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

Reactions of 2-furoyl chloride and 2-thiophene carbonyl chloride with substituted pyrazoles produced the modified pyrazolyl compounds: {(3,5-Me ₂pzCO)-2-C₄H₃O} (L1), {(3,5-Me ₂pzCO)-2-C₄H₃S} (L2), {(3,5-t-Bu ₂pzCO)-2-C₄H₃O} (L3), {(3,5-t-Bu ₂pzCO)-2-C₄H₃S} (L4), {(3,5-Ph ₂pzCO)-2-C₄H₃S} (L5), and {(pzCO)-2-C ₄H₃O} (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.