Reactions of First-row Transition Metal Complexes in Bis(alkoxide) Ligand Environments with Diazoalkanes

Reactions of First-row Transition Metal Complexes in Bis(alkoxide) Ligand Environments with Diazoalkanes
Author: Amanda Grass
Publisher:
Total Pages: 231
Release: 2020
Genre: Chemistry, Inorganic
ISBN:

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This dissertation focuses on the design and reactions of novel late transition metal carbene complexes featuring alkoxide ligand environments. The high-valent cobalt carbene Co(OR)2(=CPh2) (OR = OCtBu2Ph), featuring short Co=C bond of 1.773(3) ©5, was previously reported from the reaction of Co(OR)2(THF)2 with diphenyldiazoalkane. Magnetic and spectroscopic (EPR) studies demonstrated Co(OR)2(=CPh2) to be a low-spin S = ℗ư complex. Computational studies, in agreement with experimental data, suggested that the electronic structure of Co(OR)2(=CPh2) lies between intermediate spin Co(III) anti-ferromagnetically coupled to a carbene radical and a Co(IV) alkylidene. This dissertation began with investigation of this complex in carbene transfer reactivity. Stoichiometric ketenimine formation occurs upon reaction with various isocyanides CNR0́ø (CNR0́ø =2,6-dimethylphenyl isocyanide, 4-methoxyphenyl isocyanide, 2-chloro-6-methylphenyl isocyanide, adamantyl isocyanide). The reaction is accompanied by the formation of a cobalt bis(alkoxide) bis(isocyanide) complexes Co(OR)2(CNR)2, which were independently synthesized and characterized. Excess isocyanide was required to form ketenimine due to formation of the bis(isocyanide) complex. DFT calculations suggest the mechanism proceeds through isocyanide binding to cobalt, in contrast to nucleophilic attack at the carbene carbon. This is followed by intramolecular insertion into the Co-carbene bond to form the ketenimine complex. Dissociation of free ketenimine from cobalt then leads to the bis(isocyanide) complex. Catalytic formation of ketenimines was investigated at room temperature by exposing the mixtures of the carbene precursors and isocyanides to Co(OR)2(THF)2. The carbene precursors investigated included both diazoalkane (diphenyldiazomethane) and diazoesters (methyl phenyldiazoacetate, and ethyl diazoacetate).

Synthesis of Low-coordinate Transition Metal Bis(alkoxide) Complexes and Their Reactivity Toward Small Molecules

Synthesis of Low-coordinate Transition Metal Bis(alkoxide) Complexes and Their Reactivity Toward Small Molecules
Author: Maryam Yousif
Publisher:
Total Pages: 217
Release: 2017
Genre: Chemistry, Inorganic
ISBN:

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This dissertation focused on the synthesis of new transition metal complexes in bis(alkoxide) ligand environments and the investigation of their reactivity in nitrene-group transfer catalysis and small molecule activation. Treatment of Cr(N(SiMe3)2)2(THF)2 with two equivalents of HOR (OR = OCtBu2Ph) led to the formation of the chromium(II) alkoxide dimer, Cr2(OR)4. Upon the reaction with bulky aryl and alkyl azides, Cr2(OR)4 led to the stable Cr(IV) mono(imido) complexes, Cr(OR)2(NR), featuring trigonal planar metal centers. In contrast, less bulky aryl azides led to the formation of chromium (VI) bis(imido) complexes Cr(OR)2(NR)2, independent of the amount of azide used. Chromium(IV) mono(imido) species Cr(OR)2(NR) is capable of nitrene transfer to isocyanides to form asymmetric carbodiimides (RNCNR'). When excess isocyanide is added to Cr(OR)2(NR), a new chromium(II) complex, Cr(OR)2(CNR2)4, was identified by X-ray crystallography. This tetrakis(isocyanide) chromium(II) complex is also capable of forming carbodiimide when azide is introduced. Efficient catalytic formation of carbodiimides was obtained using 2.5 mol% of Cr2(OR)4 for the mixtures of bulky organoazides and isocyanides; no catalytic reactivity was observed for the non-bulky aryl azides. DFT calculations suggest that trigonal CrIV(OR)2(NR) intermediate is the key species in the reaction mechanism as, due to it coordinative unsaturation, it allows isocyanide binding to the metal, which enables subsequent C-N bond formation. A bulkier alkoxide ligand HOR' (R = CtBu2(3,5-Ph2Ph)) was synthesized by lithium halogen exchange reaction. The protonolysis of the metal complexes, M(N(SiMe3)2(THF)x, with HOR' enabled an easy isolation of new bis(alkoxide) precursors M(OR')2(THF)2 (where M = Cr, Co, Fe) featuring cis-divacant octahedral geometry. The chemistry of the iron bis(alkoxide) compound Fe(OR')2(THF)2 with the variety of aryl azides was investigated.

