Pyrazolyl iron, cobalt, nickel, and palladium complexes: synthesis, molecular structures, and evaluation as ethylene oligomerization catalysts

Reactions of [2-(3,5-dimethyl-pyrazol-1-yl)-ethanol] (L1) and [1-(2-chloro-ethyl)-3,5-dimethyl-1H-pyrazole] (L2) with Fe(II), Co(II), Ni(II), and Pd(II) salts gave the complexes [(L1)2FeCl2] (1), [(L1)2CoCl2] (2), [(L1)2NiBr2] (3), [(L1)2Pd(Me)Cl] (5), [(L2)2CoCl2] (6), and [(L2)2NiBr2] (7). Whereas L2 behaves as a monodentate ligand, L1 can behave as either a monodentate or bidentate ligand depending on the nature of the metal centre. For palladium, L1 is monodentate in the solid state structure of 5 but bidentate in the structure of 4, obtained during attempts to crystallize 3. While the activation of iron, cobalt and palladium complexes with EtAlCl2 did not produce active ethylene oligomerization catalysts, the nickel complexes 3 and 7 produced active ethylene oligomerization catalysts. Activities as high as 4329 kg/mol Ni h were obtained. Catalyst 3 produced mainly butenes (57%) and hexenes (43%); of which a combined 20% were converted to Friedel-Crafts alkylated-toluene. Catalyst 7, on other hand, produced mainly butenes (90%) and small amounts of hexenes (10%) which were then completely converted to the corresponding Friedel-Crafts alkylated-toluene products. This difference in product distribution in catalysis performed by complexes 3 and 7 is indicative of the role of the OH functionality in L1 on the EtAlCl2 co-catalysts.

Graphical abstractTwo nickel complexes, 3 and 7 are readily activated with EtAlCl2 to produce active ethylene oligomerization catalysts. Whereas 3 catalysed the oligomerization of ethylene to butenes and hexenes, followed by conversion of the ethylene oligomers to small amounts of Friedel-Crafts alkylated-toluene products (20%); 7 oligomerized ethylene to mainly butenes and small amounts of hexenes both of which were then completely converted to the corresponding Friedel-Crafts alkylated-toluene products.Figure optionsDownload full-size imageDownload as PowerPoint slide