Second Coordination Sphere Effects in an Evolved Ru Complex Based on Highly Adaptable Ligand Results in Rapid Water Oxidation Catalysis

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.

Adaptive water oxidation catalysis on a carboxylate-sulfonate ligand with low onset potential

A water oxidation catalyst Ru-bcs (bcs = 2,2'-bipyridine-6'-carboxylate-6-sulfonate) with a hybrid ligand was reported. Ru-bcs utilizes the electron-donating properties of carboxylate ligands and the on-demand coordination feature of sulfonate ligands to enable a low onset potential of 1.21 V vs. NHE and a high TOF over 1000 s-1 at pH 7. The adaptive chemistry uncovered in this work provides new perspectives for developing molecular catalysts with high efficiency under low driving forces.

Removing the Barrier in O-O Bond Formation Via the Combination of Intramolecular Radical Coupling and the Oxide Relay Mechanism

The Ru(tda) catalyst has been a major milestone in the development of molecular water oxidation catalysts due to its outstanding performance at neutral pH. The role of the noncoordinating carboxylate group is to act as a nucleophile, donating an oxygen atom to the oxo group, thereby acting as an oxide relay (OR) mechanism for O-O bond formation. A substitution of the carboxylates for phosphonate groups has been proposed, resulting in the Ru(tPaO) catalyst, which has shown even more efficient performance in experimental characterization. In this study, we explore the feasibility of the OR mechanism in the newly reported Ru(tPaO) molecular catalyst. We investigated the catalytic cycle using density functional theory and identified a variation of the OR mechanism that involves radical oxygen atoms in O-O bond formation. We have also determined that the subsequent hydroxide nucleophilic attack is the sole rate-limiting step in the catalytic cycle. All activation free energies are very low, with a free-energy barrier of 2.1 kcal/mol for O-O bond formation and 4.2 kcal/mol for OH- nucleophilic attack.

Pivotal Role of Geometry Regulation on O-O Bond Formation Mechanism of Bimetallic Water Oxidation Catalysts

In this study, we highlight the impact of catalyst geometry on the formation of O-O bonds in Cu2 and Fe2 catalysts. A series of Cu2 complexes with diverse linkers are designed as electrocatalysts for water oxidation. Interestingly, the catalytic performance of these Cu2 complexes is enhanced as their molecular skeletons become more rigid, which contrasts with the behavior observed in our previous investigation with Fe2 analogs. Moreover, mechanistic studies reveal that the reactivity of the bridging O atom results in distinct pathways for O-O bond formation in Cu2 and Fe2 catalysts. In Cu2 systems, the coupling takes place between a terminal CuIII -OH and a bridging μ-O⋅ radical. Whereas in Fe2 systems, it involves the coupling of two terminal Fe-oxo entities. Furthermore, an in-depth structure-activity analysis uncovers the spatial geometric prerequisites for the coupling of the terminal OH with the bridging μ-O⋅ radical, ultimately leading to the O-O bond formation. Overall, this study emphasizes the critical role of precisely adjusting the spatial geometry of catalysts to align with the O-O bonding pathway.

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.

Electrocatalytic Water Oxidation by Mononuclear Copper Complexes of Bis-amide Ligands with N4 Donor: Experimental and Theoretical Investigation

The present work describes electrocatalytic water oxidation of three monomeric copper complexes [CuII(L1)] (1), [CuII(L2)(H2O)] (2), and [CuII(L3)] (3) with bis-amide tetradentate ligands: L1 = N,N'-(1,2-phenylene)dipicolinamide, L2 = N,N'-(4,5-dimethyl-1,2-phenylene)bis(pyrazine-2-carboxamide), L3 = N,N'-(1,2-phenylene)bis(pyrazine-2-carboxamide), for the production of molecular oxygen by the oxidation of water at pH 13.0. Ligands and all complexes have been synthesized and characterized by single crystal XRD, analytical, and spectroscopic techniques. X-ray crystallographic data show that the ligand coordinates to copper in a dianionic fashion through deprotonation of two -NH protons. Cyclic voltammetry study shows a reversible copper-centered redox couple with one ligand-based oxidation event. The electrocatalytic water oxidation occurs at an onset potential of 1.16 (overpotential, η ≈ 697 mV), 1.2 (η ≈ 737 mV), and 1.23 V (η ≈ 767 mV) for 1, 2, and 3 respectively. A systematic variation of the ligand scaffold has been found to display a profound effect on the rate of electrocatalytic oxygen evolution. The results of the theoretical (density functional theory) studies show the stepwise ligand-centered oxidation process and the formation of the O-O bond during water oxidation passes through the water nucleophilic attack for all the copper complexes. At pH = 13, the turnover frequencies have been experimentally obtained as 88, 1462, and 10 s-1 (peak current measurements) for complexes 1, 2, and 3, respectively. Production of oxygen gas during controlled potential electrolysis was detected by gas chromatography.