Design of First-row Transition Metal Bis(alkoxide) Complexes and Their Reactivity Toward Nitrene and Carbene Transfer

Design of First-row Transition Metal Bis(alkoxide) Complexes and Their Reactivity Toward Nitrene and Carbene Transfer
Author: James Bellow
Publisher:
Total Pages: 231
Release: 2016
Genre: Chemistry, Inorganic
ISBN:

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The novel alkoxide ligand [OCtBu2Ph], or [OR], was synthesized in a single step as a lithium salt. It was then reacted with a series of first-row transition metal(II) halides, with widely varying results. Upon reaction with chromium, manganese, iron, or cobalt(II) chloride, dimeric complexes of the form M2(OR)4Li2Cl2 were formed, which displayed rare seesaw geometry at the metal. This unusual geometry was confirmed by various spectroscopic and computational studies. Computational studies also indicate that the steric bulk of the ligand, as well as the inclusion of lithium atoms in the molecules, are what lead to the seesaw geometry. Reaction of [OR] with nickel(II) halides generates monomeric species of the form Ni(OR)2XLi(THF)2 (X = Cl, Br), which display distorted trigonal planar geometry at three-coordinate nickel. Dimerization likely does not occur for nickel due to its smaller size. DFT studies support preference for nickel to form the monomer. Reaction of [OR] with copper(II) halides leads to reduction of the copper center by one electron, generating the tetramer Cu4(OR)4. Reduction of copper(II) by an alkoxide is a novel transformation. Spectroscopic studies to probe the mechanism suggest that Cu(OR)2XLi(THF)2 may be an intermediate prior to reduction. Observation by NMR of the ketone Ph(C=O)tBu and ROH suggest that alkoxide reduces the copper to give an alkoxide radical, which then decomposes via ß-scission. To form the desired bis(alkoxide) system, the halide-containing alkoxide complexes were reacted with thallium(I) hexafluorophosphate. For manganese, iron, and copper, complexes of the form M(OR)2(THF)2 were isolated. The bis(alkoxide) complexes display distorted tetrahedral geometry at the metal, with large RO-M-OR angles. Cyclic voltammetry of these species show that the iron bis(alkoxide) is the most easily reduced of the three.

Kinetics and Mechanism of Reactions of Transition Metal Complexes

Kinetics and Mechanism of Reactions of Transition Metal Complexes
Author: Ralph G. Wilkins
Publisher: Wiley-VCH
Total Pages: 480
Release: 1991-08-28
Genre: Science
ISBN: 9783527282531

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This thoroughly revised and updated edition of one of the classics of kinetics text books continues the successful concept of the 1974 edition: In its first part, a simplified approach to the determination of rate laws and mechnisms is given steadily working up to complex situations. In the following chapters the principles developed there are extensively used in a comprehensive account of reactions of transition metal complexes, including reactions of biological signifacance. The text is illustrated by numerous figures and tables. Points of further interest are highlighted in special insets. 140 problems, taken from the original literature, enable the student to apply and deepen his newly acquired knowledge and make the book highly useful for courses in inorganic and organometallic reaction mechanisms. Furthermore, a wealth of over 1700 references renders it an indispensable work for the active researcher.

