Ru-bda: Unique Molecular Water-Oxidation Catalysts with Distortion

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Perspective

Ru-bda: Unique Molecular Water-Oxidation Catalysts with Distortion Induced Open Site and Negatively Charged Ligands Biaobiao Zhang, and Licheng Sun J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b12862 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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Journal of the American Chemical Society

Ru-bda: Unique Molecular Water-Oxidation Catalysts with Distortion Induced Open Site and Negatively Charged Ligands Biaobiao Zhang,1 Licheng Sun1,2,* 1Department

of Chemistry, KTH Royal Institute of Technology, 10044 Stockholm, Sweden

2State

Key Laboratory of Fine Chemicals, Institute of Artificial Photosynthesis, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), 116024 Dalian, China Abstract: A water-oxidation catalyst with high intrinsic activity is the foundation for developing any type of water-splitting device. To celebrate its 10 years anniversary, in this Perspective we focus on the state-of-the-art molecular water-oxidation catalysts (MWOCs), the Ru-bda series (bda = 2,2′bipyridine-6,6′-dicarboxylate), to offer strategies for the design and synthesis of more advanced MWOCs. The O−O bond formation mechanisms, derivatives, applications, and reasons behind the outstanding catalytic activities of Ru-bda catalysts are summarized and discussed. The excellent performance of the Ru-bda catalyst is owing to its unique structural features: the distortion induced 7-coordination and the carboxylate ligands with coordination flexibility, proton-transfer function as well as small steric hindrance. Inspired by the Ru-bda catalysts, we emphasize that the introduction of negatively charged groups, such as the carboxylate group, into ligands is an effective strategy to lower the onset potential of MWOCs. Moreover, distortion of the regular configuration of a transition metal complex by ligand design to generate a wide open site as the catalytic site for binding the substrate as an extra-coordination is proposed as a new concept for the design of efficient molecular catalysts. These inspirations can be expected to play a great role in not only wateroxidation catalysis but also other small molecule activation and conversion reactions involving artificial photosynthesis, such as CO2 reduction and N2 fixation reactions. 1.

Introduction

Development of molecular catalysts is the best way to understand the structure-activity relationship to promote intrinsic catalytic activity because: i) molecular catalysts are readily characterized with clear structures, identifiable active sites and revealed catalytic mechanisms; ii) molecular catalysts are highly variable in both steric configuration and electronic structure through the ligand design; iii) each molecular catalyst is a unique single active site with a respective intrinsic activity; iv) molecular catalysts exhibit metal-atom economy with several orders of magnitude higher metal usage than material catalysts. The development of MWOCs has attracted much attention in the past decades, and

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hundreds of MWOCs based on different transition metals have been reported.1 Among these, Rubased MWOCs are not only the earliest and most extensively studied but also the most efficient ones, with a TOF higher than 1000 s−1, a turnover number (TON) over 105 and an onset overpotential less than 200 mV.2 Investigations on Ru-based MWOCs have provided us with the details and deep insights into water-oxidation catalysis, and offer guidance on how to design more advanced MWOCs.3 In this Perspective, we present our personal views of the family of Ru-bda MWOCs. The progression of Ru-based MWOCs from the pioneering catalyst “blue dimer” reported in 1982 to the current stateof-the-art catalysts, the Ru-bda complexes, is broadly summarized. The factors that can effectively enhance the performance of MWOCs are discussed, and we emphasize that the introduction of carboxylate group and distortion induced open site as catalytic site are promising strategies to design efficient MWOCs with low overpotential and high TOF. Finally, we conclude with an outlook on the future development of MWOCs. 2.

Principles for the design of efficient MWOCs guided by mechanisms of water oxidation

Inspired by natural enzymes,4-6 redox-active transition metal complexes, with variable coordination site as catalytic site, are generally studied as synthetic MWOCs.3 Figure 1 shows a general mechanism of water oxidation by MWOCs. After binding a substrate H2O, three main stages successively occurred till oxygen evolution: i) generation of high-valent metal-oxo, ii) OO bond formation, and iii) O2 evolution. In the first stage, H2O binding on the catalytic site is oxidatively activated through several steps of proton-coupled electron transfer (PCET), to form a high-valent metal-oxo M=O species. The PCET is important to make catalyst access to the high oxidation state smoothly with a low energy requirement.7-10 The formed high-valent M=O species triggers the second stage, OO bond formation, which is a crucial rate-determining step for most MWOCs.11 The nature of high-valent M=O species determines the activity and stability of a catalyst, and the OO bond formation pathway. There are two general pathways for OO bond formation (Figure 1): water nucleophilic attack (WNA) and interaction of two MO units (I2M).12 For the WNA pathway, the high-valent M=O species, which normally is electrophilic with low spin density on the oxo moiety and has a low lowest unoccupied molecular orbital (LUMO) level, is attacked by one substrate H2O to form the OO bond, generating a low oxidation-state metal hydroperoxide intermediate (MOOH). For the I2M pathway, the high-valent M=O species ordinarily has a distinct LUMO compared with the highest occupied molecular orbital (HOMO) of H2O, and the oxo moiety with more electron spin 2


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density displays more radical character. With these properties, two radical-like high-valent M=O couple with each other to form the OO bond by formation of a MOOM intermediate. O2 evolution is the final step to complete the water-oxidation catalytic cycle. One more oxidation is regularly demanded prior to a fast O2 evolution.11 The O2 evolution step usually is not a catalytic rate-determining step, requiring lower energy compared to the previous steps.

Figure 1. Schematic representation of general mechanisms for water oxidation catalyzed by metal complexes. The nature of high-valent M=O is important factor that directly influences the catalytic performance of MWOC, because it is the key intermediate involved in the water-oxidation catalysis. Both stability and reactivity of the high-valent M=O need to be considered to design an efficient MWOC. A highvalent M=O that is too active or too stable should be avoided. The stability and reactivity of a highvalent M=O are dominated by the metal center and the coordination environment provided by ligands. Frontier molecular orbital diagrams of M=O complexes can facilitate the selection of suitable metals to make active MWOCs. Coordination of an oxo ligand (O2−) to the metal center leads to the configuration distortion from Oh symmetry to C4v symmetry because of the shortness of the M=O bond, which causes the destabilization of the dz2 orbital (Figure 2).7 With this proposed electronic structure, the stability and reactivity of the M=O can be reasonably predicted by its d electron count.7 Transition metals with moderate d electron counts (e.g. MnV [d2], FeV [d3], FeIV [d4] and RuV [d3]) can generate reactive oxoes, because they have suitable empty d orbitals to accept the 2px and 2py electrons of the oxo ligand. However, the early transition metals with very low electron counts (e.g., VV [d0], TiIV [d0] and ZrIV [d0]) become highly stable and inactive to form an OO bond. In contrast, for the late transition metals, such as Co, Ni and Cu, the e(dxz, dyz) orbital sets are filled. It is difficult to support a multiple metal-oxo bond because the pπ electron pairs of the oxo cannot be

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easily accepted by metal d orbital. This explains why Mn is chosen by nature for water oxidation catalysts,13 and Ru, Ir and Fe are wildly applied to the development of synthetic MWOCs.1

Figure 2. Original d-orbital splitting diagrams for the M=O complex with a tetragonal ligand field. In addition, the design of ligands plays a great role to regulate the stability and reactivity of a highvalent M=O. To design a promising ligand for MWOCs, a basic requirement is that the ligand needs to be oxidatively stable under aqueous conditions. Both primary and secondary coordination spheres provided by the ligand need to be considered. The primary coordination spheres includes ligand properties such as coordination number, geometric arrangement of active site, rigidity, net charge effect, /π-donating ability, π-accepting ability, and conjugation. The secondary coordination sphere includes electronic effect of substituent, steric hindrance, hydrogen bonding effect, and hydrophilicity. Owing to the sufficient variability of the organic ligand framework, it is promising to generate efficient MWOCs through careful ligand design to precisely adjust the stability and reactivity of the high-valent M=O that are affected by the d electron count and electronic geometry. 3.

Development of Ru-based MWOCs − from “blue dimer” to Ru-bda

3.1 Development before the family of Ru-bda MWOCs Ru-based molecular complexes are the earliest and most studied MWOCs, started in 1982 when Meyer et al. reported the first MWOC cis,cis-[[RuII(bpy)2(H2O)]2(μO)]4+ (3, Figure 3), which is well known as the “blue dimer” (BD).14 The studies on BD have provided many theories and methods for the following development of MWOCs.15 These early studies have emphasized the importance of PCET for water-oxidation catalysis,8 and confirmed the superiority of polypyridine ligands, which are stable towards hydrolysis and capable of tolerating harsh oxidative conditions, as a scaffold to develop MWOCs.

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The success of BD, together with the multi-metallic Mn4CaO5 core of the OEC in nature, made scientists believe for a long time that multiple metal centers were necessary for synthetic MWOCs to bear the accumulation of the four oxidizing equivalents required by the oxidation of water to oxygen. Instead of using a μO bridge, Llobet and coworkers reported another important Ru dimer MWOC, the [Ru2II(bpp)(tpy)2(H2O)2]3+ (4, Hbpp = 2,6-bis(pyridyl)pyrazole, tpy = 2,2':6',2''-terpyridine), which employed a rigid backbone ligand, the Hbpp.16 With a similar strategy, Thummel et al. studied a series of Ru dimer catalysts containing a rigid polypyridyl-based equatorial ligand and two pyridine-based axial ligands, for example, complex 5.17-18

Figure 3. Historical overview of the development of Ru-based MWOCs with representative catalysts. The TOF and TON are measured using a CeIV oxidant at pH 1. However, the most interesting observation by the Thummel group was that the mononuclear reference complex 6 also displayed activity for water-oxidation catalysis.17 Further mechanistic investigations revealed that the OO bond formed through the WNA pathway with the involvement of a [RuV=O]3+ species.19 These studies indicated that a single metal center is sufficient for wateroxidation catalysis. Later, Meyer and coworkers further proved that one site was adequate for water oxidation with the complex [Ru(tpy)(bpy)(OH2)]2+ (7) and a clear catalytic mechanism.20 The awareness that a single metal site can catalyze water oxidation represented a breakthrough for the field and opened a new door for the design of MWOCs, not only Ru-based catalysts but also MWOCs based on other transition metal complexes. With only one metal site, synthetic flexibility was achieved, and reaction mechanisms could be deeply studied using much simpler platforms. Thus, the catalog of MWOCs has been significantly expanded upon by this breakthrough.2, 21 5



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3.2 The birth of Ru-bda — introduction of carboxylate ligands and 7-coordination In the early work of mimicking the OEC in PS II, it was found that phenolate and carboxylate ligands can effectively stabilize high-valent Mn states.22 Based on this point, Sun and coworkers introduced carboxylate ligands to Ru-based MWOCs aiming for an access high-valent Ru=O species under low oxidation potential. Their first design was the modification of a pyridazine-based polypyridyl equatorial backbone with two carboxylate groups to produce a carboxylate-containing ligand 3,6-bis(6′-carboxypyrid-2′-yl)pyridazine (H2cppd).23 Unexpectedly, a trans-dimeric complex 8 was obtained in the coordination of H2cppd with a Ru precursor, with a TON of 4700 and TOF of 0.28 s1 for water oxidation using CeIV as oxidant.23-24 Subsequently, they succeeded to construct a cis-configuration dimeric Ru complex 9 (TON of 10400 and TOF of 1.2 s1) by modifying the pyridazine ring of the H2cppd ligand with a benzene ring to provide steric hindrance.24 Complex 9 is closely related to 5. The contrast of properties between 5 and 9 can well demonstrate the effects of the introduction of carboxylate groups.25 Under the same conditions used for evaluation of 5, 9 showed a TON of 3540, over five times higher than 5. Moreover, the redox potentials for Ru2II,II/ Ru2II,III and Ru2II,III/ Ru2III,III of 9 are 0.9 V and 1.4 V vs. NHE, which are ~250 mV lower than those of 5. A low onset potential at 1.15 V vs. NHE at neutral pH was observed for 9. The outstanding activities and low oxidation potentials of 8 and 9 clearly demonstrate that the introduction of carboxylate groups is an effective strategy to produce efficient WOCs with low overpotentials.

