PCNCP Ligands in the Chromium‐Catalyzed Oligomerization of Ethylene: Tri‐ versus Tetramerization

Chromium(III) complexes bearing R′N(CH2PR2)2 (PCNCP) ligands have been prepared. Upon activation with methylaluminoxane, these complexes proved to be effective in the selective tri‐ and tetramerization of ethylene. The formation of either 1‐hexene or 1‐octene was found to be highly dependent on the steric bulk of the substituents R on the phosphine moieties. This observation was rationalized by using density functional theory calculations on selected steps of the metallacyclic mechanism of the ethylene oligomerization reaction. Steric bulk directs selectivity: DFT methods have been used to rationalize the observed selectivity towards either 1‐hexene and/or 1‐octene in the chromium‐catalyzed ethylene oligomerization reaction employing bis(phosphinomethyl)amine ligands with different degrees of steric bulk on the phosphorus moieties (see scheme).