Water Oxidation Initiated by In Situ Dimerization of the Molecular Ru

Research Article Cite This: ACS Catal. 2018, 8, 4375−4382

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Water Oxidation Initiated by In Situ Dimerization of the Molecular Ru(pdc) Catalyst Quentin Daniel,† Lele Duan,† Brian J. J. Timmer,† Hong Chen,† Xiaodan Luo,‡ Ram Ambre,† Ying Wang,§ Biaobiao Zhang,† Peili Zhang,† Lei Wang,† Fusheng Li,∥ Junliang Sun,‡ Mårten Ahlquist,§ and Licheng Sun*,†,∥

ACS Catal. 2018.8:4375-4382. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 10/08/18. For personal use only.

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School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, 10044 Stockholm, Sweden ‡ College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China § School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, 106 91 Stockholm, Sweden ∥ State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), 116012 Dalian, China S Supporting Information *

ABSTRACT: The mononuclear ruthenium complex [Ru(pdc)L3] (H2pdc = 2,6-pyridinedicarboxylic acid, L = N-heterocycles such as 4-picoline) has previously shown promising catalytic efficiency toward water oxidation, both in homogeneous solutions and anchored on electrode surfaces. However, the detailed water oxidation mechanism catalyzed by this type of complex has remained unclear. In order to deepen understanding of this type of catalyst, in the present study, [Ru(pdc)(py)3] (py = pyridine) has been synthesized, and the detailed catalytic mechanism has been studied by electrochemistry, UV−vis, NMR, MS, and X-ray crystallography. Interestingly, it was found that once having reached the RuIV state, this complex promptly formed a stable ruthenium dimer [RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+. Further investigations suggested that the present dimer, after one pyridine ligand exchange with water to form [RuIII(pdc)(py)2-O-RuIV(pdc)(py)(H2O)]+, was the true active species to catalyze water oxidation in homogeneous solutions. KEYWORDS: solar fuels, water oxidation, electrochemistry, ruthenium dimer, mechanism of O−O bond formation

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RuIII(H2O)(bpy)2]4+ for water oxidation, known as the “blue dimer”.6 Later, works on designing dimeric ruthenium complexes in order to improve the stability and the activity toward water oxidation have been realized.7−9 However, it has been noticed that several monomeric transition metal complexes can also perform catalytic water oxidation, opening the ways to different ligand designs and reaction mechanism studies.10−13 Previously, our group has developed several ruthenium-based complexes with anionic ancillary ligands. Oxide donor ligands compared to conventional nitrogen donor ligand (N-pyridine type), enriched the electron density on the ruthenium core, facilitating the formation of high valence species required for water oxidation.14 This concept has been successfully used to develop the Ru(bda)L2 (H2bda = 2,2′bipyridine-6,6′-dicarboxylic acid, L= N-heterocycles such as 4-picoline) type complex which are, to the best of our

INTRODUCTION The rapid growth of the world’s population and its requirement in energy are leading toward a depletion of fossil fuels and environmental/ecological disasters such as climate change. To meet those incoming challenges, a green and sustainable energy source is highly desired. So far, H2 as a fuel produced by water splitting is seen as the most promising approach.1−3 The combustion of H2 fuel produces only water as the final product, and the energetic cycle is carbon-free. Therefore, this fuel does not have any negative impact on the environment. Water oxidation can be considered as two half reactions: (1) water oxidation 2H2O → O2 + 4H+ + 4e− and (2) proton reduction 2H+ + 2e− → H2. Research on water splitting started in the late 70s, partially due to the oil crisis of 1973.4,5 It has been stated that the water oxidation half reaction is the bottleneck for the whole water splitting process due to the required transfer of multiple electrons and protons. Several molecular catalysts have been developed throughout the years to decrease the energetics of this reaction. In 1982, Meyer and co-workers reported the first molecular catalyst cis,cis- [(bpy) 2 (H 2 O)Ru III -O© 2018 American Chemical Society

Received: November 4, 2017 Revised: February 16, 2018 Published: March 28, 2018 4375

