Chemistry

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Browsing Chemistry by Author "Bezuidenhout, Ben"

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    Bulky metal complexes as model nanoscale catalysts
    (University of the Free State, 2012-11) Young, Cyril; Roodt, Andreas; Bezuidenhout, Ben
    English: The lifestyle of modern society has created a massive demand for various chemicals such as fuels, chlorine-free refrigerants, high-strength polymers, stain-resistant fibres, cancer treatment drugs and thousands of other products. The demand for these compounds can only be met through the use of catalysts. Heterogeneous catalysis has become a fundamental part of the industrial scale production of these chemicals. Although heterogeneous catalysis is better suited for these processes than its homogeneous counterpart, some of the systems are plagued by poor distribution of the active metal species throughout the support. The aim of this study was to investigate the feasibility of synthesising robust, planar, bridging ligands that could act as spacers between active metal species in the deposition of active catalysts onto heterogeneous supports. By choosing different building blocks, for the simple Schiff base reaction, the distance and proximity between active metal species could theoretically be controlled for a desired application. 1,10-Phenanthroline and diamide type ligands (Figure 1) were identified as possible candidates for this application Figure 1: Different ligand systems identified as possible dispersion spacers. (A) represents the diamide type ligands and (B) the 1,10-phenanthroline ligands. The aim of this study was pursued by the identification and synthesis of building blocks such as 5,6-diamino-1,10-phenanthroline and 1,10-phenanthroline-5,6-dione which could act as bridging ligands and could be used to construct larger bridging systems. The bridging ligands and building blocks were coordinated to square planar metal centres such as platinum and palladium. This would enhance the possibility of creating a single layer network on the surface of the support. The ligands and complexes were characterised using solid state techniques and single crystal X-Ray Diffractometry to investigate the planarity of these species and the coordination mode to some of the diamide type complexes that have not found many applications in this field. The Heck coupling was identified as a standard reaction which could be utilised to test the catalytic properties of the palladium species. The catalytic activity of a range of diamide and 1,10-phenanthroline type ligands was evaluated after the optimisation of the Heck coupling. It was found that reducing the electron density on the five and six position of the phenanthroline ring drastically enhances the catalytic capabilities of these compounds. The diamide type complexes and larger bridging ligands showed less promising results.
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    Nanoscale model catalysts supported by metal organic frameworks
    (University of the Free State, 2009-11) Young, Cyril; Roodt, Andreas; Bezuidenhout, Ben
    English: The aim of this study was to use square planar platinum(II) and palladium(II) complexes for the synthesis of bridged complexes and as building blocks for the subsequent construction of two dimensional metal organic frameworks. For this purpose, pyrazine, 2,2‟ -bipyrimidine (bpym) and 4,4‟-bipyridine (4,4-bipy) were chosen as rigid bridging ligands. 4,5-Diazafluoren-9-one (Daf) was used as a non-bridging ligand in some of the investigations. The optimum conditions for crystallization of these bridged species were determined in order to co-crystallize the complexes with synthesized crystalline inorganic supports, which consisted of titanium and tungsten oxide. Characterization of the complexes was done by Infrared spectroscopy and Nuclear Magnetic Resonance spectroscopy (NMR). Single crystal X-ray crystallographic structure determinations of the following compounds were completed: cis-[Pd(PPh3)2(Daf)2].-(Trif)2.CH3NO2, cis- [Pd2BpymCl4].DMF and cis-[Pd(PPh3)2(NO3)2].CO(CH3)2 . cis-[Pd(PPh3)2(Daf)2].(Trif)2.CH3NO2 crystallized in the monoclinic space group P2/n with Z =4 in a unit cell with dimensions: a = 15.184(1) Å, b = 17.112(1) Å, c = 22.256(2) Å, α = 90 °, β = 99.676(4) °, γ = 90 ° at 100 K. Least-squares refinement led to a value of the conventional R index of 0.0750 and Rw = 0.1930 for 12345 reflections having I > 2σ(I). The palladium ion is coordinated cis with two phosphorus and two nitrogen atoms. The formal geometry of the palladium center is square planar. In addition there is an interaction between uncoordinated nitrogen atoms of 4,5-diazafluoren-9-one ligand in the axial positions, which results in the formation of a pseudo octahedral structure. A 50% disorder is present on one of the phenyl rings and one of the triflate counter ions. The Rw value is high as the result of one carbon atom and its hydrogen atoms not being located on the disordered acetone solvate. cis-[Pd2BpymCl4].DMF crystallized in the monoclinic space group C2/m with Z = 2 in a unit cell with dimensions: a = 10.73(7) Å, b = 14.24(8) Å, c = 5.938(2) Å, α = 90 °, β = 108.229(4) °, γ = 90 ° at 100 K. Least-squares refinement led to a value of the conventional R index of 0.032 and Rw = 0.0893 for 1110 reflections having I > 2σ(I). The complex consists of two palladium centers linked by the 2,2‟-bipyrimidine ligand. Each palladium atom is coordinated by two chlorido ions, two nitrogen atoms and has a square planar geometry. A 25 % disorder is found on the DMF solvate. cis-[Pd(PPh3)2(NO3)2].CO(CH3)2 crystallized in the monoclinic space group C2/c with Z = 4 in a unit cell with dimensions: a = 40.343(2) Å, b = 9.749(1) Å, c = 19.78(1) Å, α = 90 °, β = 115.514(1) °, γ = 90 ° at 100 K. Least-squares refinement led to a value of the conventional R index of 0.0773 and Rw = 0.1873 for 8638 reflections having I > 2σ(I). The complex consists of a square planar geometry at the palladium center coordinated to two oxygen atoms and two phosphorus atoms in a cis fashion. The high Rw value is a result of one carbon and its hydrogen atoms not being located on the disordered acetone solvate. A preliminary catalytic investigation was attempted using cis-[Pd2BpymCl4].DMF in Heck coupling and the Wacker oxidation process. However, no reproducible results were obtained due to infrastructural and time constraints.