Uncovering the Role of Oxygen Atom Transfer in Ru-Based Catalytic Water Oxidation

Rational Design of Improved Ru Containing Fe-Based Metal-Organic Framework (MOF) Photoanode for Artificial Photosynthesis

Metal-Organic Frameworks (MOFs) recently emerged as a new platform for the realization of integrated devices for artificial photosynthesis. However, there remain few demonstrations of rational tuning of such devices for improved performance. Here, a fast molecular water oxidation catalyst working via water nucleophilic attack is integrated into the MOF MIL-142, wherein Fe3O nodes absorb visible light, leading to charge separation. Materials are characterized by a range of structural and spectroscopic techniques. New, [Ru(tpy)(Qc)(H2O)]+ (tpy = 2,2':6',2″-terpyridine and Qc = 8-quinolinecarboxylate)-doped Fe MIL-142 achieved a high photocurrent (1.6 × 10-3 A·cm-2) in photo-electrocatalytic water splitting at pH = 1. Unassisted photocatalytic H2 evolution is also reported with Pt as the co-catalyst (4.8 µmol g-1 min-1). The high activity of this new system enables hydrogen gas capture from an easy-to-manufacture, scaled-up prototype utilizing MOF deposited on FTO glass as a photoanode. These findings provide insights for the development of MOF-based light-driven water-splitting assemblies utilizing a minimal amount of precious metals and Fe-based photosensitizers.

Pentanuclear iron complex for water oxidation: spectroscopic analysis of reactive intermediates in solution and catalyst immobilization into the MOF-based photoanode

Photoelectrochemical water splitting can produce green hydrogen for industrial use and CO2-neutral transportation, ensuring the transition from fossil fuels to green, renewable energy sources. The iron-based electrocatalyst [FeII4FeIII(μ-3-O)(μ-L)6]3+ (LH = 3,5-bis(2-pyridyl)pyrazole) (1), discovered in 2016, is one of the fastest molecular water oxidation catalysts (WOC) based on earth-abundant elements. However, its water oxidation reaction mechanism has not been yet fully elucidated. Here, we present in situ X-ray spectroscopy and electron paramagnetic resonance (EPR) analysis of electrochemical water oxidation reaction (WOR) promoted by (1) in water-acetonitrile solution. We observed transient reactive intermediates during the in situ electrochemical WOR, consistent with a coordination sphere expansion prior to the onset of catalytic current. At a pre-catalytic (~+1.1 V vs. Ag/AgCl) potential, the distinct g~2.0 EPR signal assigned to FeIII/FeIV interaction was observed. Prolonged bulk electrolysis at catalytic (~+1.6 V vs. Ag/AgCl) potential leads to the further oxidation of Fe centers in (1). At the steady state achieved with such electrolysis, the formation of hypervalent FeV=O and FeIV=O catalytic intermediates was inferred with XANES and EXAFS fitting, detecting a short Fe=O bond at ~1.6 Å. (1) was embedded into MIL-126 MOF with the formation of (1)-MIL-126 composite. The latter was tested in photoelectrochemical WOR and demonstrated an improvement of electrocatalytic current upon visible light irradiation in acidic (pH=2) water solution. The presented spectroscopic analysis gives further insight into the catalytic pathways of multinuclear systems and should help the subsequent development of more energy- and cost-effective catalysts of water splitting based on earth-abundant metals. Photoelectrocatalytic activity of (1)-MIL-126 confirms the possibility of creating an assembly of (1) inside a solid support and boosting it with solar irradiation towards industrial applications of the catalyst.

Enhancement of Reactivity of a RuIV-Oxo Complex in Oxygen-Atom-Transfer Catalysis by Hydrogen-Bonding with Amide Moieties in the Second Coordination Sphere

