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.