DOI: 10.1021/acscatal.7b03768 ACS Catal. 2018, 8, 4375−4382

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Physical Methods. The 1H NMR spectra were recorded with either 400 or 500 MHz Bruker Avance spectrometer. Elemental analysis was performed with a Thermoquest-Flash EA 1112 apparatus. Mass spectrometry was performed on a Finnigan LCQ Advantage MAX mass spectrometer. For [RuIII(pdc)(py)2-OH2] and [RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+, during the bulk electrolysis, a small amount of solution was sampled from the reaction solution, diluted with pH 1 nitric acid solution and then directly injected into the mass spectrometer by syringe. Electrochemical measurements were performed with a Vertex potentiostat. For cyclic voltammetry measurement, a glassy carbon disk (ϕ = 3 mm) was used as the working electrode, a platinum wire as the counter electrode, and an Ag/AgCl (3 M) electrode as the reference electrode. For bulk electrolysis, a carbon foam (30 PPI) was used as working electrode, a platinum net as counter electrode and an Ag/AgCl (3 M) electrode as the reference electrode. The latter two were each separated from the bulk solution by a glass salt bridge electrode with a porous junction filled with a pH = 1 nitric acid aqueous solution. All potentials reported herein were referenced to NHE (E(Ag/AgCl) = 0.21 V vs NHE). UV−vis measurement: UV/vis spectra (800−250 nm) were recorded at different time intervals using a PerkinElmer Lambda 750 UV/vis spectrometer. Oxygen evolution measurement: To a mixture of ammonium cerium(IV) nitrate (CAN) (80 μmol, 43.86 mg) and a pH 1 solution of nitric acid in water (1.8 mL), in a flask of 49.9 mL sealed with a septum, was added 200 μL of the catalyst solution (after bulk electrolysis, 1 mM in pH 1 nitric acid containing 10% CF3CH2OH). The mixture was stirred for 2−3 h at ambient temperature, after which 400 μL of the headspace was injected into a gas chromatograph (GC-2014, Shimadzu). The total amount of formed oxygen was determined by subtraction of the quantity of oxygen in air. Single crystals of [RuIII(pdc)(py)2-O-Ru IV (pdc)(py) 2]+ (PF6)− and [RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+ (PF6)−* were obtained with the addition of ammonium hexafluorophosphate (NH4PF6) in the bulk solution after electrolysis at desired potential (respectively 1.25 V until full conversion and 1.6 V for 10 min), followed by a slow evaporation of the solution at room temperature. Suitable crystals were selected and mounted on a Bruker diffractometer with Mo Kα (λ = 0.71073 Å) fine-focus sealed X-ray tube. The crystal was kept at 298 K during data collection. The crystal structures were solved with ShelXT in Olex2 package22 using intrinsic phasing methods and refined with the ShelXL23 using least-squares minimization algorithm. All the noncarbon atoms had been refined anisotropically, and all the hydrogen atoms have been refined isotropically with riding mode. For the [RuIII(pdc)-

knowledge, the most efficient synthetic molecular catalysts designed for water oxidation under acidic conditions.15,16 However, modification of the backbone bda ligand leads to a large decrease of catalytic water oxidation activity for complexes of this family.17 Analogues to this type of catalyst, Ru(pdc)L3 (H2pdc = 2,6-pyridinedicarboxylic acid) type complex, are not only decently active toward water oxidation but also allow the modification of the backbone ligand without loss of catalytic activity.18,19 This allows these type of catalysts to be crafted on the electrode’s surface to form efficient and robust electrochemical anodes.18,19 On the basis of our previous study, here, we investigate the catalytic behavior and water oxidation mechanisms of [Ru(pdc)(py)3] in homogeneous solutions (Scheme 1).20,21 The advantages of pyridine ligands used here, Scheme 1. Chemical Structure of [RuII(pdc)(py)3]

in comparison with the conventional 4-picoline, is to avoid the oxidation of the methyl group. Additionally, with pyridine ligands, the solubility of the complex in aqueous solution increases allowing better in situ spectroscopic characterizations.

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EXPERIMENTAL SECTION Synthesis and Characterization. The synthesis of the ruthenium complex [RuII(pdc)(py)3] was performed in two steps. First, 164 mg (1 mmol) of H2pdc, 300 mg (0.5 mmol) of dicholoro(p-cymene)ruthenium(II) dimer, and 0.4 mL of triethylamine were mixed in 10 mL of dry ethanol. The solution was Argon purged in a microwave vessel and heated at 140 °C for 40 min. To the dark red solution, 1.5 mL (19 mmol) of pyridine was then added, and the solution was repurged with Argon prior heating at 140 °C for 30 min. The dark red solution was finally purified by a silica gel column (1/ 10 methanol/CH2Cl2) to isolate the desired complex (Yield 45%). MSexp: 504.13 m/z, MScalculated: 504.05 m/z. 1H NMR (500 MHz, CDCl3) δ 8.92, 8.91 (d, 2H), 8.32, 8.31 (d, 4H), 7.99, 7.97 (d, 2H), 7.74, 7.73, 7.71 (t, 1H), 7.62, 7.60, 7.59 (t, 1H), 7.52, 7.50, 7.49 (t, 2H), 7.25 (t, 2H overlapping with CDCl3), 7.08, 7.07, 7.05 (t, 4H). For crystal preparations and characterizations, see Physical Methods below.

