A range of novel N-substituted diphosphinoamine (PNP) ligands Ph 2 PN(R)PPh 2 [R = F 2 CHCH 2 (1); R = Me 2 CHCH 2 (2); R = Me 2 CHCH 2 CH 2 (3)] have been synthesized via one-step salt elimination reaction. The ligand-coordinated chromium carbonyls [Ph 2 PN(R)PPh 2 ]Cr(CO) 4 (4−6) were further synthesized, and X-ray crystallography analysis of complex 6 revealed the κ 2 -P,P bidentate binding mode of Cr center and the molecular structure of PNP ligand 3. Then the catalytic ethylene oligomerization behaviors of PNP ligands 1−3 bridging chromium chloride complexes {[Ph 2 PN(R)PPh 2 ]CrCl 2 (μ-Cl)} 2 (7−9) were further discussed in depth. Experimental results showed that complex 7 with the strong electron-withdrawing F 2 CHCH 2 group can promote the nonselective ethylene oligomerization, while both complex 8 and complex 9 with the electron-donating Me 2 CHCH 2 and Me 2 CHCH 2 CH 2 groups can significantly enhance the selective ethylene tri/tetramerization. The good catalytic activity of 198.3 kg/(g Cr•h), the selectivity toward 1-hexene and 1-octene of 76.4%, and the low PE content of 0.2% were simultaneously achieved with the Al/Cr molar ratio of 600 using the complex 8/MMAO system at 45 °C and 45 bar. These excellent results were mainly attributed to the fact that the β-branching of bridging ligand 2 increased the steric bulk of the N-moiety for complex 8.

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. Xray 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.