Ethylene Trimerization Catalysts Based on Chromium Complexes with a Nitrogen-Bridged Diphosphine Ligand Having <i>o</i><i>rtho-</i>Methoxyaryl or <i>o</i><i>rtho-</i>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 2 Cl ( 7 )) having a bis(diphenylphosphino)amine ligand ( o- CD 3 OC 6 H 4 ) 2 PN(CH 3 )P( o- CD 3 OC 6 H 4 ) 2 ((PNP OMe - d 12 ) = 1 ) have been prepared and characterized. A thioether analogue ( o- CD 3 SC 6 H 4 ) 2 PN(CH 3 )P( o- CD 3 SC 6 H 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 2 H 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 2 H NMR time scale; two distinct coalescence processes are observed by variable-temperature 2 H 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.