Abstract
A series of Cr-based complexes 6-10 bearing aminophosphine (P,N) ligands Ph(2)P-L-NH(2) [L = CH(2)CH(2) (1), L = CH(2)CH(2)CH(2) (2), and L = C(6)H(4)CH(2) (3)] and phosphine-imine-pyrryl (P,N,N) ligands 2-(Ph(2)P-L-N=CH)C(4)H(3)NH [L = CH(2)CH(2)CH(2) (4) and L = C(6)H(4)CH(2) (5)] were prepared, and their catalytic properties were examined for ethylene tri/tetramerization. X-ray crystallographic analysis of complex 8 indicated the κ(2)-P,N bidentate coordination mode at the Cr(III) center and the distorted octahedral geometry of monomeric P,N-CrCl(3). Upon activation by methylaluminoxane (MAO), complexes 7-8 bearing P,N (PC(3)N backbone) ligands 2-3 showed good catalytic reactivity for ethylene tri/tetramerization. On the other hand, complex 6 bearing the P,N (PC(2)N backbone) ligand 1 was found active for non-selective ethylene oligomerization, while complexes 9-10 bearing P,N,N ligands 4-5 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 7 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.