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

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journal contribution
posted on 2018-10-10, 00:00 authored by Fakhre Alam, Le Zhang, Wei Wei, Jiadong Wang, Yanhui Chen, Chunhua Dong, Tao Jiang
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.