Ethylene Oligomerization and Propylene Dimerization Using Cationic (α-Diimine)nickel(II) Catalysts

Abstract

Nickel-catalyzed ethylene oligomerization and propylene dimerization reactions are described. A series of aryl-substituted α-diimine ligands 1 (ArNC(R)C(R)NAr, R,R ≡ 1,8-naphth-diyl, Ar ≡ XC6H4, X = p-CF3 (a), p-H (b), p-Me (c), p-OMe (d), o-Me (e)) and their corresponding Ni(II) dibromide complexes 2 were prepared. Treatment of the Ni(II) dibromide complexes 2 with aluminum alkyl activators such as MAO (methylalumoxane), modified MAO (MMAO), or Et2AlCl in toluene generates active cationic catalysts in situ that oligomerize ethylene to a Schulz−Flory distribution of linear α-olefins. Reaction conditions can be adjusted that lead to selectivities as high as 96% for linear α-olefins. These catalysts are highly active, with ethylene turnover frequencies as high as 1.4 × 105 mol of C2H4/((mol of Ni)h) observed. Schulz−Flory α values range from 0.59 to 0.81 and are dependent on the reaction temperature, ethylene pressure, and nature of the aluminum cocatalyst. The active catalysts dimerize propylene, generatin...