Repository logo

Doctoral Degrees (Chemistry)

Permanent URI for this collection


Recent Submissions

Now showing 1 - 20 of 197
  • Item
    Cytotoxic and mechanistic studies of novel phenanthroline-derived oxovanadium(IV) complexes.
    (2022) Barry, Kristy Lyn.; Munro, Orde Quentin.
    In this work, a new series of ternary oxovanadium(IV) complexes of the type [VO(ONO)(DPQ/DPPZ)], [VO(ONN)(PHEN/DPQ)](PF6) and [VO(ONN)(DPPZ)](Cl) have been synthesised and characterised for the purpose of developing novel anticancer agents. ONO represents a tridentate Schiff base ligand derived from salicylaldehyde and various 4-substituted-2-aminophenols. ONN represents a tridentate Schiff base ligand derived from 1-methyl-2-imidazolecarboxaldehyde and various 4-substituted-2-aminophenols. PHEN represents 1,10-phenanthroline, DPQ represents dipyrido[3,2-d:2′,3′-f]quinoxaline and DPPZ represents dipyrido[3,2-a:2′,3′-c]phenazine. The bidentate N,N-donor polypyridyl co-ligands were incorporated to enhance DNA binding and to stabilise the vanadium(IV) metal centre. The known N,N-bidentate ligands were synthesised and characterised by ESI-mass, 1H and 13C NMR spectroscopy. The tridentate O,N,O′ and O,N,N′ ligands were synthesised and characterised by ESI-mass, NMR, IR, and UV/visible spectroscopic techniques, elemental analysis and single crystal X-ray crystallography. Single crystal X-ray diffraction, 1H NMR and DFT simulations confirmed the O,N,N′ ligands in this work form dimeric hydrogen-bonded supramolecular structures that are stable in solution. The oxovanadium(IV) complexes were characterised by elemental analysis, UV/visible, FT-IR and EPR spectroscopy as well as mass spectrometry. Elemental analysis and mass spectrometry confirmed the identity and purity of the complexes. EPR spectroscopy confirmed the monomeric VIVO-bound species. IR and UV/visible spectroscopy confirmed coordination of the ligands to the metal centre. Highresolution solid-state structures were elucidated for the cationic complexes with PHEN coligands and the 4-chloro and 4-tert-butyl derivatives of the [VO(ONO)(DPQ)] complexes. The crystal structures of the complexes showed a monomeric vanadium(IV) species with the bidentate N,N-donor co-ligand and respective dianionic O,N,O′- or monoanionic O,N,N′-tridentate Schiff base ligand coordinated to the oxovanadium centre in a distorted octahedral geometry. The respective neutral VIVO3N3 and monocationic VIVO2N4+ species were formed. A least-squares fit of the solid-state and DFT-simulated (B3LYP-/6-311G(dp)) structures of the oxovanadium chelates indicate the experimental and simulated structures are in good agreement. DFT-simulated structures were determined for those complexes where X-ray data are not available. The geometry-optimised structures for the neutral and cationic complexes all indicate that the respective bidentate polypyridyl ligands are free from steric hindrance by the tridentate ligand and should be available to bind DNA, which is their proposed cellular target. DFT simulations indicated the neutral complexes have larger HOMO-LUMO energy gaps than the corresponding cationic complexes, suggesting that the neutral complexes are more stable with respect to ligand substitution than the cationic complexes. Experimental mass spectrometry and UV/visible spectroscopy confirmed slower solvolysis processes for the neutral complexes versus the cationic analogues. 51V NMR studies indicate partial oxidation of the vanadium(IV) species in DMSO to VVO2(ONO/ONN)(DMSO) analogues. The cationic complexes with PHEN and DPQ co-ligands were deemed suitable to proceed with absorption DNA binding studies. The cytotoxicity screening of the oxovanadium complexes in this work revealed that, in general, the neutral complexes with DPQ co-ligands are cytotoxic against the triple-negative breast cancer MDA-MB and neuroblastoma SH-SY5Y tumour cell lines and non-toxic towards the cervical cancer HeLa cell line. The charge of the complexes was found to influence the cytotoxic properties. The cationic complexes with PHEN and DPQ co-ligands are cytotoxic towards the HeLa cell line as well as the MDA-MB and SH-SY5Y cell lines. The neutral DPQ and cationic complexes with PHEN and DPQ co-ligands were found to be more cytotoxic towards MDA-MB cell lines than cisplatin and the cationic complexes were found to be more cytotoxic towards the HeLa cell line than cisplatin. Steric bulk of the Schiff base functional group influences cytotoxicity with larger functional groups, such as tert-butyl and sulfonyl, leading to lower cytotoxicity. The N,N-donor co-ligand and steric bulk of the Schiff base functional group also influenced the selectivity index of the cationic complexes. The cationic oxovanadium-DPQ complex with a methyl substituent on the tridentate ligand is significantly more toxic to the carcinoma cell lines than the healthy renal cell line HEK293. In comparison, the cationic oxovanadium-PHEN analogue with a methyl substituent and the cationic oxovanadium-DPQ analogue with a bulky tert-butyl substituent are less selective in their cytotoxicity. The DNA binding studies show that the neutral and cationic DPQ compounds do have an affinity for DNA. A positive correlation between antitumour activity and DNA binding affinity was found. The [VO(ONN)(DPQ)](PF6) analogue with a bulky tert-butyl substituent has a lower intrinsic ct-DNA binding constant than the [VO(ONN)(DPQ)](PF6) analogue with a methyl substituent (1.3 × 104 M–1 and 2.8 × 104 M–1 respectively). The cationic DPQ derivatives also bind more strongly to DNA than the cationic complexes with PHEN co-ligands. The steric effect is also evident in the neutral complexes. The [VO(ONO)(DPQ)] complex with a tert-butyl substituent has a lower apparent binding constant than the [VO(ONO)(DPQ)] complex with no substituents on the Schiff base ligand. The cationic charge also led to a higher apparent binding constant for the [VO(ONO)(DPQ)](PF6) complex with a tert-butyl functional group than for the corresponding neutral [VO(ONO)(DPQ)] analogue with a tert-butyl functional group. Absorption and fluorescence spectroscopic and DNA viscosity studies indicate at least a partial DNA intercalative ability for the cationic oxovanadium-DPQ derivatives and the neutral oxovanadium-DPQ complexes with less bulky substituents. Molecular docking studies further highlighted the affinity of the metal chelates towards DNA, including interactions between DNA and the tridentate ligand. The lowest energy molecular docking poses range from ca. -48 to -67 kJ mol–1. Gel electrophoresis studies showed the cationic vanadium complexes with DPQ co-ligands (unlike the neutral DPQ and cationic PHEN analogues) were able to cleave plasmid DNA without adding external oxidising or reducing reagents. Experimental data suggest a singlet oxygen pathway is the most likely. It was also shown that the combination of metal ion and ligand is needed to induce DNA cleavage. The neutral [VO(ONO)(DPQ)] derivative with a methyl functional group on the Schiff base was shown to oxidatively cleave supercoiled plasmid DNA in the presence of H2O2 through the generation of hydroxyl radicals. EPR spintrapping studies with DMPO further support the idea that hydroxyl radicals are formed from reaction of the oxovanadium complex and H2O2. In summary the charge of the complex, type of substituent on the tridentate ligand and the identity of the N,N-donor heterocyclic ligand affected the stability, cytotoxic properties, selectivity, DNA binding, DNA cleavage abilities and DNA cleavage mechanism of the oxovanadium compounds in this study.
  • Item
    Comparison of extraction methods efficiency for the extraction of polycyclic aromatic hydrocarbons and phenolics in water matrices, sludge and sediment: sources of origin and ecological risk assessment.
    (2023) Ndwabu, Sinayo.; Mahlambi, Precious Nokwethemba.; Malungana, Mncedisi.
    Polycyclic aromatic hydrocarbons (PAHs) and phenolic compounds (PCs) are persistent and environmentally toxic compounds. This study therefore aimed to determine the levels of both PAHs and PCs in river water, wastewater, sludge and sediment samples. The evaluation of their origin source and ecological risk was also determined. The status of both these contaminants in South African environment is still not fully investigated, which is a gap this study intended to fill together with previous studies that have been carried-out. The PAHs and PCs were extracted using different extraction methods which include a solid phase extraction (SPE) and dispersive liquid-liquid micro-extraction (DLLME) in water matrices. The microwave assisted extraction (MAE) and Ultrasonication (UE) coupled with either filtering (F) or F + SPE as a clean-up technique was used for extraction of solid samples. The analytes extracted form water or sediment were determined using GC-MS. The PAH %recoveries obtained under optimum conditions in liquid samples were determined to be 72.1 - 118% for SPE and 70.7 – 88.4% for DLLME while the LOD and LOQ were 5.00 – 18.0 ng/L and 10.0 – 44.0 ng/L for SPE while they were 6.00 – 20.0 ng/L and 11.0 – 63.0 ng/L for DLLME. The recovery test for PAHs in solid samples gave a range of 93.7% - 121% for UE and 79.6% - 122% for MAE while the LOD and LOQ ranged from 0.0250 μg/kg to 1.21 μg/kg & 0.0800 μg/kg to 3.54 μg/kg for MAE and from 0.0840 μg/kg to 0.215 μg/kg & 0.0190 μg/kg to 0.642 μg/kg for UE respectively. The LOD and LOQs for PCs in both water and solid matrices were 0.01 – 2.00 μg/L and 0.02 – 6.07 μg/L for SPE, 0.05 – 1.20 μg/kg and 0.17 – 3.17 μg/kg for MAE and 0.09 – 1.33 μg/kg and 0.26 - 3.54 μg/kg for UE correspondingly, their %recovery test gave ranges of 75.2 – 112% (SPE), 80.9 – 110% (MAE) and 79.3 – 119% (UE).The optimization and validation of these methods indicated that they can be used for the extraction of PAHs or PCs in liquid samples, however, SPE when compared to DLLME showed to be more accurate and sensitive. Moreover, in solid samples the clean-up method was a deciding factor, with F + SPE cleaned samples giving higher concentrations of both PCs and PAHs than the filtered ones in both MAE and UE. The concentrations of PAHs ranged from nd (not detected) to 1046 ng/L in river water and nd to 778 ng/L in wastewater samples with naphthalene showing dominance over all other PAHs in both water matrices. The PC concentrations at 4.12 to 1134 μg/L for wastewater and nd to 98.0 μg/L for river water were high but still within the maximum allowable limit except for 2.4-DCP (2.4 dichlorophenol) at Wdv4. The concentrations obtained from F + SPE cleaned samples were higher for both PAHs and PCs with a range from 95.96 to 926.0 μg/kg and 1.30 to 310 μg/kg compared to concentrations from filtered only samples at 21.61 to 380.6 μg/kg and 0.90 to 266 μg/kg respectively. Pyrene showed dominance over all other PAHs in both sludge and sediments while 2.4-DCP and PCP dominated the sludge and sediment samples respectively. PAHs were determined to be of petrogenic (water matrices) and pyrolytic (solid samples) origin and on average posed low (water matrices) and a medium to high (solid matrices) ecological risk. The ILCRderm values at 4.98 x 10-1 and 2.62 x 10-1 (DahA) and 5.92 x 10-2 and 5.34 x 10-2 (PCP) were highfor adults compared to that of children at 1.92 x 10-1 and 1.01 x 10-1 (DahA) and 1.39 x 10-2 and 1.26 x 10-2 (PCP) for both sediment and sludge samples respectively. The low values of ILCRderm for children indicates that the have a high risk exposure even at low concentrations of the contaminants. The findings of this study showed that both areas (uMsunduzi river and Darvill wastewater works (WWW) of interest are polluted with PAHs and PCs therefore, more regulations such as the National Environmental Management: Waste Act (NEMWA) are needed to ensure environmental, human and animal safety.
