Use of PCCP\nLigands with a <i>P</i>‑Ph(alkyl)\nGroup in Chromium-Catalyzed Ethylene Tri-/Tetramerization

Cr(III) complexes supported by the ligands of the type\nPh<sub>2</sub>PC(R)CHPPh(alkyl) were prepared and tested in\nethylene tri-/tetramerization.\nAll the complexes exhibited predominant tetramerization selectivity.\nIt is found that not only the steric properties of both the backbone\nsubstituent and the <i>P</i>-substituent but also the electronic\nproperties of the backbone substituent affect the catalytic performance.\nPrecatalyst <b>6</b> bearing an isopropyl group on the ligand\nbackbone achieved the highest combined 1-C<sub>6</sub>/1-C<sub>8</sub> selectivity (84.8 wt %) with a high 1-octene selectivity (56.1 wt\n%), maintaining a high activity of 3662 kg/g Cr·h. Precatalyst <b>2</b> bearing a 3-pentyl group on the ligand backbone exhibited\nexcellent high-temperature stability at 100 °C, achieving a high\nactivity of up to 2213 kg/g Cr·h and a high combined 1-C<sub>6</sub>/1-C<sub>8</sub> selectivity (87.9 wt %) with a low PE formation\n(0.08 wt %). Catalysis using the ligands with a primary <i>P</i>-alkyl substituent led to a low combined 1-C<sub>6</sub>/1-C<sub>8</sub> selectivity (69.4–75.2 wt %). Introduction of β-branching\nat the primary <i>P</i>-alkyl substituent to give precatalyst <b>14</b> with a <i>P</i>-benzyl group could improve the\noverall catalytic performance, giving a markedly increased activity\nwith an improved combined 1-C<sub>6</sub>/1-C<sub>8</sub> selectivity\nof 83.3 wt %. Moreover, the incorporation of an electron-withdrawing\nphenyl group to the ligand backbone was found to deteriorate the catalytic\nperformance of Cr in the tri-/tetramerization.