The group 14 Zintl clusters [E₉]⁴⁻ (E = Ge, Sn, Pb) have been reacted with organometallic reagents in solution in the presence of alkali metal cation sequestering agents. The synthesis, characterisation and reactivity studies of the resultant complexes are reported herein. These negatively charged clusters reductively cleave one of the M–C bonds in the group 12 homoleptic organometallic reagents MR₂ (M = Zn, Cd; R = Ph, mes, ⁱPr) to yield η⁴-coordinated functionalised clusters closo-[E₉MR]³⁻. They can also activate both of the M–C bonds in Cd(mes)₂ to form metal-bridged dimers [Ge₉CdGe₉]⁶⁻ and [Pb₉CdCdPb₉]⁶⁻. Investigating the reactivity of the functionalised cluster [E₉CdPh]³⁻ (E = Sn, Pb) towards HSn(ⁿBu)₃ results in the synthesis of the novel closo-clusters [E₉CdSn(ⁿBu)₃]³⁻. The reaction of K₄Ge₉ with the heteroleptic organometallic reagent Fe(COT)(CO)₃ yields the metalated cluster anion [Ge₈Fe(CO)₃]³⁻, in which the nuclearity of the Zintl anion is altered upon coordination of the [Fe(CO)₃] moiety. Two side products have also been isolated as [K(2,2,2-crypt)]⁺ salts of [Fe(η³-C₈H₈)(CO)₃]⁻ and [Fe₂(η³, η′³-C<sub<16</sub<H₁₆)(CO)₆]²⁻. In the presence of highly reduced Zintl anions, all the M–C bonds in homoleptic mid-row transition metal organometallic reagents can also be cleaved. These ‘naked’ metal atoms have templated the formation of the endohedral clusters [Fe@Ge₁₀]³⁻, [Fe@Sn₁₀]³⁻ and [Mn@Pb₁₂]³⁻. These clusters adopt very different geometries and the electronic origin of their structures has been investigated in-depth by DFT calculations. Structural characterisation of some side products is also reported for: [E₉(mes)]³⁻ (E = Ge, Sn) and [Ge₉Mn(mes)]³⁻.

The work presented in this thesis mainly describes the development of new reactions of &beta;-lithiooxyphosphonium ylides to access stereodefined substituted alkenes in a highly convergent fashion. Firstly, &beta;-lithiooxy ylides prepared from aldehydes and phosphonium ylides were shown to react with halogen electrophiles to provide a highly stereoselective route to E-alkenyl bromides and iodides. This methodology was successfully applied to the first total synthesis of naturally occurring (5E,9Z)-6-bromohexadeca-5,9-dienoic acid. Secondly, an experimentally straightforward method was developed for the stereocontrolled formation of trisubstituted Z-allylic esters by in situ trapping of &beta;-lithiooxyphosphonium ylides with readily available halomethyl esters. The synthetic utility of this methodology was demonstrated with the synthesis of plaunotol [(2Z,6E)-2-((E)-4,8-dimethylnona-3,7-dien-1-yl)-6-methylocta-2,6-diene-1,8-diol] and the first asymmetric synthesis of the naturally occurring geranylgeraniol-derived diterpene (6S,7R,Z)-7-hydroxy-2-((E)-6-hydroxy-4-methylhex-4-enylidene)-6,10-dimethylundec-9-enyl acetate. Furthermore, the chemistry of &beta;-lithiooxyphosphonium ylides was expanded to access synthetically useful disubstituted Z-allylic esters. The synthetic utility of Z-allylic esters was also demonstrated in a versatile and diastereoselective Ireland-Claisen rearrangement to access &gamma;,&delta;-unsaturated acids. Finally, the synthesis of the side-chain of the 6,7-dideoxysqualestatin H5 was also investigated. It was demonstrated that the side-chain of 6,7-dideoxysqualestatin H5 could be accessed by a convergent and stereoselective organozinc-based strategy.

