Department of Chemistry and Polymer Science

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Browsing Department of Chemistry and Polymer Science by Subject "Acylthioureas"

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    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.