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

[摘要] ENGLISH ABSTRACT: The partial double bond character of the carbon-nitrogen bond of the (S)C-NRR' moiety results in unsymmetricaldialkyl-substituted N-alkyl-N-alkyl(aryl)-N'-acylthioureas, RC(O)NHC(S)NRR' (HL) displaying E,Zconfigurational isomerism in solution. The isomerism manifests itself by the duplication of resonances of the N-alkylgroups in the 1H and 13C NMR spectra. In one class of these ligands where R and R' groups are non-equivalent alkylgroups the isomerism is easily observable at 298 K in chloroform. In the other class where R' is still an alkyl groupand R is a para-substituted phenyl group the isomerism is only observable at much lower temperatures due to a lowerbarrier to rotation around the (S)C-N(alkyl)(para-X-Ph) bond (X = O-CH3, H and NO2). The electron-withdrawingnature 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 observableeven at 198 K in dichloromethane. The distribution of E and Z isomers of the unbound ligands vary depending onthese 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 of1.472(5) Å.The E,Z configurational isomerism in the unbound ligands is passed on to the Pt(II) chelates derived from theseligands. 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 bymeans of 195Pt NMR spectroscopy which shows three well resolved resonances, and this can be confirmed by 1H and13C NMR spectra of these complexes. The 195Pt nuclei are spatially linked to the 13C nuclei, four bonds awayresulting in 4J(195Pt-13C) couplings with N-CH2- or N-CH3 carbons in a W pathway. The 195Pt NMR spectra are alsolinked to N-CH2- or N-CH3 proton resonances by means of the ZZ, EZ and EE isomer distributions. Assignment ofthese configurational isomers was then achieved by means of a combination of low magnetic field 13C NMR spectraand 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] complexeswas observed to follow the order: (electron-withdrawing group) NO2 < H < (electron-donating group) O-CH3. TheZZ isomer was observed to be favoured over the EZ and EE isomers in this order of the para-substituent on the Nphenylgroup. 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 bysolvent polarity and the temperature at which the distributions are determined apart from the electronic influence ofthe 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 Nalkylsubstituent (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 isomersupon 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 theEZ 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 inthe 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 groupand 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 asymmetricallydisubstituted ligands to be reported.

[发布机构] Stellenbosch University