Ethylene Tri-/Tetramerization Catalysts Supported by Phenylene-Bridged Mixed Alkyl/Aryl Diphosphine Ligands

Cr(III) complexes supported by phenylene-bridged diphosphine ligands with P-alkyl substitution were prepared and evaluated in ethylene tri-/tetramerization. The ligand steric and electronic properties have significantly influenced the catalytic activity. The Cr complex 1 based on a bulky PCy2 phosphine ligand L1 having a stronger σ-donor ability exhibited the highest activity, which is more reactive than its PPh2 counterpart 8. The selectivity toward ethylene tri-/tetramerization is mainly determined by the steric property of ligands. Reducing the ligand steric bulk switched the selectivity from predominant trimerization into mixed tri-/tetramerization. 1 achieved a high 1-hexene selectivity of 82.5 wt % with a 1-octene selectivity of 9.7 wt %, while 4 based on L4 bearing a less bulky PPhCy group gave a considerable 1-octene selectivity of 43.3 wt % with a very high activity of 1874 kg·gCr–1·h–1. 1 also exhibited a high activity even at a relatively low temperature of 40 °C, while both 4 and 5 based on a PPh(alkyl) diphosphine ligand gave poor activities at 40 °C. More importantly, these Cr complexes showed a very low PE selectivity of ≤0.1 wt %.