Browsing by Author "Barnard, Ilse"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    59Co NMR, a tool for the study of the structure, reactivity and supramolecular chemistry of Co(III) complexes derived from a series of N-alkyl-N-alkyl(aryl)-N’-acyl(aroyl)thioureas
    (Stellenbosch : Stellenbosch University, 2020, 2020-03) Barnard, Ilse; Koch, Klaus Robert; Gerber, Wilhelmus Jacobus; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: A large library of Co(III) complexes, derived from selected monopodal and bipodal N,N- dialkyl-N’-acyl(aroyl)thioureas, have been synthesized and characterized. These ligands form tris(chelated) complexes in the thermodynamically more stable fac geometry with the cobalt metal. The monopodal ligands, and their corresponding Co(III) complexes, were separated into two groups based on the two R substituents of the C(S)-NRR’ function. The first group were named the symmetrically substituted acylthiourea derivatives, where R = R’. Due to the increased stability provided by the chelate effect as well as the selective population of the lower-energy d-orbitals, such complexes are generally expected to be kinetically stable in solution. Nonetheless, the ‘symmetrical’ Co(III) complexes were utilized for the study of the unexpected, slow and spontaneous ligand exchange reaction between non-identical homoleptic pairs of low-spin d⁶ octahedral fac-[Co(LA-κS,O)3] and fac-[Co(LB-κS,O)3] complexes in organic solvents. The exchange reaction between these complexes result in mixtures of their corresponding heteroleptic fac-[Co(LA-κS,O)2(LB-κS,O)] and fac-[Co(LA-κS,O)(LB-κS,O)2] complexes in solution. This discovery was followed by a quantitative determination of the ligand exchange equilibria as well as a relative rate study, as a function of temperature, of the reaction using rp-HPLC. It was then established that the high chemical shift sensitivity of ⁵⁹Co NMR is a powerful tool for the easy characterization of the Co(III) complexes derived from symmetrically substituted acylthioureas in solution. The utility of ⁵⁹Co NMR as a spectroscopic tool was exemplified after utilizing this technique for further additional studies of factors shown to dramatically affect the relative rate of ligand exchange in these complexes, specifically light and solvent. The second group of monopodal acylthiourea ligands investigated were named the asymmetrically substituted acylthioureas, where R ≠ R’. The partial double bond character in the C-N bond of the C(S)-NRR’ function results in the E,Z configurational isomerism of these ligands in solution. The isomerism is expressed as duplicate resonances in the ¹H NMR spectra of the uncoordinated ‘asymmetrical’ acylthiourea ligands. The Z to E isomer ratio varies depending on the two R and R’ substituents of the thiourea moiety. Notably, the isomerism in the uncoordinated ligands is passed on to the Co(III) chelates after coordination. As for the symmetrically substituted acylthiourea Co(III) complexes, we find that the ⁵⁹Co NMR chemical shift is very sensitive to the structure of the asymmetrical fac-[Co(Lⁿ-κS,O)3] complexes in solution. Significantly, the presence of the four, theoretically possible, fac- [Co(EEE-Lⁿ-κS,O)3], fac-[Co(EEZ-Lⁿ-κS,O)3], fac-[Co(EZZ-Lⁿ-κS,O)3] and fac-[Co(ZZZ-Lⁿ- κS,O)3] isomers is readily observable by means of ⁵⁹Co NMR, which shows four well-resolved resonances. From the relative peak integrals of the ⁵⁹Co NMR peaks, a semi-quantitative estimate of the relative amounts of the configurational isomers in solution was possible, although assignment of the isomers is not trivial. The assignment of the ⁵⁹Co peaks to each of the EEE, EEZ, EZZ and ZZZ configurational isomers of the asymmetrical fac-[Co(Lⁿ-κS,O)3] complexes were based on the relative E/Z ratio of the uncoordinated ligands, which was established from their ¹H NMR spectra. However, this assignment assumes that the relative E/Z ratio does not vary significantly during coordination to cobalt and remains therefore ambiguous. The distribution of the EEE, EEZ, EZZ and ZZZ configurational isomers was shown to be dependent on the solvent used during the analyses. Moreover, we evaluated the temperature needed to lift the barrier to rotation in the C-N bond of the coordinated ligands. Finally, the ⁵⁹Co NMR trends of several complexes isolated for single crystal X-ray diffraction analysis were complemented by DFT linear transit calculations of their configurational isomers. Finally, we investigate the interesting coordination chemistry of the well-ordered and multinuclear coordination systems of the bipodal acylthiourea analogues, namely aroylbis(N,N-dialkylthioureas). Two ligands were selected for this purpose, including isophthaloylbis(N,N-diethylthiourea) and a bipodal ligand with a modified aromatic spacer derived from catechol. These were utilized as pre-programmed chelating ligands to form metallamacrocyclic octahedral facial Cobalt(III) complexes via self-assembly. The latter catechol-spaced ligand was used for the synthesis of a number of oligometallic Co(III) complexes by one-pot reactions of the ligand, Co³⁺ and a variety of monovalent cations, to form metallacryptates of type {M⁺ ⊂ [Co2(L-κS,O))3]}(PF6), M⁺ = K⁺, Rb⁺, Cs⁺ and NH4+. A significant discovery, owing to the unique sensitivity of the ⁵⁹Co nucleus, was the instrumental role ⁵⁹Co NMR played in our attempt to characterize the metallacryptates and to determine their conditional stabilities in solution. This was achieved by way of several biphasic exchange experiments of cations between an aqueous and non-aqueous phase. This is a novel discovery that has allowed for the study of various factors shown to effect the stability of the metallacryptates in solution.
  • Thumbnail Image
    Item
    Maatskaplikewerk-hulpverlening aan die dagga- en mandrax dwelmgebruiker
    (Stellenbosch : Stellenbosch University, 1993-12) Barnard, Ilse; Botha, N. J.; Stellenbosch University. Faculty of Arts and Social Sciences. Dept. of Social Work.
    ENGLISH ABSTRACT: Drug abuse and drug dependence, especially with regard to dagga and Mandrax, is a distressing phenomenon in the current South African society. Because of an increase in this phenomenon, that is the combined use of dagga and Mandrax, there is a growing need to treat this social problem as effectively as possible. The study deals with the social worker's rendering of help to the dagga- and Mandrax user. The purpose of the study is to compile guide-lines with regard to assessing the needs of social workers. A questionnaire survey was done by social workers actively involved in family organisations to assess their needs for knowledge and skills in working with the druguser. Pre-graduate training as well as insufficient furhter training after graduation add to social worker's limited knowledge and skills in working with the druguser. Social workers are largely emphatic and supportive towards the druguser, but they do however experience feeling of uncertainty, powerlessness, ambivalance and resignation. Although quite motivated in rendering help to the druguser, the social worker's motivation rests mainly on the druguser's motivation to change his own behaviour and to co-operate. The majority of respondents' needs with regard to knowledge and skills were highly indicated. Although the family-care worker does not specialise in dependence it is nevertheless necessary that she has sufficient knowledge and skills with regard to dependence. Her attitude towards the druguser and toward the phenomenon of dependence is therefore important in rendering help effectively. Guide-lines were compiled and suggestions were made to ensure that the social worker's tasks is more sufficient.
  • Thumbnail Image
    Item
    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.