We have synthesized and characterized a RuII-OH2 complex (2), which has a pentadentate ligand with two pivalamide groups as bulky hydrogen-bonding (HB) moieties in the second coordination sphere (SCS). Complex 2 exhibits a coordination equilibrium through the coordination of one of the pivalamide oxygens to the Ru center in water, affording a η6-coordinated complex, 3. A detailed thermodynamic analysis of the coordination equilibrium revealed that the formation of 3 from 2 is entropy-driven owing to the dissociation of the axial aqua ligand in 2. Complex 2 was oxidized by a CeIV salt to produce the corresponding RuIII(OH) complex (5), which was characterized crystallographically. In the crystal structure of 5, hydrogen bonds are formed among the NH groups of the pivalamide moieties and the oxygen atom of the hydroxo ligand. Further 1e--oxidation of 5 yields the corresponding RuIV(O) complex, 6, which has intramolecular HB of the oxo ligand with two amide N-H protons. Additionally, the RuIII(OH) complex, 5, exhibits disproportionation to the corresponding RuIV(O) complex, 6, and a mixture of the RuII complexes, 2 and 3, in an acidic aqueous solution. We investigated the oxidation of a phenol derivative using complex 6 as the active species and clarified the switch of the reaction mechanism from hydrogen-atom transfer at pH 2.5 to electron transfer, followed by proton transfer at pH 1.0. Additionally, the intramolecular HB in 6 exerts enhancing effects on oxygen-atom transfer reactions from 6 to alkenes such as cyclohexene and its water-soluble derivative to afford the corresponding epoxides, relative to the corresponding RuIV(O) complex (6') lacking the HB moieties in the SCS.

Morphological and Coordination Modulations in Iridium Electrocatalyst for Robust and Stable Acidic OER Catalysis

Proton exchange membrane water splitting (PEMWS) technology has high-level current density, high operating pressure, small electrolyzer-size, integrity, flexibility, and has good adaptability to the volatility of wind power and photovoltaics, but the development of both active and high stability of the anode electrocatalyst in acidic environment is still a huge challenge, which seriously hinders the promotion and application of PEMWS. In recent years, researchers have made tremendous attempts in the development of high-quality active anode electrocatalyst, and we summarize some of the research progress made by our group in the design and synthesis of PEMWS anode electrocatalysts with different nanostructures, and makes full use of electrocatalytic activity points to increase the inherent activity of Iridium (Ir) sites, and provides optimization strategies for the long-term non-decay of catalysts under high anode potential in acidic environments. At this stage, these research advances are expected to facilitate the research and technological progress of PEMWS, and providing some research ideas and references for future research on efficient and inexpensive PEMWS anode electrocatalysts.

Electrocatalytic water oxidation by heteroleptic ruthenium complexes of 2,6-bis(benzimidazolyl)pyridine Scaffold: a mechanistic investigation

Three monomeric ruthenium complexes with anionic ligands [Ru^II(L)(L¹)(DMSO)][ClO₄] (1), [Ru^II(L)(L²)(DMSO)] [PF₆] (2), and [Ru^II(L)(L³)(DMSO)][PF₆] (3) [L = pyrazine carboxylate, L¹ = 2,6-bis(1H-benzo[d]imidazol-2-yl)pyridine, L² = 4,5-dmbimpy = 2,6-bis(5,6-dimethyl-1H-benzo[d]imidazol-2-yl)pyridine, L³ = 4-Fbimpy = 2,6-bis(5-fluoro-1H-benzo[d]imidazol-2-yl)pyridine, DMSO = dimethyl sulfoxide] as electrocatalysts for water oxidation are reported herein. The single crystal X-ray structure of the complexes reveals the presence of a DMSO molecule, which is supposed to be the labile group undergoing water exchange under the experimental condition of electrocatalysis. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) study shows the appearance of the catalytic wave for water oxidation at Ru(IV/V) oxidation. LSV, CV, and bulk electrolysis technique has been used to study the redox properties of the complexes and their electrocatalytic activity. A systematic variation on the ligand scaffold has been found to display a profound effect on the rate of electrocatalytic oxygen evolution. Electrochemical and theoretical (density functional theory) studies support the O-O bond formation during water oxidation passes through water nucleophilic attack (WNA) for all the ruthenium complexes. At pH 1, the maximum turnover frequency (TOF_max) has been experimentally obtained as 17556.25 s^-1, 31648.41 s^-1, and 39.69 s^-1 for complexes 1, 2, and 3, respectively, from the foot of wave analysis (FOWA). The high value of TOF_max for complex 2 indicates its efficiency as an electrocatalyst for water oxidation in a homogeneous medium.

