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Electrocatalysis in metal-organic frameworks is an interplay between the diffusion of charges, the intrinsic catalytic rate, and the mass-transport of reactants through the pores. Here a systematic study is carried out to investigate the role of the electrolyte nature and concentration on the oxygen reduction reaction (ORR) with the PCN-224(Co) MOF in aqueous electrolyte. It was found that the ORR activity is slightly influenced by the nature of the ions in solution, providing that the ionic strength is high enough to minimize the resistivity during the measurement. The ORR activity was found to be 1.3-1.5 times lower in lithium acetate compared to sodium acetate, while the ORR activity in cesium acetate was 1.3-1.6 times higher compared to the activity in sodium acetate. Moreover, there was no dependency found of the ORR catalysis on the size of the anion, buffer concentration, or oxygen concentration. These findings suggest that ORR catalysis in PCN-224(Co) is limited by the intrinsic ORR rate at the active site rather than charge transport through the porous structure or substrate transport in the pores. Therefore, optimization of ORR catalysis with this MOF might be achieved by the optimization of the electronics at the cobalt active site., (© 2025 The Author(s). ChemSusChem published by Wiley-VCH GmbH.)

The terms biomimetic and bioinspired are very relevant in the field of bioinorganic chemistry and have been widely applied. Although they were defined by the International Organization for Standardization in 2015, these terms have at times been used rather ambiguously in the literature. This may be due to the inherent complexity of bioinorganic systems where, for example, a structural model of an enzyme active site may not replicate its function. Conversely, the function of an enzyme may be reproduced in a system where the structure does not resemble the enzyme's active site. To address this, we suggest definitions for the terms biomimetic and bioinspired wherein structure and function have been decoupled. With the help of some representative case studies we have outlined the challenges that may arise and make suggestions on how to apply terminology with careful intention.

Electrochemical oxygen reduction is a promising and sustainable alternative to the current industrial production method for hydrogen peroxide (H 2 O 2 ), which is a green oxidant in many (emerging) applications in the chemical industry, water treatment, and fuel cells. Low solubility of O 2 in water causes severe mass transfer limitations and loss of H 2 O 2 selectivity at industrially relevant current densities, complicating the development of practical-scale electrochemical H 2 O 2 synthesis systems. We tested a flow-by and flow-through configuration and suspension electrodes in an electrochemical flow cell to investigate the influence of electrode configuration and flow conditions on mass transfer and H 2 O 2 production. We monitored the H 2 O 2 production using Cu-tmpa (tmpa = tris(2-pyridylmethyl)amine) as a homogeneous copper-based catalyst in a pH-neutral phosphate buffer during 1 h of catalysis and estimated the limiting current density from CV scans. We achieve the highest H 2 O 2 production and a 15-20 times higher geometrical limiting current density in the flow-through configuration compared to the flow-by configuration due to the increased surface area and foam structure that improved mass transfer. The activated carbon (AC) material in suspension electrodes, which have an even larger surface area, decomposes all produced H 2 O 2 and proves unsuitable for H 2 O 2 synthesis. Although the mass transfer limitations seem to be alleviated on the microscale in the flow-through system, the high O 2 consumption and H 2 O 2 production cause challenges in maintaining the initially reached current density and Faradaic efficiency (FE). The decreasing ratio between the concentrations of the O 2 and H 2 O 2 in the bulk electrolyte will likely pose a challenge when proceeding to larger systems with longer electrodes. Tuning the reactor design and operating conditions will be essential in maximizing the FE and current density., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Sustainable alternatives for the energy intensive synthesis of H 2 O 2 are necessary. Molecular cobalt catalysts show potential but are typically restricted by undesired bimolecular pathways leading to the breakdown of both H 2 O 2 and the catalyst. The confinement of cobalt porphyrins in the PCN-224 metal-organic framework leads to an enhanced selectivity towards H 2 O 2 and stability of the catalyst. Consequently, oxygen can now be selectively reduced to hydrogen peroxide with a stable conversion for at least 5 h, illustrating the potential of catalysts confined in MOFs to increase the selectivity and stability of electrocatalytic conversions., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Changes in the electronic structure of copper complexes can have a remarkable impact on the catalytic rates, selectivity, and overpotential of electrocatalytic reactions. We have investigated the effect of the half-wave potential ( E 1/2 ) of the Cu II /Cu I redox couples of four copper complexes with different pyridylalkylamine ligands. A linear relationship was found between E 1/2 of the catalysts and the logarithm of the maximum rate constant of the reduction of O 2 and H 2 O 2 . Computed binding constants of the binding of O 2 to Cu I , which is the rate-determining step of the oxygen reduction reaction, also correlate with E 1/2 . Higher catalytic rates were found for catalysts with more negative E 1/2 values, while catalytic reactions with lower overpotentials were found for complexes with more positive E 1/2 values. The reduction of O 2 is more strongly affected by the E 1/2 than the H 2 O 2 rates, resulting in that the faster catalysts are prone to accumulate peroxide, while the catalysts operating with a low overpotential are set up to accommodate the 4-electron reduction to water. This work shows that the E 1/2 is an important descriptor in copper-mediated O 2 reduction and that producing hydrogen peroxide selectively close to its equilibrium potential at 0.68 V vs reversible hydrogen electrode (RHE) may not be easy.

