PCCP ligands with a semi-rigid backbone for chromium-catalyzed selective ethylene tri-/tetramerization†

A series of semi-rigid PCCP ligands (R′′PhPC(CRR′)CH₂PPhR′′′) containing a rigid olefinic moiety and a flexible methylene moiety within the ligand backbone was prepared, and their application to chromium-catalyzed selective ethylene tri-/tetramerization has been explored. These PCCP ligands could be easily modified at both the olefinic moiety and two P-substituents. Among the ligands bearing two PPh₂ groups, L³, with a phenyl substituent at the olefinic site of the ligand backbone, in combination with chromium exhibited the highest activity (644 kg g⁻¹ Cr h⁻¹) and the highest 1-C₆ selectivity, as well as the highest combined 1-C₆/1-C₈ selectivity at 60 °C. Complex 3, based on L³, also exhibited high thermal stability, giving a high activity of 1259 kg g⁻¹ Cr h⁻¹ at 100 °C. Complex 4, containing L⁴ with a PPhCy group adjacent to the rigid olefinic site, showed improved activity compared to its PPh₂ counterpart 1. At 80 °C, complex 4 exhibited a high combined 1-C₆/1-C₈ selectivity of 91.0 wt%, with almost no PE formation. The addition of hydrogen was shown to be capable of significantly improving the catalytic performance. In the presence of hydrogen, the catalytic activity increased 2.3–3.2-fold for 3 and 4, giving a very high activity of up to 2160 kg g⁻¹ Cr h⁻¹ with significantly reduced PE formation (0.2 wt%).