Effect of ligand substituents on the reactivity pathways of copper(II) complexes towards electrocatalytic water oxidation

The electrocatalytic water oxidation activity of three copper(II) complexes [Cu(L1H)(L1)](ClO4) (1), [Cu(L2H)(L2)(H2O)](ClO4) (2) and [Cu(L3H)(L2)](ClO4) (3) with aryl oxime ligands L1H, L2H and L3H [L1H = 1-(pyridin-2-yl)methanone oxime, L2H = 1-(pyridin-2-yl)ethanone oxime and L3H = 1-(pyridin-2-yl)propanone oxime] was investigated. All the three ligands have in common a pyridyl group attached to the carbon centre of the oxime moiety and differ in the second substituent attached to the carbon centre. Electrochemical investigation of the catalytic activity of complexes 1, 2 and 3 shows that the nature of the substituent attached to the carbon centre has an influence on the catalytic pathway and overall catalytic activity of these complexes.

Spectroscopic characterization and reactivity studies of a copper(II) iminoxyl radical complex

A CuII complex (1) of a bis-pyridine-dioxime ligand and its one-electron oxidized analog (1-ox) were thoroughly characterized by various spectroscopic techniques, including X-ray absorption spectroscopy. 1-ox was found to be a CuII complex of a ligand iminoxyl radical and represents the first example of such a type. Reorganization energy (λ) of 2.12 eV was determined for the 1-ox/1 couple, which is considerably higher than the type 1 protein and synthetic CuIII/II(OH) complexes.

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.

Soluble Gd6Cu24 clusters: effective molecular electrocatalysts for water oxidation

The water oxidation half reaction in water splitting for hydrogen production is extremely rate-limiting. This study reports the synthesis of two heterometallic clusters (Gd6Cu24-IM and Gd6Cu24-AC) for application as efficient water oxidation catalysts. Interestingly, the maximum turnover frequency of Gd6Cu24-IM in an NaAc solution of a weak acid (pH 6) was 319 s-1. The trimetallic catalytic site, H2O-GdIIICuII2-H2O, underwent two consecutive two-electron two-proton coupled transfer processes to form high-valent GdIII-O-O-CuIII2 intermediates. Furthermore, the O-O bond was formed via intramolecular interactions between the CuIII and GdIII centers. The results of this study revealed that synergistic catalytic water oxidation between polymetallic sites can be an effective strategy for regulating O-O bond formation.

Impact of the hybridization form of the coordinated nitrogen atom on the electrocatalytic water oxidation performance of copper complexes with pentadentate amine-pyridine ligands

The field of molecular catalysts places a strong emphasis on the connection between the ligand structure and its catalytic performance. Herein, we changed the type of coordinated nitrogen atom in pentadentate amine-pyridine ligands to explore the impact of its hybridization form on the water oxidation performance of copper complexes. In the electrochemical tests, the copper complex bearing dipyridine-triamine displayed an apparently higher rate constant of 4.97 s-1, while the copper complex with tripyridine-diamine demonstrated overpotential reduction by 56 mV and better long-term electrolytic stability.

Unusual Water Oxidation Mechanism via a Redox-Active Copper Polypyridyl Complex

To improve Cu-based water oxidation (WO) catalysts, a proper mechanistic understanding of these systems is required. In contrast to other metals, high-oxidation-state metal-oxo species are unlikely intermediates in Cu-catalyzed WO because π donation from the oxo ligand to the Cu center is difficult due to the high number of d electrons of CuII and CuIII. As a consequence, an alternative WO mechanism must take place instead of the typical water nucleophilic attack and the inter- or intramolecular radical-oxo coupling pathways, which were previously proposed for Ru-based catalysts. [CuII(HL)(OTf)2] [HL = Hbbpya = N,N-bis(2,2'-bipyrid-6-yl)amine)] was investigated as a WO catalyst bearing the redox-active HL ligand. The Cu catalyst was found to be active as a WO catalyst at pH 11.5, at which the deprotonated complex [CuII(L-)(H2O)]+ is the predominant species in solution. The overall WO mechanism was found to be initiated by two proton-coupled electron-transfer steps. Kinetically, a first-order dependence in the catalyst, a zeroth-order dependence in the phosphate buffer, a kinetic isotope effect of 1.0, a ΔH⧧ value of 4.49 kcal·mol-1, a ΔS⧧ value of -42.6 cal·mol-1·K-1, and a ΔG⧧ value of 17.2 kcal·mol-1 were found. A computational study supported the formation of a Cu-oxyl intermediate, [CuII(L•)(O•)(H2O)]+. From this intermediate onward, formation of the O-O bond proceeds via a single-electron transfer from an approaching hydroxide ion to the ligand. Throughout the mechanism, the CuII center is proposed to be redox-inactive.

