Switchable ethylene tri-/tetramerization with high catalytic performance: Subtle effect presented by P-alkyl substituent of PCCP ligands

Electron-rich Ph2PC(R″) = CHPRR′ ligands with a P-alkyl group were prepared, and, in conjunction with chromium, showed very high activities in ethylene tri-/tetramerization, leading to, in most cases, a very low PE selectivity of ≤0.1 wt%. The electronic property of ligands has a remarkable influence on the catalytic activity: the stronger the σ-donor ability, the higher the activity. Dialkyl phosphine ligands show a stronger σ-donor ability, and thus exhibited higher activities than their mixed alkyl/Ph phosphine counterparts. Reducing the ligand steric bulk could convert the selectivity from predominant trimerization into mixed tri-/tetramerization with 1-octene:1-hexene ratio of >1 without compromising high combined 1-C6/1-C8 selectivity (>90 wt%). Furthermore, the ligands with a dialkyl phosphine showed high low-temperature activity at 25 °C with a relatively high 1-C8 selectivity. Notably, under a lower pressure of 20–30 bar, using PPhiPr-PCCP 7 showed almost no formation of PE with high activity and selectivity.