Mononuclear and Dendritic Nickel(II) Complexes ContainingN,N‘-Iminopyridine Chelating Ligands:  Generation Effects on the Catalytic Oligomerization and Polymerization of Ethylene

Abstract

A series of carbosilane dendritic compounds Gn-ONNMe m , containing one (n = 0), four (n = 1), eight (n = 2), or 16 (n = 3) terminal pyridylimine ligands, substituted with m methyl groups (m = 0, 2, 3), and nickel complexes Gn-ONNMe m NiBr2, comprising monometallic to metallodendritic structures, have been synthesized. The nickel complexes, in combination with methylaluminoxane (MAO), are active catalysts for the concurrent transformation of ethylene into mixtures of toluene-insoluble polyethylene and oily oligomers. Oligomers consist of mixtures of olefins that follow a Schulz−Flory distribution, and polymers are found to be highly branched low molecular weight polyethylene. The variation of the pyridylimine ligand framework by methyl substituents has a decisive influence on the activities of the nickel compounds. Also, the size (i.e., generation n) of the dendritic precursor acutely affects the catalyst performance and the microstructure of the insertion products. Thus, higher generation catalysts show superior oligomerization activities and produce less branched polyethylene polymers with higher molecular weights.

Olefin oligomerization, homopolymerization and copolymerization by late transition metals supported by (imino)pyridine ligands

“…The vast majority of 2-organylimino-pyridyl ligands are straightforwardly prepared by condensing 2-acetyl or 2-formyl-pyridine systems with an equivalent amount of the proper amine [13][14][15][16][17][18] or aniline [2,3,[5][6][7]16,[19][20][21][22][23][24][25][26][27][28][29][30], possibly in the presence of an acid cocatalyst (Scheme 2a). The synthesis of 6-organyl-2-iminopyridyl ligands generally requires a more complex procedure and the Schiff-base condensation step just concludes the synthetic path.…”

“…The synthesis of late transition metal catalyst precursors (mostly from the first transition row) is a straightforward process, which involves the plain addition of the solid ligands to n-BuOH, THF, toluene or CH 2 Cl 2 solutions of either anhydrous or hydrated dihalides (Scheme 8) [1,2,[9][10][11][12]14,15,20,21,25,26,32,33,37,38,[47][48][49]53,54]. Irrespective of the metal, the dihalides are sparingly soluble in aromatic hydrocarbons, while they dissolve fairly well in polar solvents.…”

“…Generally, these compounds are microcrystalline air-stable solids, whereas they decompose in solution either for prolonged times or unless protected by an inert atmosphere. They can be isolated as mononuclear complexes or, depending on both the metal halides and degree of substitution of the pyridine ring, as centrosymmetric dimers [13,20,22,28,53,[58][59][60]. The IR spectra of all compounds show a red shift of (C N) by ca.…”

“…The molecular structures of several Ni II derivatives have been determined by single-crystal X-ray diffraction techniques (Fig. 5) [2,20,22,27,28,32,60]. The coordination geometry around the metal center determines also the magnetic properties of the resulting complexes.…”

Oligomerization of Ethylene to Produce Linear α-Olefins Using Heterogeneous Catalyst Prepared by Immobilization of α-Diiminenickel(II) Complex into Fluorotetrasilicic Mica Interlayer

“…The structure of the α-diiminenickel complex, which shows an activity for the ethylene polymerization/oligomerization, has a distorted tetrahedral geometry, and the ratio of the ligand to the nickel ion is 1 (LNiX 2 , L: bidentate α-diimine ligand, X: halogen). Meanwhile, the coordination of the two bidentate iminopyridine ligands to one nickel ion (L 2 NiX 2 ) affords a complex with the distorted octahedral geometry, and this octahedral complex shows no significant activity for the ethylene polymerization compared to the tetrahedral one [19]. It is considered that L1 more readily formed the L 2 Ni 2+ -type complex than L5, and as a result, the amount of the LNi 2+ -type complex dramatically decreased when the less-hindered ligand L1 was used.…”