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.