Studies of Hydrogen-hydrogen and Carbon-sulfur Bond Cleavage; Lewis Acid Modified Molybdenum Sulfide Complexes; and Syntheses and Reactions of Pyrrole Complexes . Final Report PDF Download
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Languages : en
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The reactions of hydrogen with dinuclear cyclopentadienyl molybdenum complexes containing bridging sulfido ligands have been identified, and detailed kinetic and mechanistic studies have been carried out on selected systems. The work has led to a detailed proposal for the mechanism of heterolytic hydrogen activation by these homogeneous dinuclear complexes. The hydrogen activation reactions have been further applied in reactions with heterocycles and other small molecules, and pathways for the cleavage of carbon-nitrogen, carbon-sulfur, and carbon-oxygen bonds have been determined. The studies have established well characterized pathways for these transformations which involve reactions at the sulfido ligands of the complexes. These systems provide potential models for important hydrotreating reactions which occur on heterogeneous metal sulfide catalysts. In a related project fundamental factors which might promote the activation of nitrogen heterocycles in the hydrotreating process have been explored. New transition metal complexes containing pyrrole and indole have been synthesized, and the coordination modes of the cyclic ligands and their resulting reactivity have been characterized. Complexes which activate the rings toward both nucleophilic addition and nucleophilic substitution reactions have been identified, and further reductions of the cyclic ligands have also been observed.
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Languages : en
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Book Description
The reactions of hydrogen with dinuclear cyclopentadienyl molybdenum complexes containing bridging sulfido ligands have been identified, and detailed kinetic and mechanistic studies have been carried out on selected systems. The work has led to a detailed proposal for the mechanism of heterolytic hydrogen activation by these homogeneous dinuclear complexes. The hydrogen activation reactions have been further applied in reactions with heterocycles and other small molecules, and pathways for the cleavage of carbon-nitrogen, carbon-sulfur, and carbon-oxygen bonds have been determined. The studies have established well characterized pathways for these transformations which involve reactions at the sulfido ligands of the complexes. These systems provide potential models for important hydrotreating reactions which occur on heterogeneous metal sulfide catalysts. In a related project fundamental factors which might promote the activation of nitrogen heterocycles in the hydrotreating process have been explored. New transition metal complexes containing pyrrole and indole have been synthesized, and the coordination modes of the cyclic ligands and their resulting reactivity have been characterized. Complexes which activate the rings toward both nucleophilic addition and nucleophilic substitution reactions have been identified, and further reductions of the cyclic ligands have also been observed.
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Languages : en
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The research project proposed to synthesize new metal complexes with sulfido, disulfido and other types of reactive sulfur ligands, and to explore the joint reactivity of metal and sulfur ligands with hydrogen and organic molecules. The overall objective was to investigate reaction pathways relevant to those observed for the heterogeneous metal sulfide catalysts which promote hydrogen activation, hydrogenation-dehydrogenation of organic substrates, and hydrogenolysis of carbon-heteroatom bonds. Particular emphasis was placed on CpRe derivatives (where Cp might be C5H5 or alkylated versions) so that comparisons could be made with the previously studied CpMo complexes, which showed extensive reactivity at the sulfur ligands. Heterogeneous rhenium sulfides generally show higher catalytic activity than molybdenum sulfides, and this is attributed, in part, to the weaker Re-S bond strength, relative to the moybdenum-sulfur bond. In our studies of discrete Re-sulfide complexes, we have also observed evidence for weaker Re-S bonds relative to the molybdenum systems. In addition we have characterized novel hydrogen activation by rhenium sulfido complexes, as well as carbon-hydrogen, carbon-sulfur and metal sulfur bond cleavage reactions. Hydrogen Activation. The complex Cpà ̄Â?