This Thesis describes the use of alkaline earth and rare earth complexes bearing phenolate ligands as catalysts in the amine-initiated, immortal ring opening polymerisation (ROP) of cyclic esters. Mechanistic elucidation was performed and two propagation pathways are presented. Chapter One introduces cyclic esters and catalytic routes to polyesters by ROP. Common techniques for polymer characterisation are described and an overview of relevant phenolate-supported ROP catalysts is given. Reversible chain transfer in ROP is also discussed. Chapter Two describes the synthesis and characterisation of zwitterionic Group 3 complexes bearing bis(phenolate)-amino ligands and the development of the amine-initiated, immortal ROP methodology using this class of catalyst. Detailed studies into the ROP of rac-lactide using amines and a zwitterionic yttrium complex are presented and the mechanism of amine-initiated, immortal ROP was derived. Chapter Three documents further amine-initiated, immortal ROP studies using a zwitterionic yttrium complex as the catalyst. The preparation of multiarm polymers is described and further investigations using the cyclic esters, ε-caprolactone and rac-β-butyrolactone are presented. Chapter Four describes the use of Group 2 and lanthanide phenolate complexes in the amine-initiated ROP of rac-lactide. Bulk polymerisation studies revealed the generality of the amine-initiated, immortal ROP methodology and an alternative propagation pathway was derived from mechanistic studies. Chapter Five details the synthesis and characterisation of Group 3 amide complexes supported by phenolate-amino ligands. Each complex was screened for ROP capability and amine co-initiators were employed. Chapter Six contains experimental details and characterisation data for the new complexes and polymer products described in this Thesis. CD Appendix contains crystallography .cif files, supporting information for each Chapter and spreadsheets containing polymerisation data.

This Thesis reports the synthesis and reactions of new hydrazide, alkoxyimide and benzimidamide complexes (L)Ti=NX (X = NAr2, NOtBu or C(Ar)NO^tBu; L = dianionic supporting ligand or ligand set). The work is supported by DFT calculations which are used to rationalise the reaction outcomes observed and, in one case, the bonding in alkoxyimide complexes. <b>Chapter One</b> provides a background to hydrazide complexes, starting with their relevance to nitrogen fixation. In addition, Group 4 imide, alkylidene hydrazide and alkoxyimide complexes are also reviewed. The Chapter focuses in particular on the synthesis, structure, and stoichiometric and catalytic reactions of these complexes with unsaturated substrates. <b>Chapter Two</b> describes the development of the virtually unexplored 1,2-diamination reaction. The substrate scope and isolation of the vinylamine products are discussed. The protonation of the vinylimide complex Ti(N2N^Me){NC(Ph)C(Me)NPh2}(py) and the overall diamination reaction itself is then explored through an in-depth experimental and computational study. <b>Chapter Three</b> details the synthesis of cyclopentadienyl-amidinate supported alkoxyimide complexes. The first detailed reactivity study, supported by structural and computational studies, of any alkoxyimide complex is reported. Novel reactivity at Ti=Nα and, in one instance, Nα–Oβ reductive bond cleavage is observed. <b>Chapter Four</b> describes the reactivity of the benzimidamide complex Cp*Ti{PhC(NⁱPr)2}{NC(Ar^F5)NO^tBu} with a range of substrates including heterocumulenes, aldehydes, isonitriles and B(Ar^F5)3. Novel reactivity at Ti=Nα, and 3-component coupling is presented, and the experimental results supported by structural and computational studies. <b>Chapter Five</b> presents full experimental procedures and characterising data for the new complexes reported.