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 (H 2 O)] 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 Ru V/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 (H 2 O)] 2+ compounds follow different design rules and scaling relations, as has been reported for every other oxygen evolution catalyst thus far.

Electrocatalysis is to play a key role in the transition towards a sustainable chemical and energy industry and active, stable and selective redox catalysts are much needed. Porous structures such as metal organic frameworks (MOFs) are interesting materials as these may influence selectivity of chemical reactions through confinement effects. In this work, the oxygen reduction catalyst Cu-tmpa was incorporated into the NU1000 MOF. Confinement of the catalyst within NU1000 steers the selectivity of the oxygen reduction reaction (ORR) towards water rather than peroxide. This is attributed to retention of the obligatory H 2 O 2 intermediate in close proximity to the catalytic center. Moreover, the resulting NU1000|Cu-tmpa MOF shows an excellent activity and stability in prolonged electrochemical studies, illustrating the potential of this approach., (© 2023 The Authors. ChemSusChem published by Wiley-VCH GmbH.)

Mononuclear copper complexes relevant to the active site of copper nitrite reductases (CuNiRs) are known to be catalytically active for the reduction of nitrite. Yet, their catalytic mechanism has thus far not been resolved. Here, we provide a complete description of the electrocatalytic nitrite reduction mechanism of a bio-inspired CuNiR catalyst Cu(tmpa) (tmpa = tris(2-pyridylmethyl)amine) in aqueous solution. Through a combination of electrochemical studies, reaction kinetics, and density functional theory (DFT) computations, we show that the protonation steps take place in a stepwise manner and are decoupled from electron transfer. The rate-determining step is a general acid-catalyzed protonation of a copper-ligated nitrous acid (HNO 2 ) species. In view of the growing urge to convert nitrogen-containing compounds, this work provides principal reaction parameters for efficient electrochemical nitrite reduction. This contributes to the investigation and development of nitrite reduction catalysts, which is crucial to restore the biogeochemical nitrogen cycle., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Understanding how multicopper oxidases (MCOs) reduce oxygen in the trinuclear copper cluster (TNC) is of great importance for development of catalysts for the oxygen reduction reaction (ORR). Herein, we report a mechanistic investigation into the ORR activity of the dinuclear copper complex [Cu 2 L(μ-OH)] 3+ ( L = 2,7-bis[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine). This complex is inspired by the dinuclear T3 site found in the MCO active site and confines the Cu centers in a rigid scaffold. We show that the electrochemical reduction of [Cu 2 L(μ-OH)] 3+ follows a proton-coupled electron transfer pathway and requires a larger overpotential due to the presence of the Cu-OH-Cu motif. In addition, we provide evidence that metal-metal cooperativity takes place during catalysis that is facilitated by the constraints of the rigid ligand framework, by identification of key intermediates along the catalytic cycle of [Cu 2 L(μ-OH)] 3+ . Electrochemical studies show that the mechanisms of the ORR and hydrogen peroxide reduction reaction found for [Cu 2 L(μ-OH)] 3+ differ from the ones found for analogous mononuclear copper catalysts. In addition, the metal-metal cooperativity results in an improved selectivity for the four-electron ORR of more than 70% because reaction intermediates can be stabilized better between both copper centers. Overall, the mechanism of the [Cu 2 L(μ-OH)] 3+ -catalyzed ORR in this work contributes to the understanding of how the cooperative function of multiple metals in close proximity can affect ORR activity and selectivity., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