Figure

4.

Crystal

structures

of

the

[RuII(bda)(pic)2],

[RuIV(bda)(pic)2]2·[HOHOH] complexes. 6


[RuIII(bda)(pic)2]·ClO4

and

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Based on the same design strategy, the Sun group also reported at the same time a mononuclear Rubased catalyst, the [RuII(bda)(pic)2] (10, H2bda = 2,2′-bipyridine-6,6′-dicarboxylic acid, pic = 4picoline) with two carboxylate groups in the equatorial ligands.26 Complex 10 catalyzes water oxidation with a TON of 2000 and TOF of 41 s1 using CeIV as the driving force at pH 1.27 In the electrochemical study, initial oxygen evolution by 10 was observed with an overpotential of 180 mV.28 In addition to the excellent catalytic performance, 10 displayed many interesting and unique properties.26 First, the crystal structure of 10 revealed that it was a 6-coordinated Ru complex with a highly distorted octahedral configuration, of which the angle of O2−Ru1−O3 is 123°, much bigger than 90° of an ideal octahedral configuration (Figure 4). No solvent, substrate water or other viable monodentate ligands showed up in the structure of 10. Second, water was catalytically oxidized on the large open site of 10 as the seventh ligand. Third, the water-oxidation reaction catalyzed by 10 exhibited a second order kinetics to the catalyst, which indicated that the intermolecular interaction is involved in the rate determining step. These special catalytic features were strongly supported by the crystal structure of an isolated RuIV dimer intermediate with a seventh-coordinate bridging ligand [HOHOH]− (Figure 4). Owing to the excellent activity, special configuration, simplicity of structure and unique catalytic mechanism, Ru-bda complexes were a new family of mononuclear MWOCs, and opened a new era for research on advancing MWOCs. Ru-bda type MWOCs rapidly attracted extensive attention worldwide and they were intensely investigated by Sun’s and many other groups. 4.

Mechanisms of water oxidation by Ru-bda catalysts

In the first report of the Ru-bda catalyst in 2009, Sun and coworkers already revealed that water oxidation by the Ru-bda catalyst was through a binuclear catalytic process by many essential observations, such as the second-order kinetics (Figure 5a) and the crystal structure of the [RuIV(bda)(pic)2(OH)]+ intermediate (Figure 4).26 Based on these results and the later computational studies by Privalov et al., a direct OO coupling of two RuIV−O˙ radicals (i.e., I2M pathway) through a [RuIV−O˙…˙O−RuIV] transition state with an activation energy of 12 kcal mol−1 was proposed for the OO bond formation mechanism of Ru-bda catalysts.29 The subsequent release of O2 from a peroxo intermediate does not need to overcome a high-energy barrier.

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Figure 5. Catalytic mechanisms for water oxidation by Ru-bda catalysts. a) Plot of kobs vs. [Ru-bda]2. b) DPV of 10 in pH 1 aqueous solution. Data extracted from Ref [30]. c) Pourbaix diagram of complex 10. d) Proposed catalytic cycle for water oxidation by Ru-bda catalysts under acidic conditions and deactivation mechanism under neutral conditions. e) Proposed full O−O bond formation mechanism by Ru-bda catalyst. Reproduced with permission from Refs [30] and [31]. Copyright Nature Publishing Group, 2012, and American Chemical Society, 2018, respectively. A complete catalytic cycle was further proposed for the Ru-bda catalyst in a separate work by Sun, Llobet and Privalov in 2012 with detailed electrochemical and stopped-flow UV-vis spectroscopy monitored kinetic studies.30 The differential pulse voltammogram (DPV) and Pourbaix diagrams of 10 clearly showed that the Ru-bda catalyst went through the following proton/electron transfer process during water-oxidation catalysis under acidic conditions (pH < 5): RuII−OH2 → RuIII−OH2 → RuIV−OH → RuV=O, and RuV=O was the active species that triggers OO bond formation and oxygen evolution (Figure 5b and 5c). Moreover, the stopped-flow UV-vis spectroscopy monitored all the key reaction steps with rate constants, including RuIV−OH → RuV=O, OO bond formation 2RuV=O → [RuIV−O−O−RuIV], and even O2 release from [RuIV−O−O−RuIV]. A comprehensive catalytic cycle was reasonably proposed as shown in Figure 5d. Note that when the driving force is limited, for example with stoichiometric CeIV, O2 slowly releases from the [RuIV−O−O−RuIV] species, which is considered as the rate determining step (first order kinetics), while when the driven force is sufficient, [RuIV−O−O−RuIV] is easily further oxidized to [RuIV−O˙−O−RuIV], which

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immediately evolves O2 and returns to RuIII and RuIV, then OO coupling of two RuIV−O˙ radicals is the rate determining step (second order kinetics).30, 32 To propose a credible water-oxidation mechanism, it is important to obtain structural identification of intermediates. The Sun group identified [RuIII(bda)(pic)2]+ by NMR and HR-MS.26 Later, Fujita reported the crystal structure of the RuIII-bda species.33 The [RuIV(bda)(pic)2(OH)]+ intermediate was well characterized by Sun group using NMR spectroscopy, HR-MS and crystal structure.26 The 7coordinate attribute is the most interesting feature of this state. For the [RuV(bda)(pic)2(O)]+ states, Copéret and Pushkar trapped a 7-coordinate RuV=O intermediate (Ru−O distance of 1.75±0.02 Å) at 1.34 V vs. NHE at pH 1.0 by in situ X-ray absorption spectroscopy.34 In addition, based on stoppedflow experiments, the calculated UV-vis absorption spectra of all the intermediates in the catalytic cycle are available. However, the isolation and crystal structure of the RuV-bda species has not been obtained thus far, and the identification of the [RuIV−O−O−RuIV] species remains a challenge. Because the 6-coordinate RuII-bda and RuIII-bda already have 18 and 17 electrons respectively, the specific structures of the Ru-bda at low oxidation states during water-oxidation catalysis are interesting and debatable. As drawn in Figure 5d, the coordination of H2O with RuII-bda and RuIIIbda are in thermodynamic equilibrium along with the adjustment of coordination.35 Transition behaviors of the RuII−bda−H2O species have been investigated in detail by Llobet and coworkers recently.36 Sun and coworkers studied the RuIII−bda−H2O state through EPR . They observed a metastable 7-coordinate RuIII−bda−H2O by a reliable EPR signal. An equilibrium was found between the original 6-coordinate RuIII-bda species and the 7-coordinate RuIII−bda−H2O.37 Meyer’s group studied electrochemical water oxidation by Ru-bda with pH 7 phosphate buffers. With the involvement of a small amount of CH3CN (4% v/v) in the electrolyte solution, the redox potential of RuII/RuIII anodically shifted, and when pH is above 6, RuII was directly oxidized to RuIV=O through a two-electron oxidation.38 Under their electrocatalytic conditions, the second-order reaction of OO coupling was not observed as the rate-limiting step. This may be because the energy barriers for the OO coupling step indeed is not high, so the other first-order steps, for example the oxidation of RuIV=O to RuV=O without PCET, may become the rate-limiting step, which leads to the observed first-order kinetics. A color change of the Ru-bda solid, Ru-bda solution or Ru-bda modified transparent electrode in stock from red to green is often observed in the research involving Ru-bda catalysts.33, 39 Fujita and coworkers observed a green species involved in the catalysis of water oxidation by Ru-bda catalysts, with a high absorptivity at λmax = 688 nm. Based on this interesting absorption feature, they 9



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speculated that a blue-dimer-like Ru−O−Ru species was formed during the catalytic water oxidation by Ru-bda, and proposed a catalytic cycle with the involvement of this Ru−O−Ru dimer intermediate.33 However, Sun’s group later crystallographically characterized the green species formed in water-oxidation catalysis by Ru-bda catalysts, with a strong absorption at λmax = 690 nm, as a RuIII–O–RuIV–O–RuIII type trinuclear species. The disproportionation of unstable RuIII–OH into RuII–OH2 and RuIV=O, and the subsequent reaction of RuIV=O with RuII–OH2 or RuIII–OH resulted in the RuIII–O–RuIV–O–RuIII type trimer (Figure 5d).40 Because the formation of the green Ru-trimer leads to dissociation of one molecule of the Ru-bda catalyst, it was suggested as a deactivation pathway of the Ru-bda catalyst (Figure 5d). Based on the fundamental understanding of the water-oxidation mechanism by Ru-bda, researchers are challenged by the questions why and how Ru-bda takes the intermolecular I2M pathway for the OO bond formation. Ahlquist’s group very recently found that the intrinsic barrier between the oxo groups of the RuIV−O˙ (equal to RuV=O) moieties is negligible.41-42 Therefore, they investigated the full process of the OO bond formation between two free [RuIV−O˙]: i) random diffusion, ii) occasional encountering between two [RuIV−O˙] monomer to form the pre-reactive dimer, and (iii) the actual O−O coupling step, by empirical valence bond (EVB) and molecular dynamics approaches (Figure 5e).31 The results indicated that the origin of the barrier in the OO bond formation is actually involved in the step to form the pre-reactive dimer from two solvated [RuIV−O˙] monomers, owing to the inter- and intramolecular strain in the rest of molecule, which is inevitable to get the oxo groups in close contact. In this case, they also found that solvation effects play a great role in the binuclear reacting pathway, and the RuV=O is hydrophobic at the oxo moiety;42 consequently two RuV=O monomers favor meeting each other face-to-face in water, which facilitates the formation of the pre-reactive dimer. These very fresh results can partly explain why and how Ru-bda takes the intermolecular I2M pathway for OO bond formation. 5.

Substituted derivatives of Ru-bda catalysts

5.1 Effects of axial-ligand variation The easy manipulation of the axial ligands allows the design and synthesis of various Ru-bda catalysts to study the effects of axial ligands and to construct Ru-bda catalysts based devices. So far, dozens of Ru-bda catalysts with different axial ligands have been reported. A successful pioneering work, by Sun and coworkers, on exploring better axial ligands for Ru-bda catalysts was the introduction of π-extended hydrophobic isoquinoline (isoq) as axial ligands to replace 4-picoline, which obtained [Ru(bda)(isoq)2] (11).30 In comparison with the [Ru(bda)(pic)2], the [Ru(bda)(isoq)2] 10


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displays dramatically increased catalytic performance for water oxidation, with a TOF of 303 ± 9 s−1, which is an order of magnitude higher than that of 32 s−1 for [Ru(bda)(pic)2] (Figure 6a). The remarkable enhancement of catalytic activity is attributed to the isoquinoline introducing a π-π stacking interaction and the hydrophobic effects between two ruthenium molecules that lower the energy barrier of the OO coupling step and stabilize the pre-reactive dimer intermediate. Ahlquist’s group further found that the proposed π-π stacking interaction was driven by the water medium.42 In the study of the [Ru(bda)(isoq)2] catalyst, Sun and coworkers discovered that the dissociation of the axial ligand is a possible deactivation pathway that limits the stability of Ru-bda catalysts. They therefore carried out DFT calculations to find more suitable axial ligands to make more stable Rubda catalysts by analyzing the correlation between the HOMO energy of the aromatic N-heterocycles axial ligand and the stability of Ru-bda catalyst.43 As displayed in Figure 6b, Ru-bda complexes with axial ligands of phthalazine, cinnoline, pyridazine and 4,5-dimethoxypyridazine are far more stable in acidic solution compared with the complexes with ligands of 4-picoline, pyrazine and pyrimidine. The catalytic results fully verified the theoretical prediction. Complex 12 with a TON of 55,000 is much more stable than 11 with a TON of 8,000, and 13 (TON of 4,500) is remarkably more stable than 14 (TON of 400).