Figure 1. 1H NMR spectrum of [RuII(pdc)(py)3] in CDCl3. 4376

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Figure 2. Successive CV scans of [RuII(pdc)py3] in pH 1 solution (left), CV after electrolysis at 0.9 V vs NHE (right).

Figure 3. Mass spectra of (left) [RuIII(pdc)py3]+, (upper) generated by bulk electrolysis at 0.9 V in pH 1 solution before the ligand exchange occurred, (lower) calculated mass spectra. (Right) [RuIII(pdc)(py)2-OH2]+ (upper) generated by bulk electrolysis at 0.9 V in pH 1 solution after the ligand exchange occurred, (lower) calculated mass spectra.

(py)2-O-RuIV(pdc)(py)2]+ (PF6)−* crystal data, significantly larger anisotropic temperature factors were observed on two of the pyridine rings during the refinement; therefore, occupancy of the atoms on the pyridines were refined carefully. All Density Functional Theory (DFT) calculations were carried out with Jaguar 7.9 program package by Schrödinger LLC. For the optimization of solvated geometry, the B3LYPD3 functional was used with the LACVP** core potential and basis set augmented with two f functions on the metal.24

The electrochemical characteristic of the [RuII(pdc)(py)3] complex was studied in a mixture solution of 2,2,2trifluoroethanol (TFE)/water (10 v/v%) acidified to pH 1 by nitric acid. D2O was used as substitute of water in case of the 1 H NMR study and acidified by CF3SO3H instead. The selection of TFE, as opposed to the commonly used acetonitrile as cosolvent, is due to the high lability of the carboxylate ligand. Indeed, as a result of strong coordination ability, acetonitrile can substitute a carboxylate ligand in such type of system.15,26 Therefore, the use of a less-coordinating solvent such as TFE can prevent this issue facilitating a better electrochemical study of the complex. Figure 2 (left) presents multiple cyclic voltammetry scans from 0.2 to 1 V vs NHE. It can be noticed that the first scan presents only one oxidation peak around 0.63 V, assigned to [RuII(pdc)py3]/[RuIII(pdc)py3] couple. During the reverse scan, the reduction around 0.56 V is assigned to [RuIII(pdc)py3]/[RuII(pdc)py3]; however, an extra reduction peak appears at 0.42 V. During the second scan a new oxidation peak can be observed at 0.50 V which is reversible with the above-mentioned reduction peak. The new redox couple current increases with multiple scans, while the one related to [RuII(pdc)py3]/[RuIII(pdc)py3] decreases, indicating the formation of a new species after the oxidation [RuII(pdc)py3] to [RuIII(pdc)py3] redox. To shed light on the new species, the full conversion from [RuII(pdc)py3] to this new species was realized by bulk electrolysis at 0.9 V vs NHE of the starting [RuII(pdc)py3] complex (Figure S2). The Figure 2 (right) represents the cyclic voltammogram of the solution after the electrolysis. As observed, only the new redox couple, presenting perfect reversibility, is present, whereas the [Ru II (pdc)py 3 ]/

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RESULTS AND DISCUSSION The 1H NMR spectrum of the [RuII(pdc)(py)3] complex in CDCl3 (Figure 1) and the COSY spectrum (Figure S1) present 18 proton resonances in the aromatic region. The equatorial pdc ligand can be identified by the doublet at δ = 7.97 ppm and the triplet at δ = 7.60 ppm. The two axial pyridine ligands can be identified by the doublet at δ = 8.31 ppm, the triplet at δ = 7.50 ppm and the triplet at δ = 7.07 ppm, while the equatorial pyridine ligand is observed by the doublet at δ = 8.91 ppm, the triplet at δ = 7.73 ppm, and the doublet at δ = 7.25 ppm (partially overlapped with the CDCl3 signal). It is interesting to notice that the equatorial pyridine ligand appears at lower field compared to the two axial ones. This can be explained by the high electron deficiency of the pyridine part of the pdc ligand (induced by the presence of the two electron-withdrawing groups at the 1 and 6 positions), increasing the back-donating effect with the ruthenium core. This will disfavor the backdonating effect in the antiposition, affecting directly the equatorial pyridine. Therefore, this pyridine will be more electron deficient, and its protons will be less shielded and shifting toward the lower field.25 4377

