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.

Facilitating Multi-electron Reactivity at Low-coordinate Cobalt Complexes Using Redox-active Ligands

Facilitating Multi-electron Reactivity at Low-coordinate Cobalt Complexes Using Redox-active Ligands
Author: Aubrey L. Smith
Publisher:
Total Pages:
Release: 2011
Genre: Charge exchange
ISBN:

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In this study, we describe a detailed investigation of cobalt complexes containing redox-active ligands. We have prepared an electronic series of the complex in three oxidation states: [CoIII(ap)2]-, CoIII(isq)(ap), and [CoIII(CH3CN)(isq)2]+. Characterization shows that the metal center remains cobalt(III) through the redox changes and indicates that the oxidation state changes occur with gain or loss of electrons from the ligand set. While CoIII(isq)(ap) reacts with halide radicals to form a new cobalt-halide bond in a single electron reaction, [CoIII(ap)2]- appears to be prone to multi-electron reactivity in reactions with sources of "Cl+". Both reactions occur with electrons derived from the ligand set. Mechanistic studies suggest a single, two electron step is responsible for the bond-formation. Similarly, [CoIII(ap)2]- reacts with alkyl halides to pseudo-oxidatively add the alkyl at the cobalt center. The product of the reaction can be isolated and fully characterized and was found to be best assigned as CoIII(alkyl)(isq)2. This assignment indicates that the reaction occurs, again, with the new bond formed with two electrons formally derived from the ligand set and with no change in oxidation state at the metal center. Mechanistic investigations of the pseudo-oxidative addition suggest the reaction is SN2-like. The reaction occurs with a wide scope of alkyl halides, including those containing beta-hydrogens. : The cross-coupling reaction of CoIII(alkyl)(isq)2 with RZnX forms a new carbon-carbon bond. Similarly, the two electron oxidized complex [CoIII(CH3CN)(isq)2]+ reacts with organozinc reagents to couple two carbon nucleophiles and form a new carbon-carbon bond. Both reactions are successful with both sp2 and sp3 carbons. When followed substoichiometrically, the homocoupling reaction can be observed to form CoIII(alkyl)(isq)2. This indicates that the homocoupling and cross-coupling reactions proceed by the same mechanism. However, both reactions have low yields. The yield of the reactions are decreased by steric bulk of the alkyl or aryl fragments or around the metal center created by substituents on the ligand. Also, while the steric congestion disfavors the addition of the first alkyl fragment, the addition of the second alkyl fragment and subsequent rapid elimination of the coupling product is almost completely inhibited. This result also implies that the coupling of the two alkyl fragments is entirely inner-sphere requiring installation of both for coupling.

The Coordination Chemistry of Thioether-supported, Low-valent Cobalt Complexes

The Coordination Chemistry of Thioether-supported, Low-valent Cobalt Complexes
Author:
Publisher:
Total Pages:
Release: 2006
Genre: Cobalt compounds
ISBN: 9780542449864

