Investigation of the Role of Hydrides in Zirconocene Catalyzed Olefin Polymerization

The structure and reactivity of zirconocene hydrides in the presence of aluminum alkyls is investigated for both neutral species and cationic species. Unbridged zirconocene dichlorides react with HAliBu2 to yield trihydride dialuminum clusters of the general formula (RnC5H5-n)2Zr(μ-H)3(AliBu2)3(μ-Cl)2. Bridged zirconocenes instead predominantly yield a dihydride monoaluminum cluster of the general form Me2E(RnC5H4-n)2Zr(Cl)(μ-H)2AliBu2 where E = Si or C. For tert-butyl substituted zirconocenes the terminal Cl is replaced by a H. It is shown that steric factors dictate which hydride is formed. A single type of cationic trihydride dialuminum cluster of general formula [(RnC5H4-n)2Zr(μ-H)3(AliBu2)+] is formed for all zirconocene hydrides upon addition of [Ph3C][B(C6F5)4] regardless of which class of neutral hydride was formed. For {(SBI)Zr} and {(Me2Si)2(C5H3)2Zr} the resulting cations were crystallographically characterized where SBI stands for Me2Si(indenyl)2. [(SBI)Zr(μ-H)3(AliBu2)]+ reacts with propene to make isotactic polypropene while the Me-substituted analogue [(SBI)Zr(μ-H)3(AliMe2)]+ is a catalyst for hydroalumination. These trihydride cations are shown to be dormant species in polymerization reactions. [(SBI)Zr(μ-H)3(AliBu2)]+ is identified as the hydride observed by Babushkin and Brintzinger (Babushkin, D. E.; Brintzinger, H. H. Chem. Eur. J. 2007, 13, 5294) upon addition of AliBu3 or HAliBu2 to a mixture of (SBI)ZrCl2 and methylaluminoxane.