Chemistry

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Browsing Chemistry by Author "Botha, Ebrahiem"

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    Quantum computational, structural and electrochemical properties of substituted dithizones and photochromic dithizonato phenylmercury complexes
    (University of the Free State, 2010) Botha, Ebrahiem; Von Eschwege, Karel Grobler; Conradie, Jeanet
    English: For the purpose of investigating the influence on the photochromic reaction of its phenylmercury complexes, a series of dithizone derivatives were at first synthesized. This includes symmetrical dithizone derivatives with electron-donating ability; ortho, meta & para-methyl and methoxy, as well as the 3,4-dimethyl derivative, including some of its disulphide and dehydrodithizone oxidation products. The synthesis of extended aromatic ring dithizone derivatives was attempted, i.e. instead of a phenyl ring the dithizone was extended to have napthyl, fluorenyl, anthracenyl and pyrenyl functional groups. Synthetic procedures were optimized, while conveniently utilizing ammoniumsulphide instead of NH3(g) and H2S(g) as reagent. Phenylmercury(II) complexes were prepared from the successfully synthesized ortho and meta-methoxy, and ortho, meta and para-methyl, and napthyldithizone derivatives. All except the ortho-methyl and naphthyl dithizonates were observed to be photochromic. The ortho-methoxy dithizonate, for the first time, exhibited an alternative color reaction; instead of the usual orange to blue, this compound changed from pink to purple. From the corresponding substituted dithizone the disulphide of ortho-fluorodithizone was synthesized through auto-oxidation, chemical oxidation and electrochemical oxidation. Successful growth of a sizeable crystal of the latter compound ended in solving, for the first time, the X-ray crystal structure of a dithizone disulphide, giving conclusive evidence in support of the long standing proposal of an unstable disulphide intermediate in the oxidation reaction of dithizone. Solving also the X-ray crystal structure of ortho-methoxydehydrodithizone gave indirect structural evidence in support of the foregoing synthesis product. A comprehensive ADF DFT quantum computational study included the successfully and unsuccessfully prepared dithizones and dithizonatophenylmercury complexes. Results unambiguously showed the symmetric form of dithizone and its analogues to be the most stable. TDDFT calculations simulate the trends seen in experimental UV/visible spectra. Graphical representations show molecular orbitals (HOMO & LUMO) extending over the entire length of the ligand, confirming the molecule’s structural importance in the photochromic reaction. Extended dithizonates pose a steric hindrance for photochromism to take place. Electrochemical studies were performed on the above successfully synthesized ligands and complexes, as well as some fluoro-substituted dithizones, exploring the electronic borders within which photochromism ultimately takes place. Complete redox schemes are proposed for the observable redox waves in the cyclic voltammograms of all species involved. An approximate linear relation between experimental oxidation data of the dithizonates and the calculated HOMO energies was found (R2 = 0.89).
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    A structural, electrochemical and thermal study of new mono- and bimetallic long chain carboxylates
    (University of the Free State, 2015-07) Botha, Ebrahiem; Erasmus, Elizabeth
    English: Two methods were used for the synthesis of the mixed-metal carboxylatido complexes. The first method involves the reaction of one equivalent of [Pd3 II(μ-OOC(CH2)nCH3)6] where n = 4, 6, 8, or 10 with three equivalents of the relevant [MII(OOC(CH2)nCH3)2] where n = 4, 6, 8, or 10 or [CeIII(OOC(CH2)8CH3)3], which results in [PdIIMII(μ-OOC(CH2)nCH3)4] where n = 4, 6, 8, or 10 or [PdIICeIII(μ-OOC(CH2)8CH3)4]+ with yields between 56 to 95 %. The second method involves a ligand exchange type of reaction where [PdIIMII(μ-OOCH3)4] or [PdIICeIII(μ-OOCH3)4]+ is reacted with the desired long chain carboxylic acid. This results in [PdIIMII(μ-OOC(CH2)nCH3)4] where n = 4, 6, 8, or 10 or [PdIICeIII(μ-OOC(CH2)8CH3)4]+, with yields between 66 to 99 %. The mono-metal and mixed-metal complexes were characterised using ATR-FTIR. This study indicated that the mixed-metal complexes have more than one binding mode, namely the unidentate, bidentate, tridendate, bridging (syn-syn) binding mode and ionic binding mode. The single crystal X-ray structures of [PdIICoII(μ-OOC(CH2)8CH3)4] [21] (Z = 2, space group P21/c), [PdIIZnII(μ-OOC(CH2)8CH3)4] [36] (Z = 4, space group P21/c), and [PdIINiII(μ- OOC(CH2)8CH3)4] [38] (Z = 2, space group P-1) were solved and confirmed the binding modes observed in the ATR-FTIR studies. Selected complexes were subjected to thermal analysis using DSC and TGA-MS. Liquid crystal properties was observed for PdCo(C8)4 [20], PdCo(C10)4 [21] and PdZn(C10)4 [36]. Polymorphism was observed for PdCd(C10)4 [34], PdMn(C10)4 [39] and PdCu(C10)4 [40]. Variable temperature polarized light microscopy studies was used to shed light on the processes observed using DSC. TGA-MS analysis indicated volatile decomposition products were methane, hydroxide ions, water, carbon monoxide, oxygen, methanol, propyne, carbon dioxide and other products. Non-volatile decomposition product residues obtained were metal oxides. Cyclic Voltammetry, Osteryoung Square Wave Voltammetry and Linear sweep voltammetry was performed on selected complexes and electronic communication between the metals was observed. The length of the carbon chain had an influence on the position of the oxidation wave of the palladium cerium paddlewheel carboxylatido complexes. By increasing the carboxylatido carbon chain length from two to ten, the Epa decreased from 514 mV for [PdIICeIII(μ-OOCCH3)4]+ [32], to 297 mV for [PdIICeIII(μ-OOC(CH2)8CH3)4]+ [41]. Selected mixed-metal paddlewheel complexes were spin coated onto modified silicon wafers using either acetone or DCM as solvent. The pre-catalyst was activated by oxidation in a stream of oxygen at 450 °C. This results in palladium oxide and metal oxide being deposited on the modified silicon wafer surface. The catalysts were tested in the solvent-free aerobic oxidation of 1-octadecanol to 1- octadecanoic acid. The reaction was monitored by following the appearance of the carbonyl stretching frequencies at 1730 and 1710 cm-1 using ATR-FTIR. Turn over frequencies (TOF) between 0.8 to 2 molecules s-1 were obtained for catalysts prepared from short-chain mixed-metal complexes. TOF’s between 4 to 7 molecules s-1 were obtained for catalysts prepared from long-chain mixed-metal complexes. XPS analysis of the catalysts revealed that the PdO and MO (metal oxide) ratio was close to 1:1 and also 1:1.5.