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Author: Debashis Basu
Publisher:
ISBN:
Category : Chemistry
Languages : en
Pages : 279
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Book Description
We designed several redox-active ligand architectures to optimize and understand the redox, electronic, and catalytic properties of their respective cobalt complexes. Ligand design was varied from pentadentate donor phenolate to tetradentate acceptor oxime in order to reduce the overpotential of hydrogen generation in organic solvents. We altered the substitution, axial ligands and axial ligand substitutions to vary electronic and catalytic properties for such tetra- or pentadentate ligand systems. Knowledge of the nature of the active species for catalysis enabled us to design the pentadentate oxime ligand which exhibited rich reaction chemistry along with suitable catalytic property in organic solvent. Presence of several polar groups like -OH and -NH and the absence of any aromatic rings make this complex water soluble which is an added advantage. Additionally, this complex exhibited excellent catalytic properties in water with low onset overpotential and high turnover number. We developed similar redox-active-acceptor pentadentate phenylene-bridged pyridine-rich ligand which provided extremely versatile reaction chemistry after complexation with cobalt. These complexes displayed catalytic properties at moderate to low-overpotential in acetonitrile with good turnover numbers. Furthermore, the water solubility and tunability of such complexes make them suitable candidates for water reduction. Therefore, water reduction was carried out with these complexes showing low onset overpotentials and high turnover numbers. Finally, we incorporated [Ru(bpy)2]2+-based photosensitized with one of the catalytic module (cobalt tetradentate oxime) to generate heterobimetallic [RuIICoIII] species which displayed quenching of CoIII upon electron transfer from RuII excited state.
Author: Debashis Basu
Publisher:
ISBN:
Category : Chemistry
Languages : en
Pages : 279
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Book Description
We designed several redox-active ligand architectures to optimize and understand the redox, electronic, and catalytic properties of their respective cobalt complexes. Ligand design was varied from pentadentate donor phenolate to tetradentate acceptor oxime in order to reduce the overpotential of hydrogen generation in organic solvents. We altered the substitution, axial ligands and axial ligand substitutions to vary electronic and catalytic properties for such tetra- or pentadentate ligand systems. Knowledge of the nature of the active species for catalysis enabled us to design the pentadentate oxime ligand which exhibited rich reaction chemistry along with suitable catalytic property in organic solvent. Presence of several polar groups like -OH and -NH and the absence of any aromatic rings make this complex water soluble which is an added advantage. Additionally, this complex exhibited excellent catalytic properties in water with low onset overpotential and high turnover number. We developed similar redox-active-acceptor pentadentate phenylene-bridged pyridine-rich ligand which provided extremely versatile reaction chemistry after complexation with cobalt. These complexes displayed catalytic properties at moderate to low-overpotential in acetonitrile with good turnover numbers. Furthermore, the water solubility and tunability of such complexes make them suitable candidates for water reduction. Therefore, water reduction was carried out with these complexes showing low onset overpotentials and high turnover numbers. Finally, we incorporated [Ru(bpy)2]2+-based photosensitized with one of the catalytic module (cobalt tetradentate oxime) to generate heterobimetallic [RuIICoIII] species which displayed quenching of CoIII upon electron transfer from RuII excited state.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 19
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Book Description
Mononuclear metalloenzymes in nature can function in cooperation with precisely positioned redox-active organic cofactors in order to carry out multielectron catalysis. Inspired by the finely tuned redox management of these bioinorganic systems, we present the design, synthesis, and experimental and theoretical characterization of a homologous series of cobalt complexes bearing redox-active pyrazines. These donor moieties are locked into key positions within a pentadentate ligand scaffold in order to evaluate the effects of positioning redox non-innocent ligands on hydrogen evolution catalysis. Both metal- and ligand-centered redox features are observed in organic as well as aqueous solutions over a range of pH values, and comparison with analogs bearing redox-inactive zinc(II) allows for assignments of ligand-based redox events. Varying the geometric placement of redox non-innocent pyrazine donors on isostructural pentadentate ligand platforms results in marked effects on observed cobalt-catalyzed proton reduction activity. Electrocatalytic hydrogen evolution from weak acids in acetonitrile solution, under diffusion-limited conditions, reveals that the pyrazine donor of axial isomer 1-Co behaves as an unproductive electron sink, resulting in high overpotentials for proton reduction, whereas the equatorial pyrazine isomer complex 2-Co is significantly more active for hydrogen generation at lower voltages. Addition of a second equatorial pyrazine in complex 3-Co further minimizes overpotentials required for catalysis. The equatorial derivative 2-Co is also superior to its axial 1-Co congener for electrocatalytic and visible-light photocatalytic hydrogen generation in biologically relevant, neutral pH aqueous media. Density functional theory calculations (B3LYP-D2) indicate that the first reduction of catalyst isomers 1-Co, 2-Co, and 3-Co is largely metal-centered while the second reduction occurs at pyrazine. Taken together, the data establish that proper positioning of non-innocent pyrazine ligands on a single cobalt center is indeed critical for promoting efficient hydrogen catalysis in aqueous media, akin to optimally positioned redox-active cofactors in metalloenzymes. In a broader sense, these findings highlight the significance of electronic structure considerations in the design of effective electron-hole reservoirs for multielectron transformations.
