Alkylaluminum-complexed zirconocene hydrides: identification of hydride-bridged species by NMR spectroscopy

Reactions of unbridged zirconocene dichlorides, (R_nC_5H_5−n)_2ZrCl_2 (n = 0, 1, or 2), with diisobutylaluminum hydride (HAl^iBu_2) result in the formation of tetranuclear trihydride clusters of the type (R_nC_5H_5−n)_2Zr(μ-H)_3(Al^iBu_2)_3(μ-Cl)_2, which contain three [Al^iBu_2] units. Ring-bridged ansa-zirconocene dichlorides, Me_2E(R_nC_5H_4−n)_2ZrCl_2 with E = C or Si, on the other hand, are found to form binuclear dihydride complexes of the type Me_2E(R_nC_5H_4−n)_2Zr(Cl)(μ-H)_2Al^iBu_2 with only one [Al^iBu_2] 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 HAl^iBu_2 or Al^iBu_3, are proposed to be species of the type Me_2Si(ind)_2Zr(Me)(μ-H)_2Al^iBu_2, stabilized by interaction of their terminal Me group with a Lewis acidic site of MAO.