Intermolecular O-O Bond Formation between High-Valent Ru-oxo Species

Despite extensive research on water oxidation catalysts over the past few decades, the relationship between high-valent metal-oxo intermediates and the O-O bond formation pathway has not been well clarified. Our previous study showed that the high spin density on O in RuV=O is pivotal for the interaction of two metal-oxyl radical (I2M) pathways. In this study, we found that introducing an axially coordinating ligand, which is favorable for bimolecular coupling, into the Ru-pda catalyst can rearrange its geometry. The shifts in geometric orientation altered its O-O bond formation pathway from water nucleophilic attack (WNA) to I2M, resulting in a 70-fold increase in water oxidation activity. This implies that the I2M pathway is concurrently influenced by the spin density on oxo and the geometry organization of the catalysts. The observed mechanistic switch and theoretical studies provide insights into controlling reaction pathways for homogeneous water oxidation catalysis.

O-O bond formation via radical coupling in a dinuclear iron water oxidation catalyst with high catalytic activity

The use of iron-based catalysts for the water oxidation reaction is highly attractive due to the high abundance of iron. While many molecular catalysts have been made, most show limited activity and short lifetimes. An exception with higher activity was presented by Thummel and co-workers in 2015. Herein we present a study on the feasibility of the coupling of two O centered radicals originating from the two subunits of the dinuclear catalyst. The reaction pathway includes the oxidation to the active species FeIV-O-FeIV but avoids further high potential oxidations which previous mechanistic proposals have relied on.

The impact of secondary coordination sphere engineering on water oxidation reactivity catalysed by molecular ruthenium complexes: a next-generation approach to develop advanced catalysts

Water oxidation is the bottleneck for producing hydrogen from the water-splitting reaction. Developing efficient water oxidation catalysts (WOCs) has recently been of paramount interest. Ruthenium-based WOCs have gained much attention due to their enriched redox property, robust nature, and superior catalytic performances compared to other transition metal-based molecular catalysts. The performance of a catalyst is highly dependent on the design of the ligand framework. In nature, the secondary coordination sphere around the active site of a metalloenzyme plays a vital role in catalysis. This principle has been employed in the recent development of efficient catalysts. With the aid of secondary interactions, some landmark Ru-based WOCs, producing remarkable turnover frequencies (TOFs) in the order of 10⁴ s^-1, have been developed. In this account, we have discussed the underlying chemistry related to the effect of secondary interactions (such as hydrogen-bonding, π-π stacking, electrostatic interaction, hydrophobic-hydrophilic environment, etc.) on the kinetics of the water oxidation reaction catalysed by molecular Ru-complexes. The use of secondary interactions (such as π-π and C-H⋯π) in anchoring the molecular catalyst onto the solid conducting surface has also been discussed. We aim to provide a brief overview of the positive impact of outer-sphere engineering on water oxidation reactivity, which may offer guidelines for developing the next generation of advanced catalysts.

Computational comparison of Ru(bda)(py)2 and Fe(bda)(py)2 as water oxidation catalysts

Ru(bda)(py)₂ (bda = 2,2'-bipyridine-6,6'-dicarboxylate, py = pyridine) has been a significant milestone in the development of water oxidation catalysts. Inspired by Ru(bda)(py)₂ and aiming to reduce the use of noble metals, iron (Fe) was introduced to replace the Ru catalytic center in Ru(bda)(py)₂. In this study, density functional theory (DFT) calculations were performed on Fe- and Ru(bda)(py)₂ catalysts, and a more stable 6-coordinate Fe(bda)(py)₂ with one carboxylate group of bda disconnecting with Fe was found. For the first time, theoretical comparisons have been conducted on these three catalysts to compare their catalytic performances, such as reduction potentials and energy profiles of the radical coupling process. Explanations for the high potential of [Fe^III(bda)(py)₂-H₂O]⁺ and reactivity of [Fe^V(bda)(py)₂-O]⁺ have been provided. This study can provide insights on Fe(bda)(py)₂ from a computational perspective if it is utilized as a water oxidation catalyst.

