Masters Degrees (Chemistry)

Permanent URI for this collectionhttps://hdl.handle.net/10413/6597

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Determination of neonicotinoid insecticides in water, soil and sediment samples: acute and chronic risk assessment.
(2022) Ngomane, Nkosinathi Chris.; Mahlambi, Precious Nokwethemba.
Neonicotinoids are a type of insecticides pesticides widely used worldwide as a result of their low vertebrates toxicity, relative environmental stabilities, good bioavailability and high level of selectiveness. These insecticides are commonly employed in agricultural activities, in grass management and horticulture as well as in households to control domestic pet flea. Due to neonicotinoids intensive usage, they are continuously introduced to the water bodies where they can adversely affect the aquatic life and accumulate in sediments. Moreover, they can end up in drinking and unintentionally consumed by human beings resulting to health effects. With this regard, this work reports for the first time on the occurrence of neonicotinoids in sediment, soil tap, sludge, wastewater and river water samples from the province of KwaZulu-Natal. Also, the ecological risk of neonicotinoids in water sources was also assessed for the first time in the samples from this province.The liquid chromatography coupled with a photo-diode array detector (LC-PDA) method was modified and applied for the simultaneous detection of neonicotinoids (clothianidin, thiamethoxam and imidacloprid). Ultrasonic extraction (UE), soxhlet extraction (SE) and solid-phase extraction (SPE) methods were developed and applied for the extraction of nitro-guanidine neonicotinoids in water, soil and sediment samples. The SPE, SE, and UE parameters that influence the recoveries of the analytes were first optimized before application to real samples for the analytes recovery improvement. The SPE was used for the extraction of neonicotinoids in sludge and water samples, while SE and UE were both used to extract soil and sediment samples. The extraction conditions optimized for SPE were conditioning solvent and sample volume. While for the UE were extraction time, extraction solvent, and the solvent volume. And for SE method, extraction solvent and the extraction solvent volume were optimized. The LC-PDA method used for detection was also first optimized to improve peak separation, retention times, detection limits and quantification limits. The optimized parameters for the LC-PDA method were the mobile phase, flow rate, and the PDA detection wavelength. Optimum water recoveries of the neonicotinoids ranged from 79 to 112%. The detection and quantification limits of the analytes in water samples were 0.013 - 0.031 μg/L and 0.041 - 0.099 μg/L, respectively. The obtained analytes concentration ranged from 0.061 - 0.10 μg/L, 0.077- 3.76 μg/L and 0.99 - 15 μg/L in tap, river and wastewater, respectively. Analyte recoveries ranged from 85 - 102% in soil and 92 - 103% in sediment for the ultrasonic extraction method. The neonicotinoid recoveries ranged from 83 to 109% in soil and between 84 to 94% in sediment samples for the Soxhlet extraction method. The method’s detection limits and quantification limits in solid samples ranged from 40 - 80 μg/kg and 140 - 270 μg/kg, respectively. The relative standard deviation was less than 4%. The concentration determined in real environmental samples were 47 to 410 μg/kg in soil and 25 to 410 in sediment. The toxicity studies showed that clothianadin pose a high risk towards daphnia species in the river. Imidacloprid, clothianidin and thiamethoxam posed medium risk against algae, daphnia and fish species in the effluent receiving water bodies. These results imply the necessity to continuously monitor these neonicotinoids in the water sources. In South Africa there is limited data concerning the environmental occurrence of neonicotinoids, therefore this work will contribute towards the information available for the analysis of neonicotinoids. This will assist the policy makers to establish the MRL values that are precise for the African continent.
Syntheses of (pyridyl) pyrazine carboxamide palladium(II) complexes, their DNA/BSA interactions, and cytotoxicity studies.
(2023) Mvelase, Sabathile Thandeka.; Ojwach, Stephen Otieno.
