Chromium-Based Complexes Bearing Aminophosphine and Phosphine-Imine-Pyrryl Ligands for Selective Ethylene Tri/Tetramerization.

A series of Cr-based complexes bearing aminophosphine (P,N) ligands PhP-L-NH [L = CHCH (), L = CHCHCH (), and L = CHCH ()] and phosphine-imine-pyrryl (P,N,N) ligands 2-(PhP-L-N=CH)CHNH [L = CHCHCH () and L = CHCH ()] were prepared, and their catalytic properties were examined for ethylene tri/tetramerization. X-ray crystallographic analysis of complex indicated the κ-P,N bidentate coordination mode at the Cr(III) center and the distorted octahedral geometry of monomeric P,N-CrCl. Upon activation by methylaluminoxane (MAO), complexes bearing P,N (PCN backbone) ligands showed good catalytic reactivity for ethylene tri/tetramerization. On the other hand, complex bearing the P,N (PCN backbone) ligand was found active for non-selective ethylene oligomerization, while complexes bearing P,N,N ligands only produced polymerization products. In particular, the high catalytic activity of 458.2 kg/(g·Cr·h), excellent selectivity of 90.9% (1-hexene and 1-octene combined), and extremely low PE content of 0.1% were obtained with complex in toluene at 45 °C and 45 bar. These results suggest that rational control of P,N and P,N,N ligand backbones, including a carbon spacer and rigidity of a carbon bridge, can lead to the high-performance catalyst for the ethylene tri/tetramerization process.