Â[1/2]ReCl2S3 was synthesized in ca 70% yield and characterized by an X-ray diffraction study which confirms that the complex contains a h2-trisulfide ligand. The cyclic voltammogram of Cpà ̄Â?Â[1/2]ReCl2S3 shows a wide window of redox stability with an irreversible reduction wave at à ̄Â?Â[1/2]0.97 V and an irreversible oxidation at +1.03 V vs Fc. Nevertheless, the complex undergoes a facile reaction with hydrogen at 50oC to form H2S and a new dinuclear sulfido bridged rhenium complex. This reaction is of interest because it is the first example of the hydrogenolysis of a discrete metal polysulfide complex to produce H2S, a reaction also observed for heterogeneous rhenium sulfides. The reaction contrasts with those of related Cp-molybdenum complexes with sulfide ligands, which also activate hydrogen, but generally form hydrosulfido products without H2S elimination. C-H and C-S Cleavage Reactions. New mononuclear Cp'Re(dithiolate) complexes such as Cpà ̄Â?Â[1/2]ReCl2(SC2H4S), 1, have been prepared and characterized and have been found to display a very interesting range of reactions. The thermal reaction of 1 involves the dehydrogenation of the alkanedithiolate ligand to form Cp'ReCl2(SCH=CHS), 2 as well as a competing elimination of olefin from the dithiolate ligand in 1. On the basic of kinetic and related studies, the mechanism is proposed to involve a sequential series of reactions. In the first reaction, the olefin extrusion is proposed to produce a reactive Re-disulfide or Re-bis(sulfido) intermediate, CpReCl2S2 which serves as an oxidant for the dithiolate complex 1. The ability of the bis sulfido complex to dehydrogenate hydrocarbons is a unique feature and several additional dehydrogenation reactions with this system have been characterized, including the oxidation of other dithiolate complexes, of tetrahydro-naphthalene and of cyclohexadiene. Precedents for the role of metal sulfides in dehydrogenation reactions have been reported for heterogeneous metal sulfide surfaces. This work has begun to provide information about the electronic and structural features necessary for such reactivity. Carbon Sulfur Bond Formation. When the thermal reaction of 1 was carried out in the presence of excess dry ethene a new reaction was observed in which the dithiolate ligand is displaced by incoming olefin to form the cyclic organic product, 1,4-dithiane. The Re product is identified as Cpà ̄Â?Â[1/2]Re(alkene)Cl2 on the basis of NMR and mass spectroscopic data. Similar reactions with alkynes have been found to form unsaturated 6-membered rings and reactions with 1,3 dithiolate complexes form the organic 7-membered rings. To our knowledge the formation of cyclic bis-thioethers by the reactions of an alkene or alkyne with a discrete dithiolate metal complex has not been reported previously. Conclusions. A series of new rhenium complexes with sulfido, disulfido, and thiolate ligands have been synthesized and characterized. The complexes have shown novel patterns of reactivity at the sulfur ligands that include hydrogen activation and addition, dehydrogenations, and carbon -sulfur bond formation. Systematic studies of sulfur ligand reactivity as the coligands in these rhenium complexes were varied have established electronic features that favor the novel ligand based reactivity. This series of complexes provides a model system for aspects of rhenium-sulfide surface reactivity, and have led to the suggestion of new mechanistic possibilities.