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Figure 4. 1H NMR spectra of (top) [RuII(pdc)(py)2(H2O)] in pD 1 solution with ascorbic acid. (bottom) Free pyridine in pD 1 solution with ascorbic acid.

[RuIII(pdc)py3] redox couple has completely disappeared. This indicates that total conversion to the new species, after bulk electrolysis at 0.9 V, was achieved. The new species was characterized by MS (Figure 3). The left spectrum represents the starting complex [RuII(pdc)py3] where a typical ruthenium monomer isotope distribution can be observed. The right spectrum, representing the newly produced species after bulk electrolysis, also manifests a typical ruthenium monomer isotopic distribution. The mass difference between the two suggests that the new species formed could be [RuIII(pdc)(py)2-OH2]+ where a water molecule has replaced a pyridine ligand. To understand the coordination chemistry of [RuIII(pdc)(py)2-OH2]+ and to determine which pyridine has been substituted, 1H NMR in pD 1 solution (Figure 4 top) after addition of ascorbic acid to generate a diamagnetic species (RuII) has been performed. The new spectrum also presents 18 proton resonances in the aromatic region. The pdc ligand can be identified by the triplet at δ = 7.53 ppm and the doublet at δ = 7.78 ppm, while the 2 equiv of axial pyridine presents two triplets at δ = 7.23 ppm and at δ = 7.68 ppm and a doublet at δ = 8.03 ppm. The remaining signal was then assigned to a protonated pyridine (pyridinium ion) by correspondence with the 1H NMR in pD1 solution of pyridine (pKa([py-H]+): 5.23) (Figure 4, bottom). This proves the dissociation of the equatorial pyridine ligand on [RuIII(pdc)py3]+, which is in agreement with previous work.20,27−29 Moreover, as it could be expected from the geometry, the [RuII(pdc)py3], as synthesized, does not present an open site for coordination of a water molecule to the ruthenium core and therefore is unlikely to go through a seventh coordinated species. The decoordination of the equatorial pyridine can then create a site for the water to coordinate on the metal center for further oxidation. After the full conversion to [RuIII(pdc)(py)2OH2]+ (Figure 2, right), the electrochemical properties of the bulk solution was further investigated by multiple CV scans (Figure 5). Several interesting features can be observed from the CV scans of [RuIII(pdc)(py)2OH2]+. First, during the first scan, the oxidation to [RuIII(pdc)(py)2OH2]+ is followed by at least two other oxidations (ca. Eox1: 1.30 V and Eox2: 1.49 V) before catalytic water oxidation (Eonset: 1.56 V). The reduction part of the CV scan does not present direct evidence of a redox couple in relation to the two previously mentioned oxidation peaks. The reduction peak observed at EredA: 0.98 V is relatively too low to be in relation with Eox1 (ΔEox1/redA: 0.32 V which is too large for the expected system where the oxidation should involve only one electron transfer coupled with one or two

Figure 5. Successive CVs of [RuIII(pdc)(py)2-OH2]+ in pH 1 solution.

protons). Furthermore, the reduction of [Ru III (pdc)(py)2OH2]+ is now broader (EredB), compared with that of Figure 2 (right), and is followed by a third reduction EredC: 0.15 V. The second scan shows a decay of the current peak of the oxidation to [RuIII(pdc)(py)2OH2]+, which can be interpreted by a lower amount of [RuII(pdc)(py)2(OH2)] available for the oxidation, potentially due to the formation of a new species during the first CV scan. Moreover, the intensity of the oxidation peak Eox1, similarly to [RuII(pdc)(py)2(OH2)] oxidation peak, also diminished while the current peak of Eox2 seems constant. The reduction process of the second scan shows only a negligible increase of the current peak EredA and EredC while EredB seems constant. Further scans reproduce the pattern of the second one. It is compelling to see a similar current peak decay for the oxidation of [RuII(pdc)(py)2(OH2)] and Eox1. Indeed, as mentioned previously, oxidation of [RuIII(pdc)(py)2OH2]+ is expected to produce [RuIV(pdc)(py)2OH] or more probably [RuIV(pdc)(py)2O].29 However, those two species are also expected to be simply reducible which does not seem to be the case here. To fully understand those redox peaks, CV scans at different potential were performed. Figure S3 shows the CV scans from 0.8 to 1.4 V, the peak current decay of Eox1 under this scan range is more pronounced compared with Figure 5. Therefore, it can be assumed that the oxidation occurring in this range is not reversible and thus leads to the formation of a new species. Moreover, under this conditions, the reduction peak of EredA is present but its intensity is meager compared to the one of Figure 5, thus the species responsible for this reduction is not produced with Eox1 but more probably with Eox2 (notice that at 4378

