The aim of this study was to investigate the mechanism of phosphine induced migratory carbonyl insertion in the monocyclopentadienyliron(II) carbonyl complex, [η5- (C5H5)Fe(CO)2Me], upon variation of different parameters such as the type and the concentration of the phosphine ligand and the solvent. The mechanism that agrees with the results obtained is presented below. / Prof. A. Roodt

Please read the abstract in the section 00front of this document / Thesis (PhD (Chemistry))--University of Pretoria, 2006. / Chemistry / unrestricted

Thermolysis of Cp*W(NO)(CH₂CMe₃)₂ (2.1) in halo, methoxy, or phenylethynyl-substituted benzenes leads to the formation of the alkylidene intermediateCp*W(NO)(=CHCMe₃) which selectively activates ortho C-H bonds of the organicsubstrates. The ortho-regioselectivity diminishes as the size of the substituent increasesfrom F (97 %) to C-=CPh (51 %). In the solid-state structure of all complexes the ortho-substituent is not coordinated to the metal centre; rather, the metal centre is engaged inagostic interactions with a neopentyl methylene C-H bond. Mechanistic studies on the chlorobenzene reaction reveal that the ortho-C-H-activation product is preferentially formed via thermal isomerization from the meta / para-C-H-activation isomers. Reactions between Cp*W(NO)(CH₂EMe₃)Cl (E = C or Si) and a variety of bis(allyl)magnesium reagents lead to the expected formation of Cp*W(NO)(alkyl)(allyl)complexes. Cp*W(N0)(CH₂CMe₃)(η³-CH₂CHCH₂) (3.5), Cp*W(N0)(CH₂CMe₃)(η³-CH₂CMeCH₂) (3.6), Cp*W(N0)(CH₂CMe₃)(η³-CH₂CHCHMe) (3.7),Cp*W(N0)(CH₂CMe₃)(η³-CH₂CHCHPh) (3.8) and Cp*W(N0)(CH₂SiMe₃)(η³-CH₂CHCHMe) (3.9) have thus been synthesized in moderate yields. The solid-state molecular structures of 3.5 and 3.7-3.9 feature a σ-π distorted ally! ligand in the endoconformation. Complex 3.5 reacts with pyrrolidine at RT to form Cp*W(NO)(NC₄H8)(CHMeCH₂NC₄H8) (3.10), a nucleophilic-attack product. Complexes 3.6-3.9 effect the concurrent N-H and α-C-H activation of pyrrolidine at RT and form alkyl-amido complexes analogous to the previously known Cp*W(N0)(CH₂EMe)(NC₄H₇-2-CMe₂CH=CH₂) (3.12). Thermolysis of Cp*W(N0)(CH₂CMe₃)(η³-CH₂CHCHMe) (3.7) at RT leads to the loss of neopentane and the formation of the η²-diene intermediate Cp*W(N0)(η²-CH₂=CHCH=CH₂) (A) which has been isolated as a PMe₃ adduct. In the presence of saturated organic substrates, C-H activation occurs exclusively at the methyl positions of the molecule. Reactions between intermediate A and unsaturated substrates lead to coupling between the coordinated η²-diene and the unsaturation on the organic molecule.Treatment of Cp*W(N0)(n-C₅H₁₁)(η³-CH₂CHCHMe) (4.1) with I₂ at -60 °C produces n-C₅H₁₁ I in moderate yields. Thermolysis of Cp*W(N0)(CH₂CMe₃)(η³-CH₂CHCHPh) (3.8) in benzene at 75 °C for one day leads to the exclusive formation of Cp*W(N0)(H)(η³-PhCHCHCHPh) (5.1).Trapping, labelling, and monitoring experiments suggest that 5.1 is formed via 1) the loss of neopentane and the generation of the allene intermediate Cp*W(N0)(η²-CH₂=C=CHPh), 2) the C-H activation of benzene resulting in a phenyl phenylallyl complex, and 3) the thermal isomerization of this latter species to 5.1. / Science, Faculty of / Chemistry, Department of / Graduate

Despite the well-established use of organomagnesium reagents, research into the chemistry of the alkaline earth metals has received an increase in popularity over the last two decades, largely due to their applications as reagents and catalysts. Recently, sterically-demanding A-donor ligands have become more popular because of their ability to provide stable metal-donor interactions and achieve kinetic stabilisation of the complex by crowding the metal centre. Furthermore, the isolation of subvalent magnesium and calcium complexes which challenged established knowledge of the metals prompted further investigations into their stabilisation in low oxidation states.

