Water oxidation with transition metal catalysts with non-innocent ligands and its mechanisms

Advances in Electrocatalyzed Water Oxidation by Molecular Complexes of First Row Transition Metals

Energy transition toward sustainable, alternative, and affordable solutions is likely to be one of the major challenges of the anthropocene era. The oxygen evolution reaction (OER) is a pivotal electrocatalytic process essential for advancing renewable energy conversion and storage technologies, including water splitting, artificial photosynthesis, metal-air batteries, and fuel cells. Electrocatalytic pathways can significantly reduce the overall energy requirements of these devices, particularly focusing on the energy demands associated with water splitting for hydrogen production. This review, after introducing the state of the art in heterogeneous catalysis, will be devoted to the description of molecular water oxidation electrocatalysts (MWOCs), focusing on the recent advancements on catalysts composed of various metals, including Mn, Co, Cu, Ni, and Fe, in combination with a range of mono- and multidentate ligands. Critical insights are presented and discussed to provide readers with suggestions for ligand design in assisted catalysis. These observations aim to identify synergistic solutions that could enhance technological maturity by reducing energy absorption while improving stability and efficiency.

Complete ligand substitution and oxidation of the metal center in the photochemical reaction of CpM(CO)2I (M = Fe, Ru) with chelating β-diketones

The visible-light irradiation of solutions of CpFe(CO)2I (Cp = (η5-C5H5)), acetylacetone (or other β-diketones), and diisopropylamine in toluene led to the substitution of all ligands with β-diketonate anions and the oxidation of FeII to FeIII. The only product was Fe(β-diketonate)3, which exclusively formed even when using an equimolar ratio of CpFe(CO)2I and diketone. The reaction occurred under both anaerobic and aerobic conditions. The proposed reaction mechanism assigns the key role to the non-innocent behavior of the β-diketonate ligands. Dioxygen or the acetylacetone acts as oxidant. The ruthenium complex CpRu(CO)2I reacts with acetylacetone in a similar way to the corresponding iron complex, but irradiation with UV light is required to assure an acceptable yield of the product. All obtained complexes were analyzed by NMR, FT-IR, and ESI-MS, and one was subjected to single-crystal and powder X-ray diffraction analysis, which revealed its mer stereochemistry.

Chemical design of metal complexes for electrochemical water oxidation under acidic conditions

The development of viable, stable, and highly efficient molecular water oxidation catalysts under acidic aqueous conditions (pH < 7) is challenging with Earth-abundant metals in the field of renewable energy due to their low stability and catalytic activity. The utilization of these catalysts is generally considered more cost-effective and sustainable relative to conventional catalysts relying on precious metals such as ruthenium and iridium, which exhibit outstanding activities. Herein, we discussed the effectiveness of transition metal complexes for electrocatalytic water oxidation under acidic conditions. We focus on important aspects of 3d first-row metal complexes as they relate to the design of water oxidation systems and emphasize the importance of the fundamental coordination chemistry perspective in this field, which can be applied to the understanding of catalytic activity and fundamental structure-function relationships. Finally, we identified the scientific challenges that should be overcome for the future development and application of water oxidation electrochemical catalysts.

Exploring the Cooperation of the Redox Non-Innocent Ligand and Di-Cobalt Center for the Water Oxidation Reaction Catalyzed by a Binuclear Complex

Water oxidation is a crucial reaction in the artificial photosynthesis system. In the present work, density functional calculations were employed to decipher the mechanism of water oxidation catalyzed by a binuclear cobalt complex, which was disclosed to be a homogeneous water oxidation catalyst in pH=7 phosphate buffer. The calculations showed that the catalytic cycle starts from the CoIII,III-OH2 species. Then, a proton-coupled electron transfer followed by a one-electron transfer process leads to the generation of the formal CoIV,IV-OH intermediate. The subsequent PCET produces the active species, namely the formal CoIV,V=O intermediate (4). The oxidation processes mainly occur on the ligand moiety, including the coordinated water moiety, implying a redox non-innocent behavior. Two cobalt centers keep their oxidation states and provide one catalytic center for water activation during the oxidation process. 4 triggers the O-O bond formation via the water nucleophilic attack pathway, in which the phosphate buffer ion functions as the proton acceptor. The O-O bond formation is the rate-limiting step with a calculated total barrier of 17.7 kcal/mol. The last electron oxidation process coupled with an intramolecular electron transfer results in the generation of O2.

