Synthesis and characterization of group 4 complexes and olefin polymerization catalysts supported by chelating σ-aryl ligands

Abstract

A series of Group 4 post-metallocene bis(benzyl) complexes supported by tridentate pyridine-2-phenolate-6-(σ-aryl) [O,N,C] ligands, featuring various substituents on the σ-aryl group (including fluoro, trifluoromethyl, fused benzo [C4H4], phenyl, and substituted aryl) directly adjacent to the pyridine ring (referred to as ‘py-adjacent’), have been prepared. The purpose of this study was to investigate primarily the steric impact of the py-adjacent substituent upon chemical and catalytic reactivity. Multinuclear (including 1H, 13C, and 19F) NMR characterization of the complexes has been performed. For example, η2 coordination of the benzyl moieties is indicated for the Ti derivatives, which implies high electrophilicity at the corresponding metal center. Besides, the diastereotopic methylene hydrogens of the Ti and Zr complexes bearing 1-naphthyl as the py-adjacent substituent are observed as four discrete doublets in the 1H NMR spectrum, which is unusual and interesting for such organometallic complexes. Ethylene polymerization studies have been undertaken for the [O,N,C] catalysts, in conjunction with methylaluminoxane (MAO) or [Ph3C][B(C6F5)4] as cocatalyst, and high activities (over 100 g of polymer per mmol of catalyst per h) have been achieved for the Ti catalysts. Based on the framework of highly active titanium(IV) [O,N,C] catalysts featuring fused benzo or trifluoromethyl substituents on the σ-aryl group, a number of new Ti bis(benzyl) complexes have been successfully prepared which replaces the tert-butyl group ortho to the phenolate functionality with a substituent displaying different steric and electronic properties, namely trifluoromethyl, cumyl, and 1,1-diphenylethyl. This approach allows direct evaluation of the impact of this phenolate substituent upon catalytic behavior, which includes activities and longevity at elevated temperatures, molecular weight of polymer and polydispersity. In particular, the cumyl-derived Ti catalyst in conjunction with [Ph3C][B(C6F5)4] exhibits excellent ethylene polymerization activities (over 2.5 x 103 g mmol-1 h-1) with very high Mn (up to 4 x 106 g mol-1 from GPC, Mw/Mn = 2 The ethylene polymerization activities of previously prepared group 4 complexes containing (σ-aryl)-2-phenolate-6-pyridyl [O,C,N] ligands were significantly lower than their [O,N,C] analogues. Therefore, group 4 bis(benzyl) complexes supported by new [O,C,N] frameworks have been designed and synthesized in order to improve catalytic activity. All complexes have been characterized by NMR spectroscopy. In the 1H NMR spectra of the Ti complex where the pyridyl moiety [N] = F-substituted isoquinolinyl, one of the two methylene doublets appears as a doublet of doublets, which collapses to an usual doublet upon 19F-decoupling, indicating that the coupling is occurring via weak intramolecular C-H···F-C interactions. Furthermore, [1H,19F]-HMBC experiments indicated a significant and dominant scalar component for this coupling, which confirms that the interaction is real and taking place with chemical connectivity. Similarly, the Ti and Zr complexes where [N] = Cl-substituted pyridyl, the downfield doublet for the diastereotopic methylene hydrogens is apparently broadened, which may be indicative of weak intramolecular C–H···Cl–C interactions. For ethylene polymerization using MAO, the activity of the Ti-[O,C,N] complex where [N] = substituted isoquinolinyl is moderate and enhanced compared with the congener where [N] = Cl-substituted pyridyl, although other catalysts displayed lower and/or negligible activities.