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 Cr^II ([CrCl₂(thf)₂]) or Cr^III ([CrCl₃(thf)₃] or [Cr­(Me)­Cl₂(thf)₃]) precursors. The solid-state structures of these four complexes show square-planar Cr^II centers, with two trans chloride and two monodentate C_NHC 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 Cr^III coordination, comprising a chelating ligand through the C_NHC 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 Cr^III/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.