Manipulating the Rate and Overpotential for Electrochemical Water Oxidation: Mechanistic Insights for Cobalt Catalysts Bearing Noninnocent Bis(benzimidazole)pyrazolide Ligands

Electrochemical water oxidation is known as the anodic reaction of water splitting. Efficient design and earth-abundant electrocatalysts are crucial to this process. Herein, we report a family of catalysts (1-3) bearing bis(benzimidazole)pyrazolide ligands (H 2 L1-H 2 L3). H 2 L3 contains electron-donating substituents and noninnocent components, resulting in catalyst 3 exhibiting unique performance. Kinetic studies show first-order kinetic dependence on [3] and [H2O] under neutral and alkaline conditions. In contrast to previously reported catalyst 1, catalyst 3 exhibits an insignificant kinetic isotope effect of 1.25 and zero-order dependence on [NaOH]. Based on various spectroscopic methods and computational findings, the L3Co2 III(μ-OH) species is proposed to be the catalyst resting state and the nucleophilic attack of water on this species is identified as the turnover-limiting step of the catalytic reaction. Computational studies provided insights into how the interplay between the electronic effect and ligand noninnocence results in catalyst 3 acting via a different reaction mechanism. The variation in the turnover-limiting step and catalytic potentials of species 1-3 leads to their catalytic rates being independent of the overpotential, as evidenced by Eyring analysis. Overall, we demonstrate how ligand design may be utilized to retain good water oxidation activity at low overpotentials.

Coordination-induced O-H/N-H bond weakening by a redox non-innocent, aluminum-containing radical

Several renewable energy schemes aim to use the chemical bonds in abundant molecules like water and ammonia as energy reservoirs. Because the O-H and N-H bonds are quite strong (>100 kcal/mol), it is necessary to identify substances that dramatically weaken these bonds to facilitate proton-coupled electron transfer processes required for energy conversion. Usually this is accomplished through coordination-induced bond weakening by redox-active metals. However, coordination-induced bond weakening is difficult with earth's most abundant metal, aluminum, because of its redox inertness under mild conditions. Here, we report a system that uses aluminum with a redox non-innocent ligand to achieve significant levels of coordination-induced bond weakening of O-H and N-H bonds. The multisite proton-coupled electron transfer manifold described here points to redox non-innocent ligands as a design element to open coordination-induced bond weakening chemistry to more elements in the periodic table.

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.

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.

Sulphur vs NH Group: Effects on the CO2 Electroreduction Capability of Phenylenediamine-Cp Cobalt Complexes

The cobalt complex (I) with cyclopentadienyl and 2-aminothiophenolate ligands was investigated as a homogeneous catalyst for electrochemical CO₂ reduction. By comparing its behavior with an analogous complex with the phenylenediamine (II), the effect of sulfur atom as a substituent has been evaluated. As a result, a positive shift of the reduction potential and the reversibility of the corresponding redox process have been observed, also suggesting a higher stability of the compound with sulfur. Under anhydrous conditions, complex I showed a higher current enhancement in the presence of CO₂ (9.41) in comparison with II (4.12). Moreover, the presence of only one -NH group in I explained the difference in the observed increases on the catalytic activity toward CO₂ due to the presence of water, with current enhancements of 22.73 and 24.40 for I and II, respectively. DFT calculations confirmed the effect of sulfur on the lowering of the energy of the frontier orbitals of I, highlighted by electrochemical measurements. Furthermore, the condensed Fukui function f ^- values agreed very well with the current enhancement observed in the absence of water.

Understanding the Exchange Interaction between Paramagnetic Metal Ions and Radical Ligands: DFT and Ab Initio Study on Semiquinonato Cu(II) Complexes

The exchange coupling, represented by the J parameter, is of tremendous importance in understanding the reactivity and magnetic behavior of open-shell molecular systems. In the past, it was the subject of theoretical investigations, but these studies are mostly limited to the interaction between metallic centers. The exchange coupling between paramagnetic metal ions and radical ligands has hitherto received scant attention in theoretical studies, and thus the understanding of the factors governing this interaction is lacking. In this paper, we use DFT, CASSCF, CASSCF/NEVPT2, and DDCI3 methods to provide insight into exchange interaction in semiquinonato copper(II) complexes. Our primary objective is to identify structural features that affect this magnetic interaction. We demonstrate that the magnetic character of Cu(II)-semiquinone complexes are mainly determined by the relative position of the semiquinone ligand to the Cu(II) ion. The results can support the experimental interpretation of magnetic data for similar systems and can be used for the in-silico design of magnetic complexes with radical ligands.

