Synthesis, structure, and catalytic ethylene oligomerization of nickel complexes bearing 2-pyrazolyl substituted 1,10-phenanthroline ligands

A series of nickel(II) halide complexes [NiCl2(L)] (1a–12a) and [NiBr2(L)] (1b–12b) bearing 2-pyrazolyl substituted 1,10-phenanthroline derivatives (L1–L12) have been synthesized and characterized by elemental analysis, IR spectroscopy and X-ray diffraction analysis (for 1a′, 10b and 12b). Upon activation with methylaluminoxane (MAO), these complexes display good catalytic activities in ethylene oligomerization (up to 3.01 × 105 g mol(Ni)−1 h−1 at 10 atm of ethylene for 12a with L12 = 2-(3,5-diphenylpyrazol-1-yl)-9-mesityl-1,10-phenanthroline). Complexes with bulkier aryl groups in the 9-position of the phenanthroline ring and the 3- and 5-position of the pyrazolyl ring give higher activities. This is explained with the formation of a more directional environment, a reaction channel around the nickel center which enhances the probability of insertion over dissociation after ethylene coordination to Ni. Also, bulkier aryl substituents can suppress chain-transfer reactions (typically β-H elimination) and therefore increase the activity.

Graphical abstractA series of nickel(II) complexes bearing 2-pyrazole substituted 1,10-phenanthroline ligands have been synthesized, which show good catalytic activities for ethylene oligomerization upon activation with methylaluminoxane (MAO). The effects of the coordination environment and reaction conditions on the catalytic behavior have been discussed.Figure optionsDownload full-size imageDownload as PowerPoint slide