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

A range of novel
N-substituted diphosphinoamine (PNP) ligands Ph2PN­(R)­PPh2 [R = F2CHCH2 (1); R =
Me2CHCH2 (2); R
= Me2CHCH2CH2 (3)] have
been synthesized via one-step salt elimination reaction. The ligand-coordinated
chromium carbonyls [Ph2PN­(R)­PPh2]­Cr­(CO)4 (4–6) were further synthesized, and X-ray
crystallography analysis of complex 6 revealed the κ2-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 {[Ph2PN­(R)­PPh2]­CrCl2(μ-Cl)}2 (7–9) were further
discussed in depth. Experimental results showed that complex 7 with the strong electron-withdrawing F2CHCH2 group can promote the nonselective ethylene oligomerization,
while both complex 8 and complex 9 with
the electron-donating Me2CHCH2 and Me2CHCH2CH2 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.