Theoretical Understanding of Reactions of Rhenium and Ruthenium Tris(thiolate) Complexes with Unsaturated Hydrocarbons: Noninnocent Nature of the Ligand, Mechanism, and Origin of Differential Reactivity

In retrospect to the complexity induced by the noninnocent ligands in identifying the transition metal's oxidation state and correlating the ligand's noninnocence with reactivity, the reactions of alkene/alkyne addition to rhenium/ruthenium tris(thiolate) complexes are particularly good cases for shedding light on the chemistry of the dppbt ligand, including its noninnocent nature, ligand-centered mechanism, and origin of differential reactivity. Density functional theory (DFT) combined with the high-level ab initio calculations performed herein demonstrates that, upon alkene/alkyne addition, the orbital symmetry properly regulates the reaction to form ligand-centered cis-interligand dithioethers as the most favorable pathway. The neutral and cationic Re and Ru dithioethers are revealed via DFT calculations to be in a low-spin ground state; on the contrary, high-level ab initio methods confirm that the dicationic Re-dithioethers exhibit obvious multireference character with antiferromagnetic coupling between Re-d_yz and S1-p_y. The metal-stabilized thiyl radicals play a pivotal role in delivering the reactivity of [RuL₃]⁺ and [ReL₃]^+/2+ toward alkene/alkyne rather than [ReL₃], where [RuL₃]⁺ and [ReL₃]^+/2+ present significant radical characters on ligand S2, yet neutral [ReL₃] has little such feature, from which differential reactivity arises. Faster styrene addition to Ru tris(thiolate) in contrast to Re tris(thiolate) has been properly interpreted using DFT calculations with major products assigned. The deeper understanding gained in this work would illuminate further experimental exploration in adding alkene/alkyne to other metal-stabilized thiyl radicals.

Synthesis, Characterization, and Study of Catalytic Activity of Chiral Cu(II) and Ni(II) Salen Complexes in the α-Amino Acid C-α Alkylation Reaction

A new family of Cu(II) and Ni(II) salen complexes was synthesized and fully characterized through various physicochemical methods. Their catalytic activity was evaluated in the phase transfer C_α-alkylation reaction of the Schiff bases of D,L-alanine ester and benzaldehyde derivatives. It was found that the introduction of a chlorine atom into the ortho- and para-positions of the phenyl ring of the substrate resulted in an increase in both the chemical yield and the asymmetric induction (ee 66-98%). The highest enantiomeric excess was achieved in the case of a Cu(II) salen complex based on (S,S)-cyclohexanediamine and salicylaldehyde at -20 °C. The occurrence of a bulky substituent in the ligand present in the complexes led to a drastic decrease in ee and chemical yield. For instance, the introduction of bulky substituents at positions 3 and 5 of the phenyl ring of the catalyst resulted in a complete loss of the stereoselectivity control in the alkylation reaction.

Dinuclear Cobalt Complexes for Homogeneous Water Oxidation: Tuning Rate and Overpotential through the Non-Innocent Ligand

In this study, dinuclear cobalt complexes (1 and 2) featuring bis(benzimidazole)pyrazolide-type ligands (H₂ L and Me₂ L) were prepared and evaluated as molecular electrocatalysts for water oxidation. Notably, 1 bearing a non-innocent ligand (H₂ L) displayed faster catalytic turnover than 2 under alkaline conditions, and the base dependence of water oxidation and kinetic isotope effect analysis indicated that the reaction mediated by 1 proceeded by a different mechanism relative to 2. Spectroelectrochemical, cold-spray ionization mass spectrometric and computational studies found that double deprotonation of 1 under alkaline conditions cathodically shifted the catalysis-initiating potential and further altered the turnover-limiting step from nucleophilic water attack on (H₂ L)Co^III ₂ (superoxo) to deprotonation of (L)Co^III ₂ (OH)₂ . The rate-overpotential analysis and catalytic Tafel plots showed that 1 exhibited a significantly higher rate than previously reported Ru-based dinuclear electrocatalysts at similar overpotentials. These observations suggest that using non-innocent ligands is a valuable strategy for designing effective metal-based molecular water oxidation catalysts.

