Amphiphilic Oxo-Bridged Ruthenium "Green Dimer" for Water Oxidation.

In 1982, an oxo-bridged dinuclear ruthenium(III) complex, known as "blue dimer," was discovered to be active for water oxidation. In this work, a new amphiphilic ruthenium "green dimer" 2, obtained from an amphiphilic mononuclear Ru(bda) (N-OTEG) (L1) (1; N-OTEG = 4-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-pyridine; L1 = vinylpyridine) is reported. An array of mechanistic studies identifies "green dimer" 2 as a mixed valence of RuII-O-RuIII oxo-bridged structure. Bearing the same bda2- and amphiphilic axial ligands, monomer 1 and green dimer 2 can be reversibly converted by ascorbic acid and oxygen, respectively, in aqueous solution. More importantly, the oxo-bridged "green dimer" 2 was found to take water nucleophilic attack for oxygen evolution, in contrast to monomer 1 via radical coupling pathway for O-O bond formation. This is the first report of an amphiphilic oxo-bridged catalyst, which possesses a new oxygen evolution pathway of Ru-bda catalysts.

Introduction

In natural photosystem II (PSII), a tetra-manganese-calcium molecular cluster (Mn4CaO5) serves as the active center to split water for oxygen evolution with generation of four protons and electrons (2H2O → O2 + 4H+ + 4e−) (Ferreira et al., 2004, Umena et al., 2011, Yano and Yachandra, 2014, Suga et al., 2019). The astonishing activity and stability of the oxygen evolution complex (OEC-PSII) stimulate the scientific community to develop artificial molecular catalysts that are sufficiently rugged and efficient to remove four electrons and four protons for water oxidation (Concepcion et al., 2009, Inoue et al., 2011, Garrido-Barros et al., 2017). Since the first discovery by Meyer and coworkers that an oxo-bridged dinuclear ruthenium(III) complex RuIII-O-RuIII (Gersten et al., 1982), known as “blue dimer,” was active for water oxidation, a large number of molecular catalysts have been identified and considerable knowledge with respect to their activity has been generated (Kärkäs et al., 2014, Blakemore et al., 2015, Zong and Thummel, 2005, Concepcion et al., 2008, Shigeyuki and Ken, 2009, Duan et al., 2009). A survey of these molecular catalysts reported to date revealed that Ru-bda (bda = 2,2′-bipyridine-6,6′-dicarboxylic acid) complex is the most efficient water oxidation catalyst (Duan et al., 2009, Duan et al., 2012), which had been demonstrated to operate O-O bond formation via coupling two oxo-radical (I2M) pathway, rather than via water nucleophilic attack (WNA) pathway (Zhan et al., 2017, Hessels et al., 2017, Xie et al., 2018, Pushkar et al., 2018). From a mononuclear Ru(bda) (pic)2 (pic = 4-picoline), Sun and Sakai independently reported an oxido-bridged trinuclear species RuIII-O-RuIV-O-RuIII (Zhang et al., 2016, Tsubonouchi et al., 2016). During photocatalytic water oxidation, Sun et al. obtained the trimer under alkaline solution. Sakai et al. suggested that the oxido-bridged trinuclear species obtained in situ in the air conditions could serve as an active catalyst for photochemical oxygen evolution. Given that recent reports have renewed interest toward the construction of oxo-bridged water oxidation catalysts (Concepcion et al., 2015, Daniel et al., 2018, Jiang et al., 2018, Lopez et al., 2014), we become particularly interested in designing oxo-bridged molecular catalysts and understanding the cooperative action of substrate water access, proton release, and dioxygen formation in oxo-bridged molecular clusters.