Olefin metathesis is the most powerful, versatile reaction in current use for the formation of new carbon-carbon bonds. While metathesis has been known for over 60 years, it has only recently been implemented into pharmaceutical and specialty chemical manufacturing. The slow uptake of olefin metathesis can be attributed in part to low catalyst productivity, a consequence of short catalyst lifetime. Improving catalyst activity is critical for the advancement of metathesis. This improvement can be achieved through greater understanding of the catalysts and their limitations. The ability to perform metathesis in aqueous media is desirable, but as yet largely unrealized, for the modification of water-soluble, biologically-relevant substrates. At present, high catalyst loadings are necessary even for less demanding metathesis reactions in water. The limited mutual solubility of the catalyst and substrate in water are one limitation. Examined in this thesis are more fundamental challenges associated with catalyst deactivation by water. The impact of water on catalyst productivity was assessed for both the second-generation Grubbs catalyst GII, and the phosphine-free Hoveyda catalyst HII, in ring-closing and cross-metathesis reactions. Water was shown to have a negative impact on metathesis productivity, owing to catalyst decomposition. The decomposition pathway was catalyst-dependent: GII was found to decompose through a pathway in which water accelerated abstraction of the methylidene ligand by dissociated phosphine. For HII, water was found to decompose the metallacyclobutane intermediate. A β-hydride transfer mechanism was proposed, to account for the organic decomposition products observed. Chapter 4 focuses on problems encountered during the synthesis of ruthenium catalysts, and presents improved methods. An updated method was developed for the synthesis of phenyldiazomethane, the principal source of the alkylidene ligand required in synthesis of GI. Challenges in use of the phosphine-scavenging resin Amberlyst-15 resin are discussed. Improving synthetic routes to the important first- and second-generation Grubbs catalysts will aid in expansion of olefin metathesis methodologies, particularly in the industrial context, in which batch-to-batch reproducibility is paramount.

The development of new, efficient methods for the formation of carbon-carbon bonds using transition metal catalysis has broad applications in the field of organic chemistry and is the key to efficient chemical synthesis. Many efforts had been made to develop efficient ways to make these linkages particularly with the aid of metals such as Rh, Pd, Ni, Ru and Cu. Our group is primarily focused on exploring how these transition metals can activate typically inert functional groups, paving way to new synthetic routes to construct more complex molecules. Chapter 1 describes attempts that were conducted to achieve hydroacylation between an aldehyde and a non-conjugated alkene via a metal hydride intermediate. The use of RuHCl(CO)(PPh3)3 proved to be the most efficient catalyst for this transformation thus far. Mechanistic investigations were conducted to explore different possibilities to enable this transformation. This chapter also identifies a new self-aldol domino reaction, which consists of a self-aldol condensation of an aldehyde, followed by oxidation and decarbonylation giving rise to a ketone product. Finally, the use of a simple and direct method to access deuterated aldehydes using RuHCl(CO)(PPh3)3 as a catalyst and D2O as a deuterium source is outlined. Chapter 2 describes a novel Suzuki-Miyaura system that couples esters and boronic esters to form the corresponding ketone product. It was found that an NHC-based Pd catalyst is crucial in the transformation wherein it activates the C(acyl)-O bond of the ester. It is notable that this transformation takes place with the absence of decarbonylation. Reactivity under water in the presence of surfactants was also discovered. Results in aqueous media were demonstrated to be milder than in organic conditions, while achieving similar yields. This system was also applied to coupling of esters and anilines.

There are numerous classes of N-heterocyclic carbenes (NHCs) that have been synthesized since the discovery of stable NHCs in 1988. Their application as ligands in metal complexes has received much attention because of their strong sigma-donor and poor pi-acceptor properties. Within these NHC metal complexes, we are interested in studying zirconium metal complexes with pincer NHC ligands. Recently, achiral CCC-NHC pincer zirconium complexes were synthesized and their catalytic activity in intramolecular hydroamination of aminoalkenes were reported. Herein is reported new reaction conditions which yield pure, chiral CCC-NHC Zr pincer mono(amido) dibromo complex. The enantiopure crystal structure of the same complex is reported. Attempts to synthesize chiral CCC-NHC Zr pincer bis- and tris- amido complexes with the iodo salt of the ligand precursor are summarized. Moreover, syntheses of chiral bis(imidazolinium) ligand precursors with different counter anions are reported with optimized reaction conditions.