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Figure 6. Mass spectrum of [RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+ (upper) generated by bulk electrolysis at 1.25 V in pH 1 solution and the calculated mass spectrum (lower).

1.4 V the oxidation Eox2 has already begun to occur). Similarly, enlarging the scanning window until the reduction of [RuIII(pdc)(py)2OH2]+ (from 0.4 to 1.4 V) (Figure S4) produces an analogue effect on the Eox1 and EredA peaks. It can also be noticed that the oxidation peak of [RuII(pdc)(py)2(OH2)] decays significantly between the first and the second scan, analogous to Figure 5. In Figure S5 the CV scans in a range of 0 to 1.4 V are represented. Under these conditions, the oxidation peak of [RuIII(pdc)(py)2OH2]+ and Eox1 suffer only of a very small decay. Moreover, the broadening of EredB is present as well as the reduction peak EredC. Those observations demonstrate that the Eox1 peak is relatively steady along different scans only if the reduction at EredC has occurred. Similarly, the oxidation of [RuII(pdc)(py)2(OH2)] is relatively steady after oxidation of the solution to Eox1 only if the reduction occurring at EredC has happened. These results suppose the formation of a new species, once the oxidation peak Eox1 has been reached, which is not [RuIV(pdc)(py)2-OH] neither [RuIV(pdc)(py)2O]. Furthermore, this new species can presumably return to [RuII(pdc)(py)2(OH2)] once EredC is reached, and no water oxidation has occurred. In the case where water oxidation has occurred (Figure 5), the oxidation of [RuII(pdc)(py)2(OH2)] as well as Eox1 still decay even when the reduction at EredC occurred. Thus, it can be assumed that the species responsible for water oxidation in this case may not be reversible to [RuII(pdc)(py)2(OH2)]. To further characterize this new species formed at Eox1 oxidation peak, bulk electrolysis at 1.25 V was performed, and the solution content analyzed during the electrolysis by 1H NMR and MS. The 1H NMR spectrum (Figure S6) of the bulk solution presents only the presence of the pyridium ion, resulting from the pyridine ligand decoordinated previously from [RuIII(pdc)py3]. No new proton signal or an increase of the pyridinium ion amount on the 1H NMR leads to conclude that the newly formed species does not lose an extra ligand to get formed. From the MS spectrum, a new species with a distinctive peak at 864.99 m/z displays an isotopic pattern of a ruthenium dimer species (Figure 6). According to the spectrum and the previous observation of the conservation of all the coordinated ligands, the new species was hypothesized to be the dimer [RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+.30−32 The formation of such dimer concurs with the decay of the oxidation peaks for [RuII(pdc)(py)2(OH2)] after multiple CV scans. The formation of the dimer was also monitored by UV− vis spectroscopy (Figure S7), from which it was concluded that

the reaction is second order with respect to the starting [RuIII(pdc)(py)2(OH2)] (Figure S8). Furthermore, the addition of ammonium hexafluorophosphate in the bulk solution after the electrolysis followed by a slow evaporation of the solvent produced light-brownish needle-like crystals, the crystal refinement is presented in Figure 7.

Figure 7. Crystal structure of [RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+ (PF6)− (ellipsoid at 50% probability; PF6− was removed for clarity).

