Chemistry of Low and Medium-valent Niobium Complexes Supported by Anionic Amides PDF Download
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Author: Maryam Tayebani
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Languages : en
Pages : 0
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Reaction of [NbCl3(TMEDA)]2 with LDA in a stoichiometric ratio of 1:3 Nb/Lithium amide, resulted in a new diamagnetic niobium (II) cluster (TMEDA)3Nb2Cl5Li (2.1) with a very short NbNb bond. Similarly, reaction of NbCl4(THF) 2 with 2 equiv. of Cy2NLi caused the reduction of the metal center to produce a diamagnetic and tetranuclear cluster, [Nb4Cl 12Li2(THF)8] (2.2). Further reaction of [NbCl3(TMEDA)]2 or (TMEDA)3Nb2Cl 5Li (2.1) with (3,5-Me2Ph)(Ad)NLi in THF yielded the tetravalent, dinuclear and diamagnetic {[(3,5-Me2Ph)(Ad)N]Nb(3,5Me 2Ph)}2(mu-NAd)2.2(ether) (2.3). In the reaction of Nb2Cl6(TMEDA)2 with azobenzene cleavage of the N=N bond results in the formation of the niobium imido complex, NbCl3(C6H5N)(TMEDA) (2.4). The reaction of Nb2Cl6(TMEDA)2 with the potassium salt of tripyrrolylmethane in a 1:1 ratio afforded two products which have been isolated and characterized. The first compound is the dinuclear {[H-C(C4H3N)3]Nb(THF)}2 2THF (3.1) with two trivalent and diamagnetic metals bridged by two tripyrrolyl trianions. The second product of the reaction is {([H-C(C 4H3N)3]2NbK)2}{Nb4 Cl11[K(THF)2]2}2THF (3.2 ) which displays some unusual features. The complex is ionic with the cationic {Nb4Cl11[K (THF)2]2} 2+ unit containing the metal in a mixed valence state and the cationic moiety are balanced by two identical anionic {([H-C(C4H3N) 3]2NbK)2}- units each containing Nb in the tetravalent state. A similar reaction carried out with the lithium salt of the tripyrrolide anion led instead to a monomeric tetravalent {[H-C(C 4H3N)3]2Nb}{Li (THF)4} 2 (3.3) complex. Reaction of Nb2Cl5Li(TMEDA)3 with Ph 2NK in THF afforded a complex reaction from which three complex were isolated and characterized. The first compound Cl2Nb 2(mu-Cl)3(TMEDA)2 (4.1) was obtained in analytically pure form as dark purple, paramagnetic and moderately air-sensitive crystals. The second compound is a trivalent and dinuclear {[(Ph2N) 2Nb]2[mu-NPh(mu-eta1:eta2-C 6H4)](mu-H)}{Li(TMEDA)2}.toluene (4.2 ) which was formed via oxidative addition of the two Nb(II) centers to the C--H sigma-bond of one aromatic ring. This species thermally rearranges into the dinuclear and diamagnetic (Ph2N) 2Nb{[mu-NPh(eta6-C6H5)] 2Nb}.ether (4.3). (Abstract shortened by UMI.).
Author: Maryam Tayebani
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Languages : en
Pages : 0
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Reaction of [NbCl3(TMEDA)]2 with LDA in a stoichiometric ratio of 1:3 Nb/Lithium amide, resulted in a new diamagnetic niobium (II) cluster (TMEDA)3Nb2Cl5Li (2.1) with a very short NbNb bond. Similarly, reaction of NbCl4(THF) 2 with 2 equiv. of Cy2NLi caused the reduction of the metal center to produce a diamagnetic and tetranuclear cluster, [Nb4Cl 12Li2(THF)8] (2.2). Further reaction of [NbCl3(TMEDA)]2 or (TMEDA)3Nb2Cl 5Li (2.1) with (3,5-Me2Ph)(Ad)NLi in THF yielded the tetravalent, dinuclear and diamagnetic {[(3,5-Me2Ph)(Ad)N]Nb(3,5Me 2Ph)}2(mu-NAd)2.2(ether) (2.3). In the reaction of Nb2Cl6(TMEDA)2 with azobenzene cleavage of the N=N bond results in the formation of the niobium imido complex, NbCl3(C6H5N)(TMEDA) (2.4). The reaction of Nb2Cl6(TMEDA)2 with the potassium salt of tripyrrolylmethane in a 1:1 ratio afforded two products which have been isolated and characterized. The first compound is the dinuclear {[H-C(C4H3N)3]Nb(THF)}2 2THF (3.1) with two trivalent and diamagnetic metals bridged by two tripyrrolyl trianions. The second product of the reaction is {([H-C(C 4H3N)3]2NbK)2}{Nb4 Cl11[K(THF)2]2}2THF (3.2 ) which