Alkylaluminum-Complexed Zirconocene Hydrides: Identification of Hydride-Bridged Species by NMR Spectroscopy
Steven M. Baldwin,John E. Bercaw*† and Hans H. Brintzinger*‡
Reactions of unbridged zirconocene dichlorides, (RnC5H5−n)2ZrCl2 (n = 0, 1, or 2), with diisobutylaluminum hydride (HAliBu2) result in the formation of tetranuclear trihydride clusters of the type (RnC5H5−n)2Zr(μ-H)3(AliBu2)3(μ-Cl)2, which contain three [AliBu2] units. Ring-bridged ansa-zirconocene dichlorides, Me2E(RnC5H4−n)2ZrCl2 with E = C or Si, on the other hand, are found to form binuclear dihydride complexes of the type Me2E(RnC5H4−n)2Zr(Cl)(μ-H)2AliBu2 with only one [AliBu2] 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 C5-ring ligands. Alkylaluminum-complexed zirconocene dihydrides, previously observed in zirconocene-based precatalyst systems activated by methylalumoxane (MAO) upon addition of HAliBu2 or AliBu3, are proposed to be species of the type Me2Si(ind)2Zr(Me)(μ-H)2AliBu2, stabilized by interaction of their terminal Me group with a Lewis acidic site of MAO.