Synthesis, Characterization, and Reactivity Studies of Iron Complexes Supported by the Redox-active [ONO] Ligand

Synthesis, Characterization, and Reactivity Studies of Iron Complexes Supported by the Redox-active [ONO] Ligand
Author: Janice Lin Wong
Publisher:
Total Pages: 123
Release: 2014
Genre:
ISBN: 9781321094466

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The work reported herein primarily focuses on the development of new platforms for multi-electron reactivity using iron complexes supported by a redox-active pincer-type ligand. This dissertation details the synthesis, characterization, and reactivity of iron complexes coordinated to the redox-active [ONO] ([ONO]H3 = bis(3,5-di-[tert]-butyl-2-phenol)amine) ligand. Chapter 1 provides a general background on ligand-centered and metal-centered redox reactivity. Specifically, the characteristics of redox-active ligands and their ability to promote multi-electron reactivity at redox-inert metal centers is presented. In addition, iron-catalyzed organic transformations in which the metal center undergoes redox changes is also discussed. Finally, ligand-enabled redox reactions mediated by iron complexes containing redox-active ligands is described. Chapter 2 reports on the complexation of bis(3,5-di-[tert]-butyl-2-phenoxy)amine, [ONHO], and the redox-active [ONO] ligands by iron centers to afford a new family of iron complexes. Characterizations of each compound through a battery of analytical techniques reveal the oxidation states of the metal center and ligand. Furthermore, the electronic properties of each complex were investigated in order to evaluate their potential to facilitate multi-electron reactivity. Chapter 3 details the reactivity of the [ONO]Fe platform. Metathesis reactions are conducted with [ONO [superscript q] Fe [superscript III] X2 (X = Cl, N[SiMe3]2 complexes, demonstrating the capability of the fully-oxidized [ONO [superscript q]−1 to act as a two-electron acceptor to generate the fully reduced [ONO [superscript cat]3− that is coordinated to an iron(III) center. Similarly, oxidation of [ONO[superscript cat] Fe [superscript III] (py)3 (py = pyridine) using dihalogens result in two-electron oxidations of the tridentate ligand while the metal oxidation state remains the same. These redox reactions showcase the ability of the [ONO] ligand platform to undergo reversible two-electron oxidation state changes, allowing multi-electron reactivity to occur at the iron center. The synthesis and characterization of two novel bimetallic complexes of the form [ONO]M'[ONO]2 M (M' = Fe, Zn; M = Fe) are presented in Chapter 4. The rich redox profiles of both complexes suggest that they can potentially impart unique cooperative bimetallic reactivity. The synthetic techniques developed to prepare these complexes lay the foundation for a general method to access new bimetallic combinations that could be promising for multi-electron reactivity. Finally, Chapter 5 discusses the synthesis, characterization, and electronic comparisons between two homoleptic tris-iminosemiquinonate chromium(III) compounds. While one is coordinated to three N,N'-bis(3,5-dimethylphenyl)acenapthenediimino-semiquinonate, (dmp-ADI [superscript sq])1−, ligands, the other contains three N,N'-bis(3,5-dimethylphenyl)phenanthrenediimino-semiquinonate, (dmp-PDIsq)1− ligands. The differences in the electronic properties between each complex likely stems from variation in the diimine ligand backbones. However, further investigation is required to completely understand the complicated behaviors of such complexes, both of which apparently exhibit intramolecular anti-ferromagnetic properties.

Synthesis, Characterization and Applications of Complexes Involving Redox-active Ligands

Synthesis, Characterization and Applications of Complexes Involving Redox-active Ligands
Author: Romain Kunert
Publisher:
Total Pages: 0
Release: 2022
Genre:
ISBN:

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We developed a new type of redox-active ligand, involving two N-heterocyclic carbenes and two phenolate units. The strong electron donating properties of the NHC moieties were utilized to stabilize complexes in high oxidation states while the phenolate groups were used as redox-active units. Group 10 metal complexes were developed. Their oxidation chemistry showed the formation of phenoxyl radicals, reaching up to two-electron oxidized, bis(phenoxyl) complexes. We also report the first Ni(III) complex with NHC donors. The ligand was also used to develop metal complexes with nitride ligands. The nitridomanganese complex proved to be unstable and degraded in successive intramolecular nitride-NHC reductive couplings. The degradation afforded a peculiar organic salt with three fused rings forming a central triazone pattern. Conversely, the chromium nitride complex could be isolated and thoroughly characterized. The one-electron oxidized product was generated reversibly at low temperature but readily evolved at room temperature. In a parallel work, we also developed new types of sterically hindered salen ligands to form distorted copper salen complexes as molecular models of galactose oxidase. The copper salen catalysts showed to be effective for the aerobic oxidation of non-activated alcohol substrates.

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.