displays some unusual features. The complex is ionic with the cationic {Nb4Cl11[K (THF)2]2} 2+ unit containing the metal in a mixed valence state and the cationic moiety are balanced by two identical anionic {([H-C(C4H3N) 3]2NbK)2}- units each containing Nb in the tetravalent state. A similar reaction carried out with the lithium salt of the tripyrrolide anion led instead to a monomeric tetravalent {[H-C(C 4H3N)3]2Nb}{Li (THF)4} 2 (3.3) complex. Reaction of Nb2Cl5Li(TMEDA)3 with Ph 2NK in THF afforded a complex reaction from which three complex were isolated and characterized. The first compound Cl2Nb 2(mu-Cl)3(TMEDA)2 (4.1) was obtained in analytically pure form as dark purple, paramagnetic and moderately air-sensitive crystals. The second compound is a trivalent and dinuclear {[(Ph2N) 2Nb]2[mu-NPh(mu-eta1:eta2-C 6H4)](mu-H)}{Li(TMEDA)2}.toluene (4.2 ) which was formed via oxidative addition of the two Nb(II) centers to the C--H sigma-bond of one aromatic ring. This species thermally rearranges into the dinuclear and diamagnetic (Ph2N) 2Nb{[mu-NPh(eta6-C6H5)] 2Nb}.ether (4.3). (Abstract shortened by UMI.).
Author: Valerie Anne Williams
Publisher:
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Category :
Languages : en
Pages : 269
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The low-valent complex (silox)3NbPMe3 (silox = tBu3SiO) was synthesized and tested for reactivity towards small molecules. It was discovered that exposure to CO generated the complexes (silox)3Nb=C=C=O (3-Nb), (silox)3Nb=O (2-Nb), and [(silox)3Nb]2([mu]-C2) (4-Nb) in various ratios depending on reaction stoichiometry. The formation of [(silox)3Nb]2([mu]-CO) (5-Nb) was discovered as a byproduct in most reactions. Treatment of (silox)3NbPMe3 with potassium under a dinitrogen atmosphere afforded the dinitrogen complex (silox)3NbNNNb(silox)3 (8-Nb). Reaction of (silox)3NbCl (10-Nb) with NH3 afforded (silox)3NbIV(NH3)Cl, with ammonia binding parameters of [INCREMENT]H = 20.4 ± 2.3 kcal/mol and [INCREMENT]S = 39 ± 7 e.u. The nickel complex {dmp(PI)2}Ni (dmp(PI)2 = Me2C(CH2=Npy)2) was synthesized and subjected to chemical oxidation and reduction to synthesize a 5membered redox series. Through electronic structure study, it was determined that in these complexes the metal center typically remained NiII, with the possible exception of the cationic complex [{dmp(PI)2}Ni](OTf), and redox changes were primarily ligand-centered. The chemistry of dmp(PI)2 was extended to iron, and {dmp(PI)2}FePMe3 was synthesized. As with the nickel analogue, the complex was determined to be a MII center bound to a dianionic ligand framework, with similar redox behavior as the nickel species. A related tetradentate [beta]-diketiminate-based ligand incorporating two pyridinemethylene units, 2,4-bis[(E)-(2-pyridyl)methylideneamino]pentane (H{nn(PM)2}), was synthesized and metallated to form {nn(PM)2}FeN(TMS)2 (1-N(TMS)2). 1-N(TMS)2 could be further derivatized to form {nn(PM)2}FeX (X = Cl, N3). One of the methylene fragments of the ligand backbone was prone to facile deprotonation and allowed synthesis of {nn(PM)(PI)}FeLL' (LL' = (PMe3)2, (PMe2Ph)2, (PMe3)CO; 2LL') and {nn(PM)(PI)}FeL (L = PMe3, PMePh2, PPh3, CO; 3-L). Electronic structure studies suggested 2-LL' and 3-L existed as FeII metal centers bound to dianionic {nn(PM)(PI)}2-, with strong metal-ligand covalency in 3-L. Oxidation studies on 2(PMe3)2 showed two successive ligand-based 1e- oxidations. The same tetradentate [beta]diketiminate-based ligand was applied to other transition metals of the first row and allowed synthesis of {nn(PM)2}VCl2 (1-VCl2) and {nn(PM)2}Co (1-Co), both of which formed through unusual ligand exchange or disproportionation events, and {nn(PM)(PI)}M (M = Cr, 4-Cr; M = Ni, 4-Ni), which were generated through double deprotonation of the H{nn(PM)2} ligand.