Author: Yuichiro Himeda
Publisher: John Wiley & Sons
ISBN: 3527346635
Category : Technology & Engineering
Languages : en
Pages : 322
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Book Description
A guide to the effective catalysts and latest advances in CO2 conversion in chemicals and fuels Carbon dioxide hydrogenation is one of the most promising and economic techniques to utilize CO2 emissions to produce value-added chemicals. With contributions from an international team of experts on the topic, CO2 Hydrogenation Catalysis offers a comprehensive review of the most recent developments in the catalytic hydrogenation of carbon dioxide to formic acid/formate, methanol, methane, and C2+ products. The book explores the electroreduction of carbon dioxide and contains an overview on hydrogen production from formic acid and methanol. With a practical review of the advances and challenges in future CO2 hydrogenation research, the book provides an important guide for researchers in academia and industry working in the field of catalysis, organometallic chemistry, green and sustainable chemistry, as well as energy conversion and storage. This important book: Offers a unique review of effective catalysts and the latest advances in CO2 conversion Explores how to utilize CO2 emissions to produce value-added chemicals and fuels such as methanol, olefins, gasoline, aromatics Includes the latest research in homogeneous and heterogeneous catalysis as well as electrocatalysis Highlights advances and challenges for future investigation Written for chemists, catalytic chemists, electrochemists, chemists in industry, and chemical engineers, CO2 Hydrogenation Catalysis offers a comprehensive resource to understanding how CO2 emissions can create value-added chemicals.
Author: Tatiana Straistari
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
This work focuses on the synthesis, the characterization and the catalytic evaluation in the reduction of protons into dihydrogen, of new complexes of Ni(II), Co(III), Cu(II) and Pd(II) based ligands Type thiosemicarbazone. The catalytically active species during the process of the proton reduction was studied by cyclic voltammetry and mechanisms were formulated on the basis quantum chemical calculation.The first chapter introduces the scientific context, the goals and the main objectives of this work. The second chapter concerns the synthesis and the characterization of the N2S2 ligands and their associated mononuclear complexes, Ni, Cu and Pd. The third chapter presents the synthesis and the characterization of binuclear Co and trinuclear Ni based on N2S2 ligand.Electrochemical studies of these complexes in DMF in the presence of a proton source (trifluoroacetic acid), allowed us to evaluate their catalytic efficiency. Our results show that Cu and Pd complexes have a specific irreversible wave for the reduction of protons, but decomposition is observed during electrolysis, which makes these uninteresting complexes for the reduction of protons.On the contrary, Ni and Co complexes showed an electrochemical stability and good catalytic performances. In particular, the new mononuclear Ni complex exhibits remarkable catalytic properties that rank it among the best catalysts for the reduction of protons reported in the literature. All this work provided a complete description of the electrochemical behavior of N2S2 thiosemicarbazone ligands complexed to transition metals. It allows considering future developments in improving the catalytic properties of these complexes.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 15
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Book Description
Three new heteroaxial cobalt oxime catalysts, namely [CoIII(prdioxH)(4tBupy)(Cl)]PF6 (1), [CoIII(prdioxH)(4Pyrpy)(Cl)]PF6 (2), and [CoIII(prdioxH)(4Bzpy)(Cl)]PF6 (3) have been studied. These species contain chloro and substituted tert-butyl/pyrrolidine/benzoyl-pyridino ligands axially coordinated to a trivalent cobalt ion bound to the N4-oxime macrocycle (2E,2'E,3E,3'E)-3,3'-(propane-1,3-diylbis(azanylylidene))bis(butan-2-one)dioxime, abbreviated (prdioxH)- in its monoprotonated form. Emphasis was given to the spectroscopic investigation of the coordination preferences and spin configurations among the different 3d6 CoIII, 3d7 CoII, and 3d8 CoI oxidation states of the metal, and to the catalytic proton reduction with an evaluation of the pathways for the generation of H2via CoIII-H- or CoII-H- intermediates by mono and bimetallic routes. The strong field imposed by the (prdioxH)- ligand precludes the existence of high-spin configurations, and 6-coordinate geometry is favored by the LSCoIII species. Species 1 and 3 show a split CoIII/CoII electrochemical wave associated with partial chemical conversion to a [CoIII(prdioxH)Cl2] species, whereas 2 shows a single event. The reduction of these CoIII complexes yields LSCoII and LSCoI species in which the pyridine acts as the dominant axial ligand. In the presence of protons, the catalytically active CoI species generates a CoIII-H- hydride species that reacts heterolytically with another proton to generate dihydrogen. The intermediacy of a trifluoroacetate-bound CoIII/CoII couple in the catalytic mechanism is proposed. Finally, these results allow for a generalization of the behavior of heteroaxial cobalt macrocycles and serve as guidelines for the development of new catalysts based on macrocyclic frameworks.
