mL. In concentration-dependent experiments a TON was calculated for each concentration and the highest TON is reported. For calculation of TOF, the initial rate of catalysis was determined again at each concentration by the linear regression of the oxygen evolution curve during the first two seconds of reaction. TOF was then determined from the slope of the plot of the initial rates vs. catalyst amount.

Photocatalytic water oxidation
An Oxygraph Plus Clark-electrode system (Hansatech Instruments) was used for oxygen detection in photocatalytic water oxidation experiments. Samples were irradiated using a 150 W xenon lamp (Newport) equipped with a 400 nm cutoff filter. Irradiation was calibrated to 100 mW cm -1 , unless otherwise stated, using a PM 200 optical power meter with a S121C sensor (Thorlabs) in combination with a CCS 200/M wide range spectrometer (Thorlabs). For each measurement, a stock solution of the respective PS and sodium persulfate in the indicated solvent mixture was prepared in the dark. An aliquot of this solution was then mixed with the catalyst at variable concentrations and transferred to the transparent reaction chamber while kept in the dark. Irradiation was started at 50 s to allow thermal equilibration of the sample in the temperature-controlled chamber at 20 °C. TON was calculated based on the maximum amount of oxygen evolved during catalysis divided by the amount of catalyst present. In concentration-dependent experiments a TON was calculated for each concentration and the highest TON is reported. For calculation of TOF, the initial rate of catalysis was determined at each concentration by linear regression of the oxygen evolution curve during the first five to ten seconds of reaction direct after the initial induction period (~1 s). TOF was then determined from the slope of the plot of the initial rates vs. catalyst amount.

Theoretical simulations
For the metadynamics simulations the electronic structure of the trinuclear Ru macrocycles has been described by the semiempirical PM6 method 17 using the MOPAC2016 program package 18 version 17.279L. The Newtonian equations of motion were integrated for a total of 1 ns in time steps of 2 fs using the velocity Verlet algorithm. 19 During the dynamics, the temperature was kept constant using a velocity-rescaling thermostat at 300 K allowing for canonical sampling. 20 For the evaluation of the torsional distortion of the axial bridging ligands, the two torsion angles between the aromatic rings in one of the bridging ligands have been used as collective variables. Gaussians of 20° width and 0.1 kcal mol -1 height were added to the metadynamics potential every 200 time steps. For the rotation of the Ru-bda moieties according to the collective variable visualized in Fig. S52, Gaussians with a width of 7.5° and a height of 0.015 kcal mol -1 were added every 1000 time steps. In order to account for the S6 slower energy deposition rate, total trajectory lengths of 2 ns have been realized in this case. All metadynamics simulations were performed using the metaFALCON python package. 21 Additionally, structure optimizations have been performed on the semiempirical level as described before, as well as using density functional theory (DFT) in Turbomole V7.0 22 employing the PBE exchange-correlation functional 23 together with the def2-SVP basis set 24 and the corresponding effective core potentials (ECP) 25 on ruthenium atoms. In all calculations, solvation was treated implicitly using the COSMO model for water. 26

Powder X-ray diffraction (PXRD)
PXRD was performed in reflection mode on a Bruker D8 Discovery diffractometer with positionsensitive 1D-Lynxeye detector using Cu-Kα radiation. Crystalline samples of m-F-MC3 were dried under high vacuum at 60 °C.
Afterwards, water (250 mL) was added and the organic phase extracted with ethyl acetate (4 x 100 mL). The combined organic phases were washed with brine and dried over anhydrous Na2SO4. The residue was purified by column chromatography (SiO2, hexane/EtOAc 10:1) and washed with MeOH to yield 1 as a white solid (3.17 g, 9.61 mmol, 76%).

m-F-MC3
Ru(bda)(dmso)2 (250 mg, 500 µmol, 1.1 equiv) and m-F-bpb     3. X-ray crystal structure analysis Fig. S2 Comparison of the X-ray crystal structures of m-F-MC3 and p-F-MC3 regarding the torsion angle between the terminal pyridyl rings within a single bridging ligand (a) and the torsion between two axial ligands coordinated to one Ru center (b). Only one Ru center with its coordinated pyridyl rings is shown for each macrocycle in b) for clarity (ORTEP diagram with thermal ellipsoids set at 50% probability; grey: carbon, white: hydrogen, red: oxygen, lavender: nitrogen, turquoise: ruthenium, green-yellow: fluorine).                                                 Fig. 8a in main article) at Ru II 3 oxidation state were determined on the basis of PM6/COSMO model.

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To determine whether the solid-state structure of m-F-MC3 remained porous after removal the solvent molecules, crystals of this macrocycle were dried under high vacuum prior to analysis by PXRD. As shown in Fig. S55, the crystal packing of m-F-MC3 clearly depended on the presence of solvent molecules, since upon their removal a collapse of the ordered structure was observed by comparison with the calculated pattern from the X-ray structure of this compound.