Synthesis, Reactivity and Catalytic Applications of Two-Coordinate First Row Transition Metal Complexes

Synthesis, Reactivity and Catalytic Applications of Two-Coordinate First Row Transition Metal Complexes
Author: Michael Isaac Lipschutz
Publisher:
Total Pages: 139
Release: 2015
Genre:
ISBN:

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Recently, the development of more sustainable catalytic systems based on abundant first-row metals, especially nickel, for organic transformations has attracted significant interest. This thesis describes the development of new synthetic methods for the preparation of two-coordinate complexes of a variety of first row transition metals. The physical and spectroscopic properties of these complexes are discussed and the reactivity and catalytic applications of these compounds are also explored. Species of this type are found to catalyze a variety of useful organic transformations using inexpensive metals and ligands.

Oxidation State Roulette

Oxidation State Roulette
Author: Brandon Fitchett
Publisher:
Total Pages:
Release: 2018
Genre:
ISBN:

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The use of rare and expensive noble metals in the chemical industry as organometallic catalysts has grown exponentially in the past few decades due to their high activity, selectivity and their ability to catalyze a wide range of reactions. With this growth in use has also come a proportional growth in concern as these toxic metals inevitably leach into the environment and their negative effects on public health and our ecosystems are becoming better understood. First-row transition metal catalysts provide both environmental and economic benefits as alternatives to these noble metals due to their lower toxicity and cheaper costs. The two-electron chemistry that makes the noble metals so attractive however, is more challenging to accomplish with first-row transition metals. Intelligently designing the ligand scaffold which surrounds the metal can mitigate or even eliminate some of the shortfalls of these first-row metals. Some key features that should be considered when designing a ligand are: 1) a strong chelating ability so the ligand can stay attached to the metal, 2) incorporation of strong donors to favour low-spin complexes, 3) inclusion of hemilabile groups to allow for substrate activation and metal stabilization throughout various oxidation states, 4) redox activity to be able to donate or accept electrons, and 5) inclusion of Lewis base functionalities which are able to assist the substrate activation. Ligands which incorporate these features are known as bifunctional ligands as they can accomplish more than one function in the catalytic cycle. Developing first-row transition metal complexes containing these ligands may enable these species to replicate the reactivity and selectivity generally associated with the precious metals. Being able to replace the noble metals used in industry with these catalysts would have tremendous environmental and economic benefits. The objective of this thesis is to advance the field of bifunctional catalysis by examining the behaviour of two sterically svelte, tridentate SNS ligands containing hard nitrogen and soft sulphur donors when bonded to cobalt. Previous work with iron provides a template of the ligand behaviour to which cobalt can be compared, allowing us to contrast the effects exerted by the different metals. After an introduction to bifunctional catalysis in Chapter 1, Chapter 2 describes the reactivity of the amido ligand, SMeNHSMe, with precursors ranging from Co(I) to Co(III), all of which yielded the 19e- pseudooctahedral cobalt(II) bis-amido complex, Co(SMeN-SMe)2 characterized by 1H NMR spectroscopy, single-crystal X-ray crystallography and cyclic voltammetry. Although this complex has a similar structure as the Fe analogue, the cobalt bis-amido complex did not exhibit the same hemilabile behaviour that allowed for simple ligand substitution of one of the thioether groups. Instead it reacted reversibly with 2,2'-bipyridine while 1,2-bis(dimethylphosphino)ethane (DMPE) and 2,6-dimethylphenyl isocyanide both triggered additional redox chemistry accompanied by the loss of protonated SMeNHSMe. In contrast, protonation gave the cobalt(II) amido-amine cation, [Co(SMeNSMe)(SMeNHSMe)](NTf2), which allowed for substitution of the protonated ligand by acetonitrile, triphenylphosphine and 2,2'-bipyridine based on 1H NMR evidence. The ability of Co(SMeNSMe)2 to act as a precatalyst for ammonia-borane dehydrogenation was also probed, revealing that it was unstable under these conditions. Addition of one equivalent of DMPE per cobalt, however, resulted in better activity with a preference for linear aminoborane oligomers using ammonia-borane and, surprisingly, to a change in selectivity to prefer cyclic products when moving to methylamine-borane. Chapter 3 delves into the chemistry of the thiolate ligand, SMeNHS, which formed a new 18e- cobalt(III) pseudooctahedral complex, Co(S-NC-)(SMe)(DEPE), from oxidative addition of the Caryl-SMe bond. Scaling up this reaction resulted instead in formation of an imine-coupled [Co(N2S2)]- anion which was characterized by 1H NMR/EPR spectroscopy, single-crystal X-ray diffraction, cyclic voltammetry and DFT studies. The latter revealed an interesting electronic structure with two electrons delocalized in the ligand, demonstrating the non-innocent nature of the N2S2 ligand. While the analogous iron complex proved to be an effective pre-catalyst for the hydroboration of aldehydes with selectivity against ketones, this behaviour was not observed with [Co(N2S2)]- which gave a slower rate and less selectivity. The knowledge acquired from this thesis work has advanced the field of bifunctional catalysis by extending the application of these two SNS ligands from iron to cobalt, revealing unpredictable differences in reactivity between the metals. By comparing the behaviour of these ligands with iron and cobalt, we gain a better understanding of the chemistry that is accessible by these ligands and the applications for which they may be used. This increased knowledge contributes to our long-term goal of replacing expensive and toxic noble metals with more benign first-row transition metals, improving the sustainability of the chemical industry.

