Ethylene Oligomerization Catalyzed by a Unique Phosphine−Oxazoline Palladium(II) Complex. Propagation and Chain Transfer Mechanisms

Palladium(II)-catalyzed ethylene oligomerization reactions using a new [(P∧N)Pd(CH3)(NCArF)][SbF6] complex, 2b (P∧N = phosphine−oxazoline, ArF = 3,5-(CF3)2C6H3), are described. Analysis of the X-ray crystal structure of the (P∧N)Pd(CH3)Cl precatalyst, 2a, reveals a unique axial Pd···H interaction (2.33 Å) with a ligand Csp3−H bond. Exposure of this complex to ethylene in methylene chloride produces a Schulz−Flory distribution of α-olefins ranging from C4 to C24. Effects of varying ethylene pressure and reaction temperature on oligomerizations are reported. The turnover frequency exhibits saturation behavior as ethylene pressure is increased; carrying out a Lineweaver−Burk analysis provides a barrier of 17.5 kcal/mol for migratory insertion of the palladium alkyl ethylene species. The invariance of the Schulz−Flory constant with added quantities of nitrile suggests that chain transfer occurs through a process best described as chain transfer to monomer. The catalyst resting state is shown to vary as a function of ethylene pressure and nitrile concentration. Oligomerization experiments performed with a related phosphine−oxazoline complex lacking Pd···H−Csp3 axial interaction lead only to ethylene dimerization.