Figure 6. Derivatives of Ru-bda catalysts with various axial ligands and with substituted bda2 backbone ligands. Reproduced with permission from Refs [30] and [43]. Copyright Nature Publishing Group, 2012, and National Academy of Sciences, 2012, respectively. 11



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To investigate the electronic effect and the hydrophobic effect, Sun’s group synthesized two series of Ru-bda catalysts with various substituted pyridine axial ligands.35,

44

The studies on the Ru-bda

catalysts with axial ligands of py (18), 4-Br-py (19), 4-Ph(CH2)3-py (20), 4-MeO-py (21), 4-NMe2py (22), 4-EtOOC-py (23) and 3-SO3−-py (24) showed that 19 and 23, with electron-withdrawing groups at the 4-position, exhibited a much higher activity (TON 4,500 and TOF 115 s−1 for 19; TON 4,800 and TOF 119 s−1 for 23) than the catalysts that had an axial ligand with electron-donating groups (TON less than 800 and TOF less than 30 s−1), indicating that electron-withdrawing groups on the axial ligands enhance the catalytic performance. This conclusion was further proved by the investigations on Ru-bda catalysts with 4-Me-py (10), 4-Cl-py (25), 4-tBu-py (26), 4-CF3-py (27) and 4-py-py (29) for chemical and photocatalytic water oxidation, where 25, 27 and 29 with electron-withdrawing groups displayed a much better catalytic performance.44 The enhancement of activity by the hydrophobic effect was showed in another work by Sun’s group, where the halogen-substitutes F and Br were introduced to the isoquinoline and phthalazine axial ligands, respectively.32 Under the optimized catalytic conditions, both the Ru-bda catalysts with 6-Fisoq (32) and 6-Br-ptz (33) showed much higher catalytic performances than their mother structures. TOF of 32 was as high as 1,000 s−1, and a record TON of 100,000 was achieved by 33. In addition, TON of 33 (100,000) over four times higher than 32 (24,000) further confirmed the prediction of the stable axial ligand in Figure 6b, i.e., ptz is a suitable axial ligand to obtain robust Ru-bda catalysts. Detailed and systematic investigations on the electronic effect and non-covalent effects of Ru-bda catalysts by varying axial ligands have also been carried out by the Murata and Concepcion groups.45-46 Murata et al. studied the physical properties and chemical/photochemical catalytic activities of Ru-bda catalysts with axial ligands from strong electron-donating pyridine to strong electron-withdrawing pyridine, including 4-MeO-py (21), 4-Me-py (10), py (18), 4-Br-py (19), MeOOC-py (23) and 4-CF3-py (27), with Hammett constants of −0.27, −0.17, 0, 0.23, 0.45 and 0.54 respectively.45 Concepcion et al. focused on two series of halogen substituted Ru-bda catalysts, [Ru(bda)(4-X-py)2] and [Ru(bda)(6-X-isoq)2] (X = H, F, Cl, Br, I), to systematically reveal the noncovalent effects induced by the halogen substituent (e.g., halogen−aromatic interactions, dipole−induced dipole, hydrophobic effect).46 The big size and easily polarized iodine can facilitate the intermolecular interactions to accelerate the OO bond formation. However, for the isoquinoline series, when the halogen atom is bigger than F atom, the involvement of the heavy and large halogen atoms obstructs the π-π overlap, leading to a decrease of activity. Guided by the DFT calculations, Concepcion and coworkers obtained Ru-bda catalysts with 6-MeO-isoq (34) and 6,7-MeO-isoq (37) axial ligands with TOF for water oxidation over 1000 s−1. Indeed, 34 was earlier reported by Llobet 12


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et al. with a TOF of 900 s−1, together with an extremely slow Ru-bda catalyst with a positively charged axial ligand (30, TOF less than 1.0 s−1).47 The poor performance of 30 is attributed to the restraint of intermolecular interaction by the electrostatic repulsion. In a recent work, Gao and coworkers reported 46a and 46b to demonstrate the effect of a dangling intramolecular base.48 In addition to the pyridine-like axial ligand, Sun’s group explored several Ru-bda catalysts with axial ligands of five-membered aromatic N-heterocycles, including imidazole (38, 39, 40),27 N-heterocycle carbene (41, 42)49 and pyrazole (43, 44, 45).50 In general, a Ru-bda catalyst possessing the imidazole or pyrazole axial ligand can also have comparable performance (highest TOF of 180 s−1 for 40d and 500 s−1 for 44) for water-oxidation catalysis as the typical Ru-bda catalysts with pyridine-like axial ligands, with similar electrochemical properties and catalytic mechanism. However, with the Nheterocycle carbene, 42 is determined with a TOF of only 0.04 s−1 and the WNA mechanism for OO bond formation.49 Interestingly, the author suggested that the catalytic active site was not the H2O in the axial position but the substrate H2O coordinated at the open site as the seventh coordination site. The above reported structural modification of Ru-bda catalysts fully exhibited the variability and significance of the axial ligand of a Ru-bda catalyst. Because water-oxidation mechanism by Ru-bda catalysts goes through a bimolecular I2M pathway, the π-π stacking effect, hydrophobic effect, electrostatic effect and other non-covalent effects, which can effectively impact the intermolecular interaction, are predominant factors to the catalytic performance. The electronic effect can effectively affect the RuII/RuIII redox potential of Ru-bda; however, it plays only a negligible role to the RuIII/RuIV and RuIV/RuV redox processes;30, 32, 35, 46 consequently it shows secondary influence to the catalytic efficiency. The studies on the effect of an axial ligand of Ru-bda catalysts have been extensively conducted, and principles to choose suitable axial ligands for efficient Ru-bda catalysts have been revealed. 5.2 Ru-bda derivatives with substituted bda2 backbone The studies of [RuII(tpy)(bpy)(OH2)]2+ by Berlinguette’s group indicated that substituents trans to the oxo moiety of the intermediate RuV=O had a more pronounced effect.51 Because the active site of the Ru-bda catalyst is in the equatorial coordination plane, modification of the bda2− backbone is expected to have a remarkable electronic effect on the catalytic active site. However, the amount of research on the structural modifications on the bda-ligand is much less than that on the axial ligand, due to the challenge of synthesizing substituted bda-ligands.

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Gao et al. studied the effects of Br substituents on the bda2 backbone with complexes 47a-c.52 The catalytic performances of 47a-c are obviously lower than the relevant unsubstituted catalysts. Interestingly, the authors found that non-integral reaction orders were determined for 47a and 47c, which indicated that the introduction of Br substituents to the bda2 backbone maybe lead to a partial conversion from an I2M to a WNA mechanism for the OO bond formation. Concepcion and coworkers investigated the effects of CF3 substituents on the bda2 backbone with complexes 48ad.53 The authors emphasized the observation of two kinetics regimes depending on catalyst concentration based on the kinetics investigations by the decay of [CeIV]. They stated that the ratedetermining step was a proton coupled oxidation of the catalyst by CeIV at a relatively high catalyst concentration (ca. ≥10−4 M), according to the observations of first-order kinetics in both CeIV and catalyst, and of direct kinetic isotope effects (KIE > 1). Richmond et al. modified the bda2 backbone with two benzene rings to form a biisoquinoline dicarboxylate ligand (biqa).54 After coordinating with a Ru center, the obtained Ru complexes (49a and 49b) have very similar first coordination configuration to the parent Ru-bda complexes, but a strain is formed on the backbone. It is concluded that inducing non-planarity in the equatorial coordination plane plays a negative role on the catalytic activity, which led to a transformation from the I2M mechanism to the WNA mechanism. However, we recently researched a Ru-bda catalyst with two sterically hindered carboxylate groups at the 3,3′-position of the bda2 backbone (50). Interestingly, complex 50 adopted the I2M pathway for OO bond formation. This difference may be because it has less strain than in the case of 49, but this may also arise from other factors that facilitate intermolecular interactions. Complexes 51a and 51b with carboxylic groups were synthesized for photochemical, electrochemical and photoelectrochemical water oxidation by being immobilized on the conductive and functionalized electrodes.39 Complex 52 with phosphonophenyl groups was anchored on a porous ITO electrode for in situ X-ray absorption to track the key RuV=O intermediate.34 Although detailed evaluations on catalytic activity under CeIV-driven water-oxidation experiments were not conducted, these modified Ru-bda catalysts on the bda2 backbone still showed similar catalytic features, such as low overpotential and relevant redox potentials, as the unsubstituted Ru-bda parent structure. In general, introduction of substituents to the bda2 backbone will influence the catalytic activity of Ru-bda catalysts, not only the efficiency but also mechanistic changes. Owing to the deficiency in the number of investigations, it is still too early to give a credible conclusion about how the 14


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electronic effect of substituents on the bda2 backbone impacts the catalysis of water oxidation by Ru-bda catalysts, and if there are any other effects that influence the intermolecular interactions of Ru-bda catalysts during water-oxidation catalysis. 6.

