Structural and Spectroscopic Characterization of Reaction Intermediates Involved in a Dinuclear Co–Hbpp Water Oxidation Catalyst

Evolution in the Design of Water Oxidation Catalysts with Transition-Metals: A Perspective on Biological, Molecular, Supramolecular, and Hybrid Approaches

Increased demand for a carbon-neutral sustainable energy scheme augmented by climatic threats motivates the design and exploration of novel approaches that reserve intermittent solar energy in the form of chemical bonds in molecules and materials. In this context, inspired by biological processes, artificial photosynthesis has garnered significant attention as a promising solution to convert solar power into chemical fuels from abundantly found H2O. Among the two redox half-reactions in artificial photosynthesis, the four-electron oxidation of water according to 2H2O → O2 + 4H+ + 4e- comprises the major bottleneck and is a severe impediment toward sustainable energy production. As such, devising new catalytic platforms, with traditional concepts of molecular, materials and biological catalysis and capable of integrating the functional architectures of the natural oxygen-evolving complex in photosystem II would certainly be a value-addition toward this objective. In this review, we discuss the progress in construction of ideal water oxidation catalysts (WOCs), starting with the ingenuity of the biological design with earth-abundant transition metal ions, which then diverges into molecular, supramolecular and hybrid approaches, blurring any existing chemical or conceptual boundaries. We focus on the geometric, electronic, and mechanistic understanding of state-of-the-art homogeneous transition-metal containing molecular WOCs and summarize the limiting factors such as choice of ligands and predominance of environmentally unrewarding and expensive noble-metals, necessity of high-valency on metal, thermodynamic instability of intermediates, and reversibility of reactions that create challenges in construction of robust and efficient water oxidation catalyst. We highlight how judicious heterogenization of atom-efficient molecular WOCs in supramolecular and hybrid approaches put forth promising avenues to alleviate the existing problems in molecular catalysis, albeit retaining their fascinating intrinsic reactivities. Taken together, our overview is expected to provide guiding principles on opportunities, challenges, and crucial factors for designing novel water oxidation catalysts based on a synergy between conventional and contemporary methodologies that will incite the expansion of the domain of artificial photosynthesis.

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.

Characterization of Reaction Intermediates Involved in the Water Oxidation Reaction of a Molecular Cobalt Complex

Molecular cobalt(III) complexes of bis-amidate-bis-alkoxide ligands, (Me₄N)[Co^III(L₁)] (1) and (Me₄N)[Co^III(L₂)] (2), are synthesized and assessed through a range of characterization techniques. Electrocatalytic water oxidation activity of the Co complexes in a 0.1 M phosphate buffer solution revealed a ligand-centered 2e^-/1H⁺ transfer event at 0.99 V followed by catalytic water oxidation (WO) at an onset overpotential of 450 mV. By contrast, 2 reveals a ligand-based oxidation event at 0.9 V and a WO onset overpotential of 430 mV. Constant potential electrolysis study and rinse test experiments confirm the homogeneous nature of the Co complexes during WO. The mechanistic investigation further shows a pH-dependent change in the reaction pathway. On the one hand, below pH 7.5, two consecutive ligand-based oxidation events result in the formation of a Co^III(L^2-)(OH) species, which, followed by a proton-coupled electron transfer reaction, generates a Co^IV(L^2-)(O) species that undergoes water nucleophilic attack to form the O-O bond. On the other hand, at higher pH, two ligand-based oxidation processes merge together and result in the formation of a Co^III(L^2-)(OH) complex, which reacts with OH^- to yield the O-O bond. The ligand-coordinated reaction intermediates involved in the WO reaction are thoroughly studied through an array of spectroscopic techniques, including UV-vis absorption spectroscopy, electron paramagnetic resonance, and X-ray absorption spectroscopy. A mononuclear Co^III(OH) complex supported by the one-electron oxidized ligand, [Co^III(L^3-)(OH)]^-, a formal Co^IV(OH) complex, has been characterized, and the compound was shown to participate in the hydroxide rebound reaction, which is a functional mimic of Compound II of Cytochrome P450.

