Group 4 Post-Metallocenes Supported by [OCH2N,C(σ-aryl)] Auxiliaries Bearing a Seven-Membered Metallacycle : Synthesis, Characterization, and Catalysts for Olefin Polymerization

Research output: Journal Publications and ReviewsRGC 21 - Publication in refereed journalpeer-review

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Original languageEnglish
Pages (from-to)2963-2971
Number of pages9
Journal / PublicationOrganometallics
Volume38
Issue number15
Online published1 Aug 2019
Publication statusPublished - 12 Aug 2019

Abstract

A series of pyridine-2-phenolate-6-(σ-aryl) [OCH2N,C] group 4 bis(benzyl) precatalysts, featuring a flexible O,N-donor chelate, have been prepared and characterized by multinuclear NMR spectroscopy. These complexes adopt C1 symmetry, and variable-temperature 1H NMR experiments for a difluoro-substituted σ-aryl Ti(IV) derivative indicate a fluxional process involving inversion of the seven-membered chelate ring accompanied by conformational changes for the benzyl groups (ΔG = 62.6 kJ mol-1). Scrutiny of the molecular structure of a Zr(IV) relative, determined by X-ray crystallography, revealed severely distorted "quasi-meridional" coordination for the [OCH2N,C] ligand and virtually perpendicular orientation of the benzyl moieties. Olefin polymerization studies have been undertaken in conjunction with methylaluminoxane (MAO) and trityl borate. Using dried MAO (250 equiv) as cocatalyst and 7 atm of ethylene pressure, a CF3-substituted Ti(IV) catalyst displays good activity (680 g mmol-1 h-1 at 75 °C; Mw = 3.4 × 102 kg mol-1, Mw/Mn = 2.6), while the difluoro-substituted analogue produces high molecular-weight polymers (Mw = 1.34 × 103 kg mol-1 at 50 °C; Mw/Mn = 2.4). For the latter derivative, a weak C-H···F-C interaction between a CH2(benzyl) proton and the fluorine atom ortho to the Ti-C(σ-aryl) bond is detected by [1H,19F]-HMQC experiments. These results open up the possibility that such stabilizing interactions could potentially occur for the catalytic species to suppress β-H elimination and hence chain termination processes.

Citation Format(s)

Group 4 Post-Metallocenes Supported by [OCH2N,C(σ-aryl)] Auxiliaries Bearing a Seven-Membered Metallacycle: Synthesis, Characterization, and Catalysts for Olefin Polymerization. / Liu, Cham-Chuen; Liu, Qian; Yiu, Shek-Man et al.
In: Organometallics, Vol. 38, No. 15, 12.08.2019, p. 2963-2971.

Research output: Journal Publications and ReviewsRGC 21 - Publication in refereed journalpeer-review