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

Improvement of electrocatalytic water oxidation activity of novel copper complex by modulating the axial coordination of phosphate on metal center

The structure of organic ligand scaffolds of copper complexes critically affects their electrocatalytic properties toward water oxidation, which is widely regarded as the bottleneck of overall water splitting. Herein, two novel mononuclear Cu complexes, [Cu(dmabpy)](ClO4)2 (1, dmabpy = 6,6'-bis(dimethylaminomethyl)-2,2'-bipyridine) and [Cu(mabpy)](ClO4)2 (2, mabpy = 6,6'-bis(methylaminomethyl)-2,2'-bipyridine), with four-coordinated distorted planar quadrilateral geometry were synthesized and explored as efficient catalysts for electrochemical oxygen evolution in phosphate buffer solution. Interestingly, complex 1 with a tertiary amine group catalyzes water oxidation with lower onset overpotential and better catalytic performance, while complex 2 containing a secondary amine fragment displays much lower catalytic activity under identical conditions. The water oxidation catalytic mechanism of the two complexes is proposed based on the electrochemical test results. Experimental methods indicate that phosphate coordinated on the Cu center of the two complexes inhibits their reaction with substrate water molecules, resulting in lower activity toward water oxidation. Electrochemical tests reveal that the structure of the coordinated nitrogen atom improves the catalytic performance of the Cu complexes by modulating the coordination of phosphate on the Cu center, indicating that a minor alteration of the coordinating nitrogen atom of the ligand has a detrimental effect on the catalytic performance of electrochemical WOCs based on transition metal 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.

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.

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.

Rhodium Diamidobenzene Complexes: A Tale of Different Substituents on the Diamidobenzene Ligand

Diamidobenzene ligands are a prominent class of redox-active ligands owing to their electron reservoir behaviour, as well as the possibility of tuning the steric and the electronic properties of such ligands through the substituents on the N-atoms of the ligands. In this contribution, we present Rh(iii) complexes with four differently substituted diamidobenzene ligands. By using a combination of crystallography, NMR spectroscopy, electrochemistry, UV-vis-NIR/EPR spectroelectrochemistry, and quantum chemical calculations we show that the substituents on the ligands have a profound influence on the bonding, donor, electrochemical and spectroscopic properties of the Rh complexes. We present, for the first time, design strategies for the isolation of mononuclear Rh(ii) metallates whose redox potentials span across more than 850 mV. These Rh(ii) metallates undergo typical metalloradical reactivity such as activation of O₂ and C–Cl bond activations. Additionally, we also show that the substituents on the ligands dictate the one versus two electron nature of the oxidation steps of the Rh complexes. Furthermore, the oxidative reactivity of the metal complexes with a [CH₃]⁺ source leads to the isolation of a unprecedented, homobimetallic, heterovalent complex featuring a novel π-bonded rhodio-o-diiminoquionone. Our results thus reveal several new potentials of the diamidobenzene ligand class in organometallic reactivity and small molecule activation with potential relevance for catalysis.

Diamidobenzene ligands are versatile platforms in organometallic Rh-chemistry. They allow the isolation of tunable mononuclear ate-complexes, and the formation of a unprecedented homobimetallic, heterovalent complex.

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.