In the present work, we report a new type of oxo-bridged ruthenium catalyst, RuII-O-RuIII (2), formed by air oxidation of designed amphiphilic mono-ruthenium(II) catalyst Ru(bda) (N-OTEG) (L1) (1) (Figures S1–S4). The color of monomer 1 in aerobic aqueous solution gradually changed from red to green as shown in Figure 1. Studies using mass spectrometry (MS), nuclear magnetic resonance (NMR), Raman spectrum, UV visible spectroscopy (UV-vis), electron paramagnetic resonance (EPR), and X-ray absorption near edge structure (XANES) identified that green species from 1 is a more stable oxo-bridged dinuclear structure of RuII-O-RuIII (2). More importantly, “green dimer” 2 is catalytically active with a surprising mechanism change for O-O bond formation from radical coupling pathway for monomer 1 to water nucleophilic attack pathway.

Figure 1

Monomer 1 and Green Dimer 2

Results and Discussion

Amphiphilic monomer 1, containing hydrophilic ether chain and vinylpyridine as axial ligands and bda2- segment as equatorial ligand, was synthesized by a two-step reaction (Duan et al., 2009). UV-vis spectra of monomer 1 in aerobic aqueous solution showed a decrease of metal-to-ligand charge transfer absorption bands from 350 nm to 550 nm and an increase at 695 nm along with time (Figure 2B). The solution of 1 turned from red to green within several hours in aerobic aqueous solution (Figure 2A), but no change was observed in water under nitrogen atmosphere (Figure S8). To identify the new species, we refluxed 1 with water under air for 24 h. The obtained green species exhibited a new signal of m/z at 1,416.22, which is in agreement with a Ru-O-Ru structure of (2, Calcd, 1,416.25). The experimental isotopic peak of ESI-MS spectrum was consistent with the simulation results (Figures S5 and S6). Considering that Sakai and Sun assigned a similar absorption band at ~690 nm in the oxido-bridged trinuclear species RuIII-O-RuIV-O-RuIII to intervalence charge transfer absorption of RuIII→RuIV in buffer solution (Zhang et al., 2016, Tsubonouchi et al., 2016), we tentatively attributed the absorption at 695 nm in our case to intervalence charge transfer character in water. With addition of ascorbic acid from 0.1 equivalent to 1.0 equivalent (as the manner of the concentration of Ru center) to the solution, the 695 nm peak intensity of 2 decreased and kept constant with further addition of ascorbic acid (Figure 2C). Upon exposure to air, however, the absorption at 695 nm gradually emerged (Figures 2A and S12). The fact that the same ESI-MS signal at 691.13 as that of monomer 1 was detected for the reduced system of 2 (Figure S7) suggested that green dimer 2 can be easily synthesized by oxidation of monomer 1 with oxygen and returned to 1 by ascorbic acid in water.

Figure 2

The Transition of Monomer 1 and Green Dimer 2

(A) The solution color change of 1 (1.0×10−3 M) from left to right in an oxygen saturated aqueous solution over time.

(B) UV-vis spectral changes of 1 (2.0×10−5 M) in an oxygen saturated aqueous solution along with time.

(C) UV-vis spectral of 2 (1.0×10−5 M) after addition of 0–1.0 equivalent ascorbic acid (as the manner of the concentration of Ru center) in water.

Amphiphiles with hydrophilic and hydrophobic moieties are capable of self-assembling in solutions or at interfaces, thus enabling 1 and 2 water soluble (Yang et al., 2016). Dynamic light scattering (DLS) measurements showed that monomer 1 and dimer 2 aggregated in water to form assemblies with diameters around 50 nm (see Table S2), in accordance with scanning electron microscope (SEM) images (Figure S13). The UV-vis absorption spectra of different concentrations of monomer 1 blue shifted in water, consistent with the aggregation behaviors (Figure S14). To determine the critical aggregation concentration (CAC) of monomer 1 and dimer 2, organic dye Nile red was selected (Yang et al., 2016). When the concentration is higher than the CAC, Nile red would like to go inside the aggregates to show strong fluorescence, whereas Nile red shows no absorption and fluorescence in water. From the triggered strong fluorescence of Nile red even at a low concentration of monomer 1 and dimer 2 (Figures S15–S17), we inferred that both 1 and 2 at the catalytic concentration has existed as the aggregation forms in water. Possible due to the amphiphilic self-assembly, crystal structures of monomer 1 and dimer 2 as well as multi-core Ru-bda structure of RuIII-O-RuIV-O-RuIII reported in literature (Zhang et al., 2016, Tsubonouchi et al., 2016) were not obtained.

