The Effect of Substituents in Pnpn-H Ligands on Activity and Selectivity of A Chromium-Based Catalyst System for Highly Selective Ethylene Trimerization

The influence of ligand substituents on a highly selective homogeneous catalyst system for the trimerization of ethylene to 1-hexene consisting of chromium(III) acetylacetonate, a ligand with a PNPNH backbone, tetraphenylphosphonium chloride as chlorine source, and triethyl aluminum as activator was investigated. These N-modified and P-modified ligands, which differ strongly in their electronic and steric properties of the substituents, were tested in the ethylene trimerization at three temperatures (50, 70, and 90 degrees C). Interestingly, the substituents at the N atoms of the PNPNH-backbone had only little effect on the catalytic activity, on 1-hexene selectivity, and on thermal stability of the catalyst system, whereas the substituents at the P atoms significantly influenced the efficiency of the system. This is even more surprising as the PNP ligand from SASOL for ethylene tetramerization is much more sensitive against substituent variation at its N atom.