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 Cu II and Cu III . 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. [Cu II (H L )(OTf) 2 ] [H L = Hbbpya = N , N -bis(2,2'-bipyrid-6-yl)amine)] was investigated as a WO catalyst bearing the redox-active H L ligand. The Cu catalyst was found to be active as a WO catalyst at pH 11.5, at which the deprotonated complex [Cu II ( L - )(H 2 O)] + 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 Δ H ⧧ value of 4.49 kcal·mol -1 , a Δ S ⧧ value of -42.6 cal·mol -1 ·K -1 , and a Δ G ⧧ value of 17.2 kcal·mol -1 were found. A computational study supported the formation of a Cu-oxyl intermediate, [Cu II ( L • )(O • )(H 2 O)] + . 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 Cu II center is proposed to be redox-inactive.

Artificial redox catalysts are typically limited by unfavorable scaling relations of reaction intermediates leading to a significant overpotential in multi-electron redox reactions such as for example the oxygen reduction reaction (ORR). The multicopper oxidase laccase is able to catalyze the ORR in nature. In particular the high-potential variants show a remarkably low overpotential for the ORR and apparently do not suffer from such unfavorable scaling relations. Although laccases are intensively studied, it is presently unknown why the overpotential for ORR is so low and a clear description regarding the thermodynamics of the catalytic cycle and the underlying design principles is lacking. In order to understand the laccase catalyzed ORR from an electrochemical perspective, elucidation of the free energy scheme would be of high value. This article reviews the energetics of the proposed laccase catalyzed ORR mechanisms based on experimental and computational studies. However, there are still remaining challenges to overcome to elucidate the free energy scheme of laccase. Obtaining thermodynamic data on intermediates is hard or even impossible with analytical techniques. On the other hand, several computational studies have been performed with significantly different parameters and conditions, thus making a direct comparison difficult. For these reasons, a consensus on a clear free energy scheme is still lacking. We anticipate that ultimately conquering these challenges will result in a better understanding of laccase catalyzed ORR and will allow for the design of low overpotential redox catalysts., Competing Interests: The authors declare no conflict of interest., (© 2022 The Authors. ChemCatChem published by Wiley-VCH GmbH.)

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(H L ) (H L = 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(H L ) interacts with the electrode during electrocatalysis, the formation of CoO x was not observed. Instead, a molecular deposit of Co(H L ) 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., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

