Masters Degrees (Chemistry and Polymer Science)

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Browsing Masters Degrees (Chemistry and Polymer Science) by Author "Barnard, Ilse"

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    Synthesis, characterization and 59Co NMR study of novel Co(III) complexes with selected N-acyl-N’,N’-dialkylthiourea ligands: an assessment of spontaneous metathesis by multinuclear NMR spectroscopy and rp-HPLC
    (2016-12) Barnard, Ilse; Koch, Klaus; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: A number of ligands of the type N,N-dialkyl-N’-acyl(aroyl)thiourea as well as their corresponding complexes with cobalt(III) have been synthesized and characterized by means of various spectroscopic techniques including 1H NMR, 13C NMR, mass spectrometry and infrared spectroscopy. These ligands have been found to form relatively stable complexes of the type fac-[Co(L-S,O)3]. X-ray diffraction results supported the assumption that these complexes favour the facial conformation and coordination of the ligands occurs by means of the bidentate mode via the S and O donor atoms. Considering some of the more favourable properties of 59Co NMR spectroscopy, which include its high receptivity and chemical shift sensitivity, the technique was thus utilized for studying the effect of varying the structure of the acylthiourea ligands on the octahedral-d6 cobalt(III) complexes. By selecting one of the compounds under consideration as an external reference, the extent with which the shielding of the 59Co nucleus is effected by various factors such as concentration, temperature, solvent and ligand structure could be determined. The sensitivity of the 59Co nucleus would also prove to be beneficial for the identification and separation of possible isomers present in the complex. This includes not only various possible E/Z configurational isomers, but also the separation of more than one possible diastereomer present in solution. The possibility of ligand exchange occurring amongst these relatively inert Co(III) complexes was also investigated. A brief study followed on the extent and relative rate of ligand exchange occurring amongst two different homoleptic [Co(Ln-S,O)3] and [Co(Lm-S,O)3] complexes. The reaction was monitored by means of 59Co NMR spectroscopy as well as rp-HPLC. For the ligand exchange reaction study by means of 59Co NMR spectroscopy, the reactivity of two different sets of complexes was compared in a solution of chloroform. The ligand exchange reactions were very slow and both sets of complexes reached equilibrium after only approximately 30 days, at room temperature. The spectra initially gave two peaks corresponding to the two homoleptic complexes initially added in the solution. Two new peaks began to appear, after about 24h, which over time grew in intensity whilst that of the first mentioned complexes decreased in overall intensity. These new peaks were attributed to the formation of the heteroleptic species, and the four peaks in the 59Co NMR spectra was assigned as: [Co(Ln-S,O)3], [Co(Ln-S,O)2(Lm-S,O)], [Co(Ln-S,O)(Lm-S,O)2] and [Co(Lm-S,O)3]. The reaction between two different homoleptic [Co(Ln-S,O)3] and [Co(Lm-S,O)3] complexes were also monitored by means of liquid chromatography in a solution of acetonitrile. The same trend could be seen as for the 59Co NMR investigation, wherein a state of equilibrium would eventually be achieved between the homoleptic complexes initially added in the solution and the newly formed heteroleptic complexes. The relative rate of ligand exchange in acetonitrile on the other hand was reduced by almost half the time compared to the 59Co NMR study in chloroform, with the only differences between subsequent experiments being the reaction medium and concentration used. Unambiguous identification of the various peaks in the chromatogram was done by means of Liquid Chromatography-Mass Spectrometry. In conclusion, it was found by means of 59Co NMR and rp-HPLC that complexes of the type fac- [Co(L-S,O)3] could be well characterized with these techniques. The 59Co NMR chemical shift proved a good and sensitive tool for the study of complexes under investigation, whereby the most significant finding showed the possibility of ligand exchange occurring upon mixing two different homoleptic fac-[Co(L-S,O)3] complexes together in solution. The result was the formation of a mixture of heteroleptic complexes and occurred in a solution of chloroform and acetonitrile respectively. To our knowledge, no such ligand exchange regarding fac-[Co(L-S,O)3] complexes have previously been demonstrated.