Energy Landscape for the Electrocatalytic Oxidation of Water by a Single-Site Oxomanganese(V) Porphyrin

A cationic manganese porphyrin, Mn^III-TDMImP, is an efficient, homogeneous, single-site water oxidation electrocatalyst at neutral pH. The measured turnover frequency for oxygen production is 32 s^-1. Mechanistic analyses indicate that Mn^V(O)(OH₂), the protonated form of the corresponding trans-Mn^V(O)₂ species, is generated from the Mn^III(OH₂)₂ precursor in a 2-e^- two-proton process and is responsible for O-O bond formation with a H₂O molecule. Chloride ion is a competitive substrate with H₂O for the Mn^V(O)(OH₂) oxidant, forming hypochlorous acid with a rate constant that is 3 orders of magnitude larger than that of water oxidation. The data allow the construction of an experimental energy landscape for this water oxidation catalysis process.

Strategies for Electrochemically Sustainable H2 Production in Acid

Acidified water electrolysis with fast kinetics is widely regarded as a promising option for producing H2 . The main challenge of this technique is the difficulty in realizing sustainable H2 production (SHP) because of the poor stability of most electrode catalysts, especially on the anode side, under strongly acidic and highly polarized electrochemical environments, which leads to surface corrosion and performance degradation. Research efforts focused on tuning the atomic/nano structures of catalysts have been made to address this stability issue, with only limited effectiveness because of inevitable catalyst degradation. A systems approach considering reaction types and system configurations/operations may provide innovative viewpoints and strategies for SHP, although these aspects have been overlooked thus far. This review provides an overview of acidified water electrolysis for systematic investigations of these aspects to achieve SHP. First, the fundamental principles of SHP are discussed. Then, recent advances on design of stable electrode materials are examined, and several new strategies for SHP are proposed, including fabrication of symmetrical heterogeneous electrolysis system and fluid homogeneous electrolysis system, as well as decoupling/hybrid-governed sustainability. Finally, remaining challenges and corresponding opportunities are outlined to stimulate endeavors toward the development of advanced acidified water electrolysis techniques for SHP.

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.

Tuning Ruthenium Carbene Complexes for Selective P-H Activation through Metal-Ligand Cooperation

The use of iminophosphoryl-tethered ruthenium carbene complexes to activate secondary phosphine P-H bonds is reported. Complexes of type [(p-cymene)-RuC(SO2 Ph)(PPh2 NR)] (with R = SiMe3 or 4-C6 H4 -NO2 ) were found to exhibit different reactivities depending on the electronics of the applied phosphine and the substituent at the iminophosphoryl moiety. Hence, the electron-rich silyl-substituted complex undergoes cyclometallation or shift of the imine moiety after cooperative activation of the P-H bond across the M=C linkage, depending on the electronics of the applied phosphine. Deuteration experiments and computational studies proved that cyclometallation is initiated by the activation process at the M=C bond and triggered by the high electron density at the metal in the phosphido intermediates. Consistently, replacement of the trimethylsilyl (TMS) group by the electron-withdrawing 4-nitrophenyl substituent allowed the selective cooperative P-H activation to form stable activation products.

Efficient Electrochemical Water Oxidation Mediated by Pyridylpyrrole-Carboxylate Ruthenium Complexes

Spurred by the rapid growth of Ru-based complexes as molecular water oxidation catalysts (WOCs), we propose novel ruthenium(II) complexes bearing pyridylpyrrole-carboxylate (H₂ppc) ligands as members of the WOC family. The structure of these complexes has 4-picoline (pic)/dimethyl sulfoxide (DMSO) in [Ru(ppc)(pic)₂(dmso)] and pic/pic in [Ru(ppc)(pic)₃] as axial ligands. Another ppc^2- ligand and one pic ligand are located at the equatorial positions. [Ru(ppc)(pic)₂(dmso)] behaves as a WOC as determined by electrochemical measurement and has an ultrahigh electrocatalytic current density of 8.17 mA cm^-2 at 1.55 V (vs NHE) with a low onset potential of 0.352 V (vs NHE), a turnover number of 241, a turnover frequency of 203.39 s^-1, and k_cat of 16.34 s^-1 under neutral conditions. The H₂O/pic exchange of the complexes accompanied by oxidation of a ruthenium center is the initial step in the catalytic cycle. The cyclic voltametric measurements of [Ru(ppc)(pic)₂(dmso)] at various scan rates, Pourbaix diagrams (plots of E vs pH), and kinetic studies suggested a water nucleophilic attack mechanism. HPO₄^2- in a phosphate buffer solution is invoked in water oxidation as the proton acceptor.

Electrochemical water oxidation by a copper complex with an N4-donor ligand under neutral conditions

A mononuclear copper(ii) complex [Cu(H2L)](NO3)2 with an N4-donor redox-active ligand is found to be an efficient homogeneous catalyst for electrochemical water oxidation with the assistance of ligand oxidation.