To shed more light on the structure of 2, Raman spectra of 1 and 2 were examined by 532 nm and 633 nm excitations, which are close to the maximal absorption band to have Raman spectrum with high intensity. In the 150–1,500 cm−1 region, the spectrum of 1 in H216O was dominated by the intense absorption bands near 500 cm−1, 700 cm−1, and 1,500 cm−1 (Figure S18). Above 1,000 cm−1, a series of bands of 1 arises from ring vibrations of the bda2− and substituted pyridine ligand. Treatment of 1 with 16O2 in H216O to synthesize 2 showed a new band at 374 cm−1, 462 cm−1, and 785 cm−1, which was tentatively assigned to the symmetric Ru-16O-Ru stretches of the bridge, νsym (Ru-16O-Ru) (Jurss et al., 2012, Moonshiram et al., 2012), whereas the bands above 1,000 cm−1 were weak in a relative sense. When green dimer 2 was generated from the aerobic solution of H218O with 16O2, the 374 cm−1, 462 cm−1, and 785 cm−1, bands of νsym (Ru-O-Ru) shifted to 359 cm−1, 409 cm−1, and 741 cm−1 corresponding to the isotopic shift value (see Table S3) (Polyansky et al., 2011). When green dimer 2 was obtained from 1 in H216O under 18O2 atmosphere, the slight shifts to 369 cm−1, 430 cm−1, and 771 cm−1 indicated that oxygen atom of the oxo-bridge Ru-O-Ru is dominantly from H2O with interference of O2 (Wilson and Jain, 2018, Moonshiram et al., 2016).

X-ray absorption near edge structure (XANES), powerful in monitoring the valence state of metal complexes, was performed to affirm the valence state of monomer 1 and dimer 2 (Moonshiram et al., 2012, Pushkar et al., 2014, Yang et al., 2017, Lebedev et al., 2017). As shown in Figure 3A, the absorption edge of the Ru metal center of 2 shifted to a higher energy at 22,129.0 eV, indicating an increase in the average valence of Ru center in 2. Adding 1.0 equivalent ascorbic acid (as the manner of the concentration of 2) to green dimer 2 aqueous solution returned the K-edge shift to the initial state at 22,127.9 eV of RuII. The derivative K-edges of Ru(bda) (pic)2, RuCl3, RuO2, monomer 1, dimer 2, and dimer 2 reduced by 1.0 equivalent ascorbic acid (relative to the concentration of 2) are shown in Figures S19 and S20 and Table S4. Each of the standard samples has an ~1 eV difference, so do monomer 1 and dimer 2. Besides, EPR experiment was consistent with the expected RuII state of monomer 1 (Figure S26). A solution of 1 under argon atmosphere was EPR-silent, whereas green dimer 2 showed a signal at g = 2.46 and 2.33 corresponding to the RuIII species (Pushkar et al., 2014, Murakami et al., 2011). After adding 1.0 equivalent ascorbic acid to the solution of dimer 2, the RuIII species of 2 was reduced to return EPR silent form.

Figure 3

The Valence State of Monomer 1, Dimer 2, and Standard Samples by X-Ray Absorption Spectroscopic Observation

(A) Normalized Ru K-edge XANES of 1, 2, and 2 (5.0×10−3 M) reduced with 1.0 equiv. (as the manner of the concentration of 2) ascorbic acid in water. The powder of ruthenium was used as the standard sample of Ru(bda) (pic)2; RuCl3 and RuO2 were showed in company as standard sample of RuII, RuIII, and RuIV, respectively.

(B) Corresponding k3-weighted Fourier transform (FT) of Ru EXAFS of Ru(bda) (pic)2 (powder), 1 and 2 (5.0×10−3 M).

