A Chelating N-Heterocyclic Carbene Ligand in Organochromium Chemistry

Abstract

Several chromium complexes supported by a chelating N-heterocyclic carbene ligand have been prepared in an attempt to study their ability to catalyze the polymerization of ethylene. LCrCl 3 (THF) (3) (L ) 1,1′-methylene-3,3′-di-2,6-diisopropylphenylimidazole-2,2′-diylidene (2)) was found to polymerize ethylene when activated by either excess methylaluminoxane (MAO) or excess Et 2 AlCl. Chloride abstraction from 3 with Na + BARF -(BARF ) tetrakis(3,5-di-trifluoromethylphenyl)borate) yielded a dinuclear, chloridebridged product, [(LCrCl) 2 (µ-Cl) 3 ] + BARF -(4). In a similar fashion, reaction of 3 with 3.0 equiv of AlMe 3 gave the dinuclear, chloride-bridged alkyl cation [(LCrMe) 2 (µ-Cl) 3 ] + AlX 4 -(X ) Me, Cl) (5), which is isostructural to 4. A triaryl complex was also prepared by reaction of 2 with CrPh 3 (THF) 3 , giving LCrPh 3 (6) in good yield. Similarly, a series of alkyl and aryl chloride complexes of the form LCrRCl 2 (THF) (R ) Me (7a), Ph (7b), tolyl (7c)) have also been made from the appropriate CrRCl 2 -(THF) 3 starting material. Further treatement of 7a with Na + BARFresulted in the formation of the BARFsalt of complex 5. Complexes 3-6 and 7a-c catalyze the polymerization of ethylene when activated by MAO. Preliminary studies with Cr(II) have shown that LCrX 2 (THF) (X ) Cl (8a), Br (8b)) does not polymerize ethylene upon activation by either MAO or alkylaluminum reagents. Complexes 3-6, 7c, and 8b have been structurally characterized by X-ray diffraction methods.

N-heterocyclic carbene complexes of Group 6 metals

“…Theopold et al [176] reported the reaction of a di-NHC ligand 1,1'-methylene-3,3'-di-2,6-diisopropylphenylimidazol-2,2'-diylidene with CrCl 2 in THF giving Cr-69a ( 3 with the 'pincer' 2,6-bis(imidazolylidene)pyridine whilst the latter from the reaction of (CH-N-CH)Br 2 with Cr[N(SiMe 3 ) 2 ] 2 (THF) 2 with further amine elimination [179]. Complex Cr-81d has the similar geometry to Cr-81a with Cr-C carbene bond lengths of 2.098(3) Å.…”

“…11 Cr(II)(III) complexes with di-NHC ligands[176].The first tetracarbene Cr complex Cr-77[178] (Figure 4.12) was reported by Jenkins et al in 2012. It was prepared from a transmetallation method with a dinuclear silver complex with macrocyclic tetra-NHC.…”

“…10 Cr(II)-NHC complexes containing di-NHC ligand[176,177] In the same paper[176],Theopold et al also reported the synthesis of Cr(III)-NHC complex Cr-71 (Figure 4.11) bearing the same di-NHC ligand. In Cr-71, Cr(III) centre shows a nearperfect octahedral geometry with the Cr-C carbene bond lengths of 2.134(5) and 2.109(5) Å. Chloride abstraction of Cr-71 with Na + BARF -(tetrakis-(3,5-bis-trifluoromethylphenyl)borate) affords a dinuclear chloride-bridged complex Cr-72 (Figure 4.11) with Cr-Cr distance of 3.1116(12) Å.…”

“…Cr-71 gives moderate activities with Et 2 AlCl. With MAO, Cr-72 and Cr-73 are highly active as compared to Cr-71and Cr-76(a-c) which only show moderate activities[176].…”

Synthesis, characterization, and norbornene polymerization of η3-benzylnickel(II) complexes of N-heterocyclic carbenes

“…As a consequence, NHC complexes are more stable towards dissociation or degradation. There have been some reports of NHC metal complexes utilized as catalyst precursors for olefin polymerizations [3][4][5][6][7][8][9]. In this study, we report the preparation, characterization, and olefin polymerization reactivity of g 3 -benzylnickel complexes of the NHC ligands, 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene (IiPr) and 1,3-bis-(2,6-diisopropylphenyl)imidazolin-2-ylidene (SIiPr).…”