A multinuclear magnetic resonance spectroscopy study of E/Z configurational isomers of unsymmetrical N-alkyl-N-alkyl(aryl)-N'-acylthioureas of platinum(II) complexes

Mtongana, Sibusiso (Stellenbosch : Stellenbosch University, 2006-12)

Thesis

ENGLISH ABSTRACT: The partial double bond character of the carbon-nitrogen bond of the (S)C-NRR’ moiety results in unsymmetrical dialkyl-substituted N-alkyl-N-alkyl(aryl)-N’-acylthioureas, R”C(O)NHC(S)NRR’ (HL) displaying E,Z configurational isomerism in solution. The isomerism manifests itself by the duplication of resonances of the N-alkyl groups in the 1H and 13C NMR spectra. In one class of these ligands where R and R’ groups are non-equivalent alkyl groups the isomerism is easily observable at 298 K in chloroform. In the other class where R’ is still an alkyl group and R is a para-substituted phenyl group the isomerism is only observable at much lower temperatures due to a lower barrier to rotation around the (S)C-N(alkyl)(para-X-Ph) bond (X = O-CH3, H and NO2). The electron-withdrawing nature of the nitro group in N-methyl-N-(4-nitro-phenyl)-N’-2,2-dimethylpropanoylthiourea, HL3A and N-pentyl-N- (4-nitro-phenyl)-N’-2,2-dimethylpropanoylthiourea, HL3D result in the E,Z isomerism of these ligands not observable even at 198 K in dichloromethane. The distribution of E and Z isomers of the unbound ligands vary depending on these R and R’ groups. Several E isomers of these ligands have been isolated and structurally characterised and the (S)C-NRR’ bond falls in the range [1.343(3) – 1.329(3) Å] which shorter than the average C-N single bond of 1.472(5) Å. The E,Z configurational isomerism in the unbound ligands is passed on to the Pt(II) chelates derived from these ligands. The presence of cis-[Pt(ZZ-L-S,O)2], cis-[Pt(EZ-L-S,O)2] and cis-[Pt(EE-L-S,O)2] is readily observable by means of 195Pt NMR spectroscopy which shows three well resolved resonances, and this can be confirmed by 1H and 13C NMR spectra of these complexes. The 195Pt nuclei are spatially linked to the 13C nuclei, four bonds away resulting in 4J(195Pt-13C) couplings with N-CH2- or N-CH3 carbons in a W pathway. The 195Pt NMR spectra are also linked to N-CH2- or N-CH3 proton resonances by means of the ZZ, EZ and EE isomer distributions. Assignment of these configurational isomers was then achieved by means of a combination of low magnetic field 13C NMR spectra and high-resolution gHSQC (1H/13C) NMR experiments. 1H NMR rotational dynamics study showed that the barrier to rotation, ΔG≠, around the (S)C-N(Me)(para-X-Ph) bond in cis-bis(N-methyl-N-(4-methoxy-phenyl)-N’-2,2-dimethylpropanoylthioureato)platinum(II), cis-[Pt(L1AS, O)2]; cis-bis(N-methyl-N-phenyl-N’-2,2-dimethylpropanoylthioureato)platinum(II), cis-[Pt(L2A-S,O)2] and cisbis( N-methyl-N-(4-nitro-phenyl)-N’-2,2-dimethylpropanoylthioureato)platinum(II), cis-[Pt(L3A-S,O)2] complexes was observed to follow the order: (electron-withdrawing group) NO2 < H < (electron-donating group) O-CH3. The ZZ isomer was observed to be favoured over the EZ and EE isomers in this order of the para-substituent on the Nphenyl group. The 1H dynamic NMR trends about the barrier to rotation, ΔG≠, around the (S)C-N(Me)(para-X-Ph) bond were also complemented by DFT linear transit calculations. The isomer distributions were also influenced by solvent polarity and the temperature at which the distributions are determined apart from the electronic influence of the para-substituent of the N-phenyl group. The ZZ, EZ and EE isomers of complexes derived from N-alkyl-N-(para-X-Ph)-N’-acylthioureas with varying Nalkyl substituent (methyl, isopropyl, cyclohexyl and n-pentyl) were determined from the 195Pt NMR spectra which were measured under identical conditions. The ZZ isomer was observed to be favoured over the EZ and EE isomers upon methyl group substitution with a bulkier alkyl group in the order: methyl < isopropyl < cyclohexyl < n-pentyl. Qualitatively it has been shown that a bulkier N-pentyl group increases the barrier to rotation around the (S)CN( alkyl)(para-X-Ph) bond over the N-methyl group and this leads to higher concentrations of the ZZ isomer over the EZ and EE isomers. The combined effects of the electron-donating substituent (X = O-CH3) on the N-(para-X-Ph) group and the bulkier N-alkyl group (n-pentyl) result in highest ZZ concentration (76 %) over EZ and EE isomers in the complex cis-bis(N-pentyl-N-(4-methoxy-phenyl)-N’-2,2-dimethylpropanoylthioureato)platinum(II), cis-[Pt(L1DS, O)2]. The lowest concentration ZZ (27 %) is obtained in the complex cis-bis(N-methyl-N-(4-nitro-phenyl)-N’-2,2- dimethylpropanoylthioureato)platinum(II), cis-[Pt(L3A-S,O)2] when the coordinated ligand has both N-methyl group and N-(4-nitro-Ph) group which both lower the barrier to rotation around the (S)C-N(alkyl)(para-X-Ph) bond. A crystal of the complex cis-bis(N-pentyl-N-(4-methoxy-phenyl)-N’-2,2-dimethylpropanoylthioureato)platinum(II), cis-[Pt(L1D-S,O)2] has been isolated and structurally characterised and was shown to be in the ZZ configuration, which is the major component (76 %) in chloroform. This is the first example of Pt(II) chelates with asymmetrically disubstituted ligands to be reported.

