Zirconocene dichloride


Zirconocene dichloride is an organozirconium compound composed of a zirconium central atom, with two cyclopentadienyl and two chloro ligands. It is a colourless diamagnetic solid that is somewhat stable in air.

Preparation and structure

Zirconocene dichloride may be prepared from zirconium chloride-THF complex and sodium cyclopentadienide:
The closely related compound Cp2ZrBr2 was first described by Birmingham and Wilkinson.
The compound is a bent metallocene: the Cp rings are not parallel, the average Cp-M-Cp angle being 128°. The Cl-Zr-Cl angle of 97.1° is wider than in niobocene dichloride and molybdocene dichloride. This trend helped to establish the orientation of the HOMO in this class of complex.

Reactions

Schwartz's reagent

Zirconocene dichloride reacts with lithium aluminium hydride to give Cp2ZrHCl Schwartz's reagent:
Since lithium aluminium hydride is a strong reductant, some over-reduction occurs to give the dihydrido complex, Cp2ZrH2; treatment of the product mixture with methylene chloride converts it to Schwartz's reagent.

Negishi reagent

Zirconocene dichloride can also be used to prepare the Negishi reagent, Cp2Zr, which can be used as a source of Cp2Zr in oxidative cyclisation reactions. The Negishi reagent is prepared by treating zirconocene dichloride with n-BuLi, leading to replacement of the two chloride ligands with butyl groups. The dibutyl compound subsequently undergoes beta-hydride elimination to give one η2-butene ligand, with the other butyl ligand promptly lost as butane via reductive elimination.

Carboalumination

Zirconocene dichloride catalyzes the carboalumination of alkynes by trimethylaluminum to give a dimethylalane, a versatile intermediate for further cross coupling reactions for the synthesis of stereodefined trisubstituted olefins.