Chromium catalysts stabilized by alkylphosphanyl PNP ligands for selective ethylene tri-/tetramerization

The alkyl substituent(s) in the scaffold of the alkylphosphanyl PNP ligands of the form Ph2PN(cyclopentyl)PPhR, Ph2PN(cyclopentyl)PR2, and RPhPN(cyclopentyl)PPhR have been observed to have a marked impact on the catalytic performance of Cr(III) catalysts in ethylene oligomerization reactions. Precatalyst 6, bearing diisopropylphosphanyl substituent, afforded the highest catalytic activity of 4490 kg(product)·g(Cr)−1·h−1 and 51.0% C8 selectivity with good product purity under suitable conditions. Precatalyst 7, having diethylphosphanyl substituent in the PNP scaffold, exhibited 72.0% high C8 selectivity with 1717 kg(product)·g(Cr)−1·h−1 improved catalytic activity. Precatalysts based on monoalkylphosphanyl PNP (2–4) and symmetrical alkylphosphanyl PNP (9–10) exhibited poor catalytic performance toward selective ethylene oligomerization. Single crystal analysis suggested that the alkyl substituents in the scaffold of the alkylphosphanyl PNP ligands may effectively induce the complex structure and consequently the catalytic performance. Density functional theory (DFT) calculations revealed that in comparison to the trimerization pathway, the rate-determining states in the tetramerization pathways of precatalysts 6–7 may face low energy barriers and therefore may offer high C8 selectivity.