Structural and Spectroscopic Characterization of Copper(III) Complexes and Subsequent One-Electron Oxidation Reaction and Reactivity Studies

The formation of Cu(III) species are often invoked as the key intermediate in Cu-catalyzed organic transformation reactions. In this study, we synthesized Cu(II) (1) and Cu(III) (3) complexes supported by a bisamidate-bisalkoxide ligand consisting of an ortho-phenylenediamine (o-PDA) scaffold and characterized them through an array of spectroscopic techniques, including UV-visible, electron paramagnetic resonance, X-ray crystallography, and ¹H nuclear magnetic resonance (NMR) and X-ray absorption spectroscopy. The Cu-N/O bond distances in 3 are ∼0.1 Å reduced compared to 1, implying a significant increase in 3's overall effective nuclear charge. Further, a Cu(III) complex (4) of a bisamidate-bisalkoxide ligand containing a trans-cyclohexane-1,2-diamine moiety exhibits nearly identical Cu-N/O bond distances to that of 3, inferring that the redox-active o-PDA backbone is not oxidized upon one-electron oxidation of the Cu(II) complex (1). In addition, a considerable difference in the 1s → 4p and 1s → 3d transition energy was observed in the X-ray absorption near-edge structure data of 3 vs 1, which is typical for the metal-centered oxidation process. Electrochemical measurements of the Cu(II) complex (1) in acetonitrile exhibited two consecutive redox couples at -0.9 and 0.4 V vs the Fc⁺/Fc reference electrode. One-electron oxidation reaction of 3 further resulted in the formation of a ligand-oxidized Cu complex (3a), which was characterized in depth. Reactivity studies of species 3 and 3a were explored toward the activation of the C-H/O-H bonds. A bond dissociation free energy (BDFE) value of ∼69 kcal/mol was estimated for the O-H bond of the Cu(II) complex formed upon transfer of hydrogen atom to 3. The study represents a thorough spectroscopic characterization of high-valent Cu complexes and sheds light on the PCET reactivity studies of Cu(III) complexes.

Modulating the electrocatalytic activity of mononuclear nickel complexes toward water oxidation by tertiary amine group

Water oxidation is the bottleneck of water splitting, which is a promising strategy for hydrogen production. Therefore, it is significant to develop efficient water oxidation catalysts. Herein, electrochemical water oxidation catalyzed by three nickel complexes, namely [Ni(bptn)(H₂O)](ClO₄)₂ (1), [Ni(mbptn)(CH₃CN)](ClO₄)₂ (2), and [Ni(tmbptn)(H₂O)](ClO₄)₂ (3) (bptn = 1,9-bis(2-pyridyl)-2,5,8-triazanonane, mbptn = 5-methyl-1,9-bis(2-pyridyl)-2,5,8-triazanonane, and tmbptn = 1,9-bis(2-pyridyl)-2,5,8-triazanonane), is studied under near-neutral condition (pH 9.0). Meanwhile, the homogeneous catalytic behaviors of the three mononuclear nickel complexes were investigated and confirmed by scanning electron microscopy, energy dispersive spectrometry, X-ray photoelectron spectroscopy and electrochemical method. Complex 1 stabilized by a pentadentate ligand with three N-H fragments homogeneously catalyzes water oxidation to oxygen with the lowest onset overpotential. Complex 2 stabilized by a similar ligand with two N-H groups and one N-CH₃ group exhibits relatively higher onset overpotential but higher catalytic current and turnover frequency. However, complex 3 with three N-CH₃ coordination environment shows the highest onset overpotential and the highest catalytic current at higher potential. Comparison of catalytic behaviors and ligand structure of the three complexes reveals that the methyl group on the polypyridine amine ligand affects the water oxidation activity of the complexes obviously. The electronic effect of N-CH₃ coordination environment leads to higher redox potential of the metal center and potential demand for water oxidation, while it leads to higher reaction activity of high-valent intermediates, which account for higher catalytic current and efficiency of water oxidation. This work reveals that electrocatalytic water oxidation performance of nickel complexes can be finely modulated by constructing suitable N-CH₃ coordination.

On the Homogeneity of a Cobalt-Based Water Oxidation Catalyst

The homogeneity of molecular Co-based water oxidation catalysts (WOCs) has been a subject of debate over the last 10 years as assumed various homogeneous Co-based WOCs were found to actually form CoO_x under operating conditions. The homogeneity of the Co(HL) (HL = N,N-bis(2,2′-bipyrid-6-yl)amine) system was investigated with cyclic voltammetry, electrochemical quartz crystal microbalance, and X-ray photoelectron spectroscopy. The obtained experimental results were compared with heterogeneous CoO_x. Although it is shown that Co(HL) interacts with the electrode during electrocatalysis, the formation of CoO_x was not observed. Instead, a molecular deposit of Co(HL) was found to be formed on the electrode surface. This study shows that deposition of catalytic material is not necessarily linked to the decomposition of homogeneous cobalt-based water oxidation catalysts.

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.