Promoting Proton Transfer and Stabilizing Intermediates in Catalytic Water Oxidation via Hydrophobic Outer Sphere Interactions

The outer coordination sphere of metalloenzyme often plays an important role in its high catalytic activity, however, this principle is rarely considered in the design of man-made molecular catalysts. Herein, four Ru-bda (bda=2,2'-bipyridine-6,6'-dicarboxylate) based molecular water oxidation catalysts with well-defined outer spheres are designed and synthesized. Experimental and theoretical studies showed that the hydrophobic environment around the Ru center could lead to thermodynamic stabilization of the high-valent intermediates and kinetic acceleration of the proton transfer process during catalytic water oxidation. By this outer sphere stabilization, a 6-fold rate increase for water oxidation catalysis has been achieved.

Solvent Effects on the Menshutkin Reaction

The Menshutkin reaction is a methyl transfer reaction relevant in fields ranging from biochemistry to chemical synthesis. In the present work, the energetics and solvent distributions for NH₃+MeCl and Pyr+MeBr reactions were investigated in explicit solvent (water, methanol, acetonitrile, benzene, cyclohexane) by means of reactive molecular dynamics simulations. For polar solvents (water, methanol, and acetonitrile) and benzene, strong to moderate catalytic effects for both reactions were found, whereas apolar and bulky cyclohexane interacts weakly with the solute and does not show pronounced barrier reduction. The calculated barrier heights for the Pyr+MeBr reaction in acetonitrile and cyclohexane are 23.2 and 28.1 kcal/mol compared with experimentally measured barriers of 22.5 and 27.6 kcal/mol, respectively. The solvent distributions change considerably between reactant and TS but comparatively little between TS and product conformations of the solute. As the system approaches the transition state, correlated solvent motions occur which destabilize the solvent-solvent interactions. This is required for the system to surmount the barrier. Finally, it is found that the average solvent-solvent interaction energies in the reactant, TS, and product state geometries are correlated with changes in the solvent structure around the solute.

The Role of Counterions in Intermolecular Radical Coupling of Ru-bda Catalysts

Intermolecular radical coupling (also interaction of two metal centers I2M) is one of the main mechanisms for O–O bond formation in water oxidation catalysts. For Ru(bda)L2 (H2bda = 2,2′-bipyridine-6,6′-dicarboxylate, L = pyridine or similar nitrogen containing heterocyclic ligands) catalysts a significant driving force in water solution is the hydrophobic effects driven by the solvent. The same catalyst has been successfully employed to generate N2 from ammonia, also via I2M, but here the solvent was acetonitrile where hydrophobic effects are absent. We used a classical force field for the key intermediate [RuVIN(bda)(py)2]+ to simulate the dimerization free energy by calculation of the potential mean force, in both water and acetonitrile to understand the differences and similarities. In both solvents the complex dimerizes with similar free energy profiles. In water the complexes are essentially free cations with limited ion paring, while in acetonitrile the ion-pairing is much more significant. This ion-pairing leads to significant screening of the charges, making dimerization possible despite lower solvent polarity that could lead to repulsion between the charged complexes. In water the lower ion pairing is compensated by the hydrophobic effect leading to favorable dimerization despite repulsion of the charges. A hypothetical doubly charged [RuVIIN(bda)py2]2+ was also studied for deeper understanding of the charge effect. Despite the double charge the complexes only dimerized favorably in the lower dielectric solvent acetonitrile, while in water the separated state is more stable. In the doubly charged catalyst the effect of ion-pairing is even more pronounced in acetonitrile where it is fully paired similar to the 1+ complex, while in water the separation of the ions leads to greater repulsion between the two catalysts, which prevents dimerization.

