Browsing by Author "Nkabyo, Henry Ane"

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    A study on the reversible photo-induced isomerisation of platinum(II) and palladium(II) complexes of the N,N-dialkyl-N’-acyl(aroyl)thioureas with reversed-phase HPLC separation from related rhodium(III), ruthenium(III) and iridium(III) complexes
    (Stellenbosch : Stellenbosch University, 2014-04) Nkabyo, Henry Ane; Koch, Klaus R.; Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science.
    ENGLISH ABSTRACT: See item for full text
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    Reversible photo-induced isomerization of cis-[M(L-κS,O)2] (M=PdII, PtII) complexes with N,N-dialkyl-N’-acyl(aroyl)thiourea ligands (HL): Key to the isolation of novel geometric trans-[M(L-κS,O)2] and trans-[M(L-κS,N)2] isomers
    (Stellenbosch : Stellenbosch University, 2018-03) Nkabyo, Henry Ane; Koch, Klaus R.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: The reaction of a series of selected N,N-dialkyl-N’-acyl(aroyl)thioureas to PtII or PdII formed exclusively cis-[M(Ln -κS,O)2] complexes which were fully characterized by melting point determination, 1H, 13C{1H} NMR, FT-IR and single-crystal X-ray diffraction analyses. In acetonitrile solutions and under polychromatic light irradiation, the cis-[M(Ln -κS,O)2] (M = Pt(II) or Pd(II)) complexes undergo photo-induced isomerization to their respective trans- [M(Ln -κS,O)2] counterparts. The cis→trans isomerization was monitored by 1H NMR and RPHPLC due to differences in chemical shifts and retention times of the two geometric isomers respectively. In the photo-irradiated acetonitrile solution, the trans-[M(Ln -κS,O)2] complexes were less soluble than their thermodynamically stable cis-[M(Ln -κS,O)2] counterparts. Consequently, either controlled vapour diffusion-induced crystallization or slow evaporation of irradiated cis-[Pd(Ln -κS,O)2] complexes in acetonitrile led to isolation of novel trans- [Pd(Ln -κS,O)2] isomers in high yields > 60%. The trans-κS,O configuration in the resulting complexes was confirmed by single-crystal X-ray diffraction. Also, significant differences in melting points and 1H NMR chemical shifts were observed between the cis-trans isomers. For cis-bis(N,N-diethyl-N’-1-naphthoylthioureato-κ 2 S,O)-palladium(II) cis-[Pd(L7 -κS,O)2], an unprecedented photo-induced isomerization occurred, yielding a novel four-membered trans- [Pd(L7 -κS,N)2] isomer in addition to trans-[Pd(L7 -κS,O)2]. This unusual trans-[Pd(L7 -κS,N)2] complex was further characterized by RP-HPLC, LC-MS and represents the first example of a trans-κS,N coordination complex of Pd with the N,N-dialkyl-N’-acyl(aroyl)thioureas. In general, the trans-[Pd(Ln -κS,O)2] and trans-[Pd(Ln -κS,N)2] complexes were found to have significantly higher melting points compared to their cis-[Pd(Ln -κS,O)2] isomers, in addition to the relative upfield shift of their 1H NMR resonances. Also, the Pd-S bond distances in the trans complexes were longer, while the Pd-O bonds experienced significant shortening compared to that of the cis complexes. The addition of Pt(II) or Pd(II) solutions to asymmetrically substituted N,N-dialkyl-N’- acyl(aroyl)thioureas generated cis-[M(Ln -κS,O)2] complexes which were found to exist in chloroform as cis-[M(ZZ-Ln -κS,O)2], cis-[M(EE-Ln -κS,O)2] and cis-[M(EZ-Ln -κS,O)2] isomers. Assignment of these configurational isomers was carried out by 1H, 13C, HMBC and 1D NOESY NMR spectroscopy. Although significant overlap was observed between the cisEZ and cis-EE isomers, for cis-[Pt(Ln -κS,O)2] complexes all the ZZ, EZ and EE configurational isomers were represented by three well-resolved 195Pt{1H} resonances in the chemical shift range -2650 to -2750 ppm. The chemical shift and relative distribution of the E and Z isomers in chloroform were found to be dependent on the nature of ligand substituents. The presence of long chain N-alkyl or N-phenyl groups favoured a higher relative distribution of the cis- [Pt(ZZ-L-κS,O)2] isomer compared to complexes of ligands with smaller N-alkyl groups. In chloroform, cis-[Pt(EE-L8 -κS,O)2] complexes of PtII and PdII were isolated for HL8 = Nmethyl,N-ethyl, N’-benzoylthiourea, while a cis-[Pt(ZZ-L15 -κS,O)2] structure crystallized for HL15 = N-4-methoxy, N-isopropyl-N’-(2,2-dimethyl-pronanoyl)thiourea as revealed by singlecrystal X-ray diffraction studies. Photo-induced isomerization of a mixture of cis-[Pt(EE-Ln - κS,O)2], cis-[Pt(EZ-Ln -κS,O)2] and cis-[Pt(ZZ-Ln -κS,O)2] isomers resulted in trans-[Pt(EE-Ln - κS,O)2], trans-[Pt(EZ-Ln -κS,O)2] and trans-[Pt(ZZ-Ln -κS,O)2] isomers in chloroform as represented by appearance of three additional 195Pt{1H} resonances after light irradiation. The 195Pt resonances corresponding to the trans configurational isomers were significantly shifted downfield relative to their cis-counterparts by ca 750 ppm. By slow evaporation and under polychromatic light irradiation of an acetonitrile solution of cis-[Pd(L8 -κS,O)2] (HL8 = Nmethyl, N-ethyl, N’-benzoythiourea), a trans-[Pd(L8 -κS,O)2] complex was isolated as a mixture of trans-[Pd(ZZ-L8 -κS,O)2] and trans-[Pd(EZ/EE-L8 -κS,O)2] configurational isomers. In the absence of light, all isolated trans-[Pd(Ln -κS,O)2] complexes were found to undergo a spontaneous trans→cis isomerization in chloroform. The presence of different ligand substituents showed no significant influence on the relative rates of trans→cis isomerization. Moreover, no consistent trend in rates of isomerization based on electronic effect of ligand substituents was observed in acetonitrile and chloroform. The relative rates of trans→cis isomerization were significantly higher in acetonitrile compared to chloroform indicating the role of coordinating acetonitrile solvent in assisting the reverse isomerization process. Addition of trace amounts of free N,N-dialkyl-N’-acylthioureas catalysed the trans→cis isomerization leading to a rapid dissapearance of trans 1H NMR resonances and RP-HPLC peaks. In contrast, the presence of N,N,N’,N’-tetramethyl-1,8-naphthalene diamine appeared to remove free ligands in solution leading to a decrease in the rate of trans→cis isomerization. When the concentration of added ligand in solution was increased, a first-order increase in the rate of trans→cis isomerization was observed. The ligand exchange mediated trans→cis isomerization resulted in the formation of mixed-ligand complexes of the type cis-[Pd(La,b - κS,O)2] evident from RP-HPLC chromatograms. Further support of an associative ligand exchange process during trans→cis isomerization was provided by temperature dependence in the range 25-55°C in chloroform during which large negative activation entropy and positive activation enthalpy values were obtained.