Group 4 complexes bearing chelating σ-Aryl, σ-Alkenyl and alkyl ancillary ligands : synthesis, spectroscopic characterization and olefin polymerization catalysis

基於 σ-芳基, σ-烯基和烷基型為輔助配體的第四族絡合物 : 合成, 光譜表徵及烯烴聚合催化反應

Student thesis: Doctoral Thesis

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Author(s)

  • Cham Chuen LIU

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Awarding Institution
Supervisors/Advisors
Award dateJun 2015

Abstract

Chapter 1 gives an introduction to group 4 catalysts for olefin polymerization, and a brief review of recent developments in post-metallocene catalysts. Chapter 2 describes general experimental procedures.

Group 4 post-metallocene complexes supported by tridentate pyridine-2-phenolate-6-(σ-aryl) [O,N,C(σ-aryl)] ligands, containing methyl substituent(s) on the σ-aryl group, are described in Chapter 3. The principal aim is to investigate the steric and electronic effects upon chemical and catalytic reactivity. The synthesized complexes have been characterized by multinuclear NMR spectroscopy, and η2-coordination of benzyl groups is indicated for all titanium and zirconium complexes. Moreover, the impact of the "py-adjacent" substituent (σ-aryl substituent adjacent to pyridyl ring) upon the C(σ-aryl) atom was indicated by 13C NMR spectroscopy and X-ray crystallography. The polymerization properties of the [O,N,C(σ-aryl)] catalysts have been studied. Interestingly, one of the catalysts produced polyethylene with narrow molecular weight distribution (Mn = 1.3 × 105, Mw/Mn = 2.1), implying single-site character. Theoretical calculations have been performed to consider energy-minimized calculated (Gaussian) structures of selected catalytic species.

In Chapter 4, ligands with a more flexible chelating-[O,N] moiety have been developed, and a series of group 4 post-metallocene complexes bearing [OCH2N,C(σ-aryl)] ligands has been synthesized and characterized by multinuclear NMR spectroscopy. Unlike [O,N,C(σ-aryl)] complexes which display C2v symmetry, these complexes adopt C1 symmetry. Variable-temperature 1H NMR experiments suggest a fluxional process concerning the seven-membered [OCH2N] ring, and the associated activation barrier was calculated. The polymerization properties of [OCH2N,C(σ-aryl)] catalysts have been examined. However, these catalysts are less active compared with [O,N,C(σ-aryl)] analogues. A [O(methoxide),N,C] system bearing a five-membered [O,N] chelate ring was also developed. The catalytic capabilities of these complexes have been tested, but all are inactive for ethylene polymerization.

The synthesis, spectroscopic characterization and olefin polymerization behavior of a series of group 4 catalysts bearing aryl-substituted chelating C(sp3)-donor ligands are described in Chapter 5. The [O,N,C(alkyl)] complexes adopt C1 symmetry, which is confirmed by solution NMR spectroscopy and X-ray crystal structures. Interestingly, metalation between M(CH2Ph)4 and the (bis-pyridyl)-substituted ligand gives [O,N,N] products, which are structurally different from [O,N,C(alkyl)] complexes. The [O,N,N] complexes are of C2v symmetry, and interesting resonance structures have been proposed based on spectroscopic observations. The polymerization properties of [O,N,C(alkyl)] catalysts have been studied. Notably, excellent catalytic activities (up to 5127 g mmol1 h1) was observed in conjunction with [Ph3C][B(C6F5)4]/iBu3Al at 22 oC. DFT calculations have been performed to gain insight into catalytic behavior.

The development of chelating σ-alkenyl ligands to support group 4 post-metallocene complexes are described in Chapter 6. Mononuclear complexes bearing a σ-cyclohexenyl moiety, and unusual dimeric complexes featuring a novel μ-η3:η3 bonding mode for the C4-bridging "butadienoid" moiety, have been synthesized and characterized by multinuclear NMR spectroscopy. The binuclear complexes, which are readily formed by dimerization processes, are of C2 symmetry. Careful examination of the chemical shifts of the carbon and proton NMR resonances for the C4-bridge suggests that contributions from the proposed resonance structures may vary for different binuclear complexes and metal centers. UV-vis absorption studies have been performed in order to probe the charge transfer nature of these complexes, and infrared spectroscopy was employed to study the bonding of the C4-bridging moiety. Investigations into their catalytic characteristics in ethylene polymerization have been undertaken in conjunction with MAO or [Ph3C][B(C6F5)4].