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

Abstract

The reaction of the ethene tetramerization catalyst, ((C₆H₅)₂P)₂N^iPrCrCl₃(THF) (complex 1), and ethene trimerization catalyst, ((o-C₆H₄OMe)₂P)₂N^MeCrCl₃ (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 Cr^II alkene complex, providing evidence that catalysis could proceed via cationic Cr^II/Cr^IV intermediates. This is supported by DFT calculations, where it is shown that a mechanism proceeding via dicationic Cr^II/Cr^IV intermediates explains the observed product selectivity.