By simulation of the multiple scattering paths from the crystal coordinates of Ru(bda) (pic)2 (Duan et al., 2009) and {[RuIII(bda)-(pic)2(m-O)]2RuIV(pic)2(H2O)2}2+ (Tsubonouchi et al., 2016), the molecular structure and ligand environment of metal center in monomer 1 and dimer 2 were further analyzed by extended X-ray absorption fine structure (EXAFS) (Figure 3B) (Creus et al., 2016, Lebedev et al., 2017, Moonshiram et al., 2012, Moonshiram et al., 2016). The resulting Ru-N and Ru-O distances of Ru(bda) (pic)2, monomer 1, and dimer 2 are in a good agreement with the average Ru-N and Ru-O distances of the first coordination shell. As shown in Figures S21–S23 and Table S5, the fitting Ru-N distance of 1.91 Å and Ru-O distance of 2.06 Å in monomer 1 are in line with the average Ru-N and Ru-O distance in crystallographic Ru(bda) (pic)2. Analysis of dimer 2 referring to {[RuIII(bda)-(pic)2(m-O)]2RuIV(pic)2(H2O)2}2+ (Tsubonouchi et al., 2016) resolved Ru-N interaction at 1.97 Å, a larger Ru-O distance at 2.13 Å and a Ru-O-Ru distance at 3.51 Å (Tables S5 and Figure S24). The strong peak at 3.51 Å for Ru(bda) (pic)2 and monomer 1 was assigned to 10 forward triangle scattering paths of Ru-C. One Ru-O-Ru forward triangle scattering path, together with another 10 forward triangle scattering paths of Ru-C, was involved to fit EXAFS data of dimer 2. Due to the increased coordination numbers (Zhang, 2014), the intensity of dimer 2 at ~1.5 and ~3.5 Å was increased. The distance above 3 Å corresponding to the distance of Ru-O-Ru originated from the Ru-Ru interaction across the μ-oxo bridge.

A notable oxygen evolution was observed when the water oxidation by amphiphilic monomer 1 and green dimer 2 was performed at room temperature using the water displacement method with Ce(NH4)2(NO3)6 (CAN) as the sacrificial electron acceptor at pH 2.0. All the solutions were degassed by nitrogen before use. Monomer 1 and green dimer 2 have good stabilities in water under nitrogen atmosphere (Figures S8 and S9). When adding monomer 1 (7.0×10−5 M) into a solution of 0.5 M CAN (Figure S25), more than 2,300 equivalent molecular oxygen was obtained (turnover number(TON) of 2,302). Under the same condition, oxygen was released by dimer 2 (7.0×10−5 M) with a TON of 2,608 (as the manner of the concentration of 2). Clearly, the formation of the RuII-O-RuIII structure with a μ-oxido bridge is an active species for water oxidation. To clarify the oxygen source of 1 and 2, we conducted the mass spectrometric analysis of the evolved O2 using 97% H218O as solution (Murakami et al., 2011). The molecular ion peak at m/z = 36 assignable to 18O2 exhibited the largest intensity of 89.17% for monomer 1 and 91.24% for dimer 2 among molecular ion peaks at m/z = 32, 34, and 36 (Table S6). The observed ratios of 1 and 2 agreed well with the calculated ratios, indicating that the oxygen atom in the bridge of RuII-O-RuIII 2 did not involve in the O-O bond formation. Furthermore, the water-oxidation kinetics as a function of concentration was investigated by Clark oxygen electrode. The fastest oxygen evolution time period of 10 s was chosen to avoid the conversion of monomer 1 to green dimer 2 during the reaction time. As shown in Figures 4A and 4C, catalyst 1 had a linear fitting correlation (R2 = 0.99) between concentration square and initial rate, indicating the radical coupling mechanism (I2M) for O-O bond formation (Wang et al., 2012, Yang et al., 2016). However, green dimer 2, plotting against the initial rate of oxygen evolution, suggested a water nucleophilic attack pathway (WNA) in the rate-determining step of O-O bond formation (Figures 4B and 4D). Surprisingly, the O-O bond formation pathways altered from radical coupling pathway to water nucleophilic attack when monomer 1 was replaced by dimer 2. The kinetic isotope effect (KIE) of 1 at different concentration was calculated as 1.19, in agreement with the previously established radical coupling pathway for O-O bond formation of Ru(bda) catalysts (Figure 4C), whereas the O-O bond formation of 2 with a KIE of 2.18 (Figure 4D) is similar to the famous blue dimer for O-O bond formation via water nucleophilic attack (Schulze et al., 2016, Shaffer et al., 2016, Khan et al., 2015, Yu et al., 2018).

