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-2*Zr(CH3)(2) (1) with [(C6H5)(3)C][B(C6F5)(4)] in CD2Cl2 at - 78 degreesC proceeds with the unexpected formation of [Cp-2*Zr(CH3)eta-C6H5C(C6H5)(2)CH3][B(C6F5)(4)] (2). Evidence for the coordination of one of the phenyl rings to zirconium comes from H-1- and C-13-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 degreesC, where decomposition to intractable products occurs. Attempts to disfavor coordination of the arene ring by employing a more sterically protective isobutyl substituent in Cp-2*Zr(CH3)CH2CH(CH3)(2) (7) led to rapid, quantitative beta-hydride elimination producing isobutylene and [Cp-2*Zr(H)eta-C6H5C(C6H5)(2)CH3][B(C6F5)(4)] (8) even at temperatures as low as - 135 degreesC. 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. (C) 2002 Elsevier Science B.V. All rights reserved.