In the following thesis, new methodologies towards the synthesis of a range of sulfonyl (-SO₂-) containing functional groups are documented. These methods utilise easy-to-handle sulfur dioxide surrogates, such as DABSO (vide infra), and exploit palladium catalysis as a new mechanistic protocol for the incorporation of the -SO2- unit. <b>Chapter 1</b> is a literature review surveying sulfur dioxide in organic synthesis, the established uses of SO₂ surrogates and the importance of the sulfonyl moiety in chemistry. Palladium-catalysed (carbonylative) cross-couplings are also broadly discussed as they provide inspiration for, and mechanistic similarities with, the proposed chemistry. <b>Chapter 2</b> describes a de novo synthesis of the sulfonamide functional group; a three-component and convergent methodology coupling (hetero)aryl and alkenyl halides with sulfur dioxide (provided by easy-to-handle surrogates such as DABSO) and hydrazine nucleophiles, is documented. This is achieved through the action of a readily available palladium catalytic system and is the first example of a metal-catalysed sulfonylative cross-coupling of halide based electrophiles. <b>Chapter 3</b> presents a new method of generating (hetero)aryl and alkenyl sulfones. The ability of organometallic reagents to add to sulfur dioxide (supplied via DABSO) is applied to deliver the corresponding metal sulfinate salt. This in situ derived sulfinate is coupled with an (hetero)aryl or alkenyl (pseudo)halide using palladium catalysis to form the desired sulfone. An electronically modified XantPhos-type ligand was designed for the reaction in order to suppress unwanted aryl-aryl exchange. <b>Chapter 4</b> documents the generation of (hetero)aryl and alkenyl sulfinates from the corresponding halide and DABSO through a palladium-catalysed sulfination protocol, obviating the need for organometallic reagents. A mild set of conditions using IPA as both a solvent and reductant together with a low loading of palladium catalyst offers an attractive route to sulfonyl compounds thanks to the in situ derived sulfinates being converted into a broad variety of functional groups via established onwards reactivity. <b>Chapter 5</b> discusses the conclusion of the research and the potential for future work. <b>Chapter 6</b> presents the experimental data.

This thesis discusses the intermolecular hydroacylation reaction using cationic rhodium bis- phosphine complexes as catalysts. A series of small bite-angle rhodium bis-phosphine complexes have been prepared and characterised. The reactivity of these complexes has been investigated in order to gather information about the effect of subtle changes in the ligand design and they are compared to the previously reported catalysts. Chapter 2 presents the challenges involved in the synthesis of small bite-angle isopropyl and cyclohexyl PNP and PCP bis-phosphine ligand containing rhodium complexes. These complexes have been fully characterised and screened in intermolecular hydroacylation reaction using 2- (methylthio)benzaldehyde (<strong>E</strong>) and 1-octene or 1-octyne as substrates. The formed complexes were shown to be very efficient and regioselective alkyne hydroacylation catalysts. The mechanism of the hydroacylation reaction was investigated using the isopropyl PNP complex [Rh(ⁱPrPNMePⁱPr₂)(C₆H₅F)][BAr^F₄] (<strong>11b</strong>). Chapter 3 concentrates on developing new rhodium bis-phosphine complexes that involve a ligand incorporating the small bite-angle motif with the one of hemilability. The PNP complex [Rh((2-OMe-C₆H₄)₂PNMeP(2-OMe-C₆H₄)2)(C₆H₅F)][BAr^F₄] (<strong>41</strong>) was synthesised and analytically characterised. <strong>41</strong> was shown to be an active alkyne hydroacylation catalyst with more stability towards the catalyst deactivation pathway, reductive decarbonylation, compared to the previously investigated <strong>11b</strong>. Additionally mechanistic studies using <strong>41</strong> were carried out. The final chapter moves on to study the C-S activation ability of small bite-angle rhodium bis- phosphine complexes to remove the sulfur tether from the hydroacylation products at the end of the hydroacylation reaction. A screening is conducted to compare the reactivity of different small bite-angle ligands. Additionally, a detailed investigation is carried out to see the effect the C-S activation has on the hydroacylation reaction.

