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Cobaloxime Complex Salts: Synthesis, Patterning on Carbon Nanomembranes and Heterogeneous Hydrogen Evolution Studies.

ABSTRACT: Cobaloximes are promising, earth-abundant catalysts for the light-driven hydrogen evolution reaction (HER). Typically, these cobalt(III) complexes are prepared in situ or employed in their neutral form, for example, [Co(dmgH)₂ (py)Cl], even though related complex salts have been reported previously and could, in principle, offer improved catalytic activity as well as more efficient immobilization on solid support. Herein, we report an interdisciplinary investigation into complex salts [Co(dmgH)₂ (py)₂ ]⁺ [Co(dmgBPh₂ )₂ Cl₂ ]^- , TBA+[Co(dmgBPh2)2Cl2]- and [Co(dmgH)₂ (py)₂ ]⁺ BArF^- . We describe their strategic syntheses from the commercially available complex [Co(dmgH)₂ (py)Cl] and demonstrate that these double and single complex salts are potent catalysts for the light-driven HER. We also show that scanning electrochemical cell microscopy can be used to deposit arrays of catalysts [Co(dmgH)₂ (py)₂ ]⁺ [Co(dmgBPh₂ )₂ Cl₂ ]^- , TBA+[Co(dmgBPh2)2Cl2]- and [Co(dmgH)₂ (py)Cl] on supported and free-standing amino-terminated ∼1-nm-thick carbon nanomembranes (CNMs). Photocatalytic H₂ evolution at such arrays was quantified with Pd microsensors by scanning electrochemical microscopy, thus providing a new approach for catalytic evaluation and opening up novel routes for the creation and analysis of "designer catalyst arrays", nanoprinted in a desired pattern on a solid support.

SUBMITTER: Oswald E

PROVIDER: S-EPMC9299159 | biostudies-literature |

REPOSITORIES: biostudies-literature

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A novel ruthenium(II)-cobaloxime supramolecular complex for photocatalytic H2 evolution: synthesis, characterisation and mechanistic studies.

Project description:We report the synthesis and characterization of novel mixed-metal binuclear ruthenium(II)-cobalt(II) photocatalysts for hydrogen evolution in acidic acetonitrile. First, 2-(2'-pyridyl)benzothiazole (pbt), 1, was reacted with RuCl(3)·xH(2)O to produce [Ru(pbt)(2)Cl(2)]·0.25CH(3)COCH(3), 2, which was then reacted with 1,10-phenanthroline-5,6-dione (phendione), 3, in order to produce [Ru(pbt)(2)(phendione)](PF(6))(2)·4H(2)O, 4. Compound 4 was then reacted with 4-pyridinecarboxaldehyde in order to produce [Ru(pbt)(2)(L-pyr)](PF(6))(2)·9.5H(2)O, 5 (where L-pyr = (4-pyridine)oxazolo[4,5-f]phenanthroline). Compound 5 was then reacted with [Co(dmgBF(2))(2)(H(2)O)(2)] (where dmgBF(2) = difluoroboryldimethylglyoximato) in order to produce the mixed-metal binuclear complex, [Ru(pbt)(2)(L-pyr)Co(dmgBF(2))(2)(H(2)O)](PF(6))(2)·11H(2)O·1.5CH(3)COCH(3), 6. [Ru(Me(2)bpy)(2)(L-pyr)Co(dmgBF(2))(2)(OH(2))](PF(6))(2), 7 (where Me(2)bpy = 1,10-phenanthroline, 4,4'-dimethyl-2,2'-bipyridine) and [Ru(phen)(2)(L-pyr)Co(dmgBF(2))(2)(OH(2))](PF(6))(2), 8 were also synthesised. All complexes were characterized by elemental analysis, ESI MS, HRMS, UV-visible absorption, (11)B, (19)F, and (59)Co NMR, ESR spectroscopy, and cyclic voltammetry, where appropriate. Photocatalytic studies carried out in acidified acetonitrile demonstrated constant hydrogen generation longer than a 42 hour period as detected by gas chromatography. Time resolved spectroscopic measurements were performed on compound 6, which proved an intramolecular electron transfer from an excited Ru(II) metal centre to the Co(II) metal centre via the bridging L-pyr ligand. This resulted in the formation of a cobalt(I)-containing species that is essential for the production of H(2) gas in the presence of H(+) ions. A proposed mechanism for the generation of hydrogen is presented.

| S-EPMC3482109 | biostudies-literature

Electrocatalytic Hydrogen Evolution from a Cobaloxime-Based Metal-Organic Framework Thin Film.