H 2 O 2 is a bulk chemical used as "green" alternative in a variety of applications, but has an energy and waste intensive production method. The electrochemical O 2 reduction to H 2 O 2 is viable alternative with examples of the direct production of up to 20% H 2 O 2 solutions. In that respect, we found that the dinuclear complex Cu 2 (btmpa) (6,6'-bis[[bis(2-pyridylmethyl)amino]methyl]-2,2'-bipyridine) reduces O 2 to H 2 O 2 with a selectivity up to 90 % according to single linear sweep rotating ring disk electrode measurements. Microbalance experiments showed that complex reduction leads to surface adsorption thereby increasing the catalytic current. More importantly, we kept a high Faradaic efficiency for H 2 O 2 between 60 and 70 % over the course of 2 h of amperometry by introducing high potential intervals to strip deposited copper ( dep Cu). This is the first example of extensive studies into the long term electrochemical O 2 to H 2 O 2 reduction by a molecular complex which allowed to retain the high intrinsic selectivity of Cu 2 (btmpa) towards H 2 O 2 production leading to relevant levels of H 2 O 2 ., Competing Interests: The authors declare no conflict of interest., (© 2022 The Authors. ChemElectroChem published by Wiley-VCH GmbH.)

Upon the electrochemical reduction of an in situ generated 5-diazo-1,10-phenanthroline ion, phenanthroline was covalently attached to a gold electrode. The grafted molecules act as a ligand when brought in contact with a copper-containing electrolyte solution. As the ligands are limited in spatial movement, the exclusive formation of the active species with only one phenanthroline ligand coordinated was expected. The in situ generated complexes have been investigated for activity in the oxygen reduction reaction, for which an overpotential of 800 mV is observed. During catalysis, initially a thick copper layer is formed on top of an organic layer that is still present on the gold surface. Upon deterioration of the organic layer underneath the copper over time, the amount of copper on the electrode and thereby the electrocatalytic activity decreases., Competing Interests: The authors declare no conflict of interest., (© 2022 The Authors. ChemElectroChem published by Wiley-VCH GmbH.)

Moving towards a hydrogen economy raises the demand for affordable and efficient catalysts for the oxygen reduction reaction. Cu-bmpa (bmpa = bis(2-picolyl)amine) is shown to have moderate activity, but poor selectivity for the 4-electron reduction of oxygen to water. To enhance the selectivity towards water formation, the cooperative effect of three Cu-bmpa binding sites in a single trinuclear complex is investigated. The catalytic currents in the presence of the trinuclear sites are lower, possibly due to the more rigid structure and therefore higher reorganization energies and/or slower diffusion rates of the catalytic species. Although the oxygen reduction activity of the trinuclear complexes is lower than that of mononuclear Cu-bmpa, the selectivity of the copper mediated oxygen reduction was significantly enhanced towards the 4-electron process due to a cooperative effect between three copper centers that have been positioned in close proximity. These results indicate that the cooperativity between metal ions within biomimetic sites can greatly enhance the ORR selectivity.

The presence of defects and chemical dopants in metal-free carbon materials plays an important role in the electrocatalysis of the oxygen reduction reaction (ORR). The precise control and design of defects and dopants in carbon electrodes will allow the fundamental understanding of activity-structure correlations for tailoring catalytic performance of carbon-based, most particularly graphene-based, electrode materials. Herein, we adopted monolayer graphene - a model carbon-based electrode - for systematical introduction of nitrogen and oxygen dopants, together with vacancy defects, and studied their roles in catalyzing ORR. Compared to pristine graphene, nitrogen doping exhibited a limited effect on ORR activity. In contrast, nitrogen doping in graphene predoped with vacancy defects or oxygen enhanced the activities at 0.4 V vs the reversible hydrogen electrode (RHE) by 1.2 and 2.0 times, respectively. The optimal activity was achieved for nitrogen doping in graphene functionalized with oxygenated defects, 12.8 times more than nitrogen-doped and 7.7 times more than pristine graphene. More importantly, oxygenated defects are highly related to the 4e - pathway instead of nitrogen dopants. This work indicates a non-negligible contribution of oxygen and especially oxygenated vacancy defects for the catalytic activity of nitrogen-doped graphene., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