Electrochemical Water Oxidation and CO2 Reduction with a Nickel Molecular Catalyst

Mimicking the photosynthesis of green plants to combine water oxidation with CO2 reduction is of great significance for solving energy and environmental crises. In this context, a trinuclear nickel complex, [NiII3(paoH)6(PhPO3)2]·2ClO4 (1), with a novel structure has been constructed with PhPO32- (phenylphosphonate) and paoH (2-pyridine formaldehyde oxime) ligands and possesses a reflection symmetry with a mirror plane revealed by single-crystal X-ray diffraction. Bulk electrocatalysis demonstrates that complex 1 can homogeneously catalyze water oxidation and CO2 reduction simultaneously. It can catalyze water oxidation at a near-neutral condition of pH = 7.45 with a high TOF of 12.2 s-1, and the Faraday efficiency is as high as 95%. Meanwhile, it also exhibits high electrocatalytic activity for CO2 reduction towards CO with a TOF of 7.84 s-1 in DMF solution. The excellent electrocatalytic performance of the water oxidation and CO2 reduction of complex 1 could be attributed to the two unique µ3-PhPO32- bridges as the crucial factor for stabilizing the trinuclear molecule as well as the proton transformation during the catalytic process, while the oxime groups modulate the electronic structure of the metal centers via π back-bonding. Therefore, apart from the cooperation effect of the three Ni centers for catalysis, simultaneously, the two kinds of ligands in complex 1 can also synergistically coordinate the central metal, thereby significantly promoting its catalytic performance. Complex 1 represents the first nickel molecular electrocatalyst for both water oxidation and CO2 reduction. The findings in this work open an avenue for designing efficient molecular electrocatalysts with peculiar ligands.

Correlations between the Electronic Structure and Energetics of the Catalytic Steps in Homogeneous Water Oxidation Catalysis

The development of an efficient electrocatalyst for the water oxidation reaction is limited by unfavorable scaling relations between catalytic intermediates, resulting in an overpotential. In contrast to heterogeneous catalysts, the electronic structure of homogeneous catalysts can be modified to a great extent due to a tailored ligand design. However, studies utilizing the tunability of organic ligands have rarely been conducted in a systematic manner and, as of yet, have not produced catalytic paths that avoid the aforementioned unfavorable scaling relations. To investigate the influence of electron-donating groups (EDGs) or electron-withdrawing groups (EWGs) on elementary steps in electrochemical water oxidation catalysis, cis-[Ru(bpy)2(H2O)]2+ (bpy = 2,2'-bipyridine) was selected as the scaffold that was modified with methyl, methoxy, chloro, and trifluoromethyl groups. This catalyst can undergo several electron transfer (ET), proton transfer (PT), and proton-coupled electron transfer (PCET) steps that were all probed experimentally. In this systematic study, it was found that PCET steps are relatively insensitive with respect to the presence of EDGs or EWGs, while the decoupled ET and PT steps are more heavily affected. However, the influence of the substituents decreases with an increasing oxidation state of Ru due to a lack of d-electrons available at the Ru center for π-backbonding to the bipyridine ligand. Therefore, the RuV/VI redox couple appears to be relatively unaffected by the substituent. Nevertheless, the implementation of EWGs can shift all oxidation events to a very narrow potential window. Not only do our findings illustrate how electronic substituents affect the entire potential energy landscape of the catalytic water oxidation reaction, but they also show that the cis-[Ru(bpy)2(H2O)]2+ compounds follow different design rules and scaling relations, as has been reported for every other oxygen evolution catalyst thus far.

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.

Bioinspired Active Site with a Coordination-Adaptive Organosulfonate Ligand for Catalytic Water Oxidation at Neutral pH

Many enzymes use adaptive frameworks to preorganize substrates, accommodate various structural and electronic demands of intermediates, and accelerate related catalysis. Inspired by biological systems, a Ru-based molecular water oxidation catalyst containing a configurationally labile ligand [2,2':6',2″-terpyridine]-6,6″-disulfonate was designed to mimic enzymatic framework, in which the sulfonate coordination is highly flexible and functions as both an electron donor to stabilize high-valent Ru and a proton acceptor to accelerate water dissociation, thus boosting the catalytic water oxidation performance thermodynamically and kinetically. The combination of single-crystal X-ray analysis, various temperature NMR, electrochemical techniques, and DFT calculations was utilized to investigate the fundamental role of the self-adaptive ligand, demonstrating that the on-demand configurational changes give rise to fast catalytic kinetics with a turnover frequency (TOF) over 2000 s^-1, which is compared to oxygen-evolving complex (OEC) in natural photosynthesis.

