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.

以四氢呋喃（THF）为溶剂，由二氯(环戊二烯基)铬四氢呋喃加合物（CpCrCl₂(thf)）与相应配体反应，以高收率合成了一系列负载苯氧膦[O,P]配体（3a~3f）或苯氧膦氧化物[O,P=O]配体（4b~4e）的新型半三明治型铬配合物。对配合物3b、3c、3e及4c开展的X射线晶体结构分析显示，此类配合物均采取三脚钢琴凳型几何构型。经改性甲基铝氧烷活化后，配合物3a~3f在乙烯聚合反应中表现出中等至较高的催化活性，所得聚合物为高分子量且具有单峰分子量分布。令人惊喜的是，配合物3c可高效催化乙烯与降冰片烯（NBE）的共聚反应，在优化反应条件下可便捷地获得高降冰片烯含量、高分子量的共聚产物。负载[O,P=O]配体的铬配合物的催化活性优于[O,P]配体对应物，但其制备的聚合物分子量显著更低。此类聚合行为的差异可能源于金属中心周围空间位阻与电子效应的共同变化。