This thesis explores the area of the intermolecular hydroacylation reaction, catalysed by rhodium diphosphine complexes. A range of latent low-coordinate rhodium diphosphine complexes have been synthesised, and their catalytic activity for the hydroacylation reaction has been investigated. In particular, emphasis has been placed on understanding how subtle changes in diphosphine steric properties affect, and can be used to control, selectivity of this catalysis. Chapter 2 presents investigations into rhodium complexes incorporating the potentially hemilabile P-O-P ligands: POP’, XANTphos and Xphos. The resulting complexes have been fully characterised and their activity for the catalytic intermolecular hydroacylation of aldehyde I (HCOC₂H₄SMe) and alkene II (H₂C=CHCO₂Me) established and compared to the DPEphos system. Further reactivity of Xphos for aromatic aldehyde V (HCOC₆H₄SMe) and alkene II, and aldehyde V and alkyne XI [HC≡CC₆H₃(CF₃)₂] has also been explored, and compared with the catalytic activity of {Rh(PPh₃)₂}⁺. Focus moved from potentially hemilabile ligands to chelating diphosphine ligands of the type PPh₂(CH₂)nPPh₂ (where n = 2-5), and then on to ortho-substituted bulky analogues of the type P(₀-C₆H₅R)₂(CH₂)₂P(₀-C₆H₅R)₂ (where R = Me and ⁱPr) complexed to rhodium. Chapter 3 outlines the complexes synthesised, and their activity for the catalytic intermolecular hydroacylation of aldehyde I and alkene II, aromatic aldehyde V and alkene II or aldehyde V and alkyne XI. Possible explanations for the observed switch in selectivity from alkene to aldehyde hydroacylation, and linear alkyne to branched alkyne hydroacylation, have been explored and are detailed. The final chapter concerns the structure of an interesting catalytic intermediate: the branched alkenyl species for the {Rh(DPEphos)}+ catalysed hydroacylation of aldehyde V and alkyne XI. Investigations into the kinetic and catalytic behaviour of this system were carried out, and a reaction scheme has been proposed which correlates well with kinetic modelling undertaken by Prof. Guy Lloyd-Jones of the University of Bristol.

This thesis examines the effects of coordinating highly sterically demanding and strongly electron donating saturated N-heterocyclic carbenes (NHCs) at late transition metal centres. Chapter III details the synthesis of a range of iridium complexes of the type (NHC)2IrHxCly [x = 1, 2; y = 0, 1], bearing the saturated NHCs 5-Mes, 6-Mes and 7-Mes. Unusually facile activation chemistry is observed in the reaction of [Ir(COE)2Cl]2 with 6-Mes and 7-Mes to form the doubly cyclometallated species (6-Mes')2IrH and (7-Mes')2IrH, which were fully characterised. The responses of these complexes to the addition of dihydrogen and HCl were studied, leading to the controlled synthesis of range of precursors to 14-electron iridium cations. In Chapter IV the formation of low valent iridium cations with weakly coordinating anions is targeted. Isolation of the cationic complexes [(NHC)(NHC')IrH][BArf4] and [(NHC)2IrH2][BArf4] (NHC = 6-Mes, 7-Mes) showcases the stabilising power offered by these expanded ring systems. This allowed the study the interaction of these low valent species with a range of amine-borane substrates which are known to be readily dehydrogenated. Thermodynamic data on the C-H bond activation processes occurring at these iridium centres were able to be obtained due to facile, reversible oxidative addition of C-H bonds across the 14-electron iridium. Chapter V focuses on the effects of increasing the steric bulk of these NHCs to limit the coordination of multiple ligands at the metal centre. Use of 2,6-diisopropyl-phenyl (Dipp) groups on the expanded ring NHCs, instead of mesityl groups, leads to an unprecedented mode of reactivity with [Ir(COE)2Cl]2. Activation and cleavage of C-N bonds in the carbene ring is observed, resulting in an open chain ligand chelating to the metal centre. Activation of the backbone in this manner has allowed the synthesis of saturated NHCs bearing a weakly coordinating anion on the ring. Here the first example of an anionic, saturated NHC is reported. In Chapter VI these highly sterically demanding NHCs are exploited to stabilise active species in low valent gold chemistry. The extreme steric bulk of the 6-Dipp ligand disfavours reduction of Au(I) to Au(0), however the resulting cation is observed to interact strongly with the weakly coordinating anion, [BArf4]-. Thus, attempts were made to optimise the anion and conditions to isolate a catalytically relevant intermediate. The strong donating power of these expanded ring NHCs is also exploited to activate gold hydride complexes of the type (NHC)AuH (NHC = 6-Dipp, 7-Dipp). Analogues of [H3]+ containing gold atoms ([{LAu}2H]+ and [LAuH2]+) supported by expanded ring NHCs were also targeted.

