The Selective Oligomerization of Ethylene Using Chromium Diphosphine Catalysts

A series of bis(diphenylphosphino)amine ligands with a donor group attached to the nitrogen linker have been prepared. Metalation of these ligands with chromium trichloride provides precursors to highly active, relatively stable, and selective catalysts for trimerization and tetramerization of ethylene. It has been demonstrated in oligomerization reactions performed at 1 and 4 atm of ethylene that these new systems increase total productivity by enhancing catalyst stability, as compared with those lacking a donor group on the diphosphine ligand. The product distributions and minor byproducts provide information relevant to mechanistic issues surrounding these types of reactions. Catalyst decomposition follows second-order kinetics, and does not involve diphosphine dissociation. Furthermore, the solvent effects in the trimerization and tetramerization of ethylene to 1-hexene and 1-octene with an aluminoxane-activated chromium catalyst bearing a bis(diphenylphosphino)amine ligand are also investigated. While reactions in non-polar solvents exhibit poor activity at lower ethylene pressures, those in more polar solvents, such as chlorobenzenes and fluorobenzenes, are highly active and generate very little undesired polymer. Varying the solvent has a significant impact on 1-hexene/1-octene selectivity. Experiments with potentially coordinating additives result in a higher tendency for 1-octene formation. Changes in the aluminoxane co-catalyst have a notable effect on catalyst productivity, however selectivity remains unaffected. The results presented in this work reflect the high tunability of this system by simple modifications of the reaction medium.