Selective Ethylene Oligomerization with Chromium Complexes Bearing Pyridine–Phosphine Ligands: Influence of Ligand Structure on Catalytic Behavior

Chromium
complexes bearing a series of pyridine–phosphine
ligands have been synthesized and examined for their catalytic behavior
in ethylene oligomerization. The choice of solvent, toluene versus
methylcyclohexane, shows a pronounced influence on the catalytic activity
for all these complexes. Variations of the ligand system have been
introduced by modifying the phosphine substituents affecting ligand
bite angles and flexibility. It has been demonstrated that minor differences
in the ligand structure can result in remarkable changes not only
in catalytic activity but also in selectivity toward α-olefins
versus polyethylene and distribution of oligomeric products. Ligand
PyCH2N­(Me)­PiPr2,
in combination with CrCl3(THF)3 afforded selective
ethylene tri- and tetramerization, giving 1-hexene and 1-octene with
good overall selectivity and high purity, albeit with the presence
of small amounts of PE.