New stable aryl-substituted acyclic imino-N-heterocyclic carbene: synthesis, characterisation and coordination to early transition metals

Abstract

The synthesis of the bulky 1-(1-arylimino-2,2-dimethylpropyl)-3-(aryl)imidazolium salt from the corresponding imidazole and the activated imidoyl chloride is presented. The absence of acidic protons adjacent to the iminic carbon allowed for the first isolation of an imino-N-heterocyclic carbene of this ligand class. The free carbene was isolated, structurally characterised, and coordinated to titanium, zirconium, hafnium and chromium. The resulting metal halide complexes were fully characterised and were tested at room temperature and atmospheric pressure for their activity as ethylene polymerisation catalysts. The Zr(IV) complex was found to be the most active with a productivity of 140 kg PE mol M(-1) h(-1).

Coordination and reactivity study of titanium phenoxo complexes containing a bulky bidentate imino-N-heterocyclic carbene ligand

“…A decrease in the C]N stretching frequency from 1662 cm À1 for the free ligand to 1609 cm À1 for 1 suggests coordination of the ligand to the metal centre in a bidentate mode through both the carbenoid carbon and the imine nitrogen atoms. A comparable decrease in stretching frequency was also observed in TiCl 4 (C^Imine) and other related early transition metal complexes [12].…”

“…The catalytic activities of these new complexes towards ethylene polymerisation were assessed. TiCl 4 (C^Imine) showed promising activities and thereby warranted further tailoring of the steric and electronic parameters of the complex through ligand variations [12]. We herein report the synthesis and isolation of the corresponding titanium aryloxo complexes and the attempted synthesis of titanium imido complexes.…”

“…1) [9e11]. Our initial work with early transition metals focussed on the synthesis of group 4 and 6 metal halide complexes coordinated by C^Imine, the first free carbene of this class ever reported (Scheme 1) [12]. The catalytic activities of these new complexes towards ethylene polymerisation were assessed.…”

N-heterocyclic carbene complexes of Group 6 metals

“…Both complexes are not particularly active towards ethylene polymerization (in toluene at atmospheric pressure and room temperature in the presence of 1000 equivalents methylaluminoxane (MAO) as cocatalyst)[183]. 16 Cr complexes with acyclic imino NHC ligands[183] In2012, Braunstein et al reported the Cr complexes containing the anionic NHC amidinato ligands (Cr-86 and Cr-87, Figure 4.17) [184]. The ligand is bidentate through NHC and amidine nitrogen donation.…”

“…15 Cr(III) complexes bearing C^S and C^S^C types of hybrid NHC ligands[180] In 2011, Lavoie et al[183] reported the synthesis of complexes containing aryl-substituted acyclic imino NHC ligand (Cr-84 and Cr-85,Figure 4.16) from the reaction of CrCl 2 (THF) -imidazol-2-ylidene carbene[183]. Cr-85 has a highly distorted octahedral geometry with Cr-C carbene bond length of 2.041(4) Å.…”