The coordination- and organometallic chemistry of uranium complexes bearing the non-carbocyclic ancillary ligand XA2 (4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) has been developed as a major focus of this thesis. A number of air-sensitive actinide chloro complexes and alkyl derivatives featuring reactive An–C bonds were prepared, and investigated using a variety of structural and spectroscopic analytical techniques, including X-ray diffraction, NMR spectroscopy, elemental analysis, and electrochemical methods. The research described in this thesis serves to expand the currently underdeveloped, fundamental chemistry of actinide complexes supported by non-carbocyclic (i.e. non-cyclopentadienyl) ligands. For example, the use of the prototypical xanthene-based ligand XA2 has led to neutral dialkyl uranium(IV) complexes which a) react with alkyl anions to yield anionic trialkyl ‘ate’ complexes, b) C–H activate neutral pyridines to yield organouranium(IV) species featuring cyclometalated pyridine-based ligands, and c) react with Lewis acids to yield rare examples of cationic monoalkyl uranium(IV) complexes featuring coordinated arene ligands. By altering the nature of the arene solvent/ligand, latent catalytic ethylene polymerization behaviour has also been unlocked in cationic XA2 uranium and thorium complexes, and this development may offer industrial relevance. Additionally, new NON-donor ligand designs featuring bulky terphenyl-based substituents (the "XAT" ligand) as well as 1-adamantyl groups (the "XAd" ligand) have been developed; a family of crystallographically-characterized dipotassium XAT complexes have been prepared which feature unprecedented potassium–alkane interactions, and the XAd ligand has been employed for the development of new organometallic thorium chemistry. The developments described in this thesis contribute to an emerging field and delineate new reactivities and structural motifs, providing important steps forward in organoactinide chemistry. / Thesis / Doctor of Philosophy (PhD)

<p>While crucial advancements have been made in understanding transition metal−nitrogen interactions, the actinides have not been studied in such depth as their transition metal counterparts. Uranium has shown to catalyze the Haber−Bosch process to produce NH₃ but more attention has turned to transition metals such as iron due to their low cost and accessibility. It is thought that transition metal imido species are essential intermediates to this process; therefore, it is critical to understand NH bond cleavage and formation on the metal. To study the potential that uranium has, it is important to bridge the knowledge gap of uranium with its transition metal counterparts and further understand NH bond cleavage and formation on the metal to make the suspected imido intermediate.</p> <p>Redox neutral methods have been popular and effective for synthesizing uranium imido complexes such as starting with a uranium(IV) amide and deprotonating it with a base to yield its respective uranium(IV) imido. It was of interest to understand if the bisTp* uranium(III) system would be amenable to a deprotonation pathway. To test this, the reactivity of Tp*₂UBn with bulky 4-(2,6-di(pyridin-2-yl)pyridin-4-yl)benzenamine (terpy-aniline) and sterically smaller p-toluidine (ptol-aniline) was explored to first synthesize uranium(III) anilido species. Following successful synthesis, their reactivity is explored to yield respective uranium(IV) imido species by oxidative deprotonation.</p> <p>In addition to redox neutral methods, synthetic processes that rely on redox reactions at the uranium center have also been successful but are less common since the starting material must be a stable, low-valent uranium species. Our group has explored this method to make uranium(IV) imido species where the addition of 1 equivalent of organic azide to trivalent Tp*₂UBn or one equivalent of organic azide and potassium graphite to Tp*₂UI results in the formation of uranium(IV) imido species. The downside to this is azides are explosive and their synthesis could inhibit synthesis of diverse complexes. A redox method that eliminated usage of explosive azides is of interest so the reactivity of hydrogen atom transfer (HAT) reagents, Gomberg’s dimer or the 2,4,6-tri-tBu-phenoxy radical (·OMes*), with uranium(III) anilido complexes of varying steric bulk and electronic profile was explored. Conversion to their respective uranium(IV) imido species was achieved and this method was also explored with uranium(III) amides smaller than a phenyl since their respective azide are too dangerous to synthesize.</p> <p>Following isolation of uranium(III) anilido complexes and exploring reactivity it was of interest to understand how they compare to phosphorus analogues and how reactivity and interactions might be similar. Reactivity of Tp*₂UBn with phosphines of various steric bulk and electronic profile allowed for the isolation of uranium(III) phosphido complexes and their reactivity showed to be different than previously explored uranium(III) anilido counterparts. The electronic differences of the pnictogens were also observed in the crystal structures.</p> <p>With the differences in reactivity and electronic effects between the nitrogen and phosphorous complexes having been observed, our curiosity expanded to explore more uranium-pnictogen interactions. Therefore, synthesis of bis-substituted arsine and bis-substituted phosphine ligands were conducted for reactivity with Tp*₂UBn. Preliminary data reveals these bonds are more unstable and reactive relative to uranium(III) anilido species, likely due to the electronic mismatch between oxophilic uranium and soft pnictogens. Where applicable, compounds were characterized by multinuclear NMR spectroscopy, infrared spectroscopy, electronic absorption spectroscopy, single crystal X-ray crystallography, and quantum chemical calculations.</p>