  • Item
    Targeting the tumour extracellular environment through rational modification of the SNX class of HSP90 inhibitors.
    (2023) Mathenjwa, Gciniwe Sindiswa.; Akerman, Matthew Piers.; Veale, Clinton Garenth Lancaster.; Bode, Moira Leanne.
    HSP90 remains a valuable target for cancer therapy. Unfortunately, targeting intracellular HSP90 has proven not to be a viable chemotherapeutic approach. Compensatory HSR induction and HSP70 overexpression are the main limitations of this approach. A growing body of evidence suggests that targeting the extracellular environment would be of advantage and devoid of the drawbacks observed with intracellular HSP90 inhibition. As a result, the development of extracellular HSP90 inhibitors represents a novel opportunity for cancer therapeutics. In view of this hypothesis, we aimed to design and synthesise extracellular inhibitors and to assay these compounds against HSP90. To develop extracellular HSP90 inhibitors, cell-impermeable analogues of the well-developed benzamide HSP90 inhibitor (SNX 2112) were designed, synthesised and biologically evaluated. The desired target compounds were synthesized using developed methodology, as well as modified methodology. In Chapter 3 we compared and evaluated a variety of reported synthetic methods to deliver the analogues of SNX 2112. Interested in a general procedure for the synthesis of our analogues, we initially attempted to afford both the methyl and the trifluoromethyl containing analogues via a β- triketone mediated procedure. Despite the success observed with the methyl analogues, the instability of a trifluoromethyl containing β-triketone, deemed this procedure not feasible for this class of compounds. Our continued effort towards a general procedure led to the investigation of a tosylhydrazone mediated tetrahydroindazolone condensation; unexpectedly attempts to synthesise the methyl containing analogues via this procedure led to a 1—5 nitrogen to carbon tosyl migration, which was further investigated for varying substrates, and these results are explained in detail in this thesis. It then became apparent that each of the reported methods had its merits and shortcomings, there was no one best method, rather the synthetic approach was mainly determined by the C-3 substituent. The key intermediates were then converted into the desired targeted compounds by tethering the HSP90 pharmacophore to flexible alkyl groups, attached to polar sulfonate and phosphonate functionalities. Hypothetically, introduction of polar alkyl groups, would inhibit cell penetration thus limiting them to the extracellular environment. Based on the goals of our study we were interested in three biological evaluations; to confirm that our modified compounds were still capable of inhibiting HSP90s ATPase activity, to evaluate if our modifications reduced intracellular HSP90 activity, whether they stimulated the pro-oncogenic HSR, and to evaluate their cytotoxicity. Preliminary biological assessment of our compounds was consistent with our hypothesis. Here we showed that our compounds did not inhibit intracellular HSP90, and did not stimulate HSP70 expression, a marker of induction of the compensatory HSR. Furthermore, our analogues displayed cytotoxicity in the nanomolar range against the HeLa cell line. These preliminary data support the feasibility of targeting extracellular HSP90 as a novel anticancer strategy.
  • Item
    Syntheses of mixed donor homogeneous and immobilized palladium(II) complexes catalysts for methoxycarbonylation and hydrogenation reactions.
    (2021) Akiri, Saphan Owino.; Ojwach, Stephen.
    Reactions of ligands (E)-N'-(2,6-diisopropylphenyl)-N-(4-methylpyridin-2-yl)benzimidamide (L1), (E)-N'-(2,6-diisopropylphenyl)-N-(6-methylpyridin-2-yl)benzimidamide (L2), (E)-N'-(2,6-dimethylphenyl)-N-(6-methylpyridin-2-yl)benzimidamide (L3), (E)-N'-(2,6-dimethylphenyl)-N-(4-methylpyridin-2-yl)benzimidamide (L4) and (E)-N-(6-methylpyridin-2-yl)-N'-phenylbenzimidamide (L5) with [Pd(NCMe)2Cl2] furnished the corresponding palladium(II) pre-catalysts (Pd1-Pd5), in good yields. Molecular structures of Pd2 and Pd3 revealed an N^N bidentate coordination mode to afford square planar compounds. Activation of the palladium(II) complexes with para tolyl sulfonic acid (PTSA) afforded active catalysts in the alkenes methoxycarbonylation. The resultant catalytic activities were controlled by both the complex structure and alkene substrate. While aliphatic substrates favoured the formation of linear esters (>70%), styrene substrate resulted in predominantly branched esters of up to 91%. The water-soluble ligands; sodium 4-hydroxy-3-((phenylimino)methyl)benzenesulfonate (L6), sodium 3-(((2,6-dimethylphenyl)imino)methyl)-4-hydroxybenzenesulfonate (L7) and sodium 3-(2,6-diisopropylphenyl)imino)methyl)-4-hydroxybenzenesulfonate (L8) reacted with with Pd(OAc)2 afford their respective palladium(II) complexes [Pd(6)2] (Pd6), [Pd(L7)2] (Pd7) and [Pd(L8)2] (PdL8). In addition, treatment of the non-water-soluble ligands 2-((phenylimino)methyl)phenol (L9), 2-(((2,6-dimethylphenyl)imino)methyl)phenol (L10) and 2-((2,6 diisopropylphenyl)imino)methyl)phenol (L11) with Pd(OAc)2 yielded complexes [Pd(L9)2] (Pd9), [Pd(10)2] (Pd10) and [Pd(L11)2] (Pd11), respectively in good yields. Solid-state structures of compounds Pd6 and Pd9 revealed bis(chelated) square planar neutral compounds. All the complexes formed active catalysts in the methoxycarbonylation of 1- hexene, affording yields of up to 92% within 20 h and regioselectivity of 73% in favour of linear esters. The activities and selectivities of the compounds depended on the steric encumbrance around the coordination centre. The water-soluble complexes displayed comparable catalytic behaviour to the non-water-soluble systems. The complexes could be recycled five times with minimal changes in both the catalytic activities and regio-selectivity. Reactions of (amino)phenyl ligands, (E)-N-((Z)-4-(phenylamino)pent-3-en-2-ylidene)aniline (L12) and N,N'E,N,N'E)-N,N'-(3-(3 (triethoxysilyl)propyl)pentane-2,4-diylidene)dianiline (L13) with [Pd(NCMe)2Cl2] led to the formation of homogeneous complexes Pd13 and Pd14. Besides, supporting of complex Pd14 with either MCM-41, SBA-15, or Fe3O4 magnetic nanoparticles gave immobilized complexes P15-Pd17, respectively. Using varying metal loading in the MCM-41 immobilization of complex Pd14 produced complexes Pd18 and Pd 19. In addition, calcination of complex Pd16 at 150oC and 200oC led to the formation of complexes Pd20 and Pd21, respectively. All the complexes were received in good yields. The catalytic activities and selectivities of the homogeneous complexes were influenced by the coordination sphere, with the complexes predominantly forming linear esters. On the other hand, the catalytic behaviours of the immobilized catalysts depended on the nature of support and calcination temperatures. In addition, the catalytic activities were observed to depend on the reaction temperature, catalyst loading, amounts of PPh3 and acid promoters. The immobilized complexes Pd15, Pd16 and Pd17, were recycled up to five times. The homogeneous and silica immobilized palladium(II) complexes of ligands (2-phenyl-2-((3(triethoxysilyl)propyl)imino)ethanol) (L14), (4-methyl-2-((3(triethoxysilyl)propyl)imino)methyl)phenol ) (L15 ), [L14-MCM-41 (L16), and [L15- MCM-41 (L17)]. The homogeneous complexes [Pd(L14)2] (Pd22), [Pd(L14)2] (Pd23), [Pd(L14)(Cl2)] (Pd24), [Pd(L15)(Cl2)] (Pd25) were obtained from homogenous ligands L14, L15, L16 and L17 respectively. In addition, the silica immobilized compounds [Pd(L14)2]-MCM-41] (Pd26) and [Pd(L15)2)-MCM-4] (Pd27) were obtained through convergence immobilization of complexes Pd22 and Pd23, respectively. Comparatively, immobilized complexes [Pd (L14)(Cl2)-MCM-41] (Pd28) and [Pd(L15)(Cl2)]-MCM-41] (Pd29) were obtained from the complexation of immobilized ligands L16 and L17. Both sets of complexes gave active catalysts in molecular hydrogenation of alkenes, alkynes and functionalized benzenes. The catalytic activities and product distribution in these reactions were largely dictated by the nature of the substrate. The kinetic studies revealed reaction orders dependence on styrene for both the homogeneous and supported catalysts. Significantly, the selectivity of both sets of catalysts was comparable in the hydrogenation of alkynes and multi-functionalized benzenes. The supported catalysts could be recycled up to four times with minimum reduction in catalytic activities and showed the absence of any leaching from hot filtration experiments. Kinetics and poisoning studies established the presence of active homogeneous species for complexes Pd22-Pd5 and Pd(0) nanoparticles for the immobilized complexes Pd26-Pd29, respectively.
