Highly active half-sandwich chromium(iii) catalysts bearing bis(imino)pyrrole ligands for ethylene (co)polymerization†
A series of half-sandwich Cr(III) complexes bearing bis(imino)pyrrole ligands, Cp′[2,5-C4H2N(CHNAr)2]CrCl [Cp′ = C5H5, Ar = C6H5 (2a), 2,6-Me2C6H3 (2b), 2,6-iPr2C6H3 (2c), C6F5 (2d); Cp′ = C5Me5, Ar = C6H5 (3a), 2,6-iPr2C6H3 (3c)] were synthesized with good yields. The complexes were characterized by FTIR and mass spectrometry in addition to elemental analyses. X-ray structural analyses for 2a–c showed that the Cr complexes have a pseudo-octahedral coordination environment with a three-legged “piano stool” geometry. One of the imino nitrogen atoms is coordinated with the Cr metal. On activation with methylaluminoxane, the Cp-based complexes showed high catalytic activities for ethylene polymerization. High molecular weight polymers with unimodal molecular weight distributions were obtained, indicating that the nature of the polymerization was single site. The copolymerization of ethylene with norbornene by the pre-catalysts 2a–d was also explored in the presence of methylaluminoxane. The catalytic activity, co-monomer incorporation and the properties of the resultant polymers can be controlled over a wide range by tuning the catalyst structures and reaction parameters.