Metamorphic oxygen-evolving molecular Ru and Ir catalysts

Today sustainable and clean energy conversion strategies are based on sunlight and the use of water as a source of protons and electrons, in a similar manner as it happens in Photosystem II. To achieve this, the charge separation state induced by light has to be capable of oxidising water by 4 protons and 4 electrons and generating molecular oxygen. This oxidation occurs by the intermediacy of a catalyst capable of finding low-energy pathways via proton-coupled electron transfer steps. The high energy involved in the thermodynamics of water oxidation reaction, coupled with its mechanistic complexity, is responsible for the difficulty of discovering efficient and oxidatively robust molecules capable of achieving such a challenging task. A significant number of Ru coordination complexes have been identified as water oxidation catalysts (WOCs) and are among the best understood from a mechanistic perspective. In this review, we describe the catalytic performance of these complexes and focus our attention on the factors that influence their performance during catalysis, especially in cases where a detailed mechanistic investigation has been carried out. The collective information extracted from all the catalysts studied allows one to identify the key features that govern the complex chemistry associated with the catalytic water oxidation reaction. This includes the stability of trans-O-Ru-O groups, the change in coordination number from CN6 to CN7 at Ru high oxidation states, the ligand flexibility, the capacity to undergo intramolecular proton transfer, the bond strain, the axial ligand substitution, and supramolecular effects. Overall, combining all this information generates a coherent view of this complex chemistry.

Computational Analysis of Structure - Activity Relationships in Highly Active Homogeneous Ruthenium-based Water Oxidation Catalysts

Linear free energy scaling relationships (LFESRs) and regression analysis may predict the catalytic performance of heterogeneous and recently, homogenous water oxidation catalysts (WOCs). This study analyses twelve homogeneous Ru-based catalysts - some, the most active catalysts studied: the Ru(tpy-R)(QC) and Ru(tpy-R)(4-pic)2 catalysts, where tpy is 2,2:6,2-terpyridine, QC is 8-quinolinecarboxylate and 4-pic is 4-picoline. Typical relationships studied among heterogenous and solid-state catalysts cannot be broadly applied to homogeneous catalysts. This subset of structurally similar catalysts with impressive catalytic activity deserves closer computational and statistical analysis of energetics correlating with measured catalytic activity. We report general methods of LFESR analysis yield insufficiently robust relationships between descriptor variables. However, volcano plot-based analysis grounded in Sabatier's principle reveals ranges of ideal relative energies of the RuIV=O and RuIV-OH intermediates and optimal changes in free energies of water nucleophilic attack on RuV=O. A narrow range of RuIV-OH to RuV=O redox potentials corresponding with the highest catalytic activities suggests facile access to the catalytically competent high-valent RuV=O state, often inaccessible from RuIV=O. Our work introduces experimental oxygen evolution rates into approaches of LFESR and Sabatier principle-based analysis, identifying a narrow yet fertile energetic landscape to bountiful oxygen-evolution activity, leading future rational design.

Systematic Influence of Electronic Modification of Ligands on the Catalytic Rate of Water Oxidation by a Single-Site Ru-Based Catalyst

Catalytic water oxidation is an important process for the development of clean energy solutions and energy storage. Despite the significant number of reports on active catalysts, systematic control of the catalytic activity remains elusive. In this study, descriptors are explored that can be correlated with catalytic activity. [Ru(tpy)(pic)₂ (H₂ O)](NO₃ )₂ and [Ru(EtO-tpy)(pic)₂ (H₂ O)](NO₃ )₂ (where tpy=2,2' : 6',2"-terpyridine, EtO-tpy=4'-(ethoxy)-2,2':6',2"-terpyridine, pic=4-picoline) are synthesized and characterized by NMR, UV/Vis, EPR, resonance Raman, and X-ray absorption spectroscopy, and electrochemical analysis. Addition of the ethoxy group increases the catalytic activity in chemically driven and photocatalytic water oxidation. Thus, the effect of the electron-donating group known for the [Ru(tpy)(bpy)(H₂ O)]²⁺ family is transferable to architectures with a tpy ligand trans to the Ru-oxo unit. Under catalytic conditions, [Ru(EtO-tpy)(pic)₂ (H₂ O)](NO₃ )₂ displays new spectroscopic signals tentatively assigned to a peroxo intermediate. Reaction pathways were analyzed by using DFT calculations. [Ru(EtO-tpy)(pic)₂ (H₂ O)](NO₃ )₂ is found to be one of the most active catalysts functioning by a water nucleophilic attack mechanism.