Copper Complex Catalyzed Two-Electron and Proton Shuttle Mechanism of O-O Bond Formation from DFT-Based Metadynamics Simulations

We performed first-principles metadynamics simulations to explore the mechanistic pathway of oxygen-oxygen bond formation catalyzed by cis-bis(hydroxo) and cis-(hydroxo)oxo copper complexes. The ligands of considered complexes involve modified bipyridine ligands with oxo and hydroxo groups on 6, 6' positions. The study focuses on the kinetics and thermodynamics of the oxygen-oxygen bond formation. The individual migration of the proton to the hydroxyl group and hydroxide to the oxo and hydroxo moieties of the complexes was examined. The proton transfer requires more kinetic barrier than the hydroxide migration. The nature of the electronic density was analyzed with the help of spin population analysis. The molecular orbitals and natural orbital analysis were carried out to examine the nature of the orbitals involved in the oxygen-oxygen bond formation. The σ*(d_x²_-y²-p_x) molecular orbital of the Cu-O or Cu-OH bond overlaps with the p_z orbital of the hydroxide ion in forming the oxygen-oxygen bond. The two-electron two-centered (2e^--2C) bond is observed in the oxygen-oxygen bond formation. In the oxidation process, these ligands stabilize the electron density from the water or hydroxide ion. These redox-active ligands also help stabilize the formed hydrogen peroxide or peroxide complexes.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Theoretical study on the mechanism of water oxidation catalyzed by a mononuclear copper complex: important roles of a redox non-innocent ligand and HPO4 2- anion

The water oxidation reaction is the bottleneck problem of the artificial photosynthetic system. In this work, the mechanism of water oxidation catalyzed by a mononuclear copper complex in alkaline conditions was studied by density functional calculations. Firstly, a water molecule coordinating with the copper center of the complex (Cuii, 1) generates Cuii-H2O (2). 2 undergoes two proton-coupled electron transfer processes to produce intermediate (4). The oxidation process occurs mainly on the ligand moiety, and 4 (˙L-Cuii-O˙) can be described as a Cuii center interacting with a ligand radical antiferromagnetically and an oxyl radical ferromagnetically. 4 is the active species that can trigger O-O bond formation via the water nucleophilic attack mechanism. This process occurs in a step-wise manner. The attacking water transfers one of the protons to the HPO4 2- coupled with an electron transfer to the ligand radical, which generates a transient OH˙ interacting with the oxyl radical and H2PO4 -. Then the O-O bond is formed through the direct coupling of the oxo radical and the OH radical. The triplet di-oxygen could be released after two oxidation processes. According to the Gibbs free energy diagram, the O-O bond formation was suggested to be the rate-limiting step with a calculated total barrier of 19.5 kcal mol-1. More importantly, the copper complex catalyzing water oxidation with the help of a redox non-innocent ligand and HPO4 2- was emphasized.

Substituents make a difference: 6,6″-modified terpyridine complexes with helix configuration and enhanced emission

A series of complexes L22-M (L2: 6,6″-bis(4-methoxyphenyl)-4'-phenyl-2,2':6',2″-terpyridine, M: Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺) were synthesized by coordinating p-methoxyphenyl 6,6″-substituted terpyridine ligand with first-row transition metal ions and characterized by NMR, ESI-MS, and X-ray single crystal diffraction techniques. Single-crystal structures demonstrated that the steric hindrance of p-methoxyphenyl substituents endowed complexes L22-M with obvious longer coordination bond lengths and larger bond angles and dihedral angles compared with unmodified L12-M (L1: 4'-phenyl-2,2':6',2″-terpyridine). The chiral helix geometry was observed for L22-M, in which 2,2':6',2″-terpyridine moiety dramatically twisted to a spiral form in comparison to the nearly coplanar structure of the parent L12-M, resulting in plentiful intramolecular and intermolecular π-π interactions. Also, the appealing racemic (P and M) double helix packed structure for 6,6″-modified bisterpyridine complex L22-Cu was formed in the crystal. The consequent appealing charge transfer (CT) emission for L22-Zn in the solution and solid were investigated via UV-vis and fluorescence spectroscopy techniques and time-dependent density functional theory (TD-DFT) calculations. This work afforded a new method to achieve intriguing chiral geometry and CT optical properties via the subtle design and modification of terpyridine ligands.