Author: Mary C. Rakowski-DuBois
Publisher:
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Languages : en
Pages :
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The research project proposed to synthesize new metal complexes with sulfido, disulfido and other types of reactive sulfur ligands, and to explore the joint reactivity of metal and sulfur ligands with hydrogen and organic molecules. The overall objective was to investigate reaction pathways relevant to those observed for the heterogeneous metal sulfide catalysts which promote hydrogen activation, hydrogenation-dehydrogenation of organic substrates, and hydrogenolysis of carbon-heteroatom bonds. Particular emphasis was placed on CpRe derivatives (where Cp might be C5H5 or alkylated versions) so that comparisons could be made with the previously studied CpMo complexes, which showed extensive reactivity at the sulfur ligands. Heterogeneous rhenium sulfides generally show higher catalytic activity than molybdenum sulfides, and this is attributed, in part, to the weaker Re-S bond strength, relative to the moybdenum-sulfur bond. In our studies of discrete Re-sulfide complexes, we have also observed evidence for weaker Re-S bonds relative to the molybdenum systems. In addition we have characterized novel hydrogen activation by rhenium sulfido complexes, as well as carbon-hydrogen, carbon-sulfur and metal sulfur bond cleavage reactions. Hydrogen Activation. The complex Cp{prime}ReCl2S3 was synthesized in ca 70% yield and characterized by an X-ray diffraction study which confirms that the complex contains a {eta}2-trisulfide ligand. The cyclic voltammogram of Cp{prime}ReCl2S3 shows a wide window of redox stability with an irreversible reduction wave at -0.97 V and an irreversible oxidation at +1.03 V vs Fc. Nevertheless, the complex undergoes a facile reaction with hydrogen at 50 C to form H2S and a new dinuclear sulfido bridged rhenium complex. This reaction is of interest because it is the first example of the hydrogenolysis of a discrete metal polysulfide complex to produce H2S, a reaction also observed for heterogeneous rhenium sulfides. The reaction contrasts with those of related Cp-molybdenum complexes with sulfide ligands, which also activate hydrogen, but generally form hydrosulfido products without H2S elimination. C-H and C-S Cleavage Reactions. New mononuclear Cp{prime}Re(dithiolate) complexes such as Cp{prime}ReCl2(SC2H4S), 1, have been prepared and characterized and have been found to display a very interesting range of reactions. The thermal reaction of 1 involves the dehydrogenation of the alkanedithiolate ligand to form Cp{prime}ReCl2(SCH=CHS), 2 as well as a competing elimination of olefin from the dithiolate ligand in 1. On the basic of kinetic and related studies, the mechanism is proposed to involve a sequential series of reactions. In the first reaction, the olefin extrusion is proposed to produce a reactive Re-disulfide or Re-bis(sulfido) intermediate, CpReCl2S2 which serves as an oxidant for the dithiolate complex 1. The ability of the bis sulfido complex to dehydrogenate hydrocarbons is a unique feature and several additional dehydrogenation reactions with this system have been characterized, including the oxidation of other dithiolate complexes, of tetrahydro-naphthalene and of cyclohexadiene. Precedents for the role of metal sulfides in dehydrogenation reactions have been reported for heterogeneous metal sulfide surfaces. This work has begun to provide information about the electronic and structural features necessary for such reactivity. Carbon Sulfur Bond Formation. When the thermal reaction of 1 was carried out in the presence of excess dry ethene a new reaction was observed in which the dithiolate ligand is displaced by incoming olefin to form the cyclic organic product, 1,4-dithiane. The Re product is identified as Cp{prime}Re(alkene)Cl2 on the basis of NMR and mass spectroscopic data. Similar reactions with alkynes have been found to form unsaturated 6-membered rings and reactions with 1,3 dithiolate complexes form the organic 7-membered rings. To our knowledge the formation of cyclic bis-thioethers by the reactions of an alkene or alkyne with a discrete dithiolate metal complex has not been reported previously. Conclusions. A series of new rhenium complexes with sulfido, disulfido, and thiolate ligands have been synthesized and characterized. The complexes have shown novel patterns of reactivity at the sulfur ligands that include hydrogen activation and addition, dehydrogenations, and carbon-sulfur bond formation. Systematic studies of sulfur ligand reactivity as the coligands in these rhenium complexes were varied have established electronic features that favor the novel ligand based reactivity. This series of complexes provides a model system for aspects of rhenium-sulfide surface reactivity, and have led to the suggestion of new mechanistic possibilities.
Author: Gregory J. Kubas
Publisher: Springer Science & Business Media
ISBN: 0306475979
Category : Science
Languages : en
Pages : 479
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According to R.H. Crabtree, Metal Dihydrogen and sigma-Bond Complexes is described as `the definitive account of twentieth-century work in the area of sigma complexation'. It covers not only Kubas' discovery of dihydrogen coordination and the study of its structure and general properties but also discusses both the theoretical beliefs and experimental results of bonding and activation of dihydrogen on metal centers and the coordination and activation of C-H, B-H, X-H, and X-Y bonds, giving an overview of `one of the hottest areas in chemistry'.