Reactivity and Thermochemistry of First-row Transition Metal Complexes with Stable Organic Radicals

Reactivity and Thermochemistry of First-row Transition Metal Complexes with Stable Organic Radicals
Author: Thomas R. Porter
Publisher:
Total Pages: 179
Release: 2014
Genre:
ISBN:

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Reactions involving transition metals and organic free radicals are critically important in a variety of chemical and biological processes. Because of their prevalence, there is a fundamental interest in better understanding these types of reactions to fully realize their potential for new applications. The work presented in this dissertation describes the free radical reactivity and thermochemistry of several different transition metal systems with stable organic radicals. Chapter 1 provides an introduction to transition metal reactivity involving organic free radicals. Chapter 2 describes the catalytic disproportionation of a hydroxylamine by (TMP)Fe[superscript III]-OH (TMP = meso-tetramesityl porphyrin) and some of the radical reactions that make up the catalytic cycle. Chapter 3 describes the preparation, structural characterization and thermochemistry of a previously unreported stable organic radical, [superscript t]Bu2NPArO* (2,6-di-tert-butyl-4-(4'-nitrophenyl)phenoxyl). Chapter 4 describes the preparation of several [Tp[superscript tBu]Cu[superscript II]]+ (Tp[superscript tBu] = hydro-tris(3-tert-butyl-pyrazolyl)borate) and [Tp[superscript tBuMe]Cu[superscript II]]+ (Tp[superscript tBu]Me = hydro-tris(3-tert-butyl-5-methyl-pyrazolyl)borate) alkoxide compelxes as models for potential intermediates in copper/radical alcohol oxidation catalysis. Treating these complexes with stable radicals such a [superscript t]Bu3ArO* (2,4,6-tri-tert-butyl-phenoxyl) did not result in alkoxide oxidation despite having a large driving force. From these studies, we conclude driving force is not a primary predictor for copper/radical alcohol oxidation. Chapter 5 discusses the coordination chemistry of [Tp[supercscript tBu]Cu[superscript II]]+ and [Tp[superscript tBu]Zn[superscript II]]+ with 4-nitro-phenols. With the bulky 2,6-disubstituted 2,6-di-tert-butyl-4-nitro-phenoxide, coordination to either metal occurs through a nitronate resonance form. The 2,6-unsubstituted 4-nitro-phenol binds through the phenoxide resonance form. Chapter 6 highlights the large kinetic barrier for ketone reduction or oxidation by titanocene(III/IV) and the hydrogen atom donor/acceptor, [superscript t]Bu3ArO(-H). Finally, Chapter 7 describes the selective and stoichiometric reduction of aromatic and aliphatic nitro groups by photoreduced titanium dioxide nanoparticles in acidic aqueous solutions. From thermochemical analysis, it is likely that these reactions proceed through a rate determining H+/e- transfer.