| dc.description.abstract | Reactions of furan, thiophene and alkyl carbonyl chlorides with Unsubstituted and substituted pyrazoles produced the desired pyrazolyl ligands, [R2pyrazole-CO-R/] (R= H, Me, tBu, Ph, R/ = furan, thiophene, Me or tBu) in good yields. The reaction of these synthons with Pd(NCMe)2Cl2 as the metal precursor afforded the respective palladium(II) complexes in moderate to high yields. All compounds synthesised were characterised by
a combination of 1H NMR, 13C NMR and IR spectroscopic techniques for structure elucidation. Microanalyses were performed to confirm the purity of the compounds. Single crystal X-ray crystallography of complexes bis{2-(3,5-dimethylpyraz olyl-1-carbonyl)furan}PdCl2 ,1, and bis{2-(3,5-dimethylpyrazolyl-1-carbonyl)thiophene} PdCl2, 2, was performed to determine the molecular structures of the compounds. The two structures show that the complexes formed are mononuclear. The ligands exhibit
monodentate coordination with trans-geometry being favoured in the square planar complexes. Activation of some of the complexes, 1-4, with the methylaluminoxane (MAO) produced active catalysts for ethylene polymerisation at elevated temperatures giving high-density linear polyethylene. The optimum Al:Pd ratio was found to be 5000:1. The nature of the substituent on the pyrazolyl moiety affects the activity of these catalysts in ethylene polymerisation. In general, increase in steric bulk resulted in decreased catalytic activity and low molecular weight polyethylene. The use of the Lewis acid, B(C6F5)3, with catalyst 2 resulted in enhanced catalytic activity towards ethylene polymerisation. When ethylaluminium dichloride, EtAlCl2, was used as the co-catalyst, these catalyst systems were found to be active towards ethylene oligomerisation producing mostly C10 and C12 oligomers. An optimum Al:Pd ratio of 1000: in the catalytic activity of catalyst 2 was observed. The nature of the alkyl substituent on the pyrazolyl system also shows significant influence in the oligomerisation process. The activity of the catalysts as well as the nature of the oligomers produced greatly depends on the oligomerisation
conditions. For example, increase in both temperature and pressure resulted on significant increase in turnover number of catalyst 1 and 2. Longer reaction times resulted in drastic drop in the catalytic activity of the complexes. When acetic acid was reacted with the appropriate pyrazole in the presence of a phase transfer catalyst, the bidentate bis-pyrazolyl acetic acid ligands, {(pyrazole)2CHCO2H}, L14, {(Me2pyrazole)2CHCO2H, L15, and {(tBu2pyrazole)2CHCO2H} L16 were obtained in good yields. The reaction of these ligands with Pd(NCMe)2Cl2 in a 1:1 ratio afforded
the corresponding yellow complexes, Pd(L14)Cl2, 9, Pd(L15)Cl2, 10, and Pd(L16)Cl2, 11, in moderate yields. 1H NMR, 13C NMR and IR spectroscopy have been used to characterise these compounds. Microanalysis confirmed the purity and empirical formulae of the complexes. Reaction of L14 with ethanol under acidic conditions produced the respective ester, bis(pyrazol-1-yl)ethyl acetate, (L17) in good yield. The acid dissociation constants of the ligands and their palladium(II) complexes have been determined by titration using dilute NaOH. Ka values within the range of 1.8 × 10-4 to
1.75 × 10-2 were obtained. Acidity was found to decrease with increase in the electron donor ability of the alkyl pyrazolyl substituents. In general, the palladium(II) complexes were more acidic than their corresponding ligands. | en_US |