Figure 4

The Kinetic Study of Monomer 1 and Dimer 2 for Water Oxidation

Oxygen evolution of monomer 1 (A) and dimer 2 (B) as a function of concentration (0.3–1.5×10−6 M) in water.The plots of the initial oxygen evolution rate in the first 10 s reaction system of 1 (C) and 2 (D) in H2O and D2O, respectively. The average k values from three sets were used to obtain KIE value.

When 1.0 equivalent CAN was added to the system of 1, an EPR signal at g = 2.30, 2.19 and 1.82 character showed up (Figure S27). Further addition of 3.0 equivalent CAN caused a new signal at g = 2.28, 2.09, and 1.85. These EPR signals of monomer 1 fit quite well with that reported in literature for RuIII and RuV species (Pushkar et al., 2014, Erdman et al., 2017, Pineda-Galvan et al., 2019). With 1.0 equivalents of CAN, dimer 2 showed a signal at g = 2.28, 2.16, 1.84 of RuIII species (Figure S28). However, the poor signals for dimer 2 with 5.0 equivalents of CAN deterred us from immediately attributing these peaks to RuV species. Additionally, spectroelectrochemistry was carried out. With increasing the potential from 0 to 1.3 V vs. SCE (Figures S29 and S30), the metal-to-ligand charge transfer (MLCT) absorption bands of monomer 1 at 395 nm and 520 nm decreased along with the consumption of RuII species, whereas the intervalence charge transfer absorption bands of dimer 2 decreased at 695 nm accompanying with an increase at 445 nm that is consistent with the conversion of [RuII-O-RuIII→ RuIII-O-RuIII] process.

To further understand the electron and proton release processes for water oxidation, differential pulse voltammograms (DPV) of monomer 1 and dimer 2 were studied (Figures S31–S34). Three oxidation peaks in the DPV curve of monomer 1 were observed at 446, 826, and 1022 mV versus SCE (saturated calomel electrode) at pH 2.0, which was assigned to [RuII→RuIII],[RuIII→RuIV] and [RuIV→RuV] couples, respectively (Figure S31). The DPV curve of green dimer 2 showed five oxidation peaks at 132; 470; 808; 1,039; and 1,280 mV vs SCE corresponding to the five oxidation processes of 2 as [RuII-O-RuIII→RuIII-O-RuIII], [RuIII-O-RuIII→RuIII-O-RuIV], [RuIII-O-RuIV→RuIV-O-RuIV], [RuIV-O-RuIV→RuV-O-RuIV], and [RuV-O-RuIV→RuV-O-RuV], respectively. From the Pourbaix diagram of 1 within the arrangement of pH 1–4, the [RuII→RuIII] couple is pH-independent, whereas the [RuIII→RuIV] and the [RuIV→RuV] processes have the slopes of 58 mV/pH and 61 mV/pH, respectively (Figures 5A and S33), indicating an 1e−/1H+ process (Wasylenko et al., 2010, Murakami et al., 2011). Thus, the three oxidation processes of monomer 1 at pH 2.0 are [RuII-OH2→RuIII-OH2], [RuIII-OH2→RuIV-OH], and [RuIV-OH→RuV=O]. As for the electron and proton transfer of mixed-valence dimer 2 during water oxidation by CAN at pH 2.0, the three processes of [RuII-O-RuIII→RuIII-O-RuIII], [RuIII-O-RuIII→RuIII-O-RuIV], and [RuIV-O-RuIV→RuV-O-RuIV] involved an 1e−/1H+ process, whereas the [RuIII-O-RuIV→RuIV-O-RuIV] and [RuV-O-RuIV→RuV-O-RuV] processes were pH independent (Figures 5B and S34). Therefore, the water oxidation processes of dimer 2 could be summarized as [OH2-RuII-O-RuIII-OH→OH-RuIII-O-RuIII-OH], [OH-RuII-O-RuIII-OH→O=RuIV-O-RuIII-OH], [O=RuIV-O-RuIII-OH→O=RuIV-O-RuIV-OH], [O=RuIV-O-RuIV-OH→O=RuV-O-RuIV= O], and [O=RuIV-O-RuIV=O→O=RuV-O-RuV=O], respectively.

