N-Heterocyclic Carbene Chelated Metal Complexes as Polymerization Catalysts
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N-Heterocyclic Carbene Chelated Metal Complexes as Polymerization Catalysts Amer A. El-Batta1 1 Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia ABSTRACT The synthesis of titanium and zirconium complexes ligated by bidentate “salicylaldiminelike” N-heterocyclic carbenes (NHCs) is reported. These complexes are rare examples of group IV transition metal NHC adducts. In the presence of methylaluminoxane these complexes are useful initiators for the polymerization of ethylene and the copolymerization of ethylene with norbornene and 1-octene. Linear high density polyethylene could be produced. The Ti complexes are also utilized to yield highly syndiotactic polystyrene. INTRODUCTION N-Heterocyclic carbenes (NHCs) are a class of stable carbenes [1] that have found increasing use as ligands in transition metal catalysis [2]. The distinctive electronic properties of NHCs, namely their high σ-basicity and low π-acidity [3], often produce enhanced activity in transition-metal catalyzed processes. Recently was reported a series of unsymmetrically substituted NHC ligands that feature a chelating phenoxide moiety appended to one of the NHC N atoms (Figure 1) [4]. These ligands, which are analogous to the salicylaldimine (sal) framework, form robust organometallic fragments with late metals such as Ni and Pd [5]. The synthesis of early metal complexes of these “sal-like” NHCs was previously described as well as their activity in ethylene polymerization [6]. O
M
R
N
Ar
Ar H
Salicylaldimine framework
O
M
R
N N
Proposed chelating carbene
Figure 1. An NHC analogue of sal ligands. EXPERIMENTAL DETAILS General All reactions involving metal complexes were conducted in oven-dried glassware under a nitrogen atmosphere using standard glovebox techniques. Solvents were prepared by passage through alumina. All commercially obtained reagents were used as received. Organic reagents were purchased from Sigma-Aldrich and metal salts obtained from Strem. The co-catalyst, methaluminoxane (MAO) was supplied by Albermale as 1.2 M toluene solution, and the
remaining trimethylaluminum was evaporated in vacuo prior to use. 1H and 13C NMR spectra were recorded on a Varian Mercury 500 spectrometer (at 500 MHz and 125 MHz respectively) and are reported as parts per million relative to Me4Si (δ 0.0). Preparation of 1-(2,6-Diisopropylphenyl)-3-(2-hydroxy-3-(adamant-1-yl)-5-methylphenyl)4,5-dihydro-imidazolyl titanium(IV) dichloride (2a) 1-(2,6-Diisopropylphenyl)-3-(2-hydroxy-3-(adamant-1-yl)-5-methylphenyl)-4,5-dihydroimidazolium chloride (1) (262 mg, 0.520 mmol, 1.00 equiv) and potassium hexamethyldisilazide (216 mg, 1.04 mmol, 2.10 equiv) were weighed together in a vial in the glovebox. THF (~5 mL) was added to the mixture of solids to provide a light yellow solution with a light precipitate. This was added to a round-bottomed flask and allowed to stir for 10 min. At this point, a suspension of (thf)TiCl4 (86 mg, 0.260 mmol, 0.50 equiv) in THF (~5 mL) was added. The resulting yellow suspensi
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