Chromium N-phosphinoamidine ethylene tri-/tetramerization catalysts: Designing a step change in 1-octene selectivity

Abstract

Steric reduction principles were applied in N-phosphinoamidine ligand (PN) design to significantly improve 1-octene selectivity in ethylene tri-/tetramerization chromium catalysis. Novel phosphole- and phospholane-based PN-ligands were prepared and used to synthesize discrete chromium complexes which were fully characterized. The highest 1-octene producing Cr/PN/MMAO catalyst system compares favorably with the benchmark Cr/PNP/MMAO system generating a higher overall product selectivity at equivalent yields. Characterization of the structurally related cyclic phosphine-based Cr/PN pre-catalysts revealed that 1-octene selectivity varied inversely with the ligand bite angle, albeit over a small range. In contrast, the acyclic phosphine-based catalyst produces significantly less 1-octene than expected based on its small bite angle. This disparity signals a breakdown of bite angle as the main 1-octene selectivity geometric descriptor and suggests that the key selectivity driver occurs further away from the metal center requiring more sophisticated modeling efforts to properly account for the differences.