New Avenues for Redox-active Ligands

New Avenues for Redox-active Ligands
Author: Daniël Laurens Johannes Broere
Publisher:
Total Pages: 0
Release: 2016
Genre:
ISBN: 9789402801767

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"Many homogeneous and heterogeneous catalyst systems contain one or more transition metals. The widespread employment of these metals as catalysts is ascribed to their accessible d-orbitals to activate chemical bonds, and the ability to undergo metal-based oxidation state changes to facilitate desirable chemical transformations. The fine-tuning of homogeneous catalyst systems is commonly achieved by the coordination of (spectator) ligands, which can vary greatly in steric bulk or electron-donating ability. For such ligands the energy required for oxidation or reduction of the ligand is much bigger than that needed to change the oxidation state of the metal. Accordingly, the redox changes required for bond making and breaking processes typically occur at the metal center. Redox-active ligands have more energetically accessible levels for reduction and/or oxidation upon coordination to a metal. As a result, either solely ligand-centered redox processes can occur, with the metal center remaining in the same oxidation state, or more diffuse scenarios, wherein both the ligand and metal change oxidation states in a synergistic fashion. Although initially thought of as a spectroscopic curiosity, redox-active ligands are nowadays recognized for their ability to induce new reactivity at metal centers. Within this thesis we have shown that o-aminophenol derived architectures can give fascinating spectroscopic properties upon coordination to late transition metals. Moreover, these ligands can expand upon a metal's "common" reactivity by actively taking part in intramolecular redox processes. We have demonstrated that intramolecular single-electron transfer processes can facilitate homolytic bond cleaving reactions and the generation of reactive nitrogen-centered radicals."--Samenvatting auteur.

Synthesis, Reactivity, and Multi-electron Redox Behavior of a Bis(phenoxy)amide Ligand Coordinated to a Tantalum Metal Center

Synthesis, Reactivity, and Multi-electron Redox Behavior of a Bis(phenoxy)amide Ligand Coordinated to a Tantalum Metal Center
Author: Ryan A. Zarkesh
Publisher:
Total Pages: 150
Release: 2012
Genre:
ISBN: 9781267171108

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This dissertation describes the synthesis and reactivity of tantalum metal complexes containing a tridentate redox-active ligand. Fundamental studies have focused on utilizing the redox-active ligand to store multiple electron equivalents for oxidative addition and reductive elimination reactions. Chapter 1 provides an introduction to the characteristics of redox-active ligands and provides an overview of group transfer reactions involving redox-active ligands. The previous published results of bidentate redox-active ligands coordinated to Group IV d0 metals are discussed in terms of their decomposition side reactions. Chapter 2 describes the coordination of a known tridentate redox-active bis(phenoxy)amide ligand, (ONO), to a d0 tantalum(V) metal center and the examination of the redox properties of the resulting chloro oxidation products by electrochemical and spectroscopic methods. Chapter 3 examines the reactivity of the (ONO)TaR2 complexes in the general context of organometallic chemistry with a focus on protonolysis and reactivity with aryl azides, a known source of nitrene fragments upon oxidation. Chapter 4 examines the reactivity of the (ONO)TaX2 (X = Me, Cl) compounds with bulky diazoalkanes, a known carbene transfer reagent. The (ONO)TaCl2 complex proved to be a competent catalyst to generate cyclopropanes from styrene and the corresponding diazoalkane. Chapter 5 explores the utilization of the (ONO) ligand to store electron equivalents for the catalytic nitrene-nitrene coupling reactions with organoazides to afford organodiazenes. Finally, Chapter 6 addresses the electronic considerations of a related redox-active triamido ligand in an effort to tune the ligand's redox potentials.

Organic Redox Chemistry

Organic Redox Chemistry
Author: Jun-Ichi Yoshida
Publisher: John Wiley & Sons
Total Pages: 260
Release: 2022-03-07
Genre: Science
ISBN: 352734487X

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Organic Redox Chemistry Explore the most recent advancements and synthesis applications in redox chemistry Redox chemistry has emerged as a crucial research topic in synthetic method development. In Organic Redox Chemistry: Chemical, Photochemical and Electrochemical Syntheses, some key researchers in this field, including editors Dr. Frédéric W. Patureau and the late Dr. Jun-Ichi Yoshida, deliver an insightful exploration of this rapidly developing topic. This book highlights electron transfer processes in synthesis by using different techniques to initiate them, allowing for a multi-directional perspective in organic redox chemistry. Covering a wide array of the important and recent developments in the field, Organic Redox Chemistry will earn a place in the libraries of chemists seeking a one-stop resource that compares chemical, photochemical, and electrochemical methods in organic synthesis.

Redox-Active Therapeutics

Redox-Active Therapeutics
Author: Ines Batinić-Haberle
Publisher: Springer
Total Pages: 709
Release: 2016-10-13
Genre: Science
ISBN: 3319307053

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This essential volume comprehensively discusses redox-active therapeutics, focusing particularly on their molecular design, mechanistic, pharmacological and medicinal aspects. The first section of the book describes the basic aspects of the chemistry and biology of redox-active drugs and includes a brief overview of the redox-based pathways involved in cancer and the medical aspects of redox-active drugs, assuming little in the way of prior knowledge. Subsequent sections and chapters describe more specialized aspects of central nervous system injuries, neurodegenerative diseases, pain, radiation injury and radioprotection (such as of brain, lungs, head and neck and erectile function) and neglected diseases (e.g., leishmaniasis). It encompasses several major classes of redox-active experimental therapeutics, which include porphyrins, salens, nitrones, and most notably metal-containing (e.g., Mn, Fe, Cu, Zn, Sb) drugs as either single compounds or formulations with nanomaterials and quantum dots. Numerous illustrations, tables and figures enhance and complement the text; extensive references to relevant literature are also included. Redox-Active Therapeutics is an invaluable addition to Springer’s Oxidative Stress in Applied Basic Research and Clinical Practice series. It is essential reading for researchers, clinicians and graduate students interested in understanding and exploring the Redoxome—the organism redox network—as an emerging frontier in drug design, redox biology and medicine.