Project description:Molecular hydrogen evolution catalysts (HECs) are synthetically tunable and often exhibit high activity, but they are also hampered by stability concerns and practical limitations associated with their use in the homogeneous phase. Their incorporation as integral linker units in metal-organic frameworks (MOFs) can remedy these shortcomings. Moreover, the extended three-dimensional structure of MOFs gives rise to high catalyst loadings per geometric surface area. Herein, we report a new MOF that exclusively consists of cobaloximes, a widely studied HEC, that act as metallo-linkers between hexanuclear zirconium clusters. When grown on conducting substrates and under applied reductive potential, the cobaloxime linkers promote electron transport through the film as well as function as molecular HECs. The obtained turnover numbers are orders of magnitude higher than those of any other comparable cobaloxime system, and the molecular integrity of the cobaloxime catalysts is maintained for at least 18 h of electrocatalysis. Being one of the very few hydrogen evolving electrocatalytic MOFs based on a redox-active metallo-linker, this work explores uncharted terrain for greater catalyst diversity and charge transport pathways.

| S-EPMC6803166 | biostudies-literature

Pyridine vs. Imidazole Axial Ligation on Cobaloxime Grafted Graphene: Hydrogen Evolution Reaction Insights.

Project description:While cobaloximes have been protagonists in the molecular (photo)catalytic hydrogen evolution reaction field, researchers originally shed light on the catalytically active metallic center. However, the specific chemical environment of cobalt, including equatorial and axial ligation, has also a strong impact on the catalytic reaction. In this article, we aim to demonstrate how pyridine vs. imidazole axial ligation of a cobaloxime complex covalently grafted on graphene affects the hydrogen evolution reaction performance in realistic acidic conditions. While pyridine axial ligation mirrors a drastically superior electrocatalytic performance, imidazole exhibits a remarkable long-term stability.

| S-EPMC9458012 | biostudies-literature

Microsecond X-ray Absorption Spectroscopy Identification of Co(I) Intermediates in Cobaloxime-Catalyzed Hydrogen Evolution.

Project description:Rational development of efficient photocatalytic systems for hydrogen production requires understanding the catalytic mechanism and detailed information about the structure of intermediates in the catalytic cycle. We demonstrate how time-resolved X-ray absorption spectroscopy in the microsecond time range can be used to identify such intermediates and to determine their local geometric structure. This method was used to obtain the solution structure of the Co(I) intermediate of cobaloxime, which is a non-noble metal catalyst for solar hydrogen production from water. Distances between cobalt and the nearest ligands including two solvent molecules and displacement of the cobalt atom out of plane formed by the planar ligands have been determined. Combining in situ X-ray absorption and UV/Vis data, we demonstrate how slight modification of the catalyst structure can lead to the formation of a catalytically inactive Co(I) state under similar conditions. Possible deactivation mechanisms are discussed.

| S-EPMC5493984 | biostudies-literature

A Poly(cobaloxime)/Carbon Nanotube Electrode: Freestanding Buckypaper with Polymer-Enhanced H2-Evolution Performance.

Project description:A freestanding H2-evolution electrode consisting of a copolymer-embedded cobaloxime integrated into a multiwall carbon nanotube matrix by π-π interactions is reported. This electrode is straightforward to assemble and displays high activity towards hydrogen evolution in near-neutral pH solution under inert and aerobic conditions, with a cobalt-based turnover number (TON(Co)) of up to 420. An analogous electrode with a monomeric cobaloxime showed less activity with a TON(Co) of only 80. These results suggest that, in addition to the high surface area of the porous network of the buckypaper, the polymeric scaffold provides a stabilizing environment to the catalyst, leading to further enhancement in catalytic performance. We have therefore established that the use of a multifunctional copolymeric architecture is a viable strategy to enhance the performance of molecular electrocatalysts.

| S-EPMC4794774 | biostudies-literature

COMPLEX EVOLUTION OF BILE SALTS IN BIRDS.

Project description:Bile salts are the major end-metabolites of cholesterol and are important in lipid digestion and shaping of the gut microflora. There have been limited studies of bile-salt variation in birds. The purpose of our study was to determine bile-salt variation among birds and relate this variation to current avian phylogenies and hypotheses on the evolution of bile salt pathways. We determined the biliary bile-salt composition of 405 phylogenetically diverse bird species, including 7 paleognath species. Bile salt profiles were generally stable within bird families. Complex bile-salt profiles were more common in omnivores and herbivores than in carnivores. The structural variation of bile salts in birds is extensive and comparable to that seen in surveys of bile salts in reptiles and mammals. Birds produce many of the bile salts found throughout nonavian vertebrates and some previously uncharacterized bile salts. One difference between birds and other vertebrates is extensive hydroxylation of carbon-16 of bile salts in bird species. Comparison of our data set of bird bile salts with that of other vertebrates, especially reptiles, allowed us to infer evolutionary changes in the bile salt synthetic pathway.

| S-EPMC2990222 | biostudies-literature

Light-driven hydrogen evolution catalyzed by a cobaloxime catalyst incorporated in a MIL-101(Cr) metal-organic framework.