Graphic Abstract

Switching the O-O Bond Formation Pathways of Ru-pda Water Oxidation Catalyst by Third Coordination Sphere Engineering

Water oxidation is a vital anodic reaction for renewable fuel generation via electrochemical- and photoelectrochemical-driven water splitting or CO₂ reduction. Ruthenium complexes, such as Ru-bda family, have been shown as highly efficient water-oxidation catalysts (WOCs), particularly when they undergo a bimolecular O-O bond formation pathway. In this study, a novel Ru(pda)-type (pda^2- =1,10-phenanthroline-2,9-dicarboxylate) molecular WOC with 4-vinylpyridine axial ligands was immobilized on the glassy carbon electrode surface by electrochemical polymerization. Electrochemical kinetic studies revealed that this homocoupling polymer catalyzes water oxidation through a bimolecular radical coupling pathway, where interaction between two Ru(pda)-oxyl moieties (I2M) forms the O-O bond. The calculated barrier of the I2M pathway by density-functional theory (DFT) is significantly lower than the barrier of a water nucleophilic attack (WNA) pathway. By using this polymerization strategy, the Ru centers are brought closer in the distance, and the O-O bond formation pathway by the Ru (pda) catalyst is switched from WNA in a homogeneous molecular catalytic system to I2M in the polymerized film, providing some deep insights into the importance of third coordination sphere engineering of the water oxidation catalyst.

Electrostatic Interactions Accelerating Water Oxidation Catalysis via Intercatalyst O-O Coupling

Intercatalyst coupling has been widely applied in the functional mimics for binuclear synergy in natural metal enzymes. Herein, we introduce two facile and effective design strategies, which facilitate the coupling of two catalytic units via electrostatic interactions. The first system is based on a catalyst molecule functionalized with both a positively charged and a negatively charged group in the structure being able to pair with each other in an antiparallel manner arranged by electrostatic interactions. The other system consists of a mixture of two different of catalysts modified with either positively or negatively charged groups to generate intermolecular electrostatic interactions. Applying these designs to Ru(bda) (H₂bda = 2,2'-bipyridine-6,6'-dicarboxylic acid) water-oxidation catalysts improved the catalytic performance by more than an order of magnitude. The intermolecular electrostatic interactions in these two systems were fully identified by ¹H NMR, TEM, SAXS, and electrical conductivity experiments. Molecular dynamics simulations further verified that electrostatic interactions contribute to the formation of prereactive dimers, which were found to play a key role in dramatically improving the catalytic performance. The successful strategies demonstrated here can be used in designing other intercatalyst coupling systems for activation and formation of small molecules and organic synthesis.

From Ru-bda to Ru-bds: a step forward to highly efficient molecular water oxidation electrocatalysts under acidic and neutral conditions

Significant advances during the past decades in the design and studies of Ru complexes with polypyridine ligands have led to the great development of molecular water oxidation catalysts and understanding on the O−O bond formation mechanisms. Here we report a Ru-based molecular water oxidation catalyst [Ru(bds)(pic)₂] (Ru-bds; bds²⁻ = 2,2′-bipyridine-6,6′-disulfonate) containing a tetradentate, dianionic sulfonate ligand at the equatorial position and two 4-picoline ligands at the axial positions. This Ru-bds catalyst electrochemically catalyzes water oxidation with turnover frequencies (TOF) of 160 and 12,900 s⁻¹ under acidic and neutral conditions respectively, showing much better performance than the state-of-art Ru-bda catalyst. Density functional theory calculations reveal that (i) under acidic conditions, the high valent Ru intermediate Ru^V=O featuring the 7-coordination configuration is involved in the O−O bond formation step; (ii) under neutral conditions, the seven-coordinate Ru^IV=O triggers the O−O bond formation; (iii) in both cases, the I2M (interaction of two M−O units) pathway is dominant over the WNA (water nucleophilic attack) pathway.