Author: Thomas Lucien Gianetti
Publisher:
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Category :
Languages : en
Pages : 170
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In this dissertation is presented the synthesis and reactivity of Nb(III) imido complexes supported by the BDI ligand (BDI = &beta-diketimate). Chapter I. The discovery of a Nb(III)-mediated catalytic hydrogenation of internal alkynes to Z-alkenes is reported and found to proceed through an unprecedented mechanism. The mechanistic proposal involves initial reduction of the alkyne by the Nb(III) complex (BDI)Nb(NtBu)(CO)2 to provide a Nb(V) metallacyclopropene, itself capable of &sigma-bond metathesis reactivity with H2. The resulting alkenyl hydride species then undergoes reductive elimination to provide the Z-alkene product and regenerate a metal complex in the Nb(III) oxidation state. Support for the proposed mechanism is derived from i) the dependence of product selectivity on the relative concentrations of CO and H2, ii) the isolation of complexes closely related to those proposed to lie on the catalytic cycle, iii) H/D crossover experiments, and iv) DFT studies on multiple possible reaction pathways. Chapter II. Monometallic niobium arene complexes [Nb(BDI)(NtBu)(&mu-RC6H5)] (R = H and Me) were synthesized and were found to slowly converted into the diniobium inverted arene sandwich complexes [[(BDI)Nb(NtBu)]2(&mu-RC6H5)] (R = H and Me) in solution. The kinetics of this reaction were followed by 1H NMR spectroscopy, and is in agreement with a dissociative mechanism. These compounds showed a lack of reactivity towards small molecules - even at elevated temperatures - which is unusual in the chemistry of inverted sandwich complexes. However, protonation of the BDI ligands occurred readily on treatment with [H(OEt2)][B(C6F5)4], resulting in the mono-protonated cationic inverted sandwich complex [[(BDI#)Nb(NtBu)][(BDI)Nb(NtBu)](&mu-C6H5)][B(C6F5)4] and the dicationic complex [[(BDI#)Nb(NtBu)]2(&mu-RC6H5)][B(C6F5)4]2 (BDI# = (ArNC(Me))2CH2). NMR and UV-vis spectroscopies were used to characterize this unique series of diamagnetic molecules as a means of determining how best to describe the Nb-arene interactions. The X-ray crystal structures, UV-visible spectra, arene 1H NMR chemical shifts and large JCH coupling constants provide evidence for donation of electron density from the Nb d-orbitals into the antibonding [pi] system of the arene ligands. However, the lack of sp3 hybridization of the arene carbon indicate that the Nb-arene donation is not accompanied by an increase in formal oxidation state, and suggest that 4d2 electronic configurations are appropriate to describe the Nb atoms in all four complexes. Chapter III. Inverted sandwich complexes have seen interesting recent developments both in the nature of their bonding and in their use as chemical reactants. Although discussions concerning the electronic delocalization lend credit to their use as potential electronic and spintronic devices, mixed valent inverted sandwich complexes are rarely reported. We show in this work that the selective single electron oxidation of a neutral benzene inverted sandwich complex of niobium leads to an isolable cationic mixed valent benzene inverted sandwich complex. The latter complex shows unique structural features elucidated through studies with an arsenal of physical methods, including cyclic voltammetry, 1H NMR, UV-Vis, magnetism, EPR spectroscopies, in addition to DFT calculations. These analyses indicate that although delocalization is allowed over the benzene ring from both niobium atoms, the single electron is unequally shared between the two metal centres. Under certain conditions, this complex reforms the neutral benzene complex along with a highly reactive Nb(IV) species, which is of great interest for potential chemical reactivity. Chapter IV. All three C-F bonds in CF3-substituted arenes are activated by a niobium imido complex, driven by the formation of strong Nb-F bonds. The mechanism of this transformation was studied by NMR spectroscopy which revealed the involvement of Nb(III). Attempts to extend this chemistry to non-aromatic CF3 groups led to intramolecular reactivity.The mechanism of activation of C-F bonds in fluoroarenes using a well-defined niobium (III) imido complex has been investigated. Chapter V. The Nb(III) arene species [BDI]Nb(NtBu)(C6H6), reacts stoichiometrically with fluoroarenes to yield niobium (V) aryl fluorides. Spectroscopic analysis supported by DFT calculations revealed the critical involvement of a Nb(III) fluoroarene-bound species. In contrast to previous reports of related reactivity, we found that perfluorinated arenes (i.e. those normally assumed to bear more `activated' C-F bonds) are, in the present system, much less reactive towards C-F bond cleavage than mono- or difluoro-substituted arenes. In addition to demonstrating stoichiometric hydrodefluorination reactions, we also describe an efficient and mild hydrodefluorination of mono- and di-fluoroarenes that is catalytic in niobium.
