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.

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.

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.

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.

Molecular Mechanism of the Mononuclear Copper Complex-Catalyzed Water Oxidation from Cluster-Continuum Model Calculations

Cluster-continuum model calculations were conducted to decipher the mechanism of water oxidation catalyzed by a mononuclear copper complex. Among various O-O bond formation mechanisms investigated in this study, the most favorable pathway involved the nucleophilic attack of OH^- onto the ^.+ L-Cu^II -OH^- intermediate. During such process, the initial binding of OH^- to the proximity of ^.+ L-Cu^II -OH^- would result in the spontaneous oxidation of OH^- , leading to OH⋅ radical and Cu^II -OH^- species. The further O-O coupling between OH⋅ radical and Cu^II -OH^- was associated with a barrier of 14.8 kcal mol^-1 , leading to the formation of H₂ O₂ intermediate. Notably, the formation of "Cu^III -O^.- " species, a widely proposed active species for O-O bond formation, was found to be thermodynamically unfavorable and could be bypassed during the catalytic reactions. On the basis the present calculations, a catalytic cycle of the mononuclear copper complex-catalyzed water oxidation was proposed.

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.

Efficient homogeneous electrochemical water oxidation by a copper(ii) complex with a hexaaza macrotricyclic ligand

A copper complex [CuII(L)](ClO4)2 with a hexaaza macrotricyclic ligand is found to be an efficient homogeneous electrocatalyst for water oxidation with onset overpotential of 480 mV and a turnover frequency of 3.65 s−1.

Mechanistic Insight into the O2 Evolution Catalyzed by Copper Complexes with Tetra- and Pentadentate Ligands

The mononuclear complexes ([(bztpen)Cu] (BF₄)₂ (bztpen = N-benzyl-N,N',N'-tris (pyridin-2-yl methyl ethylenediamine))) and ([(dbzbpen)Cu(OH₂)] (BF₄)₂ (dbzbpen = N,N'-dibenzyl-N,N'-bis(pyridin-2-ylmethyl) ethylenediamine)) have been reported as water oxidation catalysts in basic medium (pH = 11.5). We explore the O₂ evolution process catalyzed by these copper catalysts with various ligands (L) by applying the first-principles molecular dynamics simulations. First, the oxidation of catalysts to the metal-oxo intermediates [LCu(O)]²⁺ occurs through the proton-coupled electron transfer (PCET) process. These intermediates are involved in the oxygen-oxygen bond formation through the water-nucleophilic addition process. Here, we have considered two types of oxygen-oxygen bond formation. The first one is the transfer of the hydroxide of the water molecule to the Cu═O moiety; the proton transfer to the solvent leads to the formation of the peroxide complex ([LCu(OOH)]⁺). The other is the formation of the hydrogen peroxide complex ([LCu(HOOH)]²⁺) by the transfer of proton and hydroxide of the water molecule to the metal-oxo intermediate. The formation of the peroxide complex requires less activation free energy than hydrogen peroxide formation for both catalysts. We found two transition states in the well-tempered metadynamics simulations: one for proton transfer and another for hydroxide transfer. In both cases, the proton transfer requires higher free energy. Following the formation of the oxygen-oxygen bond, we study the release of the dioxygen molecule. The formed peroxide and hydrogen peroxide complexes are converted into the superoxide complex ([LCu(OO)]²⁺) through the transfer of proton, electron, and PCET processes. The superoxide complex releases an oxygen molecule upon the addition of a water molecule. The free energy of activation for the release of the dioxygen molecule is lesser than that of the oxygen-oxygen bond formation. When we observe the entire water oxidation process, the oxygen-oxygen bond formation is the rate-determining step. We calculated the rates of reaction by using the Eyring equation and found them to be close to the experimental values.

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.

Copper catalysts for photo- and electro-catalytic hydrogen production

Square planar 1, square pyramidal 2 and trigonal bipyramidal 3 copper complexes are poor catalysts for hydrogen evolution (HER) under photocatalytic conditions, whereas 1 is, or forms, a good and enduring electrocatalyst for HER, but 2 and 3 do not.

Cobalt Carbonate as an Electrocatalyst for Water Oxidation

Co^II salts in the presence of HCO₃ ^- /CO₃ ^2- in aqueous solutions act as electrocatalysts for water oxidation. It comprises of several key steps: (i) A relatively small wave at E_pa ≈0.71 V (vs. Ag/AgCl) owing to the Co^III/II redox couple. (ii) A second wave is observed at E_pa ≈1.10 V with a considerably larger current. In which the Co^III undergoes oxidation to form a Co^IV species. The large current is attributed to catalytic oxidation of HCO₃ ^- /CO₃ ^2- to HCO₄ ^- . (iii) A process with very large currents at >1.2 V owing to the formation of Co^V (CO₃ )₃ ^- , which oxidizes both water and HCO₃ ^- /CO₃ ^2- . These processes depend on [Co^II ], [NaHCO₃ ], and pH. Chronoamperometry at 1.3 V gives a green deposit. It acts as a heterogeneous catalyst for water oxidation. DFT calculations point out that Coⁿ (CO₃ )₃ ^n-6 , n=4, 5 are attainable at potentials similar to those experimentally observed.

Dynamic Role of Cluster Cocatalysts on Molecular Photoanodes for Water Oxidation

While loading of cocatalysts is one of the most widely investigated strategies to promote the efficiency of photoelectrodes, the understanding of their functionality remains controversial. We established new hybrid molecular photoanodes with cobalt-based molecular cubane cocatalysts on hematite as a model system. Photoelectrochemical and rate law analyses revealed an interesting functionality transition of the {Co(II)₄O₄}-type cocatalysts. Their role changed from predominant hole reservoirs to catalytic centers upon modulation of the applied bias. Kinetic analysis of the photoelectrochemical processes indicated that this observed transition arises from the dynamic equilibria of photogenerated surface charge carriers. Most importantly, we confirmed this functional transition of the cocatalysts and the related kinetic properties for several cobalt-based molecular and heterogeneous catalysts, indicating wide applicability of the derived trends. Additionally, complementary analytical characterizations show that a transformation of the applied molecular species occurs at higher applied bias, pointing to a dynamic interplay connecting molecular and heterogeneous catalysis. Our insights promote the essential understanding of efficient (molecular) cocatalyzed photoelectrode systems to design tailor-made hybrid devices for a wide range of catalytic applications.

Pentanuclear iron catalysts for water oxidation: substituents provide two routes to control onset potentials

The development of robust and efficient molecular catalysts based on earth-abundant transition metals for water oxidation reactions is a challenging research target. Our group recently demonstrated the high activity and stability of a pentairon-based water oxidation electrocatalyst (M. Okamura, M. Kondo, R. Kuga, Y. Kurashige, T. Yanai, S. Hayami, V. K. K. Praneeth, M. Yoshida, K. Yoneda, S. Kawata and S. Masaoka, Nature, 2016, 530, 465-468). However, the development of strategies to decrease onset potentials for catalysis remains challenging. In this article, we report the construction of a series of pentanuclear iron complexes by introducing electron-donating (methyl) and electron-withdrawing (bromo) substituents on the ligand. Two newly synthesized complexes exhibited five reversible redox processes, similar to what is seen with the parent complex. These complexes can also serve as homogeneous catalysts for water oxidation reactions, and the faradaic efficiencies of the reactions were high. Additionally, the onset potentials of the newly developed complexes were lower than that of the parent complex. Mechanistic insights revealed that there are two methods for decreasing onset potentials: control of the redox potentials of the pentairon complex and control of the reaction mechanism.

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.