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-MeOC_6H_4)_2PN(CH_3)P(o-MeOC_6H_4)_2, P^(t-Bu)N^(i-amyl)P_(OMe)) = (2-MeO-4-t-BuC_6H_3)_2PN(i-amyl)P(2-MeO-4-t-BuC_6H_3)_2) has been explored. Chromium(0) carbonyl complexes have been synthesized by CO displacement with diphosphine. Oxidation of Cr(CO)_4{κ^2-(P,P)-(PNP^(OMe))} with I_2, Br_2, and PhICl_2 generates the corresponding chromium(III) halide complexes. Chromium(III) complexes CrCl_3{(κ^3-(P,P,O)-(PNP^(OMe))}, CrCl_3{κ^3-(P,P,O)-(P^(t-)^(Bu)N^(i-amyl)P^(OMe))}, and CrCl_2(CH_3){κ^3-(P,P,O)-(PNP^(OMe))} can be synthesized by metalation with CrCl_3(THF)_3 or CrCl_2(CH_3)(THF)_3. Reaction of CrCl_3{κ^3-(P,P,O)-(PNP^(OMe))} with o,o‘-biphenyldiyl diGrignard affords CrBr(o,o‘-biphenyldiyl){κ^3-(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