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

Abstract

A series of semi-rigid PCCP ligands (RR’C=C(PPhR’’)(CH2PPhR’’’)) containing a rigid olefinic moiety and a flexible methylene moiety within the ligand backbone were prepared and their application in the chromium-catalyzed selective ethylene tri-/tetramerization has been explored. These PCCP ligands could be easily modified at both olefinic moiety and two P-substituents. Among the ligands bearing two PPh2 groups, L3with a phenyl substituent on the olefinic site of ligand backbone in combination with chromium exhibited the highest activity (644 kg/g Cr·h) and the highest 1-C6 selectivity as well as the highest combined 1-C6/1-C8 selectivity at 60 oC. Complex 3 based on L3also exhibited high thermal stability, giving a high activity of 1259 kg/g Cr·h at 100 oC. Complex 4 containing L4 with a PPhCy group adjacent to the rigid olefinic site showed an improved activity compared to its PPh2 counterpart 1. At 80 oC, complex 4 exhibited a high combined 1-C6/1-C8 selectivity of 91.0 wt % with almost no PE formation. Addition of hydrogen was proved to be capable of improving significantly 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 Cr·h with a significantly reduced PE formation (0.2 wt %).