<p>  </p> <p>This thesis will focus on two broad topics: the synthesis and characterization of various diruthenium aryl compounds and of mono- and bis-alkynyl unsymmetric compounds based on Ru(II)(dppm)₂ and Ru(II)(dppe)₂ bridges (dppm = 1,2-bis(diphenylphosphino)methane; dppe = 1,2-bis(diphenylphophino)ethane).</p> <p>Chapters 1–3 focus on multiply bonded metal–metal compounds, utilizing four different ‘paddlewheel’ motifs (dinuclear ruthenium units that are supported by four bidentate ligands). These highly stable mono- and bis-aryl diruthenium compounds are readily prepared using lithium-halogen exchange reactions. Two different oxidation states have been accessed, Ru₂(II,III) and Ru₂(III,III), through modification of the paddlewheel ligands or coordination of a small, π-accepting ligand at the vacant ruthenium site in Ru₂(<em>ap</em>)₄(Ar) compounds (<em>ap</em> = 2-anilinopyridinate; Ar = aryl). Chapter 1 discusses the modification of the bidentate ligand to yield two unique Ru₂(<em>ap</em>')₄(Ar) series, which both exhibit improved solubility over the previously reported un-modified Ru₂(<em>ap</em>)₄(Ar) series, and the structural, electronic, and optical characterizations of the compounds within these two new Ru₂(II,III) series. Chapter 2 builds upon our lab’s previous studies on electron transfer between the two ruthenium centers in [Ru₂(<em>ap</em>)₄]₂(μ-C≡C)_x compounds and applies this towards synthesizing and characterizing mixed-valency within a Ru₂(III,III) phenylene bridged compound [(NC)Ru₂(<em>ap</em>)₄]₂(μ-1,4-C₆H₄). Chapter 3 highlights the synthesis and characterization of bis-aryl and bis-alkynyl Ru2(III,III) compounds, Ru₂(amtfmp)₄(Y)₂ (Y = -C≡CPh, -Ph), supported with the electron-withdrawing paddlewheel ligand amtfmp (amtfmp = 2-amino-3-(trifluoromethyl)pyridinate). </p> <p>Chapters 4 and 5 are focused on the synthesis and characterization of both mono- and bis-alkynyl unsymmetric compounds to study photo-induced electron transfer (PET) processes. Chapter 4 features as an introduction to the synthesis of these Ru(II)(dppm)₂ and Ru(II)(dppe)₂alkynyl compounds along with some material applications. Chapter 5 discusses the mono- and bis-alkynyl compounds based on Ru(II)(dppm)₂ and Ru(II)(dppe)₂ bridges that utilized a highly electron-withdrawing chromophore ‘acceptor’ ligand, NAP^R (R = isopropyl, mesityl), to generate the <em>B-A</em> (mono-alkynyl) and <em>D-B-A</em> (unsymmetric bis-alkynyl) compounds.</p>