Catalytic Systems Based on Chromium(III) Silylated-Diphosphinoamines for Selective Ethylene Tri-/Tetramerization

The N-substituents on the backbone of Ph₂PN­(R)­Si­(CH₃)₂CH₂PPh₂- and [Ph₂PCH₂Si­(CH₃)₂]₂N­(R)-type silylated-diphosphinoamine (Si-PNP) systems have been observed to have a significant impact on their catalytic performance in ethylene oligomerization reactions. Cr precatalyst 3, bearing an isopropyl (ⁱPr) substituent, affords the most efficient catalytic system and exhibited the highest selectivity (83%) toward 1-octene (1-C₈) and showed a catalytic activity of more than 76,700 g­(product)·g­(Cr)⁻¹·h^–1 under experimental conditions. Single-crystal analysis results revealed the influence of steric constraints around the catalytically active center and established a relationship between the product selectivity and the P–Cr–P bite angle. Furthermore, DFT calculations indicate that the catalytic system based on precatalyst 3 faces a low energy barrier in the formation of 1-C₈ and therefore shows high selectivity toward the 1-C₈ fraction. Modification in the backbone length may alter the binding mode of the ligands from mononuclear-bidentate (k²-P, P) to mononuclear-tridentate (k³-P, N, P), which consequently switches the ethylene tetramerization systems to ethylene trimerization systems.