  • Item
    Carboxamide ruthenium(II) and manganese(II) complexes: structural, kinetic, and mechanistic studies in the transfer hydrogenation of ketones.
    (2022) Kumah, Robert Tettey.; Ojwach, Stephen.
    The carboxamide ligands N-(benzo[d]thiazol-2-yl)pyrazine-2-carboxamide (HL1), N-(1H-benzo[d]imidazol-2-yl)pyrazine-2-carboxamide (HL2), were prepared by condensation of pyrazine-carboxylic acid and appropriate heteroaromatic amines. Reactions of HL1 and HL2 with ruthenium(II) precursors, [RuH(CO)Cl(PPh3)3] and [RuH2(CO)(PPh3)3] afforded the mononuclear complexes [RuL1(PPh3)2(CO)Cl] (Ru1), [RuL1(PPh3)2(CO)H] (Ru2), [RuL2(PPh3)2(CO)Cl] (Ru3), [RuL2(PPh3)2(CO)H] (Ru4). The solid-state structures of complexes Ru1, Ru2, and Ru4 reveal bidentate modes of coordination of the ligands and distorted octahedral geometries around the Ru(II) centre. The complexes formed active catalysts in the transfer hydrogenation of ketones and achieved turnover number (TON) up to 530 in 6 h. The ruthenium(II)–hydride complexes, Ru2 and Ru4, were capable of catalysing transfer hydrogenation of ketones reactions under base free reaction conditions and demonstrated higher catalytic activities compared to the corresponding non-hydride analogues (Ru1 and Ru3). An inner sphere monohydride mechanism involving dissociation of one PPh3 group was proposed from in situ 31P{1H} NMR spectroscopy studies. Dipicolinamide ligand system, N,N'-(1,4 phenylene)dipicolinamide (H2L3), N,N'-(1,2-phenylene)dipicolinamide (H2L4), N,N'-(4,5-dimethyl-1,2-phenylene)dipicolinamide (H2L5), N,N'-(4-methoxy-1,2-phenylene)dipicolinamide (H2L6) were synthesised following a similar protocol described for HL1 and HL2. Treatment of the ligands H2L3 and H2L4 with RuH(CO)Cl(PPh3)3 afforded bimetallic complexes [Ru2(H2L3)(PPh3)4(CO)2][2Cl] (Ru5), [Ru2(H2L3)(H)2(PPh3)4(CO)2] (Ru5b), [Ru2(HL4)(PPh3)3(CO)2Cl3] (Ru6) and a mononuclear complex [RuCl2L4(PPh3)2(CO)] (Ru7). The solid-sate structure of the dinuclear ruthenium(II) complexes confirmed a bidentate coordinate mode, with PPh3, CO, and chlorido auxiliary ligands occupying the remaining coordinating sites to afford distorted trigonal bipyramidal geometries (Ru5 and Ru6) while the mononuclear complex Ru7 adopted a distorted octahedral geometry around its ruthenium(II) atom. The reaction of the ligands H2L4-H2L6 with the [RuCl2-η6-p-cymene]2 precursor produces half-sandwich diruthenium complexes [{Ru(η6-p-cymene)}2-μ-Cl(L4)][Ru(η6-p-cymene)Cl3] (Ru8), [{Ru(η6-p-cymene)}2-μ-Cl(L4)][PF6] (Ru9), [{Ru(η6-p-cymene)}2-μ-Cl(L5)][PF6] (Ru10), and [{Ru(η6-p-cymene)}2-μ-Cl (L6)][PF6] (Ru11). The molecular structure of cationic complexes, Ru8-Ru11, was confirmed by single-crystal X-ray crystallography analysis. The complexes Ru8-Ru11 display a bidentate Npyridine ^ Namidate mode of coordination to give pseudo-octahedral geometry (piano-stool-like geometry). The ruthenium(II) complexes demonstrated remarkable enhanced catalytic activity (TON values up to 1.71 x 104) in the transfer hydrogenation of ketones at a very low catalyst loading of 2.75 x10-2 mol% (275 ppm). The dinuclear ruthenium(II) complexes showed higher catalytic activity compared to the corresponding mononuclear complex Ru5. The half-sandwich diruthenium complexes Ru8-Ru11 displayed relatively higher catalytic activity than the ruthenium complexes Ru5 and Ru6 bearing the PPh3 co-ligands. Monohydride inner-sphere catalytic cycle was proposed for the transfer hydrogenation of ketones catalysed by both Ru1 and Ru9, and the formation of the reactive intermediates was supported with low-resolution mass spectrometry data. The dinuclear ruthenium complexes of pyridine and pyrazine-carboxamide bearing quinolinyl motif were synthesised by reacting, N-(quinolin-8-yl)pyrazine-2-carboxamide, (HL7), 5-methyl-N-(quinolin-8-yl)pyridine-2-carboxamide, (HL8), 5-chloro-N-(quinolin-8-yl)pyridine-2-carboxamide, (HL9), and 2-pyrazine-carboxylic acid (HL10) with equimolar [RuCl2(η6-p-cymene)]2 to afford the dinuclear complexes [{Ru(η6-p-cymene)}2Cl3(L10)] (Ru12), [{Ru(η6-p-cymene)Cl}2(L7)] [PF6] (Ru13), [{Ru(η6-p-cymene)Cl}2(L8)][Ru(L8)Cl3] (Ru14), and [{Ru(η6-p-cymene)Cl}2(L9)][PF6] (Ru15), respectively. The solid-state structures of the dinuclear complexes Ru12 and Ru13 reveal a typical piano-stool geometry around the Ru(II) ions. The dinuclear ruthenium complexes Ru12-Ru15 were used as catalysts in the transfer hydrogenation of a broad spectrum of aldehydes and ketones and demonstrated excellent catalytic activity, TON values up to 4.8 x 104, using catalyst loading of 2.0 x10-3 mol% (20 ppm). The catalytic performance of the complexes was affected by the ligand architecture and the substituents on the pyridyl ring. Complexes Ru13-15 exhibited higher catalytic activities compared to the complex Ru12 which could be ascribed to the role of quinoline in stabilising the complexes. The pyridine and pyrazine motifs have a significant impact on the reactivity and the catalytic activity of the complexes. In-situ low-resolution ESI-MS analyses of the reactive intermediates aided in proposing a monohydride inner-sphere mechanism for the transfer hydrogenation of ketones catalysed by Ru15. To develop a more sustainable, environmentally compatible and cost-efficient protocol for transfer hydrogenation of ketones, a new catalytic system based on manganese(II) metal was synthesised. New manganese(II) complexes Mn1-Mn4, ligated on dipicolinamide ligands were synthesized by treating the N,N'-(1,4-phenylene)dipicolinamide (H2L3), N,N'-(1,2-phenylene)dipicolinamide (H2L4), N,N'-(4-methoxy-1,2-phenylene)dipicolinamide (H2L5) and N,N'-(4,5-dimethyl-1,2-phenylene)dipicolinamide (H2L6) with MnCl2.4H2O salt to afford dinuclear manganese(II) complexes [Mn2(H2L3)2Cl4] (Mn1), [Mn2(H2L4)2Cl4] (Mn2), [Mn2(H2L5)2(Cl)4] (Mn3) and [Mn2(H2L6)2Cl4] (Mn4). The solid-state structure of complex Mn2 showed a six-coordinate dinuclear complex with the two Mn(II) ions adopting a distorted octahedral environment surrounded by two tetradentate ligands and chlorido co-ligands, respectively. The Mn(II) complexes formed active catalysts in transfer hydrogenation of ketones to achieve TON values up to 5.12 x 104. The presence of electron-donating substituents -OCH3 and -CH3 in complexes Mn3 and Mn4 displayed minor effects in the transfer hydrogenation of ketones. The new carboxamide-manganese(II) complexes are among the most active manganese-based catalysts capable of hydrogenating a large scope of ketones ranging from aliphatic to aromatic ketones. A dihydride catalytic cycle has been proposed and supported with in-situ low-resolution mass spectrometry data.
  • Item
    Assessment of antiretroviral drugs uptake by vegetables from contaminated soil and their adsorption by exfoliated graphite in river and wastewater.
    (2022) Kunene, Philisiwe Nganaki.; Mahlambi, Precious Nokwethemba.