Theoretical Study on the Electro-Reduction of Carbon Dioxide to Methanol Catalyzed by Cobalt Phthalocyanine

Density functional theory (DFT) calculations have been conducted to investigate the mechanism of cobalt(II) tetraamino phthalocyanine (CoPc-NH₂) catalyzed electro-reduction of CO₂. Computational results show that the catalytically active species 1 (⁴[Co^II(H₄L)]⁰) is formed by a four-electron-four-proton reduction of the initial catalyst CoPc-NH₂. Complex 1 can attack CO₂ after a one-electron reduction to give a [Co^III-CO₂^2-]^- intermediate, followed by a protonation and a one-electron reduction to give intermediate [Co^II-COOH]^- (4). Complex 4 is then protonated on its hydroxyl group by a carbonic acid to generate the critical species 6 (Co^III-L^•--CO), which can release the carbon monoxide as an intermediate (and also as a product). In parallel, complex 6 can go through a successive four-electron-four-proton reduction to produce the targeted product methanol without forming formaldehyde as an intermediate product. The high-lying π orbital and the low-lying π* orbital of the phthalocyanine endow the redox noninnocent nature of the ligand, which could be a dianion, a radical monoanion, or a radical trianion during the catalysis. The calculated results for the hydrogen evolution reaction indicate a higher energy barrier than the carbon dioxide reduction. This is consistent with the product distribution in the experiments. Additionally, the amino group on the phthalocyanine ligand was found to have a minor effect on the barriers of critical steps, and this accounts for the experimentally observed similar activity for these two catalysts, namely, CoPc-NH₂ and CoPc.

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.

Understanding the factors governing the water oxidation reaction pathway of mononuclear and binuclear cobalt phthalocyanine catalysts

The rational design of efficient catalysts for electrochemical water oxidation highly depends on the understanding of reaction pathways, which still remains a challenge. Herein, mononuclear and binuclear cobalt phthalocyanine (mono-CoPc and bi-CoPc) with a well-defined molecular structure are selected as model electrocatalysts to study the water oxidation mechanism. We found that bi-CoPc on a carbon support (bi-CoPc/carbon) shows an overpotential of 357 mV at 10 mA cm⁻², much lower than that of mono-CoPc/carbon (>450 mV). Kinetic analysis reveals that the rate-determining step (RDS) of the oxygen evolution reaction (OER) over both electrocatalysts is a nucleophilic attack process involving a hydroxy anion (OH⁻). However, the substrate nucleophilically attacked by OH⁻ for bi-CoPc is the phthalocyanine cation-radical species (Co^II–Pc–Pc˙⁺–Co^II–OH) that is formed from the oxidation of the phthalocyanine ring, while cobalt oxidized species (Pc–Co^III–OH) is involved in mono-CoPc as evidenced by the operando UV-vis spectroelectrochemistry technique. DFT calculations show that the reaction barrier for the nucleophilic attack of OH⁻ on Co^II–Pc–Pc˙⁺–Co^II–OH is 1.67 eV, lower than that of mono-CoPc with Pc–Co^III–OH nucleophilically attacked by OH⁻ (1.78 eV). The good agreement between the experimental and theoretical results suggests that bi-CoPc can effectively stabilize the accumulated oxidative charges in the phthalocyanine ring, and is thus bestowed with a higher OER performance.

bi-CoPc can stabilize accumulated oxidative charges in phthalocyanine ring, which leads to the OER proceeding through a nucleophilic attack of OH^- on the phthalocyanine cation-radical species that is formed from the oxidation of phthalocyanine ring.