Reaction of pyrazine-2,3-dicarboxylic acid with a respective amine in the presence of triphenyl phosphite afforded the corresponding carboxamide ligands: [N2, N3-bis(pyridin 2-yl)pyrazine2,3-dicarboxamide] (L1), [N2, N3-bis(6-methylpyridin-2-yl)pyrazine-2,3-dicarboxamide] (L2), [N2,N3-bis(4-methylpyridin-2-yl)pyrazine-2,3-dicarboxamide] (L3), and [N2, N3bis(quinoline-8-yl)pyrazine-2,3-dicarboxamide] (L4). Treatments of the corresponding (pyridyl)pyrazine carboxamide ligands with [PdCl2(NCCH3)2 afforded new mononuclear and dinuclear palladium(II) complexes with a general formula, [Pd2(L1)2Cl2] (Pd1), [Pd2(L2)2Cl2] (Pd2), [Pd2(L3)2Cl2] (Pd3) and [Pd(L4)Cl] (Pd4). The identities and coordination nature of the palladium(II) complexes were established through a combination of characterization techniques such as NMR, FT-IR spectroscopy, mass spectrometry, elemental analysis, and single X-ray crystallography. The molecular structures of the dinuclear Pd1 and Pd3 complexes reveal that the two (pyridyl) pyrazine carboxamide ligands coordinate with the palladium atom via one arm, while the other arm remains non-coordinating. The ligands are bridged by two palladium atoms to form dinuclear palladium(II) complexes. While one ligand coordinates to the palladium in a bidentate fashion via the nitrogen atoms of the pyrazine and amide groups, the other ligand unit coordinates to the palladium through the pyridine nitrogen atom to give two ligand units and two palladium atoms in the complex coordination sphere. On the other hand, ligand L4 binds to palladium atom in a tridentate fashion via the pyrazine, amide, and pyridine nitrogen atoms to give complex Pd4 as a mononuclear species. The interaction of the palladium complexes (Pd1-Pd4) with calf thymus DNA (CT-DNA) was monitored using UV-Vis, and fluorescence spectroscopy. Absorption spectroscopic studies revealed that complexes Pd1-Pd4 interact with CT-DNA via intercalative mode, and the computed intrinsic binding constant (Kb) values range from (4.28-13.12) x 106 M-1. In addition, Ksv values of (1.82-28.41) x 106 M-1 and KF values of (1.01-53.44) x 104 M-1 determined in competitive binding studies confirmed the intercalative binding mode. The interaction of the complexes with CT-DNA decrease in the order of Pd3 > Pd2 > Pd1 > Pd4. Furthermore, bovine serum albumin (BSA) interaction was evaluated using fluorescence studies and the results revealed the existence of a static quenching mechanism with bimolecular constant, kq range of (0.66-13.99) x 1014 M-1 s-1. The Ksv values of (1.48-29.67) x 106 M-1 and KF values of (0.10-16.10) x 105 M-1 confirmed the interaction between BSA and palladium complexes. The interaction follows this order Pd2 > Pd1 > Pd3 > Pd4, which is inconsistent with the CT-DNA trend. In general, ligands bearing electron-donating methyl groups (L2 and L3) contributed to higher binding constants in their respective complexes Pd2 and Pd3 compared to the unsubstituted complex Pd1. In addition, the mononuclear complex Pd4 showed the weakest interactions with both the DNA and BSA, pointing to some beneficial effects of increased metal atoms in the complexes. The cytotoxic effect of the ligands L1-L3 and complexes Pd1-Pd4 were examined against a human breast cancer cell line (MCF-7) using 3-(4,5-Dimethyl-2thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. In general, the ligands displayed poor cytotoxicity L1 (IC50 > 400 μM), L2 (IC50 = 182.4 μM), L3 (IC50 = 80.2 μM), when compared to their respective palladium(II) complexes Pd2 (IC50 = 154.9 μM), Pd3 (IC50 = 230.1 μM). Complexes Pd1 (IC50 = 11.4 μM), and Pd4 (IC50 = 61.5 μM) displayed high and moderate cytotoxic activity which was attributed to the planarity of the complexes.
Synthesis and the application of molecularly imprinted polymers as solid-phase extraction and dispersive solid-phase extraction sorbents in the extraction of antiretroviral drugs in water.
