Alkylaluminum-Complexed Zirconocene Hydrides: Identification of Hydride-Bridged Species by NMR Spectroscopy

Reactions of unbridged zirconocene dichlorides, (RnC5H5-n)(2)ZrCl2 (n = 0, 1, or 2), with diisobutylaluminum hydride ((HAlBu2)-Bu-i) result in the formation of tetranuclear trihydride clusters of the type (RnC5H5 (n))(2)Zr(mu-H)(3)((AlBu2)-Bu-i)(3)(mu-Cl)(2), which contain three ((AlBu2)-Bu-i) units. Ring-bridged ansa-zirconocene dichlorides, Me2E(RnC5H4 (n))(2)ZrCl2 with E = C or Si, on the other hand, are found to form binuclear dihydride complexes of the type Me2E(RnC5H4 (n))(2)Zr(Cl)(mu-H)(2)(AlBu2)-Bu-i with only one [(AlBu2)-Bu-i] unit. The dichotomy between unbridged and bridged zirconocene derivatives with regard to tetranuclear versus binuclear product formation is proposed to be connected to different degrees of rotational freedom of their C-5-ring ligands. Alkylaluminum-complexed zirconocene dihydrides, previously observed in zirconocene-based precatalyst systems activated by methylalumoxane (MAO) upon addition of (HAlBu2)-Bu-i or (AlBu3)-Bu-i, are proposed to be species of the type Me2Si(ind)(2)Zr(Me)(mu-H)(2)(AlBu2)-Bu-i, stabilized by interaction of their terminal Me group with a Lewis acidic site of MAO.