Developing efficient molecular water oxidation catalysts for artificial photosynthesis is a challenging task. Here the authors introduce a ruthenium based complex with negatively charged sulfonate groups to effectively drive water oxidation under both acidic and neutral conditions.

A Calix[4]arene-Based Cyclic Dinuclear Ruthenium Complex for Light-Driven Catalytic Water Oxidation

A cyclic dinuclear ruthenium(bda) (bda: 2,2'-bipyridine-6,6'-dicarboxylate) complex equipped with oligo(ethylene glycol)-functionalized axial calix[4]arene ligands has been synthesized for homogenous catalytic water oxidation. This novel Ru(bda) macrocycle showed significantly increased catalytic activity in chemical and photocatalytic water oxidation compared to the archetype mononuclear reference [Ru(bda)(pic)2 ]. Kinetic investigations, including kinetic isotope effect studies, disclosed a unimolecular water nucleophilic attack mechanism of this novel dinuclear water oxidation catalyst (WOC) under the involvement of the second coordination sphere. Photocatalytic water oxidation with this cyclic dinuclear Ru complex using [Ru(bpy)3 ]Cl2 as a standard photosensitizer revealed a turnover frequency of 15.5 s-1 and a turnover number of 460. This so far highest photocatalytic performance reported for a Ru(bda) complex underlines the potential of this water-soluble WOC for artificial photosynthesis.

Dispersion forces drive water oxidation in molecular ruthenium catalysts

Rational design of artificial water-splitting catalysts is central for developing new sustainable energy technology. However, the catalytic efficiency of the natural light-driven water-splitting enzyme, photosystem II, has been remarkably difficult to achieve artificially. Here we study the molecular mechanism of ruthenium-based molecular catalysts by integrating quantum chemical calculations with inorganic synthesis and functional studies. By employing correlated ab initio calculations, we show that the thermodynamic driving force for the catalysis is obtained by modulation of π-stacking dispersion interactions within the catalytically active dimer core, supporting recently suggested mechanistic principles of Ru-based water-splitting catalysts. The dioxygen bond forms in a semi-concerted radical coupling mechanism, similar to the suggested water-splitting mechanism in photosystem II. By rationally tuning the dispersion effects, we design a new catalyst with a low activation barrier for the water-splitting. The catalytic principles are probed by synthesis, structural, and electrochemical characterization of the new catalyst, supporting enhanced water-splitting activity under the examined conditions. Our combined findings show that modulation of dispersive interactions provides a rational catalyst design principle for controlling challenging chemistries.

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

Ruthenium-based catalysts for water oxidation: the key role of carboxyl groups as proton acceptors

In this work, the mechanism of water oxidation catalyzed by an Ru-based complex [Ru(L)]⁺ (L = 5,5-chelated 2-carboxy-phen, 2,2′;6′,2′′-terpyridine) was studied by density functional theory (DFT) calculations.

Understanding the performance of a bisphosphonate Ru water oxidation catalyst

Water oxidation catalysts (WOCs) are a key part of generating H₂ from water and sunlight, consequently, it is a promising process for the production of clean energy.

Artificial photosynthesis: opportunities and challenges of molecular catalysts

Molecular catalysis plays an essential role in both natural and artificial photosynthesis (AP). However, the field of molecular catalysis for AP has gradually declined in recent years because of doubt about the long-term stability of molecular-catalyst-based devices. This review summarizes the development history of molecular-catalyst-based AP, including the fundamentals of AP, molecular catalysts for water oxidation, proton reduction and CO2 reduction, and molecular-catalyst-based AP devices, and it provides an analysis of the advantages, challenges, and stability of molecular catalysts. With this review, we aim to highlight the following points: (i) an investigation on molecular catalysis is one of the most promising ways to obtain atom-efficient catalysts with outstanding intrinsic activities; (ii) effective heterogenization of molecular catalysts is currently the primary challenge for the application of molecular catalysis in AP devices; (iii) development of molecular catalysts is a promising way to solve the problems of catalysis involved in practical solar fuel production. In molecular-catalysis-based AP, much has been attained, but more challenges remain with regard to long-term stability and heterogenization techniques.