(2022) Xolo, Thabiso.; Mahlambi, Precious Nokwethemba.
A multi-template molecularly imprinted polymer was synthesized, characterized, and applied to real water samples for the specific extraction of antiretroviral drugs (ARVDs), abacavir, nevirapine, and efavirenz. A MIP was synthesized by bulk polymerization at 60 ºC using abacavir, nevirapine and efavirenz (templates), 2-vinylpyridine (functional monomer), 1,1’- azobis-(cyclohexanecarbonitrile) (initiator), ethylene glycol dimethacrylate (cross-linker) and toluene: acetonitrile (9:1, v/v) (porogenic solvent mixture) for 16 hours. The temperature was then increased to 80 ℃ for 24 hours to ensure complete polymerization. The synthesized MIP was washed with acetic acid: acetonitrile (1:9, v/v) via soxhlet extraction until all three ARVDs were undetected in the washing solvent using high performance liquid chromatography coupled with a photo-diode array detector. A non-imprinted polymer (NIP) was synthesised using the same reagents and quantities except for the templates. Both MIP and NIP were characterized using Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscope (SEM), N2 physisorption analysis, and elemental analysis. The FTIR results showed that the polymers were similar in structure and BET showed that they were mesoporous. The SEM showed that the MIP surface was rougher when compared to the NIP and characterization with TGA showed that they were both thermally stable. The synthesized MIP was used to study its adsorption kinetics and isotherms. Kinetics modelling revealed that the Pseudo-second-rate order kinetics was the best fitting model with correlation coefficient of 1 compared to Pseudo-first-rate order kinetics which had a correlation efficient of 0.81-0.983 for all target analytes. The best fitting adsorption isotherm was the Freundlich model with a correlation range of 0.9451-0.986 for compared to the Langmuir model which had correlation efficient of 0.6692-0.93390.0198-0.6782 for all target analytes. The traditional solid phase extraction and the MIP-based solid phase extraction methods were applied in distilled water samples spiked with 1 mg.L-1 of ARVDs and recoveries obtained were ranging from 91.68-94.59% and 97.20 to 99.68%, respectively. The MIP-based dispersive solid phase extraction method was successfully optimized with recoveries ranging from 100.28% to 102.60% for all three analytes. Selectivity studies were conducted using both the NIP and MIP with lamivudine and diclofenac as competitors. The recoveries obtained for the MIP ranged between 92% to 98% for the target analytes while they were 63% to 79% for competitors. These results showed good selectivity and strong affinity of the polymer towards the target analytes than the competitors. This is justified by the presence of imprinted recognition sites that have the same functional groups, size, and shape as the target analytes/templates hence recoveries were low for competitors. The MIP was more selective towards analytes of interest compared to the NIP (recoveries ranged from 87.9% to 91%) for the analytes of interest which indicates successful imprinting on the MIP. Reusability studies showed that the MIP can be reused for up to 8 cycles with recoveries above 92% for all target analytes. The developed, adopted, and validated methods were then applied to wastewater, tap water and river water samples from around KwaZulu-Natal. The concentrations obtained for abacavir, nevirapine and efavirenz were 10.65-295.90 μg.L-1 in wastewater, 1.95-13.15 μg.L-1 in river water, and 2.17-6.27 μg.L-1 in tap water. Efavirenz was the most dominant and consistently detected ARVD in all samples. The MIP-DSPE was the most sensitive and selective extraction technique compared to SPE and MIP-SPE. Umhlathuzana and Amanzimtoti were the most ARVD’s polluted wastewater treatment plants, whilst Northern wastewater water works was the least polluted. Camps Drift was the most ARVD’s polluted sampling point in Msunduzi river. Napierville and Scottsville showed the most contaminated tap water samples from suburbs around Pietermaritzburg.
Synthesis of ketones through aerobic transition metal-catalyzed cross-coupling of thioesters.
(2021) Mthombothi, Zabenguni Zakithi.; Sithebe, Siphamandla.