Computational Scaling Relationships Predict Experimental Activity and Rate-Limiting Behavior in Homogeneous Water Oxidation

While computational screening with first-principles density functional theory (DFT) is essential for evaluating candidate catalysts, limitations in accuracy typically prevent the prediction of experimentally relevant activities. Exemplary of these challenges are homogeneous water oxidation catalysts (WOCs) where differences in experimental conditions or small changes in ligand structure can alter rate constants by over an order of magnitude. Here, we compute mechanistically relevant electronic and energetic properties for 19 mononuclear Ru transition-metal complexes (TMCs) from three experimental water oxidation catalysis studies. We discover that 15 of these TMCs have experimental activities that correlate with a single property, the ionization potential of the Ru(II)-O₂ catalytic intermediate. This scaling parameter allows the quantitative understanding of activity trends and provides insight into the rate-limiting behavior. We use this approach to rationalize differences in activity with different experimental conditions, and we qualitatively analyze the source of distinct behavior for different electronic states in the other four catalysts. Comparison to closely related single-atom catalysts and modified WOCs enables rationalization of the source of rate enhancement in these WOCs.

Highly selective oxidation of organic contaminants in the RuIII-activated peroxymonosulfate process: The dominance of RuVO species

Ruthenium (Ru^III)-activated peroxymonosulfate (the Ru^III/PMS process) is one of the most efficient PMS-based advanced oxidation processes for the abatement of organic contaminants. Here we interestingly found that phenyl methyl sulfoxide (PMSO) was significantly oxidized to phenyl methyl sulfone (PMSO₂) in the Ru^III/PMS process at the pH range of 3.0-6.0, with the conversion ratio of ΔPMSO to ΔPMSO₂ was close to 100%, which favored the dominance of high-valent ruthenium-oxo species (Ru^VO) instead of the widely-recognized radicals (i.e, hydroxyl radical and sulfate radical). Scavenging experiments further indicated that Ru^VO was unreactive to tert-butyl alcohol, but could be scavenged by methanol and dimethyl sulfoxide. Besides, sulfamethoxazole, acetaminophen, carbamazepine, diclofenac, 2,4,6-trichlorophenol were readily degraded in the Ru^III/PMS process, but atrazine, ibuprofen, benzoic acid and 4-nitrobenzoic acid were barely removed, suggesting the high selectivity of Ru^VO species. This study enriched the understandings on the mechanism of Ru^III-mediated PMS activation and the nature of Ru^VO species.

Identification of intermediates of a molecular ruthenium catalyst for water oxidation using in situ electrochemical X-ray absorption spectroscopy

This is the first study on a Ru(bda) (bda: 2,2'-bipyridine-6,6'-dicarboxylic acid) catalyst in solution using a home-built electrochemical cell, in combination with an energy-dispersive X-ray absorption spectroscopy setup. The oxidation state and coordination number of the catalyst during electrocatalysis could be estimated, while avoiding radiation damage from the X-rays.

Catalytic water oxidation by an in situ generated ruthenium nitrosyl complex bearing a bipyridine-bis(alkoxide) ligand

Oxidative degradation and transformation of catalysts are commonly observed in water oxidation by molecular catalysts, especially when a highly oxidizing reagent such as (NH₄)₂[Ce(NO₃)₆] [Ce(IV)] is used. We report herein the synthesis of a ruthenium(III) complex bearing an oxidative resistant bipyridine-bis(alkoxide) ligand, [Ru(bdalk)(pic)₂]⁺ (1, H₂bdalk = 2,2'-([2,2'-bipyridine]-6,6'-diyl)bis(propan-2-ol), pic = 4-picoline) as a water oxidation catalyst (WOC). A ruthenium(II) nitrosyl complex [Ru(Hbdalk)(NO)(pic)₂]²⁺ (3) was also formed during the water oxidation process by 1/Ce(IV), and was isolated and structurally characterized. Complex 3 was found to be an active WOC, with the nitrosyl group remaining intact during water oxidation.