Electrocatalytic Water Oxidation Activity of Molecular Copper Complexes: Effect of Redox-Active Ligands

Two molecular copper(II) complexes, (NMe₄)₂[Cu^II(L₁)] (1) and (NMe₄)₂[Cu^II(L₂)] (2), ligated by a N₂O₂ donor set of ligands [L₁ = N,N'-(1,2-phenylene)bis(2-hydroxy-2-methylpropanamide), and L₂ = N,N'-(4,5-dimethyl-1,2-phenylene)bis(2-hydroxy-2-methylpropanamide)] have been synthesized and thoroughly characterized. An electrochemical study of 1 in a carbonate buffer at pH 9.2 revealed a reversible copper-centered redox couple at 0.51 V, followed by two ligand-based oxidation events at 1.02 and 1.25 V, and catalytic water oxidation at an onset potential of 1.28 V (overpotential of 580 mV). The electron-rich nature of the ligand likely supports access to high-valent copper species on the CV time scale. The results of the theoretical electronic structure investigation were quite consistent with the observed stepwise ligand-centered oxidation process. A constant potential electrolysis experiment with 1 reveals a catalytic current density of >2.4 mA cm^-2 for 3 h. A one-electron-oxidized species of 1, (NMe₄)[Cu^III(L₁)] (3), was isolated and characterized. Complex 2, on the contrary, revealed copper and ligand oxidation peaks at 0.505, 0.90, and 1.06 V, followed by an onset water oxidation (WO) at 1.26 V (overpotential of 560 mV). The findings show that the ligand-based oxidation reactions strongly depend upon the ligand's electronic substitution; however, such effects on the copper-centered redox couple and catalytic WO are minimal. The energetically favorable mechanism has been established through the theoretical calculation of stepwise reaction energies, which nicely explains the experimentally observed electron transfer events. Furthermore, as revealed by the theoretical calculations, the O-O bond formation process occurs through a water nucleophilic attack mechanism with an easily accessible reaction barrier. This study demonstrates the importance of redox-active ligands in the development of molecular late-transition-metal electrocatalysts for WO reactions.

Repurposing a peptide antibiotic as a catalyst: a multicopper–daptomycin complex as a cooperative O–O bond formation and activation catalyst

A peptide antibiotic, daptomycin, was repurposed to a multicopper catalyst presenting cooperative rate enhancement in O–O bond formation and activation reactions.

A low-cost commercial Cu(ii)–EDTA complex for electrocatalytic water oxidation in neutral aqueous solution

A low-cost commercial Cu complex [Cu(EDTA)(H2O)] is developed as a molecular catalyst for OER with high efficiency and durable stability.

Electrocatalytic water oxidation by a water-soluble copper complex with a pentadentate amine-pyridine ligand

A water-soluble copper complex with a diamine-tripyridine ligand was synthesized successfully and well characterized. It was found to be catalytically active for the water oxidation reaction under basic conditions. Based on the electrochemical test result, this copper complex displayed an apparent rate constant (k_cat) of 0.81 s^-1 for the oxygen evolution reaction in 0.1 M phosphate buffer solution at pH 11.0. More importantly, the copper complex remained stable over 3 h of a bulk electrolysis experiment at 1.60 V with a Faradaic efficiency of 90.7% for O₂ evolution, and the decrement of current density was only 1.9%. These results suggest that the pentadentate copper complex is an efficient and durable homogeneous Earth-abundant electrocatalyst for water oxidation.

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.

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.

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.

Copper Coordination Complexes for Energy-Relevant Applications

Copper coordination complexes have emerged as a group of transition metal complexes that play important roles in solar energy conversion, utilization and storage, and have the potential to replace the quintessential commonly used transition metals, like Co, Pt, Ir and Ru as light sensitizers, redox mediators, electron donors and catalytic centers. The applications of copper coordination compounds in chemistry and energy related technologies are many and demonstrate their rightful place as sustainable, low toxicity and Earth-abundant alternative materials. In this perspective we show the most recent impact made by copper coordination complexes in dye-sensitized solar cells and other energy relevant applications.

Copper-based homogeneous and heterogeneous catalysts for electrochemical water oxidation

Water oxidation is currently believed to be the bottleneck in the field of electrochemical water splitting and artificial photosynthesis. Enormous efforts have been devoted toward the exploration of water oxidation catalysts (WOCs), including homogeneous and heterogeneous catalysts. Recently, Cu-based WOCs have been widely developed because of their high abundance, low cost, and biological relevance. However, to the best of our knowledge, no review has been made so far on such types of catalysts. Thus, we have summarized the recent progress made in the development of homogeneous and heterogeneous Cu-based WOCs for electrochemical catalysis. Furthermore, the evaluations of catalytic activity, stability, and mechanism of these catalysts are carefully concluded and highlighted. We believe that this review can summarize the current progress in the field of Cu-based electrochemical WOCs and help in the design of more efficient and stable WOCs.

Redox-active ligand assisted electrocatalytic water oxidation by a mononuclear cobalt complex

A cobalt complex bearing a redox-active monoanionic amidate ligand is shown to act as an efficient molecular electrocatalyst for water oxidation at a moderate overpotential (∼500 mV) in mildly alkaline medium.