Ligand effect on ethylene trimerisation with [NNN]-heteroscorpionatepyrazolylCr(III) catalysts

Abstract

Cr(III) complexes with [NNN]-heteroscorpionate pyrazolyl ligands of the type Pz’₂CHX (Pz’ = pyrazol-l-yl (Pz), or 3,5-dimethylpyrazol-1-yl (Pz^Me); X = N-containing heterocyclic ring or amine CH₂NR¹R²; R¹, R² = H or alkyl) have been prepared. Upon activation with MAO, they are active for selective ethylene trimerisation to 1-hexene. The effects of the hetero-functional group and chelate ring size on the catalytic performance have been examined. The pre-catalysts with an N-heterocycle substituent show highest activity [32400–53000 g/(g Cr h⁻¹)] and total C₆ selectivities (>97.6%) as well as 1-hexene selectivity (>96.0%) among hexenes. The X-ray single-crystal crystallographic analysis of CrCl₃[Pz^Me₂CH₂NCH₂Ph] and CrCl₃[Pz^Me₂CH₂NCH₂Fc] (Fc = ferrocenyl) shows a tridentate coordination on the fac-octahedral Cr(III) sphere with the Cr-N bond length dependent on the N-substituent.