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

Abstract

The chromium(II) complexes [CrCl 2 ( t-Bu NHC,P-κC) 2 ] (1), [CrCl 2 ( Mes NHC,P-κC) 2 ] (2), [CrCl 2 ( Dipp NHC,P-κC) 2 ] (3), and [CrCl 2 (P,NHC,P-κC) 2 ] (4) containing the N-phosphanyl-or N,N′-diphosphanyl-substituted N-heterocyclic carbene (NHC) hybrid ligands t-Bu NHC,P (1-(di-tert-butylphosphino)-3-tert-butylimidazol-2-ylidene), Mes NHC,P (1-(di-tert-butylphosphino)-3-mesitylimidazol-2-ylidene), Dipp NHC,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 ([CrCl 2 (thf) 2 ]) or Cr III ([CrCl 3 (thf) 3 ] or [Cr(Me)Cl 2 (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 ( Dipp NHC,P-κC,κP)] (5) and [Cr(benzyl) 3 (P,NHC,P-κC,κP)] (6), respectively, with fivecoordinate 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 C 2 H 4 /((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 AlEtCl 2 (EADC) as cocatalyst, polyethylene was predominately formed.

Cu(i), Ag(i), Ni(ii), Cr(iii) and Ir(i) complexes with tritopic NimineCNHCNamine pincer ligands and catalytic ethylene oligomerization

“…However, related phenomena have been previously encountered when Cr(II) complexes were reacted with Grignard reagents. 27 A better understanding of the often subtle factors governing the formation/stabilization of Cr(II)/(III) complexes is clearly relevant to their use in catalytic ethylene oligomerization. In the case of Ir(I) chemistry, the coordination of the N imine C NHC N amine ligand could be modified from C NHC monodentate behaviour, with both imine and amine donors remaining pendent, to bidentate C NHC ,N imine coordination after halide abstraction to liberate an additional coordination site.…”

Architecture and synthesis of P,N-heterocyclic phosphine ligands

“…Application References pyridyl phosphines OLEDs [13] Heck coupling [14] metal organic frameworks [15] polymerization of lactides [16] alkene hydroxylation [17] addition reaction [18] ethylene oligomerization [19] synthesis of pyrazolines [20] triazolyl phosphines Suzuki cross coupling [21] asymmetric hydrogenation [22] luminescence [23] hydroformylation [24] pyrazolyl phosphines coordination polymers [25] Heck coupling [26] imidazolyl phosphines Suzuki coupling [27] hydroamination [28] OLEDs [29] ethylene oligomerization [30] amination [31] olefin metathesis [32] hydroformylation [33] pyrrolyl phosphines hydroformylation [34,35] ethylene polymerization [36] oxazolyl phosphines asymmetric cycloaddition [37] asymmetric hydrogenation [38] carbonylation of alkynes [39] allylic substitution [40] asymmetric addition [41] allylic amination [42,43] The presence of soft donor atoms such as phosphorus results in the formation of hemilabile ligands. These are multidentate ligands having hard P-donor and soft N-and/or O-donor atoms [44].…”

Chromium Complexes Supported by Phenyl Ether‐Pyrazolyl [N,O] Ligands as Catalysts for the Oligo‐ and Polymerization of Ethylene

“…[ 13–16 ] In this way, significant efforts have been dedicated to the synthesis of new families of ligands based on a wide variety of donor‐group combinations aiming to generate more efficient chromium catalyst systems that are capable of selectively forming α‐olefins with high productivities. [ 17–81 ] In particular, chromium catalysts stabilized by pyrazolyl‐based tridentate ligands have been successfully synthetized and their catalytic application in ethylene oligomerization explored in details. [ 51,81–91 ] Selected examples are presented in Chart 1.…”