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The tridentate S3- donor ligand, phenyltris((tert-butylthio)methyl)borate, [PhTttBu]-, has been used to investigate the coordination chemistry of cobalt. A series of coordinatively and electronically unsaturated organocobalt(II) complexes [PhTttBu]Co(R) (R = CH3, CH2CH3, C6H5, CH2C6H5, C3H5) was synthesized through reaction of the starting material [PhTttBu]COCl, with the appropriate Grignard reagent. The resultant pseudotetrahedral complexes (allyl derivative is square pyramidal) proved to be both air and moisture sensitive. The complexes were characterized spectroscopically and crystal lographically. The small molecule reactivity of these complexes was assayed through reactions with CO and NO. The former reagent showed sensitivity to the identity of the organocobalt while the latter gave rise to the same product, regardless of the organic ligand. When R = CH3, CH2CH 3, C6H5, reaction of [PhTttBu]Co(R) with CO yielded red, five-coordinate [PhTttBu]Co(CO)(C(O)R). These square-pyramidal complexes contain apical thioether ligation with the remaining four ligands in the basal plane. Alternatively, when R = CH 2C6H5, C3H5, reaction with CO resulted in Co-C bond homolysis with formation of the brown Co(I) complex, [PhTtBu]Co(CO)2. In all cases, reaction of the organocobalt(II) complexes with NO resulted in the formation of the & kappa;2-dinitrosyl complex, [& kappa;2-PhTttBu]Co(NO)2. These results were confirmed both spectroscopically and crystallographically. A series of cobalt(I) phosphine complexes, [PhTttBu ]Co(PR3), PR3 = PMe3, PEt3, PMe2Ph, PMePh2, PPh3, P(OPh3), was prepared by reduction of [PhTttBu]COCl in the presence of the phosphine. These four-coordinate complexes were characterized spectroscopically and crystal lographically. The structures may be grouped into two classes, pseudo-tetrahedral and cis-divacant (i.e. an octahedron where two cis ligands have been removed, & alpha; = 54.7° for this complex; a & alpha; is the degree to which L lies off of the linear vector). The former class, for which PR 3 = PMe3, PEt3, PMe2Ph, the phosphine donor resides approximately on the B ... Co vector. The latter class, for which PR3 = PMePh2, PPh3, P(OPh) 3, the phosphine is significantly 'off-axis.' These complexes exhibited varying a angles allowing for insight into the structural choice of the complex based on the & sigma;-donating and & pi;-accepting properties of the phosphine donor. The cobalt and nickel isocyanide complexes, [PhTttBu]M(CNBu t) are readily alkylated with MeI, leading to the corresponding cationic iminoacyl complexes, [[PhTttBu]M(C(Me)NBut)]I.

Novel [beta]-diketiminates and [beta]-diketiminate Derivatives for the Synthesis of Main Group Complexes

Novel [beta]-diketiminates and [beta]-diketiminate Derivatives for the Synthesis of Main Group Complexes
Author: Brant James Maitland
Publisher:
Total Pages: 480
Release: 2015
Genre:
ISBN:

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This thesis is primarily concerned with the synthesis of low oxidation state alkaline earth metal complexes featuring novel [beta]-diketiminate ligands and their derivatives. The syntheses of a variety of different [beta]-diketiminates possessing extreme steric bulk, as well as those that have been electronically modified are detailed. These ligands have been used to produce a variety of different alkali metal, alkaline earth metal and lanthanide complexes.Chapter 1 provides a general introduction to low oxidation state alkaline earth metal chemistry. Particular attention is given to metal complexes featuring [beta]-diketiminates, given their central role in the research making up this thesis.Chapter 2 deals with the synthesis of a range of novel, sterically hindered [beta]-diketimines. These pro-ligands were targeted in the belief that their extreme bulk would allow the synthesis of currently unknown structural motifs.Chapter 3 is primary concerned with the exploitation of the developed sterically hindered [beta]-diketimines in the synthesis of a variety of alkali metal, alkaline earth metal and lanthanide complexes. A range of three coordinate magnesium complexes were produced using the developed ligands, most notably, the first crystallographically characterised [beta]-diketiminato magnesium hydride. In our quest for a calcium(I) dimer, a complex type which is currently unknown, a variety of [beta]-diketiminato calcium iodides were produced. The attempted reduction of these heteroleptic metal iodide complexes resulted not in the targeted low oxidation state complexes, but in two different calcium(II) dimers.In Chapter 4, the [beta]-diketimine ligand was adapted, resulting in two separate classes of pro-ligands. The first, a 1,3,5-triazapentadiene, has had the central carbon replaced by nitrogen. The second, instead of having the typical methyl groups on the ligand backbone, has dialkylamino groups. In each instance, it was envisaged that the ligands would donate additional electron density, and bind more strongly to the metal centre in coordination complexes, when compared to the standard [beta]-diketiminate ligand class. Six separate heteroleptic magnesium(II) iodide complexes have been developed from these ligands, the majority of which show promise as precursors to low oxidation state magnesium complexes.Chapter 5 details several new secondary amines which have been produced as pro-ligands for low oxidation state main group chemistry. In addition, preliminary studies have shown that these ligands have potential in coordination chemistry.