Application of Ru-bda catalysts in water-splitting devices

Since the Ru-bda catalyst was reported in 2009,26 many functionalized Ru-bda catalysts have been applied to modify photoanodes and anodes for efficient photoelectrochemical and electrochemical water oxidation. Thanks to the extremely high catalytic rate and low overpotential of Ru-bda catalyst, the photoanodes and anodes modified with Ru-bda catalysts have displayed outstanding performances compared with other previously reported MWOC decorated electrodes. 6.1 Ru-bda catalysts-modified water-oxidation photoanodes For a long time, the development of dye-sensitized PEC cells (DSPEC cells) was largely limited by the lack of promising MWOCs with fast catalytic rate and low overpotential.55 The presence of Rubda catalysts has made many designs for visible light-driven water oxidation possible, and has dramatically improved the performances of DSPEC cells. Sun and Li’s groups reported homogeneous photocatalytic water oxidation using three-component molecular systems (Figure 7a),28, 56 two-component supramolecular assembly systems (Figure 7b)57-58 and two-component host– guest interaction systems.59 A remarkable quantum efficiency of 84% was achieved by the twocomponent host–guest interaction systems. Based on the success of homogeneous photocatalytic water oxidation by Ru-bda catalysts, DSPEC cells have also been fabricated with Ru-bda catalysts-modified photoanodes. Immobilization of [RuII(bda)(pic)] by Nafion on a dye-sensitized TiO2 film results in a photoanode for visible lightdriven water oxidation with a photocurrent density of 30 μA cm−2.60 Later, Sun and Gao’s groups reported a photoanode in 2013 by self-assembled co-adsorption of a Ru-based dye and Ru-bda WOC on TiO2 film, which made a breakthrough in improving the performance of a MWOCs-modified photoanode (Figure 7c). With an external bias of 0.2 V vs. NHE, the photoanode showed a high initial photocurrent density of more than 1.7 mA cm−2 and a steady photocurrent density of 0.8 mA cm−2 for water oxidation.61 Obviously, the remarkable performance of the photoanode arose from the excellent catalytic activity of the Ru-bda catalyst. The simple but effective self-assembled coadsorption method is also essential, when compared with the poor performance of the above photoanode, in which the Ru-bda catalyst was entrapped in a Nafion polymer layer. 60

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Figure 7. Photocatalytic systems for homogeneous water-oxidation by Ru-bda catalysts and Ru-bda catalysts modified photoanodes. Reproduced with permission from Refs [28], [57], [61], [62] and [63]. Copyright American Chemical Society, 2010, Wiley, 2012, and American Chemical Society 2013, 2016 and 2017, respectively. Later, Sun’s group developed more Ru-bda catalysts-modified photoanodes to further investigate the behavior of Ru-bda catalysts with different structure design64-65 and to explore new strategies for stable immobilization of Ru-bda catalysts on photoanodes.66-67 The works of Sun and coworkers fully displayed the benefits of using state-of-the-art Ru-bda MWOCs in DSPEC cells. Subsequently, Meyer’s group and other groups also started to employ Ru-bda catalysts in their research of DSPEC cells to promote the device performances.62-63,

68-71

For example, Concepcion et al. reported a

photoanode with a covalently linked Ru-based chromophore-Ru-bda-catalyst assembly (Figure 7d).62 Meyer and coworkers achieved photocatalytic water oxidation by using Ru-bda catalysts and surface-bound organic chromophores (Figure 7e).63 However, the most challenging problem thus far is to enhance the stability of these DSPEC cells. The deeper understanding of the catalytic mechanisms of Ru-bda catalysts, the promotion of activities of Ru-bda catalysts and the advance of modification strategies are expected to provide more efficient and more stable Ru-bda catalystsmodified photoanodes for DSPEC cells. 16


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6.2 Ru-bda catalysts-engineered water-oxidation anode materials Anodes with low overpotential and high efficiency are still required for water electrolysis technologies, especially for the proton exchange membrane (PEM) electrolyzer.72 Ru-bda catalysts with a fast catalytic rate and low overpotential can generate promising anode materials for watersplitting electrolyzers. In 2011, Sun’s group successfully immobilized a Ru-bda catalyst on a carbon nanotubes deposited ITO electrode through ππ stacking of pyrene groups (Figure 8a).73 For the obtained hybrid anode, the molecular nature of the Ru-bda catalyst was retained at the heterogeneous surface with the clear redox couples of RuII/RuIII at 0.59 V and RuIII/RuIV at 0.91 V, which resulted in a remarkable efficiency of water oxidation with low overpotential. The introduction of highly conductive carbon nanotubes for fast electron transfer is essential to display the complete features of the Ru-bda catalyst. This proposal was later supported by the findings of Meyer et al.. They found that an electron mediator, for example, [Ru(bpy)3]2+, is needed for the oxidative activation of the Rubda catalysts (Figure 8b).74-75 The behaviors of the pyrene functionalized Ru-bda catalyst on carbon nanotubes have recently been simulated by theoretical calculations based on the EVB model (Figure 8c).76 The ππ stacking strategy has also been widely adopted in the subsequent works for modification of MWOCs on carbon materials.77-78

Figure 8. a) Ru-bda catalyst immobilized on carbon nanotube electrodes and the CVs in Na2SO4 solution. b) Schematic diagram showing the key effect of modifying an electron mediator on a metal oxide electrode. c) EVB simulations for the hybrid material shown in a). Reproduced with permission from Refs [74] and [76]. Copyright American Chemical Society, 2015 and 2018, respectively.

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To achieve a more stable modification and to increase the loading amount of Ru-bda catalysts, Sun and coworkers further functionalized Ru-bda catalysts with pyrrole, thiophene or vinyl groups to directly electrochemically polymerize the Ru-bda catalysts on electrodes to form anodes for water oxidation.67,

79-80

The loading amounts of the Ru-bda catalysts were dramatically increased, and a

much higher catalytic current density was achieved, but the stabilities of these anodes were still not good enough to be used in applicable devices. The decomposition of the Ru-bda catalyst covalently anchored on glass carbon electrodes was carefully studied by Llobet et al..81 We noted that this fast decomposition could be attributed to the covalent anchoring method, which prevented the OO coupling step. Recently, many classical protocols have been adopted to produce Ru-bda catalysts based materials, such as metal-organic frameworks (MOFs)82-83 and a vertical step-growth polymerization.84 Nevertheless, we suggest that an appropriate evaluation method is urgently required to compare MWOCs-generated materials with commercial catalysts. The lack of standard test conditions and evaluation systems has limited the development of MWOCs-generated materials for water-oxidation anodes. 6.3 Taking advantage of the fast bimolecular catalysis feature of Ru-bda catalysts Low overpotential, high catalytic TOF and O-O bond formation through I2M are the three important features that make the Ru-bda catalyst standout from all reported MWOCs. The first two points are no doubt greatly favorable for assembling efficient water-oxidation electrodes. However, the bimolecular catalytic feature is considered a drawback of the Ru-bda catalyst for its application in water-splitting devices, because in contrast to the MWOCs that exhibit mononuclear catalysis through the WNA mechanism, where the catalytic TOF is independent of the catalyst concentration, a certain concentration of the catalyst is required to achieve a promising catalytic rate, as shown in Figure 9a. However, the OO coupling step may be dramatically prevented when the Ru-bda is anchored on the electrode surface inappropriately, thereby making the immobilization of Ru-bda catalyst on an electrode more challenging.

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Figure 9. a) Dependence of catalytic TOF with catalyst concentration. b) Axial ligand effect enhanced intermolecular interaction. c) Supramolecular binuclear and trinuclear Ru-bda catalysts. d) Ru-bda based MWOCs confined in a nanocage of SBA-16. e) Ru-bda catalysts entrapped in an assembled guanidinium nanosphere. f) Self-assembled amphiphilic Ru-bda catalysts. Reproduced with permission from Ref [85], [86] and [87]. Copyright Royal Society of Chemistry, 2012, and Wiley, 2018 and 2016, respectively. However, it has recently been realized that the energy barrier for the formation of an OO bond through the pre-reactive [RuIV−O−O−RuIV] intermediate is very low, so the main barrier for the oxygen evolution by a Ru-bda catalyst is from the collision, namely OO coupling of two RuV=O (i.e., RuIV−O˙) to form the pre-reactive [RuIV−O−O−RuIV], especially under extremely diluted conditions.31,

41, 46

In this case, a super-high catalytic rate can be expected if the intramolecular

interaction is realized suitably or a high local concentration is facilitated as a TOF of 1000 s−1 has been determined with a high concentration of 32 in CeIV-driven homogeneous water oxidation.32

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Several strategies have been carried out to accelerate the rate of the OO coupling step to acquire a high catalytic rate for water oxidation, such as by enhancing the intermolecular interaction of Ru-bda catalysts through a π-π stacking effect and other non-covalent effects of axial ligands (Figure 9b),30, 32, 43, 46-47

by forming binuclear or multinuclear Ru-bda catalysts to switch water-oxidation catalysis

from intermolecular catalysis to intramolecular catalysis (Figure 9c)88-93 and by entrapping the Rubda catalysts into nanocages to achieve high local concentrations (Figures 9d-f).85-87 Note that although the designs in catalyst 58 switched the I2M mechanism to a WNA mechanism, a high TOF was still obtained thanks to the intramolecular H2O and H+ networks.88-89,

92-93

More designs and

investigations are necessary to provide a sufficient basis of protocols for the development of Ru-bda catalyst based applicable water-splitting devices. 7.

Carboxylate ligand and 7-coordination − the reasons behind the low overpotential and high catalytic rate

With the intensive studies on the family of Ru-bda catalysts during the past ten years, there is now more evidence for why Ru-bda catalysts exhibit an outstandingly high catalytic performance and extremely low overpotential, which will certainly enlighten the design of more efficient and robust MWOCs. As previously discussed in Section 2, the performance of a catalyst is influenced by the stability and reactivity of the high-valent M=O intermediate, which is dominated by the electronic nature of the metal center and the coordination environment offered by the ligands. Therefore, the challenge to obtain an efficient WOC is to generate high-valent M=O species with good stability and reactivity, by fine design of the molecule structure. A systematic comparison of Ru-bda catalysts with other less active Ru-based MWOCs in the structural distinctions as well as the differences of generation, stability and reactivity of RuV=O can help to address the above questions. Selected typical Ru-based MWOCs are listed from left to right according to the types of the generated reactive RuV=O (e.g., 6-coordinate axial oxo, 6-coordinate equatorial oxo and 7-coordinate equatorial oxo), and from top to bottom according to the number of carboxylate groups in the ligands (Figure 10). By comparing the performance of catalysts in the first two rows, it generally indicates that the 6-coordinate equatorial oxo is more reactive than the 6-coordinate axial oxo. The different reactivities of the 6-coordinate axial oxo and 6-coordinate equatorial oxo can be explained by the cistrans-effect.51, 94 The ligand exchange of 62 and 65 takes place on the equatorial coordination instead of the axial coordination, and the water oxidation occurring on the equatorial site of 42 can also be a good sign that the equatorial site is more variable and preferable as an active site.49 However, since water oxidation by 62, 64 and 65 involve ligand exchange of pyridine,95-96 contradistinctive

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investigations on the corresponding aqua coordinated complexes are required to further confirm this conclusion.

Figure 10. Selected different types of representative Ru-based MWOCs for comparison. Ru-based MWOCs 7,51 59,97 60,98 61,95 62,95 63,99 64,96 65,96 10.27, 30 Catalysts in the last column are special with a new open site, the seventh coordination site, as catalytic active site owing to the highly distorted configuration. The reactive high-valent Ru=O involved in the catalysis by these catalysts are 7-coordinate equatorial oxo with distinctive pentagonal bipyramidal geometry. The unique 7-coordinate equatorial oxo may have a very different reactive nature, making it more reactive. Nevertheless, complexes 60 and 63 showed no outstanding performance for water oxidation compared with the other catalysts with 6-coordinate RuV=O, because 60 changed its structure by oxidation of the side pyridine to pyridine nitrogen oxide, during water-oxidation catalysis, and 63 may also suffer from similar unexpected structural change.98 On the other hand, the introduction of carboxylate groups obviously leads to a decrease of the oxidation potential for the generation of high-valent Ru=O species, and it has been widely accepted that negatively charged groups can invrease the stability of high-valent M=O.3, 11, 100 Consequently, the catalytic activities are remarkably increased for MWOCs with negative charged ligands, not only for the examples shown in Figure 10 but also many other Ru-based WOCs reported since Sun’s group verified this strategy in 2009.101-104 The Ru-bda at the bottom right corner with the seven-

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coordination feature and two carboxylate groups showed remarkably outstanding activity for water oxidation. In general, these two structural features result in the highly efficient activity of the Ru-bda catalyst.