Green Energy by Hydrogen Production from Water Splitting, Water Oxidation Catalysis and Acceptorless Dehydrogenative Coupling

In this review, we want to explain how the burning of fossil fuels is pushing us towards green energy. Actually, for a long time, we have believed that everything is profitable, that resources are unlimited and there are no consequences. However, the reality is often disappointing. The use of non-renewable resources, the excessive waste production and the abandonment of the task of recycling has created a fragile thread that, once broken, may never restore itself. Metaphors aside, we are talking about our planet, the Earth, and its unique ability to host life, including ourselves. Our world has its balance; when the wind erodes a mountain, a beach appears, or when a fire devastates an area, eventually new life emerges from the ashes. However, humans have been distorting this balance for decades. Our evolving way of living has increased the number of resources that each person consumes, whether food, shelter, or energy; we have overworked everything to exhaustion. Scientists worldwide have already said actively and passively that we are facing one of the biggest problems ever: climate change. This is unsustainable and we must try to revert it, or, if we are too late, slow it down as much as possible. To make this happen, there are many possible methods. In this review, we investigate catalysts for using water as an energy source, or, instead of water, alcohols. On the other hand, the recycling of gases such as CO2 and N2O is also addressed, but we also observe non-catalytic means of generating energy through solar cell production.

Three-Electron Two-Centered Bond and Single-Electron Transfer Mechanism of Water Splitting via a Copper-Bipyridine Complex

We report the atomistic and electronic details of the mechanistic pathway of the oxygen-oxygen bond formation catalyzed by a copper-2,2'-bipyridine complex. Density functional theory-based molecular dynamics simulations and enhanced sampling methods were employed for this study. The thermodynamics and electronic structure of the oxygen-oxygen bond formation are presented in this study by considering the cis-bishydroxo, [Cu^III(bpy)(OH)₂]⁺, and cis-(hydroxo)oxo, [Cu^IV(bpy)(OH)(═O)]⁺, complexes as active catalysts. In the cis-bishydroxo complex, the hydroxide transfer requires a higher kinetic barrier than the proton transfer process. In the case of [Cu^IV(bpy)(OH)(═O)]⁺, the proton transfer requires a higher free energy than the hydroxide one. The peroxide bond formation is thermodynamically favorable for the [Cu^IV(bpy)(OH)(═O)]⁺ complex compared with the other. The hydroxide ion is transferred to one of the Cu-OH moieties, and the proton is transferred to the solvent. The free energy barrier for this migration is higher than that for the former transfer. From the analysis of molecular orbitals, it is found that the electron density is primarily present on the water molecules near the active sites in the highest occupied molecular orbital (HOMO) state and lowest unoccupied molecular orbital (LUMO) of the ligands. Natural bond orbital (NBO) analysis reveals the electron transfer process during the oxygen-oxygen bond formation. The σ*Cu(d_xz)-O(p) orbitals are involved in the oxygen-oxygen bond formation. During the bond formation, three-electron two-centered (3e^--2C) bonds are observed in [Cu^III(bpy)(OH)₂]⁺ during the transfer of the hydroxide before the formation of the oxygen-oxygen bond.

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.

Pillared-MOF@NiV-LDH Composite as a Remarkable Electrocatalyst for Water Oxidation

Oxygen evolution reaction (OER) represents a highly important electrochemical transformation in energy storage and conversion technologies. Considering the low rate of this four-electron half-reaction, there is a demand for efficient, stable, and noble-metal-free electrocatalysts to improve the kinetic and economical parameters. In this work, a new pillared-MOF@NiV-LDH nanocomposite based on a Co^II metal-organic framework (pillared-MOF) and heterometallic Ni/V-layered double hydroxide (NiV-LDH) was assembled via a simple protocol, characterized, and explored as an electrocatalyst in OER. A remarkable electrocatalytic efficiency of pillared-MOF@NiV-LDH in 1 M KOH is evidenced by a low overpotential (238 mV at 10 mA cm^-2 current density) and a small value of the Tafel slope (62 mV dec^-1). These parameters are very close to those of the reference IrO₂ electrocatalyst and are superior to the majority of the LDH- and MOF-based systems previously applied for OER. Excellent stability of pillared-MOF@NiV-LDH was confirmed by the chronopotentiometry tests for 70 h and linear-sweep voltammetry after 7000 cycles. Features such as rich electroactive sites, porous structure, high surface area, and synergic effect between pillared-MOF and NiV-LDH are likely responsible for the remarkable electrocatalytic efficiency of this electrocatalyst in OER. Despite prior reports on the application of NiV-LDH in OER, the present study describes the first example where this type of LDH is blended with MOF to generate a nanocomposite material. The interface between the two components of the composite can improve the electronic structure and, in turn, the electrocatalytic behavior. The introduction of this composite paves the way toward the synthesis of other multicomponent materials with potential applications in different energy fields.