    This study was directed toward vegetable uptake of the commonly used antiretroviral drugs (ARVDs), abacavir, nevirapine, and efavirenz. Antiretroviral drugs are used to treat the human immune-deficiency virus (HIV). South Africa (SA) is one of the countries with a high number of infected people on ARV therapy, therefore, the ARVDs are anticipated to be existing at high concentrations in the South African environment than in other countries worldwide. In recent years, the presence of ARVDs in the environment has drawn attention; hence studies have reported their presence in aquatic environments while very few studies have been conducted on their uptake using vegetables. This work was therefore based on the optimization and application of sensitive, simple, cost-effective, and robust techniques for quantifying ARVDs in vegetables. Based on this information, ultrasonic extraction (UE) and microwave-assisted extraction (MAE) were used to isolate target compounds from vegetable samples to the aqueous phase. Dispersive liquid-liquid microextraction (DLLME) and solid-phase extraction (SPE) were utilized to preconcentration and clean up the extracts from UE and MAE, respectively. A liquid chromatography photodiode array detector (LC-PDA) was utilized to detect and quantify the extracted compounds. The UE with and without DLLME cleanup were compared with each other, also, MAE with and without SPE cleanup were compared with each other. The methods comparison was done in terms of their detection (LOD) and quantification limits (LOQ), extraction efficiencies (%Recovery), relative standard deviations (%RSD), and concentrations of ARVDs found in vegetable samples. In comparison of UE and ultrasonic-assisted dispersive liquid-liquid microextraction (UADLLME), the LOD and LOQ obtained ranged between 0.0081 - 0.015 μg/kg and 0.027 - 0.049 μg/kg for UE and 0.0028 -0.0051 μg/kg and 0.0094 - 0.017 μg/kg for UADLLME respectively. High recoveries ranging from 93 to 113% in UE and 85 to 103% in UADLLME with less than 10% RSD in both procedures were obtained. These results indicated that UADLLME is more sensitive than the UE method, although they are both accurate and precise. The UE can be recommended for routine analysis as UADLLME showed the inability to extract analytes from root vegetables. The optimized UE and UADLLME methods were applied to extract ARVDs from vegetables bought from local fruit and veggie supermarket. Vegetables were categorized as root (carrot, potato, and sweet potatoes), leaf (cabbage and lettuce), and fruit (green paper, butternut, and tomato). The target ARVDs were quantified in most samples with concentrations up to 8.18 μg/kg. The concentrations obtained were slightly high in UADLLME than in UE as a result of its high sensitivity. Efavirenz was the most dominant drug, while the potato was the most contaminated vegetable. In the comparison of MAE and MAESPE, the obtained LOD and LOQ ranged from 0.020 to 0.032 μg/kg and 0.068 to 0.109 μg/kg for MAE and 0.019 to 0.066 μg/L and 0.065 to 0.22 μg/L for MAE-SPE. The obtained recoveries ranged from 85 to 103% for MAE and from 82 to 98 % for MAE-SPE, respectively, and the RSDs were all less than 6%. These results showed that both methods have comparable sensitivity; however, the recoveries values for MAE were slightly higher than those obtained in MAE-SPE, which signals MAE’s high accuracy. The optimized MAE and MAE-SPE methods were applied to remove ARVDs in the root (potatoes, onions, and beetroot), leaf (lettuce, and spinach), and fruit (green paper, cucumber, and eggplant) vegetables bought from local fruit and veggie supermarket. The obtained ARVDs concentration range was 1.48 ± 0.5 - 27.9 ± 1.2 μg/kg. The MAE-SPE resulted in low concentration compared to MAE without cleanup. Beetroot exhibited high concentrations of the target ARVDs, while nevirapine was found to have high concentration and as a dominant compound. The results obtained revealed that the vegetables from the studied area are contaminted with ARVDs, which could indicate their possible irrigation with wastewater effluent or the use of sludge as biosolids in the agricultural areas. This is a concern as it leads to unintentional consumption by consumers which could lead to drug resistance by the human body or have human health effects. The study was then expanded by conducting the phytoremediation approach to investigate the uptake of abacavir, nevirapine, and efavirenz by beetroot, spinach, and tomato from the contaminated soil. The three selected vegetable plants were planted and irrigated with ARVDs spiked (at 2000 and 5000 μg/L) water over a period of three months. The optimized UE and LC-PDA methods were used to extract and quantify the selected ARVDs from the target vegetables and soil. The obtained results showed that the studied vegetables have the potential to take up abacavir, nevirapine, and efavirenz from contaminated soil, be absorbed by the root, and translocate to the aerial part of the plants. Abacavir was found at high concentrations to a maximum of 40.21 μg/kg in the root, 18.43 μg/kg in the stem, and 6.77 μg/kg in the soil, while efavirenz was the highest concentrations, up to 35.44 μg/kg in leaves and 8.86 μg/kg in fruits. Spinach root accumulated more ARVDs than beetroot and tomato. The bio-accumulation factor ranged from 2.0-14 μg/kg in beetroot, 3.6 - 15 μg/kg in spinach, and 6 – 10 μg/kg in tomato. The root concentration factor range was 0.047 – 17.6 μg/kg; 0.34-5.9 μg/kg, and 0.14-2.82 μg/kg in beetroot, spinach, and tomato, respectively. The translocation factor range obtained was 0.40 – 38 μg/kg, 0.08 – 19 μg/kg, and 0.14 – 49 μg/kg in beetroot, spinach, and tomato, respectively. However, the accumulation of ARVDs in all studied plants showed that they could be used in phytoremediation. The results obtained in the phytoremediation approach revealed that the utilization of the contaminated water has an influence on the presence ARVDs in vegetables; hence this work also focused on evaluating the exfoliated graphite adsorption of ARVDs in water. Natural graphite was intercalated with acids and exfoliated with thermal shock to obtain the exfoliated graphite. The scanning electron microscopy images showed that the exfoliated graphite had increased c-axis distance between the layers with accordion-like structure which were confirmed by the lower density of exfoliated graphite material (0.0068 gmL-1) compared to the natural graphite (0.54 g mL-1). Fourier Transformed Infrared Spectroscopy results showed the C=C in natural and exfoliated graphite at 1635 cm-1 stretching. The phenolic, alcoholic, and carboxylic groups were observed from 1000 to 1700 cm-1 for the intercalated and exfoliated graphite. The Energy-dispersive X-ray results further confirmed these results, which showed carbon and oxygen peaks in the intercalated and exfoliated graphite spectrum, whereas natural graphite showed only a carbon peak. Raman spectroscopy results showed that the material’s crystallinity was not affected by the intercalation and exfoliation processes as observed from the ratios of the G and D peaks and the G' and D'. Natural, intercalated and exfoliated graphite contained the D, G, D', and G' peaks at about 1350 cm-1, 1570 cm-1, 2440 cm-1, and 2720 cm-1, respectively. The exfoliated graphite material showed the characteristic of a hexagonal phase graphitic structure by (002) and (110) reflections in the X-ray diffraction results. The exfoliated graphite adsorption method was optimized based on the pH of a solution, adsorbent dosage, and adsorption time prior to application to water samples. The optimum pH solution, adsorbent dosage, and adsorption time were 7, 30 mg, 0.01 μg/L, and 30 minutes respectively. The kinetics and isotherm studies were conducted to assess the model that best fit and explain the experimental data obtained. The kinetic model and adsorption isotherm studies showed that the experimental data fit well pseudo-second-order kinetics and is well explained by Freundlich’s adsorption isotherm. The maximum adsorption capacity of the exfoliated graphite (EG) for ARVDs ranges between 1.660-197.0, 1.660-232.5, and 1.650-237.7 mg/g for abacavir, nevirapine, and efavirenz, respectively. These results showed that under proper operating conditions, the EG adsorbent could potentially be applied as a water purifying tool for the removal of ARVDs pollutants.
  • Item
    Phytochemistry and quorum sensing inhibitory studies of four vernonia species growing in Nigeria.
    (2017) Aliyu, Abubakar Babando; Moodley, Brenda.
    Abstract This thesis contains the phytochemical analysis of four medicinal Vernonia species growing in Nigeria and used in ethnomedicine to treat a variety of medical conditions. The four Vernonia species studied were Vernonia blumeoides, Vernonia perrottetii, Vernonia ambigua and Vernonia glaberrima. The thesis also contains a comprehensive review of the sesquiterpene lactones from the genus Vernonia, their structural diversity and biosynthetic considerations. For each of the plants, the crude extracts and selected compounds were tested for their antibacterial activity using the traditional disc diffusion and broth microdilution as well as anti-quorum sensing assays. Three of the four plants studied yielded sesquiterpene lactones, Vernonia blumeoides yielded four novel eudesmanolide sesquiterpene lactones (blumeoidolides A-D), Vernonia perrottettii yielded a novel keto-hirsutinolide 13-acetoxy-1(4β),5(6)β-diepoxy-8α-(senecioyloxy)-3-oxo-1,7(11)-germacradiene-12,6-olide B1 and the known keto-hirsutinolide 13-acetoxy-1,4β-epoxy-8α-(senecioyloxy)-3-oxo-1,5,7(11)-germacratriene-12,6-olide B2 and Vernonia ambigua yielded a novel glaucolide sesquiterpene, 5,6-dehydrobrachycalyxolide. The structures of the sesquiterpenes were determined from their 1H, 13C and 2D NMR spectra along with mass spectra. The crystal structure of one of the eudesmanolide sesquiterpenes allowed the configuration of the stereocentres in the molecule to be determined. In addition to the sesquiterpene lactones, some common sterols and flavonoids were isolated from the plants: stigmasterol was isolated from V. blumeoides, lupeol was isolated from V. blumeoides, V. ambigua and V. perrottettii and lupeol acetate from V. ambigua and V. perrottetti. The flavonoid apigenin was found in V. blumeoides, V. perrottetti and V. glaberrima, luteolin in V. blumeoides and V. perrottetti, velutin in V. perrottetti and V. glaberrima and chrysoeriol in V. ambigua and V. glaberrima. Chrysin was found only in V. blumeoides and luteolin 3',4'-dimethyl ether in V. glaberrima. Several of the isolated sesquiterpene lactones showed good anti-quorum sensing inhibition (QSI). QSI ≥80% was obtained for blumeoidolide A at a concentration ≥ 0.071 mg mL-1, blumeoidolide B (≥ 3.6 mg mL-1) and B1 (1.31 mg mL-1), QSI ≥75% for B2 (0.33 mg mL-1) and QSI ≥84% for 5,6-dehydrobrachycalyxolide (2.6 mg mL-1). The sterols, lupeol and lupeol acetate, were also found to have QSI ≥84% at 2.6 mg mL-1. Molecular docking studies carried out on blumeodolides A-D in the binding sites of CviR and CviR' (transcription activator proteins) suggested that these molecules are able to bind to certain domains in the target protein, thus eliciting an effect. The current work adds to the library of sesquiterpene lactones from the genus Vernonia and provides some lead compounds to antibacterial activity via quorum sensing inhibition.
