published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

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This thesis describes the synthesis of a novel multimeric peptide ligand targeted to the human melanocortin 4 receptor. The synthesis of the peptide was attempted both by solid phase peptide synthesis and by solution phase peptide synthesis, leading to the conclusion that the necessary C- and N- terminal substituents were much easier to install via the solution phase route. The bifunctional peptide was purified and then multimerized in both protected and active amino acid forms using the copper(I)-catalyzed azide alkyne cycloaddition (CuAAC) reaction. The multimers were characterized using MS and UV-Vis spectroscopy. It was found that a large portion of the monomer cyclized under CuAAC conditions, though sufficient multimerization took place to form up to nonamers, as determined by mass spectrometry.

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The phenyl ring in the crown ether, 13-phenyl-1,4,7,10-tetraoxa-13-azacyclopentadecane (I), was used a site for functionalizing the compound. Electrophilic bromination of the ring with tribromide ion gave a 95% yield of the product substituted in the para position. This product underwent lithium-bromine exchange when reacted with n-butyllithium. The resulting anion was used to prepare PhZPX and PhPX2 derivatives, X p-C6H4NCH2(CH2OCH2)4&12 The oxide of PhZPX was completely characterized by an x-ray diffraction study which showed, in general, that the phosphorus was tetrahedral, the nitrogen planar, and the crown ether ring organized with the oxygen atoms endodentate. Several attemps were made to nitrate or nitrosate the phenyl ring in the parent crown ether. Spectroscopic evidence obtained from the products indicate that the reaction led to mixtures of mono and disubstituted products. / Department of Chemistry

An empirical approach to the study of phosphine compounds was completed using Molecular Mechanics 2 (MM2), and several computer programs written to descibe and analyze the final geometric orientations of the molecules. The calculations were performed on 64 conformers of 16 different phosphines. Results from these calculations were compared to those previously obtained for MNDO and MINDO/3 calculations, and to experimental data. Cone angles calculated from the MM2 optimized geometries, were compared to Tolman's original work, and to values obtained from semiempirical calculations, and to experimental results. In general, it was found that weighted average cone angles best represent the size of phosphine ligands. / Department of Chemistry

A series of SNS ligands have been successfully synthesised and characterised by IR, NMR and MS. The ligands are divided into two groups and represented by the general formulae: 2,6- bis(RSCH2)pyridine [R= methyl, ethyl, butyl, cyclohexyl, phenyl] and bis(RSCH2CH2)amine [R= ethyl, butyl, decyl]. Cobalt complexes of the respective ligands with the general formulae Co[2,6-bis(RSCH2)pyridine]Cl2 and Co[bis(RSCH2CH2)amine]Cl2 were synthesised and characterised by IR, elemental analysis and X-ray crystallography (for selected complexes). Thus, to investigate the electronic and steric effects of the ligand structure on the chemistry and reactivity of the complexes, the substituents bonded to the two sulfur donor atoms were sequentially varied and two different nitrogen sources were chosen. Crystal structures of Co[2,6- bis(CH2SCH2)pyridine]Cl2 (Ia), Co[2,6-bis(CH2CH2SCH2) pyridine]Cl2 (IIa) and Co[2,6- bis(CH2CH2CH2CH2SCH2)pyridine]Cl2 (IIIa) were obtained. It was found that complex Ia exists as a molecular dimer linked through two chloride bridges resulting in an octahedral geometry around each metal centre, while complexes IIa and IIIa are monomers exhibiting a trigonal bypyrimidal geometry. The complexes were tested as catalysts for the activation of paraffinic C−H bonds towards the formation of oxygenated products: octanol, octanone, octanal and octanoic acid from the substrate n-octane. Gas chromatography was utilised to quantify the products formed and also to calculate the conversion and selectivity of each catalyst system. The catalytic testing revealed that the ketone products were the most dominant with selectivities of ca. 90%. The catalyst that was the most active was Co[bis(CH2CH2SCH2CH2)amine]Cl2 (Ib) with a total n-octane conversion of 23%. / Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2012.