Asymmetric diaryl ketones are an important class of compounds in organic chemistry due to their presence in natural products, synthesis, cosmetics as well as in biological active compounds. The aim of this project was to expand the scope of thiophilic metal carboxylates catalysts that can be applied to the aerobic Liebeskind-Srogl cross coupling reaction between various thioesters and phenylboronic acids. Thioesters bearing electron neutral, withdrawing and donating groups were successfully synthesized in yields ranging from 35 to 54%Xcv. The electron neutral thioester was used in the optimization of the aerobic Liebeskind-Srogl reaction and CuMeSal proved to be the most effective catalyst in this protocol. CuMeSal was applied in the aerobic synthesis of the asymmetric diaryl ketones bearing a wide range of functional groups including CF3, SMe, OMe and F yielding up to 65% in 24hrs. Aerobic Liebeskind-Srogl cross-coupling reaction was applied in the successful synthesis of chalcones, through the coupling of commercially available phenylboronic acids with previously synthesised thioesters catalysed by CuMeSal in DMF for 24hrs,furnishing desired products in poor to excellent yields 26-89%.
Synthesis, characterization and substitution reaction studies of pyridyl N,N’-bidentate palladium(II) complexes. A kinetic and mechanistic study.
(2023) Mjwara, Pinky Ncomela.; Sithebe, Siphamandla.; Papo, Tshephiso Rose.
The influence of structural as well as electronic properties of bidentate N,N chelates with different substituents on the mononuclear Pd(II) complexes were investigated. The complexes were synthesized and characterized by various spectroscopic methods such as 1H & 13C NMR, FT-IR, LC-MS, CHN and single x-ray crystallography. For the first set of complexes (Chapter 3), we studied the unexplored kinetics and mechanistic behaviour of N,N’-pyridyl Pd(II) complexes, viz. dichloro-(N-((pyridin-2-yl)methyl)aniline)palladium(II) (PdL1), dichloro-(4-fluoro-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL2), dichloro-(4-bromo-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL3), dichloro-(4-methoxy-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL4) and dichloro-(4-ethyl-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL5). The substitution behaviour of coordinated chloride atoms by three bio-relevant thiourea nucleophiles, viz. thiourea (TU), N,N’-dimethylthiourea (DMTU) and N,N,N’,N’-tetramethylthiourea (TMTU), of different steric demands was studied in a 0.1 M solution of ultra-pure water under pseudo-first order conditions. The reactions were studied as a function of concentration and temperature using standard Stopped-Flow and UV-Vis spectrophotometric technique. The substitution of the chloride atoms from the Pd metal by thiourea nucleophiles was a two-step reaction where the chloride trans to the pyridine ligand was substituted first, since the pyridine has a stronger trans effect compared to the amine group. The reactivity of mononuclear Pd(II) complexes containing bidentate N,N’-donor ligands with different substituents depends on the electronic effects of the complexes. The reactivity of the complexes increased with the presence of electron withdrawing substituents and decreased when an electron donating group was attached on the para position of the aniline moiety. The electron withdrawing groups influence the pull of electrons from the electron deficient amine that is coordinated to the metal center which results in the loss of electron density from the ligand moiety and increases the electrophilicity of the metal center and thus the substitution reaction. The reactivity of the nucleophiles depends on steric effects, with the bulky TMTU being the least reactive. The negative entropies and second order kinetics for all the substitution reactions support an associative mode of substitution mechanism. DFT calculations were performed to account for the observed reactivity of all the complexes studied. For the second set of novel Pd(II) complexes (Chapter 4), viz. bis[N-(4-bromophenyl)pyridine-2-carboxamidato] Palladium (Pd1) and Palladium(II) [N-(4-bromophenyl)-2-pyridinecarboxamide), pyridine chloride (Pd2), crystals were obtained and the structures were studied. Pd1 crystallizes in the monoclinic crystal system and in the P21/c space group, and Pd2 crystallizes in the orthorhombic system, with the space group Pbca.
Vinyl-addition polymerization of norbornene catalyzed by (pyridyl)imine Ni(II), Pd(II), Co(II), and Fe(II) complexes.
(2023) van der Westhuizen, Barend Danielle.; Ojwach, Stephen Otieno.