Figure 11. a) Analogs of Ru-bda catalysts for comparison. b) Structural function analysis of the Rubda catalyst. Addressing the following questions can help to understand the 7-coordination of the Ru-bda catalysts. What is 7-coordination? How can one obtain 7-coordination? What is the effect of 7-coordination on water-oxidation catalysis? What are the differences between the 7-coordinate equatorial oxo and the 6-coordinate equatorial oxo? A simple way to understand 7-coordination is to add a vertex (ligand) to the regular octahedron along an octahedral edge or into an octahedral face, accompanied by a regional distortion of the octahedron, which will lead to an idealized 7-coordinate structure, such as a pentagonal bipyramid, capped octahedron and capped trigonal prism.105 The pentagonal bipyramid is the most commonly found polyhedron in 7-coordinate monomers, which normally are composed with pentadentate ligands to completely occupy the girdle.105 However, a MWOC needs at least one

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of the seven coordination sites to be variable as a catalytic site. Therefore, to make an MWOC with a 7-coordination feature, a distorted tetradentate planar ligand, as the case of bda2−, is a smart design (Figure 11b). In addition to the example of Ru-bda, Thummel et al. reported that a complex with distorted tridentate ligands can also result in a 7-coordinate intermediate and makes the complex active for catalyzing water oxidation.106

Figure 12. Schematic diagram showing the catalyst design strategy, distortion induced extracoordination as a catalytic site. Bottom: original d-orbital splitting diagrams for the M=O complex with a pentagonal-bipyramidal ligand field. Several benefits can be expected from the 7-coordination featured complexes for water-oxidation catalysis. First, the seventh coordination is much more variable that facilitating the ligands exchanging with substrate H2O; moreover, the variable seventh coordination in many cases is unoccupied, meaning that there is a ready site to direct bind the substrate H2O. Second, 7-coordinate complexes are more tolerance to oxidation. Even at a RuIV state, the metal complex still can keep an 18-electron structure.107 This makes the high-valent intermediate more accessible and more stable. Third, the nature of the 7-coordinate equatorial oxo is different from the 6-coordinate equatorial oxo, as shown by the d-orbital splitting pattern in Figure 12.105 It has a higher spin density, displays more radical character, and is more reactive for the I2M pathway, while the 6-coordinate equatorial oxo is more electrophilic because of the more required π-donating from the oxo to the metal center.99 Finally, after the O−O bond formation, the 7-coordinate intermediate can directly dissociate O2, and transforms back to the initial 6-coordinate stable structure without needing to take any compensatory ligands.11

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The conclusion can be supported by comparing the properties and activities of Ru-bda analogs, which contain two negatively charged groups or possess a similar distorted configuration as Ru-bda (Figure 11a). These studies have led to a deeper understanding of the effect of the 7-coordination and the carboxylate groups of the Ru-bda catalyst. Complex 65 has a similar electron-donating coordination sphere as the Ru-bda catalyst, but it has one pyridine less, leading to the loss of the 7coordination feature.96 In the case of 66, one more pyridine was inserted to the bda2− ligand to give a pentadentate planar ligand. As a result of this change, the crystal structure of 66 at the RuIV state showed a typical pentagonal bipyramidal configuration without the involvement of the oxidation of H2O; however, 66 can efficiently catalyze water oxidation in basic solutions, after one of the carboxylate groups is displaced by a hydroxide, which clearly indicates the importance of leaving a variable coordination site as a catalytic site.108-110 The far lower activities of these two catalysts fully support the above analysis of designing MWOCs with a 7-coordination feature as the Ru-bda catalyst. The two carboxylate groups also play an essential role in the catalytic performances of Ru-bda catalysts (Figure 11b). There are three evident effects of the carboxylate groups related to its negatively charged property. It makes the oxidation of the Ru-bda to a high-valent intermediate at low potential, and it increases the pKa value of the intermediates, e.g., RuIVOH, which also makes the high-valent intermediate more accessible through the PCET process. In addition, the negatively charged ligand is effective to stabilize high-valent M=O, and affect the nature of high-valent M=O. Thirdly, the carboxylate groups may assist proton transfer during water oxidation. Furthermore, the coordination flexibility and the small steric hindrance of carboxylate groups benefit greatly to the water oxidation by Ru-bda catalysts. The electron number of the Ru-bda complex at a lower oxidation state, RuII and RuIII, is 18 and 17 electrons, respectively. This makes the binding of substrate H2O unfavorable because of the full-filled electronic structure. This problem is solved by dissociating one of the carboxylate groups to create a lower electron filling to facilitate the substrate H2O binding, indicating the significance of the coordination flexibility of the carboxylate group. The small steric hindrance is also essential for a fast binding of substrate H2O at the seventh coordination and for a fast OO coupling of two high-valent Ru-bda-oxo intermediate. Recent research from the groups of Sun and Concepcion can demonstrate the effects of the two carboxylate groups. Replacing one of the carboxylate groups with pyridine, Sun and coworkers investigated 67 to demonstrate the necessity of having two carboxylate groups.99 With only one carboxylate group, the oxidation potential of 67 was remarkably shifted to a higher potential, for example, an anodic shift of 300 mV for the RuII/RuIII oxidation. Not only showed poor catalytic

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performance by 67, but also the catalytic mechanism switched from I2M to WNA. This proved the negative charge effects and the significance of a small steric hindrance. Instead of two carboxylate groups, Concepcion et al. reported a bda2 analog ligand, the 2,2′-bipyridine-6,6′-diphosphonic acid (bpaH4), with two phosphonate groups as the negatively charged moieties.10 After coordination with Ru ions, the formed Ru-bpaH2 complex (68) showed a similar structure as the Ru-bda complex. However, the electrochemical behavior of the Ru-bpaH2 complex was different from that of the Rubda complex. The catalytic activity dramatically decreased and the catalytic mechanism changed to the WNA mechanism. With two phosphonate groups replacing two carboxylate groups, the Ru complex loses its coordination lability, and H2O cannot bind to Ru in the +2 and +3 state. Subsequently, Concepcion and coworkers replacing only one of the carboxylate groups by one phosphonate group, forming a hybrid ligand 2,2′-bipyridine-6-phosphonic acid-6′-carboxylic acid (bpHc).111 Not only did the fast catalytic rate return for the Ru-bpHc catalyst (69), but also the presence of H2O on the Ru center with a low oxidation state was observed. These two works clearly verified the significance of the coordination flexibility of the carboxylate groups in the Ru-bda catalyst. 8.

Conclusions and future challenges

The past 10 years successful research on the Ru-bda catalysts for water oxidation, including the mechanism study, promotion of the activity by structural design and application in devices, have well demonstrated the advantages of developing molecular catalysts, which is the best way to understand the structure-activity relationships and to dramatically promote the intrinsic catalytic activities of catalysts. This has encouraged the advance of molecular catalysis. After a careful and comprehensive summary of the investigations on Ru-bda-based catalysts, we concluded that the extremely high efficiency and low onset overpotential of the Ru-bda catalyst originate from the teamwork of its unique structural features: the 7-coordination and the carboxylate groups. Inspired by the Ru-bda catalysts, we propose in this Perspective a new concept for the design of transition metal complex-based molecular catalysts. During the development of MWOCs, metal complexes with regular configurations, such as 6-coordinate octahedral geometry and 4-coordinate square planar, have extensively been designed by leaving one or two labile coordination sites as an active site for water oxidation. Instead of leaving labile coordination sites as an active site, we emphasized that distorting the regular configuration of the metal complex by ligand design to create an open site wide enough for an extra coordination (i.e., 7-coordination or 5-coordination) as a catalytic active site can lead to promising catalysts for not only the water oxidation reaction but also

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other small molecule activation and conversion reactions, such as CO2 reduction and N2 fixation reactions. In brief, to make an extra coordination position open in the catalyst structure as catalytic site is a key to create a good catalyst (Figure 12). Regarding the introduction of carboxylate groups, it has been well demonstrated, not only by the Rubda series but also many other recently reported catalysts, as an effective strategy to improve the catalytic activity of MWOCs results from their function of reducing the oxidation potential of metal centers, stabilizing high-valent intermediates, and involving the proton transfer. More extensive designs based on this strategy are necessary. In addition, we believe this strategy may not be limited to using carboxylate groups,112 but it can be extended to other negatively charged ligands, for example, sulfonic acid group, phenolic group and pyrrole. Subsequently, we suggest the σ-donating ability, steric hindrance and coordination flexibility of these substituents are necessary to be considered when they are employed for the design of MWOCs. Although high catalytic rates and low overpotential for water oxidation have been achieved by MWOCs, another challenge to fully show that the molecular catalyst is an important solution to the catalytic issues of artificial photosynthesis is the application of molecular catalysts in promising artificial photosynthesis devices. Nevertheless, many problems need to be considered and solved for the adoption of molecular catalysts into devices, such as the method of loading molecular catalyst, the effect of electrolyte, and the device design for mass transfer and proton transfer. Accordingly, the requirements of molecular catalysts are much more than simply a high intrinsic activity, which is only one of the most fundamental issues. Because the Ru-bda catalyst has been developed with a low overpotential, excellent catalytic rate, and comparatively good stability, it provides an opportunity to advance the methods and technologies for the employment of molecular catalyst in applicable devices for water-splitting. Development of efficient earth-abundant-metal based MWOCs is still one of the greatest challenges.113 An advantageous aspect is that a toolbox is now available to allow for reasonable design of more efficient MWOCs, for example, the strategies highlighted in this Perspective such as the distorted configuration induced 7-coordination and the introduction of negatively charged ligands. These well-proven principles can be used to facilitate the development of more efficient earthabundant-metal based MWOCs. Therefore, it is meaningful to further study the Ru-bda family to obtain more knowledge for the design of MWOCs, since the understanding of Ru-bda is not yet complete. For instance, the catalyst 70 has all the positive structural features as the Ru-bda catalyst; however, it possesses a very low catalytic performance and the WNA mechanism for water-oxidation,

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which indicates there must be other factors that influence the catalytic activity of the Ru-bda catalyst.114 Further research on Ru-based MWOCs designed based on the inspirations from the Rubda catalyst, for example the structures proposed in a computational survey,94 may lead to more complete principles for the design of efficient MWOCs. Corresponding Author: [email protected] Acknowledgments The authors would like to acknowledge the financial support from the Swedish Research Council (2017-00935), Swedish Energy Agency, the Knut and Alice Wallenberg Foundation, the National Natural Science Foundation of China (21120102036), and the National Basic Research Program of China (973 program, 2014CB239402). Conflict of interest The authors declare no conflict of interest. References (1) Kärkäs, M. D.; Verho, O.; Johnston, E. V.; Åkermark, B. Artificial Photosynthesis: Molecular Systems for Catalytic Water Oxidation. Chem. Rev. 2014, 114, 11863-12001. (2) Tong, L.; Thummel, R. P. Mononuclear Ruthenium Polypyridine Complexes that Catalyze Water Oxidation. Chem. Sci. 2016, 7, 6591-6603. (3) Garrido-Barros, P.; Gimbert-Surinach, C.; Matheu, R.; Sala, X.; Llobet, A. How to Make an Efficient and Robust Molecular Catalyst for Water Oxidation. Chem. Soc. Rev. 2017, 46, 6088-6098. (4) Kern, J.; Chatterjee, R.; Young, I. D.; Fuller, F. D.; Lassalle, L.; Ibrahim, M.; Gul, S.; Fransson, T.; Brewster, A. S.; Alonso-Mori, R.; Hussein, R.; Zhang, M.; Douthit, L.; de Lichtenberg, C.; Cheah, M. H.; Shevela, D.; Wersig, J.; Seuffert, I.; Sokaras, D.; Pastor, E.; Weninger, C.; Kroll, T.; Sierra, R. G.; Aller, P.; Butryn, A.; Orville, A. M.; Liang, M.; Batyuk, A.; Koglin, J. E.; Carbajo, S.; Boutet, S.; Moriarty, N. W.; Holton, J. M.; Dobbek, H.; Adams, P. D.; Bergmann, U.; Sauter, N. K.; Zouni, A.; Messinger, J.; Yano, J.; Yachandra, V. K. Structures of the Intermediates of Kok's Photosynthetic Water Oxidation Clock. Nature 2018, 563, 421-425. (5) Umena, Y.; Kawakami, K.; Shen, J. R.; Kamiya, N. Crystal Structure of Oxygen-Evolving Photosystem II at a Resolution of 1.9 Å. Nature 2011, 473, 55-60. (6) Yano, J.; Yachandra, V. Mn4Ca Cluster in Photosynthesis: Where and How Water is Oxidized to Dioxygen. Chem. Rev. 2014, 114, 4175-4205.

