Group 4 Complexes Supported by Pyridine-2-Phenolate-6-Arylmethine Ligands: Spectroscopic and Structural Characterization and Olefin Polymerization Catalysis

Abstract

A series of Ti(IV) and Zr(IV) bis(benzyl) post-metallocene precatalysts containing an unusual C(sp³)-chelating [O,N,CH(Ar)] ligand has been synthesized. The four-membered N,C-donor chelate is anticipated to engender more accessible active sites, while the aryl-substituted C(methine) moiety can introduce an element of flexibility therein. The racemic complexes have been fully characterized by multinuclear NMR spectroscopy. In particular, [¹H,¹⁹F]-COSY and -HOESY experiments were conducted to elucidate the environment around the fluorine atoms at the aryl substituent. The molecular structures of two Zr derivatives have been determined by X-ray crystallography, which confirm their C₁-symmetric nature and the relatively small tridentate bite angle. With regard to ethylene polymerization reactions in conjunction with trityl borate at 22 °C, the [O,N,CH(Ar)]-Ti catalysts exhibit high activities (up to 820 g mmol^–1 h^–1 atm^–1 for Ar = 2-fluorophenyl) and are more efficient than the C(sp²)-chelating [O,N,C(σ-aryl)] congeners, and the 2,4,6-trifluorophenyl catalyst produces a polymer (M_w = 3.3 × 10² kg mol^–1) with an M_w/M_n value of 2.3. Although evidence of C–H···F–C contacts corresponding to “weak attractive ligand–polymer interactions” has not been found for these catalysts, this study illustrates the beneficial effects arising from ortho (especially fluorine) substituents on the arylmethine moiety.