Phosphorus-containing metallacycles formed from the [2+2] cycloaddition of unsaturated substrates at the Ru-P π-bond of [Ru(η5-indenyl)(PCy2)(PPh3)] (2) were examined as possible intermediates relevant to hydrophosphination. Reagents, intermediates, products, and by-products involved in the [2+2] cycloaddition were identified and analyzed for reactivity and stability. The products, metallacycles of the form [Ru(η5-indenyl)(κ2-RCHCH2PCy2)(PPh3)] (4), were found to undergo facile cycloreversion. An ethylene η2-coordination adduct was directly observed by low temperature 31P{1H} NMR as an intermediate in the [2+2] cycloaddition mechanism. Steric and electronic effects of alkene substituents on metallacycle formation and selectivity were investigated in detail through rate constant and activation parameter determination, as well as collaborative computational DFT analyses and the construction of a Hammett plot. Preliminary attempts at releasing phosphinated products from ruthenacycle complexes via protonolysis and phosphine substitution were conducted. An unexpected metallacyclic product of one of these attempts, [Ru(η5-indenyl)(κ2-CHCHPCy2)(PPh3)] (10), was identified and characterized. / Graduate

This thesis is concerned with the synthesis and characterisation of new ansa bridged bis-η-cyclopentadienyl derivatives of niobium. Chapter 1 provides an introduction to the area of early transition metal bent metallocene derivatives and charts the development of ansa bridged metallocenes. Chapter 2 presents the synthesis and characterisation of some new ansa niobocene derivatives prepared from niobium(IV) chloride. The compounds [Nb(η-C₅H₄-CMe₂- η-C₅H₄)(η²-BH₄)],* [Nb(η-C₅H₄-CMe₂-η-C₅H₄)(PMe3)H],* [Nb(η-C₅H₄-CMe₂-η- C₅H₄)(PPh₃)H], [Nb(η-C₅H₄-CMe₂-η-C₅H₄)(SPh)₂] and [Nb(η-C₅H₄-CMe₂-η- C₅H₄)(CH2Ph)2]* are described. An asterisk indicates that the X-ray crystal structure was determined. Chapter 3 describes the synthesis and characterisation of some ansa niobocene derivatives containing ancillary imido ligands. The compounds [Nb(η-C₅H₄-CMe₂-η- C₅H₄)(NSiMe₃)Cl],* [Nb(η-C₅H₄-CMe₂-η-C₅H₄)(NBu')X] (X = Cl,* Br,* I,* Me), [Nb(η-C₅H₄-CMe₂-η-C₅H₄)(NHBu')Cl][BF₄)]and[Nb(η-C₅H₄-CMe₂-η-C₅H₄)(O)Cl]* are described. An asterisk indicates that the X-ray crystal structure was determined. Photoelectron spectra were recorded for [Nb(η-C₅H₄-CMe₂-η-C₅H₄)(NBu')X] (X = Cl, Me) and compared to a geometry optimisation calculation obtained using density functional theory. In these formally twenty electron compounds the HOMO has negligible metal contribution and the eighteen electron rule is not violated. Chapter 4 describes the results of a preliminary investigation into ansa niobium(III) bis-η-cyclopentadienyl derivatives containing phosphorus donor ligands. The compound [Nb(η-C₅H₄-CMe₂-η-C₅H₄)(PMe₃)Cl] is characterised spectroscopically. The synthesis and full characterisation details of the compound [Nb(η-C₅H₄-CMe₂-η- C₅H₄){P(OMe)₃}Cl] are given, including the X-ray crystal structure. Chapter 5 provides the experimental details for the preceding Chapters and Chapter 6 contains the characterising data for the new compounds described in this thesis. Full details of the X-ray crystallographic structure determinations are given in Appendices A -1.