Strong Bond Activation with Late Transition-metal Pincer Complexes as a Foundation for Potential Catalysis

Strong Bond Activation with Late Transition-metal Pincer Complexes as a Foundation for Potential Catalysis
Author: Yanjun Zhu
Publisher:
Total Pages:
Release: 2012
Genre:
ISBN:
GET BOOK

Download Strong Bond Activation with Late Transition-metal Pincer Complexes as a Foundation for Potential Catalysis Book in PDF, Epub and Kindle

Strong bond activation mediated by pincer ligated transiton-metal complexes has been the subject of intense study in recent years, due to its potential involvement in catalytic transformations. This dissertation has focused on the net heterolytic cleavage of B-H and B-B bonds across the N-Pd bond in a cationic (PNP)Pd fragment, the C-H oxidative addition to a (PNP)Ir center and the recent results on the C-H and C-O oxidative addition in reactions of aryl carboxylates with the (PNP)Rh fragment. Transition metal carbene and carbyne complexes are of great interest because of their role in a wide variety of catalytic reactions. Our work has resulted in the isolation of a rhodium(I) difluorocarbene. Reaction of the rhodium difluorocarbene complex with a silylium salt led to the C-F bond cleavage and the formation of a terminal fluorocarbyne complex. Reductive elimination is a critical step of cross coupling reactions. In order to examine the effect of the pincer ligand on the reductive elimination reactions from Rh(III), the first pi-accepting PNP ligand bearing pyrrolyl substituents was prepared and installed onto the rhodium center. Arylhalide (halide = Br, I) oxidative addition was achieved in the presence of donor ligands such as acetonitrile to form stable six-coordinate Rh(III) compounds. The C-O reductive elimination reactions in this system were also explored.

The Chemistry of Pincer Compounds

The Chemistry of Pincer Compounds
Author: David Morales-Morales
Publisher: Elsevier
Total Pages: 467
Release: 2011-08-11
Genre: Science
ISBN: 0080545157
GET BOOK

Download The Chemistry of Pincer Compounds Book in PDF, Epub and Kindle

Pincer complexes are formed by the binding of a chemical structure to a metal atom with at least one carbon-metal bond. Usually the metal atom has three bonds to a chemical backbone, enclosing the atom like a pincer. The resulting structure protects the metal atom and gives it unique properties.The last decade has witnessed the continuous growth in the development of pincer complexes. These species have passed from being curiosity compounds to chemical chameleons able to perform a wide variety of applications. Their unique metal bound structures provide some of the most active catalysts yet known for organic transformations involving the activation of bonds. The Chemistry of Pincer Compounds details use of pincer compounds including homogeneous catalysis, enantioselective organic transformations, the activation of strong bonds, the biological importance of pincer compounds as potential therapeutic or pharmaceutical agents, dendrimeric and supported materials. * Describes the chemistry and applications of this important class of organometallic and coordination compounds* Covers the areas in which pincer complexes have had an impact* Includes information on more recent and interesting pincer compounds not just those that are well-known

Late Transition-metal Complexes Supported by Pincer Ligands

Late Transition-metal Complexes Supported by Pincer Ligands
Author: Wilson D. Bailey
Publisher:
Total Pages: 181
Release: 2016
Genre: Alkenes
ISBN:
GET BOOK

Download Late Transition-metal Complexes Supported by Pincer Ligands Book in PDF, Epub and Kindle

