(Pyrazol-1-ylmethyl)pyridine Nickel Complexes: Ethylene Oligomerization and Unusual Friedel−Crafts Alkylation Catalysts

The reactions of 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L1) and 2,6-bis(3,5-di-tert-butylpyrazol-1-ylmethyl)pyridine (L2) with NiCl2 or NiBr2 gave the nickel(II) complexes [NiCl2(L1)] (1), [NiBr2(L1)] (2), [NiCl2(L2)] (3), and [NiBr2(L2)] (4) in high yields. Compounds 2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L3) and 2-(3,5-di-tert-butylpyrazol-1-ylmethyl)pyridine (L4) on the other hand gave either mononuclear or dinuclear nickel(II) complexes, depending on the steric bulk of the substituents on the pyrazolyl unit. While L3 gave the dinuclear complexes [Ni2(μ2-Cl)2Cl2(L3)2] (5) and [Ni2(μ2-Br)2Br2(L3)2] (6), L4 gave the mononuclear complexes [NiCl2(L4)] (7) and [NiBr2(L4)] (8). Activation of 1−8 with EtAlCl2 resulted in the oligomerization of ethylene to C4, C6, and C8 alkenes, followed by subsequent Friedel−Crafts alkylation of the toluene solvent. Activities as high as 15 660 kg of alkylated products/mol Ni/h were observed for 5 at 40 bar. However, when hexane was used as solvent, only trace amounts of alkylated toluene products were observed.