Author: Gregory J. Kubas
Publisher: Springer Science & Business Media
ISBN: 0306464659
Category : Science
Languages : en
Pages : 479
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Book Description
According to R.H. Crabtree, Metal Dihydrogen and sigma-Bond Complexes is described as `the definitive account of twentieth-century work in the area of sigma complexation'. It covers not only Kubas' discovery of dihydrogen coordination and the study of its structure and general properties but also discusses both the theoretical beliefs and experimental results of bonding and activation of dihydrogen on metal centers and the coordination and activation of C-H, B-H, X-H, and X-Y bonds, giving an overview of `one of the hottest areas in chemistry'.
Author: Arkadi Vigalok
Publisher: Springer
ISBN: 3642120733
Category : Science
Languages : en
Pages : 198
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Contents: Kilian Muñiz: Transition Metal Catalyzed Electrophilic Halogenation of C-H bonds in alpha-Position to Carbonyl Groups; Arkadi Vigalok * and Ariela W Kaspi: Late Transition Metal-Mediated Formation of Carbon-Halogen Bonds; Paul Bichler and Jennifer A. Love*: Organometallic Approaches to Carbon-Sulfur Bond Formation; David S. Glueck: Recent Advances in Metal-Catalyzed C-P Bond Formation; Andrei N. Vedernikov: C-O Reductive Elimination from High Valent Pt and Pd Centers; Lukas Hintermann: Recent Developments in Metal-Catalyzed Additions of Oxygen Nucleophiles to Alkenes and Alkynes; Moris S. Eisen: Catalytic C-N, C-O and C-S bond formation promoted by organoactinide complexes.
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The RuII complex TpRu(CO)(NCMe)Me (Tp = hydridotris(pyrazolyl)borate) initiates carbon-hydrogen bond activation at the 2-position of furan and thiophene to produce methane and TpRu(CO)(NCMe)Ar (Ar = 2-furyl or 2-thienyl). Solid-state structures have been determined for TpRu(CO)(NCMe)(2-thienyl) and [TpRu(CO)(m-C, S-thienyl)]2. The complex TpRu(CO)(NCMe)(2-furyl) serves as a catalyst for the formation of 2-ethylfuran from ethylene and furan. Similar catalytic reactivity was observed with TpRu(CO)(NCMe)(2-thienyl) for the production of 2-ethylthiophene. Density functional theory (DFT) calculations of the C-H activation of furan by {(TAB)Ru(CO)Me} (TAB = tris(azo)borate) indicate that the C-H activation sequence does not proceed through a RuIV oxidative addition intermediate. The reaction of TpRu(CO)(NCMe)Me and pyrrole forms TpRu(CO){k 2-N, N-(H)N=C(Me)(NC4H3)}. The formation of complex TpRu(CO){k2-N, N-(H)N=C(Me)(NC4H3)} involves the cleavage of the N-H bond and 2-position C-H bond of pyrrole as well as a C-C bond forming step between pyrrole and the acetonitrile ligand of {TpRu(CO)(NCMe)}. Mechanistic studies indicate that the most likely reaction pathway involves initial metal-mediated N-H activation of pyrrole to produce TpRu(CO)(NCMe)(N-pyrrolyl) followed by C-C bond formation and proton transfer. Complex TpRu(CO)(NCMe)(N-pyrrolyl) has been independently prepared. At elevated temperatures, TpRu(CO)(NCMe)(N-pyrrolyl) converts to TpRu(CO){k2-N, N-(H)N=C(Me)(NC4H3)}. Single crystal X-ray analysis has been achieved for TpRu(CO)(NCMe)(N-pyrrolyl), [TpRu(CO)(NCMe)(h1-O-OC4H8) and TpRu(CO){k2-N, N-(H)N=C(Me)(NC4H3)}. Computational studies support the suggested selectivity for initial N-H bond cleavage in preference to C-H bond activation. Rational design for a more electron-poor hydroarylation catalyst was discussed. Synthesis of Mp (Mp = tris(pyrazolyl)methane) complexes of the type [MpRu(PPh3)(CO)H]BAr'4, [MpRu(PPh3)(CO)Cl]BAr'4].
