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

Abstract

Chromium-based ethylene trimerization catalyst precursors ((PNP^(OMe)_(-d)_(12))CrPh_3 (4) and (PNP^(OMe)_(-d)_(12))CrPh_2Cl (7)) having a bis(diphenylphosphino)amine ligand (o-CD_3OC_6H_4)_2PN(CH_3)P(o-CD_3OC_6H_4)_2 ((PNP^(OMe)_(-d)_(12)) = 1) have been prepared and characterized. A thioether analogue (o-CD_3SC_6H_4)_2PN(CH_3)P(o-CD_3SC_6H_4)_2 ((PNP^(SMe)_(-d)_(12)) = 2) and its triphenylchromium complex (PNP^(SMe))CrPh_3 (5) have also been synthesized. The solid-state structures of 4 and 7 display octahedral geometries with a κ^3-(P,P,O) coordination of PNP^(OMe) ligands having chromium−oxygen bond lengths of 2.29−2.44 Å. Compound 5 differs, exhibiting (S,P,S)-κ^3 coordination of the PNP^(SMe) 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 CrCl_3(THF)_3, 1, and MAO. Both the well-defined systems and the CrCl_3(THF)_3/PNP^(OMe)/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.