Design and synthesis of titanium(IV) complexes with ethylene bridged fluorenyl–phenoxyimine ligands for ethylene polymerization: Effect of ligand architecture on catalyst performance

Abstract

A series of ethylene-bridged fluorenyl-phenoxyimine ligands, L= (E)-2-((2-(9H-fluoren-9-yl)alkylimino)methyl)-6-R1-4-R2-phenol [alkyl= Me (L1–L5), H (L6–L10)] were strategically designed and synthesized by means of an condensation reaction between 3-R1-5-R2-2-hydroxybenzaldehyde [(R1= tBu, H and R2= tBu, Et, Me, H) (6a–6e)] and the synthesized 2-(9H-fluoren-9-yl)alkanamines (5a and 5b). The formation of the ligands was confirmed by various characterization techniques, including NMR and FTIR spectroscopy, mass spectrometry and elemental analyses. Molecular structures of ligands L5, L8, L9 and L10 were further confirmed by single-crystal X-ray diffraction. The ligands formed dianionic species upon lithiation followed by reaction with TiCl4 in THF afforded half-metallocene Ti(IV) complexes (C1-C10), which were characterized by NMR, HRMS and elemental analysis. After activation by TIBA/[CPh3][B(C6F5)4], complexes C1-C10 were evaluated for ethylene polymerization by varying the Al/Ti ratio and reaction temperature. All complexes exhibited good catalytic activity, ranging from 0.72 × 105–2.16 × 105 g PE·(mol Ti)−1·h−1, and produced high molecular weight polyethylene with Mw values between 7.8 × 105 and 17.2 × 105 g·mol−1. The presence of a bulky tert-butyl group ortho to the phenoxy oxygen in complexes C1 and C6 led to significantly improved catalytic activity, with the obtained polyethylene products displaying notably higher molecular weights than those produced by C5 and C10, which lack this steric feature. By introducing various substituents on the phenoxy moiety, the electronic environment and spatial arrangement around the metal centre were finely tuned to tailor the polymer properties.