The thesis reports the syntheses, and structural characterization of (pyridyl)imine transition metal complexes and their applications as pre-catalysts in the vinyl-addition polymerization of norbornene. The bidentate ligand (E)-N-(1 phenylethyl)-1-(pyridin-2-yl)methanimine (L1) was synthesized by reactions of 2-pyridine carboxaldehyde with (R)-(+)-α-methyl benzylamine in the presence of p-TsOH and obtained in moderate yields of 64%. On the other hand, the tridentate ligand (E)-1-(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanimine (L2) was obtained by reacting 2-pyridine carboxaldehyde with 2-picolylamine in the presence of p-TsOH in high yields of 90%. Reactions of bidentate ligand L1 and tridentate ligand L2 with NiCl2, [Pd(COD)Cl2], FeCl2, and CoCl2 metal salts gave the corresponding complexes [Ni(L1)2Cl2] (Ni1), [Ni(L2)Cl2] (Ni2), [Pd(L1)Cl2] (Pd1), [Co(L1)3][2PF6] (Co1), [Co(L2)Cl2] (Co2), [Fe(L1)3][2PF6] (Fe1), [Fe(L2)Cl2] (Fe2) in low to moderate yields (18% - 60%). The identities of the isolated complexes were confirmed by characterization with FT-IR spectroscopy, mass spectrometry, elemental analysis, single crystal X-ray crystallography for Co1, and nuclear magnetic resonance where applicable. The solid state structure of complex Co1 was established as tris-chelated, containing three ligands to give an octahedral coordination environment. The metal complexes were evaluated as pre-catalysts in the vinyl-addition polymerization of norbornene to produce poly(2,3-bicyclo[2.2.1]heptene) using modified methyl aluminoxane (MMAO) as the co-catalyst. Complex Ni1 was the most active with catalytic activity of 22.7 g×10³(PNBE).mol(M)-¹.h-¹ followed by complex Pd1 which showed catalytic activity of 17.6 g×10³(PNBE).mol(M)-¹.h-¹ whereas complex Co1 showed 0.7 g×10³(PNBE).mol(M)-¹.h-¹ and complex Fe1 showed catalytic activity of 0.3 g×10³(PNBE).mol(M)-¹.h-¹ concluding that the choice of metal center is of absolute importance to achieve high catalytic activity. The number of electron donor atoms in the ligand structure influenced catalytic activity as bidentately chelated complex Ni1 showed catalytic activity of 22.7 g×10³(PNBE).mol(M)-¹.h-¹ whereas the tridentately chelated Ni2 showed catalytic activity of 81.9 g×10³(PNBE).mol(M)-¹.h-¹. The influence of reaction parameters were investigated using Ni1 and Ni2 as pre-catalysts and it was concluded that monomer/metal ratios, co-catalyst/metal ratios, reaction temperatures, reaction times, and solvent choice influenced catalytic activity. A higher monomer/metal ratio of 1250 resulted in catalytic activity of 17.5 g×10³(PNBE).mol(M)-¹.h-¹ compared to the value of 6.8 g×10³(PNBE).mol(M)-¹.h-¹ obtained from a lower monomer/metal ratio of 625. An optimum co-catalyst/metal ratio of 1500 was established and recorded catalytic activity of 33.7 g×10³(PNBE).mol(M)-¹.h-¹. Polymerization reactions at room temperature gave higher monomer conversions of 70% as opposed to lower conversions of 17% obtained at 50 °C. The choice of solvent influenced catalytic activities of the complexes, with the more polar o-chlorobenzene solvent giving the highest monomer conversion of 70% in comparison to 31% obtained in toluene solvent. Polymers formed from all complexes were of the vinyl type with possible ring-opening metathesis polymeric inserts present in the polymer backbone. Thermal gravimetric analysis and differential scanning calorimetry of the formed polymers demonstrated that polymers formed from complex Co2 and complex Fe2 displayed degradation temperatures of 492 °C and 478 °C respectively opposed to polymers formed from complex Ni2 and complex Pd2 which gave values of 478 °C and 462 °C.

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