AFRIKAANSE OPSOMMING: Die gedeeltelike dubbelbinding karakter van die koolstof-stikstof-binding van die (S)C-NRR’-moieteit lei tot onsimmetriese dialkiel-gesubstitueerde N,N-dialkiel-N’-asieltioureums, R”C(O)NHC(S)NRR’ (HL) wat E,Zkonfigurasionele isomerie in oplossing besit. Die isomerie is sigbaar in die verdubbelling van die seine van die Nalkielgroepe in die 1H- en 13C-KMR spektra. In een so klas ligande waar R- en R’-groepe nie-ekwivalente alkielgroepe is, is isomerie duidelik sigbaar by 298 K in chloroform. In die ander klas waar R’ steeds ’n alkielgroep is, en R ’n para-gesubstitueerde feniel groep, is die isomerie alleenlik sigbaar by baie laer temperature as gevolg van ’n laer rotasieversperring om die (S)C-N(alkiel)(para-X-Ph)-binding (X = O-CH3, H and NO2). Die elektrononttrekkende aard van die nitrogroep in N-metiel-N-(4-nitrofeniel)-N’-(2,2-dimetielpropanoïel)tioüreum, HL3A en N-(4-nitrofenyl)-N-pentiel-N’-(2,2-dimetielpropanoïel)tioüreum, HL3D lei daartoe dat die E,Z-isomerie van die ligande nie eers by 198 K in dichlorometaan waargeneem word nie. Die verspreiding van die E en Z isomere verskil na gelang van die R en R’ groepe. Verskeie E-isomere van hierdie ligande is geïsoleer en struktureel gekarakteriseer en die (S)C-NRR’-bindingslengte is in ‘n gebied [1.343(3) – 1.329(3) Å] wat korter is as die gemiddelde C-N-enkelbindingslengte van 1.472(5) Å. Die Pt(II) chelate wat afgelei is van die ligande is blootgestel aan die E,Z-konfigurasie isomere van die ongebinde ligande. Die teenwoordigheid van cis-[Pt(ZZ-L-S,O)2], cis-[Pt(EZ-L-S,O)2] en cis-[Pt(EE-L-S,O)2] is maklik waarneembaar deur middel van 195Pt-KMR-spektroskopy wat drie goed geresolueerde seine toon, en dit kan bevestig word met 1H- en 13C-KMR spectra van hierdie komplekse. Die 195Pt kerne is ruimtelik geskakel met die 13C kerne deur vier bindings wat aanleiding gee tot 4J(195Pt-13C)-koppelings met N-CH2- of N-CH3-koolstofatome in ‘n Wkonformasie. Die 195Pt KMR spektra word geskakel met die N-CH2- of N-CH3-protonresonansies in al drie die moontlike ZZ, EZ en EE kompleksisomere. Toekenning van die konfigurasionele isomere is dan bewerkstellig deur middel van ‘n kombinasie van lae magneetveld 13C-KMR spectra en hoë resolusie gHSQC (1H/13C) KMR experimente. 