Structure‐Activity Relationship for Di‐ up to Tetranuclear Macrocyclic Ruthenium Catalysts in Homogeneous Water Oxidation

Abstract Two di‐ and tetranuclear Ru(bda) (bda: 2,2′‐bipyridine‐6,6′‐dicarboxylate) macrocyclic complexes were synthesized and their catalytic activities in chemical and photochemical water oxidation investigated in a comparative manner to our previously reported trinuclear congener. Our studies have shown that the catalytic activities of this homologous series of multinuclear Ru(bda) macrocycles in homogeneous water oxidation are dependent on their size, exhibiting highest efficiencies for the largest tetranuclear catalyst. The turnover frequencies (TOFs) have increased from di‐ to tetranuclear macrocycles not only per catalyst molecule but more importantly also per Ru unit with TOF of 6 s−1 to 8.7 s−1 and 10.5 s−1 in chemical and 0.6 s−1 to 3.3 s−1 and 5.8 s−1 in photochemical water oxidation per Ru unit, respectively. Thus, for the first time, a clear structure–activity relationship could be established for this novel class of macrocyclic water oxidation catalysts.

All co-solvents used in electrochemical studies were of a purity of 99.8% and higher.
NMR spectroscopy 1 H NMR spectra were recorded at room temperature (ca. 22 °C) with a Bruker Avance III HD 400 spectrometer at 400 MHz. 13 C NMR spectra are proton decoupled and recorded at 100 MHz using the same instrument. DOSY spectra were recorded on a Bruker Avance III HD 600 spectrometer. Chemical shifts δ are indicated in parts per million (ppm) relative to the solvent peaks and coupling constants J in Hz. Solvent peaks of residual undeuterated solvent were used for calibration of the spectra. The following abbreviations were used to describe the observed multiplicities: s = singlet, d = doublet, t = triplet, m = multiplet, dd = doublet of a doublet.
6 (eq. S1) Here, is the diffusion coefficient which was determined from the DOSY experiments, is the Boltzman constant, is the temperature and is the dynamic viscosity.

Mass spectrometry
High-resolution mass spectra (

Spectroelectrochemistry
Measurements were performed in reflection mode using Agilent Cary 5000 spectrometer. A home built cylindrical PTFF cell with a sapphire window and an adjustable three-in-one electrode setup comprising a 6 mm platinum disk as a working electrode, a 1 mm platinum counter electrode and a pseudo-reference electrode was used. The layer thickness was set to be about 100 µm and experiments were performed in 1:1 TFE/water mixtures (pH 7, phosphate buffer). Potentials were referenced to the first oxidation event determined by DPV.

Chemical water oxidation
For chemically driven water oxidation experiments, gas tight reaction vessels of V = 20.6 mL were connected to pressure transducers (Honeywell, SSCDANN030PAAA5, absolute pressure, 0 to 30 psi). In one single experiment, cer ammonium nitrate (CAN, 1 g, 1.82 mmol) was dissolved in 3 mL of a water/acetonitrile mixture (pH 1, triflic acid, various ratios) in the reaction vessel and 400 µL of a stock solution of the catalyst dissolved in water/acetonitrile (pH 1, triflic acid, same ratio as in experiment) were injected through a septum using a Hamilton syringe. At the end of each experiment, 500 µL gas of the head space was taken with a gas tight Hamilton syringe and injected into a gas chromatograph GC-2010 Plus (Shimadzu, thermal conductivity detector at 30 mA, argon as carrier gas) for analyzing its composition. For calculating the TON, the total amount of generated oxygen during catalysis was determined and divided by the amount of catalyst present in the experiment. By applying the ideal gas law, the quantity of evolved oxygen was determined from the increase in pressure in the reaction vessel: where ∆ is the pressure increase, is the volume of the reaction vessels, is the gas constant, is the temperature and ∆ the amount of generated oxygen. In a series of concentration-dependent measurements, the highest TON obtained at one single concentration was reported. TOFs were determined for each measurement. In series with varying solvent ratios, TOFs for single experiments at various solvent ratios were calculated from the obtained initial rates by linear regression through the first linear part of the oxygen evolution curve at the very beginning of catalysis. In series of multiple measurements of different concentration at a fixed solvent ratio, the reported TOF is the slope of the linear regression of the plot of the initial rates vs the amount of catalyst.

S5
Photocatalytic water oxidation Photocatalytic water oxidation experiments were carried out in a transparent and temperaturecontrolled reaction chamber at 20 °C. The chamber was equipped with a Clark electrode (Oxygraph Plus Clark-electrode system from Hansatech) for oxygen detection in solution.
Irradiation of the samples was performed by a 150 W xenon lamp (Newport) with a 400 nm cutoff filter calibrated to an intensity of 100 mW cm -1 . Light intensity calibration inside the reaction chamber was carried out using a PM 200 optical power meter with a S121C sensor The oxygen evolution after a short induction period (~1 s) was fitted by linear regression analysis during the first five to ten seconds of catalysis to give the initial rates of catalysis. The slope of a plot of the initial rates versus the catalyst concentration gives the TOF.

OEG-MC4
Ru ( 1, 160.7, 160.3, 157.3, 152.8, 148.5, 139.4, 132.3, 126.6, 125.1, 123.2, 118.7, 114.9, 72.3, 71.1, 70.9, 70.8, 70.0, 68.4, 58.9                   The lighting symbol indicates the time at which the sample was exposed to light for the first time after stirring for 30 seconds in the dark. The light was turned off after 30 seconds of illumination and no further oxygen was produced. After 30 seconds in the dark, the light was again turned on and the catalytic process was re-initiated, indicated by the rise in oxygen detected by the Clark electrode.