The structure obtained after refinement is similar to the dimer hypothesized during the bulk electrolysis, MS and UV− vis analysis. Therefore, [RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+ is not only a kinetically favored dimer but also a thermodynamically stable one. This dimer has some interesting structural features such as an angle for Ru1−O9−Ru2 with a value of 173 degrees which is, to the best of our knowledge, the “flattest” angle reported for a RuIII−O−RuIV structure.33 It can also be noticed that the axial pyridines are facing each other (probably due to a weak π−π stacking interaction) which could have an effect on the previously mentioned angle. Finally, the bond distances of Ru1−O9 and Ru2−O9 are relatively similar (1.855 and 1.822 Å, respectively), suggesting a delocalization of the positive charge between the two ruthenium ions.34 However, in terms of water oxidation catalyst, contrary to [Ru II (pdc)(py) 2 (OH 2 )], the dimer [Ru III (pdc)(py) 2 -O4379

DOI: 10.1021/acscatal.7b03768 ACS Catal. 2018, 8, 4375−4382

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ACS Catalysis RuIV(pdc)(py)2]+ does not seem to present an open site for the coordination of a water molecule. This particularity could indicate a deactivation of the catalytic activity after the formation of this dimer. To test if the newly formed dimer remained active for the catalytic water oxidation reaction, chemically driven water oxidation was performed using CAN and the amount of oxygen produced was quantified by GC (Table S1). Moreover, new CV scans from the bulk solution electrolyzed at 1.25 V revealed a similar water oxidation as well as a modification of the redox peak features (Figure 8). First of all, it

Figure 9. Crystal structure of [RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+ (PF6)−* (ellipsoid at 50% probability; PF6− was removed for clarity).

considered to be with mixed occupancy of nitrogen atom from a pyridine and oxygen atom derived a water molecule. Finally, the refinement converged to a pyridine occupancy of 0.879 and water occupancy of 0.121, the carbon atoms present a similar temperature factor as other carbon atoms in the same molecule. Taking into consideration that the X-ray crystallography is only capable of detecting the average of the molecules crystallized in a single crystal, these observations indicate that in the obtained new single crystal, at these two positions, with 87.9% probability that it were occupied by pyridine ligands, and 12.1% probability that these positions were occupied by water molecules. These results show that the water and pyridine may undergo a dynamic equilibrium state competition at a certain oxidation number of the ruthenium core. To emphasize the assumption from the crystal structure, optimized geometries for [RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+ and [RuIII(pdc)(py)(H2O)-O-RuIV(pdc)(py)2]+ have been calculated (Figure 10), and the characteristic values are reported with those of crystal [RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+ (PF6)−* in Table 1. The DFT-optimized structure as well as the crystal structure present very close values in terms of Ru−O bond distance, Ru− O−Ru angle, and N−Ru−Ru−L (L= pyridine or water) dihedral angle. These results enforce the assumption that 2 species may coexist inside the crystals structure and their relative structures are extremely similar. This new dimer [RuIII(pdc)(py)2-O-RuIV(pdc)(py)(H2O)]+* then possesses a water molecule coordinated and thus is capable of performing catalytic water oxidation. Therefore, another pathway for catalytic water oxidation could be proposed compared to the conventional monomeric one (Figure 11). As presented in the mechanistic pathways in Figure 11, water oxidation by the monomer [RuIVO] is “poisoned” by the formation of the dimer [RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+. It is nonetheless possible that under certain conditions, such as anchored on an electrode surface, the catalyst follows a mononuclear pathway.19,35 However, under our experimental conditions, this dimerization is relatively fast, forming [RuIII(pdc)(py)2-O-RuIV(pdc)(py)(H2O)]+, which after pyridine ligand exchange with water is the real catalyst for water oxidation.

Figure 8. Successive CV scans of the electrolyzed solution at 1.25 V.

can be observed that the redox peak of [RuII(pdc)(py)2(OH2)]/[RuIII(pdc)(py)2-(OH2)] nearly disappears, in agreement with the assumption of the dimer [RuIII(pdc)(py)2-ORuIV(pdc)(py)2]+ being the major species in the solution. Upon reduction until 0.2 V, regeneration of the monomer is absent and therefore the μ-oxo bridge is considered stable over the potential in the CV scan range. At this stage, as mentioned previously, the dimer is suspected to be the real active species for water oxidation even though no vacant coordination site (open site) is available for water to coordinate. After electrolysis of the dimer [RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+ solution at 1.6 V, which is higher than the onset potential of water oxidation for several minutes (Figure S9), a new MS analysis of the bulk solution revealed the partial loss of one axial pyridine ligand from the dimer (Figure S10). Moreover, a new set of crystals has successfully been obtained with the addition of ammonium hexafluorophosphate followed by a slow evaporation of the solvent. The obtained refined structure of the new crystal is presented in Figure 9. Surprisingly, the newly obtained crystal is also a Ru dimer which shows the same rough structure model as the one crystallized from the bulk solution after electrolysis at 1.25 V presented in Figure 7. However, significantly larger anisotropic temperature factors compared with the other carbon atoms in the same molecule, have been observed on 10 carbon atoms (C13−C22) (represented on the top part of the crystal structure in Figure 9) of the two pyridine ligands, while this feature is not present on the two nitrogen atoms of these pyridine ligands. This indicates that these carbon atom positions are not fully occupied by the carbon atoms and some vacancies may be present; however, the nitrogen atoms positions remained fully occupied. Therefore, during the refinement process, we have allowed the occupancy refinement of these two pyridine rings, and the nitrogen position has been