  • Item
    NCC-based SERS substrate: fundamentals, preparation and applications.
    (2018) Ogundare, Segun Ajibola.; Van Zyl, Werner Ewald.
    This study reports the isolation and characterization of nanocrystalline cellulose (NCC) produced from discarded cigarette filters (DCF). The DCF were processed into cellulose via ethanolic extraction, hypochlorite bleaching, alkaline deacetylation, and then converted into NCC by sulphuric acid hydrolysis. The morphological structures of the isolated NCC, established with TEM, showed that the nanocrystals were needle-like with length in the range of between 62–258 nm. FEGSEM showed the morphological transition of the micro-sized DCF to a self-assembled NCC, while EDX revealed the presence of Ti (as TiO2) in DCF, which was retained in the NCC. A NCC sample that was freeze-dried showed a specific surface area of ~8 m2/g. The crystallinity of the NCC film and the freeze-dried samples were ~97% and ~94%, respectively. Crystallite sizes of the freeze-dried (8.4 nm) and film (7.6 nm) samples correlated with the mean width (8.3 nm) of the NCC, observed with the TEM. The isolated NCC was used in the dual role as a reducing- and stabilizing agent in the formation of silver nanoparticles (AgNPs). By this method, a notable size variation of the synthesized AgNPs was found over the pH range of between 5–10, ranging from 4.61 nm at pH = 9 and increasing to 19.93 nm diameter at pH = 5. The size and yield of the AgNPs were also affected by the reaction time and concentration. The spherically shaped AgNPs induced a localized surface plasmon resonance (LSPR) at around 416 nm. The Ag content in the dry AgNPs was 81.9 wt%, which correlated with 82.1 wt% mass left at 600 ºC. Further analysis showed that the dry AgNPs were macroporous with reduced surface area and porosity upon calcination. The sensitivity of the AgNPs showed excellent surface-enhanced Raman scattering (SERS) of riboflavin. The limit-of-detection (LOD) for riboflavin, based on a signal-to-background ratio of 3:1, was found to be 3 x 10-7 M. The intensities of SERS signals increased with increase in concentration. In addition, clusters of AgNPs were synthesized with NCC isolated from Whatman cellulose filter paper, which behaved as a dual reducing- and stabilizing agent, and Stӧber silica (SiO2) provided a suitable anchoring surface. The synthesized nanocomposite (AgNPs/SiO2/NCC) was evaluated as a substrate for surface-enhanced Raman scattering (SERS) of malachite green (MG) and compared with AgNPs/NCC nanocomposite. The FTIR spectra of both nanocomposites showed a weak carbonyl band (1754 cm-1), indicating partial oxidation of the NCC. The UV-vis spectrum of AgNPs/NCC showed a narrow peak at 412 nm, characteristicof LSPR of monodispersed AgNPs. However, this peak was broad with a shoulder at 490 nm in the spectrum of the AgNPs/SiO2/NCC, which indicated clustering of the plasmonic nanoparticles. TEM micrographs showed that the plasmonic nanoparticles were monodispersed with a mean diameter of 19.5 nm in AgNPs/NCC, while they aggregated into clusters on SiO2 in AgNPs/SiO2/NCC resulting in an approximately 20 nm increase in the mean diameter of the SiO2. The SEM/EDX spectra and XRD diffractograms of the nanocomposites showed Ag as the predominant element. The SERS performance of the nanocomposites was evaluated by using MG as a probe, showed AgNPs/SiO2/NCC as a superior substrate with significant improvement in intensities of Raman peaks of MG and high sensitivity as the LOD was 0.9 nm, while AgNPs/NCC showed a LOD of 5.2 nm, based on a signal-to-background ratio of 3:1. This result underscores the huge contribution of SERS “hot spots” as AgNPs assembled into clusters in contrast to monodispersed AgNPs in the absence of SiO2.
  • Item
    Production of nanocellulose composites and catalytic and microbial applications.
    (2018) Moodley, Vashen.; Van Zyl, Werner Ewald.
    This study describes the preparation, isolation, characterization and application of polysaccharide based nanocrystalline cellulose (NCC) from two source materials (filter paper and bleached pulp). The isolated NCCs were utilized as a composite/support material. Hydroxyapatite (HAp), titanium(IV) oxide (anatase phase, TiO2) and biologically synthesized silver nanoparticles were chosen as the preferred candidates for the incorporation of NCC into their respective matrices, which allowed for the preparation of three new materials. These newly prepared composites were applied in catalytic and anti-microbial studies. NCC formed the basis of this investigation and was prepared via a common acid hydrolysis treatment, using sulfuric acid as the preferred acid hydrolytic medium. The isolated NCCs were obtained in reasonable yields and were characterized using the following techniques: ATR-FTIR, XRD, TEM, HRTEM, FEGSEM equipped with EDX detector, TGA/DTA, NTA (zeta potential) and BET. The rod-shaped particles revealed a high crystallinity, small crystallite sizes and good thermal stabilities. These results led to use the prepared NCC’s as a composite material in the pursuit to prepare a new class of materials with a potential array of applications. The preparation of HAp and subsequently the NCC/HAp (in varied wt%) composite allowed for the use of this newly synthesized material as a versatile catalyst. The catalyst was well characterized and used in the preparation of the two-component one-pot synthesis of triazolidine-3-one derivatives. Preliminary reaction optimization established that the 40 w/w% NCC/HAp composite catalyst returned the best results. The eleven new triazolidine-3-one derivatives (4a - 4k) were synthesized in good yields and maintained good atom economy. The catalyst proved to be an effective tool in this protocol with the supplementary advantage of being recyclable. This approach to organic multicomponent reactions (MCRs) proved to be a cost-effective strategy and allowed for an easy work up with environment-friendly reaction conditions. Compared to non-catalytic protocols this approach required shorter reaction times. With the prospect of employing the optical properties of the prepared NCC, TiO2 was then considered for the use in the second newly prepared composite material as a potential photo-catalyst. The NCC/TiO2 material was synthesized in varying wt% and was characterized via a number of optical, spectroscopic and microscopic techniques to establish if it possessed the potential to be used as a photo-catalyst. Proceeding this evaluation, the NCC/TiO2 material was used in the solar-driven photo-degradation mineralization of o-chloranil (2,3,5,6-tetrachloro-2,5-cyclohexadiene-1,4-dione), a commonly used pesticide. The successful decomposition of o-chloranil led to the identifiable products to 2,3-dichloro-4,5-dioxohex-2-enedoic acid (DCA), 2,3-dioxosuccinic acid (DSA) and oxalic acid (OA). This proved that the 20 w/w% NCC/TiO2 composite could be employed as a successful photo-catalyst, and in particular that NCC could be used as a successful composite material together with TiO2. The rate of degradation was influenced by various parameters such as substrate concentration and photo-catalyst loading. The intermediate product (DCA) formed during the decomposition process was assumed to slow down the progression of the reaction and provided a useful insight into the degradation pathway of the contaminant. The final study demonstrated the synthesis of silver nanoparticles (AgNPs) via a biological (phyto-mediated) route using Lippia javanica plant extract (LPE). The preparation of the colloidal AgNPs involved a variation in the LPE (100 – 400 μL) and AgNO3 (1 – 10 mM) concentration to determine the ideal morphology of AgNPs formed. This biosynthetic approach proved successful in the formation of AgNPs in colloidal form, with superior advantages over the chemical formation. These AgNPs were used in several applications as demonstrated with the inclusion of NCC as a support. Samples were characterized via optical, spectroscopic and microscopic analyses, with the ideal colloidal solution C9 (400 μL LPE, 10 mM AgNO3) established as providing the greatest number of AgNPs with the lowest size. This colloid was chosen for the further incorporation of NCC. NCC (filter paper source) was then incorporated into the quasi-spherical shaped nanoparticle matrix and further characterized, analysed and applied as a catalyst to the synthesis of benzylidene-bis-(4-hydroxycoumarin) derivatives and as a potential bactericidal agent. All colloidal samples were biologically tested against 5 bacterial strains and demonstrated good activity, however, samples with the highest concentration of AgNPs were chosen for further Minimal Bactericidal Concentration (MBC) testing. The results showed that all samples were superior in relation to their anti-biotic counterparts used as standards. Sample C9e (1000 mg NCC, 4 mL of LPE in 10 mL of 10 mM AgNO3 and made up to 50 mL), proved to be an efficient catalyst for a three-component reaction. This led to the catalytic preparation of nine benzylidene-bis-(4-hydroxycoumarin) derivatives (6a – 6i).
  • Item
    Catalytic ring-opening polymerization of cyclic esters to biodegradable polyesters using N,N'- and N,O-ligand supported Cu(II), Mg(II) and Zn(II) complexes.
    (2018) Munzeiwa, Wisdom.; Owaga, Bernard Omondi.; Nyamori, Vincent Onserio.