27



Page 28 of 38

(7) Betley, T. A.; Wu, Q.; Voorhis, T. V.; Nocera, D. G. Electronic Design Criteria for O-O Bond Formation via Metal-Oxo Complexes. Inorg. Chem. 2008, 47, 1849-1861. (8) Dobson, J. C.; Meyer, T. J. Redox Properties and Ligand Loss Chemistry in Aqua/Hydroxo/Oxo Complexes Derived from cis- and trans-[(bpy)2RuIIOH2)2]2+. Inorg. Chem. 1988, 27, 3283-3291. (9) Huynh, M. H. V.; Meyer, T. J. Proton-Coupled Electron Transfer. Chem. Rev. 2007, 107, 5004−5064. (10) Xie, Y.; Shaffer, D. W.; Lewandowska-Andralojc, A.; Szalda, D. J.; Concepcion, J. J. Water Oxidation by Ruthenium Complexes Incorporating Multifunctional Bipyridyl Diphosphonate Ligands. Angew. Chem. Int. Ed. 2016, 55, 8067-8071. (11) Shaffer, D. W.; Xie, Y.; Concepcion, J. J. O-O Bond Formation in Ruthenium-Catalyzed Water Oxidation: Single-Site Nucleophilic Attack vs. O-O Radical Coupling. Chem. Soc. Rev. 2017, 46, 6170-6193. (12) Romanin, S.; Vigara, L.; Llobet, A. Oxygen-Oxygen Bond Formation Pathways Promoted by Ruthenium Complexes. Acc. Chem. Res. 2009, 42, 1944-1953. (13) Zhang, B.; Sun, L. Why Nature Chose the Mn4CaO5 Cluster as Water-Splitting Catalyst in Photosystem II: A New Hypothesis for the Mechanism of O-O Bond Formation. Dalton Trans. 2018, 47, 14381-14387. (14) Gersten, S. W.; Samuels, G. J.; Meyer, T. J. Catalytic Oxidation of Water by an Oxo-Bridged Ruthenium Dimer. J. Am. Chem. Soc. 1982, 104, 4029-4030. (15) Collin, J. P.; Sauvage, J. P. Synthesis and Study of Mononuclear Ruthenium(II) Complexes of Sterically Hindering Diimine Chelates. Implications for the Catalytic Oxidation of Water to Molecular Oxygen. Inorg. Chem. 1986, 25, 135-141. (16) Sens, C.; Romero, I.; Rodríguez, M.; Llobet, A.; Parella, T.; Benet-Buchholz, J. A New Ru Complex Capable of Catalytically Oxidizing Water to Molecular Dioxygen. J. Am. Chem. Soc. 2004, 126, 7798-7799. (17) Zong, R.; Thummel, R. P. A New Family of Ru Complexes for Water Oxidation. J. Am. Chem. Soc. 2005, 127, 12802-12803. (18) Deng, Z.; Tseng, H.-W.; Zong, R.; Wang, D.; Thummel, R. Preparation and Study of a Family of Dinuclear Ru(II) Complexes that Catalyze the Decomposition of Water. Inorg. Chem. 2008, 47, 1835-1848. (19) Polyansky, D. E.; Muckerman, J. T.; Rochford, J.; Zong, R.; Thummel, R. P.; Fujita, E. Water Oxidation by a Mononuclear Ruthenium Catalyst: Characterization of the Intermediates. J. Am. Chem. Soc. 2011, 133, 14649-14665.

28


Page 29 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(20) Concepcion, J. J.; Jurss, J. W.; Templeton, J. L.; Meyer, T. J. One Site is Enough. Catalytic Water Oxidation by [Ru(tpy)(bpm)(OH2)]2+ and [Ru(tpy)(bpz)(OH2)]2+. J. Am. Chem. Soc. 2008, 130, 16462-16463. (21) Wasylenko, D. J.; Palmer, R. D.; Berlinguette, C. P. Homogeneous Water Oxidation Catalysts Containing a Single Metal Site. Chem. Commun. 2013, 49, 218-227. (22) Lomoth, R.; Huang, P.; Zheng, J.; Sun, L.; Hammarström, L.; Åkermark, B.; Styring, S. Synthesis and Characterization of a Dinuclear Manganese(III,III) Complex with Three Phenolate Ligands. Eur. J. Inorg. Chem. 2002, 2965-2974. (23) Xu, Y.; Åkermark, T.; Gyollai, V.; Zou, D.; Eriksson, L.; Duan, L.; Zhang, R.; Åkermark, B.; Sun, L. A New Dinuclear Ruthenium Complex as an Efficient Water Oxidation Catalyst. Inorg. Chem. 2009, 48, 2717-2719. (24) Xu, Y.; Fischer, A.; Duan, L.; Tong, L.; Gabrielsson, E.; Åkermark, B.; Sun, L. Chemical and Light-Driven Oxidation of Water Catalyzed by an Efficient Dinuclear Ruthenium Complex. Angew. Chem. Int. Ed. 2010, 49, 8934-8937. (25) Zong, R.; Thummel, R. P. 2,9-Di-(2′-pyridyl)-1,10-phenanthroline: A Tetradentate Ligand for Ru(II). J. Am. Chem. Soc. 2004, 126, 10800-10801. (26) Duan, L.; Fischer, A.; Xu, Y.; Sun, L. Isolated Seven-Coordinate Ru(IV) Dimer Complex with [HOHOH]- Bridging Ligand as an Intermediate for Catalytic Water Oxidation. J. Am. Chem. Soc. 2009, 131, 10397-10399. (27) Wang, L.; Duan, L.; Stewart, B.; Pu, M.; Liu, J.; Privalov, T.; Sun, L. Toward Controlling Water Oxidation Catalysis: Tunable Activity of Ruthenium Complexes with Axial Imidazole/DMSO Ligands. J. Am. Chem. Soc. 2012, 134, 18868-18880. (28) Duan, L.; Xu, Y.; Zhang, P.; Wang, M.; Sun, L. Visible Light-Driven Water Oxidation by a Molecular Ruthenium Catalyst in Homogeneous System. Inorg. Chem. 2010, 49, 209-215. (29) Nyhlén, J.; Duan, L.; Åkermark, B.; Sun, L.; Privalov, T. Evolution of O2 in a SevenCoordinate Ru(IV) Dimer Complex with a [HOHOH]- Bridge: A Computational Study. Angew. Chem. Int. Ed. 2010, 49, 1773-1777. (30) Duan, L.; Bozoglian, F.; Mandal, S.; Stewart, B.; Privalov, T.; Llobet, A.; Sun, L. A Molecular Ruthenium Catalyst with Water-Oxidation Activity Comparable to that of Photosystem II. Nat. Chem. 2012, 4, 418-423. (31) Zhan, S.; Zou, R.; Ahlquist, M. S. G. Dynamics with Explicit Solvation Reveals Formation of the Prereactive Dimer as Sole Determining Factor for the Efficiency of Ru(bda)L2 Catalysts. ACS Catal. 2018, 8, 8642-8648.

29



Page 30 of 38

(32) Wang, L.; Duan, L.; Wang, Y.; Ahlquist, M. S.; Sun, L. Highly Efficient and Robust Molecular Water Oxidation Catalysts Based on Ruthenium Complexes. Chem. Commun. 2014, 50, 1294712950. (33) Concepcion, J. J.; Zhong, D. K.; Szalda, D. J.; Muckerman, J. T.; Fujita, E. Mechanism of Water Oxidation by [Ru(bda)(L)2]: The Return of the "Blue Dimer". Chem. Commun. 2015, 51, 4105-4108. (34) Lebedev, D.; Pineda-Galvan, Y.; Tokimaru, Y.; Fedorov, A.; Kaeffer, N.; Copéret, C.; Pushkar, Y. The Key Ru(V)=O Intermediate of Site-Isolated Mononuclear Water Oxidation Catalyst Detected by in Situ X-ray Absorption Spectroscopy. J. Am. Chem. Soc. 2018, 140, 451-458. (35) Duan, L.; Wang, L.; Inge, A. K.; Fischer, A.; Zou, X.; Sun, L. Insights into Ru-Based Molecular Water Oxidation Catalysts: Electronic and Noncovalent-Interaction Effects on Their Catalytic Activities. Inorg. Chem. 2013, 52, 7844-7852. (36) Matheu, R.; Ghaderian, A.; Francàs, L.; Chernev, P.; Ertem, M. Z.; Benet-Buchholz, J.; Batista, V. S.; Haumann, M.; Gimbert-Suriñach, C.; Sala, X.; Llobet, A. Behavior of Ru-bda WaterOxidation Catalysts in Low Oxidation States. Chem. Eur. J. 2018, 24, 12838-12847. (37) Daniel, Q.; Huang, P.; Fan, T.; Wang, Y.; Duan, L.; Wang, L.; Li, F.; Rinkevicius, Z.; Mamedov, F.; Ahlquist, M. S. G.; Styring, S.; Sun, L. Rearranging from 6- to 7-Coordination Initiates the Catalytic Activity: An EPR Study on a Ru-bda Water Oxidation Catalyst. Coor. Chem. Rev. 2017, 346, 206-215. (38) Song, N.; Concepcion, J. J.; Binstead, R. A.; Rudd, J. A.; Vannucci, A. K.; Dares, C. J.; Coggins, M. K.; Meyer, T. J. Base-Enhanced Catalytic Water Oxidation by a Carboxylate-bipyridine Ru(II) Complex. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 4935-4940. (39) Li, T.-T.; Zhao, W.-L.; Chen, Y.; Li, F.-M.; Wang, C.-J.; Tian, Y.-H.; Fu, W.-F. Photochemical, Electrochemical, and Photoelectrochemical Water Oxidation Catalyzed by Water-Soluble Mononuclear Ruthenium Complexes. Chem. Eur. J. 2014, 20, 13957-13964. (40) Zhang, B.; Li, F.; Zhang, R.; Ma, C.; Chen, L.; Sun, L. Characterization of a Trinuclear Ruthenium Species in Catalytic Water Oxidation by Ru(bda)(pic)2 in Neutral Media. Chem. Commun. 2016, 52, 8619-8622. (41) Fan, T.; Zhan, S.; Ahlquist, M. S. G. Why Is There a Barrier in the Coupling of Two Radicals in the Water Oxidation Reaction? ACS Catal. 2016, 6, 8308-8312. (42) Zhan, S.; Martensson, D.; Purg, M.; Kamerlin, S. C. L.; Ahlquist, M. S. G. Capturing the Role of Explicit Solvent in the Dimerization of Ru(V) (bda) Water Oxidation Catalysts. Angew. Chem. Int. Ed. 2017, 56, 6962-6965.