Author: J. A. McCleverty
Publisher: Newnes
ISBN: 0080913164
Category : Science
Languages : en
Pages : 11845
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Comprehensive Coordination Chemistry II (CCC II) is the sequel to what has become a classic in the field, Comprehensive Coordination Chemistry, published in 1987. CCC II builds on the first and surveys new developments authoritatively in over 200 newly comissioned chapters, with an emphasis on current trends in biology, materials science and other areas of contemporary scientific interest.
Author: Montserrat Diéguez
Publisher: John Wiley & Sons
ISBN: 3527804072
Category : Technology & Engineering
Languages : en
Pages : 431
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An important reference for researchers in the field of metal-enzyme hybrid catalysis Artificial Metalloenzymes and MetalloDNAzymes in Catalysis offers a comprehensive review of the most current strategies, developed over recent decades, for the design, synthesis, and optimization of these hybrid catalysts as well as material about their application. The contributors—noted experts in the field—present information on the preparation, characterization, and optimization of artificial metalloenzymes in a timely and authoritative manner. The authors present a thorough examination of this interesting new platform for catalysis that combines the excellent selective recognition/binding properties of enzymes with transition metal catalysts. The text includes information on the various applications of metal-enzyme hybrid catalysts for novel reactions, offers insights into the latest advances in the field, and contains an informative perspective on the future: Explores the development of artificial metalloenzymes, the modern and strongly evolving research field on the verge of industrial application Contains a comprehensive reference to the research area of metal-enzyme hybrid catalysis that has experienced tremendous growth in recent years Includes contributions from leading researchers in the field Shows how this new catalysis combines the selective recognition/binding properties of enzymes with transition metal catalysts Written for catalytic chemists, bioinorganic chemists, biochemists, and organic chemists, Artificial Metalloenzymes and MetalloDNAzymes in Catalysis offers a unique reference to the fundamentals, concepts, applications, and the most recent developments for more efficient and sustainable synthesis.
Author: Jean-Marie Basset
Publisher: Springer Science & Business Media
ISBN: 9400929714
Category : Science
Languages : en
Pages : 340
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Book Description
Surface organometallic chemistry is a new field bringing together researchers from organometallic, inorganic, and surface chemistry and catalysis. Topics ranging from reaction mechanisms to catalyst preparation are considered from a molecular basis, according to which the "active site" on a catalyst surface has a supra-molecular character. This. the first book on the subject, is the outcome of a NATO Workshop held in Le Rouret. France, in May. 1986. It is our hope that the following chapters and the concluding summary of recommendations for research may help to provide a definition of surface organometallic chemistry. Besides catalysis. the central theme of the Workshop, four main topics are considered: 1) Reactions of organometallics with surfaces of metal oxides, metals. and zeolites; 2) Molecular models of surfaces, metal oxides, and metals; 3) Molecular approaches to the mechanisms of surface reactions; 4) Synthesis and modification of zeolites and related microporous solids. Most surface organometallic chemistry has been carried out on amorphous high-surf ace-area metal oxides such as silica. alumina. magnesia, and titania. The first chapter. contributed by KNOZINGER. gives a short summary of the structure and reactivity of metal oxide surfaces. Most of our understanding of these surfaces is based on acid base and redox chemistry; this chemistry has developed from X-ray and spectroscopic data, and much has been inferred from the structures and reactivities of adsorbed organic probe molecules. There are major opportunities for extending this understanding by use of well-defined (single crystal) oxide surfaces and organometallic probe molecules.
