Furoyl and thiophene carbonyl linker pyrazolyl palladium(II) complexes Synthesis, characterization, and evaluation as ethylene oligomerization catalysts

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Reactions of 2-furoyl chloride and 2-thiophene carbonyl chloride with substituted pyrazoles produced the modified pyrazolyl compounds: {(3,5-Me2pzCO)-2-C4H3O} (L1), {(3,5-Me2pzCO)-2-C4H3S} (L2), {(3,5-t-Bu2pzCO)-2-C4H3O} (L3), {(3,5-t-Bu2pzCO)-2-C4H3S} (L4), {(3,5-Ph2pzCO)-2-C4H3S} (L5), and {(pzCO)-2-C4H3O} (L6) in good yields. Reactions of these synthons with [Pd(NCMe)2Cl2] afforded the corresponding mononuclear palladium(II) complexes: [Pd(L1)2Cl2] (1), [Pd(L2)2Cl2] (2), [Pd(L3)2Cl2] (3), [Pd(L4)2Cl2] (4), [Pd(L5)2Cl2] (5), and [Pd(L6)2Cl2] (6) in moderate to high yields. All compounds synthesized were characterized by a combination of 1H NMR, 13C NMR, and IR spectroscopy. Compounds L1, 1, and 2 were examined by single crystal X-ray crystallography. DFT theoretical studies at the B3LYP/6-31+G(d) level of theory with GAUSSIAN98 have been used to rationalize some of the results. When the complexes were activated with ethylaluminium dichloride (EtAlCl2), they catalysed the oligomerization of ethylene to mostly C10 and C12 oligomers. Oligomer distribution greatly depends on the oligomerization conditions; for example, an increase in temperature and pressure produced a higher percentage of C12 compared to C10.