Chromium-Based Complexes Bearing N-Substituted Diphosphinoamine Ligands for Ethylene Oligomerization

[Image: see text] 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.