A homogeneous cobalt complex mediated electro and photocatalytic O2/H2O interconversion in neutral water

The O2/H2O redox couple is vital in various renewable energy conversion strategies. This work delves into the Co(L-histidine)2 complex, a functional mimic of oxygen-carrying metalloproteins, and its electrochemical behavior driving the bidirectional oxygen reduction (ORR) and oxygen evolution (OER) activity in neutral water. This complex electrocatalyzes O2 via two distinct pathways: a two-electron O2/H2O2 reduction (catalytic rate = 250 s-1) and a four-electron O2 to H2O production (catalytic rate = 66 s-1). The formation of the key trans-μ-1,2-Co(III)-peroxo intermediate expedites this process. Additionally, this complex effectively oxidizes water to O2 (catalytic rate = 15606 s-1) at anodic potentials via a Co(IV)-oxo species. Additionally, this complex executes the ORR and OER under photocatalytic conditions in neutral water in the presence of appropriate photosensitizer (Eosin-Y) and redox mediators (triethanolamine/ORR and Na2S2O8/OER) at an appreciable rate. These results highlight one of the early examples of both electro- and photoactive bidirectional ORR/OER catalysts operational in neutral water.

Computational Insights of Selective Intramolecular O-atom Transfer Mediated by Bioinspired Copper Complexes

The stereoselective copper-mediated hydroxylation of intramolecular C-H bonds from tridentate ligands is reinvestigated using DFT calculations. The computational study aims at deciphering the mechanism of C-H hydroxylation obtained after reaction of Cu(I) precursors with dioxygen, using ligands bearing either activated (L¹ ) or non-activated (L² ) C-H bonds. Configurational analysis allows rationalization of the experimentally observed regio- and stereoselectivity. The computed mechanism involves the formation of a side-on peroxide species (P) in equilibrium with the key intermediate bis-(μ-oxo) isomer (O) responsible for the C-H activation step. The P/O equilibrium yields the same activation barrier for the two complexes. However, the main difference between the two model complexes is observed during the C-H activation step, where the complex bearing the non-activated C-H bonds yields a higher energy barrier, accounting for the experimental lack of reactivity of this complex under those conditions.

Borate Buffer as a Key Player in Cu-Based Homogeneous Electrocatalytic Water Oxidation

Borate buffer was found to have both structural and functional roles within a low-cost tri-copper electrocatalyst for homogeneous water oxidation that exhibits a high turnover frequency of 310 s^-1 . The borate buffer was shown to facilitate the catalytic activity by both bridging the three Cu ions and participating in O-O bond formation. Phosphate and acetate buffers did not show such roles, making borate a unique player in this catalytic system.

Non-heme oxoiron complexes as active intermediates in the water oxidation process with hydrogen/oxygen atom transfer reactions

In this study, we explore the water oxidation process with the help of density functional theory. The formation of an oxygen-oxygen bond is crucial in the water oxidation process. Here, we report the formation of the oxygen-oxygen bond by the N5-coordinate oxoiron species with a higher oxidation state of Fe^IV and Fe^V. This bond formation is studied through the nucleophilic addition of water molecules and the transfer of the oxygen atom from meta-chloroperbenzoic acid (mCPBA). Our study reveals that the oxygen-oxygen bond formation by reacting mCPBA with Fe^VO requires less activation barrier (13.7 kcal mol^-1) than the other three pathways. This bond formation by the oxygen atom transfer (OAT) pathway is more favorable than that achieved by the hydrogen atom transfer (HAT) pathway. In both cases, the oxygen-oxygen bond formation occurs by interacting the σ*d_z²-2p_z molecular orbital of the iron-oxo intermediate with the 2p_x orbital of the oxygen atom. From this study, we understand that the oxygen-oxygen bond formation by Fe^IVO with the OAT process is also feasible (16 kcal mol^-1), suggesting that Fe^VO may not always be required for the water oxidation process by non-heme N5-oxoiron. After the oxygen-oxygen bond formation, the release of the dioxygen molecule occurs with the addition of the water molecule. The release of dioxygen requires a barrier of 7.0 kcal mol^-1. The oxygen-oxygen bond formation is found to be the rate-determining step.

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.

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.