Author: Mahesh M. Parsutkar
Publisher:
ISBN:
Category : Cobalt
Languages : en
Pages : 0
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Book Description
Developments of new catalytic transformations by using earth-abundant metal (base-metal) catalysts have played a significant role in modern civilization and will continue to play a vital role towards maintaining and improving our quality of life. Particularly, these transformations have had a tremendous impacts on the agricultural, transport, energy, and pharmaceutical sectors. This field of base-metal catalysis would enjoy added benefits with the utilization of sustainable feedstock carbon sources for fine chemical synthesis. However, the dual problems of activation of thermodynamically stable precursors (ethylene, CO2, H2, CO, aldehydes, acrylates, HCN) and their highly stereoselective incorporation into other readily available substrates (1,3-dienes, alkynes, enynes) pose new challenges. In a nutshell, the development of benign catalysts for employing sustainable feedstock starting materials has the potential to transform inexpensive materials into valuable precursors for fine chemical synthesis. My dissertation work focuses on the development of scalable, atom-economical, and cost-effective catalytic methods for the preparation of value-added products relevant to fine chemicals. The overarching aims are to use sustainable feedstocks or readily available precursors, and environmentally benign chemistry. To achieve these goals, three efficient catalytic methods have been developed which employ complexes of an earth-abundant metal, cobalt, with ligands derived from naturally occurring amino acids or commercially available bis-phosphine ligands. The key to success was a systematic ligands investigation that inspired the design and synthesis of novel ligands to achieve high chemo-, regio-, and enantioselectivities. In the first methodology, a broadly applicable method affecting [2+2] cycloaddition between several alkynes and alkenyl derivatives to form cyclobutenes has been disclosed. A library of >70 nearly enantiopure cyclobutenes, which are ubiquitous motifs in bioactive compounds, have been synthesized in excellent yields. In the second methodology, ligand controlled regio-divergent enantioselective synthesis of primary and secondary homoallylic boronates (>50 examples) from readily available 1,3-dienes and a common boron reagent have been developed. Furthermore, the hydrofunctionalization of 1,3-dienes program has been extended to unprecedented enantioselective hydroacylation of 1,3-dienes. This method opens a realm to achieve the synthesis of enantiopure alpha- or beta-chiral center containing ketones. In all the mentioned transformations above, cationic Co(I)- species has been invoked as an active catalyst. To further corroborate the role of cationic Co(I)-complexes, a reliable protocol has been developed to synthesize, isolate discrete neutral and cationic Co(I)-complexes and characterized by X-ray crystallography. These isolated cationic complexes serve as an excellent single-component catalyst for heterodimerization, hydroboration, and hydroacylation, suggesting the key role of cationic Co(I)-complexes in these transformations. While developing these efficient methodologies, striking ligand, counterion, and solvent effects have been revealed along with a unique role of a cationic Co(I) intermediate in the reactions which advanced novel fundamental concepts. We believe that these cationic Co(I) complexes have broader utility in homogeneous catalysis. We hope that the rational evolution of a mechanism-based strategy that led to the eventual successful outcome and the attendant support studies will add to the burgeoning organometallic chemistry of cobalt and its applications with further implications beyond the synthetic reactions described in this dissertation.