Synthesis, Characterization, and Water Oxidation Activity of Isomeric Ru Complexes

The synthesis and characterization of the isomeric ruthenium complexes with the general formula cis- and trans-[Ru(trpy)(qc)X]ⁿ⁺ (trpy is 2,2':6',2″-terpyridine, qc is 8-quinolinecarboxylate, cis-1 and trans-1, X = Cl, n = 0; cis-2 and trans-2, X=OH₂, n = 1) with respect to the relative disposition of the carboxylate and X ligands are reported. For comparison purposes, another set of ruthenium complexes with general formula cis- and trans-[Ru(trpy)(pic)(OH₂)]⁺ (pic is 2-picolinate (cis-3, trans-3)) have been prepared. The complexes with a qc ligand show a more distorted geometry compared to the complexes with a pic ligand. In all of the cases, the trans isomers show lower potential values for all of the redox couples relative to the cis isomers. Complexes cis-2 and trans-2 with six-member chelate rings show higher catalytic activity than cis-3 and trans-3. Overall, it was shown that the electronic perturbation to the metal center exerted by different orientation and geometry of the ligands significantly influences both redox properties and catalytic performance.

A broad view on the complexity involved in water oxidation catalysis based on Ru-bpn complexes

A new Ru complex with the formula [Ru(bpn)(pic)₂]Cl₂ (where bpn is 2,2'-bi(1,10-phenanthroline) and pic stands for 4-picoline) (1Cl₂) is synthesized to investigate the true nature of active species involved in the electrochemical and chemical water oxidation mediated by a class of N4 tetradentate equatorial ligands. Comprehensive electrochemical (by using cyclic voltammetry, differential pulse voltammetry, and controlled potential electrolysis), structural (X-ray diffraction analysis), spectroscopic (UV-vis, NMR, and resonance Raman), and kinetic studies are performed. 1²⁺ undergoes a substitution reaction when it is chemically (by using NaIO₄) or electrochemically oxidized to Ru^III, in which picoline is replaced by an hydroxido ligand to produce [Ru(bpn)(pic)(OH)]²⁺ (2²⁺). The former complex is in equilibrium with an oxo-bridged species {[Ru(bpn)(pic)]₂(μ-O)}⁴⁺ (3⁴⁺) which is the major form of the complex in the Ru^III oxidation state. The dimer formation is the rate determining step of the overall oxidation process (k_dimer = 1.35 M^-1 s^-1), which is in line with the electrochemical data at pH = 7 (k_dimer = 1.4 M^-1 s^-1). 3⁴⁺ can be reduced to [Ru(bpn)(pic)(OH₂)]²⁺ (4²⁺), showing a sort of square mechanism. All species generated in situ at pH 7 have been thoroughly characterized by NMR, mass spectrometry, UV-Vis and electrochemical techniques. 1²⁺ and 4²⁺ are also characterized by single crystal X-ray diffraction analysis. Chemical oxidation of 1²⁺ triggered by Ce^IV shows its capability to oxidize water to dioxygen.

Facile Light-Induced Transformation of [RuII(bpy)2(bpyNO)]2+ to [RuII(bpy)3]2

Ru-based coordination compounds have important applications as photosensitizers and catalysts. [Ru^II(bpy)₂(bpyNO)]²⁺ (bpy = 2,2'-bipyridine and bpyNO = 2,2'-bipyridine-N-oxide) was reported to be extremely light-sensitive, but its light-induced transformation pathways have not been analyzed. Here, we elucidated a mechanism of the light-induced transformation of [Ru^II(bpy)₂(bpyNO)]²⁺ using UV-vis, EPR, resonance Raman, and NMR spectroscopic techniques. The spectroscopic analysis was augmented with the DFT calculations. We concluded that upon 530-650 nm light excitation, ³[Ru^III(bpyNO^-•)(bpy)₂]²⁺ is formed similarly to the ³[Ru^III(bpy^-•)(bpy)₂]²⁺ light-induced state of the well-known photosensitizer [Ru^II(bpy)₃]²⁺. An electron localization on the bpyNO ligand was confirmed by obtaining a unique EPR signal of reduced [Ru^II(bpy)₂(bpyNO^-•)]⁺ (g_xx = 2.02, g_yy = 1.99, and g_zz = 1.87 and ¹⁴N hfs A_xx = 12 G, A_yy = 34 G, and A_zz = 11 G). ³[Ru^III(bpyNO^-•)(bpy)₂]²⁺ may evolve via breaking of the Ru-O-N fragment at two different positions resulting in [Ru^IV═O(bpy)₂(bpy_out)]²⁺ for breakage at the O-|-N bond and [Ru^II(H₂O)(bpy)₂(bpyNO_out)]²⁺ for breakage at the Ru-|-O bond. These pathways were found to have comparable ΔG. A reduction of [Ru^IV═O(bpy)₂(bpy_out)]²⁺ may result in water elimination and formation of [Ru^II(bpy)₃]²⁺. The expected intermediates, [Ru^III(bpy)₂(bpyNO)]³⁺ and [Ru^III(bpy)₃]³⁺, were detected by EPR. In addition, a new signal with g_xx = 2.38, g_yy = 2.10, and g_zz = 1.85 was observed and tentatively assigned to a complex with the dissociated ligand, such as [Ru^III(H₂O)(bpy)₂(bpyNO_out)]³⁺. The spectroscopic signatures of [Ru^IV═O(bpy)₂(bpy_out)]²⁺ were not observed, although DFT analysis and [Ru^II(bpy)₃]²⁺ formation suggest this intermediate. Thus, [Ru^II(bpy)₂(bpyNO)]²⁺ has potential as a light-induced oxidizer.

