Browsing by Author "Mthiyane, Wakhiwe M."

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    Reactivity of cis-platinum(II) complexes with 2-(4-substituted)phenylthiomethyl)quinoline nonleaving ligands toward thiourea nucleophiles
    (International Journal of Chemical Kinetics, 2018-06-06) Mthiyane, Wakhiwe M.; Mambanda, Allen.; Jaganyi, Deogratius.
    The sequential substitution of aqua ligands from [Pt{2-(phenylthiomethyl)quinoline}(H2O)2]CF3SO3, Pt(L1), [Pt{2-(4-tert-butylphenylthiomethyl)quinoline}(H2O)2]CF3SO3 Pt(L2), and [Pt{2-(4-fluorophenylthiomethyl) quinoline}(H2O)2]CF3SO3 Pt(L3) by thiourea nucleophiles (Nu) was studied under pseudo–first-order conditions as a function of concentration and temperature using stopped-flow and UV-visible spectrophotometric techniques. The observed pseudo–first-order rate constants for the substitutions can be described by the rate law: kobs(1/2) = k2(1/2)[Nu], where the subscript denotes the consecutive substitution steps. The first aqua ligand was substituted opposite to the strong σ-trans-directing thioether followed by the aqua ligand opposite to the quinoline or pyridine moieties. Second-order rate constants, k2(1st), for the substitution of the first aqua ligand by thiourea nucleophiles ranged between 9 and 22 M−1 s−1 for Pt(L1), 86 and 326 M−1 s−1 for Pt(L2), and 58 –287 M−1 s−1 for Pt(L3). The ranges of the second-order rate constant, k2(2nd), were always lower than the k2(1st)’s and are 0.3-9 M−1 s-1 for Pt(L1), 2-20 M−1 s−1 for Pt(L2), and 0.3-5 M−1 s−1 for Pt(L3). Aqua substitution from Pt(L2) is slower for both steps than from Pt4, its pyridyl derivative from our previous study. A Job's method of continuous variation plot suggests a species with a metal-to-nucleophile ratio of 1:3 as the ultimate product of the chloride substitution from the Pt(II) complexes by the incoming thiourea nucleophiles. The high and negative activation entropy and low and positive activation enthalpy values support an associative mechanism of activation, characteristic of substitution reactions occurring in square-planar complexes.
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    Substitution of aqua ligands from cis-platinum(II) complexes bearing 2-(phenylthiomethyl)pyridine spectator ligands
    (Transition Metal Chemistry, 2017-09-16) Mthiyane, Wakhiwe M.; Mambanda, Allen.; Jaganyi, Deogratius.
    Cis-Pt(II) complexes, namely [Pt{2-(phenylthiomethyl)pyridine}(H2O)2](CF3SO3)2 Pt(pyS Ph ), [Pt{2-(4-tert-butylphenylthiomethyl)pyridine}(H2O)2](CF3SO3)2 Pt(pyS Ph(t-But) ) and [Pt{2-(4-fluorophenylthiomethyl)pyridine}(H2O)2](CF3SO3)2 Pt(pyS PhF ), were synthesised and characterised. The pK a1 and pK a2 values of the complexes were determined titrimetrically. Substitution of the aqua ligands from these complexes by thiourea nucleophiles was studied at a pH of 2 and ionic strength of 0.1 M under pseudo-first-order conditions using stopped-flow and UV–visible spectrophotometric techniques. Substitution of the aqua ligands depends on both the nature and concentration of the incoming ligand, with low enthalpy and negative entropy of activation values. Substitution of the first and second aqua ligands occurs sequentially and fits the rate laws: k obs (1/2) = k (1/2) [Nu]. The second-order rate constant, k 1, relates to the substitution trans to sulphur, while k 2 is the second-order rate constant for the subsequent substitution of the aqua ligand trans to pyridine. The rate of substitution of the first aqua ligand decreases in the order: Pt(pyS Ph(t-But) ) > Pt(pyS PhF ) > Pt(pyS Ph ), while that of the second decreases in the order: Pt(pyS Ph(t-But) ) > Pt(pyS Ph ) > Pt(pyS PhF ), reflecting the influence of the substituents on the spectator ligands. 195Pt NMR spectra of aged solutions of complexes with the thiourea nucleophile suggest a subsequent but rapid concentration-independent ring opening of the N,S-bidentate ligand to form a PtS 4 species. The crystal structure of Pt(pyS PhF )Cl 2 was elucidated by X-ray diffraction analysis.