Unsymmetrical (pyrazolylmethyl)pyridine metal complexes as catalysts for ethylene oligomerization reactions: Role of solvent and co-catalyst in product distribution

• Ni(II), Co(II) and Fe(II) complexes form active ethylene oligomerization catalysts.
• Oligomer products depend on the nature of the solvent and co-catalyst used.
• Reactions in toluene using EtAlCl2 resulted in alkylated toluene products.
• Reactions in hexane and chlorobenzene using EtAlCl2 gave C4 and C6 oligomers.
• Use of MAO as a co-catalyst in toluene produced mainly C4, C6 and C8 oligomers.

Reactions of 2-(chloromethyl)-6-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)pyridine (L1) and 2-(chloromethyl)-6-((3,5-diphenyl-1H-pyrazol-1-yl)methyl)pyridine (L2) with NiCl2, NiBr2, CoCl2 and FeCl2 led to the formation of their respective metal complexes [NiCl2(L1)] (1), [NiBr2(L1)] (2), [CoCl2(L1)] (3), [FeCl2(L1)] (4), [NiBr2(L2)] (5), and [CoCl2(L2)] (6) in moderate to high yields. The complexes were characterized by elemental analyses, mass spectrometry and single-crystal x-ray diffraction for 5 and 6. Solid state structures of 5 and 6 confirmed the bidentate coordination modes of L1 and L2 and formation of monometallic compounds. Complexes 1-6 formed active catalysts for the oligomerization of ethylene reactions when activated with either EtAlCl2 or methylaluminoxane (MAO). The catalytic activities of 1-6 and products formed largely depended on the co-catalyst and solvent system. While activation with EtAlCl2, in toluene produced Friedel-Crafts toluene-alkylated products, the use of hexane and chlorobenzene gave predominantly C4 and C6 oligomers. On the other hand, activation with MAO in toluene led to the formation of mainly C4, C6 and C8 oligomers. The complex structure and reaction conditions such as co-catalyst/complex ratio, time and pressure also influenced the catalytic behaviour of these pre-catalysts.

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