Author: Hyun Seo Ahn
Publisher:
ISBN:
Category :
Languages : en
Pages : 135
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Book Description
Chapter 1. Small domains of cobalt on silica (CoSBA) were prepared by the reaction of Co[N(SiMe3)2]2 and SBA-15, resulting in a range of surface structures as the cobalt loading varied from 0.27 to 5.11 wt%. X-ray absorption spectroscopy (XAS) was employed to characterize these surface structures, which range from single-site cobalt atoms to small clusters of Co3O4. The CoSBA materials exhibit photochemical water oxidation catalysis, revealing distinct catalytic activities associated with characteristic types of surface structures that are dominant in particular concentration regimes. The catalytic turnover frequency for water oxidation of an isolated single-site cobalt atom (0.0143 s−1) is much greater than that observed for a surface atom of a small cluster Co3O4 on silica (0.0006 s−1). The CoSBA catalysts were recyclable for more than seven catalytic cycles (> 200 turnovers) with additional sacrificial oxidant, and no leaching of cobalt was observed. Post-catalytic analysis of CoSBA by XAS revealed that the cobalt atoms were partially oxidized to Co3+, without exhibiting significant surface migration and aggregation of cobalt atoms. Chapter 2. Single-site cobalt atoms on various oxide surfaces (TiO2, MgO, SBA-15, AlPO, and Y-Zeolite) were prepared and evaluated as water oxidation catalysts. Superior catalytic rates were observed for cobalt sites on basic supporting oxides (TiO2 and MgO) relative to those on acidic oxides (Y-Zeolite, AlPo, and SiO2). Per-atom turnover frequencies of ca. 0.04 s−1 were achieved, giving initial rates 100 times greater than a surface atom of a Co3O4 nanoparticle. No correlation was observed between catalytic rates and oxygen atom affinities of the supporting oxides. Chapter 3. Cobalt metaphosphate Co(PO3)2 nanoparticles are prepared via the thermolytic molecular precursor (TMP) method. A Ni foam electrode decorated with Co(PO3)2 nanoparticles is evaluated as an anode for water oxidation electrocatalysis in pH 6.4 phosphate-buffered water. Catalytic onset occurs at an overpotential of ca. 310 mV, which is 100 mV lower than that observed for Co3O4 nanoparticles, with a comparable surface area under identical conditions. A per-metal turnover frequency (TOF) of 0.10 - 0.21 s−1 is observed at [eta] = 440 mV, which is comparable to the highest rate reported for a first-row metal heterogeneous catalyst. Post-catalytic characterization of the catalyst resting state by XPS and Raman spectroscopy reveals that surface rearrangement occurs, resulting in an oxide-like surface overlayer. Chapter 4. Linear trimetallic CoIII/CoII/CoIII cobalt complexes with bridging acyl-alkoxy ligands are electrocatalysts for the reduction of tosic acid in acetonitrile. The -OCMe2CH2COMe complex appears to operate homogeneously, and at an onset overpotential of only 25 mV. A turnover frequency of ca. 80 s−1 was observed at an overptotential of 150 mV.
Author: David Morales-Morales
Publisher: Elsevier
ISBN: 0128129328
Category : Science
Languages : en
Pages : 756
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Book Description
Pincer Compounds: Chemistry and Applications offers valuable state-of-the-art coverage highlighting highly active areas of research—from mechanistic work to synthesis and characterization. The book focuses on small molecule activation chemistry (particularly H2 and hydrogenation), earth abundant metals (such as Fe), actinides, carbene-pincers, chiral catalysis, and alternative solvent usage. The book covers the current state of the field, featuring chapters from renowned contributors, covering four continents and ranging from still-active pioneers to new names emerging as creative strong contributors to this fascinating and promising area. Over a decade since the publication of Morales-Morales and Jensen's The Chemistry of Pincer Compounds (Elsevier 2007), research in this unique area has flourished, finding a plethora of applications in almost every single branch of chemistry—from their traditional application as very robust and active catalysts all the way to potential biological and pharmaceutical applications. - Describes the chemistry and applications of this important class of organometallic and coordination compounds - Includes contributions from global leaders in the field, featuring pioneers in the area as well as emerging experts conducting exciting research on pincer complexes - Highlights areas of promising and active research, including small molecule activation, earth abundant metals, and actinide chemistry
Author: Jean-Michel Savéant
Publisher: John Wiley & Sons
ISBN: 047175806X
Category : Science
Languages : en
Pages : 505
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Book Description
This book is based on the George Fisher Baker Lecture given by Jean-Michel Savéant at Cornell University in Fall 2002. * The first book focusing on molecular electrochemistry * Relates to other fields, including photochemistry and biochemistry * Outlines clearly the connection between concepts, experimental illustrations, proofs and supporting methods * Appendixes to provide rigorous demonstrations to prevent an overload of algebra in the main text * Applications-oriented, focused on analyzing the results obtained rather than the methodology
Author: M. Chanon
Publisher: John Wiley & Sons
ISBN:
Category : Science
Languages : en
Pages : 440
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Book Description
Photocatalysis and related processes occupy a strategic position for the future of photochemistry. This volume provides an introduction to basic concepts and explains how applications work at the molecular level.