Late transition-metal pincer complexes of primarily palladium(II) and platinum(II) have been investigated for their application as catalysts in partial oxidation reactions. The epoxidation of higher olefins using molecular oxygen as the oxidant has been targeted, and the individual reaction steps needed to accomplish this overall transformation are described herein, including: (1) hydrogenolysis of a metal hydroxide (M-OH) species to yield a metal hydride (M-H), (2) insertion of O2 into the M-H bond to form a metal hydroperoxide (M-OOH), and (3) O-atom transfer from the M-OOH to epoxides, yielding a M-OH and completing the catalytic cycle. Previous results from our group on these individual transformations using (tBuPCP)Pd and (tBuPCO)Pd fragments are also reviewed. The requirements for O2 insertion into PdII and PtII hydrides are discussed. An array of cationic, neutral, and anionic Pd-H and Pt-H complexes supported by a tBuPNP backbone were synthesized and evaluated for O2 insertion (tBuPNP = 2,6-bis-(di-tbutylphosphinomethyl)pyridine). Metal-ligand cooperation was observed in the activation of H2 to form neutral hydride complexes. The effect of ligand protonation/deprotonation on the trans influence experienced by the hydride ligand was investigated. No reaction with O2 was observed with the cationic hydrides, while the neutral and anionic forms reacted with O2 at the tBuPNP backbone. The synthesis and characterization of mono- and dinuclear Pd-OH complexes supported by a PCNR pincer ligand (PCNR = (1-(3-((di-tert-butylphosphino)methyl)phenyl)-1H-5-R-pyrazole), R = H, Me) is presented. When R = H, ligand pyrazole "rollover" C-H activation was observed, forming a mixed ligand (PCNH)Pd(μ-OH)Pd(PCC) dinuclear structure. This "rollover" was investigated using DFT computations. The mono- and dinuclear hydroxide species were evaluated for hydrogenolysis. The dinuclear compounds {[(PCNR)Pd]2(μ-OH)}[OTf] reacted under an H2 atmosphere to yield the corresponding dinuclear hydrides {[(PCNR)Pd]2(μ-H)}[OTf]. A mechanistic study on the hydrogenolysis of the μ-bridged hydroxide {[(PCNMe)Pd]2(μ-OH)}[OTf] revealed first order kinetics in both [Pd] and [H2]. Terminal hydrides were not detected, and reduction of the mononuclear hydroxide complexes (PCNR)Pd-OH to Pd0 was observed under H2. The reduction was proposed to proceed through displacement of the pyrazole arm, and was examined by DFT computations. Lastly, a new strategy to promote O-atom transfer from M-OOH to epoxides, the final step in the targeted catalytic cycle, is proposed. Preliminary studies on NNNPyz, NNNEt, and NNMe ligated PdII and PtII are discussed (NNNPyz = 2,6-bis(5-tbutyl-1H-pyrazol-3-yl)pyridine; NNNEt = 2-(5-tbutyl-1H-pyrazol-3-yl)-6-(diethylaminomethyl)pyridine; NNMe = 2-(5-tBu-1H-pyrazol-3-yl)-6-methylpyridine). The NNNPyz ligand, containing two acidic sites in proximity to the fourth site in the square plane, was found to protonate M-O2 complexes, chelate to the metal center and oxidize phosphine substrates. Similar reactivity was observed with NNNEt and NNMe, however hemilability of these ligands resulted in undesired coordination modes.

Organometallic Pincer Chemistry

Organometallic Pincer Chemistry
Author: Gerard van Koten
Publisher: Springer
Total Pages: 356
Release: 2012-09-18
Genre: Science
ISBN: 9783642310805
GET BOOK

Download Organometallic Pincer Chemistry Book in PDF, Epub and Kindle

Gerard van Koten: The Mono-anionic ECE-Pincer Ligand - a Versatile Privileged Ligand Platform: General Considerations.- Elena Poverenov, David Milstein: Non-Innocent Behavior of PCP and PCN Pincer Ligands of Late Metal Complexes.- Dean M. Roddick: Tuning of PCP Pincer Ligand Electronic and Steric Properties.- Gemma R. Freeman, J. A. Gareth Williams: Metal Complexes of Pincer Ligands: Excited States, Photochemistry, and Luminescence.- Davit Zargarian, Annie Castonguay, Denis M. Spasyuk: ECE-Type Pincer Complexes of Nickel.- Roman Jambor and Libor Dostál: The Chemistry of Pincer Complexes of 13 - 15 Main Group Elements.- Kálmán J. Szabo: Pincer Complexes as Catalysts in Organic Chemistry.- Jun-ichi Ito and Hisao Nishiyama: Optically Active Bis(oxazolinyl)phenyl Metal Complexes as Multi-potent Catalysts.- Anthony St. John, Karen I. Goldberg, and D. Michael Heinekey: Pincer Complexes as Catalysts for Amine Borane Dehydrogenation.- Dmitri Gelman and Ronit Romm: PC(sp3)P Transition Metal Pincer Complexes: Properties and Catalytic Applications.- Jennifer Hawk and Steve Craig: Physical Applications of Pincer Complexes.