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 Ru^II-O-Ru^III oxo-bridged structure. Bearing the same bda^2- 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.

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Green dimer (Ru^II-O-Ru^III), referring to “blue dimer” of Ru^III-O-Ru^III, is disclosed

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The first amphiphilic μ-oxido-bridged catalyst is reported active for water oxidation

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The oxo-bridged “green dimer” 2 takes water nucleophilic attack for O-O bond formation

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This is the first Ru-bda catalyst, which possesses a new oxygen evolution pathway

In need of a second-hand? The second coordination sphere of ruthenium complexes enables water oxidation with improved catalytic activity

A ligand dangling arm, acting as an intramolecular proton acceptor, drastically increasing the catalytic activity of Ru-complexes for water oxidation.

Isolation and Study of Ruthenium-Cobalt Oxo Cubanes Bearing a High-Valent, Terminal RuV-Oxo with Significant Oxyl Radical Character

High-valent Ru^V-oxo intermediates have long been proposed in catalytic oxidation chemistry, but investigations into their electronic and chemical properties have been limited due to their reactive nature and rarity. The incorporation of Ru into the [Co₃O₄] subcluster via the single-step assembly reaction of Co^II(OAc)₂(H₂O)₄ (OAc = acetate), perruthenate (RuO₄^-), and pyridine (py) yielded an unprecedented Ru(O)Co₃(μ₃-O)₄(OAc)₄(py)₃ cubane featuring an isolable, yet reactive, Ru^V-oxo moiety. EPR, ENDOR, and DFT studies reveal a valence-localized [Ru^V(S = 1/2)Co^III₃(S = 0)O₄] configuration and non-negligible covalency in the cubane core. Significant oxyl radical character in the Ru^V-oxo unit is experimentally demonstrated by radical coupling reactions between the oxo cubane and both 2,4,6-tri-tert-butylphenoxyl and trityl radicals. The oxo cubane oxidizes organic substrates and, notably, reacts with water to form an isolable μ-oxo bis-cubane complex [(py)₃(OAc)₄Co₃(μ₃-O)₄Ru]-O-[RuCo₃(μ₃-O)₄(OAc)₄(py)₃]. Redox activity of the Ru^V-oxo fragment is easily tuned by the electron-donating ability of the distal pyridyl ligand set at the Co sites demonstrating strong electronic communication throughout the entire cubane cluster. Natural bond orbital calculations reveal cooperative orbital interactions of the [Co₃O₄] unit in supporting the Ru^V-oxo moiety via a strong π-electron donation.

Universal scaling relations for the rational design of molecular water oxidation catalysts with near-zero overpotential

A major roadblock in realizing large-scale production of hydrogen via electrochemical water splitting is the cost and inefficiency of current catalysts for the oxygen evolution reaction (OER). Computational research has driven important developments in understanding and designing heterogeneous OER catalysts using linear scaling relationships derived from computed binding energies. Herein, we interrogate 17 of the most active molecular OER catalysts, based on different transition metals (Ru, Mn, Fe, Co, Ni, and Cu), and show they obey similar scaling relations to those established for heterogeneous systems. However, we find that the conventional OER descriptor underestimates the activity for very active OER complexes as the standard approach neglects a crucial one-electron oxidation that many molecular catalysts undergo prior to O-O bond formation. Importantly, this additional step allows certain molecular catalysts to circumvent the "overpotential wall", leading to enhanced performance. With this knowledge, we establish fundamental principles for the design of ideal molecular OER catalysts.