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-Constraint-Induced Peroxide Activation for Electrophilic Reactivity

μ‐1,2‐peroxo‐bridged diiron(III) intermediates P are proposed as reactive intermediates in various biological oxidation reactions. In sMMO, P acts as an electrophile, and performs hydrogen atom and oxygen atom transfers to electron‐rich substrates. In cyanobacterial ADO, however, P is postulated to react by nucleophilic attack on electrophilic carbon atoms. In biomimetic studies, the ability of μ‐1,2‐peroxo‐bridged dimetal complexes of Fe, Co, Ni and Cu to act as nucleophiles that effect deformylation of aldehydes is documented. By performing reactivity and theoretical studies on an end‐on μ‐1,2‐peroxodicobalt(III) complex 1 involving a non‐heme ligand system, L1, supported on a Sn₆O₆ stannoxane core, we now show that a peroxo‐bridged dimetal complex can also be a reactive electrophile. The observed electrophilic chemistry, which is induced by the constraints provided by the Sn₆O₆ core, represents a new domain for metal−peroxide reactivity.

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.

Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives

The recent synthetic and mechanistic progress in molecular and heterogeneous water oxidation catalysts highlights the new, overarching strategies for knowledge transfer and unifying design concepts.

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

A new Ru complex containing the deprotonated 2,2':6',2''-terpyridine-6,6''-diphosphonic acid (H₄tPa) and pyridine (py) of general formula [Ru^II(H₃tPa-κ-N³O)(py)₂]⁺, 2⁺, has been prepared and thoroughly characterized by means of spectroscopic and electrochemical techniques, X-ray diffraction analysis, and density functional theory (DFT) calculations. Complex 2⁺ presents a dynamic behavior in the solution that involves the synchronous coordination and the decoordination of the dangling phosphonic groups of the tPa^4- ligand. However, at oxidation state IV, complex 2⁺ becomes seven coordinated with the two phosphonic groups now bonded to the metal center. Further, at this oxidation state at neutral and basic pH, the Ru complex undergoes the coordination of an exogenous OH^- group from the solvent that leads to an intramolecular aromatic O atom insertion into the CH bond of one of the pyridyl groups, forming the corresponding phenoxo-phosphonate Ru complex [Ru^III(tPaO-κ-N²O_PO_C)(py)₂]^2-, 4^2-, where tPaO^5- is the 3-(hydroxo-[2,2':6',2''-terpyridine]-6,6''-diyl)bis(phosphonate) ligand. This new in situ generated Ru complex, 4^2-, has been isolated and spectroscopically and electrochemically characterized. In addition, a crystal structure has been also obtained using single-crystal X-ray diffraction techniques. Complex 4^2- turns out to be an exceptional water oxidation catalyst achieving record maximum turnover frequencies (TOF_max) on the order of 16 000 s^-1. A mechanistic analysis complemented with DFT calculations has also been carried out, showing the critical role of intramolecular second coordination sphere effects exerted by the phosphonate groups in lowering the activation energy at the rate-determining step.

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.

Computational Approach to Molecular Catalysis by 3d Transition Metals: Challenges and Opportunities

Computational chemistry provides a versatile toolbox for studying mechanistic details of catalytic reactions and holds promise to deliver practical strategies to enable the rational in silico catalyst design. The versatile reactivity and nontrivial electronic structure effects, common for systems based on 3d transition metals, introduce additional complexity that may represent a particular challenge to the standard computational strategies. In this review, we discuss the challenges and capabilities of modern electronic structure methods for studying the reaction mechanisms promoted by 3d transition metal molecular catalysts. Particular focus will be placed on the ways of addressing the multiconfigurational problem in electronic structure calculations and the role of expert bias in the practical utilization of the available methods. The development of density functionals designed to address transition metals is also discussed. Special emphasis is placed on the methods that account for solvation effects and the multicomponent nature of practical catalytic systems. This is followed by an overview of recent computational studies addressing the mechanistic complexity of catalytic processes by molecular catalysts based on 3d metals. Cases that involve noninnocent ligands, multicomponent reaction systems, metal-ligand and metal-metal cooperativity, as well as modeling complex catalytic systems such as metal-organic frameworks are presented. Conventionally, computational studies on catalytic mechanisms are heavily dependent on the chemical intuition and expert input of the researcher. Recent developments in advanced automated methods for reaction path analysis hold promise for eliminating such human-bias from computational catalysis studies. A brief overview of these approaches is presented in the final section of the review. The paper is closed with general concluding remarks.

