Low-coordinate Iron and Cobalt Complexes

Low-coordinate Iron and Cobalt Complexes
Author: Thomas R. Dugan
Publisher:
Total Pages: 220
Release: 2012
Genre:
ISBN:

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"Unsaturated transition metal complexes are important in many stoichiometric and catalytic bond cleavage reactions. Therefore, low-coordinate transition metal complexes coordinated with sterically hindered ancillary ligands have been used for C-H activation, N2, and CO bond cleavage reactions. In this thesis, the coordination chemistry and reactivity of low-coordinate [beta]-diketiminate cobalt and iron complexes toward bond-breaking and bond-making reactions is explored and presented. In chapter 2, the unsaturated complex LtBuCo (LtBu = bulky [beta]-diketiminate ligand) is reported. The [beta]-diketiminate ligand in LtBuCo was ligated to cobalt in a slipped [kappa]N, [eta]6-arene mode. Addition of Lewis bases to LtBuCo yielded rapid and reversible conversion to the [kappa]2N, N' mode. The rate law of ligand binding to LtBuCo was first-order in both cobalt and substrate concentration. Therefore, ligand coordination was consistent with an associative or interchange mechanism that either preceded or occurred simultaneously to [beta]-diketiminate isomerization. In addition, LtBuCo cleaved Sn-F and aryl C-F bonds, and homolytic Sn-F bond cleavage yielded [LtBuCo([mu]-F)]2. Aryl C-F bond cleavage by LtBuCo yielded [LtBuCo([mu]-F)]2 and a cobalt(II) aryl complex in a 1:2 molar ratio. [LtBuCo([mu]-F)]2 reacted with triethylsilane (Et3SiH) to give pure hydride complex [LtBuCo([mu]-H)]2, which has different properties than previously reported. In chapter 3, treatment of LMeFeNNFeLMe with 4-tert-butylpyridine (tBupy) displaced the dinitrogen ligand to give LMeFe(tBupy)2 which is formally iron(I). However, LMeFe(tBupy)2 can be defined as high-spin iron(I) with a resonance form that is high-spin iron(II) antiferromagnetically coupled to a radical on the tBupy ligand. In contrast, treatment of LMeFeNNFeLMe with pyridine (py) resulted in the reductive coupling of pyridine via C-C bond formation to give {LMeFepy}2([mu]-C10H10N2), a complex with a bridging 4,4'-bis(hydridopyridyl) ligand. {LMeFepy}2([mu]-C10H10N2) was diiron(II) in the solid state, but C-C bond formation was rapidly reversible as the solution properties were consistent with LMeFe(py)2. Chapter 4 reports new synthetic routes to iron hydride complexes with higher purity than previously achieved. The binuclear oxidative addition of H2 to a transient iron(I) intermediate yielded [LtBuFe([mu]-H)]2. This method was adapted for the synthesis of [LMeFe([mu]-H)]2, and the deuterated isotopologues, [LtBuFe([mu]-D)]2 and [LMeFe([mu]-D)]2, were synthesized using D2. The H/D exchange of hydride ligands between isotopologues and H2/D2 was observed"--Page ix-x.

Synthesis, Structure, and Reactivity of Terminal Cobalt Imido Complexes

Synthesis, Structure, and Reactivity of Terminal Cobalt Imido Complexes
Author: Daniel Travis Shay
Publisher:
Total Pages: 134
Release: 2007
Genre:
ISBN:

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Kinetic studies of the thermal decomposition of Tpt Bu,MeCoNAd which undergoes C-H activation of the Tp ligand yielding BptBu,Me(Me-pz-CMe2CH 2N(Ad)H)Co, have been closely monitored by 1H NMR spectroscopy. Determination of the rate of C-H insertion of the imido ligand of Tp tBu,MeCoNAd at a variety of temperatures led to the generation of an Erying Plot which showed curvature. Kinetic isotope measurements were also conducted showing a temperature dependent kinetic isotope effect indicating that breaking of the C-H bond was involved in the rate determining step.

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.

Synthesis and Original Reactivity of Copper and Gold Complexes

Synthesis and Original Reactivity of Copper and Gold Complexes
Author: Maximilian Joost
Publisher:
Total Pages: 0
Release: 2014
Genre:
ISBN:

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The present work is a study concerning the coordination chemistry and reactivity of copper and gold complexes. Of particular interest was the elucidation of (i) the metal-mediated activation of s-bonds and (ii) the migratory insertion reaction. Both processes are of considerable importance in organometallic chemistry, but elusive for copper and gold. The first part of this manuscript is dedicated to the study of the coordination of s-SiH bonds to copper. By means of a chelate assistance approach, we synthesized copper complexes employing diphosphino-hydrosilane ligands (R2P(o-C6H4))2Si(R')H. This study allowed for the identification of a weak ?2-SiH coordination to copper that has been evidenced by experimental (NMR, IR, XRD) and computational (geometry optimization, NBO analysis) means. The s-SiH coordination is essentially dominated by donation of electron density from the hydrosilane moiety to copper, without significant backdonation. In the second part, we investigated the reactivity of silylgold(I) complexes towards unsaturated organic molecules. We showed that these complexes react with alkynes and allenes to yield (Beta-silyl)vinylgold complexes via migratory syn insertion into the gold-silicon bond. A mechanistic proposal consisting of a 2-step coordination insertion process was established for this transformation by means of a kinetic and computational study. In the third part, the intermolecular oxidative addition of s-SiSi, s-CC and s-CX (X=halogen) bonds with molecular gold(I) complexes was studied in detail. We showed that this reaction, usually considered to be impossible for gold, is actually highly favored, provided an adequate ligand is employed. Disilanes add to cationic monophosphine gold(I) complexes. However, the corresponding bis(silyl)gold(III) products are highly unstable. The use of bis(phosphine) gold(I) complexes, featuring a bidentate ligand with a small bite-angle, allowed for the first time for the oxidative addition of s-CC and s-CX bonds at gold(I) and proved key for the stabilization of the corresponding gold(III) products that were characterized by spectroscopic and structural means. These results shed light on an unexpected reactivity pattern of gold complexes and may be an entry point to 2-electron redox transformations mediated by this metal, opening up new perspectives in gold catalysis.

Comprehensive Coordination Chemistry II

Comprehensive Coordination Chemistry II
Author: J. A. McCleverty
Publisher: Newnes
Total Pages: 11845
Release: 2003-12-03
Genre: Science
ISBN: 0080913164

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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.