30


Page 31 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(43) Duan, L.; Araujob, C. M.; Ahlquist, M. S. G.; Sun, L. Highly Efficient and Robust Molecular Ruthenium Catalysts for Water Oxidation. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15584-15588. (44) Jiang, Y.; Li, F.; Huang, F.; Zhang, B.; Sun, L. Chemical and Photocatalytic Water Oxidation by Mononuclear Ru Catalysts. Chin. J. Catal. 2013, 34, 1489-1495. (45) Sato, Y.; Takizawa, S.-y.; Murata, S. Substituent Effects on Physical Properties and Catalytic Activities toward Water Oxidation in Mononuclear Ruthenium Complexes. Eur. J. Inorg. Chem. 2015, 2015, 5495-5502. (46) Xie, Y.; Shaffer, D. W.; Concepcion, J. J. O-O Radical Coupling: From Detailed Mechanistic Understanding to Enhanced Water Oxidation Catalysis. Inorg. Chem. 2018, 57, 10533-10542. (47) Richmond, C. J.; Matheu, R.; Poater, A.; Falivene, L.; Benet-Buchholz, J.; Sala, X.; Cavallo, L.; Llobet, A. Supramolecular Water Oxidation with Ru-bda-Based Catalysts. Chem. Eur. J. 2014, 20, 17282-17286. (48) Liu, Z.; Gao, Y.; Zhang, M.; Liu, J. Design of a Dinuclear Ruthenium Based Catalyst with a Rigid Xanthene Bridge for Catalytic Water Oxidation. Inorg. Chem. Commun. 2015, 56-59. (49) Staehle, R.; Tong, L.; Wang, L.; Duan, L.; Fischer, A.; Ahlquist, M. S.; Sun, L.; Rau, S. Water Oxidation Catalyzed by Mononuclear Ruthenium Complexes with a 2,2'-bipyridine-6,6'dicarboxylate (bda) Ligand: How Ligand Environment Influences the Catalytic Behavior. Inorg. Chem. 2014, 53, 1307-1319. (50) Daniel, Q.; Wang, L.; Duan, L.; Li, F.; Sun, L. Tailored Design of Ruthenium Molecular Catalysts with 2,2'-Bypyridine-6,6'-Dicarboxylate and Pyrazole Based Ligands for Water Oxidation. Dalton Trans. 2016, 45, 14689-14696. (51) Wasylenko, D. J.; Ganesamoorthy, C.; Koivisto, B. D.; Henderson, M. A.; Berlinguette, C. P. Insight into Water Oxidation by Mononuclear Polypyridyl Ru Catalysts. Inorg. Chem. 2010, 49, 2202-2209. (52) Liu, Z.; Gao, Y.; Yu, Z.; Zhang, M.; Liu, J. Effects of Br Substituent on Catalytic Performance of Ru-bda (H2bda = 2,2′-bipyridine-6,6′-Dicarboxylic Acid) Catalysts for Water Oxidation. Chin. J. Catal. 2015, 36, 1742-1749. (53) Shaffer, D. W.; Xie, Y.; Szalda, D. J.; Concepcion, J. J. Manipulating the Rate-Limiting Step in Water Oxidation Catalysis by Ruthenium Bipyridine-Dicarboxylate Complexes. Inorg. Chem. 2016, 55, 12024-12035. (54) Scherrer, D.; Schilling, M.; Luber, S.; Fox, T.; Spingler, B.; Alberto, R.; Richmond, C. J. Ruthenium Water Oxidation Catalysts Containing the Non-Planar Tetradentate Ligand, Biisoquinoline Dicarboxylic Acid (BiqaH2). Dalton Trans. 2016, 45, 19361-19367.

31



Page 32 of 38

(55) O'Regan, B. C.; Durrant, J. R. Kinetic and Energetic Paradigms for Dye-Sensitized Solar Cells: Moving from the Ideal to the Real. Acc. Chem. Res. 2009, 42, 1799-1808. (56) Wang, L.; Duan, L.; Tong, L.; Sun, L. Visible Light-Driven Water Oxidation Catalyzed by Mononuclear Ruthenium Complexes. J. Catal. 2013, 306, 129-132. (57) Li, F.; Jiang, Y.; Zhang, B.; Huang, F.; Gao, Y.; Sun, L. Towards a Solar Fuel Device: LightDriven Water Oxidation Catalyzed by a Supramolecular Assembly. Angew. Chem. Int. Ed. 2012, 51, 2417-2420. (58) Wang, L.; Mirmohades, M.; Brown, A.; Duan, L.; Li, F.; Daniel, Q.; Lomoth, R.; Sun, L.; Hammarström, L. Sensitizer-Catalyst Assemblies for Water Oxidation. Inorg. Chem. 2015, 54, 27422751. (59) Li, H.; Li, F.; Zhang, B.; Zhou, X.; Yu, F.; Sun, L. Visible Light-Driven Water Oxidation Promoted by Host-Guest Interaction between Photosensitizer and Catalyst with a High Quantum Efficiency. J. Am. Chem. Soc. 2015, 137, 4332-4335. (60) Li, L.; Duan, L.; Xu, Y.; Gorlov, M.; Hagfeldt, A.; Sun, L. A Photoelectrochemical Device for Visible Light Driven Water Splitting by a Molecular Ruthenium Catalyst Assembled on DyeSensitized Nanostructured TiO2. Chem. Commun. 2010, 46, 7307-7309. (61) Gao, Y.; Ding, X.; Liu, J.; Wang, L.; Lu, Z.; Li, L.; Sun, L. Visible Light Driven Water Splitting in a Molecular Device with Unprecedentedly High Photocurrent Density. J. Am. Chem. Soc. 2013, 135, 4219-4222. (62) Sherman, B. D.; Xie, Y.; Sheridan, M. V.; Wang, D.; Shaffer, D. W.; Meyer, T. J.; Concepcion, J. J. Light-Driven Water Splitting by a Covalently Linked Ruthenium-Based Chromophore–Catalyst Assembly. ACS Energy Lett. 2016, 2, 124-128. (63) Eberhart, M. S.; Wang, D.; Sampaio, R. N.; Marquard, S. L.; Shan, B.; Brennaman, M. K.; Meyer, G. J.; Dares, C.; Meyer, T. J. Water Photo-oxidation Initiated by Surface-Bound Organic Chromophores. J. Am. Chem. Soc. 2017, 139, 16248-16255. (64) Gao, Y.; Zhang, L.; Ding, X.; Sun, L. Artificial Photosynthesis-Functional Devices for Light Driven Water Splitting with Photoactive Anodes Based on Molecular Catalysts. Phys. Chem. Chem. Phys. 2014, 16, 12008-12013. (65) Zhang, L.; Gao, Y.; Ding, X.; Yu, Z.; Sun, L. High-Performance Photoelectrochemical Cells Based on a Binuclear Ruthenium Catalyst for Visible-Light-Driven Water Oxidation. ChemSusChem 2014, 7, 2801-2804.

32


Page 33 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(66) Li, H.; Li, F.; Wang, Y.; Bai, L.; Yu, F.; Sun, L. Visible-Light-Driven Water Oxidation on a Photoanode by Supramolecular Assembly of Photosensitizer and Catalyst. ChemPlusChem 2016, 81, 1056-1059. (67) Li, F.; Fan, K.; Wang, L.; Daniel, Q.; Duan, L.; Sun, L. Immobilizing Ru(bda) Catalyst on a Photoanode via Electrochemical Polymerization for Light-Driven Water Splitting. ACS Catal. 2015, 5, 3786-3790. (68) Wang, D.; Marquard, S. L.; Troian-Gautier, L.; Sheridan, M. V.; Sherman, B. D.; Wang, Y.; Eberhart, M. S.; Farnum, B. H.; Dares, C. J.; Meyer, T. J. Interfacial Deposition of Ru(II) Bipyridine-Dicarboxylate Complexes by Ligand Substitution for Applications in Water Oxidation Catalysis. J. Am. Chem. Soc. 2018, 140, 719-726. (69) Wang, D.; Eberhart, M. S.; Sheridan, M. V.; Hu, K.; Sherman, B. D.; Nayak, A.; Wang, Y.; Marquard, S. L.; Dares, C. J.; Meyer, T. J. Stabilized Photoanodes for Water Oxidation by Integration of Organic Dyes, Water Oxidation Catalysts, and Electron-Transfer Mediators. Proc. Natl. Acad. Sci. U. S. A. 2018, 115, 8523-8528. (70) Wang, D.; Sheridan, M. V.; Shan, B.; Farnum, B. H.; Marquard, S. L.; Sherman, B. D.; Eberhart, M. S.; Nayak, A.; Dares, C. J.; Das, A. K.; Bullock, R. M.; Meyer, T. J. Layer-by-Layer Molecular Assemblies for Dye-Sensitized Photoelectrosynthesis Cells Prepared by Atomic Layer Deposition. J. Am. Chem. Soc. 2017, 139, 14518-14525. (71) Yamamoto, M.; Nishizawa, Y.; Chabera, P.; Li, F.; Pascher, T.; Sundström, V.; Sun, L.; Imahori, H. Visible Light-Driven Water Oxidation with a Subporphyrin Sensitizer and a Water Oxidation Catalyst. Chem. Commun. 2016, 52, 13702-13705. (72) Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D. A Comprehensive Review on PEM Water Electrolysis. Int. J. Hydrogen Energy 2013, 38, 4901-4934. (73) Li, F.; Zhang, B.; Li, X.; Jiang, Y.; Chen, L.; Li, Y.; Sun, L. Highly Efficient Oxidation of Water by a Molecular Catalyst Immobilized on Carbon Nanotubes. Angew. Chem. Int. Ed. 2011, 50, 12276-12279. (74) Sheridan, M. V.; Sherman, B. D.; Fang, Z.; Wee, K.-R.; Coggins, M. K.; Meyer, T. J. Electron Transfer Mediator Effects in the Oxidative Activation of a Ruthenium Dicarboxylate Water Oxidation Catalyst. ACS Catal. 2015, 5, 4404-4409. (75) Sheridan, M. V.; Sherman, B. D.; Marquard, S. L.; Fang, Z.; Ashford, D. L.; Wee, K.-R.; Gold, A. S.; Alibabaei, L.; Rudd, J. A.; Coggins, M. K.; Meyer, T. J. Electron Transfer Mediator Effects in Water Oxidation Catalysis by Solution and Surface-Bound Ruthenium Bpy-bicarboxylate Complexes. J. Phys. Chem. C 2015, 119, 25420-25428.