Ru-bda: Unique Molecular Water-Oxidation Catalysts with Distortion Induced Open Site and Negatively Charged Ligands

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 water-oxidation catalysis but also other small molecule activation and conversion reactions involving artificial photosynthesis, such as CO₂ reduction and N₂ fixation reactions.

Quantum Chemical Study of the Mechanism of Water Oxidation Catalyzed by a Heterotrinuclear Ru2 Mn Complex

The heterotrinuclear complex A {[Ru^II (H₂ O)(tpy)]₂ (μ-[Mn^II (H₂ O)₂ (bpp)₂ ])}⁴⁺ [tpy=2,2':6',2''-terpyridine, bpp=3,5-bis(2-pyridyl)pyrazolate] was found to catalyze water oxidation both electrochemically and photochemically with [Ru(bpy)₃ ]³⁺ (bpy=2,2'-bipyridine) as the photosensitizer and Na₂ S₂ O₈ as the electron acceptor in neutral phosphate buffer. The mechanism of water oxidation catalyzed by this unprecedented trinuclear complex was studied by density functional calculations. The calculations showed that a series of oxidation and deprotonation events take place from A, leading to the formation of complex 1 (formal oxidation state of Ru1^IV Mn^III Ru2^III ), which is the starting species for the catalytic cycle. Three sequential oxidations of 1 result in the generation of the catalytically competing species 4 (formal oxidation state of Ru1^IV Mn^V Ru2^IV ), which triggers the O-O bond formation. The direct coupling of two adjacent oxo ligands bound to Ru and Mn leads to the production of a superoxide intermediate Int1. This step was calculated to have a barrier of 7.2 kcal mol^-1 at the B3LYP*-D3 level. Subsequent O₂ release from Int1 turns out to be quite facile. Other possible pathways were found to be much less favorable, including water nucleophilic attack, the coupling of an oxo and a hydroxide, and the direct coupling pathway at a lower oxidation state (Ru^IV Mn^IV Ru^IV ).

Quantum Chemical Modeling of Homogeneous Water Oxidation Catalysis

The design of efficient and robust water oxidation catalysts has proven challenging in the development of artificial photosynthetic systems for solar energy harnessing and storage. Tremendous progress has been made in the development of homogeneous transition-metal complexes capable of mediating water oxidation. To improve the efficiency of the catalyst and to design new catalysts, a detailed mechanistic understanding is necessary. Quantum chemical modeling calculations have been successfully used to complement the experimental techniques to suggest a catalytic mechanism and identify all stationary points, including transition states for both O-O bond formation and O₂ release. In this review, recent progress in the applications of quantum chemical methods for the modeling of homogeneous water oxidation catalysis, covering various transition metals, including manganese, iron, cobalt, nickel, copper, ruthenium, and iridium, is discussed.

Discovery of Open Cubane Core Structures for Biomimetic LnCo3 (OR)4 Water Oxidation Catalysts

Bio-mimetic catalysts such as LnCo₃ (OR)₄ (Ln=Er, Tm; OR=alkoxide) cubanes have recently been in the focus of research for artificial water oxidation processes. Previously, the remarkable adaptability with respect to ligand shell, nuclear structure as well as protonation and oxidation states of those catalysts has been shown to be beneficial for the water oxidation process. We further explored the structural flexibility of those catalysts and present here a series of novel structures in which one metal center is pulled out of the cubane cage. This leads to an open cubane core, which is to some extent reminiscent of observed open/closed cubane-core forms of the oxygen-evolving complex in nature's photosystem II. We investigate how those open cubane core models alter the thermodynamics of the water oxidation cycle and how different solvation approaches influence their stability.