Asymmetric carbon-bridged diphosphine based chromium complexes for selective ethylene tri-/tetramerization with high thermal stability

Asymmetric alkylphosphanyl carbon-bridged diphosphine Ph2PC = CPR1(R2)-type L1-L5 ligand has been observed to have a distinct effect on the catalytic performance of the corresponding Cr(III) based catalysts in ethylene oligomerization. Precatalyst 1, bearing diethylphosphanyl moiety, exhibited a high catalytic activity of 3628 kg·g−1·h−1 accompanied by 65.5% high C8 selectivity. Precatalyst 2 having diisopropylphosphanyl group afforded remarkably high catalytic activity of 4833 kg·g−1·h−1 with 91.8% combined (C6 + C8) product selectivity. Precatalysts 1–2 also showed good thermal stability and offer high catalytic activity at the high reaction temperature. Precatalyst 3 with sterically encumbered dicyclohexylphosphanyl substituent mainly offers C6 selectivity at the expense of the C8 fraction. Precatalyst 5 having isopropylphosphanyl moiety afforded poor catalytic activity and high C6 selectivity. Density functional theory (DFT) calculations of precatalyst 2 showed that the rate-determining step in the trimerization pathway of the catalyst may face a low energy barrier and therefore produce more C6 fraction.