An Asymmetric Dinuclear Bis(ansa-Zirconocene) Complex: Synthesis and Performance in Olefin (co-)Polymerization

Abstract

A synthetic strategy to access asymmetric dinuclear bis(ansa-metallocene) pre-catalysts is described. As a key step, the Pd-catalyzed Suzuki cross-coupling of 9,9-bis(trimethylsilyl)-fluoren-2-yl-boronic acid with a substituted 2-bromo-9H-fluorene generates an asymmetric 2,2′-bifluorene platform, which can be individually functionalized at the two differentiated 9-positions. Herein, as a first demonstration of this strategy, we report the asymmetric dinuclear bis(ansa-zirconocene) complex 2,2′-[{Me2C(Flu)(Cp)}ZrCl2][{Me2C(7-tBuFlu)(Cp)}ZrCl2], which has been characterized with NMR spectroscopy and high-resolution mass spectrometry. The performance of this bimetallic pre-catalyst when combined with MAO has been evaluated in ethylene, propylene, and ethylene/1-hexene (co-)polymerization. This pre-catalyst is revealed to be less productive than the mononuclear reference pre-catalyst {Me2C(2,7-tBuFlu)(Cp)}ZrCl2, likely because of higher steric hindrance induced by the linkage at the difluorenyl platform. The resulting (co-)polymers featured only slight differences in terms of molecular weights, tacticity, and comonomer incorporation. No bimodal molecular weight distribution was achieved at any produced polymer; this might have originated from the close similarity of the connected Cp/Flu moieties or a rapid chain-transfer phenomenon between the different active sites which were quite close to each other.