Masters Degrees (Chemistry)

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Application of cobalt complexes containing SNS ligands as catalysts for biomimetic paraffin activation.
(2012) Komarsamy, Lynette.; Bala, Muhammad Dabai.; Friedrich, Holger Bernhard.
A series of SNS ligands have been successfully synthesised and characterised by IR, NMR and MS. The ligands are divided into two groups and represented by the general formulae: 2,6- bis(RSCH2)pyridine [R= methyl, ethyl, butyl, cyclohexyl, phenyl] and bis(RSCH2CH2)amine [R= ethyl, butyl, decyl]. Cobalt complexes of the respective ligands with the general formulae Co[2,6-bis(RSCH2)pyridine]Cl2 and Co[bis(RSCH2CH2)amine]Cl2 were synthesised and characterised by IR, elemental analysis and X-ray crystallography (for selected complexes). Thus, to investigate the electronic and steric effects of the ligand structure on the chemistry and reactivity of the complexes, the substituents bonded to the two sulfur donor atoms were sequentially varied and two different nitrogen sources were chosen. Crystal structures of Co[2,6- bis(CH2SCH2)pyridine]Cl2 (Ia), Co[2,6-bis(CH2CH2SCH2) pyridine]Cl2 (IIa) and Co[2,6- bis(CH2CH2CH2CH2SCH2)pyridine]Cl2 (IIIa) were obtained. It was found that complex Ia exists as a molecular dimer linked through two chloride bridges resulting in an octahedral geometry around each metal centre, while complexes IIa and IIIa are monomers exhibiting a trigonal bypyrimidal geometry. The complexes were tested as catalysts for the activation of paraffinic C−H bonds towards the formation of oxygenated products: octanol, octanone, octanal and octanoic acid from the substrate n-octane. Gas chromatography was utilised to quantify the products formed and also to calculate the conversion and selectivity of each catalyst system. The catalytic testing revealed that the ketone products were the most dominant with selectivities of ca. 90%. The catalyst that was the most active was Co[bis(CH2CH2SCH2CH2)amine]Cl2 (Ib) with a total n-octane conversion of 23%.
Application of triazolium-based metal complexes for catalytic oxidation of octane.
(2014) Mncube, Siyabonga Gift.; Bala, Muhammad Dabai.
A series of related 1,2,3-triazole compounds were synthesised and characterised by spectroscopic methods including NMR, IR, MS and X-ray diffraction. N-alkylation of the triazole compounds yielded 1,4-disubstituted triazolium ionic liquids (3.1-3.6). The ionic liquids were found to act as “green” solvent systems for the dissolution of an iron-based compound for the catalytic oxidation of n-octane. Recycling and re-usage of the system was found to be reproducible for three cycles and leaching of the catalyst to the product phase was associated with decrease of catalytic activity in subsequent cycles. Synthesis and characterisation of 1,2,3-triazolium-based nickel carbene complexes (4.1-4.6) by modification of reported synthetic methods was described. The molecular structures of three metal complexes (4.1, 4.3 and 4.4) were analysed by single crystal X-ray diffraction and fully discussed. The complexes were then tested for catalytic oxidation of alkanes in the presence of various oxidants under mild reaction conditions. Catalyst (4.3) with less bulky substituents on the triazolium ring exhibited the highest catalytic activity (15%) with H2O2 as the most productive oxidant. The main goals of this study were achieved because the catalyst systems were successfully synthesised, characterised and tested for activity in alkane oxidation. Indeed there was significant activity observed with all prepared catalysts, however from the results obtained, the nickel carbene catalysts in the presence of H2O2 demonstrated the highest efficiency in this work.
An investigation into air stable analogues of Wilkinson's catalyst.
(2010) Naicker, Serina.; Bala, Muhammad Dabai.; Friedrich, Holger Bernhard.
Since the discovery of Wilkinson’s catalyst and its usefulness in the homogeneous hydrogenation of olefins many investigations have been carried out on trivalent, tertiary phosphine–rhodium complexes.¹ Studies have shown that N-Heterocyclic carbenes as ligands offer increased stability to the complex and possess similar electronic properties as phosphine ligands.² The applications of the traditional catalyst are limited due to the limited stability of its solutions and its susceptibility to attack from the environment i.e. oxygen and moisture. The hydrogenation of olefins and other unsaturated compound is of great importance for the fine chemical and petroleum industries. The aim is to produce more stable and active versions of the traditional catalyst and also to demonstrate their improved stability and activity in catalytic applications. This study involves the investigation of the effects of ligand modification on Wilkinson type hydrogenation catalysts. Five Rhodium-phosphine complexes 1a: Rh(PPh₃)₃Cl, 1b: Rh(PPh₂Me)₃Cl, 1c: Rh(PPh₂Et)₃Cl, 1d: Rh(PPhMe₂)₃Cl, 1e: Rh(PPhMe₂)₃Cl have been synthesised and characterised by means of melting point,¹H NMR, ¹³C NMR, ³¹P NMR, IR and Mass Spectroscopy. Complexes 1d and 1e have also been characterised by means of elemental analysis and single crystal XRD. Five rhodium-N-heterocyclic carbene complexes 2a: Rh(COD)ImesCl [Imes =1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] , 2b: Rh(COD)(diisopropylphenyl)₂Cl 2c: Rh(COD)(adamantyl)²Cl, 2d: Rh(COD)(diisopropyl)²Cl 2e: Rh(COD)(ditertbutyl)²Cl have been synthesised and characterised by means of melting point, ¹H NMR, ¹³C NMR, IR and Mass Spectroscopy. Five rhodium-NHC-CO complexes 3a: Rh(CO)₂ImesCl, 3b: Rh(CO)₂(diisopropylphenyl)₂Cl, 3c: Rh(CO)₂(adamantyl)₂Cl , 3d: Rh(CO)₂(diisopropyl)₂Cl, 3e: Rh(CO)₂(ditertbutyl)₂Cl, have been synthesised and characterised by means of ¹H NMR, ¹³C NMR, IR and Mass Spectroscopy. Complexes 1a, 1d, 1e, 2a, 2b, 2c, 2d, 2e were tested in the hydrogenation of simple alkenes under mild conditions. For the rhodium-phosphine complexes the catalyst efficiency based on TOF increases in the following order: 1a > 1d > 1e or RhCl₃(PPhMe₂)₃ > RhCl₃(PPhEt₂)₃ > RhCl(PPh₃)₃. For the rhodium-(COD)-NHC complexes catalyst efficiency based on TOF increases in the following order: 2d > 2b > 2e > 2a > 2c. While rhodium-phosphine complexes are far more active than rhodium-(COD)-NHC complexes, the latter seem to be active for a longer time and hence more stable under mild hydrogenation conditions.