Figure 5

Investigation of the Electron and Proton Transfer Processes of Monomer 1 and Dimer 2

Pourbiax diagram of 1 (A) and 2 (B) (1.0×10−3 M) in a solution of 0.1 M Na2SO4 using HNO3-NaOH to adjust the pH values. Conditions: glassy carbon as working electrode, Pt wire as auxiliary electrode, and saturated calomel electrode as reference electrode.

Combined with all above experimental results, the oxygen evolution of the whole catalytic cycles of monomer 1 and green dimer 2 could be summarized as follows (Figure 6). Amphiphilic mononuclear 1 could form an intermediate of RuIII-H2O with the supplementary oxidation by oxygen and then react with another molecule of 1 to generate “green dimer” 2. In the presence of CAN, monomer 1 performed a radical coupling pathway by RuV=O intermediate; however, dimer 2 with the RuII-O-RuIII structure operated a water nucleophilic attack pathway to form the O-O bond (Concepcion et al., 2015, Moonshiram et al., 2013). The μ-oxido bridge of dimer 2 may prevent the coupling from two high RuV=O species, thus resulting in water nucleophilic attack pathway to generation of oxygen. The alternation of the catalytic pathway for oxygen evolution leads to different water oxidation performance of the amphiphilic Ru-bda monomer 1 and its oxido-bridged green dimer 2.

Figure 6

Proposed mechanism for water oxidation catalyzed by monomer 1 and dimer 2 in water.

In summary, we report an amphiphilic oxo-bridged dimer catalyst 2 for the first time for water oxidation, which is easily generated by monomer 1 in aerobic aqueous solution. With detailed characterization and analysis, the structure of green dimer 2 has been identified as a RuII-O-RuIII oxo-bridged mixed-valence species, referring to the famous “blue dimer” of RuIII-O-RuIII. The amphiphilic monomer 1 and green dimer 2 can catalyze water oxidation for oxygen evolution in high efficiency. The presence of μ-oxido-bridge greatly influences the catalytic behavior on O-O bond formation for Ru-bda catalysts in water. A mechanistic change of O-O bond formation from radical coupling pathway to water nucleophilic attack reveals another important oxygen evolution pathway of Ru-bda catalysts. The conversion of monomer 1 to oxo-bridged dimer 2 may signify that the real form of the long-lived Ru-bda catalysts would be close to the nature's mastery of the multinuclear structure of Mn4CaO5 with μ-oxido bridges.

Limitations of the Study

Our results show that an amphiphilic μ-oxido-bridged catalyst is active for water oxidation for the first time. Referring to the famous “blue dimer” of RuIII-O-RuIII, green dimer (RuII-O-RuIII) (2) formed by air oxidation of amphiphilic mono-ruthenium(II) catalyst Ru(bda) (N-OTEG) (L1) (1), has been demonstrated to take water nucleophilic attack for oxygen evolution, which is distinct from monomer 1 via radical coupling pathway for O-O bond formation. Further investigation with advanced in-situ techniques may provide a deeper insight on the high-valence catalytic intermediates that may give more enlightenment to design more robust molecular catalysts for water oxidation.

Methods

All methods can be found in the accompanying Transparent Methods supplemental file.