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

[Image: see text] 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.