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

Reactions of unbridged zirconocene dichlorides, (R(n)C(5)H(5-n))(2)ZrCl(2) (n = 0, 1, or 2), with diisobutylaluminum hydride (HAl(i)Bu(2)) result in the formation of tetranuclear trihydride clusters of the type (R(n)C(5)H(5-n))(2)Zr(mu-H)(3)(Al(i)Bu(2))(3)(mu-Cl)(2), which contain three [Al(i)Bu(2)] units. Ring-bridged ansa-zirconocene dichlorides, Me(2)E(R(n)C(5)H(4-n))(2)ZrCl(2) with E = C or Si, on the other hand, are found to form binuclear dihydride complexes of the type Me(2)E(R(n)C(5)H(4-n))(2)Zr(Cl)(mu-H)(2)Al(i)Bu(2) with only one [Al(i)Bu(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(i)Bu(2) or Al(i)Bu(3), are proposed to be species of the type Me(2)Si(ind)(2)Zr(Me)(mu-H)(2)Al(i)Bu(2), stabilized by interaction of their terminal Me group with a Lewis acidic site of MAO.