Mono-/Multinuclear Water Oxidation Catalysts

Water splitting, in which water molecules can be transformed into hydrogen and oxygen, is an appealing energy conversion and transformation strategy to address the environmental and energy crisis. The oxygen evolution reaction (OER) is dynamically slow, which limits energy conversion efficiency during the water-splitting process and requires high-efficiency water oxidation catalysts (WOCs) to overcome the OER energy barrier. It is generally accepted that multinuclear WOCs possess superior OER performances, as demonstrated by the CaMn₄ O₅ cluster in photosystem II (PSII), which can catalyze the OER efficiently with a very low overpotential. Inspired by the CaMn₄ O₅ cluster in PSII, some multinuclear WOCs were synthesized that could catalyze water oxidation. In addition, some mononuclear molecular WOCs also show high water oxidation activity. However, it cannot be excluded that the high activity arises from the formation of dimeric species. Recently, some mononuclear heterogeneous WOCs showed a high water oxidation activity, which testified that mononuclear active sites with suitable coordination surroundings could also catalyze water oxidation efficiently. This Review focuses on recent progress in the development of mono-/multinuclear homo- and heterogeneous catalysts for water oxidation. The active sites and possible catalytic mechanisms for water oxidation on the mono-/multinuclear WOCs are provided.

An Overview of Significant Achievements in Ruthenium-Based Molecular Water Oxidation Catalysis

Fossil fuels (coal, oil, natural gas) are becoming increasingly disfavored as long-term energy options due to concerns of scarcity and environmental consequences (e.g., release of anthropogenic CO₂). Hydrogen gas, on the other hand, has gained popularity as a clean-burning fuel because the only byproduct from its reaction with O₂ is H₂O. In recent decades, hydrogen derived from water splitting has been a topic of extensive research. The bottleneck of the water splitting reaction is the difficult water oxidation step (2H₂O → O₂ + 4H⁺ + 4e⁻), which requires an effective and robust catalyst to overcome its high kinetic barrier. Research in water oxidation by molecular ruthenium catalysts enjoys a rich history spanning nearly 40 years. As the diversity of novel ligands continues to widen, the relationship between ligand geometry or electronics, and catalyst activity is undoubtedly becoming clearer. The present review highlights, in the authors’ opinion, some of the most impactful discoveries in the field and explores the evolution of ligand design that has led to the current state of the art.

Mechanism for O-O Bond Formation via Radical Coupling of Metal and Ligand Based Radicals: A New Pathway

Artificial photosynthesis carries promise to deliver abundant clean energy for the needs of a growing population. Deep mechanistic understanding is required to achieve rational design of fast and durable water oxidation catalysts. Here we provided first evidence for a new mechanism of the O-O bond formation via radical coupling of the oxidized metal═oxo of radicaloid character (Ru^IV═O) and ligand based radical ([ligand-NO]^+• cation radical). O-O bond formation is facilitated via spin alignment and takes place via a virtually barrier less pathway inside the single metal complex. In situ reactive intermediate conversion was monitored by mass spectrometry, resonance Raman (RR) and EPR. Computational analysis have shown that the formation of [ligand-NO]^+• happens at a lower overpotential than the formation of the [Ru^V═O(ligand)]³⁺ intermediate. Overall, the presented paradigm for O-O bond formation opens new opportunities for rational catalyst design.

Model of the Oxygen Evolving Complex Which Is Highly Predisposed to O-O Bond Formation

Light-driven water oxidation is a fundamental reaction in the biosphere. The Mn₄Ca cluster of photosystem II cycles through five redox states termed S₀-S₄, after which oxygen is evolved. Critically, the timing of O-O bond formation within the Kok cycle remains unknown. By combining recent crystallographic, spectroscopic, and DFT results, we demonstrate an atomistic S₃ state model with the possibility of a low barrier to O-O bond formation prior to the final oxidation step. Furthermore, the associated one electron oxidized S₄ state does not provide more advantages in terms of spin alignment or the energy of O-O bond formation. We propose that a high energy peroxide isoform of the S₃ state can preferentially be oxidized by Tyr _z^ox in the course of final electron transfer leading to O₂ evolution. Such a mechanism may explain the peculiar kinetic behavior of O₂ evolution as well as serve as an evolutionary adaptation to avoid release of the harmful peroxides.

