Oxygen-Bridged Hybrid Metallocene−Nonmetallocene Polymetallic Catalysts of Group 4 Metals for Bimodal Activity in Olefin Polymerization: Synthesis, Characterization, and Theoretical Investigation

Abstract

This article describes the syntheses of two covalently linked oxygen-bridged hybrid metallocene-nonmetallocene polymetallic catalysts bearing two different catalytically active group 4 metals. The reactions of Cp*2(Me)Zr(OH) (2) with Ti(NMe2)4 and Hf(NMe2)4 led to the formation of a heterobimetallic compound Cp*2(Me)Zr(micro-O)Ti(NMe2)3 (7) and a trimetallic derivative Cp*2(Me)Zr(micro-O)Hf(NMe2)2(micro-O)Zr(Me)Cp*2 (9), respectively, under the elimination of Me2NH. The crystal data confirm the molecular structures of 7 and 9, which crystallize in the space groups P and P21/n, respectively. 9 is the first example of a crystallographically characterized heterotrimetallic complex having a Zr-O-Hf-O-Zr core. 7 bearing two active catalytic centers, namely, zirconium and titanium, exhibits bimodal activity in olefin polymerization when activated with methylalumoxane (MAO). It produces polyethylene largely controlled by the zirconium center, and polystyrene seems to be formed predominantly by the titanium center. DFT calculations were performed on the supposed cationic intermediates, revealing that a cation generated on the titanium center is sterically more accessible for monomer binding, though it is energetically less-favorable than that generated on the zirconium center.