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CONCLUSIONS Aiming at understanding the catalytic water oxidation mechanisms of Ru-pdc type complexes in depth, the catalyst [RuII(pdc)(py)3] has been synthesized and reinvestigated. The electrochemical study of this complex under pH 1 acidic 4380

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Figure 10. Optimized geometries of (a) [RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+ and (b) [RuIII(pdc)(py)2-O-RuIV(pdc)(py)(H2O)]+. (Purple = Ru; Red = O; Blue = N; Gray = C; White = H).

RuIV(pdc)(py)2]+ dimer. This dimer has been successfully crystallized as [RuIII(pdc)(py)2-O-RuIV(pdc)(py)2]+ (PF6)−, and it seems responsible for the catalytic activity observed during electrochemistry. Further investigations of different crystals obtained following water oxidation presented a new intermediate that cocrystallized with the previously mentioned dimer. This new species after crystal refinement and DFT study presented a missing axial pyridine ligand replaced by a water molecule [RuIII(pdc)(py)2-O-RuIV(pdc)(py)(H2O)]+. Thus, we propose a new mechanism for water oxidation involving this ruthenium dimer complex rather than the monomeric Ru(pdc). This observation of the in situ dimerization of catalyst during water oxidation is extremely important in order to design new generation water oxidation catalysts and to provide deeper understanding of how a “simple catalyst” works for water oxidation.

Table 1. Comparison of Geometries between Calculated Structures and X-ray Crystal Structurea

X-ray crystalbExpt. L = PyridineCalc. L = waterCalc.

Ru−O bond length (Å)

Ru−O−Ru angle (deg)

N−Ru−Ru−L dihedral (deg)

1.855/1.822 1.847/1.846 1.853/1.861

173.0 175.8 172.5

20.0 21.8 20.1

a

The charge is shared by the two Ru atoms. bL = Pyridine (occ 0.879) L = Water (occ 0.121).

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ASSOCIATED CONTENT

S Supporting Information *

conditions revealed several interesting features. First, after oxidation of RuII to the RuIII state, a ligand exchange occurs between the equatorial pyridine and a water molecule to produce [RuIII(pdc)(py)2-OH2]+. Moreover, further oxidation to the expected [RuIV(pdc)(py)2O] state is not reversible, the monitoring of this oxidation by mass spectrometry and UV−vis revealed the formation of a [RuIII(pdc)(py)2-O-

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b03768. COSY-NMR spectrum and bulk electrolysis of [Ru(pdc)(py)3], CV scans of [RuIII(pdc)(py)2(OH2)], 1H NMR spectrum of electrolyzed [Ru(pdc)(py)3], UV−vis analysis of dimerization toward [RuIII(pdc)(py)2-O-

Figure 11. Proposed mechanisms for Ru(pdc)(py)3-based catalytic water oxidation (with x and y ≥ 4). 4381

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ACS Catalysis

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RuIV(pdc)(py)2]+ and oxygen evolution measurement, mass spectra before and after electrolysis, Cartesian coordinates obtained by single crystal X-ray crystallography (PDF) X-ray crystallography data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hong Chen: 0000-0003-4053-7147 Mårten Ahlquist: 0000-0002-1553-4027 Licheng Sun: 0000-0002-4521-2870 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Swedish Research Council (2017-00935), K & A Wallenberg Foundation, Wenner-Gren Foundation, the Swedish Energy Agency, the National Natural Science Foundation of China (21120102036), and the National Basic Research Program of China (973 program, 2014CB239402) for financial support of this work.

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DOI: 10.1021/acscatal.7b03768 ACS Catal. 2018, 8, 4375−4382