PNPN-H in Comparison to other PNP, PNPN and NPNPN Ligands for the Chromium Catalyzed Selective Ethylene Oligomerization

Abstract

Many examples exist for the chromium catalyzed selective ethylene oligomerization in which the influence of ligands is essential for the formation of products. Regarding the tri- and tetramerization to 1-hexene or 1-octene mostly PNP ligands are responsible for the tetra- and some of such modified ligands for the trimerization. A very special case in these reactions are PNPN−H ligands, showing in most cases highly selective trimerization of ethylene to 1-hexene. In this review all existing published information about these PNPN−H ligands is accumulated and compared to some other related PNP, PNPN and NPNPN ligands in the chromium catalyzed selective ethylene oligomerization with respect to the switch from tetra- to trimerization and back by different substituent pattern of PNP ligand. Mechanistic information and arguments are collected to explain the switch from tetra- to trimerization and back by substitution of functional groups in classical PNP to PNPN−H ligands as a result of mono- and dinuclear catalytic species.