A Dicopper Nitrenoid by Oxidation of a CuICuI Core: Synthesis, Electronic Structure, and Reactivity

A dicopper nitrenoid complex was prepared by formal oxidative addition of the nitrenoid fragment to a dicopper(I) center by reaction with the iminoiodinane PhINTs (Ts = tosylate). This nitrenoid complex, (DPFN)Cu₂(μ-NTs)[NTf₂]₂ (DPFN = 2,7-bis(fluorodi(2-pyridyl)methyl)-1,8-naphthyridine), is a powerful H atom abstractor that reacts with a range of strong C-H bonds to form a mixed-valence Cu(I)/Cu(II) μ-NHTs amido complex in the first example of a clean H atom transfer to a dicopper nitrenoid core. In line with this reactivity, DFT calculations reveal that the nitrenoid is best described as an iminyl (NR radical anion) complex. The nitrenoid was trapped by the addition of water to form a mixed-donor hydroxo/amido dicopper(II) complex, which was independently obtained by reaction of a Cu₂(μ-OH)₂ complex with an amine through a protonolysis pathway. This mixed-donor complex is an analogue for the proposed intermediate in copper-catalyzed Chan-Evans-Lam coupling, which proceeds via C-X (X = N or O) bond formation. Treatment of the dicopper(II) mixed donor complex with MgPh₂(THF)₂ resulted in generation of a mixture that includes both phenol and a previously reported dicopper(I) bridging phenyl complex, illustrating that both reduction of dicopper(II) to dicopper(I) and concomitant C-X bond formation are feasible.

Homogeneous Catalysts Based on First-Row Transition-Metals for Electrochemical Water Oxidation

Strategies that enable the renewable production of storable fuels (i. e. hydrogen or hydrocarbons) through electrocatalysis continue to generate interest in the scientific community. Of central importance to this pursuit is obtaining the requisite chemical (H+ ) and electronic (e- ) inputs for fuel-forming reduction reactions, which can be met sustainably by water oxidation catalysis. Further possibility exists to couple these redox transformations to renewable energy sources (i. e. solar), thus creating a carbon neutral solution for long-term energy storage. Nature uses a Mn-Ca cluster for water oxidation catalysis via multiple proton-coupled electron-transfers (PCETs) with a photogenerated bias to perform this process with TOF 100∼300 s-1 . Synthetic molecular catalysts that efficiently perform this conversion commonly utilize rare metals (e. g., Ru, Ir), whose low abundance are associated to higher costs and scalability limitations. Inspired by nature's use of 1st row transition metal (TM) complexes for water oxidation catalysts (WOCs), attempts to use these abundant metals have been intensively explored but met with limited success. The smaller atomic size of 1st row TM ions lowers its ability to accommodate the oxidative equivalents required in the 4e- /4H+ water oxidation catalysis process, unlike noble metal catalysts that perform single-site electrocatalysis at lower overpotentials (η). Overcoming the limitations of 1st row TMs requires developing molecular catalysts that exploit biomimetic phenomena - multiple-metal redox-cooperativity, PCET and second-sphere interactions - to lower the overpotential, preorganize substrates and maintain stability. Thus, the ultimate goal of developing efficient, robust and scalable WOCs remains a challenge. This Review provides a summary of previous research works highlighting 1st row TM-based homogeneous WOCs, catalytic mechanisms, followed by strategies for catalytic activity improvements, before closing with a future outlook for this field.

Electrocatalytic oxidation of water using self-assembled copper(ii) tetraaza macrocyclic complexes on a 4-(pyridine-4′-amido)benzene grafted gold electrode

Gold surface anchored copper(ii)tetraaza macrocyclic complex showed an excellent electrocatalytic activity towards water oxidation with an overpotential of 284 mV at a current density of 1.31 mA cm⁻² and a Tafel slope of 48 mV decade⁻¹ in neutral pH.

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.