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 Reactivities of Cobalt(iii) Alkylperoxo Complexes Bearing Quaterpyridine Ligand

Synthesis and Reactivities of Cobalt(iii) Alkylperoxo Complexes Bearing Quaterpyridine Ligand
Author: Yunzhou Chen
Publisher:
Total Pages: 0
Release: 2022
Genre: Alkanes
ISBN:

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Direct use of dioxygen (O2) in functionalizing organic molecules is highly desirable. In nature, enzymes perform alkane oxidation efficiently at ambient conditions. The transition metals involved in the active sites of enzymes play vital roles in binding with O2 and transferring electrons and protons during metabolism. Many metal-oxygen species, such as hydro(alkyl) peroxo complexes, are invoked as reactive intermediates in these biological processes. Given the complexity of enzymes, studying the reactivity of these enzymes with simple synthetic coordination compounds is one of the strategies. This thesis is mainly concerned with the oxidation of alkanes and alkenes catalyzed by tailor-made cobalt(III) alkylperoxo complexes at ambient conditions. In the first part, we report the design and synthesis of the highly electrophilic cobalt(III) alkylperoxo complex, [CoIII(qpy)(OOtBu)(NCCH3)]2+ (2), supported by a planar tetradentate quaterpyridine ligand (qpy = 2,2′:6′,2′′:6′′,2′′′-quaterpyridine). This complex activates C(sp3)–H bonds of a variety of organic molecules at ambient conditions and yields a series of alkylperoxo complexes with the general formula [CoIII(qpy)(OOR)(NCCH3)]2+ [RH = Et2O (3), THF (4), tBuOMe (5), ethylbenzene (6), toluene (7), cyclopentene (8), and 3-hexyne (9)], which have been well characterized by various spectroscopic techniques including NMR, ESI-MS, UV-vis, FT-IR, and CHN elemental analysis. The structures of these complexes have also been characterized by X-ray crystallography. In the second part, the mechanism for the alkane oxidation catalyzed by [CoIII(qpy)(OOR)(NCCH3)]2+ was extensively studied at room temperature and one atmospheric pressure. NMR study reveals the reaction stoichiometry. ESI-MS study indicates exogeneous O2 is crucial with the support of 18O-labeled experiments. Kinetics study by UV-vis and a significant kinetic isotopic effect resulted for the oxidation of ethylbenzene by 2 suggest a rate-limiting hydrogen-atom abstraction from organic substrates (R′H) by [CoIII(qpy)OOR]2+ via the proximal oxygen atom of the peroxo group (i.e., [CoIII(qpy)OOR]2+ + R′H → [CoII(qpy)]2+ + R′• + ROOH). The resulting alkyl radical R′• bound with O2 to form alkyl peroxyl radical R′OO•, which was rapidly scavenged by the [CoII(qpy)]2+ to give another alkylperoxo complex [CoIII(qpy)OOR′]2+. The proposed mechanism in the peroxidation of organic molecules b y alkyl(hydro)peroxo complexes is unprecedented. In the third part, we examine the catalytic properties of [CoIII(qpy)(OOR)(NCCH3)]2+ in aerobic oxidation of various substrates. Using ethylbenzene, cumene, cyclopentene, and cyclohexene as the substrates, [CoIII(qpy)(OOR)(NCCH3)]2+ are found to be active and robust catalysts to produce the corresponding hydroperoxides, alcohols, and ketones catalytically. A turnover of >3000 is achieved in the oxidation of cyclohexene for 7 d. In the fourth part, the reactivities of [CoIII(qpy)OOR]2+ with alkenes were explored. Alkenes with weak C–H bonds (e.g., 1,4-cyclohexadiene and cycloalkenes) resulted in C–H functionalization. In case there are no weak C–Hs in the alkenes (e.g., styrene), [CoIII(qpy)OOR]2+ catalyzes the polymerization of styrenes in O2 to produce polyalkylperoxo species. The [CoIII(qpy)(OOCH(OOtBu)CH2Ph)(NCCH3)]2+ ( bisalkylperoxo 11), has complex, been isolated and characterized by ESI-MS, NMR, and X-ray crystallography. In summary, this work demonstrates the highly electrophilic character of Co(III) alkylperoxo complexes supported by the qpy ligand. Under ambient conditions, these complexes are suitable catalysts to perform aerobic peroxidation of a variety of alkanes and alkenes