Benzylnickel(II) Complexes of 2-Iminopyrrolyl Chelating Ligands: Synthesis, Structure, and Catalytic Oligo-/Polymerization of Ethylene to Hyperbranched Polyethylene

Abstract

A family of benzyl nickel(II) complexes of 5-aryl-2-iminopyrrolyl ligands has been synthesized and characterized, and their behavior as aluminum-free catalysts for the oligo-/polymerization of ethylene is discussed. η1-Benzyl Ni(II) complexes of the type [Ni{κ2N,N′-5-R-NC4H2-2-C(H)═N(2,6-iPr2C6H3)}(η1-CH2C6H5)(PMe3)] (R = 3,5-(CF3)2C6H3 (1a), 2,4,6-iPr3C6H2 (1b)) were synthesized by the reaction of [NiCl(η3-CH2C6H5)(PMe3)] with 1 equiv of the respective 5-R-2-[N-(2,6-diisopropylphenyl)formimino]pyrrolyl potassium salt (KLa,b) in good yields. Complexes 1a,b, upon treatment with B(C6F5)3, yielded η3-benzyl Ni(II) complexes of the type [Ni{κ2N,N′-5-R-NC4H2-2-C(H)═N(2,6-iPr2C6H3)}(η3-CH2C6H5)] (R = 3,5-(CF3)2C6H3 (2a), 2,4,6-iPr3C6H2 (2b)). All complexes were characterized by 1H and 13C{1H} NMR spectroscopy and elemental analysis and 1b and 2a by X-ray diffraction. All complexes were further tested in the oligo-/polymerization of ethylene. Complexes 1a,b in the presence of 2 equiv of [Ni(COD)2] showed activities from 2.06 to 61.81 kg mol–1 h–1 bar–1, whereas complexes 2a,b alone showed activities from 0.67 to 14.44 kg mol –1 h–1 bar–1. The polyethylene products obtained with catalyst systems 1a and 2a are low-viscosity oils with molecular weights in the range 900–1200 g mol–1, and those obtained with catalyst systems 1b and 2b are high-viscosity oils with molecular weights between 10900 and 17700 g mol–1, all showing very high branching degrees (100–115 branches/1000C atoms), characteristic of hyperbranched polyethylene products.