Ethylene Trimerization Catalysts Based on Chromium Complexes with a Nitrogen-Bridged Diphosphine Ligand Having o rtho-Methoxyaryl or o rtho-Thiomethoxy Substituents:  Well-Defined Catalyst Precursors and Investigations of the Mechanism

• Published in: Organometallics Vol. 25; no. 11; pp. 2743 - 2749
• Main Authors: Schofer, Susan J, Day, Michael W, Henling, Lawrence M, Labinger, Jay A, Bercaw, John E
• Format: Journal Article
• Language: English
• Published: American Chemical Society 22.05.2006
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om0506062

Chromium-based ethylene trimerization catalyst precursors ((PNPOMe-d 12)CrPh3 (4) and (PNPOMe-d 12)CrPh2Cl (7)) having a bis(diphenylphosphino)amine ligand (o-CD3OC6H4)2PN(CH3)P(o-CD3OC6H4)2 ((PNPOMe-d 12) = 1) have been prepared and characterized. A thioether analogue (o-CD3SC6H4)2PN(CH3)P(o-CD3SC6H4)2 ((PNPSMe-d 12) = 2) and its triphenylchromium complex (PNPSMe)CrPh3 (5) have also been synthesized. The solid-state structures of 4 and 7 display octahedral geometries with a κ3-(P,P,O) coordination of PNPOMe ligands having chromium−oxygen bond lengths of 2.29−2.44 Å. Compound 5 differs, exhibiting (S,P,S)-κ3 coordination of the PNPSMe ligand. The deuteromethyl groups allow for 2H NMR characterization of these paramagnetic complexes in solution. Dynamic exchange processes occur in solution at room temperature to render all four of the methoxy or thioether groups equivalent on the 2H NMR time scale; two distinct coalescence processes are observed by variable-temperature 2H NMR spectroscopy for all compounds. The neutral species 4 and 7 react with ethylene (1 atm) by insertion into chromium−phenyl bonds with the release of styrene and ethylbenzene, but 1-hexene is not observed under these conditions. Activation of 4 by protonation and activation of 7 by halide abstraction in the presence of ethylene provide active trimerization catalysts that give turnover numbers for 1-hexene as high as 3000 mol 1-hexene·mol-1 Cr. These catalysts display comparable activity and selectivity for 1-hexene compared to the original BP system, where the catalyst is generated in situ from CrCl3(THF)3, 1, and MAO. Both the well-defined systems and the CrCl3(THF)3/PNPOMe/MAO system provide catalysts that undergo an initiation period followed by an apparent first-order decomposition process. Activated complexes 4 and 7 initiate trimerization primarily through ethylene insertion into the chromium−phenyl bond, followed by β-hydrogen elimination and reductive elimination to give the active species, rather than via reductive elimination of biphenyl.