Role of the ligand and activator in selective Cr-PNP ethene tri- and tetramerization catalysts-a spectroscopic study

The reaction of the ethene tetramerization catalyst, ((C6H5)2P)2NiPrCrCl3(THF) (complex 1), and ethene trimerization catalyst, ((o-C6H4OMe)2P)2NMeCrCl3 (complex 2), with alkylaluminum reagents (AlMe3 and MMAO) was investigated using spectroscopic techniques (Cr K-edge XAS, X-band EPR and UV-vis) and catalytic studies. In all cases the majority of chromium was reduced to the divalent oxidation state and only a minor fraction of chromium was reduced further to the monovalent oxidation state. It is demonstrated that MMAO and the ligand (through a pendant ether donor) can facilitate ion pair formation for these divalent Cr complexes, providing insights into the role of the ligand and activator in the activation process. Via the use of dienes, we succeeded in characterizing a monocationic CrII alkene complex, providing evidence that catalysis could proceed via cationic CrII/CrIV intermediates. This is supported by DFT calculations, where it is shown that a mechanism proceeding via dicationic CrII/CrIV intermediates explains the observed product selectivity.