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

\u3cp\u3eDinuclear Cu\u3csup\u3eII\u3c/sup\u3e 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)\u3csub\u3e2\u3c/sub\u3e 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 M\u3csub\u3es\u3c/sub\u3e = ±2 absorption characteristic of the Cu\u3csub\u3e2\u3c/sub\u3e(μ-DAT)\u3csub\u3e2\u3c/sub\u3e moiety. Monitoring the typical 380 nm peak with UV-Vis spectroscopy revealed that the Cu\u3csub\u3e2\u3c/sub\u3e(μ-DAT)\u3csub\u3e2\u3c/sub\u3e 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 O\u3csub\u3e2\u3c/sub\u3e 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 2p\u3csub\u3e3/2\u3c/sub\u3e and N 1s spectra were observed with respect to Cu(DAT). After ORR catalysis at pH 5, mostly Cu\u3csup\u3eI\u3c/sup\u3e and/or Cu\u3csup\u3e0\u3c/sup\u3e 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.\u3c/p\u3

Activation pathways taking place at molecular copper precatalysts for the oxygen evolution reaction

\u3cp\u3eThe activation processes of [Cu\u3csup\u3eII\u3c/sup\u3e(bdmpza)\u3csub\u3e2\u3c/sub\u3e] in the water oxidation reaction were investigated using cyclic voltammetry and chronoamperometry. Two different paths wherein CuO is formed were distinguished. [Cu\u3csup\u3eII\u3c/sup\u3e(bdmpza)\u3csub\u3e2\u3c/sub\u3e] can be oxidized at high potentials to form CuO, which was observed by a slight increase in catalytic current over time. When [Cu\u3csup\u3eII\u3c/sup\u3e(bdmpza)\u3csub\u3e2\u3c/sub\u3e] is initially reduced at low potentials, a more active water oxidation catalyst is generated, yielding high catalytic currents from the moment a sufficient potential is applied. This work highlights the importance of catalyst pre-treatment and the choice of the experimental conditions in water oxidation catalysis using copper complexes.\u3c/p\u3

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

\u3cp\u3eThe structure of the copper complex of the 6-((1-butanethiol)oxy)-tris(2-pyridylmethyl)amine ligand (Cu-tmpa-O(CH\u3csub\u3e2\u3c/sub\u3e)\u3csub\u3e4\u3c/sub\u3eSH) 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)\u3csub\u3e2\u3c/sub\u3e resulted in the formation of the anchored Cu-tmpa-O(CH\u3csub\u3e2\u3c/sub\u3e)\u3csub\u3e4\u3c/sub\u3eSH 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\u3csup\u3eII/I\u3c/sup\u3e 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\u3csup\u3eII\u3c/sup\u3e species but instead showed the presence of a Cu\u3csup\u3eI\u3c/sup\u3e 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\u3csub\u3e2\u3c/sub\u3e)\u3csub\u3e4\u3c/sub\u3eSH, the resulting structure is not identical to the homogeneous Cu\u3csup\u3eII\u3c/sup\u3e-tmpa complex. Upon anchoring, a novel Cu\u3csup\u3eI\u3c/sup\u3e 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.\u3c/p\u3

The influence of the ligand in the iridium mediated electrocatalyic water oxidation

\u3cp\u3eElectrochemical 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 \u3csub\u3e7/2\u3c/sub\u3e 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 \u3csup\u3e-2\u3c/sup\u3e once more without clear structure-activity relationships. In addition, by employing \u3csup\u3e1\u3c/sup\u3eH NMR spectroscopy we found evidence for Cp∗ breakdown products such as acetate. Electrodeposited iridium oxide from ligand free [Ir(OH) \u3csub\u3e6\u3c/sub\u3e] \u3csup\u3e2-\u3c/sup\u3e or a colloidal iridium oxide nanoparticles solution produces currents almost 2 orders of magnitude higher with a maximum current of 11 mA cm \u3csup\u3e-2\u3c/sup\u3e. Also, this deposited material contains, apart from an Ir 4f \u3csub\u3e7/2\u3c/sub\u3e 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 \u3csub\u3ex\u3c/sub\u3e 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. \u3c/p\u3