Project description:A cobaloxime H2 evolution catalyst with a hydroxo-functionalized pyridine ligand, Co(dmgH)2(4-HEP)Cl [dmgH = dimethylglyoxime, 4-HEP = 4-(2-hydroxyethyl)pyridine] was immobilized on a chromium terephthalate metal-organic framework (MOF), MIL-101(Cr), to construct a MOF-catalyst hybrid which displays good photocatalytic H2 evolution activity. The longevity of the cobaloxime catalyst is increased by MOF incorporation, but limited by the stability of the cobalt-pyridine bond under turnover conditions.

| S-EPMC6130847 | biostudies-literature

Imaging of carbon nanomembranes with helium ion microscopy.

Project description:Carbon nanomembranes (CNMs) prepared from aromatic self-assembled monolayers constitute a recently developed class of 2D materials. They are made by a combination of self-assembly, radiation-induced cross-linking and the detachment of the cross-linked SAM from its substrate. CNMs can be deposited on arbitrary substrates, including holey and perforated ones, as well as on metallic (transmission electron microscopy) grids. Therewith, freestanding membranes with a thickness of 1 nm and macroscopic lateral dimensions can be prepared. Although free-standing CNMs cannot be imaged by light microscopy, charged particle techniques can visualize them. However, CNMs are electrically insulating, which makes them sensitive to charging. We demonstrate that the helium ion microscope (HIM) is a good candidate for imaging freestanding CNMs due to its efficient charge compensation tool. Scanning with a beam of helium ions while recording the emitted secondary electrons generates the HIM images. The advantages of HIM are high resolution, high surface sensitivity and large depth of field. The effects of sample charging, imaging of multilayer CNMs as well as imaging artefacts are discussed.

| S-EPMC4578422 | biostudies-literature

Enhanced photocatalytic hydrogen evolution by combining water soluble graphene with cobalt salts.

Project description:There is tremendous effort put in the pursuit for cheap and efficient catalysts for photocatalytic hydrogen evolution systems. Herein, we report an active catalyst that uses the earth-abundant element cobalt and water-dispersible sulfonated graphene. The photocatalytic hydrogen evolution activity of the catalyst was tested by using triethanolamine (TEOA) as electron donor and eosin Y (EY) as the photosensitizer under LED irradiation at 525 nm. Hydrogen was produced constantly even after 20 h, and the turnover number (TON) reached 148 (H2/Co) in 4 h with respect to the initial concentration of the added cobalt salts was shown to be 5.6 times larger than that without graphene.

| S-EPMC4142871 | biostudies-literature

Thermodynamic Properties of Hydrogen-Producing Cobaloxime Catalysts: A Density Functional Theory Analysis.

Project description:Density functional theory calculations were carried out to study the electrochemical properties including reduction potentials, pK a values, and thermodynamic hydricities of three prototypical cobaloxime complexes, Co(dmgBF2)2 (dmgBF2 = difluoroboryl-dimethylglyoxime), Co(dmgH)2 (dmgH = dimethylglyoxime), and Co(dmgH)2(py)(Cl) (py = pyridine) in the acetonitrile (AN)-water solvent mixture. The electrochemical properties of Co(dmgBF2)2 in pure AN and pure water were also considered for comparison to reveal the key roles of the solvent on the catalytic reaction. In agreement with previous studies, hydrogen production pathways starting from reduction of the resting state of CoII and involving formation of the CoIIIH and CoIIH intermediates are the favorable ones for both bimetallic and monometallic pathways. However, we found that in pure AN, both the CoIIIH and CoIIH intermediates can react with a proton to produce H2. In the presence of water in the solvent, the reduction of CoIIIH to CoIIH is necessary for the reaction with a proton to occur to form H2. This suggests that it is possible to design catalytic systems by suitably tuning the composition of the AN-water mixture. We also identified the key role of axial coordination of the solvent molecules in affecting the catalytic reaction, which allows further catalyst design strategy. The highest hydride donor ability of Co(dmgH)2(py)(Cl) indicates that this complex displays the best catalytic hydrogen-producing performance among the three cobaloximes studied in this work.

| S-EPMC6649079 | biostudies-literature

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