New Half-Sandwich Chromium(III) Complexes Bearing Phenoxy-Phosphine (Oxide) [O,P(O)] Ligands: Synthesis, Structures, and Catalytic Properties for Ethylene (Co)Polymerization

A series of novel half-sandwich-type chromium complexes bearing phenoxy-phosphine [O,P] ligands (3a–3f) or phenoxy-phosphine oxide [O,PO] ligands (4b–4e) were synthesized in high yields from CpCrCl₂(thf) with corresponding ligands in THF. X-ray structure analyses for 3b, 3c, 3e, and 4c revealed that these complexes adopt a three-legged piano stool geometry. When activated by modified methylaluminoxane, complexes 3a–3f exhibited moderate to high activities toward ethylene polymerization, giving high molecular weight polymers with unimodal molecular weight distributions. Excitingly, complex 3c could efficiently promote ethylene/NBE copolymerization, therefore, high molecular weight copolymers with high NBE content could easily be obtained under optimized conditions. The [O,PO] complexes showed higher catalytic activities than [O,P] analogues but produced much lower molecular weight polymers. The difference in polymerization behavior might be caused by the change of both the steric bulk and the electronic effect around the metal center.