Observation of non-chelated bis(pentamethylcyclopentadienyl)zirconium-alkyl–alkene complexes is thwarted by competitive arene or amine coordination or by β-hydride elimination

The reaction of Cp*2Zr(CH3)2 (1) with [(C6H5)3C][B(C6F5)4] in CD2Cl2 at −78 °C proceeds with the unexpected formation of [Cp*2Zr(CH3)η-C6H5C(C6H5)2CH3][B(C6F5)4] (2). Evidence for the coordination of one of the phenyl rings to zirconium comes from 1H- and 13C-NMR chemical shifts and from nOe experiments showing spin saturation transfer from the Cp* methyl protons to the protons of the bound phenyl ring. No chemical exchange between the bound and free phenyl rings is observed up to 0 °C, where decomposition to intractable products occurs. Attempts to disfavor coordination of the arene ring by employing a more sterically protective isobutyl substituent in Cp*2Zr(CH3)CH2CH(CH3)2 (7) led to rapid, quantitative β-hydride elimination producing isobutylene and [Cp*2Zr(H)η-C6H5C(C6H5)2CH3][B(C6F5)4] (8) even at temperatures as low as −135 °C. Addition of propylene to cold solutions of the trityl-coordinated complexes resulted in very rapid formation of polypropylene. This polymerization resulted in no observable changes in the NMR spectra of the zirconium complexes in solution, implying a very rapid rate of propagation following a much slower first monomer insertion.