1H-KMR-rotasiedinamiek studie toon dat die rotasiegrens, ΔG≠, om die (S)C-N(Me)(para-X-Ph)-binding in cisbis( N-metiel-N-(4-metoksifeniel)-N’-2,2-dimetielpropanoïeltioureato)platinum(II), cis-[Pt(L1A-S,O)2]; cis-bis(Nfeniel- N-metiel-N’-2,2-dimetielpropanoïeltioureato)platinum(II), cis-[Pt(L2A-S,O)2] en cis-bis(N-metiel-N-(4- nitrofeniel)-N’-2,2-dimetielpropanoïeltioureato)platinum(II), cis-[Pt(L3A-S,O)2] komplekse was met die volgende orde bepaal: (elektron-ontrekkende groep) NO2 < H < (elektron-skenkende groep) O-CH3. Die ZZ-isomeer blyk by voorkeur te vorm bo die EZ- en EE-isomere in dieselfde orde as hierbo wat betref para-substituent aan die Nfenielgroep. Die 1H dinamiese KMR tendencies ten opsigte van die rotasiegrens, ΔG≠, om die (S)C-N(Me)(para-XPh)- binding is gekomplimenteer met DFT-linêre organgs berekeninge. Die isomer verspreidings blyk ook beïnvloed te word deur die oplosmiddel polariteit en die temperatuur waarby die verspreidings bepaal is, buiten die elektroniese invloed van die para-substituent aan die N-fenielgroep. Die ZZ, EZ en EE isomeer verspreiding van komplekse wat afgelei is van N-alkiel-N-(para-X-Ph)-N’-asieltioureums met veranderlike N-alkiel substituente (metiel, isopropyl, sikloheksiel, en n-pentiel) is vasgestel deur middel van die 195Pt KMR wat opgeneem is onder identiese kondisies. Die ZZ-isomeer blyk die verkose isomeer te wees bo die EZen EE-isomere waar die metiel substituent vervang word met ‘n groter alkiel groep in die orde van: metiel < isopropiel < sikloheksiel < n-pentiel. Dit is kwalitatief getoon dat die groter N-pentielgroep die rotasiegrens verhoog vir rotasie om die (S)C-N(alkiel)(para-X-Ph)-binding bo dié van die N-metielgroep wat aanleiding gee tot hoër konsentrasies van die ZZ-isomeer relatief tot die EZ- en EE-isomere. Die gekombineerde uitwerking van die electron-skenkende substituent (O-CH3) op die N-(para-X-Ph)-groep en die groter N-alkiel groep (n-pentiel) gee aanleiding tot die hoogste ZZ-konsentrasie (76%) bo EZ- en EE-isomere in die kompleks cis-bis(N-pentiel-N-(4- metoksifeniel)-N’-2,2-dimetielpropanoïeltioureato)platinum(II), cis-[Pt(L1D-S,O)2]. Die laagste konsentrasie ZZ (27%) is verkry in die kompleks cis-bis(N-metiel-N-(4-nitrofeniel)-N’-2,2-dimetielpropanoïel)tioureato)platinum(II), cis-[Pt(L3A-S,O)2] waar die gekoördineerde ligand beide die N-metiel- sowel as die N-(4-nitro-Ph)-groep, wat albei die rotasiegrens van die (S)C-N(alkiel)(para-X-Ph)-binding verlaag. ‘n Kristalstruktuur van die kompleks cis-bis(N-pentiel-N-(4-metoksifeniel)-N’-2,2-dimetielpropanoïeltioureato) platinum(II), cis-[Pt(L1D-S,O)2] wat geïsoleer is, is struktureel gekarakteriseer en is in die ZZ-konfigurasie, wat die hoofkomponent (76%) is in chloroform. Hierdie is die eerste voorbeeld van Pt(II) chelate met asimmetriese digesubstitueerde ligande om geraporteer te word.

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