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.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Mechanistic Insights into Cobalt-Based Water Oxidation Catalysis by DFT-Based Molecular Dynamics Simulations

We report the mechanistic details of the water oxidation process by the complex, [Co^II(bpbH₂)Cl₂], where bpbH₂ = N, N'-bis(2'-pyridinecarboxamide)-1,2-benzene. An experimental study reported the complex as the efficient catalyst for the water oxidation process. We performed density functional theory calculations at the M06-L level and first-principles molecular dynamics simulations to study the catalytic nature of the complex. We investigated the energetics of the total catalytic cycle, which combines the oxygen-oxygen bond formation, proton-coupled electron transfer (PCET) steps, and release of oxygen molecule. The formed peroxide and superoxide intermediates in the catalytic cycle were characterized with the help of the Mulliken spin density parameters. Mulliken spin densities of the metal-oxo bond reveal that the triplet state of Co^V═O has a double-bond nature, but the quintet state of the complex has a radical nature (Co^IV-O^•-). In an alternative way, the deprotonation of the amide groups of the ligand is also considered. The deprotonation and formation of higher oxidation metal-oxo intermediates are also possible. In addition to this, we have considered the effect of phosphate buffer on water nucleophilic addition. The oxygen-oxygen bond formation is favorable by the catalyst with the deprotonated form of the ligand, with the addition of water as the nucleophile. In the oxidation process, the C═O bonds of the ligand transfer the electron density to nitrogen atoms, stabilizing the higher order oxo, peroxide, and superoxide bonds. The oxygen-oxygen bond formation is the rate-determining step in the overall water oxidation process. This bond was further investigated using first-principles molecular dynamics at the PBE-D2 level. The dynamics of proton, hydroxide ion, and the nature of the ligand structure on the oxygen-oxygen bond were examined. We find that the oxygen molecule is released from the superoxide complex with the addition of water molecules.

Homogeneous Water Oxidation Catalyzed by First-Row Transition Metal Complexes: Unveiling the Relationship between Turnover Frequency and Reaction Overpotential

The utilization of earth-abundant low-toxicity metal ions in the construction of highly active and efficient molecular catalysts promoting the water oxidation reaction is important for developing a sustainable artificial energy cycle. However, the kinetic and thermodynamic properties of the currently available molecular water oxidation catalysts (MWOCs) have not been comprehensively investigated. This Review summarizes the current status of MWOCs based on first-row transition metals in terms of their turnover frequency (TOF, a kinetic property) and overpotential (η, a thermodynamic property) and uses the relationship between log(TOF) and η to assess catalytic performance. Furthermore, the effects of the same ligand classes on these MWOCs are discussed in terms of TOF and η, and vice versa. The collective analysis of these relationships provides a metric for the direct comparison of catalyst systems and identifying factors crucial for catalyst design.

Dangling Carboxylic Group That Participates in O-O Bond Formation Reaction to Promote Water Oxidation Catalyzed by a Ruthenium Complex: Experimental Evidence of an Oxide Relay Pathway

Two mononuclear ruthenium(II) complexes of the types [Ru(trpy)(HL¹)(OH₂)]²⁺ (1^Aq) and [Ru(trpy)(L²-κ-N²O)] (2) [where trpy = 2,2':6',2″-terpyridine, HL¹ = 2-(2-pyridyl)benzimidazole, H₂L² = 2-(pyridin-2-yl)-1H-benzo[d]imidazole-4-carboxylic acid] have been synthesized and thoroughly characterized by analytical and spectroscopic [UV-vis, NMR, high-resolution mass spectrometry, and IR] techniques. Complex 1^Aq has been further characterized by X-ray crystallography. In an acidic aqueous medium (pH 1), complex 2 undergoes carboxylate/water exchange readily to form an aqua-ligated complex, [Ru(trpy)(H₂L²-κ-N²)(OH₂)]²⁺ (2^Aq), having a dangling carboxylic group. This exchange phenomenon has been followed by IR, ¹H NMR, and UV-vis spectroscopic techniques. Electrochemical analyses of 1^Aq and 2^Aq (Pourbaix diagram) suggest the generation of a formal Ru^V═O species that can potentially promote the oxidation of water. A comparative study of the water oxidation activity catalyzed by 1^Aq and 2^Aq is reported here to see the effect of a dangling carboxylic group in the catalytic performance. Complex 2^Aq shows an enormously higher rate of reaction than 1^Aq. The pendant carboxylic group in 2^Aq participates in an intramolecular O-O bond formation reaction with the reactive formal Ru^V═O unit to form a percarboxylate intermediate and provides an electron-deficient carbon center where water nucleophilic attack takes place. The isotope labeling experiment using ¹⁸O-labeled water verifies the attack of water at the carbon center of the carboxylic group rather than a direct attack at the oxo of the formal Ru^V═O unit. The present work provides experimental evidence of the uncommon functionality of the carboxylic group, the oxide relay, in molecular water oxidation chemistry.