    Over the past decades, there has been a tremendous increase in market demand for polyesters and their co-polymers. Of interest, polycaprolactone (PCL) and polylactides (PLA) which are biodegradable have found widespread applications in the packaging and biomedical fields. Polyesters are produced via ring-opening polymerization (ROP) process using metal-based metal-catalyst/initiators, with industrial production relying on tin(II) compounds. Despite the intense research efforts devoted to this area, there are still considerable limitations. For example, in case of chiral lactides monomers very few catalytic systems are capable of stereoselective synthesis. In addition, there is also lack of control of the polymerization process to curb side reactions which results in low molecular weight polymers with broad molecular distributions. Furthermore, the toxicological effects associated with tin compounds pose a danger if polymers are applied in the biomedical field since it is difficult to completely remove remnant catalyst from the polymer matrices. Thus, this thesis investigated the synthesis of less toxic metal complexes such as zinc, copper and magnesium supported by strategically designed ligands and their application in ROP. Four different class of ligands were explored as supports namely formamidine, N-hydroxy formamidine, Schiff base phenoxide and chiral amino pyridyl ligands and thirty complexes were synthesized and reported in this thesis. The steric and electronic properties of the ligands were fine-tuned to influence the catalytic activity and the polymer properties. The effect of the nature of the metal—oxygen bond which is prerequisite for ring-opening polymerization was investigated. Complexes with acetate and alkoxide reactive ligands were synthesised where the oxygen was not part of the ligand system. N-hydroxy formamidine and Schiff base phenoxide ligands contain the oxygen heteroatom as part of the ligand backbone. All the complexes polymerized caprolactone and lactides with appreciable activity, however for hydroxy formamidine ligands the polymerization complexes were more active only in the presence of co-initiator. The effect of auxiliary ligands such as acetates, alkoxides was also investigated. The polymerization data showed that catalytic activity depended on the metal identity, steric crowding and auxiliary ligands. Generally, zinc acetate complexes were more active achieving complete monomer conversion within 68 h compared to 120 h for the copper analogues.Magnesium-amino phenolate complexes showed greater activity, attaining 99% monomer conversion in less than 32 h as compared to 55 h for the zinc analogues. The zinc pyridyl alky and alkoxide complexes showed excellent activity, achieving 100% monomer conversion within 1 min at room temperature. Bulk substituents and electron withdrawing substituent resulted in reduced catalytic activity. All catalytic systems produced low molecular weight polymers ranging from 1200 to 10 500 g mol-1 with relatively broad molecular weight distributions and PDIs that lie between 1.2 and 2 pointing to semi-living polymerization. Chiral ligand supported catalysts showed good stereoselectivity in polymerization of rac-lactide (rac-LA) with Pr values ca 0.70. The role of the solvent was studied, and it was observed that coordinating solvent such as THF retards the polymerization as they compete with the monomer for catalytic active sites. Detailed abstracts are given in each of chapters 3 to 6.
  • Item
    Phytochemical, elemental and biological studies of three ficus species (moraceae) found in KwaZulu-Natal, South Africa.
    (2017) Ogunlaja, Olumuyiwa Olufisayo.; Jonnalagadda, Sreekantha Babu
    ABSTRACT Ficus (Moraceae), with over 800 species, is one of the understudied genera in modern pharmacognosy. Rural households depend on their fruits for food while other plant parts such as leaves and bark are utilised for medicinal purposes. Phytochemical analyses and biological activities of different plant parts, as well as the nutritional value of the edible fruits of many of the species are yet to be investigated. This study aimed at investigating three Ficus species (Ficus burtt-davyi, Ficus sur Forssk and Ficus sycomorous Linn) that produce edible fruits and are indigenous to KwaZulu-Natal, South Africa, as a source of secondary metabolites and essential dietary elements, due to their claimed medicinal and nutritional value. Plant material was subjected to chromatographic analyses and isolated compounds were identified using spectroscopic techniques and by comparison with previously reported data. Fruit and soil samples that were collected from sites within KwaZulu-Natal, were digested and analysed for macro, micro and toxic elements by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES). The phytochemical investigation of F. burtt-davyi revealed the bark to be rich in pentacyclic triterpenoids (lupeol and lupeol acetate) and the known antioxidant flavonoid ((+)-catechin) whilst the leaves were rich in sterols (β-sitosterol, campesterol, and stigmasterol). Both leaves and fruits also contained phaeophytin a, lutein and α-amyrin. This is the first phytochemical report on this species. The cytotoxic results indicated that lupeol and (+)-catechin, the most abundant bioactive compounds in the stem bark, were responsible for its synergistic cytotoxic effects against breast and colorectal adenocarcinoma cell lines. This study supports the use of this plant species as a substitute for antioxidant supplements and as an alternative medicine for oxidative stress related iv non-communicable chronic diseases in vulnerable communities. The phytochemical analysis of F. sur revealed two pharmacologically active triterpenoids (lupeol and sitosterol), one pheaophytin (pheaphytin a) and one flavonoid (epicatechin). The analytical results indicated that the fruits of F. burtt-davyi, F. sur and F. sycomorous are good sources of essential dietary elements and can contribute significantly (p < 0.05) to the recommended dietary allowances (RDAs) for most nutrients. The fruits of F. sur and F. burttdavyi are good dietary sources of Se and Mn, respectively. The concentrations of As, Cd, and Pb were below the instrument detection limits in all three figs indicating that the species do not tend to accumulate these toxic elements. Data from this study showed that metal interactions in soil influenced their availability, but uptake was to a greater extent controlled by the plant. Statistical analyses revealed synergistic relationships in the plants, thereby confirming that uptake of elements is controlled to meet metabolic needs. Overall, this study validates the ethnomedicinal use of these figs and reveals the nutritional and medicinal benefits of consuming the indigenous edible fruits. It also addressed the need for analytical information on the elemental concentrations in indigenous edible fruits consumed in South Africa.
  • Item
    Final stage CO removal by oxidation or hydrogenation using supported PGM catalysts for fuel cell applications.
    (2015) Mohamed, Ziyaad.; Friedrich, Holger Bernhard.; Singh, Sooboo.
    Hydrogen has recently become a promising alternative fuel for small scale energy generation with the aid of fuel cells. The most prefered method for on-board production of pure hydrogen from methane is through a series of catalytic reactions. However, prior to entering the fuel cell stack, the CO concentration in the reformate gas must not exceed 10 ppm. Concentrations of CO greater than 10 ppm poison the Pt anode which results in the loss of activity and, the power output. Post water-gas shift reaction, two methods show promise for the effective CO removal to the desired levels of less than 10 ppm. In the first method, known as preferential oxidation (PROX), CO is oxidized to CO2, whereas in the second method, known as selective methanation (SMET), CO is hydrogenated to CH4. The catalysts for these reactions must be highly active and selective for the specific reaction (CO oxidation and/or CO hydrogenation), since unwanted side reactions could result in the additional loss of hydrogen. This study presents the synthesis, characterization and testing of Pt, Ir and Ru supported on reducible oxides, TiO2 and ZrO2, for both the oxidation and hydrogenation of CO in H2 rich streams. The effect of synthesis methods (wet impregnation and deposition precipitation), controlling the isoelectric points of the supports, the nature of the active metals (metal dispersion, particle sizes, CO chemisorption capacities) and the metal support interactions were investigated. The catalysts were characterized by ICP-OES, BET, XRD, XPS, temperature programmed studies, FTIR-CO, CO chemisorption and HRTEM. Catalytic testing of these materials included CO oxidation, CO oxidation in the presence of H2 and the hydrogenation of CO in dry and realistic water-gas shift reformate feeds. All the catalysts showed appreciable activy for the total oxidation of CO below 200 °C, but in the presence of H2, the activity decreased significantly. The Pt and Ir catalysts, although showing low CO conversions, favoured the undesired oxidation of H2, which was due to the strong metal support interactions of these materials, resulting in higher H2 spillover on the supports, reducing them and thus forming H2O. The Ru systems showed slightly better activity but tend to simultaneously hydrogenate CO and oxidize it, which is not selective or desired since increased H2 consumption takes place.CO hydrogenation, on the other hand, showed promising results for all the catalysts in the dry reactions. However, the Pt and Ir systems tested with realistic water-gas shift feeds, which included CO2 and H2O, favoured the forward and reverse water-gas shift reaction, as well as CO2 hydrogenation. The Ru systems showed the best activity towards the selective methanation of CO with realistic feeds at a temperature 100 °C lower than the Pt and Ir systems, giving 99.9 % CO conversions and 99.9 % selectivity towards CH4. CO2 methanation was only observed once all CO in the feed was converted. The superior results of the Ru systems were attributed to the active metal which has a lower heat of CO adsorption and a higher CO dissociative adsorption energy compared to that of Pt and Ir. The CO content in the feed stream was effectively reduced to less than 10 ppm over the Ru catalysts which is crucial for fuel cell applications.
  • Item
    Synthesis, characterization and antibacterial activity of benzimidazole Derivatives.
    (2018) Cheddie, Adele.; Koorbanally, Neil Anthony.