Detangling catalyst modification reactions from the oxygen evolution reaction by online mass spectrometry

\u3cp\u3eHere we showcase the synthesis and catalytic response of the anionic iridium(III) complex [IrCl\u3csub\u3e3\u3c/sub\u3e(pic)(MeOH)]\u3csup\u3e-\u3c/sup\u3e ([1]\u3csup\u3e-\u3c/sup\u3e, pic = picolinate) toward the evolution of oxygen. Online electrochemical mass spectrometry experiments illustrate that an initial burst of CO\u3csub\u3e2\u3c/sub\u3e due to catalyst degradation is expelled before the oxygen evolution reaction commences. Electrochemical features and XPS analysis illustrate the presence of iridium oxide, which is the true active species. (Chemical Equation Presented).\u3c/p\u3

Variation of salivary immunoglobulins in exercising and sedentary populations

When exposed to a potential exceeding 1.5 V versus RHE for several minutes the molecular iridium bishydroxide complex bearing a pentamethylcyclopentadienyl and a N-dimethylimidazolin-2-ylidene ligand spontaneously adsorbs onto the surface of glassy carbon and gold electrodes. Simultaneously with the adsorption of the material on the electrode, the evolution of dioxygen is detected and modifications of the catalyst structure are observed. XPS and XAS studies reveal that the species present at the electrode interface is best described as a partly oxidized molecular species rather than the formation of large aggregates of iridium oxide. These findings are in line with the unique kinetic profile of the parent complex in the water oxidation reaction

Relevance of chemical vs electrochemical oxidation of tunable carbene iridium complexes for catalytic water oxidation

\u3cp\u3eBased on previous work that identified iridium(III) Cp* complexes containing a C,N-bidentate chelating triazolylidene-pyridyl ligand (Cp* = pentamethylcyclopentadienyl, C \u3csub\u3e5\u3c/sub\u3eMe \u3csub\u3e5\u3c/sub\u3e \u3csup\u3e–\u3c/sup\u3e) as efficient molecular water oxidation catalysts, a series of new complexes based on this motif has been designed and synthesized in order to improve catalytic activity. Modifications include specifically the introduction of electron-donating substituents into the pyridyl unit of the chelating ligand (H, a; 5-OMe, b; 4-OMe, c; 4-tBu, d; 4-NMe \u3csub\u3e2\u3c/sub\u3e, e), as well as electronically active substituents on the triazolylidene C4 position (H, 8; COOEt, 9; OEt, 10; OH, 11; COOH, 12). Chemical oxidation using cerium ammonium nitrate (CAN) indicates a clear structure-activity relationship with electron-donating groups enhancing catalytic turnover frequency, especially when the donor substituent is positioned on the triazolylidene ligand fragment (TOF \u3csub\u3emax\u3c/sub\u3e = 2500 h \u3csup\u3e–\u3c/sup\u3e \u3csup\u3e1\u3c/sup\u3e for complex 10 with a MeO group on pyr and a OEt-substituted triazolylidene, compared to 700 h \u3csup\u3e–\u3c/sup\u3e \u3csup\u3e1\u3c/sup\u3e for the parent benchmark complex without substituents). Electrochemical water oxidation does not follow the same trend, and reveals that complex 8b without a substituent on the triazolylidene fragment outperforms complex 10 by a factor of 5, while in CAN-mediated chemical water oxidation, complex 10 is twice more active than 8b. This discrepancy in catalytic activity is remarkable and indicates that caution is needed when benchmarking iridium water oxidation catalysts with chemical oxidants, especially when considering that application in a potential device will most likely involve electrocatalytic water oxidation. \u3c/p\u3