A Chromium-Diphosphine System for Catalytic Ethylene Trimerization:  Synthetic and Structural Studies of Chromium Complexes with a Nitrogen-Bridged Diphosphine Ligand with ortho-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 (PNP^OMe = (o-MeOC₆H₄)₂PN(CH₃)P(o-MeOC₆H₄)₂, P^t-BuN^i-amylP^OMe = (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)-(PNP^OMe)} with I₂, Br₂, and PhICl₂ generates the corresponding chromium(III) halide complexes. Chromium(III) complexes CrCl₃{(κ³-(P,P,O)-(PNP^OMe)}, CrCl₃{κ³-(P,P,O)-(P^t-BuN^i-amylP^OMe)}, and CrCl₂(CH₃){κ³-(P,P,O)-(PNP^OMe)} can be synthesized by metalation with CrCl₃(THF)₃ or CrCl₂(CH₃)(THF)₃. Reaction of CrCl₃{κ³-(P,P,O)-(PNP^OMe)} with o,o‘-biphenyldiyl diGrignard affords CrBr(o,o‘-biphenyldiyl){κ³-(P,P,O)-(PNP^OMe)}. 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.