New pyrazolyl functionalized N-heterocyclic carbene ligand precursors and their transition metal complexes for the catalytic oxidation of paraffins.
(2021) Ndamase, Liziwe.; Bala, Muhammad Dabai.; Owaga, Bernard Omondi.
Abstract available in PDF.
Synthesis and immobilization of triazolium based ionic liquids as recyclable organocatalysts for the transfer hydrogenation of ketones.
(2015) Dhimba, George.; Bala, Muhammad Dabai.
The aim of the study was to bridge the gap between homogeneous and heterogeneous catalysis by combining the advantages offered by single phase homogeneous catalysts with the ease of separation of heterogeneous catalysts while minimizing the disadvantages of both systems. A common method used is catalyst immobilization on solid supports. Hence, triazoles and corresponding salts have been synthesized by adopting the versatile, green and regioselective Cu(Ι) catalyzed cycloaddition reaction of organic azides and terminal alkynes. The triazolium salts were characterized by various spectroscopic techniques and then tested for activity in the transfer hydrogenation of ketones. Selected triazoles were then immobilized onto straight chain polyethylene glycols (PEGs) of various chain lengths. The immobilization was achieved by the tosylation of PEGs to yield PEG ditosylates followed by N-alkylation which created binding O-N covalent bonds. Characterization by NMR and mass spectrometry confirmed successful immobilization. Salt metathesis was done using sodium iodide in dichloromethane that yielded brick red colored ionic liquids. The formed ionic liquids were then tested for activity in the transfer hydrogenation of acetophenone with isopropanol as a hydrogen donor and solvent. The effects of polymer chain length, electronic effect of the triazolium moiety, reaction temperature and time on the catalysis were investigated. Results indicate that the immobilized triazoles bearing tosylate counter ions were inactive as catalysts. However, the results indicate that for optimum reactivity, PEG600 was the ideal chain length for the immobilized systems. The ionic liquids were then tested for transfer hydrogenation of different substituted ketones. The catalyst system was easily recovered by the addition of diethyl ether after the reaction followed by simple decantation. The immobilized catalysts were recycled three times with a percentage conversion of 82% being observed in the third cycle.
The synthesis of xanthene-based transition metal complexes and their application in the oxidation reaction of n-octane.
(2014) Nyalungu, Nonjabulo.; Friedrich, Holger Bernhard.; Bala, Muhammad Dabai.
The oxidation of alkanes into valuable products such as alcohols, ketones and aldehydes is very important to industry for detergents and perfumes. One of the challenges with alkanes is their inertness which results from the strong and localized C-C and C-H bonds. There are few methods that are known to transform alkanes into products of value. Therefore in this study xanthene-based ligands were used in an attempt to transform alkanes into products of value. Xanthene-based ligands are known to produce catalysts that are highly active and selective in reactions such as hydroformylation and hydrocyanation. These ligands are bidentate and their structure consists of a xanthene backbone with two phosphorus donor atoms and a rigid backbone. Five xanthene-based ligands were synthesized, characterized and complexed to cobalt and nickel. In this study modification at position X was done by using a sulphur atom, a methyl group as well as an isopropyl group in order to observe the effect this has on the activation of noctane. Crystal structures of ligand (4,5-bis(di-p-tolylphosphino)-9,9-dimethyl xanthene) and complex (Co(4,5-bis(di-p-tolylphosphino)-9,9-dimethylxanthene)Cl2) and (Ni(4,5-bis(di-ptolylphosphino)- 9,9-dimethylxanthene)Cl2) were obtained. The five cobalt and five nickel complexes were catalytically tested in the oxidation of n-octane, using three oxidants tert-butyl hydroperoxide, hydrogen peroxide and meta-chloroperbenzoic acid. This was carried out in tetrahydrofuran solvent at varying temperatures. Hydrogen peroxide and meta-chloroperbenzoic acid gave no substantial activation, while tertbutyl hydroperoxide showed activity. Modifications to the backbone at position X brought changes to the bite angle and minor changes to activity. Selectivity at 50 and 60 °C favoured the C-2 position with 2-octanone as the dominant product. Terminal position showed no products of alkane oxygenation. Alcohols (3-octanol and 4-octanol) were observed at higher temperatures. Steric factors had no significance effect on activity while temperature had a greater effect. The temperature that was best to work with was 50 °C since all catalysts were active. Sulphur had a deactivating effect on efficacy of both cobalt and nickel catalysts.

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Browsing Masters Degrees (Chemistry) by Author "Bala, Muhammad Dabai."