Hydrogen peroxide plays an important role as an intermediate and product in the reduction of dioxygen by copper enzymes and mononuclear copper complexes. The copper(II) tris(2-pyridylmethyl)amine complex (Cu-tmpa) has been shown to produce H 2 O 2 as an intermediate during the electrochemical 4-electron reduction of O 2 . We investigated the electrochemical hydrogen peroxide reduction reaction (HPRR) by Cu-tmpa in a neutral aqueous solution. The catalytic rate constant of the reaction was shown to be one order of magnitude lower than the reduction of dioxygen. A significant solvent kinetic isotope effect (KIE) of 1.4 to 1.7 was determined for the reduction of H 2 O 2 , pointing to a Fenton-like reaction pathway as the likely catalytic mechanism, involving a single copper site that produces an intermediate copper(II) hydroxo species and a free hydroxyl radical anion in the process., Competing Interests: The authors declare no conflict of interest., (© 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH.)

The complex α-[Fe(mcp)(OTf) 2 ] (mcp = N , N '-dimethyl- N , N '-bis(pyridin-2-ylmethyl)-cyclohexane-1,2-diamine and OTf = trifluoromethanesulfonate anion) was reported in 2011 by some of us as an active water oxidation (WO) catalyst in the presence of sacrificial oxidants. However, because chemical oxidants are likely to take part in the reaction mechanism, mechanistic electrochemical studies are critical in establishing to what extent previous studies with sacrificial reagents have actually been meaningful. In this study, the complex α-[Fe(mcp)(OTf) 2 ] and its analogues were investigated electrochemically under both acidic and neutral conditions. All the systems under investigation proved to be electrochemically active toward the WO reaction, with no major differences in activity despite the structural changes. Our findings show that WO-catalyzed by mcp-iron complexes proceeds via homogeneous species, whereas the analogous manganese complex forms a heterogeneous deposit on the electrode surface. Mechanistic studies show that the reaction proceeds with a different rate-determining step (rds) than what was previously proposed in the presence of chemical oxidants. Moreover, the different kinetic isotope effect (KIE) values obtained electrochemically at pH 7 (KIE ∼ 10) and at pH 1 (KIE = 1) show that the reaction conditions have a remarkable effect on the rds and on the mechanism. We suggest a proton-coupled electron transfer (PCET) as the rds under neutral conditions, whereas at pH 1 the rds is most likely an electron transfer (ET)., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

To date, the copper complex with the tris(2-pyridylmethyl)amine ( tmpa ) ligand ( Cu - tmpa ) catalyzes the ORR with the highest reported turnover frequency (TOF) for any molecular copper catalyst. To gain insight into the importance of the tetradentate nature and high flexibility of the tmpa ligand for efficient four-electron ORR catalysis, the redox and electrocatalytic ORR behavior of the copper complexes of 2,2':6',2″-terpyridine ( terpy ) and bis(2-pyridylmethyl)amine ( bmpa ) ( Cu - terpy and Cu - bmpa , respectively) were investigated in the present study. With a combination of cyclic voltammetry and rotating ring disk electrode measurements, we demonstrate that the presence of the terpy and bmpa ligands results in a decrease in catalytic ORR activity and an increase in Faradaic efficiency for H 2 O 2 production. The lower catalytic activity is shown to be the result of a stabilization of the Cu I state of the complex compared to the earlier reported Cu - tmpa catalyst. This stabilization is most likely caused by the lower electron donating character of the tridentate terpy and bmpa ligands compared to the tetradentate tmpa ligand. The Laviron plots of the redox behavior of Cu - terpy and Cu - bmpa indicated that the formation of the ORR active catalyst involves relatively slow electron transfer kinetics which is caused by the inability of Cu - terpy and Cu - bmpa to form the preferred tetrahedral coordination geometry for a Cu I complex easily. Our study illustrates that both the tetradentate nature of the tmpa ligand and the ability of Cu - tmpa to form the preferred tetrahedral coordination geometry for a Cu I complex are of utmost importance for ORR catalysis with very high catalytic rates.

