New <i>C</i><sub>1</sub>-Symmetric Ph<sub>2</sub>C-Bridged Multisubstituted <i>ansa</i>-Zirconocenes for Highly Isospecific Propylene Polymerization: Synthetic Approach via Activated Fulvenes
作者:Evgeny Kirillov、Nicolas Marquet、Abbas Razavi、Vincenzo Belia、Frank Hampel、Thierry Roisnel、John A. Gladysz、Jean-François Carpentier
DOI:10.1021/om100289y
日期:2010.11.8
The synthesis of multisubstituted diphenylmethylene-bridged fluorenyl-cyclopentadienyl proligands Ph2C(3,6-tBu(2)FluH)(3-R-1-5-(RC5H3)-C-2) (Flu = fluorenyl; R-1 = tert-butyl, R-2 = H (2a); R-1 = tert-butyl, R-2 = Me (2b); R-1 = cumyl, R-2 = Me (2c)) was developed using a nucleophilic addition protocol based on regular and activated fulvenes. Two highly congested proligands (2b,c) were prepared in a two-step procedure, starting first by addition of [3,6-tBu(2)Flu]Li-+(-) onto 6,6'-bis(p-chlorophenyl)fulvenes, followed by a Pd-catalyzed reductive dechlorination. The X-ray crystal structures of 2a and 2b center dot CH2Cl2 were determined. These revealed a preorganized sandwich-like positioning of the fluorenyl and cyclopentadienyl plane fragments, similar to that observed in the metallocenes. The corresponding dichlorozirconium complexes Ph2C(3,6-tBu(2)Flu)(3-R-1-5-R-2-C5H2)}ZrCl2 (R-1 = tert-butyl, R-2 = H (3a); R-1 = tert-butyl, R-2 = Me (3b); R-1 = cumyl, R-2 = Me (3c)) were prepared by salt metathesis and characterized by elemental analysis, NMR spectroscopy, and X-ray crystallography (for 3a,c). These C-1-symmetric zirconocenes exist as racemic mixtures of two enantiomers, arising from planar chirality at the Cp ring. When activated with MAO, 3a,b showed high activity in the polymerization of propylene (5020 and 3580 kg of iPP mol(-1) h(-1), respectively; toluene solution, 60 degrees C), affording highly isotactic ([m(4)] 94.0%) polymers with molecular weights in the range M-w = 30 000-175 000, which are similar to those obtained with the corresponding CMe2-bridged catalysts. No activity was observed with the 3c/MAO system, which was proposed to be due to a deactivation involving a cumyl phenyl ring. This was supported by OFT computations on the cationic species [Ph2C(3,6-tBu(2)Flu)(3-cumyl-5-Me-C5H2)}ZrMe](+) which revealed facile coordination of the phenyl ring to the metal center, followed by C-H(phenyl) activation at the ortho position with concomitant elimination of methane; propylene insertion into the resulting ortho-metalated species was found to be thermodynamically unfavorable.