New C1-Symmetric Ph2C-Bridged Multisubstituted ansa-Zirconocenes for Highly Isospecific Propylene Polymerization: Synthetic Approach via Activated Fulvenes

International audienceThe synthesis of multisubstituted diphenylmethylene-bridged fluorenyl−cyclopentadienyl proligands Ph2C(3,6-tBu2FluH)(3-R1-5-R2C5H3) (Flu = fluorenyl; R1 = tert-butyl, R2 = H (2a); R1 = tert-butyl, R2 = Me (2b); R1 = cumyl, R2 = 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-tBu2Flu]+Li− onto 6,6′-bis(p-chlorophenyl)fulvenes, followed by a Pd-catalyzed reductive dechlorination. The X-ray crystal structures of 2a and 2b*CH2Cl2 were determined. These revealed a preorganized sandwichlike positioning of the fluorenyl and cyclopentadienyl plane fragments, similar to that observed in the metallocenes. The corresponding dichlorozirconium complexes {Ph2C(3,6-tBu2Flu)(3-R1-5-R2-C5H2)}ZrCl2 (R1 = tert-butyl, R2 = H (3a); R1 = tert-butyl, R2 = Me (3b); R1 = cumyl, R2 = Me (3c)) were prepared by salt metathesis and characterized by elemental analysis, NMR spectroscopy, and X-ray crystallography (for 3a,c). These C1-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 °C), affording highly isotactic ([m]4 94.0%) polymers with molecular weights in the range Mw = 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 DFT computations on the cationic species [{Ph2C(3,6-tBu2Flu)(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