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

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, generating product mixtures that have roughly equal compositions of n-hexenes and 2-methylpentenes. 2,3-Dimethylbutenes are minor products, usually comprising less than 10% of the total product distribution. Propylene dimers and their associated isomerization products are the only reaction products observed under the standard reaction conditions used. Propylene turnover frequencies as high as 2 × 104 mol of C3H6/((mol of Ni)h) were observed. Control experiments indicate that terminal olefins are slowly isomerized to internal olefins under propylene dimerization reaction conditions but dimer production is fast relative to product isomerization. Higher olefins such as 1-butene and 4-methyl-1-pentene are dimerized very slowly by these catalysts.