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.为获取乙烯三聚用铬-双膦催化体系关键特征的分子级认知，本研究探索了铬与"PNP"配体的配位化学（其中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）。通过双膦配体取代一氧化碳，成功合成了零价铬羰基配合物。以I2、Br2及PhICl2对Cr(CO)4{κ2-(P,P)-(PNPOMe)}进行氧化反应，可得到对应的三价卤化铬配合物。通过与CrCl3(THF)3或CrCl2(CH3)(THF)3发生金属化反应，可合成三价铬配合物CrCl3{(κ3-(P,P,O)-(PNPOMe)}、CrCl3{κ3-(P,P,O)-(Pt-BuNi-amylPOMe)}以及CrCl2(CH3){κ3-(P,P,O)-(PNPOMe)}。将CrCl3{κ3-(P,P,O)-(PNPOMe)}与邻联苯二基二格氏试剂反应，可得到产物CrBr(o,o‘-biphenyldiyl){κ3-(P,P,O)-(PNPOMe)}。单晶X射线衍射研究结果显示，Cr-O与Cr-P键长会随金属氧化态以及铬结合的其他配体发生显著变化。对PNP配体配位的三价铬配合物开展变温氘核磁共振（2H NMR）光谱研究，结果表明在较高温度下醚基团发生动态交换，表现出流变行为。低温下的2H NMR光谱结果与固态解析得到的溶液结构一致。