Late-Transition-Metal Complexes with Bisazaferrocene Ligands for Ethylene Oligomerization

Abstract

Herein we report the synthesis, characterization, and ethylene polymerization properties of novel C2-symmetric and unsymmetric bisazaferrocene complexes with late-transition-metal Ni(II) and Pd(II). In the designed complexes, the two sp2-hybridized nitrogen atoms in the azaferrocene rings coordinate to the transition metals with the azaferrocene architecture presenting pentamethyl or pentaphenyl cyclopentadiene (Cp* or Cp°) rings above and below the coordination plane for the purpose of preventing the associative chain transfer processes of ethylene from the axial faces. The Ni(II) and Pd(II) complexes were prepared and characterized by mass spectrometry (MS), 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, elemental analysis, and single-crystal X-ray crystallography. Upon activation with methylaluminoxane (MAO), the bisazaferrocene complexes with NiBr2 (2a) and PdCl2 (2b) showed very low activities toward ethylene polymerization. Well-defined preactivated bisazaferrocene-Pd(II) complexes (5 and 6) exhibited relatively high thermal stability for ethylene oligomerization: for example, complex 5 remains active in ethylene oligomerization at temperatures up to 120 °C. They have moderate activities in ethylene polymerizations to form relatively low molecular weight oligomers. The polyethylene oligomers formed have branching densities ranging from 20 to 60 branches/1000 carbons. To overcome the difficulty encountered in the synthesis of complex 6, a novel route was developed to make Pd(Me)Cl complexes with various dinitrogen ligands by in situ ligand substitution reaction. Finally, to gain mechanistic insight into the low reactivity of the bisazaferrocene-PdII complexes (5 and 6) as compared to their α-diimine counterparts, kinetic studies were undertaken to measure the energetic barriers for the first methyl migration and subsequent ethylene consumption. It was found that the insertion barriers (ΔG⧧) for both the first methyl migration and subsequent ethylene insertion for complex 5 were about 2−3 kcal/mol higher than the values for the α-diimine counterparts, which corresponds to a 2-order difference in the rate of polymerization. Variable-temperature experiments further revealed that the higher ethylene insertion barrier for the bisazaferrocene-Pd(II) complexes arises from both higher activation enthalpy (ΔH⧧) and smaller activation entropy (ΔS⧧).