Mechanistic study of ethylene tri- and tetramerisation with Cr/PNP catalysts: effects of additional donors†
The mechanism of ethylene trimerisation and tetramerisation with chromium–diphosphinoamine (Cr–PNP) catalysts has been studied by experimental and theoretical (DFT) methods. The effects of a pendant ether donor (ortho-methoxyaryl ligand substitution) and of anion coordination to the active species have been studied. In the former case, coordination of the ether donor to chromium favours 1-hexene by suppressing formation of the bis(ethylene) chromacyclopentane intermediate which is postulated to be the major route to 1-octene. The effect of anion coordination is similar and as the coordination strength increases, displacement of the anion by a second ethylene ligand becomes more difficult, again favouring trimerisation over tetramerisation. Hence, the experimentally observed effects of pendant donor coordination and changes in anion coordination strength can be rationalised.