    Three series of benzimidazole hybrids were synthesised, two of which contained either thiazolidinone or amino acids tethered to the benzimidazole at C-2 by a phenyl linker and the third contained a thiazolidinone moiety at C-5 on the core structure of the benzimidazole. A total of 35 new compounds were synthesised. Variety was introduced into the molecules by using different benzaldehydes when forming the thiazolidinones or different amino acids substituted on the phenyl linker. Many of the reaction steps were carried out using microwave reactions and in one series, a comparative study was carried out between conventional synthesis and microwave irradiation. In all cases, the microwave methods had many advantages over conventional methods, having shorter reaction times, improved yields and use of green solvents. The synthesized compounds were characterised using mainly Nuclear Magnetic Resonance Spectroscopy and confirmed by High Resolution Mass Spectroscopy. All compounds were tested for their antimicrobial properties against two Gram positive bacteria (Staphylococcus aureus ATCC 25923 and Staphylococcus aureus Rosenbach ATCC BAA-1683 (methicillin resistant S. aureus)) and four Gram negative bacteria (Pseudomonas aeruginosa ATCC 27853, Klebsiella pneumonia ATCC 31488, Escherichia coli ATCC 25922 and Salmonella typhimurium ATCC 14026). A disc diffusion assay was used to first screen the compounds for bacterial activity, followed by the Minimum Bactericidal Concentration (MBC) assay. Among the three series, the thiazolidinones linked to the benzimidazole via the phenyl group at C-2 showed the lowest activity in the mM range. Both the amino acid derivatives linked to the phenyl group at C-2 and the thiazolidinone attached to C-5 of the benzimidazole showed antimicrobial activity in the M range. Although the benzimidazole amino acid hybrids were inactive against the Gram positive bacteria, B7c and B7d (the methionine derivatives) showed excellent inhibitory activity against S. typhimurium (MBC = 0.25 and 0.05 μM respectively), along with compounds B7a (valine derivative) and B7j (tryptophan derivative) which were active against K. pneumoniae with MBC values of 0.27 and 0.10 μM respectively. The benzimidazole–thiazolidinone hybrids, containing a trifluoromethyl group at C-2 and a thiazolidinone group at C-5, showed excellent activity with most compounds exhibiting activity ranging from 3 to over 100 fold higher than the standards. Among these, C3d, C3f and C3j (0.14-35.46 μM), containing bromine and nitro groups, displayed broad range activity against all strains tested. These findings are a major contribution and a good lead to developing new and better antimicrobial drugs.
  • Item
    Catalytic ozonation of hazardous halogenated compounds with mixed-metal oxides.
    (2018) Gounden, Asogan Nokan.; Jonnalagadda, Sreekantha Babu.; Singh, Sooboo.
    The study compares degradation of four hazardous halogenated pollutants, namely 2,3-DBP, 1,3-DCP, 2,4,6-TBP and 2,4-DCPA Acid in water, by ozonation alone and catalytic ozonation using Co and Ni loaded on Fe respectively by co-precipitation and a simple mixing method. The brominated pollutants showed a higher reactivity during ozonation than the chlorinated pollutants. In ozonation alone dehalogenation of each compound improved with an increase in the amount of hydroxide ions. TOC removal and DBP minimization was difficult to achieve in ozonation alone, however, in catalytic ozonation with Fe:Co (Co-ppt) and Fe:Ni (Co-ppt) significant improvements were noted. Fe:Ni (Co-ppt) catalyst material showed the best activity for conversion of the pollutants, TOC removal and DBP minimization in water during ozone treatment. BET and SEM data showed that the mixed metal oxides catalyst prepared by coprecipitation had better textural properties than the mixed metal oxide catalyst prepared by simple mixing, hence more superior catalytic activity for degradation of pollutants, TOC removal and DBP minimization, however, Fe:Co (Mixed) was the only catalyst material to effectively minimize bromate formation through lowering of solution pH. The chloride ion was found be refractory towards ozone, which is an added advantage during water treatment processes. NH3-TPD analysis and pZc values reveal that Fe alone has negligible acidic sites, whereas, Fe:Co (Co-ppt) and Fe:Ni (Co-ppt) have more acidic sites than Fe:Co (Mixed) and Fe:Ni (Mixed), hence improved decomposition of ozone to hydroxyl radicals on these active sites. The presence of 𝐻2𝑂2 showed an improvement in the debromination efficiency of 2,4,6- TBP. TOC data indicated that total mineralization of OBP’s occurred in the 𝑂3/𝐻2𝑂2 process, which was not achievable in ozonation alone. Only 10% 𝐻2𝑂2 was able to effectively lessen 𝐵𝑟𝑂3 − formation. In basic water both 2,4,6-TBP conversion and TOC removal decreased with an increase in 𝐶𝑂3 2−, hence minimizing 𝐵𝑟𝑂3 − formation.
  • Item
    Synthesis, characterisation and antibacterial activity of new quinoline and quinoxaline hybrid molecules.
    (2018) Sewpersad, Shirveen.; Koorbanally, Neil Anthony.
    This thesis focussed on the synthesis of three series of hybrid molecules, all related to the quinoline or quinoxaline scaffolds. These are (i) 2-(1H-benzo[d]imidazol-2-yl)quinolines, (ii) quinoxaline-chalcones and their pyrazoline derivatives, and (iii) 5-(quinolin-2-ylmethylene)- 2-thioxothiazolidin-4-ones. The target molecules were fully characterized particularly by 2D NMR and verified by high resolution mass spectrometry. The synthesised compounds were tested for their antibacterial activity against two Gram +ve species, Staphylococcus aureus and MRSA and four Gram -ve bacteria, Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli and Salmonella typhimurium to identify lead compounds which could be developed into active pharmaceutical ingredients for antibiotics. A total of fifty-six compounds were synthesised based on these scaffolds. The 2-(1H-benzo[d]imidazol-2-yl)quinolines were synthesised by the Doebner-Miller reaction of crotonaldehyde and substituted anilines, followed by oxidation and benzimidazole formation with o-phenylenediamine. The series of compounds consisted of 6- and 8- substituted halogen, methoxy and methyl groups. A complete structural elucidation of all compounds was carried out and the effects that the different substituents had on the resonances of the quinoline scaffold reported. The quinoxaline-chalcone and quinoxaline-chalcone-quinoline hybrids were synthesised from a quinoxaline acetophenone derivative and various 2- or 4- substituted benzaldehydes and 6- or 8- substituted quinoline-2-carbaldehydes via the Claisen-Schmidt condensation. These molecules were then converted to their respective pyrazoline derivatives using hydrazine hydrate. The 5-(quinolin-2-ylmethylene)-2-thioxothiazolidin-4-ones were synthesised from the Knoevenagel reaction of various 8- and 6-substituted quinoline-2- carbaldehydes and rhodanine. The method described is a convenient way to tag a rhodanine moeity onto a quinoline ring.Of the three sets of compounds synthesised, only the quinoxaline-chalcone-quinoline hybrids showed appreciable antibacterial activity, being active against the Gram +ve S. aureus and MRSA and not against the Gram -ve species, with the activity against S. aureus being much higher than that for MRSA. Quinoxaline chalcones showed no activity against the bacterial strains tested, however, when a quinoline moiety replaced the aromatic ring, eight derivatives showed enhanced antibacterial activity, having MBC values between 0.151-0.360 M against S. aureus and 10.9-618 M against MRSA. The quinoxaline-chalcone-quinoline hybrids showed antibacterial activity two orders of magnitude greater than ciprofloxacin and levofloxacin against S. aureus and comparable activity to these standards against MRSA. Interestingly, the 4'-Br and 4'-Cl pyrazoline derivatives of the quinoxaline chalcones (having MBC values of 11 and 12 M, respectively) showed comparable activity to levofloxacin and ciprofloxacin.
  • Item
    2D3V electromagnetic particle-in-cell simulations of plasmas having kappa velocity distributions.
    (2018) Abdul, Reginald Francis.; Mace, Richard Lester.
    It is now well established that the kappa distribution is a more appropriate kinetic model for space plasmas than the Maxwellian distribution. In particular it possesses a power-law tail, frequently observed in space plasmas. The research presented in this thesis outlines the development of a two-dimensional electromagnetic particle-in-cell (PIC) simulation code, designed to run on general purpose graphics processing units (GPGPUs), and presents results from simulations of waves and instabilities obtained using it. While PIC simulations are not new, the majority have focussed on the old paradigm of initial particle loadings with a Maxwellian velocity distribution, or one of its variants. Distinguishing this research from previous PIC simulations is the use of the kappa distribution for the initial particle loading. To achieve this, a fast and e cient algorithm for generating multi-dimensional kappa distributed deviates was developed. The code is rst applied to the study of waves in an electron-ion plasma, in a stable equilibrium con guration with a constant background magnetic eld. Both species are modelled by isotropic (a) kappa and (b) Maxwellian velocity distributions. In each case, spectral analysis of the eld uctuations is performed, allowing mode identi cation. For parallel propagation, the maximum uctuation intensities follow the dispersion relations for the L and R modes, respectively, while those at perpendicular propagation follow the dispersion relations for the X, O and electromagnetic electron and ion Bernstein waves. The variation of wave intensity for the oblique angles is also investigated. For the kappa case, this yields new and important information presently unavailable by analysis alone. The e ects of the kappa distribution on wave intensity, as well as its e ect on the dispersion relations of the modes is discussed in detail. The second application is to the simulation of the electron temperature anisotropy driven whistler instability in an electron-ion plasma, where the electron species is modelled by the (a) bi-kappa and (b) bi-Maxwellian velocity distribution. For parallel propagation, the maximum eld uctuation intensities agree well with the dispersion relation for the whistler instability in a kappa plasma. While most of the wave intensity is in the parallel whistler mode, the oblique modes also contribute signi cantly to the overall uctuation spectrum, but their intensities vary with angle of propagation relative to the magnetic eld. The dependence of the growth rate on the index e of the electron kappa distribution is discussed in detail and compared with the well known Maxwellian results. Saturation of the instability via pitch angle scattering, reducing the electron temperature anisotropy, is observed.
  • Item
    Synthesis and biological activity of quinoline derivatives.
    (2018) Govender, Hogantharanni.; Koorbanally, Neil Anthony.