<div>Cyclopropanes are commonly found throughout synthetic and natural biologically active compounds. The Simmons–Smith cyclopropanation reaction is one of the most useful methods for converting an alkene into a cyclopropane. Zinc carbenoids are the active intermediate in the reaction, capable of delivering the methylene unit to a broad variety of substrates. Significant advances have been made in the field to increase overall efficiency of the reaction including the use of diethyl zinc as a precursor and allylic alcohols as directing groups.</div><div>Despite the many notable contributions in zinc carbenoid chemistry, persistent limitations of the Simmons–Smith reaction still exist. Zinc carbenoids exhibit poor steric discrimination in the presence of a polyolefin with minimal electronic bias. Additionally, due to the electrophilic nature of zinc carbenoid intermediates, the reaction performs inefficiently with electron-deficient olefins. Finally, alkyl-substituted zinc carbenoids are known to be quite unstable, limiting the potential for substituted cyclopropanation reactions.</div><div>In this work, we demonstrate that cobalt catalysis can be utilized to access novel cyclopropane products through the activation of dihaloalkanes. The content of this thesis will focus on the limitations of Zn carbenoid chemistry and addressing them with cobalt catalyzed, reductive cyclopropanations. In addition to this reactivity, we also demonstrate the dimethylcyclopropanation of activated alkenes to furnish valuable products applicable to natural product synthesis and pharmaceutically relevant compounds. Finally, we will show the unique character of the cobalt catalyzed cyclopropanation reaction through mechanistic experiments and characterization of reaction intermediates. In whole, these studies offer a complementary method to zinc carbenoid chemistry in producing novel and diverse cyclopropane products.</div>

<p>A series of uranium benzyl compounds supported by two hydrotris(3,5-dimethylpyrazolyl) borate (Tp*) ligands has been synthesized and characterized. In addition to the previously reported Tp*₂U(CH₂Ph) (<b>2-Bn</b>), examinations of both steric (<i>tert</i>-butyl, <i>iso</i>-propyl) and electronic (methoxy, picolyl) changes on the aromatic ring led to the formula Tp*₂U(CH₂Ar) (Ar = 4-<i>tert</i>-butylphenyl (<b>2-<i>^t</i>Bu</b>), 4-isopropyl (<b>2-ⁱPr</b>), 2-picolyl (<b>2-pyr</b>), 3-methoxyphenyl (<b>2-OMe</b>). Treatment of the entire series of benzyl compounds with azidotrimethylsilane results in the formation of a neutral, monomeric U(III) compound, Tp*₂U(N₃) (<b>3-N₃</b>), and substituted benzyltrimethylsilane. While there was no observed change in reactivity among the benzyl compounds and Me₃SiN₃, treatment of these compounds with triphenylphosphine oxide saw unique carbon-carbon coupling occur for three of the substituted benzyl compounds. With a single equivalent of OPPh₃, the following products were isolated: Tp*₂U[OP(C₆H₅)(C₆H₅CH₂C₆H₅)] (<b>4-Ph</b>), Tp*₂U[OP(C₆H₅)(C₆H₅-<i>p</i>-<i>i</i>PrC₆H₄)] (<b>4-ⁱPr</b>), Tp*₂U[OP(C₆H₅)(C₆H₅-<i>p</i>-<i>t</i>BuC₆H₄)] (<b>4-<i>^t</i>Bu</b>), Tp*₂U[OP(C₆H₅)(C₆H₅-<i>m</i>-OCH₃C₆H₄)] (<b>4-OMe</b>). </p> <p> A