Mechanistic Investigations into the Selective Reduction of Oxygen by a Multicopper Oxidase T3 Site-Inspired Dicopper Complex

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 [Cu2L(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 [Cu2L(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 [Cu2L(μ-OH)]3+. Electrochemical studies show that the mechanisms of the ORR and hydrogen peroxide reduction reaction found for [Cu2L(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 [Cu2L(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

Elucidation of the Electrocatalytic Nitrite Reduction Mechanism by Bio-Inspired Copper Complexes

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 (HNO2) 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

Mechanistic Investigations into the Selective Reduction of Oxygen by a MCO T3 site-inspired Copper Complex

Understanding how multicopper oxidases (MCOs) efficiently and selectively reduce oxygen in the trinuclear copper cluster (TNC) is of great importance. Previously it was reported that when the T2-site is removed from the TNC, all O2 binding activity at the dinuclear T3-site is lost. Computational studies attribute this loss of activity to the flexibility of the protein active site, where the T3-copper centers move apart to minimize electrostatic repulsions. To address the question if and how a more constrained T3-site will catalyze the reduction of oxygen, we herein report a mechanistic investigation into the oxygen reduction reaction (ORR) activity of the dinuclear copper complex [Cu2L(μ-OH)]3+ (L=2,7-bis[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine). This T3-inspired complex confines the Cu centers in a rigid scaffold in close proximity instead of the flexible scaffold found in the protein active site and we demonstrate that under these constraints the dinuclear copper site displays ORR activity. Compared to the ORR mechanism of MCOs, we show that electrochemical reduction of [Cu2L(μ-OH)]3+ follows a similar pathway as the reduction of the resting enzyme due to the presence of the Cu-OH-Cu motif. By identification of key intermediates along the catalytic cycle of [Cu2L(μ-OH)]3+ we provide for the first time evidence that metal-metal cooperativity takes place during electrocatalysis of the ORR by a copper-based catalyst, which is achieved by the ability of the rigid ligand framework to bind two copper atoms in close proximity. Electrochemical studies show that the mechanisms of the ORR and hydrogen peroxide reduction reaction (HPRR) found for [Cu2L(μ-OH)]3+ are different 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%. This selectivity is achieved by better stabilization of reaction intermediates between both copper centers, which is also essential for the ORR mechanism observed in MCOs. Overall, the mechanism of the [Cu2L(μ-OH)]3+-catalyzed ORR in this work gives insight into the ORR activity of a T3-site and contributes to understanding of how the ORR activity and selectivity are established in MCOs

Catalytic Activity of an Iron-Based Water Oxidation Catalyst: Substrate Effects of Graphitic Electrodes

The synthesis, characterization, and electrochemical studies of the dinuclear complex [(MeOH)­Fe­(Hbbpya)-μ-O-(Hbbpya)­Fe­(MeOH)]­(OTf)₄ (<b>1</b>) (with Hbbpya = <i>N,N</i>-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 <b>1</b> undergoes reversible changes under high-potential conditions, regenerating the original structure of complex <b>1</b> 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

Catalytic Activity of an Iron-Based Water Oxidation Catalyst: Substrate Effects of Graphitic Electrodes

The synthesis, characterization, and electrochemical studies of the dinuclear complex [(MeOH)­Fe­(Hbbpya)-μ-O-(Hbbpya)­Fe­(MeOH)]­(OTf)₄ (<b>1</b>) (with Hbbpya = <i>N,N</i>-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 <b>1</b> undergoes reversible changes under high-potential conditions, regenerating the original structure of complex <b>1</b> 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

Mechanically Constrained Catalytic Mn(CO)(3)Br Single Sites in a Two-Dimensional Covalent Organic Framework for CO2 Electroreduction in H2O

The development of CO2 electroreduction (CO2RR) catalysts based on covalent organic frameworks (COFs) is an emerging strategy to produce synthetic fuels. However, our understanding on catalytic mechanisms and structure-activity relationships for COFs is still limited but essential to the rational design of these catalysts. Herein, we report a newly devised CO2 reduction catalyst by loading single-atom centers, {fac-Mn(CO)(3)S}, (S = Br, CH3CN, H2O), within a bipyridylbased COF (COFbpyMn). COFbpyMn shows a low CO2RR onset potential (eta = 190 mV) and high current densities (>12 mA.cm(-2), at 550 mV overpotential) in water. TOFCO and TONCO values are as high as 1100 h-1 and 5800 (after 16 h), respectively, which are more than 10-fold higher than those obtained for the equivalent manganese-based molecular catalyst. Furthermore, we accessed key catalytic intermediates within a COF matrix by combining experimental and computational (DFT) techniques. The COF imposes mechanical constraints on the {fac-Mn(CO)(3)S} centers, offering a strategy to avoid forming the detrimental dimeric Mn-0-Mn-0, which is a resting state typically observed for the homologous molecular complex. The absence of dimeric species correlates to the catalytic enhancement. These findings can guide the rational development of isolated single-atom sites and the improvement of the catalytic performance of reticular materials