Electrochemical water oxidation is the bottleneck of electrolyzers as even the best catalysts, iridium and ruthenium oxides, have to operate at significant overpotentials. Previously, the position of a hydroxyl on a series of hydroxylpicolinate ligands was found to significantly influence the activity of molecular iridium catalysts in sacrificial oxidant driven water oxidation. In this study, these catalysts were tested under electrochemical conditions and benchmarked to several other known molecular iridium catalysts under the exact same conditions. This allowed us to compare these catalysts directly and observe whether structure-activity relationships would prevail under electrochemical conditions. Using both electrochemical quartz crystal microbalance experiments and X-ray photoelectron spectroscopy, we found that all studied iridium complexes form an iridium deposit on the electrode with binding energies ranging from 62.4 to 62.7 eV for the major Ir 4f 7/2 species. These do not match the binding energies found for the parent complexes, which have a broader binding energy range from 61.7 to 62.7 eV and show a clear relationship to the electronegativity induced by the ligands. Moreover, all catalysts performed the electrochemical water oxidation in the same order of magnitude as the maximum currents ranged from 0.2 to 0.6 mA cm -2 once more without clear structure-activity relationships. In addition, by employing 1 H NMR spectroscopy we found evidence for Cp* breakdown products such as acetate. Electrodeposited iridium oxide from ligand free [Ir(OH) 6 ] 2- or a colloidal iridium oxide nanoparticles solution produces currents almost 2 orders of magnitude higher with a maximum current of 11 mA cm -2 . Also, this deposited material contains, apart from an Ir 4f 7/2 species at 62.4 eV, an Ir species at 63.6 eV, which is not observed for any deposit formed by the molecular complexes. Thus, the electrodeposited material of the complexes cannot be directly linked to bulk iridium oxide. Small IrO x clusters containing few Ir atoms with partially incorporated ligand residues are the most likely option for the catalytically active electrodeposit. Our results emphasize that structure-activity relationships obtained with sacrificial oxidants do not necessarily translate to electrochemical conditions. Furthermore, other factors, such as electrodeposition and catalyst degradation, play a major role in the electrochemically driven water oxidation and should thus be considered when optimizing molecular catalysts., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

The structure of the copper complex of the 6-((1-butanethiol)oxy)-tris(2-pyridylmethyl)amine ligand (Cu-tmpa-O(CH 2 ) 4 SH) anchored to a gold surface has been investigated. To enable covalent attachment of the complex to the gold surface, a heteromolecular self-assembled monolayer (SAM) of butanethiol and a thiol-substituted tmpa ligand was used. Subsequent formation of the immobilized copper complex by cyclic voltammetry in the presence of Cu(OTf) 2 resulted in the formation of the anchored Cu-tmpa-O(CH 2 ) 4 SH system which, according to scanning electron microscopy and X-ray diffraction, did not contain any accumulated copper nanoparticles or crystalline copper material. Electrochemical investigation of the heterogenized system barely showed any redox activity and lacked the typical Cu II/I redox couple in contrast to the homogeneous complex in solution. The difference between the heterogenized system and the homogeneous complex was confirmed by X-ray photoelectron spectroscopy; the XPS spectrum did not show any satellite features of a Cu II species but instead showed the presence of a Cu I ion in a ∼2:3 ratio to nitrogen and a ∼2:7 ratio to sulfur. The +I oxidation state of the copper species was confirmed by the edge position in the X-ray absorption near-edge structure (XANES) region of the X-ray absorption spectrum. These results show that upon immobilization of Cu-tmpa-O(CH 2 ) 4 SH, the resulting structure is not identical to the homogeneous Cu II -tmpa complex. Upon anchoring, a novel Cu I species is formed instead. This illustrates the importance of a thorough characterization of heterogenized molecular systems before drawing any conclusions regarding the structure-function relationships.