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Category : Dissertations, Academic
Languages : en
Pages : 784
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Category : Dissertation abstracts
Languages : en
Pages : 776
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Author: Janez Košmrlj
Publisher: Springer
ISBN: 3642294294
Category : Science
Languages : en
Pages : 245
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B. R. Buckley and H. Heaney: Mechanistic Investigations of Copper(I)- Catalyzed Alkyne–Azide Cycloaddition Reactions.- J. D. Crowley and D. A. McMorran: “Click-Triazole” Coordination Chemistry: Exploiting 1,4-Disubstituted-1,2,3-Triazoles as Ligands.- S. Lee and A. H. Flood: Binding Anions in Rigid and Reconfigurable Triazole Receptors.- M. Watkinson: Click Triazoles as Chemosensors.- H.-F. Chow, C.-M. Lo and Y. Chen: Triazole-Based Polymer Gels.- T. Zheng, S. H. Rouhanifard, A. S. Jalloh, P. Wu: Click Triazoles for Bioconjugation.- S. Mignani, Y. Zhou, T. Lecourt and L. Micouin: Recent Developments in the Synthesis 1,4,5-Trisubstituted Triazoles.
Author: Fabian Mohr
Publisher: John Wiley & Sons
ISBN: 3527320865
Category : Science
Languages : en
Pages : 425
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Written by world-class authors, this most recent major book on the topic highlights new and current trends as well as future directions. It is comprehensive in its scope, covering all aspects of gold chemistry -- from homogeneous to heterogeneous catalysis, from supramolecular assemblies to sensors and medicinal applications. The result is an invaluable work for both organic and inorganic chemists working in universities and industry, as well as material scientists.
Author: L.R. Morss
Publisher: Springer Science & Business Media
ISBN: 1402035985
Category : Science
Languages : en
Pages : 4059
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The Chemistry of the Actinide and Transactinide Elements is a contemporary and definitive compilation of chemical properties of all of the actinide elements, especially of the technologically important elements uranium and plutonium, as well as the transactinide elements. In addition to the comprehensive treatment of the chemical properties of each element, ion, and compound from atomic number 89 (actinium) through to 109 (meitnerium), this multi-volume work has specialized and definitive chapters on electronic theory, optical and laser fluorescence spectroscopy, X-ray absorption spectroscopy, organoactinide chemistry, thermodynamics, magnetic properties, the metals, coordination chemistry, separations, and trace analysis. Several chapters deal with environmental science, safe handling, and biological interactions of the actinide elements. The Editors invited teams of authors, who are active practitioners and recognized experts in their specialty, to write each chapter and have endeavoured to provide a balanced and insightful treatment of these fascinating elements at the frontier of the periodic table. Because the field has expanded with new spectroscopic techniques and environmental focus, the work encompasses five volumes, each of which groups chapters on related topics. All chapters represent the current state of research in the chemistry of these elements and related fields.
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: Yoshiaki Nishibayashi
Publisher: John Wiley & Sons
ISBN: 352734425X
Category : Science
Languages : en
Pages : 496
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A comprehensive book that explores nitrogen fixation by using transition metal-dinitrogen complexes Nitrogen fixation is one of the most prominent fields of research in chemistry. This book puts the focus on the development of catalytic ammonia formation from nitrogen gas under ambient reaction conditions that has been recently repowered by some research groups. With contributions from noted experts in the field, Transition Metal-Dinitrogen Complexes offers an important guide and comprehensive resource to the most recent research and developments on the topic of nitrogen fixation by using transition metal-dinitrogen. The book is filled with the information needed to understand the synthesis of transition metal-dinitrogen complexes and their reactivity. This important book: -Offers a resource for understanding nitrogen fixation chemistry that is essential for explosives, pharmaceuticals, dyes, and all forms of life -Includes the information needed for anyone interested in the field of nitrogen fixation by using transition metal-dinitrogen complexes -Contains state-of-the-art research on synthesis of transition metal-dinitrogen complexes and their reactivity in nitrogen fixation -Incorporates contributions from well-known specialists and experts with an editor who is an innovator in the field of dinitrogen chemistry Written for chemists and scientists with an interest in nitrogen fixation, Transition Metal-Dinitrogen Complexes is a must-have resource to the burgeoning field of nitrogen fixation by using transition metal-dinitrogen complexes.
