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

Pinpointing the active species of the Cu(DAT) catalyzed oxygen reduction reaction

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

Elucidation of the structure of a thiol functionalized Cu-tmpa complex anchored to gold via a self-assembled monolayer

The structure of the copper complex of the 6-((1-butanethiol)oxy)-tris(2-pyridylmethyl)amine ligand (Cu-tmpa-O(CH2)4SH) 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(CH2)4SH 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 CuII/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 CuII species but instead showed the presence of a CuI 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(CH2)4SH, the resulting structure is not identical to the homogeneous CuII-tmpa complex. Upon anchoring, a novel CuI 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

Thermally induced oxygen vacancies in BiOCl nanosheets and their impact on photoelectrochemical performance

Oxygen vacancies (OVs) have been reported to significantly alter the photocatalytic properties of BiOCl nanosheets. However, their formation mechanism and their role in the enhancement of photoelectrochemical performance remain unclear. In this work, thermally induced oxygen vacancies are introduced in BiOCl nanosheets by calcination at various temperatures and their formation mechanism is investigated by in-situ diffuse reflectance infrared (DRIFTS) measurements. The influence of oxygen vacancies on band offset, carrier concentrations and photoelectrochemical performance are systematically studied. The results show that (1) the surface of BiOCl nanosheets is extremely sensitive to temperature and defects are formed at temperatures as low as 200 °C in inert atmosphere. (2) The formation of bulk oxygen vacancies in BiOCl is identified by XPS and EPR experiments. (3) The photocurrent of BiOCl is dominantly limited by the concentration of charge carriers and shallow defect states induced by bulk oxygen vacancies can effectively increase light absorption and carrier concentration leading to an enhancement of photoelectrochemical performance of BiOCl

Electrocatalytic Water Oxidation with α‑[Fe(mcp)(OTf)2] and Analogues

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)

CCDC 1909357: Experimental Crystal Structure Determination

Related Article: Marta Olivares, Cornelis J. M. van der Ham, Velabo Mdluli, Markus Schmidtendorf, Helge Müller-Bunz, Tiny W.G.M Verhoeven, Mo Li, Hans J. W. Niemantsverdriet, Dennis G. H. Hetterscheid, Stefan Bernhard, Martin Albrecht, J. W. Hans Niemantsverdriet|2020|Eur.J.Inorg.Chem.|2020|801|doi:10.1002/ejic.20200009

CCDC 1909358: Experimental Crystal Structure Determination

Related Article: Marta Olivares, Cornelis J. M. van der Ham, Velabo Mdluli, Markus Schmidtendorf, Helge Müller-Bunz, Tiny W.G.M Verhoeven, Mo Li, Hans J. W. Niemantsverdriet, Dennis G. H. Hetterscheid, Stefan Bernhard, Martin Albrecht, J. W. Hans Niemantsverdriet|2020|Eur.J.Inorg.Chem.|2020|801|doi:10.1002/ejic.20200009

CCDC 1909355: Experimental Crystal Structure Determination

Related Article: Marta Olivares, Cornelis J. M. van der Ham, Velabo Mdluli, Markus Schmidtendorf, Helge Müller-Bunz, Tiny W.G.M Verhoeven, Mo Li, Hans J. W. Niemantsverdriet, Dennis G. H. Hetterscheid, Stefan Bernhard, Martin Albrecht, J. W. Hans Niemantsverdriet|2020|Eur.J.Inorg.Chem.|2020|801|doi:10.1002/ejic.20200009

CCDC 1909363: Experimental Crystal Structure Determination

Related Article: Marta Olivares, Cornelis J. M. van der Ham, Velabo Mdluli, Markus Schmidtendorf, Helge Müller-Bunz, Tiny W.G.M Verhoeven, Mo Li, Hans J. W. Niemantsverdriet, Dennis G. H. Hetterscheid, Stefan Bernhard, Martin Albrecht, J. W. Hans Niemantsverdriet|2020|Eur.J.Inorg.Chem.|2020|801|doi:10.1002/ejic.20200009

CCDC 1909361: Experimental Crystal Structure Determination

Related Article: Marta Olivares, Cornelis J. M. van der Ham, Velabo Mdluli, Markus Schmidtendorf, Helge Müller-Bunz, Tiny W.G.M Verhoeven, Mo Li, Hans J. W. Niemantsverdriet, Dennis G. H. Hetterscheid, Stefan Bernhard, Martin Albrecht, J. W. Hans Niemantsverdriet|2020|Eur.J.Inorg.Chem.|2020|801|doi:10.1002/ejic.20200009