Dye-sensitized solar cells strike back

Dye-sensitized solar cells (DSCs) are celebrating their 30th birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. In recent years, DSCs and dye-sensitized photoelectrochemical cells (DSPECs) have experienced a renaissance as the best technology for several niche applications that take advantage of DSCs' unique combination of properties: at low cost, they are composed of non-toxic materials, are colorful, transparent, and very efficient in low light conditions. This review summarizes the advancements in the field over the last decade, encompassing all aspects of the DSC technology: theoretical studies, characterization techniques, materials, applications as solar cells and as drivers for the synthesis of solar fuels, and commercialization efforts from various companies.

Applying Active Learning to the Screening of Molecular Oxygen Evolution Catalysts

The oxygen evolution reaction (OER) can enable green hydrogen production; however, the state-of-the-art catalysts for this reaction are composed of prohibitively expensive materials. In addition, cheap catalysts have associated overpotentials that render the reaction inefficient. This impels the search to discover novel catalysts for this reaction computationally. In this communication, we present machine learning algorithms to enhance the hypothetical screening of molecular OER catalysts. By predicting calculated binding energies using Gaussian process regression (GPR) models and applying active learning schemes, we provide evidence that our algorithm can improve computational efficiency by guiding simulations towards candidates with promising OER descriptor values. Furthermore, we derive an acquisition function that, when maximized, can identify catalysts that can exhibit theoretical overpotentials that circumvent the constraints imposed by linear scaling relations by attempting to enforce a specific mechanism. Finally, we provide a brief perspective on the appropriate sets of molecules to consider when screening complexes that could be stable and active for this reaction.

Synthesis, Structure, and Ammonia Oxidation Catalytic Activity of Ru-NH3 Complexes Containing Multidentate Polypyridyl Ligands

Ammonia (electro)oxidation with molecular catalysts is a rapidly developing topic with wide practical applications ahead. We report here the catalytic ammonia oxidation reaction (AOR) activity using [Ru(tda-κ-N³O)(py)₂], 2, (tda^2- is 2,2':6',2''-terpyridine-6,6''-dicarboxylate; py is pyridine) as a catalyst precursor. Furthermore, we also describe the rich chemistry associated with the reaction of Ru-tda and Ru-tPa (tPa^-4 is 2,2':6',2''-terpyridine-6,6''-diphosphonate) complexes with NH₃ and N₂H₄ using [Ru^II(tda-κ-N³O)(dmso)Cl] (dmso is dimethyl sulfoxide) and [Ru^II(tPa-κ-N³O)(py)₂], 8, as synthetic intermediates, respectively. All the new complexes obtained here were characterized spectroscopically by means of UV-vis and NMR. In addition, a crystal X-ray diffraction analysis was performed for complexes trans-[Ru^II(tda-κ-N³)(py)₂(NH₃)], 4, trans-[Ru^II(tda-κ-N³)(N-NH₂)(py)₂], 5, cis-[Ru^II(tda-κ-N³)(py)(NH₃)₂], 6 (30%), and cis-[Ru^II(tda-k-N³)(dmso)(NH₃)₂], 7 (70%). The AOR activity associated with 2 and 8 as catalyst precursors was studied in organic and aqueous media. For 2, turnover numbers of 7.5 were achieved under bulk electrolysis conditions at an E_app = 1.4 V versus normal hydrogen electrode in acetonitrile. A catalytic cycle is proposed based on electrochemical and kinetic evidence.

Bio-Inspired Molecular Catalysts for Water Oxidation

The catalytic tetranuclear manganese-calcium-oxo cluster in the photosynthetic reaction center, photosystem II, provides an excellent blueprint for light-driven water oxidation in nature. The water oxidation reaction has attracted intense interest due to its potential as a renewable, clean, and environmentally benign source of energy production. Inspired by the oxygen-evolving complex of photosystem II, a large of number of highly innovative synthetic bio-inspired molecular catalysts are being developed that incorporate relatively cheap and abundant metals such as Mn, Fe, Co, Ni, and Cu, as well as Ru and Ir, in their design. In this review, we briefly discuss the historic milestones that have been achieved in the development of transition metal catalysts and focus on a detailed description of recent progress in the field.

From NiMoO4 to γ-NiOOH: Detecting the Active Catalyst Phase by Time Resolved in Situ and Operando Raman Spectroscopy