Synthesis and Characterization of Cu(II) and Mixed-Valence Cu(I)Cu(II) Clusters Supported by Pyridylamide Ligands

A group of copper complexes supported by polydentate pyridylamide ligands H₂bpda and H₂ppda were synthesized and characterized. The two Cu(II) dimers [Cu^II₂(Hbpda)₂(ClO₄)₂] (1) and [Cu^II₂(ppda)₂(DMF)₂] (2) were constructed by using neutral ligands to react with Cu(II) salts. Although the dimers showed similar structural features, the second-sphere interactions affect the structures differently. With the application of Et₃N, the tetranuclear cluster (HNEt₃)[Cu^II₄(bpda)₂(μ₃-OH)₂(ClO₄)(DMF)₃](ClO₄)₂ (3) and hexanuclear cluster (HNEt₃)₂[Cu^II₆(ppda)₆(H₂O)₂(CH₃OH)₂](ClO₄)₂ (4) were prepared under similar reaction conditions. The symmetrical and unsymmetrical arrangement of the ligand donors in ligands H₂bpda and H₂ppda led to the dramatic conformation difference of the two Cu(II) complexes. As part of our effort to explore mixed-valence copper chemistry, the triple-decker pentanuclear cluster [Cu^II₃Cu^I₂(bpda)₃(μ₃-O)] (5) was prepared. XPS examination demonstrated the localized mixed-valence properties of complex 5. Magnetic studies of the clusters with EPR evidence showed either weak ferromagnetic or antiferromagnetic interactions among copper centers. Due to the trigonal-planar conformation of the trinuclear Cu(II) motif with the μ₃-O center, complex 5 exhibits geometric spin frustration and engages in antisymmetric exchange interactions. DFT calculations were also performed to better interpret spectroscopic evidence and understand the electronic structures, especially the mixed-valence nature of complex 5.

Dicopper(ii) tetrapyridyl complexes incorporated with ancillary ligands for effective water oxidation

Water oxidation catalysis of dicopper(ii) tetrapyridyl complexes under alkaline conditions was improved by diamine ligands.

Efficient visible light-driven water oxidation catalysed by an iron(iv) clathrochelate complex

A water-stable FeIV clathrochelate complex catalyses fast and homogeneous photochemical oxidation of water to dioxygen with a turnover frequency of 2.27 s-1 and a maximum turnover number of 365. An FeV intermediate generated under catalytic conditions is trapped and characterised using EPR and Mössbauer spectroscopy.

Copper Containing Molecular Systems in Electrocatalytic Water Oxidation—Trends and Perspectives

Molecular design represents an exciting platform to refine mechanistic details of electrocatalytic water oxidation and explore new perspectives. In the growing number of publications some general trends seem to be outlined concerning the operation mechanisms, with the help of experimental and theoretical approaches that have been broadly applied in the case of bioinorganic systems. In this review we focus on bio-inspired Cu-containing complexes that are classified according to the proposed mechanistic pathways and the related experimental evidence, strongly linked to the applied ligand architecture. In addition, we devote special attention to features of molecular compounds, which have been exploited in the efficient fabrication of catalytically active thin films.

Copper-containing hybrid compounds based on extremely rare [V2Mo6O26]6– POM as water oxidation catalysts

Hybrid Cu/V/Mo compounds with rare [α-V₂Mo₆O₂₆]^6– and oxides prepared by their thermal degradation were used as catalysts for water oxidation.

The role of methylation in the copper(ii) coordination properties of a His-containing decapeptide

N-Methylation: a simple strategy to stabilize copper species lacking amidate coordination at neutral pH value.

Heterogeneous catalytic water oxidation controlled by coordination geometries of copper(ii) centers with thiolato donors

Heterometallic coordination compounds containing a thiolato dicopper(ii) core increase heterogeneous electrocatalytic activities for water oxidation in proportion to the numbers of vacant dicopper(ii) coordination sites.

An octanuclear Cu(ii) cluster with a bio-inspired Cu4O4 cubic fragment for efficient photocatalytic water oxidation

An octanuclear Cu(ii) cluster [Cu₈(dpk·OH)₈(OAc)₄](ClO₄)₄ was found to be an efficient homogeneous catalyst for photocatalytic water oxidation with an oxygen yield, TON and TOF of 35.6%, 178 and 3.6 s⁻¹, respectively.

O–O bond cleavage via electrochemical reduction of a side-on peroxo dicopper model of hemocyanin

The redox properties of the μ-η²:η² peroxo complex [Cu₂(H6M4h)(O₂)]²⁺ were elucidated.