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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.

This thesis is concerned with the problems of sample preparation arising in the study of lead isotope abundances. The importance of this study to geophysics has been amply shown by R.D. Russell, R.M. Farquhar, F.G. Houtermans, J.T. Wilson, H.F. Ehrenberg and many others. Chapter 1 gives an outline of lead isotope measurement techniques, including types of mass spectrometers generally used and some of the problems encountered. The mass spectrometer used in the present research was designed and constructed by R.D. Russell and F. Kollar and descriptions of it will be found in their publications and in F. Kollar's Ph.D. thesis. The present techniques of producing lead tetramethyl for isotopic analysis from ore samples are discussed in Chapter 2. The remaining chapters deal with the purification of lead tetramethyl for mass spectrometer analysis, using vapour phase chromatography. This technique has found immediate application in the precise intercomparison of lead samples recently carried out in the Geophysics Laboratory at the University of British Columbia by F. Kollar and others (F. Kollar, R.D. Russell and T.J. Ulrych, in press). The long range object for developing this technique is to purify lead tetramethyl prepared by free methyl radicals reacting with metallic lead (cf. A.J. Surkan 1956) prior to isotopic analysis. The presence of impurities in samples prepared this way has discouraged the development of this method in the past. The final chapter deals with this aspect of the proposed problem. This thesis is intended as a preliminary to the writer's Ph.D. research which will also deal with isotopic lead analysis. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

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.

The redox behavior of a series of organometallic complexes containing Cp'M(NO) groups (Cp' = ƞ⁵-C₅H₅(Cp) or ƞ⁵-C₅Me₅(Cp*) ; M = Mo or W) has been investigated both by cyclic voltammetry and by chemical means. The neutral 16-electron Cp'Mo(N0)X₂ compounds (X = CL, Br or I) undergo a single, essentially reversible, one-electron reduction in CH₂CL₂/O.1M [n-Bu₄N]PF₆ at relatively low potentials (<-0.1 V vs SCE). The electrochemically observed reductions can be effected on a preparative scale by employing CP₂C0 as the chemical reductant. The isolable 17-electron [Cp'Mo (NO)X₂]•⁻ radical anions are cleanly reconverted to their 16-electron neutral precursors by treatment with [Cp₂Fe]BF₄. In contrast, the Cp'W(NO)I₂ compounds undergo rapid decomposition to their [Cp'W(NO)I]₂ monohalo dimers upon electrochemical reduction. Electrophiles NE⁺ (E = O or ϱ-O₂NC₆H₄N) undergo unprecedented insertions into the Cr-C ϭ-bonds of CpCr(NO)₂R complexes (R = Me, CH₂SiMe₃ or Ph) to afford [CpCr(N0)₂{N(E)R}]⁺ cationic complexes. Present evidence is consistent with these insertions occurring via charge-controlled, intermolecular attacks by NE⁺ at the Cr-R groups in classical SE2 processes. The newly-formed N(E)R ligands function as Lewis bases through nitrogen atoms toward the formally 16-electron [CpCr(NO)₂]⁺ cations and may be displaced from the chromium's coordination sphere by the more strongly coordinating CL⁻ anion. The resulting CpCr(NO)₂CL can be reconverted to CpCr(NO)₂R. thereby completing a cycle by regenerating the initial organometallic reactant. The entire sequence of stoichiometric reactions forming the cycle thus constitutes a selective method for the formation of new carbon-nitrogen bonds, the net organic conversions mediated by the CpCr(NO)₂ group being NE⁺ + R⁻ → N(E)R. The electrophilic [Cp'M(NO)₂]⁺ cations (Cp'=Cp or Cp* ; M = Cr, Mo or W) condense with methyl propiolate and 2,3-dimethyl-2-butene to afford cationic organometallic lactone complexes. These complexes undergo facile ⍜-dealkylation to yield the neutral Cp'M(NO)₂(ƞ¹-lactone) derivatives. Furthermore, the neutral Cp'W(NO)₂(ƞ¹-lactone) compounds decompose in air to their Cp'W(O)₂(ƞ¹-lactone) dioxo products. / Science, Faculty of / Chemistry, Department of / Graduate