Water electrolysis powered by renewable energies is a promising technology to produce sustainable fossil free fuels. The development and evaluation of effective catalysts are here imperative; however, due to the inclusion of elements with different redox properties and reactivity, these materials undergo dynamical changes and phase transformations during the reaction conditions. NiMoO4 is currently investigated among other metal oxides as a promising noble metal free catalyst for the oxygen evolution reaction. Here we show that at applied bias, NiMoO4·H2O transforms into γ-NiOOH. Time resolved operando Raman spectroscopy is utilized to follow the potential dependent phase transformation and is collaborated with elemental analysis of the electrolyte, confirming that molybdenum leaches out from the as-synthesized NiMoO4·H2O. Molybdenum leaching increases the surface coverage of exposed nickel sites, and this in combination with the formation of γ-NiOOH enlarges the amount of active sites of the catalyst, leading to high current densities. Additionally, we discovered different NiMoO4 nanostructures, nanoflowers, and nanorods, for which the relative ratio can be influenced by the heating ramp during the synthesis. With selective molybdenum etching we were able to assign the varying X-ray diffraction (XRD) pattern as well as Raman vibrations unambiguously to the two nanostructures, which were revealed to exhibit different stabilities in alkaline media by time-resolved in situ and operando Raman spectroscopy. We advocate that a similar approach can beneficially be applied to many other catalysts, unveiling their structural integrity, characterize the dynamic surface reformulation, and resolve any ambiguities in interpretations of the active catalyst phase.

Consecutive Ligand-Based Electron Transfer in New Molecular Copper-Based Water Oxidation Catalysts

Water oxidation to dioxygen is one of the key reactions that need to be mastered for the design of practical devices based on water splitting with sunlight. In this context, water oxidation catalysts based on first‐row transition metal complexes are highly desirable due to their low cost and their synthetic versatility and tunability through rational ligand design. A new family of dianionic bpy‐amidate ligands of general formula H₂LNⁿ⁻ (LN is [2,2′‐bipyridine]‐6,6′‐dicarboxamide) substituted with phenyl or naphthyl redox non‐innocent moieties is described. A detailed electrochemical analysis of [(L4)Cu]²⁻ (L4=4,4′‐(([2,2′‐bipyridine]‐6,6′‐dicarbonyl)bis(azanediyl))dibenzenesulfonate) at pH 11.6 shows the presence of a large electrocatalytic wave for water oxidation catalysis at an η=830 mV. Combined experimental and computational evidence, support an all ligand‐based process with redox events taking place at the aryl‐amide groups and at the hydroxido ligands.

Ligand Radical Mediated Water Oxidation by a Family of Copper o-Phenylene Bis-oxamidate Complexes

Understanding the reactivity landscape for the activation of water until the formation of the O-O bond and O₂ release in molecular chemistry is a decisive step in guiding the elaboration of cost-effective catalysts for the oxygen-evolving reaction (OER). Copper(II) complexes have recently caught the attention of chemists as catalysts for the 4e^-/4H⁺ water oxidation process. While a copper(IV) intermediate has been proposed as the reactive intermediate species, no spectroscopic signature has been reported so far. Copper(III) ligand radical species have also been formulated and supported by theoretical studies. We found, herein, that the reactivity sequence for the water oxidation with a family of Copper(II) o-phenylene bis-oxamidate complexes is a function of the substitution pattern on the periphery of the aromatic ring. In-situ EPR, FTIR, and rR spectroelectrochemical studies helped to sequence the elementary electrochemical and chemical events leading toward the O₂ formation selectively at the copper center. EPR and FTIR spectroelectrochemistry suggests that ligand-centered oxidations are preferred over metal-centered oxidations. rR spectroelectrochemical study revealed the accumulation of a bis-imine bound copper(II) superoxide species, as the reactive intermediate, under catalytic turnover, which provides the evidence for the O-O bond formation during OER.

Ligand Controls the Activity of Light-Driven Water Oxidation Catalyzed by Nickel(II) Porphyrin Complexes in Neutral Homogeneous Aqueous Solutions

Finding photostable, first‐row transition metal‐based molecular systems for photocatalytic water oxidation is a step towards sustainable solar fuel production. Herein, we discovered that nickel(II) hydrophilic porphyrins are molecular catalysts for photocatalytic water oxidation in neutral to acidic aqueous solutions using [Ru(bpy)₃]²⁺ as photosensitizer and [S₂O₈]²⁻ as sacrificial electron acceptor. Electron‐poorer Ni‐porphyrins bearing 8 fluorine or 4 methylpyridinium substituents as electron‐poorer porphyrins afforded 6‐fold higher turnover frequencies (TOFs; ca. 0.65 min⁻¹) than electron‐richer analogues. However, the electron‐poorest Ni‐porphyrin bearing 16 fluorine substituents was photocatalytically inactive under such conditions, because the potential at which catalytic O₂ evolution starts was too high (+1.23 V vs. NHE) to be driven by the photochemically generated [Ru(bpy)₃]³⁺. Critically, these Ni‐porphyrin catalysts showed excellent stability in photocatalytic conditions, as a second photocatalytic run replenished with a new dose of photosensitizer, afforded only 1–3 % less O₂ than during the first photocatalytic run.

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.