33



Page 34 of 38

(76) Zhan, S.; Ahlquist, M. S. G. Dynamics and Reactions of Molecular Ru Catalysts at Carbon Nanotube-Water Interfaces. J. Am. Chem. Soc. 2018, 140, 7498-7503. (77) Matheu, R.; Moreno-Hernandez, I. A.; Sala, X.; Gray, H. B.; Brunschwig, B. S.; Llobet, A.; Lewis, N. S. Photoelectrochemical Behavior of a Molecular Ru-Based Water-Oxidation Catalyst Bound to TiO2-Protected Si Photoanodes. J. Am. Chem. Soc. 2017, 139, 11345-11348. (78) Creus, J.; Matheu, R.; Peñafiel, I.; Moonshiram, D.; Blondeau, P.; Benet-Buchholz, J.; GarcíaAntón, J.; Sala, X.; Godard, C.; Llobet, A. A Million Turnover Molecular Anode for Catalytic Water Oxidation. Angew. Chem. Int. Ed. 2016, 55, 15382-15386. (79) Wang, L.; Fan, K.; Chen, H.; Daniel, Q.; Philippe, B.; Rensmo, H.; Sun, L. Towards Efficient and Robust Anodes for Water Splitting: Immobilization of Ru Catalysts on Carbon Electrode and Hematite by in Situ Polymerization. Catal. Today 2017, 290, 73-77. (80) Wang, L.; Fan, K.; Daniel, Q.; Duan, L.; Li, F.; Philippe, B.; Rensmo, H.; Chen, H.; Sun, J.; Sun, L. Electrochemical Driven Water Oxidation by Molecular Catalysts in Situ Polymerized on the Surface of Graphite Carbon Electrode. Chem. Commun. 2015, 51, 7883-7886. (81) Matheu, R.; Francàs, L.; Chernev, P.; Ertem, M. Z.; Batista, V.; Haumann, M.; Sala, X.; Llobet, A. Behavior of the Ru-bda Water Oxidation Catalyst Covalently Anchored on Glassy Carbon Electrodes. ACS Catal. 2015, 5, 3422-3429. (82) Bhunia, A.; Johnson, B. A.; Czapla-Masztafiak, J.; Sá, J.; Ott, S. Formal Water Oxidation Turnover Frequencies from Mil-101(Cr) Anchored Ru(bda) Depend on Oxidant Concentration. Chem. Commun. 2018, 54, 7770-7773. (83) Zheng, T.; Li, L. {[Ru(bda)]xLy}n Cross-linked Coordination Polymers: Toward Efficient Heterogeneous Catalysis for Water Oxidation in an Organic Solvent-Free System. New J. Chem. 2018, 42, 2526-2536. (84) Zhang, J.; Du, J.; Wang, J.; Wang, Y.; Wei, C.; Li, M. Vertical Step-Growth Polymerization Driven by Electrochemical Stimuli from Electrode. Angew. Chem. Int. Ed. 2018, 57, 16698-16702. (85) Li, B.; Li, F.; Bai, S.; Wang, Z.; Sun, L.; Yang, Q.; Li, C. Oxygen Evolution from Water Oxidation on Molecular Catalysts Confined in the Nanocages of Mesoporous Silicas. Energy Environ. Sci. 2012, 5, 8229-8233. (86) Yu, F.; Poole, D., 3rd; Mathew, S.; Yan, N.; Hessels, J.; Orth, N.; Ivanović-Burmazović, I.; Reek, J. N. H. Control over Electrochemical Water Oxidation Catalysis by Preorganization of Molecular Ruthenium Catalysts in Self-Assembled Nanospheres. Angew. Chem. Int. Ed. 2018, 57, 11247-11251.

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Page 35 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(87) Yang, B.; Jiang, X.; Guo, Q.; Lei, T.; Zhang, L.-P.; Chen, B.; Tung, C.-H.; Wu, L.-Z. SelfAssembled Amphiphilic Water Oxidation Catalysts: Control of O-O Bond Formation Pathways by Different Aggregation Patterns. Angew. Chem. Int. Ed. 2016, 55, 6229-6234. (88) Kunz, V.; Schulze, M.; Schmidt, D.; Würthner, F. Trinuclear Ruthenium Macrocycles: Toward Supramolecular Water Oxidation Catalysis in Pure Water. ACS Energy Lett. 2017, 2, 288-293. (89) Kunz, V.; Schmidt, D.; Röhr, M. I. S.; Mitrić, R.; Würthner, F. Supramolecular Approaches to Improve the Performance of Ruthenium-Based Water Oxidation Catalysts. Adv. Energy Mater. 2017, 7, 1602939. (90) Jiang, Y.; Li, F.; Zhang, B.; Li, X.; Wang, X.; Huang, F.; Sun, L. Promoting the Activity of Catalysts for the Oxidation of Water with Bridged Dinuclear Ruthenium Complexes. Angew. Chem. Int. Ed. 2013, 52, 3398-3401. (91) Zhang, L. L.; Gao, Y.; Liu, Z.; Ding, X.; Yu, Z.; Sun, L. C. A Trinuclear Ruthenium Complex as a Highly Efficient Molecular Catalyst for Water Oxidation. Dalton Trans. 2016, 45, 3814-3819. (92) Kunz, V.; Lindner, J. O.; Schulze, M.; Röhr, M. I. S.; Schmidt, D.; Mitrić, R.; Würthner, F. Cooperative Water Oxidation Catalysis in a Series of Trinuclear Metallosupramolecular Ruthenium Macrocycles. Energy Environ. Sci. 2017, 10, 2137-2153. (93) Schulze, M.; Kunz, V.; Frischmann, P. D.; Wurthner, F. A Supramolecular Ruthenium Macrocycle with High Catalytic Activity for Water Oxidation that Mechanistically Mimics Photosystem II. Nat. Chem. 2016, 8, 576-583. (94) Kang, R.; Chen, K.; Yao, J.; Shaik, S.; Chen, H. Probing Ligand Effects on O-O Bond Formation of Ru-Catalyzed Water Oxidation: A Computational Survey. Inorg. Chem. 2014, 53, 7130-7136. (95) Tong, L.; Inge, A. K.; Duan, L.; Wang, L.; Zou, X.; Sun, L. Catalytic Water Oxidation by Mononuclear Ru Complexes with an Anionic Ancillary Ligand. Inorg. Chem. 2013, 52, 2505-2518. (96) Duan, L.; Xu, Y.; Gorlov, M.; Tong, L.; Andersson, S.; Sun, L. Chemical and Photochemical Water Oxidation Catalyzed by Mononuclear Ruthenium Complexes with a Negatively Charged Tridentate Ligand. Chem. Eur. J. 2010, 16, 4659-4668. (97) Duan, L.; Xu, Y.; Tong, L.; Sun, L. Ce(IV)- and Light-Driven Water Oxidation by [Ru(terpy)(pic)3]2+ Analogues: Catalytic and Mechanistic Studies. ChemSusChem 2011, 4, 238-244. (98) Liu, Y.; Ng, S.-M.; Yiu, S.-M.; Lam, W. W.; Wei, X.-G.; Lau, K.-C.; Lau, T.-C. Catalytic Water Oxidation by Ruthenium(II) Quaterpyridine (qpy) Complexes: Evidence for Ruthenium(III) qpy-N,N'''-Dioxide as the Real Catalysts. Angew. Chem. Int. Ed. 2014, 53, 14468-14471.

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Page 36 of 38

(99) Fan, T.; Duan, L.; Huang, P.; Chen, H.; Daniel, Q.; Ahlquist, M. S. G.; Sun, L. The Ru-tpc Water Oxidation Catalyst and Beyond: Water Nucleophilic Attack Pathway versus Radical Coupling Pathway. ACS Catal. 2017, 7, 2956-2966. (100) Duan, L.; Wang, L.; Li, F.; Li, F.; Sun, L. Highly Efficient Bioinspired Molecular Ru Water Oxidation Catalysts with Negatively Charged Backbone Ligands. Acc. Chem. Res. 2015, 48, 20842096. (101) Lu, Z.; Gao, Y.; Chen, H.; Liu, Z.; Sun, L. Water Oxidation Catalyzed by a Charge-Neutral Mononuclear Ruthenium(III) Complex. Dalton Trans. 2017, 46, 1304-1310. (102) Tong, L.; Wang, Y.; Duan, L.; Xu, Y.; Cheng, X.; Fischer, A.; Ahlquist, M. S.; Sun, L. Water Oxidation Catalysis: Influence of Anionic Ligands upon the Redox Properties and Catalytic Performance of Mononuclear Ruthenium Complexes. Inorg. Chem. 2012, 51, 3388-3398. (103) Kärkäs, M. D.; Åkermark, T.; Johnston, E. V.; Karim, S. R.; Laine, T. M.; Lee, B.-L.; Åkermark, T.; Privalov, T.; Åkermark, B. Water Oxidation by Single-Site Ruthenium Complexes: Using Ligands as Redox and Proton Transfer Mediators. Angew. Chem. Int. Ed. 2012, 51, 1158911593. (104) Lu, Z.; Gao, Y.; Chen, H.; Liu, Z.; Chen, L.; Sun, L. Efficient Molecular Ruthenium Catalysts Containing Anionic Ligands for Water Oxidation. Dalton Trans. 2016, 45, 18459-18464. (105) Hoffmann, R.; Beier, B. F.; Muetterties, E. L.; Rossi, A. R. Seven-Coordination. A Molecular Orbital Exploration of Structure, Stereochemistry, and Reaction Dynamics. Inorg. Chem. 1977, 16, 511-522. (106) Kaveevivitchai, N.; Kohler, L.; Zong, R.; El Ojaimi, M.; Mehta, N.; Thummel, R. P. A Ru(II) Bis-terpyridine-like Complex that Catalyzes Water Oxidation: The Influence of Steric Strain. Inorg. Chem. 2013, 52, 10615-10622. (107) Muckerman, J. T.; Kowalczyk, M.; Badiei, Y. M.; Polyansky, D. E.; Concepcion, J. J.; Zong, R.; Thummel, R. P.; Fujita, E. New Water Oxidation Chemistry of a Seven-Coordinate Ruthenium Complex with a Tetradentate Polypyridyl Ligand. Inorg. Chem. 2014, 53, 6904-6913. (108) Matheu, R.; Ertem, M. Z.; Benet-Buchholz, J.; Coronado, E.; Batista, V. S.; Sala, X.; Llobet, A. Intramolecular Proton Transfer Boosts Water Oxidation Catalyzed by a Ru Complex. J. Am. Chem. Soc. 2015, 137, 10786-10795. (109) Matheu, R.; Ertem, M. Z.; Gimbert-Suriñach, C.; Benet-Buchholz, J.; Sala, X.; Llobet, A. Hydrogen Bonding Rescues Overpotential in Seven-Coordinated Ru Water Oxidation Catalysts. ACS Catal. 2017, 7, 6525-6532.

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Page 37 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(110) Matheu, R.; Ertem, M. Z.; Pipelier, M.; Lebreton, J.; Dubreuil, D.; Benet-Buchholz, J.; Sala, X.; Tessier, A.; Llobet, A. The Role of Seven-Coordination in Ru-Catalyzed Water Oxidation. ACS Catal. 2018, 8, 2039-2048. (111) Shaffer, D. W.; Xie, Y.; Szalda, D. J.; Concepcion, J. J. Lability and Basicity of BipyridineCarboxylate-Phosphonate Ligand Accelerate Single-Site Water Oxidation by Ruthenium-Based Molecular Catalysts. J. Am. Chem. Soc. 2017, 139, 15347-15355. (112) Yang, J.; An, J.; Tong, L.; Long, B.; Fan, T.; Duan, L. Sulfur Coordination Effects on the Stability and Activity of a Ruthenium-Based Water Oxidation Catalyst. Inorg. Chem. 2019, 58, 3137-3144. (113) Liu, T.; Zhang, B.; Sun, L. Iron-Based Molecular Water Oxidation Catalysts: Abundant, Cheap, and Promising. Chem. Asian J. 2019, 14, 31-43. (114) Tong, L.; Duan, L.; Xu, Y.; Privalov, T.; Sun, L. Structural Modifications of Mononuclear Ruthenium Complexes: A Combined Experimental and Theoretical Study on the Kinetics of Ruthenium-Catalyzed Water Oxidation. Angew. Chem. Int. Ed. 2011, 50, 445-449.

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Ru-bda: Unique Molecular Water-Oxidation Catalysts with Distortion

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