family of uranium(IV) imido complexes of the form Tp*₂U(NR) (R = benzyl (<b>7-Bn</b>), <i>para</i>-tolyl (<b>7-Tol</b>), <i>para</i>-methoxyphenyl (<b>7-OMe</b>), 2,6-diethylphenyl (<b>7-detp</b>), 2,6-diisopropylphenyl (<b>7-dipp</b>)) have been generated by bibenzyl extrusion from <b>2-Bn</b>. When <b>7-Bn</b> and <b>7-Tol</b>, along with previously reported Tp*₂U(N-Ph) (<b>7-Ph</b>) and Tp*₂U(N-Ad) (<b>7-Ad</b>), are treated with isocyanates or isothiocyanates, they readily undergo [2π+2π]-cycloaddition to generate κ²-ureato and κ²-thioureato derivatives, respectively. Use of phenylisoselenocyanate with <b>7-Tol</b> and <b>7-Ph</b> generates a rare κ²-selenoureato complex. Treating <b>7-Tol</b> and <b>7-OMe</b> with benzonitrile or 4-cyanopryidine results in unusual products of multiple bond metathesis, namely κ¹-amidinate U(IV) complexes. </p> <p>A family of dinuclear bis(Tp*) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) uranium compounds with conjugated organic linkers was synthesized to explore possible electronic communication between uranium ions. Trivalent diuranium phenyl alkynyl compounds, Tp*₂UCC(1,3-C₆H₄)CCUTp*₂ (<b>14-<i>meta</i></b>) or Tp*₂UCC(1,4-C₆H₄)CCUTp*₂ (<b>14-<i>para</i></b>), and tetravalent diuranium phenylimido compounds, Tp*₂U(N-1,3-C₆H₄-N)UTp*₂ (<b>15-<i>meta</i></b>) and Tp*₂U(N-1,4-C₆H₄-N)UTp*₂ (<b>15-<i>para</i></b>), were generated from trivalent Tp*₂UCH₂Ph. All compounds were fully characterized both spectroscopically and structurally. The electronic structures of all derivatives were interrogated using magnetic measurements, electrochemistry, and were the subject of computational analyses. All of this data combined established that little electronic communication exists between the uranium centers in these trivalent and tetravalent diuranium molecules.</p> <p>Uranium mono(imido) species have been prepared via oxidation of Cp*U(^MesPDI^Me)(THF) (<b>16-Cp</b>*) and [Cp^PU(^MesPDI^Me)]₂ (<b>16-Cp^P</b>) (Cp* = <i>η</i>⁵-1,2,3,4,5-pentamethylcyclopentadienide; Cp^P = 1-(7,7-dimethylbenzyl)cyclopentadienide; ^MesPDI^Me = 2,6-((Mes)N=CMe)₂C₅H₃N, Mes = 2,4,6-trimethylphenyl) with organoazides. Treating either with N₃DIPP formed uranium(IV) mono(imido) complexes, Cp^PU(NDIPP)(^MesPDI^Me) (<b>17-Cp^P</b>) and Cp*U(NDIPP)(^MesPDI^Me) (<b>17-Cp*</b>), featuring reduced [^MesPDI^Me]^1-. Addition of electron-donating 1-azidoadamantane (N₃Ad) to <b>16-Cp*</b> generated a dimeric product, [Cp*U(NAd)(^MesHPDI^Me)]₂ (<b>18</b>), from radical coupling at the <i>para</i>-pyridine position of the pyridine(diimine) ligand and H-atom abstraction, formed through a monomeric intermediate that was observed in solution but could not be isolated. To support this, Cp*U(<i>^t</i>Bu-^MesPDI^Me)(THF) (<b>16-<i>^t</i>Bu</b>), which has a <i>tert</i>-butyl group protecting the <i>para</i>-position, was also treated with N₃Ad, and the monomeric product, Cp*U(NAd)(<i>^t</i>Bu-^MesPDI^Me) (<b>17-<i>^t</i>Bu</b>), was isolated. All isolated complexes were analyzed spectroscopically and structurally, and dynamic solution behavior was examined using electronic absorption spectroscopy. </p>