    A total of 42 compounds, which include 12 quinoline-3-carboxamides (A5a-l), 15 quinolone thiosemicarbazones (B5a-o) and 15 tetrazoloquinoline phenylhydrazones (C5a-o) were successfully synthesized and characterised. Of these 42 compounds, 36 have been prepared for the first time in this work, with 4 thiosemicarbazones and 2 phenylhydrazones being previously reported. The 2-chloroquinoline-3-carbaldehyde intermediate was the basic scaffold on which the three types of quinoline hybrids were based on. Various substituents were placed at C-6 on this substituent to create small libraries of compounds. This key intermediate was prepared using the Vilsmeier-Haack reaction, which resulted in a quinoline with a carbaldehyde at C-3. In the case of the quinoline-3-carboxamides, the aldehyde at position 3 was oxidized to the acid functionality via the Pinnick Oxidation. This was followed by the preparation of carboxamides using a coupling reaction with different substituted anilines and using the coupling reagents EDC∙HCl and HOBt in the presence of triethylamine used as a base. The thiosemicarbazones were prepared by condensing the 2-quinolone carbaldehydes with thiosemicarbazides and the tetrazolophenylhydrazones prepared by first forming a tetrazolo ring with sodium azide on the quinoline and then forming phenylhydrazones from the carbaldehyde moiety and phenylhydrazines. The quinoline scaffold was varied at C-6 with Cl, F, Br and CH3 groups and the various hybrids were varied again using different anilines, thiosemicarbazides and phenylhydrazines. The structures of the synthesised compounds were elucidated using 1D and 2D NMR spectroscopy. The synthesised compounds were tested for their antibacterial activity against two Gram positive (Staphylococcus aureus and S. aureus Rosenbach (MRSA)) and four Gram negative species, Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli and Salmonella typhimurium. Although compounds in all three classes of hybrids showed antibacterial activity, these were not as good as current drugs being used as antibiotics. The synthesised compounds showed antibacterial activity in the range of 0.80 to 36.49 mM.
  • Item
    Partitioning studies of polychlorinated biphenyls between aqueous solution and soil and sediment systems of Umngeni river, KwaZulu-Natal, South Africa.
    (2018) Adeyinka, Gbadebo Clement.; Moodley, Brenda.
    The significance of soil and sediment physicochemical properties and the environmental parameters such as pH, temperature, ionic strength, humic acid (HA) and time on the partitioning action of eight selected polychlorinated biphenyl (PCB) congeners were critically evaluated in this study to better understand the mobility, transportation, fate and distribution of hydrophobic organic pollutants in environmental media. Natural soil samples used in this study were collected along uMngeni River of KwaZulu-Natal province of South Africa. The mineral properties of soil samples were determined using the Walkley Black method, barium chloride compulsive exchange method and Brunauer-Emmet-Teller (BET) adsorption-desorption isotherm. All these were employed to observe the surface characteristics of the modeled individual soil particle sizes. Scanning electron microscopy (SEM) equipped with energy disperse X-ray (EDX) and Fourier transform infrared spectroscopy (FTIR) were used for the internal morphology and qualitative elemental analysis, as well as identification of possible functional groups in soil samples and commercial HA. Batch adsorption experiments were used for sorption studies. The results revealed that the amount of PCBs sorbed by soil was found to increase with an increase in contact time reaching equilibrium within 8 h. Among the soil chemical properties, soil organic matter was observed to correlate positively and play a more significant role in the sorption of PCBs. Soil samples with highest BET surface areas were related to the soil particle grain size. The sorption of PCBs onto soil was also found to decrease with an increase in the aqueous HA concentrations, and a change in the aqueous concentration of ionic strength was found to be less significant. Other important factors found to be more significant in the sorption were the degree of chlorination as well as stereochemistry of PCB congeners. The more hydrophobic and non-ortho (planar) congeners were found to contribute more significantly to sorption relative to the less hydrophobic and more ortho-substituted (nonplanar) congeners. Moreover, a decrease in the ratio of Si: (Al + Fe) was found to contribute positively to the sorption of PCBs. The kinetic studies on the partitioning of PCBs onto the soils was found to fit best with pseudo-second order, suggesting that the partitioning process of the selected PCBs between aqueous solution and active components in soil, involved more than onestep. Logarithmic values of organic carbon normalized sorption coefficient (log Koc) of the selected PCBs were found to decrease with an increase in the solution pH. The partitioning of PCBs onto the soils was also said to be temperature driven, where low aqueous temperatures encouraged morepartitioning of hydrophobic PCBs onto the soil. The Gibbs free energy (ΔG°) was found to be negative. Therefore, the thermodynamic studies showed that the PCB interaction with soil particle sizes was a spontaneous process. The role of initial PCB concentration on the partitioning was found to be L-type. This indicated that an increase in PCB concentration in the aqueous phase made it more difficult for PCB molecules to find a vacant site available for sorption onto the soil SOM.
  • Item
    Synthesis, characterization and antibacterial activity of curcumin and curcumin-like derivatives.
    (2018) Kannigadu, Christina.; Koorbanally, Neil Anthony.
    This thesis consists of the synthesis, characterization and antibacterial activity of three series of curcumin and curcumin-like compounds. The curcumins were synthesised from acetylacetone and benzaldehydes whilst curcumin-like compounds were synthesised from acetone and either benzaldehydes or quinoline aldehydes. The curcumins were derivatised to pyrazolines and the curcumin-like compounds to ketopyrazoles with hydrazine hydrate and to spiro barbiturates with barbituric acid. The spiro barbiturates in turn were converted to oximes using hydroxylamine hydrochloride. A total of 52 new compounds were synthesised in this work. These compounds were characterized using 1H, 13C and 2D NMR spectroscopy and mass spectrometry. Both the ketopyrazoles and the spiro barbiturates contained sterogenic centers. Single crystal XRD was used to determine the configuration of these stereogenic centers, where the pyrazole was found to be in the 5S and the spiro barbiturates in the 7R,11R configuration. The three sets of synthesised compounds was tested for their antibacterial activity against two Gram +ve strains (Staphylococcus aureus and methicillin resistant S. aureus (MRSA)) and four Gram -ve strains (Salmonella typhimurium, Pseudomonas aeruginosa, Klebsiella pneumonia and Escherichia coli). The best activity was seen by the 6-chloro, 6-bromo and 6- methyl derivatives of quinoline ketodienes, which were active against all six strains of Gram +ve and Gram -ve species at 0.98-31.3 g mL-1, with the exception of the 6-bromo derivative having lower activity against S. aureus at 250 g mL-1. This was followed by the curcumin pyrazolines where several compounds showed good antibacterial activity: the 3-Cl pyrazoline showed activity against both the Gram +ve MRSA (31.3 μg mL-1) and Gram –ve K. pneumonia (7.8 μg mL-1), the 2,4-difluoro pyrazoline showed activity against Gram –ve K. pneumonia (0.98 μg mL-1), and the 3-methoxy-4-(4-chlorobenzyloxy) curcumin derivativeand 3-methoxy-4-hydroxy pyrazoline showed activity against both S. aureus and MRSA at 31.3-62.5 μg mL-1. A further compound the 4-trifluoro curcumin pyrazoline was also active against the Gram –ve E. coli with a MBC of 31.3 μg mL-1. Amongst the ketopyrazoles, the chloro derivatives were active at low concentrations with MBC's between 15.6-62.5 g mL-1 against either S. aureus or MRSA. In addition, the 4- bromo derivative was also active against MRSA and the 2-chloro derivative against P. aeruginosa, both with a MBC value of 31.3 g mL-1. In contrast, the spiro barbiturates showed activity against the Gram –ve E. coli and P. aeruginosa with MBC values ranging from 0.98-125 g mL-1. In particular, the 4-bromo derivative showed excellent activity, better than the standards levofloxacin and ciprofloxacin with MBC values of 0.98 g mL-1. Conversion to the oximes resulted in loss of activity against all the Gram –ve bacteria. However the 4-trifluoro and 4-bromo spiro barbiturate oximes showed weak activity against MRSA.
  • Item
    Development of efficient and chemoselective quasi-heterogeneous catalytic systems for C-C cross-coupling reactions mediated by Pd/Ce based solidsolution catalysts.
    (2018) Mpungose, Philani Perfect.; Friedrich, Holger Bernhard.; Maguire, Glenn Eamonn Mitchel.
    Construction of carbon-carbon bonds is of pivotal importance in chemistry and their careful assembly can allow complex molecules such as peptides to be created. As a result, the development of carbon-carbon bond forming reactions has turned into an area appreciated by Chemists both in academia and industry. In addition, numerous Nobel prizes in chemistry have been awarded for this area of research. The C-C forming reactions have also facilitated the synthesis of natural products, pharmaceuticals, agrochemicals, conjugated polymers and nanomaterials. In this study, the intention is to synthesize heterogeneous palladium based catalysts that can efficiently catalyze Heck-Mizoroki, Sonogashira and Suzuki- Miyaura C-C cross-coupling reactions. The heterogenisation of these coupling reactions improves their overall efficiency, since it allows for easy work-up and the reusability of the expensive palladium catalysts. In this direction, we used palladium and ceria based solid-solution oxides, PdxCe1-xO2-δ, as heterogeneous nanocrystalline catalysts for the three C-C cross-coupling reactions. The PdxCe1- xO2-δ based solid solution oxides were synthesized in one-step using a ureaassisted solution combustion method. These PdxCe1-xO2-δ solid solution oxides were fully characterized by XRD, ICP-OES, BET, XPS, SEM, EDX, TEM, TGA and Raman spectroscopy. All characterization techniques strongly suggested that Pd2+ was successfully incorporated into the lattice structure of ceria. The effect of reaction conditions on the catalytic properties of the PdxCe1-xO2-δ solid-solution oxide catalysts was studied in detail with the model Heck- Mizoroki, Sonogashira and Suzuki-Miyaura cross-coupling reactions to obtain the optimum reaction conditions for each transformation. Then, a wide range of aryl halides was efficiently coupled to various alkenes or alkynes or boric acids. All the PdxCe1-xO2-δ based solid solution oxide catalysts exhibited high activity and afforded the desired products in good to excellent yields. A careful investigation through a series of suitable tests clearly showed that the C–C crosscoupling is accomplished via a quasi-homogeneous mechanism by leached palladium(0) species. Characterization of the used catalyst suggests that Pd2+ in PdxCe1-xO2-δ is reduced in situ to Pd0 when employed in the cross-coupling reactions. However, the PdxCe1-xO2-δ catalysts were found to be both recoverable and recyclable.