Plasma-Initiated Graft Polymerization of Acrylic Acid onto Fluorine-Doped Tin Oxide as a Platform for Immobilization of Water-Oxidation Catalysts

The discovery of new and versatile strategies for the immobilization of molecular water-oxidation catalysts (WOCs) is crucial for developing clean energy conversion devices [e.g., (photo)electrocatalytic cells for water splitting]. The traditional approach for surface attachment to transparent conductive oxides [e.g., fluorine doped tin oxide (FTO)] is via synthetic modification of the ligand architecture to incorporate functional groups such as carboxylic acids (-COOH) or phosphonates (-PO₃H₂) prior to immobilization. However, challenges arising from desorption and the cumbersome derivatizations steps have limited the scope and applications of surface-bound WOCs. Herein, we report the successful immobilization of underivatized Ru(II)-based WOCs (Ru-Cat1 = [Ru(tpy) (bpy) (H₂O)]²⁺ (tpy = 2,2':6'2″-terpyridine and bpy = 2,2'-bipyridine) and Ru-Cat2 = [Ru(Mebimpy) (bpy) (H₂O)]²⁺ (Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl) pyridine)) and the Ru(II) polypyridyl chromophore Ru-C3 = [Ru(bpy)₃]²⁺ onto a FTO plasma-grafted poly(acrylic acid) surface (PAA|FTO). Various characterization techniques such as attenuated total reflectance Fourier transform infrared spectroscopy, scanning electron microscopy, atomic force microscopy, and cyclic voltammetry measurements provide evidence for the plasma-induced grafted PAA|FTO film and immobilization. Surface stability and electrocatalytic properties of these new hybrid composite films upon cycling were investigated at different pH values. Immobilized Ru-Cat1 and Ru-Cat2 onto PAA|FTO displayed pH-dependent (Ru^III/Ru^II) couples and onset potentials indicative of PCET (proton-coupled electron transfer) reactions. Based on cyclic voltammetry results and spectroscopic monitoring, the immobilized WOCs Ru-Cat1 and Ru-Cat2 exhibited a higher surface stability in neutral aqueous solutions relative to Ru-C3 upon electrochemical oxidation. We attribute the surface PCET and stability to the presence of a water ligand in the coordination sphere of immobilized Ru-Cat1 and Ru-Cat2 which can H-bond with negatively charged carboxylate groups of the cross-linked PAA brushes. Our findings demonstrate that the plasma-grafted polymeric network onto FTO offers a versatile platform to directly anchor unmodified homogeneous WOCs or chromophores for potential applications in solar-to-fuel energy conversion.

A Ru diphosphonato complex with a metal-metal bond for water oxidation

Unlike [Ru2(μ-O2CCH3)4], the structurally analogous water-soluble RuII,III2 diphosphonato complex K3[Ru2(hedp)2(H2O)2] (K3·1) is only involved in stoichiometric water oxidation with a maximum 67% O2 yield under CAN/HNO3 solution (pH 1.0) for 2.5 h. The water oxidation mechanism and intermediate products were ascertained by UV-vis, ESI-MS and DFT calculation.

Mechanism and Dynamics of Formation of Bisoxo Intermediates and O-O Bond in the Catalytic Water Oxidation Process

This work elucidates the reactivity of water molecules toward the tridentate nitrogen-containing iron complex in the water oxidation process. Here, we consider the Fe^V-bisoxo complex {[Fe^V(Me₃tacn)(OH₂)(═O)₂]⁺} to be responsible for the oxygen-oxygen bond formation. This O-O bond formation happens through the addition of water as a nucleophile. The transition state was determined by the synchronous transit-guided quasi-Newton method using reactants and products and verified by intrinsic reaction coordinates (IRCs). From the IRC calculations, we observe that the Fe^V═O moiety is attacked by water and assisted by the H-bonded interaction with the oxygen atom of the bisoxo complex. The hydrogen atom is transferred to the oxygen atom of the bisoxo complex through the transition state, and subsequently, the hydroxide is transferred to another oxygen of the bisoxo complex, resulting in the formation of the oxygen-oxygen bond. This work also explains the effect of explicit water molecules on the oxygen-oxygen bond formation. Our results also show how the formation of superoxide plays an essential role in O₂ evolution. We used the potential energy scan method to compute the transition state in the oxygen evolution step. In the present work, we study the effect of chlorine on the formation of the oxygen-oxygen bond formation. In this study, the changes in the oxidation state, spin density, and spin multiplicity of the complexes are investigated for each successive step. Apart from these static theoretical calculations, we also studied the oxygen-oxygen bond formation through first-principles molecular dynamics with the aid of the well-tempered metadynamics sampling technique. From the observation of the free energy surfaces from metadynamics simulations, it is evident that the hydroxide transfer has a lesser free energetic reaction as compared to the proton transfer. This complete mechanistic study may give an idea to design a suitable water oxidation catalyst.

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.