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{κ2 N,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 (KL a ,b ) in good yields. Complexes 1a,b, upon treatment with B­(C6F5)3, yielded η3-benzyl Ni­(II) complexes of the type [Ni­{κ2 N,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.

The second coordination sphere effect of benzothiophene groups enhances chain transfer in ethylene (co)oligomerization

“…Thus, the development of synthetic approaches to ethylene oligomers is also a rewarding endeavor. [22][23][24][25][26][27][28][29][30][31][32][33] In latetransition metal catalyzed systems, the small steric hindrance substitution strategy is usually required to achieve low molecular weight ethylene oligomers. 27,28 However, this strategy often leads to unsatisfactory thermal stability and activity of the catalysts.…”

Direct synthesis of hyperbranched ethene oligomers and ethene‐MA co‐oligomers using iminopyridyl systems with weak neighboring group interactions

“…[1][2][3][4] Although this requires the catalysts to have simultaneously strong chain walking and chain transfer capabilities, the use of ethene as a sole feedstock greatly reduces the cost of synthesis and the target products yielded can be employed as high-value functional additive in surface modifiers or lubricants. 1 So far, some specially designed N,O-salicylaldiminato and N,N-iminopyrrolyl neutral Ni(II) catalysts, [1][2][3][4][5] and cationic Ni(II)-and Pd(II)-α-diimine catalysts [6][7][8] were employed in the preparation of hyperbranched ethene oligomers and co-oligomers. Recently, some new iminopyridyl Ni(II) and Pd(II) complexes have also been developed to generate hyperbranched ethene oligomers and polar functional ethene oligomers in the (co)oligomerization of ethene.…”

Ethylene (co) oligomerization using iminopyridyl Ni(II) and Pd(II) complexes bearing benzocycloalkyl moieties to access hyperbranched ethylene oligomers and ethylene-MA co-oligomers

“…In particular, a few late-transition metal catalysts have made important advances in the preparation of branched ethylene oligomers via the chain walking mechanism. For example, a few N,N-iminopyrrolyl and N,O-salicylaldiminato neutral Ni(II) catalysts with specially designed ligands ( Stephenson et al, 2014 ; Wiedemann et al, 2014 ; Falivene et al, 2018 ; Mecking and Schnitte, 2020 ; Cruz et al, 2021 ) and cationic Ni(II)- and Pd(II)-α-diimine catalysts with minor steric hindrance ( Xiang et al, 2011 ; Meduri et al, 2013 ; Guo et al, 2019a ) were used in the synthesis of hyperbranched ethylene oligomers and hyperbranched ethylene-based polar functionalized co-oligomers. Lately, a series of novel iminopyridyl Ni(II) and Pd(II) catalysts have also been developed to yield hyperbranched ethylene oligomers and ethylene-based co-oligomers ( D'Auria et al, 2017 ; Saki et al, 2020 ; Li et al, 2021 ; Yan et al, 2021 ).…”