A Chromium-Diphosphine System for Catalytic Ethylene Trimerization: Synthetic and Structural Studies of Chromium Complexes with a Nitrogen-Bridged Diphosphine Ligand with <i>o</i><i>rtho-</i>Methoxyaryl Substituents

To gain molecular-level insight into the important features of a chromium-diphosphine catalytic system for ethylene trimerization, the coordination chemistry of chromium with “PNP” ligands (PNPOMe = (o-MeOC6H4)2PN(CH3)P(o-MeOC6H4)2, Pt-BuNi-amylPOMe = (2-MeO-4-t-BuC6H3)2PN(i-amyl)P(2-MeO-4-t-BuC6H3)2) has been explored. Chromium(0) carbonyl complexes have been synthesized by CO displacement with diphosphine. Oxidation of Cr(CO)4{κ2-(P,P)-(PNPOMe)} with I2, Br2, and PhICl2 generates the corresponding chromium(III) halide complexes. Chromium(III) complexes CrCl3{(κ3-(P,P,O)-(PNPOMe)}, CrCl3{κ3-(P,P,O)-(Pt-BuNi-amylPOMe)}, and CrCl2(CH3){κ3-(P,P,O)-(PNPOMe)} can be synthesized by metalation with CrCl3(THF)3 or CrCl2(CH3)(THF)3. Reaction of CrCl3{κ3-(P,P,O)-(PNPOMe)} with o,o‘-biphenyldiyl diGrignard affords CrBr(o,o‘-biphenyldiyl){κ3-(P,P,O)-(PNPOMe)}. Single-crystal X-ray diffraction studies show that the Cr−O and Cr−P distances can vary significantly as a function of metal oxidation state and the other ligands bound to chromium. Variable-temperature 2H NMR spectroscopy studies of chromium(III) complexes supported by PNP ligands indicate fluxional behavior with the ether groups interchanging at higher temperatures. Low-temperature 2H NMR spectra are consistent with solution structures similar to the ones determined in the solid state.