Although hydrides of metal carbonyls are widely known, the number of hydrides in the related family of metal nitrosyls is extremely small. The preparation of a series of nitrosyl hydrides from the treatment of [CpW(NO)I₂]₂ (Cp=ƞ⁵-C₅H₅) with Na[H₂Al(OCH₂CH₂OCH₃)₂] is described. The addition of one or two equivalents of the aluminum reagent results in the formation of [CpW(NO)IH]₂ or [CpW(NO)H₂]₂ respectively. The reaction of [CpW(NO)IH]₂ with a Lewis base (L=P(OPh)₃, P(OMe)₃, PPh₃ or PMe₃) gives the monometallic CpW(NO)IHL, while [CpW(NO)H₂]₂ reacts with P(OPh)₃ or P(OMe)₃ to yield [CpW(NO)HL]₂ which undergoes further reaction to give CpW(NO)H₂L. Proton NMR spectroscopy shows that all bimetallic species contain bridging hydride ligands and are therefore best, formulated as [CpW(NO)1]₂(µ-H)₂, [CpW(NO)H]₂(µ-H)₂ and [CpW(NO)L]₂(µ-H)₂. The ¹H NMR spectrum of [CpW(NO)H]₂(µ-H)₂ shows that there is no hydride ligand exchange on the NMR time scale and that ¹jH(terminal)W ≃ ¹jH(bridging)w > ²jHW. From this finding, it is possible to develop new criteria for assessing the static or fluxional nature of hydride ligands for several families of organotungsten hydrides (Cp₂W, CpW(CO)₃, W(CO)₃ and CpW(NO)x (x=l or 2)). Within each family, the magnitude of ¹JHW strongly reflects the type of metal hydride bonding, i.e. [Formula Omitted] and suggests that bridge bonding involves all the atoms in the bridge and therefore the "fused" notation is introduced. Treatment of CpW(NO)(CH₂SiMe₃)₂ with low pressures of H₂ (60-80 psig) in the presence of Lewis bases (L=P(0Ph)₃, PMePh₂) gives the unusually stable alkyl hydride compounds CpW(NO)(H)(CH₂SiMe₃)L. This chemistry is then extended to the Cp* (Cp*=ƞ⁵ -C₅Me₅) analogues, including the preparation of the appropriate starting materials. Upon thermolysis of Cp*W(NO)(H)(CH₂SiMe₃)(PMe₃) in C₆H₆, the intermolecular C-H activation product Cp*W(N0)(H)(C₆H₅)(PMe₃) is cleanly formed. However, intermolecular activation of CH₄, C₆H₁₂ or n-C₆H₁₄ does not occur under similar experimental conditions. Hydrogenolysis of Cp*W(NO)(CH₂SiMe₃)₂ at high pressures (≃920 psig) with no Lewis base present results in the formation of isolable [Cp*W(NO)H]₂(µ-H)₂ and [Cp*W(N0)H](µ-H)₂[Cp*W(N0)(CH₂SiMe₃)]. The latter is a new example of the rare class of dinuclear alkyl hydride complexes. Proton NMR spin tickling experiments on this compound allow the complete assignment of all couplings in the spectrum and show that ¹jH(terminal)W' ¹JH(bridging)W and ²jHW have the same sign. / Science, Faculty of / Chemistry, Department of / Graduate

The nitrosyl complex CpW(NO)R₂ (R = CH₂SiMe₃) forms 1:1 adducts via isonitrosyl linkages to Lewis acids such as AlMe₃ and Cp₃Er, i.e. CpWR₂(NO→A) (A = AlMe₃, ErCp₃). These adducts regenerate the starting dialkyl complex when treated with water. Protonation of CpW(NO)R₂ by HBF₄⋅0Me₂ can also be effected. Whether the site of protonation is the nitrogen or the oxygen atom of the nitrosyl ligand is not known with certainty, although O-protonation is postulated by analogy with the other Lewis-acid adducts of CpW(NO)R₂. In these adducts, the nitrosyl stretching frequency is shifted to lower wavenumbers relative to that of the parent dialkyl, to an extent which increases as harder Lewis acids are employed. The colour of the adducts also ranges from red to orange to yellow as progressively harder acids are used. Treatment of CpW(NO) (CH₂SiMe₃)₂ with [Ph₃C]⁺ PF₆⁻ in Ch₂CL₂ results in electrophilic cleavage of a carbon-silicon bond to yield the mixed dialkyl CpW(NO)(CH₂SiMe₃)(CH₂CPh₃), which has been fully characterized by spectroscopic methods and by a single-crystal X-ray crystallographic study. The formation of Me₃SiF and PF₅ (coordinated to Lewis bases in the reaction mixture) as by-products of this reaction has been confirmed by ³¹P and ¹⁹F NMR spectroscopy of the reaction mixture in CD₂CL₂. Preliminary attempts to extend this novel reaction of a silicon-containing ligand by using other carbocations were unsuccessful. This is attributed to the high reactivity of the required carbocations and the large number of possible reaction sites on the metal complex. Some reactions of the mixed dialkyl CpW(NO)RR¹ (R = CH₂SiMe₃ R¹ = CH₂CPh₃) were found to be analogous to those of the parent CpW(NO)R₂, while other reactions followed a different course because of the ability of the CH₂CPh₃ ligand to orthometallate. Thus, CpW(NO)RR¹ is much less thermally stable than CpW(NO)R₂. As a solid or a solution in non-coordinating solvents, it decomposes in a matter of days at room temperatures to a mixture of products which were not identified. In acetonitrile solution, an orthometallated complex derived from CpW(NO)RR¹ can be trapped by coordination of solvent. The product CpW(NO)(CH₂C(C₆H₄)Ph₂)(NCMe) has been isolated and crystallographically characterized. Cyclic voltammograms of CpW(NO)R₂ and CpW(NO)RR¹ show that both complexes undergo an apparently chemically reversible reduction and an irreversible oxidation. The mixed dialkyl CpW(NO)RR¹ is somewhat easier both to reduce and to oxidize than CpW(NO)R₂. Like CpW(NO)R₂, CpW(NO)RR¹ reversibly forms a 1:1 adduct with PMe₃. Also analogously to CpW(NO)R₂, it reacts with 0₂ to form a 5:1 mixture of dioxoalkyl complexes CpW(0)₂R and CpW(0)₂R¹, and with NO(g) to form 2 CpW(NO)R¹(ƞ² -0₂N₂R). In this product, insertion of NO has occurred exclusively in the W-CH₂SiMe₃ bond. Upon photolysis, both complexes CpW(NO)R¹¹(ƞ²-0₂N₂R) (R¹¹ = CH₂SiMe₃ or CH₂CPh₃) form dioxo alkyls CpW(O)₂R¹¹ in an unprecedented reaction. The ability of CpW(NO)RR¹ to orthometallate also results in the formation, when this complex is treated with sulphur, of CpW(O)(CH₂C(C₆H₄)Ph₂)-(SR). No analogue to this compound can be obtained from reaction of CpW(NO)R₂ with sulphur. The sequence of reactions leading to the formation of this product is not known. / Science, Faculty of / Chemistry, Department of / Graduate