The Key RuV=O Intermediate of Site-Isolated Mononuclear Water Oxidation Catalyst Detected by in Situ X-ray Absorption Spectroscopy

Improvement of the oxygen evolution reaction (OER) is a challenging step toward the development of sustainable energy technologies. Enhancing the OER rate and efficiency relies on understanding the water oxidation mechanism, which entails the characterization of the reaction intermediates. Very active Ru-bda type (bda is 2,2'-bipyridine-6,6'-dicarboxylate) molecular OER catalysts are proposed to operate via a transient 7-coordinate Ru^V═O intermediate, which so far has never been detected due to its high reactivity. Here we prepare and characterize a well-defined supported Ru(bda) catalyst on porous indium tin oxide (ITO) electrode. Site isolation of the catalyst molecules on the electrode surface allows trapping of the key 7-coordinate Ru^V═O intermediate at potentials above 1.34 V vs NHE at pH 1, which is characterized by electron paramagnetic resonance and in situ X-ray absorption spectroscopies. The in situ extended X-ray absorption fine structure analysis shows a Ru═O bond distance of 1.75 ± 0.02 Å, consistent with computational results. Electrochemical studies and density functional theory calculations suggest that the water nucleophilic attack on the surface-bound Ru^V═O intermediate (O-O bond formation) is the rate limiting step for OER catalysis at low pH.

Photoelectrochemical Behavior of a Molecular Ru-Based Water-Oxidation Catalyst Bound to TiO2-Protected Si Photoanodes

A hybrid photoanode based on a molecular water oxidation precatalyst was prepared from TiO₂-protected n- or p⁺-Si coated with multiwalled carbon nanotubes (CNT) and the ruthenium-based water oxidation precatalyst [Ru^IV(tda)(py-pyr)₂(O)], 1(O) (tda^2- is [2,2':6',2″-terpyridine]-6,6″-dicarboxylato and py-pir is 4-(pyren-1-yl)-N-(pyridin-4-ylmethyl)butanamide). The Ru complex was immobilized by π-π stacking onto CNTs that had been deposited by drop casting onto Si electrodes coated with 60 nm of amorphous TiO₂ and 20 nm of a layer of sputtered C. At pH = 7 with 3 Sun illumination, the n-Si/TiO₂/C/CNT/[1+1(O)] electrodes exhibited current densities of 1 mA cm^-2 at 1.07 V vs NHE. The current density was maintained for >200 min at a constant potential while intermittently collecting voltammograms that indicated that over half of the Ru was still in molecular form after O₂ evolution.

X-ray Emission Spectroscopy of Biomimetic Mn Coordination Complexes

Understanding the function of Mn ions in biological and chemical redox catalysis requires precise knowledge of their electronic structure. X-ray emission spectroscopy (XES) is an emerging technique with a growing application to biological and biomimetic systems. Here, we report an improved, cost-effective spectrometer used to analyze two biomimetic coordination compounds, [Mn^IV(OH)₂(Me₂EBC)]²⁺ and [Mn^IV(O)(OH)(Me₂EBC)]⁺, the second of which contains a key Mn^IV═O structural fragment. Despite having the same formal oxidation state (Mn^IV) and tetradentate ligands, XES spectra from these two compounds demonstrate different electronic structures. Experimental measurements and DFT calculations yield different localized spin densities for the two complexes resulting from Mn^IV-OH conversion to Mn^IV═O. The relevance of the observed spectroscopic changes is discussed for applications in analyzing complex biological systems such as photosystem II. A model of the S₃ intermediate state of photosystem II containing a Mn^IV═O fragment is compared to recent time-resolved X-ray diffraction data of the same state.

Formation of N-oxide in the third oxidation of [RuII(tpy)(L)(OH2)]2+

The widely studied water oxidation catalyst [Ru^II(tpy)(L)(OH₂)]²⁺ (tpy = 2,2′:6′,2′′-terpyridine, L = bpy = 2,2′-bipyridine or L = bpm = 2,2′-bipyrimidine) is still under scrutiny.