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 (PNPOMe) (o-MeOC6H4)(2)PN(CH3)P(o-MeOC6H4)(2), (Pt-BuNi-amylPOMe) = (2-MeO-4-t-BuC6H3)(2)PN(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) {kappa(2)-(P,P)-(PNPOMe)} with I-2, Br-2, and PhICl2 generates the corresponding chromium(III) halide complexes. Chromium(III) complexes CrCl3 {(kappa(3)-(P,P,O)-(PNPOMe)}, CrCl3{kappa(3)-(P,P,O)-((Pt-BuNi-amylPOMe))}, and CrCl2(CH3){kappa(3)-(P,P,O)-(PNPOMe)} can be synthesized by metalation with CrCl3(THF)(3) or CrCl2(CH3)(THF)(3). Reaction of CrCl3{kappa(3)-(P,P,O)-(PNPOMe)} with o,o'-biphenyldiyl diGrignard affords CrBr(o,o'-biphenyldiyl){kappa(3)-(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 H-2 NMR spectroscopy studies of chromium(III) complexes supported by PNP ligands indicate fluxional behavior with the ether groups interchanging at higher temperatures. Low-temperature H-2 NMR spectra are consistent with solution structures similar to the ones determined in the solid state.