Mechanistic Insight into Dioxygen Evolution from Diastereomeric μ-Peroxo Dinuclear Co(III) Complexes Based on Stoichiometric Electron-Transfer Oxidation

Stoichiometric electron-transfer (ET) oxidation of two diastereomeric μ-peroxo-μ-hydroxo dinuclear Co(III) complexes with tris(2-pyridylmethyl)amine (TPA) was examined to scrutinize the reaction mechanism of O₂ evolution from the peroxo complexes, as seen in the final step in water oxidation by a Co(III)-TPA complex. The two isomeric Co(III)-peroxo complexes were synthesized and selectively isolated by recrystallization under different conditions. Although cyclic voltammograms of the two isomers in aqueous solutions showed one reversible wave at 1.1 V vs NHE at pH 2.0, two oxidation waves were observed at 1.0 and 1.4 V at pH 7.0 in the aqueous solutions, the latter of which is responsible for the O₂-releasing process. At pH 7, one diastereomer showed higher reactivity than the other in O₂ evolution, indicating the importance of structures of the μ-peroxo complexes in the reaction. In order to clarify the O₂-evolving mechanism, we performed electron paramagnetic resonance (EPR) and resonance Raman (RR) measurements for characterizing one-electron oxidized species: The observed EPR and RR signals supported the formation of μ-superoxo-μ-hydroxo dinuclear Co(III) complexes; however, no characteristic difference was observed between two isomers in the EPR parameters including g values and superhyperfine coupling constants. ET-oxidation rate constants of the isomers were determined to be much faster than the O₂-evolving rate constants, indicating that the O₂-releasing step is the rate-determining step in the O₂ evolution through the stoichiometric ET oxidation of the dinuclear Co(III)-μ-peroxo complexes. Therefore, the difference of reactivity in the O₂ evolution for the two isomers should be derived from the thermodynamic stability of two-electron oxidized species of the dinuclear Co(III)-μ-peroxo complexes, μ-dioxygen-μ-hydroxo dinuclear Co(III) intermediates.

Quantitative in situ13C NMR studies of the electro-catalytic oxidation of ethanol

The newly developed pouch cells offer a sensitive method to analyse various products of electrocatalytic reactions, especially of the alcohol oxidation reaction.

Computational Modeling of Cobalt-Based Water Oxidation: Current Status and Future Challenges

A lot of effort is nowadays put into the development of novel water oxidation catalysts. In this context, mechanistic studies are crucial in order to elucidate the reaction mechanisms governing this complex process, new design paradigms and strategies how to improve the stability and efficiency of those catalysts. This review is focused on recent theoretical mechanistic studies in the field of homogeneous cobalt-based water oxidation catalysts. In the first part, computational methodologies and protocols are summarized and evaluated on the basis of their applicability toward real catalytic or smaller model systems, whereby special emphasis is laid on the choice of an appropriate model system. In the second part, an overview of mechanistic studies is presented, from which conceptual guidelines are drawn on how to approach novel studies of catalysts and how to further develop the field of computational modeling of water oxidation reactions.

Armed by Asp? C-terminal carboxylate in a Dap-branched peptide and consequences in the binding of CuII and electrocatalytic water oxidation

C-Terminal carboxylate in branched peptide allows insight into water oxidation electrocatalysis by Cu-complexes, revealing differences to homologues with varied modules.

Trapping of superoxido cobalt and peroxido dicobalt species formed reversibly from CoII and O2

Superoxido cobalt(iii) and peroxido dicobalt(iii) species are formed in the temperature dependent reversible reaction of a common cobalt(ii) precursor with O₂.

How to make an efficient and robust molecular catalyst for water oxidation

The key factors to design an efficient and rugged molecular water oxidation catalyst (Mol-WOC) are reviewed and discussed.