Alkyl-heteroatom Bond Forming Reactions from the Late Transition Metals

Alkyl-heteroatom Bond Forming Reactions from the Late Transition Metals
Author: Tyler Eugene Stevens
Publisher:
Total Pages: 183
Release: 2016
Genre: Alkenes
ISBN:
GET BOOK

Download Alkyl-heteroatom Bond Forming Reactions from the Late Transition Metals Book in PDF, Epub and Kindle

C(sp3)-N bond formation is a potential high value reaction in transition metal catalysis, however, only a handful of examples of this fundamental coupling reaction step have been reported. Presented in this thesis are new efforts to promote this challenging reaction. Group 9 and 10 metals bound by the pincer ligands Pybox and PCP, respectively, were initially investigated. Evidence suggests the Ir-Et complex ([(dmPybox)Ir(Et)(OAc)][PF6] (12)) undergoes a [Beta]-H deprotonation, similar to an E2 type elimination in organic chemistry, rather than the desired SN2 nucleophilic attack. Use of strong nucleophiles, i.e. sulfur nucleophiles, resulted in the ring opening of the pybox ligand. Ir-Et complexes bound by the MeBPA ligand ([(MeBPA)Ir(OOCR)Et][PF6] (20 and 21)), however, did undergo the desired reaction to form C-N bonds upon reaction with neutral nitrogen nucleophiles. A computational investigation of the mechanism revealed that rather than the direct attack at the Ir-Et (generally observed for high valent Pd and Pt) the complex first undergoes [Beta]-hydrogen elimination to form an Ir ethylene complex. Nucleophilic attack by amine at the olefin then forms the C-N bond. In the final chapter the synthesis and characterization of new a new Rh-Me complex ((DPEphos)RhMe(I)2 (25)), which undergoes C(sp3)-I reductive elimination, is described. A kinetic investigation of the mechanism indicated two competing mechanisms involving SN2 attack by iodide at both cationic and neutral Rh-Me species. The demonstration of bond formation via nucleophilic attack at the Rh-Me allowed for the room temperature functionalization of the methyl ligand with both nitrogen and sulfur nucleophiles.

Activation and Functionalization of C-H Bonds

Activation and Functionalization of C-H Bonds
Author: Karen I. Goldberg
Publisher: ACS Symposium
Total Pages: 0
Release: 2004
Genre: Science
ISBN: 9780841238497
GET BOOK

Download Activation and Functionalization of C-H Bonds Book in PDF, Epub and Kindle

Activation and Functionalization of C-H Bonds explores recent developments in the reaction chemistry of solution-phase transition-metal based systems with simple hydrocarbons and with more complex organic molecules. More than 20 internationally leading research groups contributed to this volume, and their chapters cover such topics as fundamental theoretical and mechanistic studies of C-H bond activation by metal complexes, catalytic systems for alkane functionalization, and new applications in synthetic organic chemistry. An introductory chapter offers an overview of stoichiometric and catalytic reactions of C-H bonds with transition metal complexes. The C-H bond is the most widespread linkage in organic chemistry, present in virtually every organic molecule. Unfortunately, C-H bonds are famously resistant to selective chemical transformations. The development of methods for their selective transformations has enormous potential value in fields ranging from the chemistry of fuels (for example, the conversion of methane to methanol) to the synthesis of the most complex organic molecules.

Synthesis and Catalytic Activity of CCC-NHC Group 9 Metal Pincer Complexes

Synthesis and Catalytic Activity of CCC-NHC Group 9 Metal Pincer Complexes
Author: Sean William Reilly
Publisher:
Total Pages: 244
Release: 2015
Genre:
ISBN:
GET BOOK

Download Synthesis and Catalytic Activity of CCC-NHC Group 9 Metal Pincer Complexes Book in PDF, Epub and Kindle

N-Heterocyclic carbenes (NHCs) are one of the few ligand systems that can finely tune transition metal catalysts via sterics and electronics. The strong sigma-donating properties of these ancillary ligands allow the development of robust tridentate NHC pincer framework, which has emerged as an alternative to the phosphine pincer ligands. The combination of NHC and pincer systems has resulted in a new generation of catalytically active organometallic complexes reported throughout the literature. CCC-NHC Rh pincer complexes were found to be catalytically active in C-C and C-B bond formation via 1,4-addition reactions. In addition, the in-situ generated CCC-NHC Ir(H) pincer complex demonstrated catalytic activity in borylation of arene C-H bonds. Preliminary results are comparable to the C-H borylation results published by Hartwig and co-workers. The CCC-NHC Ir(H) pincer complex may also prove to be a suitable catalyst for alkane dehydrogenation, due to framework similarities of the highly active and durable PCP and POCOP pincer hydride systems. Expansion of group 9 metal sources for transmetalation of the CCC-NHC Zr pincer complex afforded the development of CCC-NHC Rh(CO) and CCC-NHC Co complexes. Group 9 metal carbonyl complexes have been reported as active catalysts in photocatalytic C-H activation of small molecules. Testing of Co sources for transmetalation afforded three rare Co pincer complexes, and the first examples of CCC-NHC Co pincer complexes to date. Development of CCC-NHC pincer complexes with base metals provide cost-effect alternatives to pincer systems with precious metal centers, and is reported herein.