Catalytic pathways for the reduction of dioxygen can either lead to the formation of water or peroxide as the reaction product. We demonstrate that the electrocatalytic reduction of O 2 by the pyridylalkylamine copper complex [Cu(tmpa)(L)] 2+ in a neutral aqueous solution follows a stepwise 4 e - /4 H + pathway, in which H 2 O 2 is formed as a detectable intermediate and subsequently reduced to H 2 O in two separate catalytic reactions. These homogeneous catalytic reactions are shown to be first order in catalyst. Coordination of O 2 to Cu I was found to be the rate-determining step in the formation of the peroxide intermediate. Furthermore, electrochemical studies of the reaction kinetics revealed a high turnover frequency of 1.5×10 5  s -1 , the highest reported for any molecular copper catalyst., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Dinuclear CuII complexes bearing two 3,5-diamino-1,2,4-triazole (DAT) ligands have gained considerable attention as a potential model system for laccase due to their low overpotential for the oxygen reduction reaction (ORR). In this study, the active species for the ORR was investigated. The water soluble dinuclear copper complex (Cu(DAT)) was obtained by mixing a 1 : 1 ratio of Cu(OTf)2 and DAT in water. The electron paramagnetic resonance (EPR) spectrum of Cu(DAT) showed a broad axial signal with a g factor of 2.16 as well as a low intensity Ms = ±2 absorption characteristic of the Cu2(μ-DAT)2 moiety. Monitoring the typical 380 nm peak with UV-Vis spectroscopy revealed that the Cu2(μ-DAT)2 core is extremely sensitive to changes in pH, copper to ligand ratios and the presence of anions. Electrochemical quartz crystal microbalance experiments displayed a large decrease in frequency below 0.5 V versus the reversible hydrogen electrode (RHE) in a Cu(DAT) solution implying the formation of deposition. Rotating ring disk electrode experiments showed that this deposition is an active ORR catalyst which reduces O2 all the way to water at pH 5. The activity increased significantly in the course of time. X-ray photoelectron spectroscopy was utilized to analyze the composition of the deposition. Significant shifts in the Cu 2p3/2 and N 1s spectra were observed with respect to Cu(DAT). After ORR catalysis at pH 5, mostly CuI and/or Cu0 species are present and the deposition corresponds to previously reported electrodepositions of copper. This leads us to conclude that the active species is of a heterogeneous nature and lacks any structural similarity with laccase.

The synthesis, characterization, and electrochemical studies of the dinuclear complex [(MeOH)Fe(Hbbpya)-μ-O-(Hbbpya)Fe(MeOH)](OTf) 4 ( 1 ) (with Hbbpya = N,N -bis(2,2'-bipyrid-6-yl)amine) are described. With the help of online electrochemical mass spectrometry, the complex is demonstrated to be active as a water oxidation catalyst. Comparing the results obtained for different electrode materials shows a clear substrate influence of the electrode, as the complex shows a significantly lower catalytic overpotential on graphitic working electrodes in comparison to other electrode materials. Cyclic voltammetry experiments provide evidence that the structure of complex 1 undergoes reversible changes under high-potential conditions, regenerating the original structure of complex 1 upon returning to lower potentials. Results from electrochemical quartz crystal microbalance experiments rule out that catalysis proceeds via deposition of catalytically active material on the electrode surface., Competing Interests: The authors declare no competing financial interest.

Homogeneous reactions in general are relatively easy to study with respect to heterogeneous systems since all catalytic sites are uniform and can be addressed simultaneously. The latter feature is fully out of the window in an electrochemical context, where only the few catalytic species that are sufficiently close to the electrode undergo redox reactions. Especially in the water oxidation reaction where harsh reaction conditions are employed, a clear picture of what is the active species, what products are formed, how one can steer this, and how it all depends on the exact reaction conditions is important to be able to fully unravel the key reaction paths. The combination of electrochemical experiments with on-line detection of the catalytic species and reaction products is a powerful approach to successfully address these questions. Recently, a significant progress has been made in on-line studies on molecular water oxidation catalysts during electrochemical experiments. These are reviewed here.