Use of PCCP Ligands with a P-Ph(alkyl) Group in Chromium-Catalyzed Ethylene Tri-/Tetramerization

Cr(III) complexes supported by the ligands of the type Ph2PC(R)═CHPPh(alkyl) were prepared and tested in ethylene tri-/tetramerization. All the complexes exhibited predominant tetramerization selectivity. It is found that not only the steric properties of both the backbone substituent and the P-substituent but also the electronic properties of the backbone substituent affect the catalytic performance. Precatalyst 6 bearing an isopropyl group on the ligand backbone achieved the highest combined 1-C6/1-C8 selectivity (84.8 wt %) with a high 1-octene selectivity (56.1 wt %), maintaining a high activity of 3662 kg/g Cr·h. Precatalyst 2 bearing a 3-pentyl group on the ligand backbone exhibited excellent high-temperature stability at 100 °C, achieving a high activity of up to 2213 kg/g Cr·h and a high combined 1-C6/1-C8 selectivity (87.9 wt %) with a low PE formation (0.08 wt %). Catalysis using the ligands with a primary P-alkyl substituent led to a low combined 1-C6/1-C8 selectivity (69.4–75.2 wt %). Introduction of β-branching at the primary P-alkyl substituent to give precatalyst 14 with a P-benzyl group could improve the overall catalytic performance, giving a markedly increased activity with an improved combined 1-C6/1-C8 selectivity of 83.3 wt %. Moreover, the incorporation of an electron-withdrawing phenyl group to the ligand backbone was found to deteriorate the catalytic performance of Cr in the tri-/tetramerization.