N-Phosphanyl- and N,N′-Diphosphanyl-Substituted N-Heterocyclic Carbene Chromium Complexes: Synthesis, Structures, and Catalytic Ethylene Oligomerization

The chromium(II) complexes CrCl2(t-BuNHC,P-κC)2 (1), CrCl2(MesNHC,P-κC)2 (2), CrCl2(DippNHC,P-κC)2 (3), and CrCl2(P,NHC,P-κC)2 (4) containing the N-phosphanyl- or N,N′-diphosphanyl-substituted N-heterocyclic carbene (NHC) hybrid ligands t-BuNHC,P (1-(di-tert-butylphosphino)-3-tert-butylimidazol-2-ylidene), MesNHC,P (1-(di-tert-butylphosphino)-3-mesitylimidazol-2-ylidene), DippNHC,P (1-(di-tert-butylphosphino)-3-(2,6-diisopropylphenyl)imidazol-2-ylidene), and P,NHC,P (1,3-bis(di-tert-butylphosphino)imidazol-2-ylidene), respectively, were prepared from CrII (CrCl2(thf)2) or CrIII (CrCl3(thf)3 or Cr(Me)Cl2(thf)3) precursors. The solid-state structures of these four complexes show square-planar CrII centers, with two trans chloride and two monodentate CNHC donors. Alkylation of 3 and 4 with Mg(benzyl)2(thf)2 led to the formation of the σ complexes Cr(benzyl)3(DippNHC,P-κC,κP) (5) and Cr(benzyl)3(P,NHC,P-κC,κP) (6), respectively, with five-coordinate distorted-square-pyramidal CrIII coordination, comprising a chelating ligand through the CNHC and one P donor and three benzyl groups. These complexes were used as precatalysts in ethylene oligomerization, and it was found that the nature of the cocatalyst used and the metal oxidation state have a remarkable influence on the catalytic properties. The CrIII/MAO systems displayed superior catalytic performance (TOF values up to 16320 mol of C2H4/((mol of Cr) h) for 6) and gave mostly oligomers. Interestingly, the oligomers obtained with complex 3 were almost exclusively 1-hexene and 1-butene when the reaction was initiated at 30 °C. The overall activities and selectivities were also affected by the initial reaction temperature and the nature of the solvent. With AlEtCl2 (EADC) as cocatalyst, polyethylene was predominately formed.