Synthesis and Characterization of Titanium Complexes Bearing Two β-Enaminoketonato Ligands with Electron-Withdrawing Groups and Their Behavior in Ethylene Polymerization

Abstract

A series of new titanium complexes with two asymmetric bidentate β-enaminoketonato (N,O) ligands (4b−t), [RN═C(CF3)CHC(t-Bu)O]2TiCl2 (4b, R = −C6H4F(o); 4c, R = −C6H4F(m);4d, R = −C6H4F(p); 4e, R = −C6H3F2(2,3); 4f, R = −C6H3F2(2,4); 4g, R = −C6H3F2(2,5); 4h, R = −C6H3F2(2,6); 4i, R = −C6H3F2(3,4); 4j, R = −C6H3F2(3,5); 4k, R = −C6H2F3(2,3,4); 4l, R = −C6H2F3(3,4,5); 4m, R = −C6H4CF3(o); 4n, R = −C6H4CF3(m); 4o, R = −C6H4CF3(p); 4p, R = −C6H4Cl(p); 4q, R = −C6H4I(p); 4r, R = −C6H4NO2(p); 4s, R = −CH2C6H5; 4t, R = −C6H11), have been synthesized and characterized. X-ray crystal structures suggest that complexes 4a−d, 4j, and 4m all adopt a distorted octahedral geometry around the titanium centers. Two chlorine atoms in complexes 4a−d and 4j are in cis-configuration, while those of complex 4m are trans. NMR spectra and X-ray structure analyses reveal that the conformational isomers of some complexes, such as 4b, in which the two β-enaminoketonato ligands bear asymmetrical N-phenyl rings, exist both in solution and in solid state. With modified methylaluminoxane (MMAO) as a cocatalyst, complexes 4b−l and 4n−q are highly active toward ethylene polymerization and produce high molecular weight polyethylenes. The catalytic activities are significantly enhanced by introducing electron-withdrawing groups (EWG), such as fluorine and chlorine atom(s) or the trifluoromethyl group, into suitable positions on the N-aryl rings. The titanium complex 4m is inactive toward ethylene polymerization due to the trans-configuration of the two chlorine atoms. In addition, the titanium complexes display low catalytic activity for ethylene polymerization only if the N-substituents are not aromatic.